Articles, etc., with Abstracts & Links to PDFs

Seidel, I. J., Litavecz, M. S., Putney, S., Davis, M. E., and Hubbe, M. A. “Effects of carboxymethyl starch as a papermaking additive,” TAPPI J. 23(2), 78-87.

Carboxymethyl starch (CMS) is a bio-based, anionic polymer that has potential as part of a dry-strength additive program for papermaking. Due to its negative charge, its effects can be expected to depend on its interactions with various cationic agents.  In this work, the effects of CMS were observed following its sequential addition after one of three selected cationic strength agents at different dosage levels. In selected tests, the furnish was pretreated at the 1% level by a dispersant, sodium polyacrylate, which might represent a high level of anionic contaminants in a paper mill system. Laboratory tests were conducted to show the effects on dewatering, fine-particle retention, and flocculation. These tests were supplemented with measurements of charge demand, zeta potential, and handsheet properties. Sequential addition of cationic glyoxylated acrylamide copolymers (gPAM) and CMS were found to strongly promote dewatering. Two gPAM products and a poly(vinylamine) product in sequential addition with CMS were very effective for promoting fine-particle retention. These same sequential treatments of the stock contributed to moderate fiber flocculation, though severe flocculation was caused by further treatment of the furnish with colloidal silica. Handsheet strength results were mixed.  In the default recycled copy paper furnish, the average breaking length for the sheets made with cationic additives followed by CMS was not greatly different from the blank condition. Superior strength resulted when the default furnish was treated with a dispersant alone. When the dispersant-contaminated furnish was treated with the same combinations of cationic additives and CMS, the strength returned to the baseline achieved in the absence of the dispersant. The results were discussed in terms of the charged character of the different additives and their interactions not only with the fiber surfaces but also with each other.

Putney, S., Seidel, I. J., Litavecz, M. S., and Hubbe, M. A. (2024). “Effects of tissue additives on copy paper forming and properties,”TAPPI J. 23(2), 67-77.

Laboratory tests were conducted in an effort to determine effects on paper machine process attributes and the properties of paper made from recycled copy paper furnish upon the addition of chemical agents that are commonly used in the production of hygiene tissue products. Due to continuing growth in tissue and towel grades of paper, such agents are experiencing greater usage. Charge titration test results revealed that certain dry-strength agents associated with      tissue manufacturing have the potential to shift the balance of charge in papermaking furnish to less negative or even positive values. Creping adhesive was found to contribute to fine particle retention, especially when present at relatively high levels. Release aid and a polyacrylate dispersant had the opposite effect. Low addition levels of both a creping adhesive and a debonding agent surprisingly increased a wide range of strength attributes of paper handsheets in comparison to sheets prepared from unaltered recycled copy paper furnish. The debonding agent decreased paper strength at higher levels of addition. Such effects appear to depend not only on the expected effects of agents themselves, but also on how they affect the charge balance of the wet end system.

Hubbe, M. A. (2024). “Size press practices and formulations affecting paper properties and process efficiency: A Review,” BioResources 19(1), 1925-2002. DOI: 10.15376/biores.19.1.Hubbe

Size presses on paper machines are used to apply a solution of a polymer – usually starch – to the surface of the sheet and thereby to increase the stiffness, surface strength, and printing quality of the product. This article reviews publications dealing with the size press equipment, the materials, and factors affecting both the operating efficiency and attributes of the resulting paper. The emergence of film-press equipment (e.g. blade-metering size presses) in the 1980s has greatly decreased the frequency of web breaks and increased productivity. Starch technology at the size press, though relatively mature, continues to evolve. By adjustment of starch attributes, solids levels, and incorporating other additives, modern papermakers can tune size press outcomes to meet a range of paper product requirements, including strength, hydrophobicity, and the reduction of air permeability. By application of various synthetic polymers, mineral particles, and even nanocellulose in combination with starch or other base polymers, there is potential to extend the technology to meet a range of future needs for paper products.

Shen, J., and Hubbe, M. A. (2024). “Why paper technologists use the terms ‘wet end’ and ‘wet end chemistry’,” BioResources 19(1), 19-22. DOI: 10.15376/biores.19.1.19-22

Terminology plays a crucial role in shaping our understanding of a field. In this editorial, we focus on the widely used terms “wet end” and “wet end chemistry” within the realm of papermaking. By delving into historical records, our aim is to provide a deeper understanding and a clearer perspective on these terms. It is worth noting that exploring terminology can enhance comprehension and foster a more comprehensive understanding of the subject matter.

Hubbe, M. A., Maitland, C., Nanjiba, M., Horst, T., Ahn, K., and Potthast, A. (2023). “Archival performance of paper as affected by chemical components: A Review,” BioResources 18(3), 6430-6498. DOI: 10.15376/biores.18.3.Hubbe

For about two millennia, paper has served as a main medium for preservation of people’s ideas, stories, contracts, and art.  This article reviews what is known about the various components that make up paper from the perspective of their long-term stability under typical storage conditions.  Literature evidence is considered relative to the susceptibility of different paper components to embrittlement, acid hydrolysis, microbiological attack, and discoloration, among others.  The cellulose that makes up a majority of most paper items is demonstrably stable enough to persist for many hundreds of years on the shelves of archival collections, though it is susceptible to acid-catalyzed hydrolysis, which can be accelerated by byproducts of decomposition.  Though less attention has been paid to the archival performance of various minor components of modern paper products, evidence suggests that at least some of them are subject to likely breakdown, embrittlement, or decay in the course of prolonged storage. Based on these considerations, one can envision different categories of paper that can be expected to meet different levels of storage stability: ancient recipes for handmade papermaking, e.g. washi and hanji, archival-grade paper products, ordinary modern alkaline paper products, and paper manufactured without concerns for its longevity.

Hubbe, M. A., Trovagunta, R., Zambrano, F., Tiller, P., and Jardim, J. (2023). “Self-assembly fundamentals in the reconstruction of lignocellulosic materials: A review,” BioResources 18(2), 4262-4331. DOI: 10.15376/biores.18.2.Hubbe

This review article considers processes by which the main components of wood have been reported to arrange themselves into various kinds of organized structures, at least to a partial extent.  The biosynthesis of wood provides the clearest examples of such self-organization.  For example, even before a cellulose macromolecule has been completely synthesized in a plant organism, the leading parts of the polymer chains already will have assembled themselves into organized crystals, i.e., nano-fibrils.  This review then considers a challenge that faces industrial engineers:  how to emulate the great success of natural systems when attempting to achieve favorable materials properties, process efficiency, and environmental friendliness when developing new engineered wood structures, barrier films, and other desired products composed of lignocellulosic materials.  Based on the reviewed literature, it appears that the main chemical components of wood, even after they have been isolated from each other, still have a remnant of their initial tendencies to come back together in a somewhat non-random fashion, following mechanisms that can be favorable for the production of engineered materials having potentially useful functions.

Barrios, N. A., Garland, L. J., Leib, B. D., and Hubbe, M. A. (2023). “Mechanistic aspects of nanocellulose – cationic starch – colloidal silica systems for papermaking,” TAPPI J. 22(2), 107-115. DOI: 10.32964/TJ22.2.107

Optimization of a chemical additive program for a paper machine can require attention to both colloidal charges and kinetic effects.  This work considered an additive program with two negatively charged substances (nanofibrillated cellulose and colloidal silica) and two positively charged items (cationic starch and cationic acrylamide copolymer retention aid).  Results were shown to depend on charge interactions; however that clearly was not the whole story.  Some findings related to cationic demand, dewatering, fine-particle retention, and flocculation among fibers were best explained in terms of at least partly irreversible complexation interactions between the charged entities.  Adjustments in ratios between oppositely charged additives, their sequences of addition, and effects of hydrodynamic shear levels all affected the results.  In general, the most promising results were obtained at a cationic starch level of 0.25 to 0.5% based on sheet solids in systems where the cationic starch was used as a pretreatment for nanofibrillated cellulose.

Hubbe, M. A. (2023). “How to make cellulose hate water – Change it, cover it, confuse it, or accept it as it is,” BioResources 18(2), 2531-2534. DOI: 10.15376/biores.18.2.2531-2534

In many of its current and potential applications, technologists treat the surface of cellulose to render it more hydrophobic. By use of a variety of hydrophobic sizing treatment strategies, the bulk cellulose phase becomes covered up with a layer having lower polarity and less inclination to interact with water. Often, the goal is to use a relatively low amount of additive to cover up or change just the surface of the cellulosic material, while still benefiting from the strength, recyclability, relatively low cost, and other favorable features of the bio-based material. But what often gets forgotten is that the hydrophilic nature of pure cellulose is not very high, and there are ways to manipulate such characteristics without reacting the material or covering it up. Sometimes reacting the cellulose with hydrophobic substituent groups appears to make it more water-loving. So, when thinking broadly of processing options for new applications, there are several contrasting approaches to consider.

Owens, L. P., and Hubbe, M. A. (2023). “Performance factors for filtration of air using cellulosic fiber-based media: A review,” BioResources 18(1), 2440-2519. DOI: 10.15376/biores.18.1.Owens

The filtration of air has attracted increasing attention during recent waves of viral infection.  This review considers published literature regarding the usage of cellulose-based materials in air filtration devices, including face masks.  Theoretical aspects are reviewed, leading to models that can be used to predict the relationship between structural features of air filter media and the collection efficiency for different particle size classes of airborne particulates.  Collection of particles can be understood in terms of an interception mechanism, which is especially important for particles smaller than about 300 nm, and a set of deterministic mechanisms, which become important for larger particles.  The effective usage of cellulosic material in air filtration requires the application of technologies including pulp refining and chemical treatments with such additives as wet-strength agents and hydrophobic sizing agents.  By utilization of high levels of refining, in combination with freeze drying and related approaches, there are opportunities to achieve high levels of interception of fine particles.  A bulky layer incorporating nanofibrillated cellulose can be used in combination with a coarser ply to achieve needed strength in a filter medium.  Results of recent research show a wide range of development opportunities for diverse air filter devices containing cellulose.

This chapter focuses on poly(lactic acid) (PLA) composites containing cellulose or nanocellulose particles, as well as hybrid composites in which two or more types of fillers are used in combination within a PLA matrix.  In addition to the eco-friendly attributes of both the cellulose and the PLA matrix polymer, their formulation as composites holds potential to achieve superior properties.  As recent reviews of this subject already exist, emphasis in this chapter is placed on certain challenging questions that can be expected to influence the paths of future commercial development of PLA cellulose composites and hybrid composites.  These questions consider, for example: the likely scale of production; societal expectations related to end of life and composite biodegradability; the suitability of PLA composites for outdoor applications; the blending of PLA with other polymers; effects related to the size of filler particles; and the role of pilot-scale processing of PLA cellulose composites.  Based on recent literature, the authors expect that the most rapid development and implementation of PLA-cellulose composites and hybrid composites will be in applications having high consumer interactions, e.g. packaging films and 3D printing.  However, multiple developmental paths are likely to shape the uptake and application of PLA composites in the coming years.

Garland, L. J., Leib, B. D., Barrios, N. A., and Hubbe, M. A. (2022). “Nanocellulose-cationic starch-colloidal silica systems for papermaking: Effects on process and paper properties,” TAPPI J. 21(10), 563-570. DOI: 10.32964/TJ21.10.563

Laboratory tests were conducted to better understand effects on the papermaking process and handsheets when recycled copy paper furnish was treated with combinations of nanofibrillated cellulose (NFC), cationic starch, colloidal silica, and cationic retention aid (cPAM).  Dosage-response experiments helped to define conditions leading to favorable processing outcomes, including dewatering rates and the efficiency of fine-particle retention during papermaking.  Effects were found to depend on the addition amounts of cationic starch and colloidal silica added to the system.  It was shown that the presence of a polymer additive such as cationic starch was essential in order to achieve large strength gains with simultaneous usage of NFC.

Leib, B. D., Garland, L. J., Barrios, N. A., and Hubbe, M. A. (2022). “Effects of orders of addition in nanocellulose-cationic starch-colloidal silica systems for papermaking,” TAPPI J. 21(10), 572-579.  DOI: 10.32964/TJ21.10.572

Two orders of addition were compared when preparing paper handsheets from recycled copy paper furnish in combination with nanofibrillated cellulose (NFC), cationic starch, colloidal silica, and cationic retention aid (cPAM).  Faster dewatering and higher fine-particle retention were obtained at equal optimized dosages of additives when the colloidal silica was added last, after addition of the cPAM.  The same order of addition also provided a higher gain in the paper’s tensile strength.  However, higher paper stiffness was achieved when the colloidal silica was instead added to the NFC, after its pretreatment with cationic starch.  Results were consistent with the principle that papermaking additives added shortly before sheetforming tend to have the largest effects on drainage and retention.  The results also demonstrated a sensitivity to the relative dosages of positively and negatively charged additives.

Hubbe, M. A., Szlek, D. B., and Vera, R. E. (2022). “Detergency mechanisms and cellulosic surfaces: A review,” BioResources 17(4), Page numbers to be added. DOI: 10.15376/biores.17.4.Hubbe

The release of soils and impurities from cellulosic surfaces plays a critical role in such processes as the laundering of clothes and the deinking of wastepaper pulps. This article reviews publications that provide evidence about factors that affect such release and the mechanisms by which such factors operate. In general, cellulosic substrates provide advantages for the release of contaminants due to their hydrophilic nature and due to their permeability, allowing the transport of surfactants to contact interfaces with dirt. However, the same permeability of cellulosic material also provides opportunities for contaminants to work themselves into internal crevices and pores, from which they are difficult to remove. The article also reviews aspects of theory related to detergency and how those theories relate to the laundering, deinking, and purifying of substrates based on cellulose and related plant materials. Cellulose and some of its derivatives also can play a role in detergent formulation, especially as builders or as finishes placed on textile surfaces, which sometimes aid in the release of dirt.

This is a book about process additives and papermaking. Process additives help papermakers achieve the needed levels of efficiency and productivity. Key topics covered in this book include influent water treatment, charge control agents, acids and bases, defoamers, corrosion control, organic deposit control, and effluent water treatment.

Process efficiency and final product quality provide motivations for papermakers to employ chemical additives to affect charge and pH. This chapter describes widely used chemical agents for the adjustment of these two variables. As will be shown, there is considerable overlap between the topics of charge and pH with respect to the chemistry of papermaking.

            Common terms such as charge, cationic demand, and anionic character refer to the capacity of either a pulp suspension or process water from a paper mill to interact with and to neutralize a standard cationic titrant. Due to its high chemical stability and pH-independent nature, a commonly used cationic titrant is poly(diallyldimethylammonium chloride) (poly-DADMAC). In less-common situations where a papermaking system may have a net-positive charge, anionic titrants such as the potassium salt of poly(vinyl sulfate) (PVSK) can be used in a titration to determine the anionic demand. Titration methods and charge control technology were covered in the book Advances in Papermaking Wet End Chemistry Application Technologies that is available from TAPPI [Hubbe and Waetzig 2018]. The present chapter emphasizes the chemical agents and their use.

Chen, Y., Li, Y.-H., Zang, C.-Z., Qi, H.-S., and Hubbe, M. A. (2022). “Holocellulosic fibers and nanofibrils using peracetic acid pulping and sulfamic acid esterification,” Carbohyd. Polym. 295(1), article no. 119902.  DOI: 10.1016/j.carbpol.2022.119902

Cellulose provides promising alternatives to synthetic plastics to achieve a low carbon footprint and biodegradable materials, which have significant positive impacts on environmental protection and on human health. In this work, sulfated holocellulose fibers and sulfated holocellulose nanofibrils (SHCNFs) are prepared using a combination of delignification with derivatization to achieve high fiber yield, superior recycling performance, and less energy consumption of the final products by means of preserving hemicellulose. Derivatization of the surface with sulfate groups provides a further means to avoid excessive aggregation between adjacent cellulose surfaces. Interestingly, hemicellulose increases the accessibility of holocellulose fibers and reduces the embodied energy during sulfate esterification. The presence of hemicellulose imparts high optical transmittance, mechanical performance (ultimate strength, 390 MPa; Young’s modulus, 33 GPa), and recyclability for SHCNFs. This combination of two treatments can unlock the greater potential of cellulose as a sustainable material over its entire life cycle.

Li, Z.-Y., Liu, C.-Z., Hong, S., Lian, H.-L., Mei, C.-T., Lee, J.-Y., Wu, Q.-L., Hubbe, M. A., and Li, M.-C. (2022). “Recent advances in extraction and processing of chitin using deep eutectic solvents,”Chem. Eng. J., early availability, DOI: 10.1016/j.cej.2022.136953

Chitin, as the most abundant non-wood biopolymer on earth, exists widely in the shells of crustaceans, the cell walls of fungi, and the cuticles of insects. It consists of N-acetyl-D-glucosamine and D-glucosamine units with various molar ratios linked by β-(1 → 4)-glycoside bonds. The inherently recalcitrant structure of chitin-rich raw materials and the high density of hydrogen bonding between chitin’s molecules result in difficulties in the extraction and further processing. Currently, the mineral acids and bases are dominant in the extraction and processing of chitin; however, they are harmful to our human health as well as the environment. In recent years, the use of deep eutectic solvents (DESs) to extract and process chitin has attracted considerable interest due to its superior sustainability, low toxicity, cost-effectiveness, facile preparation, biodegradability and recyclability. The present review provides a critical overview of recent advances in the utilization of DESs for the extraction and processing of chitin, including isolation, dissolution, surface modification, and nanomaterial production. Particular emphases are placed on the mechanism, characterization method, and governing factors. We also outline the crucial challenges and limitations in this field, and then propose perspectives and future directions. It is anticipated that this review will provide some insights into the structure–function relationship of the extraction and processing of chitin using DESs.

Tyagi, P., Gutierrez, J. N., Lucia, L. A., Hubbe, M. A., and Pal, L. (2022). “Evidence for antimicrobial activity in hemp hurds and lignin-containing nanofibrillated cellulose materials,”Cellulose __(_), early availability online.

Antimicrobial activity is one of the most well-studied properties of hemp, with numerous contradictions and anecdotal information. However, most of the studies on antimicrobial activity of hemp have been focused on extractives, obtained by use of polar or nonpolar solvents such as petroleum ether, acetone, methanol, and ethanol, etc. This work compared the antimicrobial activity of unprocessed hemp hurds powder, processed hemp hurds-lignin
containing nano-fibrillated cellulose (LNFC), and their ethanol extractives. The hemp hurd shives were pulped (defibrillated) to obtain fibers by employing a hydrothermal, carbonate and kraft pulping process. The obtained cellulose fibers were further processed into LNFC using intensive mechanical processing. LNFC films were prepared using a solvent casting
method. The extractives of hemp hurd powder and differently treated pulps were obtained using ethanol and benzene. The obtained extractives were used to treat selected sterilized paper discs and hydrothermal pulped LNFC films. Extractives from the hemp hurds were characterized for the presence of antimicrobial active compounds using GC–MS. The LNFC films and extractives-treated paper substrate were tested against E. coli for their antimicrobial activity. The extractives-treated paper showed a signifcant reduction in bacterial growth and resulted in a zone of bacterial inhibition up to 1.85 mm in disk diffusion
assays. The antimicrobial activity of extractives was confirmed by doing a colony-forming assay, which showed a bacterial inhibition by 98% colony forming units (CFU). However, no significant antimicrobial activity of unprocessed hemp hurd powder or LNFC films was observed. On the contrary, treating LNFC films with obtained extractives led to a reduction of CFU by 99.7%. These results of using hemp extractives and LNFC as antimicrobial coatings creates great potential for valorizing the industrial hemp residues for sustainable antimicrobial applications

Debnath, M., Sarder, R., Pal, L., and Hubbe, M. A. (2022). “Molded pulp products for sustainable packaging: Production rate challenges and product opportunities,”BioResources 17(2), 3810-3870. DOI: 10.15376/biores.17.2.Debnath

Molded cellulosic pulp products provide eco-friendly alternatives to various petroleum-based packaging systems. They have a long history of reliable usage for such applications as egg trays and the shipping of fruits. They have recently become increasingly used for the packaging of electronics, wine bottles, and specialty items. Molded pulp products are especially used in applications requiring cushioning ability, as well as when it is important to match the shapes of the packed items. Their main component, cellulosic fibers from virgin or recycled wood fibers, as well as various nonwood fibers, can reduce society’s dependence on plastics, including expanded polystyrene. However, the dewatering of molded pulp tends to be slow, and the subsequent evaporation of water is energy-intensive. The article reviews strategies to increase production rates and to lower energy consumption. In addition, by applying chemical treatments and processing approaches, there are opportunities to achieve desired end-use properties, such as grease resistance. New manufacturing strategies, including rapid prototyping and advances in tooling, provide opportunities for more efficient form factors and more effective packaging in the future.

Szlek, D. B., Reynolds, A. M., and Hubbe, M. A. (2022). “Hydrophobic molecular treatments of cellulose-based or other polysaccharide barrier layers for sustainable food packaging: A Review,”BioResources 17(2), 3551-3673. DOI: 10.15376/biores.17.2.Szlek

Paper, nanocellulose, and other polysaccharide-based materials can be excellent candidates for food packaging barrier layers, except that they tend to be vulnerable to moisture. This article reviews published research describing various chemical treatments having the potential to render hydrophobic character to such layers. Emphasis is placed on systems in which hydrophobic monomers are used to treat either particles or sheets comprised largely of polysaccharides. A goal of this review is to identify combinations of materials and procedures having promise for scale-up to industrial production, while providing effective resistance to moisture. The idea is to protect the underlying polysaccharide-based barrier layers such that they can continue to impede the transfer of such permeants as oxygen, greases, flavor compounds, and water vapor. A further goal is to minimize any adverse environmental impacts associated with the treatments. Based on the research articles considered in this review, promising hydrophobic treatments can be achieved involving silanes, ester formation, other covalent interactions, plasma treatments, and to some extent by various treatments that do not require formation of covalent bonds. The article is designed such that readers can skip ahead to items of particular interest to them.

Lu, Y., Liu, C.-Z., Mei, C.-T., Sun, J.-S., Lee, J.-Y., Wu, Q.-L., Hubbe, M. A., and Li, M.-C. (2022). “Recent advances in metal organic framework and cellulose nanomaterial composites,”Coordination Chem. Rev. 461, article no. 214496. DOI: 10.1016/j.ccr.2022.214496

Metal organic frameworks (MOFs) have been widely used in various emerging fields due to their attractive characteristics, such as large specific surface area, highly porous structure, tunable porosity and pore size, versatile surface chemistry, diverse topological structure, and high chemical and thermal stability. However, nanoscale MOFs are prone to agglomeration, and their inherently crystalline structure leads to poor flexibility, processability and recyclability, which seriously limit the performance and application of MOFs. To address these deficiencies, MOFs have been composited with other materials through different strategies. One such attractive material is cellulose nanomaterials (CNMs), the most abundant and
sustainable biomass on the earth. Herein, recent advances in the MOF/CNM composites in terms of preparation approaches, general properties, and emerging applications are overviewed, aiming to provide some useful guidance to researchers on the rational design of high-performance MOF/CNM composites in different forms for advanced applications in the future. Particularly, MOFs and CNMs are usually compounded in aqueous solutions through two main strategies, i.e., in-situ synthesis and ex-situ blending. Further processing of as-prepared MOF/CNM aqueous mixtures can generate MOF/CNM composites in
four forms, i.e., hydrogel, powder, membrane and aerogel. Benefitted from advantages of both MOFs and CNMs, MOF/CNM composites hold exceptional high specific surface area, hierarchically porous structure, as well as superior electrochemical, mechanical and antibacterial properties, which can be further modulated and enhanced through optimizing type and composition of MOFs and CNMs, preparation method, and addition of other functional components. These exceptional properties offer huge potential in a wide range of application fields

Hubbe, M. A., Piner, E. V., Lavoine, N., and Lucia, L. A. (2022). “Barrier properties of bionanocomposite films,” in: Polymer Based Bio-nanocomposites, C. Muthukumar et al. (eds.), Ch. 6, Springer Nature Publ.

This chapter reviews research about biodegradable bionanocomposite barrier films, which have the potential to replace petroleum-based barrier films that currently have a dominant role in the packaging and serving of food products.  In addition to describing the state of research in this field, a series of pivotal questions are considered that may affect which materials and processes will emerge as the most successful options.  Brittleness is a key issue that is considered in the light of published findings.  Progress also has been achieved in the mechanistic modeling of bionanocomposite film performance.  Bacterial cellulose stands out as a promising reinforcing material that has advantages for food contact, pharmaceutical, and medical applications.  Finally, progress has been made in being able to form barrier films at relatively high speeds using procedures that can be scaled up and used industrially.

Koo, M. S., Wu, Q. L., Lee, S. Y., and Hubbe, M. A. (2021). “Rheological properties of lignocellulosic nanomaterial aqueous suspensions as influenced by water-soluble biopolymer additives,”ACS Sustain. Chem. Eng. 9(50), 17049-17060. DOI: 10.1021/acssuschemeng.1c05946

Rheological properties including gel strength, steady-state viscosity, and oscillatory rheology of four types of cellulose nanomaterials (CNMs) under the influence of two different biopolymers were studied. The processed CNMs had distinct fiber morphology (individual fibril vs bundles with/without lignin particles), composition (lignin content varying from 52.88% to 6.86%), and surface chemical properties (zeta potential values from -28.62 mV to -13.4 mV). The presence of lignin in lignin-containing nanofibrillated cellulose (LNFC) decreased a suspension’s gel strength, viscosity, and dynamic modulus. The chosen bleached NFC contained large fibril bundles, and the processed material with more individualized fibril and enhanced fluid rheology was demonstrated after regrinding the original NFC. Biopolymer xanthan gum (XG) showed a much larger effect in modifying gel strength, viscosity, and dynamic moduli compared with polyanionic cellulose (PAC). Among the three chosen rheological models, the Nasiri-Ashrafizadeh model fitted the measured shear stress and shear rate data the best for each fluid system with correlation coefficients larger than 0.99. The use of biopolymers (e.g., XG and PAC) helped reduce negative effect of lignin on gel strength and other rheological properties for LNFC. The much-improved rheological performance for biopolymer-modified LNFC suspensions open new opportunities for CNMs to be used as more environmentally friendly fluids for the energy industry.

Koo, M. S., Lee, D., Lee, S., Hubbe, M. A., and Wu, Q. L. (2021). “Salt sensitivity of low solid content bentonite suspension as influenced by lignocellulosic nanomaterial and polyanionic cellulose,” MRS Communications.  11(6), 726-738.  DOI: 10.1557/s43579-021-00133-w

Effectiveness of lignocellulose nanomaterials and polyanionic cellulose (PAC) on microstructure, rheology, filtration, and salt tolerance performance of bentonite (BT) water-based fluid under two saline conditions (NaCl and CaCl2) was studied. The fluid with cellulose nanocrystals (CNCs) and nanofibrillated cellulose (NFC) exhibited salt-dependent increase in rheological properties. However, the fluid with lignin-containing NFC (LNFC) showed only minor reduction in yield stress and gelation behavior with improved filter cake properties. All CNC-, NFC-, and LNFC-BT fluids showed an increase in filtration losses with added salt ions. Adding PAC enhanced salt tolerance for the fluids, mainly in the NaCl-added system.

Hubbe, M. A. (2021). “Energy efficiency challenges in pulp and paper manufacture: A tutorial review,”BioResources 16(4), 8567-8639.

Wood-processing facilities, including pulp, paper, lumber, and engineered wood facilities, use large amounts of energy for such purposes as evaporative drying and the curing of adhesives. Much of that energy is already being supplied by the incineration of biomass, and there is opportunity to increase the proportion of renewable energy that is used. Specific changes can be made within such factories that allow them to come closer to what is thermodynamically possible in terms of avoiding the wastage of exergy, which can be defined as useful energy. Savings in exergy are often obtained by optimization of a network of heat exchangers within an integrated system. No steam should be allowed to leak to the atmosphere; rather the latent heat (due to phase transitions) and sensible heat (due to temperature changes) are recovered during the heating up of incoming air and water, ideally at a similar range of temperatures. Thus, by a combination of process integration and full utilization of cellulosic residues generated from the process, even biobased industries can be made greener.

Zubair, N. A., Moawla, R. M., Nasef, M. M., Hubbe, M., and Zakeri, M. (2021). “A critical review on natural fibers modification by graft copolymerization for wastewater treatment,”J. Polym. Environ. DOI: 10.1007/s10924-021-02269-1

Graft copolymerization is a distinctive approach to modify the inherently cheap natural fibers (NFs) using different initiators to incorporate synthetic polymer side chains allowing development of novel types of hybrid materials. This method has been widely applied to develop a variety of NFs based adsorbents for decontamination of toxic pollutants from the aqueous environment. However, the development of high-performance adsorbents from NFs is steady challenged by the need to preserve the sustainability during graft modifications and applications. This article critically reviews the progress on modifications of NFs by graft copolymerization of polar monomers on NFs using various initiating methods and their applications in wastewater treatment. Particularly, the applications of the grafted NFs in removal of heavy metal ions, synthetic dyes, oil spills and extraction of precious metals from wastewater are elaborated. The critical challenges to the viability and sustainability of NFs-based adsorbents with respect to functionalization by graft copolymerization and environmental impacts are discussed and the future research directions are also outlined.

Tyagi, P., Salem, K. S., Hubbe, M. A., and Pal, L. (2021). “Advances in barrier coatings and film technologies for achieving sustainable packaging of food products – A review,” Trends Food Sci. Technol. 115, 461-485. DOI: 10.1016/j.tifs.2021.06.036

Background: The technology of food packaging is responding to significant market dynamics such as the rapid growth in e-commerce and preservation of fresh food, a sector that accounts for over 40% of plastic waste. Further, mandates for sustainability and recent changes in national governmental policies and regulations that include banning single-use plastic products as observed in sweeping reforms in Europe, Asia, and several US States are forcing industries and consumers to find alternative solutions. Scope and approach: This review highlights an ongoing shift of barrier coatings from traditional synthetic polymers to sustainable breakthrough materials for paper-based packaging and films. Advantages, challenges and adapting feasibility of these materials are described, highlighting the implications of selecting different materials and processing options. A brief description on progress in methods of coating technologies is also included. Finally, the end fate of the barrier materials is classified depending on the packaging type, coating materials used and sorting facility availability. Key findings and conclusions: Different types of coatings, such as water-based biopolymers, due to their greater environmental compatibility, are making inroads into more traditional petroleum-based wax and plastic laminate paperboard products for fresh food bakery, frozen food, and take-out containers applications. In addition, nano-biocomposites have been studied at an accelerating pace for developing active and smart packaging. Based on the momentum of recent developments, a strong pace of continuing developments in the field can be expected.

Tyagi, P., Gutierrez, J. N., Nathani, V., Lucia, L. A., Rojas, O. J., Hubbe, M. A., and Pal, L. (2021). “Hydrothermal and mechanically generated hemp hurd nanofibers for sustainable barrier coatings/films,” Indust. Crops Prod. 168, article no. 113582. DOI: 10.1016/j.indcrop.2021.113582

Residual hemp (Cannabis sativa) hurd fibers obtained from hydrothermal, carbonate, and kraft treatments were the resources used to obtain lignocellulosic nanofibers (LCNF) by using an ultra-fine friction grinder. The morphological, crystallinity, and chemical characteristics of the nanocellulose films were carried out using SEM, XRD, EDX and ToF-SIMS. Water barrier properties of the same were measured in terms of water contact angle, water vapor permeability (WVP) and water absorption. The barrier properties were found to be dependent not only on the lignin content and lignin distribution, but also on the film density and porous structure. LCNF films and coatings showed much higher water contact angle (WCA) (80 degrees-102 degrees) than films produced from the bleached CNF. WVP was found to be more dependent on the density of films than lignin content. Overall, LCNF-based films and coatings derived from hemp hurd residual fibers can contribute to a circular economy and sustainability.

Hubbe, M. A. (2021). “Contributions of polyelectrolyte complexes and ionic bonding to performance of barrier films for packaging: A review,” BioResources 16(2), 4544-4605.

Barrier films that are used on packages play an important role, especially in the protection of food products. Research is being carried out at an accelerating pace to replace petroleum-based plastic films, which do not biodegrade and are difficult to recycle. This review article considers publications related to the use of polyelectrolyte complexes (PECs) in barrier films as a strategy to decrease the permeation of oxygen and other substances into and out from packages. Research progress has been achieved in using combinations of positively and negatively charged polymers, sometimes together with platy mineral particles, as a way to restrict diffusion through packaging materials. In principle, the ionic bonds within PECs contribute to a relatively high cohesive energy density within such a barrier film, which can resist diffusion of various gases and greasy substances. Resistance to water vapor, as well as aqueous substances, represent important challenges for barrier concepts that depend on ionic bond contributions. Factors affecting barrier performance of PEC-based films are discussed in light of research findings.

Ma, Q.-Y., Lu, X.-M., Wang, W.-X., Hubbe, M. A., Liu, Y. Q., Mu, J.- L., Wang, J., Sun, J. F., and Rojas, O. J. (2021). “Recent developments in colorimetric and optical indicators stimulated by volatile base nitrogen to monitor seafood freshness,”Food Packaging and Shelf Life 28, article no. 100634. DOI: 10.1016/j.fpsl.2021.100634

Seafood spoilage could cause food waste and also serious foodborne disease because of the specific organisms and their metabolites. Therefore, it is important to monitor their freshness in the supply chain. In most cases, their spoilage has been determined by the content of volatile base nitrogen, adenosine triphosphate or the total viable count. However, these destructive detection methods are tedious and time consuming. Hence, it is highly desirable to develop easily operated and non-destructive sensing technologies for real-time detection. A lot of freshness indicators have emerged up to now. Colorimetric indicators are particularly beneficial to accompany use-by dates in food packaging as real-time indicators because they produce visible color changes to the naked eye, which can be easily understood by consumers and non-specialists. Numerous studies have considered the colorimetric indicators to monitor seafood freshness in recent years. This paper mainly focused on colorimetric indicators stimulated by volatile base nitrogen. The most traditional materials used in these indicators are also discussed. The challenges and opportunities in the systems are introduced in this context as well based on the published literature.

Hubbe, M. A. (2021). “Insisting upon meaningful results from adsorption experiments,” Separation & Purification Reviews 51(2), 121-225, article ID LSPR 1888299. DOI: 10.1080/15422119.2021.1888299

A rigorous approach to adsorption studies is advocated in this review article as a means to improving the experience of researchers and improving the practical value of published work in the field. Three broad areas of concern are considered in this article: mistakes related to the experimental conditions selected, mistakes related to modeling of results, and mistakes related to the mind-set of the investigators. Adsorption experiments can be perceived as an excellent training ground within which to hone the skills of researchers, including experimental methods, the use of statistics, and the ability to find suitable equations by referring to published literature. There is potential to increase the value to society of the results of such work by insisting upon the acquisition of truly meaningful results, in addition to accurately following research strategies laid out in published work. Examples related to adsorption onto cellulosic materials are emphasized in this article.

Li, M.-C., Wu, Q., Moon, R. J., Hubbe, M. A., and Bortner, M. J. (2020). “Rheological aspects of cellulose nanomaterials: Governing factors and emerging applications,” Advanced Materials 33(21), article no. 2006052. DOI: 10.1002/adma.202006052

Cellulose nanomaterials (CNMs), mainly including nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNCs), have attained enormous interest due to their sustainability, biodegradability, biocompatibility, nanoscale dimensions, large surface area, facile modification of surface chemistry, as well as unique optical, mechanical, and rheological performance. One of the most fascinating properties of CNMs is their aqueous suspension rheology, i.e., CNMs helping create viscous suspensions with the formation of percolation networks and chemical interactions (e.g., van der Waals forces, hydrogen bonding, electrostatic attraction/repulsion, and hydrophobic attraction). Under continuous shearing, CNMs in an aqueous suspension can align along the flow direction, producing shear-thinning behavior. At rest, CNM suspensions regain some of their initial structure immediately, allowing rapid recovery of rheological properties. These unique flow features enable CNMs to serve as rheological modifiers in a wide range of fluid-based applications. Herein, the dependence of the rheology of CNM suspensions on test protocols, CNM inherent properties, suspension environments, and postprocessing is systematically described. A critical overview of the recent progress on fluid applications of CNMs as rheology modifiers in some emerging industrial sectors is presented as well. Future perspectives in the field are outlined to guide further research and development in using CNMs as the next generation rheological modifiers.

Bildik Dal, A. E., and Hubbe, M. A. (2021). “Hydrophobic copolymers added with starch at the size press of a paper machine: A review of findings and likely mechanisms,” BioResources 16(1), 2138-2180.

This article reviews publications with the goal of understanding the role of hydrophobic copolymers added to size-press starch as a means to make paper products more resistant to penetration by aqueous fluids. The underlying technology is considered, including background related to starch, size-press equipment, and various hydrophobic copolymers and latex products that have been evaluated. The resulting hydrophobization of the paper has been reported to depend not only on the dosage of the hydrophobic additive, but also on its molecular mass and ionic form. The mechanism appears to rely on an ability of starch to serve as a temporary host for hydrophobic compounds in aqueous solution. It has been proposed that hydrophobic copolymers added with size press starch tend to migrate to the air interface during drying of the starch film, thus allowing the low-energy functional groups, such as styrene or alkyl chains, to face outwards. Further research is needed to address various mechanistic questions. There may be opportunities to further raise the performance of this type of technology as practiced within paper production factories.

Hubbe, M. A., Lavoine, N., Lucia, L. A., and Dou, C. (2021). “Formulating bioplastic composites for biodegradability, recycling, and performance: A review,” BioResources 16(1), 2021-2083.

Society’s wish list for future packaging systems is placing some daunting challenges upon researchers: In addition to protecting contents during storage and shipping, the material must not bio-accumulate, and it should be readily recyclable by using practical processing steps. This article considers strategies employing bio-based plastics and reviews published information relative to their performance. Though bioplastics such as poly(lactic acid) (PLA) and poly(hydroxybutyrate) (PHB) can be prepared from plant materials, their default properties are generally inferior to those of popular synthetic plastics. In addition, some bioplastics are not easily decomposed in soil or seawater, and the polymers can undergo chemical breakdown during recycling.  This review considers strategies to overcome such challenges, including the use of biodegradable cellulose-based reinforcing particles.  In addition to contributing to strength, the cellulose can swell the bioplastic, allowing enzymatic attack.  The rate-controlling step in bioplastic degradation also can be abiotic, i.e. not involving enzymes.  Though there is much more work to be done, much progress has been achieved in formulating bioplastic composites that are biodegradable, recyclable, and higher in strength compared to the neat polymer. Emphasis in this review is placed on PLA and PHB, but not to the exclusion of other bioplastic matrix materials.

Price, C., and Hubbe, M. A. (2021). “Spraying starch on the Fourdrinier – An option between wet end starch and the size press,” TAPPI J. 20(1), 21-26.

Technology to apply suspensions of starch grains to the wet surface of paper, during the dewatering process, is reviewed. Though the technology is not new, it continues to attract the attention of papermakers as a means to increase bonding strength. Starch grains that are sprayed onto the wet-web of paper can be retained at levels exceeding what can be effectively added to the fiber suspension at the wet end. Unlike adding a starch solution at a size press, no additional drying capacity is required on the paper machine. To be effective, the starch needs to be able to swell and develop bonding during the paper drying process. Paperboard applications with recycled fibers appear to be a good fit. There is potential to increase bonding by processes that favor fuller gelatinization of the starch grains by the time the paper becomes dry.

Hubbe, M. A. (2021). “When defects dominate: Rheology of nanofibrillated cellulose suspensions,” BioResources 16(1), 16-18.

Conventional rheological tests can be difficult to carry out in the case of suspensions of nanofibrillated cellulose (NFC). Such suspensions tend to migrate away from the walls of a rheometer device, leaving a low-viscosity layer. The very high aspect ratio of typical nanofibrillated cellulose particles favors formation of tangled clusters. But application of hydrodynamic shear can cause fragmentation of those clusters. It is proposed in this essay that some focus be placed on the fragments of entangled clusters of NFC and interactions between them at their fractured surfaces. The condition of near-uniform, defect-free structures of nanocellulose spanning the volume within a sheared suspension might be regarded as an unlikely dream. Isaac Newton started with a very simple equation to start to understand rheology. It is proposed that a similarly bold and simplified approach may be needed to account for the effects of broken entangled clusters of NFC on flow phenomena, their assessment, and their consequences related to industrial processes.

Ayoub, A., Treasure, T., Hansen, L., Nypelo, T., Jameel, H., Khan, S., Chang, H.-m., Hubbe, M. A., and Venditti, R. A. (2021). “Effect of plasticizers and polymer blends for processing softwood kraft lignin as carbon fiber precursors,”Cellulose 28(2), 1039-1053. DOI: 10.1007/s10570-020-03571-2

Plasticizers depress the glass transition temperature (T-g) of polymers and produce a flowable material at lower temperatures. The use of plasticizers to depress T-g of lignin is important, since at high processing temperatures lignin crosslinks, making it intractable. The goal of this study was to assess plasticizers and polymer blends for the ability to retard a commercial softwood kraft lignin from crosslinking and also serve as thermal and rheological property modifiers during thermal processing in the attempt to produced moldable and spinnable lignin for lignin and carbon fiber products. The T-g of the lignin and the lignin mixed with various amounts of plasticizers and with different thermo-mechanical mixing were determined using differential scanning calorimetry. The T-g and the change in heat capacity at the glass transition (Delta C-p) decreased and increased, respectively, about linearly within this plasticizers range with increased plasticizer weight percentage. Gel permeation chromatography results for extruded lignin as well as extruded lignin-plasticizer blends with glycerol, N-allyurea, citric acid with and without sodium hypophosphite, and oleic acid indicate that the presence of these materials reduced the rate of molecular weight increase at temperatures between 100 and 200 degrees C. Continuous, homogenous films and fibers could be produced by thermal processing with plasticized lignin samples and plasticized lignin-polymer blends, but not with lignin alone. These fibers could be carbonized, yielding up to about 50% of carbon. The present findings have shown the advantages of plasticizers in thermally processing a commercial softwood kraft lignin.

Hubbe, M. A. (2020). “What would J. Willard (Gibbs) do?” Int. J. Wettability Sci. Technol. 1, 169-170.

For years I have been in awe of J. Willard Gibbs, who over years of careful analysis and intuition came up with the foundations of thermodynamics. Gibbs’ work has provided a firm foundation upon which subsequent scientists and technologists have built not only theories, but also practical equipment and engines that power our modern society. So the question arises, if thermodynamics has been so reliable, why do researchers such as Makkonen keep stumbling across situations where those concepts do not seem to work for contact angle and wettability issues? I wonder what Gibbs would be thinking if he were to read articles published in the last 60 years in this academic discipline.

Bildik Dal, A. E., Hubbe, M. A., Pal, L., and Gule, M. E. (2020). “Crude wood rosin and its derivatives as hydrophobic surface treatment additives for paper and packaging,” ACS Omega 5(49), 31559-31566.  DOI: 10.1021/acsomega.0c03610

The aim of this work is to obtain better water resistance properties with additives to starch at the size press. A further goal is to replace petroleum-based additives with environmentally friendly hydrophobic agents obtained by derivatization of wood rosin. A crude wood rosin (CWR) sample was methylated and analyzed with gas chromatography-mass spectrometry (GC-MS). Methyl abietate, dehydroabietic acid, and abietic acid were the main constituents of the sample. The crude wood rosin samples were fortified with fumaric acid and then esterified with pentaerythritol. Fortified and esterified wood rosin samples were dissolved in ethanol and emulsified with cationic starch to make them suitable as hydrophobic additives for surface treatment formulations in mixtures with starch. These hydrophobic agents (2% on a dry weight basis in a cationic starch solution) were applied to paperboard, bleached kraft paper, and test liner paper using a rod coater with a target pickup of 3-5 gsm. The solution pickup was controlled by varying the rod number. The amounts of hydrophobic material applied in the preparation of the paper samples were 32.2, 48.6, and 35.1 lb/ton pickup compared to three types of base papers. Basic surface features of fortified and fortified and esterified rosin-treated paper were compared with base paper and paper treated with starch alone. Lower Cobb(60) values were obtained for fortified and esterified samples than for linerboard samples that had been surface-sized just by starch. Thus, as novel hydrophobic additive agents, derivatives of CWR can be a green way to increase hydrophobicity while reducing starch consumption in papermaking.

Kropat, M., Hubbe, M. A., and Laleicke, F. (2020). “Natural, accelerated, and simulated weathering of wood: A review,” BioResources 15(4), 9998-10062.

This review considers three aspects of the weathering of wood natural weathering, accelerated weathering, and simulated weathering. Natural weathering begins when unprotected wood, such as an unpainted board, is exposed to cycles of solar radiation and rain. Unpainted barns and fenceposts take on a gray coloration and their surfaces may become rough, loosened, or checked with the passage of time. The underlying causes of such changes involve ultraviolet light, the effects of cyclic wetting and drying, and the action of certain fungi. Accelerated weathering tests have been used not only to evaluate the effectiveness of varnishes and paints, but also to aid in the understanding of factors affecting natural weathering. Simulated weathering usually has the goal of quickly and conveniently changing the appearance of fresh wood to give the impression of weathering. This might increase its appeal for various decorative purposes. Information about simulated weathering, though largely absent from the scientific literature, is very much alive in social media. This article considers the science behind all three types of weathering in the light of published accounts.

Horst, T. H., Smith, R. D., Potthast, A., and Hubbe, M A. (2020). “Accelerated Aging of Deacidified and Untreated Book Paper in 1967 Compared with 52 Years of Natural Aging,”Restaurator – Int. J. Preservation Library Archiv. Mater. 41(3), 131-152. DOI: 10.1515/res-2020-0006

Three copies of a book that had been optionally deacidified using two different procedures in 1967, and then subjected to accelerated aging, were tested again after 52 years of natural aging. Matched copies of the book Cooking the Greek Way, which had been printed in Czechoslovakia on acidic paper, were evaluated. Nonaqueous treatment of two of the copies with magnesium methoxide dissolved in chlorofluorocarbon solvent had been found in 1967 to have decreased the susceptibility to embrittlement, as evidenced by the results of the accelerated aging, followed by folding endurance tests. Retesting of the same books in 2019, after 52 years of room temperature storage, showed that the deacidification treatments had achieved the following benefits in comparison to the untreated book: (a) higher brightness; (b) higher folding endurance; (c) tensile breaking length higher in the cross-direction of the paper; (d) substantial alkaline reserve content, (e) an alkaline surface pH in the range 7.1-7.4, and (f) higher molecular mass of the cellulose. Remarkably, some of the folding endurance results matched those of unaged samples evaluated in 1967. Scanning electron micrographs showed no differences between the treated and untreated books.

Lu, X.-M., Sundberg, A., Strand, A., and Hubbe, M. A. (2020). “Effects of metal ions and wood pitch on retention and physical properties of TMP,” Nordic Pulp Paper Res. J. 35(4), 649-659.  DOI: 10.1515/npprj-2020-0036

The influence of metal ions, Ca2+ and Mg2+, on wood pitch retention at pH 8 was studied by investigating the pitch content of TMP-papers using both short- and long-column gas chromatography (GC). The effects of two different drying methods, air-drying and freeze-drying, on pitch retention were also compared in this work. The pitch emulsion was prepared with fatty acids, resin acids, and triglycerides at a certain ratio to simulate the pitch composition in closed water system in paper mill. At increasing pH, some of the resin and fatty acids will dissolve into the water phase as metal soaps. In this work, the retention of colloidal pitch in TMP-papers by metal ions at pH 8 was determined. The amount of 5 mM metal ions was found to retain more pitch. The tensile index decreased by the addition of metal ions, and the decrease became more pronounced as the increase of the metal ions concentration. Besides, the hydrophobicity of handsheets was found being changed only a little, which might because the morphology and pitch retained on the surface both affected the hydrophobicity.

Hubbe, M. A., Sjöstrand, B., Nilsson, L., Kopponen, A., and McDonald, J. D. (2020). “Rate-limiting mechanisms of water removal during the formation, vacuum dewatering, and wet-pressing of paper webs: A review,” BioResources 15(4), 9672-9755.

Because some of the critical events during the removal of water before the dryer section on a paper machine happen very rapidly within enclosed spaces such as wet-press nips there have been persistent challenges in understanding the governing mechanisms. In principle, a fuller understanding of the controlling mechanisms, based on evidence, should permit progress in achieving both higher rates of production of paper and more reliable control of paper attributes. In addition, energy can be saved, reducing environmental impacts. The goal of this article is to review published work dealing both with the concepts involved in water removal and evidence upon which existing and new theories can be based. The scope of this review includes all of the papermaking unit operations between the jet coming from the headbox and the final wet-press nip of an industrial-scale paper machine. Published findings support a hypothesis that dewatering rates can be decreased by densification of surface layers, plugging of drainage channels by fines, sealing effects, flocculation, and rewetting. Ways to overcome such effects are also reviewed.

Bildik Dal, A. E., Hubbe, M. A., and Pal, L. (2020). “Cationic emulsions of maleic anhydride derivatives of oleic acid and abietic acid for hydrophobic sizing of paper,”TAPPI J. 19(6), 281-290.

Ordinary rosin sizing agents are mixtures of resin acids that include abietic acid and related compounds obtained from softwoods such as pine. Fatty acids, which are another byproduct of the kraft pulping of softwood species, also may have hydrophobic effects, but their use as sizing agents has seldom been considered. In the current study, abietic acid and oleic acid, in the absence of other components, were first modified by reaction with maleic acid anhydride. Then the maleated derivatives (maleated oleic acid (MOA) and maleated abietic acid (MAA)), which were emulsified with cationic starch at the 1:1 and 3:2 ratio respectively, were added to fiber furnish containing aluminum sulfate (papermaker’s alum). The prepared sheets were dried with a rotating drum on one side at 100 °C at low pressure to cure the sizing agents. The chemical, optical, strength and absorption properties were measured. The presence of the sizing material was confirmed using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and the retention of the sizing agent on fibers was supported by evidence of hydrocarbons on the paper surface. In addition to achieving sufficient water resistance features with MAA, a lesser hydrophobic character was obtained when using MOA. Compared to commercial applications, to obtain a sufficient sizing degree, relatively large amounts of sizing agent were used. The MOA required 5% addition to achieve a similar sizing degree as MAA at the 2% level. The sizing treatments also resulted in substantial increases in tensile index value. Since cationic starch was used in the formulation of the sizing agents, the increase in tensile index may have been due to the influence of cationic starch. Contributions to paper strength from a combination of ionic complexation and mutual association of hydrophobic groups is also proposed. Depending on the amount of sizing agent, the yellowness increased, especially when sizing with MOA.

Hubbe, M. A., and Rosencrance, S. (eds.) (2020). Make Paper Products Stand Out. Strategic Use of Wet End Chemical Additives, TAPPI Press, Atlanta, GA, 285 pp.

The title of this book presents a challenge for those involved in the manufacturing of paper. How, in practice, can a papermaker make paper stand out? Regardless of the grade being produced, customers often regard paper as being a commodity — something that they can obtain from multiple suppliers, all of whom presumably can meet their specifications. Papermakers routinely expend great efforts to ensure that their products comply with strict specifications. As will be discussed in the chapters of this book, papermakers have various approaches and tools that can be used to differentiate their products in a highly competitive marketplace. Many times this preferred status of their product is achieved by delivering paper that consistently meets all specifications.

Hubbe, M. A., and Rosencrance, S. (2020). “Introduction: Making your paper special,” in: Make Paper Products Stand Out. Strategic Use of Wet End Chemical Additives, M. A. Hubbe and S. Rosencrance, S. (eds.) TAPPI Press, Atlanta, GA, Ch. 1, pp. 1-11.

The title of this book presents a challenge for those involved in the manufacturing of paper. How, in practice, can a papermaker make paper stand out? Regardless of the grade being produced, customers often regard paper as being a commodity — something that they can obtain from multiple suppliers, all of whom presumably can meet their specifications. Papermakers routinely expend great efforts to ensure that their products comply with strict specifications. As will be discussed in the chapters of this book, papermakers have various approaches and tools that can be used to differentiate their products in a highly competitive marketplace. Many times this preferred status of their product is achieved by delivering paper that consistently meets all specifications.

Hubbe, M. A. (2020). “Security papers: Trust but verify,” in: Make Paper Products Stand Out. Strategic Use of Wet End Chemical Additives, M. A. Hubbe and S. Rosencrance, S. (eds.) TAPPI Press, Atlanta, GA, Ch. 6, pp. 129-154.

Security paper products are all around us in the form of currency, lottery tickets, passports, stamps, and check forms, among others [Martin 2014]. A key attribute of such products is that the accepted value of the sheet or stub can greatly exceed the value of the materials and the effort involved in manufacture. That would seem to be an ideal product line for a paper manufacturer! However, no other paper product is so routinely abused through folding, crumpling, soiling, and even occasional laundering [Bureau of Engraving and Printing 2006]. Also, the tonnages generally are not large compared with some other grades of paper. Because banknotes present some of the greatest challenges for the manufacturer, such products will be emphasized in this chapter.

Lu, C., Rosencrance, S., Swales, D., Covarrubias, R., and Hubbe, M. A. (2020). “Dry strength: Strategies for stronger paper,” in: Make Paper Products Stand Out. Strategic Use of Wet End Chemical Additives, M. A. Hubbe and S. Rosencrance, S. (eds.) TAPPI Press, Atlanta, GA, Ch. 7, pp. 155-196.

The dry strength of paper can be defined as the ability of a dry paper specimen to resist a variety of external forces. Common paper dry strength properties can be grouped into in-plane strength properties, out-of-plane strength properties, and structural mechanical properties, based on the type of external force. Paper is a web of cellulose fibers. Many of the strength properties of paper are directly related to fiber packing density, bonding strength among fibers, and the strength of individual fibers. Paper strength is also a function of paper’s uniformity of formation. Failure modes for dry paper are often initiated from a defect-based heterogeneity in sheet structure. This chapter focuses on the various chemical approaches to intensify inter-fiber bonds. By strengthening these bonds, the overall paper product is imparted with more desirable characteristics. To begin the discussion, the next section describes some of the key challenges that modern papermakers face with regards to paper strength. The third section concerns the ways in which paper fails, along with methods for the evaluation of paper strength. The final two sections deal with conventional and non-conventional ways to increase paper’s strength, again with the emphasis being placed on the use of chemical additives at the paper machine wet end.

Yin, Y.-Y., Lucia, L. A., Pal, L., Jiang, X., and Hubbe, M. A. (2020). “Lipase-catalyzed laurate esterification of cellulose nanocrystals and their use in reinforcement in PLA composites,” Cellulose 27, 6263-6273. DOI: 10.1007/s10570-020-03225-3

Cellulose nanocrystals (CNCs) were surface-modified using immobilized lipase (Novozyme 435) to catalyze the formation of laurate ester groups on the CNC surface and thus facilitate homogeneous dispersion within a poly(lactic acid) (PLA) matrix. Results of dynamic contact angle measurement revealed that the modified CNCs (CNC-LAA) presented greater hydrophobicity than their CNC controls. FT-IR, C-13-NMR, XRD, and TGA were used to characterize the structure and stability of the CNCs before and after modification. PLA reinforced with the modified CNCs were prepared, and the effect on mechanical properties was studied. SEM micrographs suggested uniform dispersion of modified CNCs throughout the PLA matrix at a maximum 1 wt% loading, which was found to be the optimal loading level. The CNC-LAA were able to achieve a 0.4 maximum degree of substitution. DSC indicated enhanced crystallization of PLA chains due to the inclusion of CNC-LAA, indicating that the nanofillers behaved as nucleating agents and also revealed improved compatibility. Tensile strength tests showed slightly improved mechanical properties for breaking strength and elongation at breakage.

Hubbe, M. A., McLean, D. S., Stack, K. R., Lu, X. M., Strand, A., and Sundberg, A. (2020). “Self-assembly of alkyl chains of fatty acids in papermaking systems: A review of related pitch issues, hydrophobic sizing, and pH effects,”BioResources 15(2), 4591-4635. DOI: 10.15376/biores.15.2.4591-4635

This review article considers the role of fatty acids and the mutual association of their long-chain (e.g. C18) alkyl and alkenyl groups in some important aspects of papermaking. In particular, published findings suggest that interactions involving fatty acids present as condensed monolayer films can play a controlling role in pitch deposition problems. Self-association among the tails of fatty acids and their soaps also helps to explain some puzzling aspects of hydrophobic sizing of paper. When fatty acids and their soaps are present as monolayers in papermaking systems, the pH values associated with their dissociation, i.e. their pK(a) values, tend to be strongly shifted. Mutual association also appears to favor non-equilibrium multilayer structures that are tacky and insoluble, possibly serving as a nucleus for deposition of wood extractives, such, as resins and triglyceride fats, in pulp and paper systems.

Zambrano, F., Starkey, H., Wang, Y., Abbiti de Assis, C., Venditti, R., Pal, L., Jameel, H., Hubbe, M. A., Rojas, O. J., and Gonzolez, R. (2020). “Using micro- and nanofibrillated cellulose as a means to reduce weight of paper products: A review,”BioResources 15(2), 4553-4590. DOI: 10.15376/biores.15.2.4553-4590

Based on publications related to the use of micro- and nanofibrillated cellulose (MNFC) in papermaking applications, three sets of parameters (intrinsic and extrinsic variables, furnish composition, and degree of dispersion) were proposed. This holistic approach intends to facilitate understanding and manipulation of the main factors describing the colloidal behavior in systems comprising of MNFC, pulp fibers, and additives, which directly impact paper product performance. A preliminary techno-economic assessment showed that cost reductions driven by the addition of MNFC in paper furnishes could be as high as USD 149 per ton of fiber (up to 20% fiber reduction without adverse effects on paper’s strength) depending on the cost of papermaking fibers. It was also determined that better performance in terms of strength development associated with a higher degree of MNFC fibrillation offset its high manufacturing cost. However, there is a limit from which additional fibrillation does not seem to contribute to further strength gains that can justify the increasing production cost. Further research is needed regarding raw materials, degree of fibrillation, and combination with polyelectrolytes to further explore the potential of MNFC for the reduction of weight of paper products.

Hubbe, M. A., and Grigsby, W. (2020). “From nanocellulose to wood particles: A review of particle size vs. the properties of plastic composites reinforced with cellulose-based entities,” BioResources 15(1), 2030-2081. DOI: 10.15376/biores.15.1.2030-2081

This review article considers published evidence regarding effects of particle size on mechanical properties of plastic matrix materials filled with cellulose-based reinforcements. Cellulosic or wood-based reinforcements in plastic matrices can contribute to higher modulus, lower density, and less tendency to sag in comparison with the matrix phase by itself, while still allowing the resulting material to be cut or milled. Although cellulosic materials are generally too hydrophilic to adhere well to common thermoplastic materials such as polyethylene, such deficiencies can be overcome by use of compatibilizers, e.g. polyethylene-maleic anhydride. Recently many researchers have evaluated nanocellulose in plastic composites. The higher surface areas of nanocellulose generally imply a higher cost of compatibilizer to achieve good interfacial adhesion. This review first examines results of a large number of studies all involving high-density polyethylene as the matrix. Then, to get a more detailed mechanistic view, studies are considered that compare different particle sizes of cellulose-based reinforcements within the same conditions of preparation of composites prepared with various matrix polymers. To summarize the findings, there does not appear to be any consistent and dependable advantage of using nano-sized cellulosic reinforcements when trying to achieve higher values of composite strength or modulus.

Hubbe, M. A., and Pruszynski, P. (2020). “Greaseproof paper products: A review emphasizing ecofriendly approaches,”BioResources 15(1), 1978-2004. DOI: 10.15376/biores.15.1.1978-2004

A cost-effective, eco-friendly, and health-promoting packaging system that prevents the passage of greases and oils would fulfill an urgent need. This review discusses what is known about the highly divergent technological paths that have been studied to achieve these objectives. Before the emergence of plastic films, the paper industry addressed these objectives in two ways, by parchmentizing and by high levels of refining of the fibers. Parchmentizing means passing the paper through a bath of concentrated sulfuric acid, followed by rinsing out the acid and drying the sheet. Though both parchmentized paper and highly refined greaseproof paper products are still made, they have been substantially displaced by oil-repellent fluorocarbon treatments of paper. The fluorocarbon treatments have allowed papermakers to achieve greaseproof properties with ordinary paper machine equipment at ordinary refining levels and without a need to immerse the paper in strong acid. Now, however, due to environmental concerns and regulations, the paper industry needs more options. Some promising directions in published research include advances in chemistry, superoleophobic surfaces, nanocellulose films, and systems to protect nanocellulose films from the effects of moisture.

Hubbe, M. A. (2019). “Review of the mechanistic roles of nanocellulose, cellulosic fibers, and hydrophilic cellulose derivatives in cellulose-based absorbents,” in: Cellulose-Based Superabsorbent Hydrogels, Md. I. H. Mondal (ed.), series title: Polymers and Polymeric Composites: A Reference Series, Springer Intl. Publ., Vol. 1, Ch. 5, pp. 123-153.  DOI: 10.1007/978-3-319-76573-0_8-1

Cellulose – either in solid form or as a highly hydrophilic chemical derivative of cellulose – can serve multiple and synergistic roles in the preparation of absorbent materials to meet the requirements of diverse absorbent products. Progress in the preparation of nanocellulose products, including nanocrystalline cellulose (CNC), nanofibrillated cellulose (NFC), and bacterial cellulose (BC), is opening up new possibilities for the reinforcement of hydrogels. Conventional cellulosic fibers, including kraft pulp fibers (e.g. fluff pulp), mechanically pulped lignocellulosic fibers, and recycled paper fibers can provide a structure to fine-tune the mechanical and drainage properties of products that can include superabsorbent materials. Carboxymethylcellulose (CMC) is an especially strong candidate for preparation of the swellable phase of a hydrogel. The high content of carboxylic acid groups in CMC gives rise to a strong swelling tendency, especially at neutral to alkaline pH values. The uptake of water can be understood based on concepts of osmotic pressure, in addition to any salinity in the fluid that is being absorbed. The swelling can be adjusted by the choice and amount of a cross-linking agent. Notably, some of the needed cross-linking effect can be optionally provided by nanocellulose or conventional cellulosic fibers. Combinations of solid cellulose entities and water-soluble cellulose-based polyelectrolytes can be used to prepare completely bio-based products that offer an alternative to the presently available disposable absorbents, which are based mainly on petroleum-based superabsorbent hydrogels. Chemical and physical aspects of cellulose and its derivatives also help determine what happens during drying of absorbent products; some swelling ability may be lost irreversibly due to highly organized hydrogen bonding and coalescence of the cellulose-based macromolecular chains. Since cellulose can be involved in both the structural and chemical aspects of highly absorbent products, there will be unique mechanistic roles governing water uptake, water holding, and even the environmental impacts of cellulose-based absorbent products.

Hubbe, M. A. (2019). “Why, after all, are chitosan films hydrophobic?“BioResources 14(4), 7630-7631.

Chitosan has a molecular structure very similar to that of cellulose, except that one of the -OH groups on each repeating unit (at the C2 position) is replaced by an amine group. Since chitosan has abundant water-loving groups and is soluble in weakly acidic aqueous solution, one might expect films prepared from casting of chitosan solutions to be hydrophilic. Experiments have shown wide variability, often indicating a hydrophobic character of the chitosan films. A 2008 article by Cunha et al. presented evidence suggesting that the apparent hydrophobicity was attributable to impurities. However, not all the evidence was consistent. In particular, extraction of chitosan film with methanol failed to increase the polar component of surface free energy. It is proposed in this editorial that the explanation can be found in a differing water-affinity of chitosan polymer segments, depending on their orientation. This explanation, if valid, is consistent with differences in the hydrophilic or hydrophilic character of different crystalline faces of cellulose.

Hubbe, M. A., Tyagi, P., and Pal, L. (2019). “Nanopolysaccharides in barrier composites,” in: Advanced Functional Materials from Nanopolysaccharides, N. Lin, J. T. Tang, A. Dufresne, and M. K. C. Tam (eds.), Springer, Ch. 10. DOI: 10.1007/978-981-15-0913-1_9

The purpose of a barrier layer or film in a packaging product is to slow down or essentially eliminate the progress of oxygen, water vapor, or other molecules, thereby extending the shelflife, safety, and maybe also the taste of products – especially in the case of foods. This chapter discusses progress in the preparation of barrier composite films that include nanopolysaccharides, such as nanochitin, nanostarch, and nanocellulose. The reviewed research shows that these eco-friendly components in the resulting films often can improve barrier properties. While nanocellulose has attracted more research attention, nanostarch particles can be prepared under less aggressive chemical conditions, and particles related to chitin might possibly be preferred when one of the goals is to achieve antimicrobial effects. Nanopolysaccharides are also likely to find future applications in barrier films containing montmorillonite clay (nanoclay) and in multi-layer barrier film systems.

Zhang, H., Sun, X. H., Hubbe, M. A., and Pal, L. (2019). “Highly conductive carbon nanotubes and flexible cellulose nanofibers composite membranes with semi-interpenetrating networks structure,” Carbohydrate Polymers 222, article no. UNSF 115013. DOI: 10.1016/j.carbpol.2019.115013

Highly conductive multi-walled carbon nanotubes (MWCNTs) and flexible cellulose nanofibers (CNF) membranes with a structure of semi-interpenetrating networks were fabricated using the typical paper-making method, which was simple and cost-effective. Scanning electron microscope (SEM), Fourier-transfrom infrared (FT-IR), and thermal gravimetric analysis (TGA) were used to estimate the morphology, chemical structure, and thermal stability of the membranes. The mechanical, optical, and electrical properties of the membranes were characterized with a uniaxial tensile testing machine, ultraviolet visible spectroscope, and digital multimeter, respectively. The results indicated that the membranes containing 10 wt% of MWCNTs showed a high conductivity value of 37.6 S/m, and the sheet resistances of the membranes were stable at different bending states. Furthermore, we demonstrated the electrical features of membrane-based capacitive pressure sensors based on CNF/MWCNTs. The proposed method for fabricating CNF/MWCNTs membranes can simplify the production process and have great practical potential in various electronics applications such as touch screens.

Hubbe, M. A., Azizian, S., and Douven, S. (2019). “Implications of apparent pseudo-second-order adsorption kinetics onto cellulosic materials. A review,”BioResources 14(3), 7582-7626. DOI: 10.15376/biores.14.3.7582-7626

The pseudo-second-order (PSO) kinetic model has become among the most popular ways to fit rate data for adsorption of metal ions, dyes, and other compounds from aqueous solution onto cellulose-based materials. This review first considers published evidence regarding the validity of the mechanistic assumptions underlying application of the PSO model to adsorption kinetics. A literal interpretation of the model requires an assumption that different adsorption sites on a solid substrate randomly collide with each other during a rate-limiting mechanistic step. Because of problems revealed by the literature regarding the usual assumptions associated with the PSO model, this review also considers how else to account for good fits of adsorption data to the PSO model. Studies have shown that adsorption behavior that fits the PSO model well often can be explained by diffusion-based mechanisms. Hypothetical data generated using the assumption of pseudo-first-order rate behavior has been shown to fit the PSO model very well. In light of published evidence, adsorption kinetics of cellulosic materials is expected to mainly depend on diffusion-limited processes, as affected by heterogeneous distributions of pore sizes and continual partitioning of solute species between a dissolved state and a fixed state of adsorption.

Tyagi, P., Joyce, M., Agate, S., Hubbe, M. A., and Pal, L. (2019). “Citrus-based hydrocolloids: A water retention aid and rheology modifier for paper coatings,”TAPPI J. 18(7), 443-450. DOI: 10.32964/TJ18.7.443

Zhang, H., Sun, X. H., Hubbe, M., and Pal, L. (2019). “Flexible and pressure-responsive sensors from cellulose fibers coated with multiwalled carbon nanotubes,”ACS Appl. Electronic Mater. 1(7), 1179-1188.DOI: 10.1021/acsaelm.9b00182

Composite cellulose-based materials have shown increasing potential in wearable and flexible hybrid electronics for large-scale deployment because of their flexible, cost effective, abundant, and biodegradable features. A flexible cotton cellulose-incorporated multi-walled carbon nanotubes (MWCNTs) based pressure sensor was fabricated. First, the cotton cellulose fibers were swelled in water solution containing sodium hydroxide and urea. Second, the MWCNTs were dispersed uniformly in the cellulosic matrix. Then, cellulosic pressure sensors were formed with a water bath process. The porous and interlaced conductive networks in the pressure sensors containing 10 wt% of MWCNTs exhibited a sensitivity about -0.0197 kPa-1, a response time about 20 ms, a recovery time about 20 ms, and a wide workable pressure range from 0 kPa to 20 kPa. Further, the practical piezoresistivity of sensor specimens was investigated. The proposed pressure sensors are perspective in various applications including smart clothing, electronic skins, medical diagnosis and treatment.

Zhang, H., Dou, C., Pal, L., and Hubbe, M. A. (2019). “Review of electrically conductive composites and films containing cellulosic fibers or nanocellulose,” BioResources 14(3), 7494-7542. DOI: 10.15376/biores.14.3.7494-7542

Strategic combinations of cellulosic materials with electrically conductive polymers or nanoconductors offer important potential advantages for technological advances, light-weight inexpensive products, applications of novel form factors, and more eco-friendly alternatives to certain forms of smart packaging and electronics. This review of the literature focuses on how such electrically conductive composite system work, the roles that cellulosic materials can provide in such structures, processes by which electrically-conductive cellulose-based composites and films can be manufactured, and various potential applications that have been demonstrated. Several advantages of cellulose, such as ease of fabrication, compatibility with conductive agents, and sustainability, allow its integration with conductive agents in making conductive composites. Applications of electrically conducting cellulose-based composites for strain sensors, energy storage, solar cells, electrodes, supercapacitors, and smart packaging are discussed.

Park, J., Yoo, S., Lim, K. H., Rojas, O. J., Hubbe, M. A., and Park, S. (2019). “Impact of oxidative carbonization on structure development of loblolly pine-derived biochar investigated by nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy,” Diamond and Related Materials 96, 140-147. DOI: 10.1016/j.diamond.2019.05.001

Biochar produced at an oxidative atmosphere shows distinct chemical properties compared to those of biochar produced at an inert atmosphere. However, there has been little investigation on the relationship between the oxidative atmosphere and the structure development of biochar, which can be useful information for the utilization of derived products. In this study, the effect of the oxygen atmosphere on the structure development of loblolly pine-derived biochar during thermal treatment was investigated. Quantitative analysis using solid-state direct polarization/magic angle spinning C-13 nuclear magnetic resonance spectroscopy presented the existence of large fractions of aromatic and non-protonated carbons in the biochars treated at an oxidative atmosphere, implying highly condensed aromatic structures with large cluster sizes. Simultaneous thermogravimetic analyzer-differential scanning calorimetry was employed to demonstrate the difference in heat flow during the thermal treatment at two different atmospheres. Relatively large exothermic heat flow was observed when woody biomass was treated at 350 degrees C under an oxidative atmosphere which might be responsible for the difference in structural alteration. The series of observations suggested that compared to inert atmospheric conditions, admitting a limited amount of oxygen during thermal treatment of woody biomass could promote the evolution of condensed aromatic carbon structures.

Hubbe, M. A. (2019). “Nanocellulose, cationic starch and paper strength,” Appita J. 72(2), 82-93.

The purpose of this work is to help understand the effects of adding nanocellulose – in combination with a dry-strength agent such as cationic starch – on the strength properties of paper. Recently it has been found that combinations of nanofibrillated cellulose (NFC) with high levels of cationic starch can achieve much greater increases in paper’s tensile strength and stiffness compared to either additive on its own. This paper considers how the observed effects can be accounted for based on capillary forces, the forming of hydrogen bonds during paper drying, and forming of polyelectrolyte complexes. Recent research showed that adding pre-mixed cationic starch and NFC to papermaking furnish made it possible to maintain tensile strength and stiffness while decreasing the refining level and achieving much lower levels of apparent density. The resulting paper was bulkier but still strong. It is proposed here that the observed effects involve formation of dense attachments at the nano-scale, but that a “spring-back” effect of the NFC-starch complex avoids the overall densification that occurs when papermakers rely on refining of the main furnish to achieve paper strength objectives. The initial, nano-scale densification is predicted by the capillary force theories of Campbell and Page. Elastic forces appear to facilitate re-expansion of clusters of nanofibrillated cellulose, allowing the paper to regain some of its bulk after wet-pressing. It appears that NFC can play a synergistic role as a kind of “extender” for cationic starch, allowing the starch to behave as if it were a long, fibrillar material capable of filling the pore spaces within typical paper. Pronounced decreases in air-permeability of the resulting paper, even when the apparent density has been decreased relative to a default paper sheet, provide supporting evidence for such a concept. Because the cationic starch and NFC surfaces have opposite charge, the concepts of polyelectrolyte complexes may apply.

Hubbe, M. A., Chandra, R. P., Dogu, D., and van Velzen, S. T. J. (2019). “Analytical staining of cellulosic materials: A Review,”BioResources 14(3), 7387-7464. DOI: 10.15376/biores.14.3.7387-7464

Numerous dyes and fluorescent compounds, as reported in the literature, exhibit specificity in the staining of materials associated with lignocellulosic fibers and their chemical components, including cellulose, hemicellulose, and lignin. Such effects long have provided analysts with convenient ways to identify cellulosic fiber types, products of different pulping methods, degrees of mechanical refining, estimates of accessibility to enzymes, and localization of chemical components within microscopic sections of cellulosic material. Analytical staining procedures allow for the facile estimation or quantification using simple methods such as light microscopy or UV-vis spectroscopy. More recent developments related to confocal laser micrometry, using fluorescent probes, has opened new dimensions in staining technology. The present review seeks to answer whether the affinity of certain colored compounds to certain cellulose-related domains can improve our understanding of those stained materials – either in terms of their fine-scale porous structure or their ability to accommodate certain colored compounds having suitable solubility characteristics. It is proposed here that successful staining ought to be viewed as being a three-dimensional phenomenon that depends on both the physical dimensions of the colored compounds and also on functional groups that influence their interactions with different components of lignocellulosic materials. Published information about the mechanisms of staining action as well as characteristics of different stain types is reviewed.

Salas, C., Hubbe, M., and Rojas, O. J. (2019). “Nanocellulose applications in papermaking,” in: Production of Materials from Sustainable Biomass Resources, Z. Fang, R. L. Smith, Jr., and X.-F. Tian (eds.), Biofuels and Biorefineries Ser. 9, Springer, New York, Chapter 3, pp. 61-96. DOI: 10.1007/978-981-13-3768-0_3

Research on the utilization of biomass feedstocks has evolved rapidly in the past decades. Key developments include the production of materials with a more sustainable footprint than those derived from petrochemicals. Among associated materials, nanocelluloses have been produced from different sources and routes, such as high shear fibrillation and hydrolysis (chemical or enzymatic) or their combinations. The unique properties of nanocelluloses have sparked a myriad of uses including those related to the fields of oil and gas, adhesion, film formation, coating, packaging, food and composite processing. High end uses include the development of advanced lightweight materials, biosensors and energy harvesting systems; however, central to this review, are uses closer to the source itself, namely fiber processing and, particularly, papermaking. In this chapter, the literature in these latter applications is discussed with emphasis on the use of nanocellulose to achieve favorable strength and barrier properties as well as in coating and paper sheet-forming.

Bildik, A. E., Hubbe, M. A., and Gule, M. E. (2019). “Neutral/alkaline sizing of paper with fortified, saponified wood rosin premixed with alum and retained using cationic polymer,”APPITA J. 72(1), 39-49.

Rosin, as a traditional internal sizing agent, has not been extensively used under neutral or alkaline pH due to the ineffectiveness of aluminum sulfate (alum) under such conditions. Rosin has some inherent advantages relative to the cellulose-reactive agents alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA). Rosin is generally not subject to problems with paper-surface slipperiness, sizing reversion, or hydrolysis. Rosin products are widely used on account of their favorable price, but they are pH-sensitive and their emulsions can become unstable and lose homogeneity during storage. Developing a rosin-sizing system from domestic sources is a primary objective of this study. Crude rosin was obtained from mixed pine stumps by hexane extraction. Fortification was carried out with maleic acid anhydride to alter the structure of the rosin. The material was then saponified until reaching an acid number of 26. Part of the rosin with premixed with alum, using a blender. To make the alum-premixed sizing formulation positively charged, a high-charged cationic polymer (poly-DADMAC) was added. Each sizing agent formulation was tested with three pulp types. Charge demand values of bleached chemical pulp, unbleached kraft pulp and recycled pulp were determined, and the sizing behavior of the resulting paper was compared. For comparison, paper was also sized conventionally with alum and rosin. Greater sizing performance was obtained with the pre-mixed rosin formation than with the conventional sizing. Moreover, it was observed that the yellowish coloration of the paper was decreased with increasing pre-mixed rosin sizing agent usage in the case of the bleached furnish.

Hubbe, M., Alén, R., Paleologou, M., Kannangara, M., and Kihlman, J. (2019). “Lignin recovery from spent alkaline pulping liquors using acidification, membrane separation, and related processing steps: A Review,” BioResources 14(1), 2300-2351. DOI: 10.15376/biores.14.1.2300-2351

The separation of lignin from the black liquor generated during alkaline pulping is reviewed in this article with an emphasis on chemistry. Based on published accounts, the precipitation of lignin from spent pulping liquor by addition of acids can be understood based on dissociation equilibria of weak acid groups, which affects the solubility behavior of lignin-related chemical species. Solubility issues also govern lignin separation technologies based on ultrafiltration membranes; reduction in membrane permeability is often affected by conditions leading to decreased solubility of lignin decomposition products and the presence of colloidal matter. Advances in understanding of such phenomena have potential to enable higher-value uses of black liquor components, including biorefinery options, alternative ways to recover the chemicals used to cook pulp, and debottlenecking of kraft recovery processes.

Tyagi, P., Mathew, R., Opperman, C., Jameel, H., Gonzalez, R., Lucia, L., Hubbe, M., and Pal, L. (2019). “High-strength antibacterial chitosan-cellulose nanocrystal composite tissue paper,”Langmuir 35, 104-112. DOI: 10.1021/acs.langmuir.8b02655

Heightened need to control the spread of infectious diseases necessitates improvement in the quality of hygiene products. A functionalized and innovative antimicrobial tissue paper was developed with a hydrophobic spray-coating of chitosan-cellulose nanocrystals (CNC). It was hypothesized that the hydrophobic nature of chitosan could be counterbalanced by the addition of CNC. Light-weight tissue handsheets were prepared, spray coated with chitosan and CNC, and tested for antimicrobial activity against gram-negative bacteria E. coli and a microbial sample from a human hand after using the restroom. Water absorption and strength properties were also analyzed. To activate the surface of cationized tissue paper, a helium gas atmospheric plasma treatment was also employed on the best performing antimicrobial tissue papers. The highest bactericidal activity was observed with CNC-chitosan coated tissue paper, inhibiting up to 98% microbial growth. Plasma treatment further improved the antimicrobial activity of the coatings. Water absorption properties were reduced with chitosan but increased with CNC. This “self-disinfecting” bactericidal tissue has the potential to be one of the most innovative products for hygiene industry since it can dry, clean, and disinfect surfaces simultaneously providing significant societal benefits.

Tyagi, P., Lucia, L. A., Hubbe, M. A., and Pal, L. (2019). “Nanocellulose-based multilayer barrier coatings for gas, oil, and grease resistance,”Carbohydrate Polymers 206, 281-288. DOI: 10.1016/j.carbpol.2018.10.114

Nanocellulose materials derived from renewable sources have high surface areas, crystallinity, mechanical strength, tunable chemistry, and are resistant to solvents. These characteristics can dramatically impact many commercial markets including packaging, composites, and coatings. However, nanocellulose-based materials are generally susceptible to the presence of high humidity or moisture. Through this work, we develop a new pathway for sustainable packaging materials by formulating a multi-component cellulose nanocrystal (CNC) barrier coating having superior properties. Our research introduces additives including high-aspect ratio nanoclay, kaolin clay, alkylketene dimer (AKD), and protein into the nanocellulose formulation in order to prepare a uniformly-distributed coating layer. Formulations with nanocellulose and additives with specialized functionalities were obtained in the laboratory and applied on three different packaging papers using a Mayer rod. Coated paper samples were characterized to determine surface morphology, barrier, and mechanical properties. The developed bio-based coatings presented a hydrophobic surface and a more complex and tortuous path, which was found to block gas, moisture, water, and oil permeation, while simultaneously improving stiffness and preserving the look and feel of the paper. Nanocellulose barrier coatings prepared with montmorillonite and other components showed a significant reduction in water absorption (up to 71% compared to surfaces with no coating and up to 27% for surfaces with a CNC-only coating), water vapor transmission rate (up to 27% compared to surfaces with no coating and up to 6% for surfaces with a CNC-only coating), and resistance to air permeation (up to 88% compared to surfaces with no coating and up to 44% for surfaces with a CNC-only coating). The Kit test for oil and grease resistance showed a rating of 6 for the surface with a multi-component CNC barrier coating, while a sample with a CNC-only coating and another without any coating failed for kit test number 1. Stiffness also increased by 20% compared to surfaces without coating. As expected, barrier performance was also impacted by the base paper itself. Overall, the use of blended nanocellulose barrier coatings provides a great opportunity to reduce costs by light weighting of packaging liner grades while sustainably improving barrier characteristics. Future studies will include finding a way to compatibilize other nano-scale additives with CNCs by plasticizing such substances into the matrix and precision multi-layer higher solids coating to further improve resistance to gases, water, water vapor, oil, and grease as well as improve mechanical characteristics for commercial applications.

Hubbe, M. A., Becheleni, E. M. A., Lewis, A. E., Peters, E. M., Gan, W., Nong, G., Mandal, S., and Shi, S. Q. (2018). “Recovery of inorganic compounds from spent alkaline pulping liquor by eutectic freeze crystallization and supporting unit operations: A Review,”BioResources 13(4), 9180-9219. DOI: 10.15376/biores.13.4.Hubbe

After the kraft or soda pulping of lignocellulosic materials to produce pulp suitable for papermaking, the spent pulping liquor typically has been recovered by multi-effect evaporation, followed by incineration in a recovery boiler. This review article considers one unit operation, eutectic freeze crystallization (EFC), that may have potential to save some of the energy that is presently consumed in the evaporation step during recovery of inorganic chemicals from spent pulping liquor. Based on a review of the literature it appears that EFC can be employed to obtain relatively pure sodium sulfate and sodium carbonate, along with relatively pure water (in the form of ice) from the spent liquor, under the assumption that lignin previously has been removed by acidification and precipitation. Issues of inorganic scale formation, during the operation of an EFC process applied to lignin-free black liquor, will require research attention. The chemical reactions to regenerate the active pulping chemicals sodium hydroxide and sodium sulfide from sodium carbonate, sodium sulfate, and other compounds isolated by EFC can be carried out either in a separate operation or by returning the materials to the feed of an existing recovery boiler.

Rudi, H., Ghorgannazhad, P., and Hubbe, M. A. (2018). “Optimizing the mechanical properties of papers reinforced with refining and layer-by-layer treated recycled fibers using response surface methodology,” Carbohydr. Polym. 200, 391-399. DOI: 10.1016/j.carbpol.2018.08.006

Layer-by-layer (LbL) treated recycled fibers were investigated in mixtures with refined pulp relative to the mechanical properties of paper. The LbL treatments were conducted to assemble consecutive cationic and anionic starch layers on the fibers of old corrugated container (OCC) pulp. Fibers zeta potential was measured to examine the success of LbL treatment. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to reveal the differences between treated and untreated fibers surface and network. Subsequently, the LbL-treated fibers were added to the refined OCC pulps. Optimization of paper (60 +/- 3 g m(-2) basis weight) strength properties including: tensile index, Scott bond (internal bonding), and ring crush test (RCT) was carried out by response surface methodology (RSM). The meaningful change of zeta potential substantiated cationic/anionic starch layers construction. The AFM results showed that the surface of fibers were covered with starch, which was consistent with deposition of polyelectrolyte multi-layers (PEMs). The surfaces of the LbL-treated fibers were rough in comparison with untreated fibers. The optimization of mechanical parameters using RSM indicated that refining time significantly affected the paper’s mechanical properties. The property values of 44.5 N.m/g tensile index, 149 J/m(2) Scott bond, 32 mN RCT, and 245% strain at break were achieved at optimal conditions of 16 min refining time and the addition ratio of 17.6% LbL treated pulp respectively.

Rice, M. C., Pal, L., Gonzalez, R., and Hubbe, M. A. (2018). “Wet-end addition of nanofibrillated cellulose pretreated with cationic starch to achieve paper strength with less refining and higher bulk,”TAPPI J. 17(7), 395-403. DOI: 10.32964/TJ17.07.395

Nanofibrillated cellulose (NFC) treated with cationic starch was evaluated as a bonding system to permit lower degrees of refining and lower apparent density of high-mass handsheets made from bleached kraft pulp. Mixed pulp (70% hardwood, 30% softwood) was formed into sheets with the optional addition of 5% by dry mass of NFC. The default addition of NFC was compared with a system in which the NFC had been pretreated either with cationic starch (at various levels) or optionally followed by colloidal silica. Comparative tests also were carried out with separate addition of cationic starch to the main furnish. Unrefined fibers (514 mL CSF) were compared with low-refined (473 mL CSF) and high-refined (283 mL CSF) pulp mixtures. The NFC that had been pretreated with cationic starch at a high level was especially effective at boosting the tensile strength and stiffness of sheets prepared from pulp that had been refined at a low level, thus achieving improved strength at relatively low apparent density (high bulk) of the handsheets. The results support a strategy, for applicable grades of paper, of using cationic starch-pretreated NFC in place of refining energy applied to the main fiber furnish. It was further established that colloidal silica can be employed as a further pretreatment of the cationic starch-treated NFC as a means of promoting dewatering in the combined system.

Hubbe, M. A., Henniges, U., Potthast, A., Ahn, K., and Smith, R. (2018). “Nonaqueous solution deacidification treatments to prolong the storage life of acidic books: A review of mechanistic and process aspects,”BioResources 13(3), 7096-7136. DOI: 10.15376/biores.13.3.7096-7136

According to published studies, certain nonaqueous solution-based treatments can be highly effective for prolonging the useful lives of bound volumes, within which the paper had been formed under acidic papermaking conditions. Such treatments, which typically use reactive alkoxide-based organometallic compounds dissolved in low-surface-tension liquids, have been shown to decrease the tendency of the paper to become brittle during long storage or during accelerated aging. This article reviews published evidence concerning the underlying mechanisms of such treatments. Evidence suggests that dissolved alkoxides and related carbonated alkoxide-based compounds are able to react directly with acidic species within acidic paper during treatment of books. Such reactions help explain the demonstrated effectiveness of nonaqueous solution-based deacidification treatments.

Yin, Y. Y., Lou, C. Q., Tian, X. Z., Jiang, X., Wang, H. B., Gao, W. D., and Hubbe, M. A. (2018). “Synergy of silane and polyacrylate treatments to prepare thermally stable and hydrophobic cellulose nanocrystals,” Chemistry Letters 47(10), 1272-1275. DOI: 10.1246/cl.180559

Cellulose nanocrystals (CNCs) were modified by surface grafting with silane and polyacrylate without disturbing the material morphologies. The modified CNCs exhibited improved hydrophobicity and thermal stability as a consequence of the grafted material on CNCs surface as well as the displacement of sulfate half-ester groups.

Tyagi, P., Hubbe, M. A., Lucia, L., and Pal, L. (2018). “High performance nanocellulose-based composite coatings for oil and grease resistance,” Cellulose 25(6), 3377-3391. DOI: 10.1007/s10570-018-1810-7

A sustainable packaging system with excellent liquid- and gas-barrier properties and enhanced strength properties was highlighted by a composite coating containing a mixture of cellulose nanocrystals (CNC), a high-aspect ratio nano-filler montmorillonite clay, an amphiphilic binder soy protein, and a surface-active agent alkyl ketene dimer. They were tested on various surfaces of commercially available packaging papers and compared with the appropriate control (i.e., CNC-only coating) to determine surface morphology, chemical composition, barrier, and mechanical properties. Scanning electron microscopy image analysis showed a compact matrix whose defects (cracks) were significantly attenuated compared to the control while FTIR showed fewer exposed hydroxyl (-OH) groups. The compact structure and reduced proportion of -OH groups are attributed to the plate-like structure, high aspect ratio of clay, and uniform distribution of additives to help inhibit gas, moisture, water, oil, and grease permeation. The base paper used also had a significant impact on how coatings interacted with various fluids. Overall, sustainable CNC-composite barrier coatings with relatively low-cost additives were fabricated and showed improved barrier and strength characteristics with a strong potential as barrier coatings for packaging.

Nelson, L., Park, S., and Hubbe, M. A. (2018). “Thermal depolymerization of biomass with emphasis on gasifier design and best method for catalytic hot gas conditioning,”BioResources 13(2), 4630-4727. DOI: 10.15376/biores.13.2.Nelson

This paper reviews ways that biomass can be converted by thermal depolymerization to make synthetic gas, i.e. syngas.  Biomass, being carbon neutral, is considered as a form of solar energy stored during the growing season by photosynthesis.  An effective biomass is one with low moisture and ash content, high lignin content, high calorific value, and small particle size.  Woody biomass with low ash content (<1%), nut shells with high lignin content (30 to 40%), and municipal solid waste with synthetic polymers are effective at creating value-added synthetic gases.  An allothermal downdraft gasifier produces a low tar syngas (99.9% tar conversion) at 850 oC and provides a simple and low-cost process.  Integrated gasification combined cycle (IGCC) improves thermodynamic efficiency.  To avoid thermal loss, a hot gas filtration system uses trona sorption material for sulfur and halogen compounds.  Secondary systems can use multiple cyclones followed by reactors employing calcined dolomite, olivine, and others for adsorption or reaction with residual sulfur, ammonia, metals, and halogens.  Reforming of residual tar to syngas can take place within chambers with ceramic tubes doped with nano-nickel particles.  Syngas can then be used in boilers, gas turbines for production of electricity or production of chemicals by Fischer-Tropsch conversion.

Hubbe, M. A., and Rosencrance, S. (eds.) (2018). Advances in Papermaking Wet End Chemistry Application Technologies, TAPPI Press, Atlanta, pp. 7-42.

Papermaking can be viewed as a process by which a mixture of cellulosic fibers and many other potential components in an aqueous suspension are combined into a sheet, by the removal of water. This definition is as relevant today as it was in the year 105 when Cai Lun, an official in the Han dynasty of China, left the first written account of the papermaking process. What is different today are the speed, sophistication, and automation of the equipment. The purpose of this book is to highlight ways in which recent progress in equipment, automation, and procedures is providing improvements in efficiency and effectiveness of chemical additives used in the papermaking process. Such advances can be keys to achieving advantages of the paper product uniformity and production rates, as well as imparting improved functionality and end-use performance to the resulting paper products. This book will focus on various devices, procedures, and systems that can improve the performance of chemical additives used during the production of paper, as well as reduce variation in the desired benefits of additives introduced into the process. It is important that engineers engaged in improving the efficiency of operations, as well as the quality and uniformity of the products, have a solid grasp of how these applications technologies work.

Jackson, L., Chen, J., Hubbe, M., and Rosencrance, S. (2018). “Handling, dilution, and pumping of papermaking additives,” in: Advances in Papermaking Wet End Chemistry Application Technologies, TAPPI Press, Atlanta, Ch. 2, pp. 7-42.

Chemical additives are introduced into the papermaking process for many different reasons. These additives can be classified in various ways. One common approach is to separate them into the broad categories of functional additives and process additives. Functional additives are used to directly modify and improve the properties of paper, whereas process additives are generally added to modify or improve the operational efficiency of the process. This chapter will present the basics of handling, dilution, and pumping for selected additives from each of these two main classes of chemical additives. Specifically, we will discuss polymer-based additives as well as sizing additives. Emphasis here will be placed on some of the most widely used additives, noting that similar addition strategies often can be used for other additives.

Dölle, K., and Hubbe, M. A. (2018). “Paper machine white-water systems and the paper machine wet end,” in: Advances in Papermaking Wet End Chemistry Application Technologies, M. A. Hubbe and S. Rosencrance (eds.), TAPPI Press, Atlanta, Chapter 5, pp. 103-131.

This chapter deals with equipment used for blending, cleaning, and other unit operations leading to the formation of paper. A brief look-back at history can help place the modern process in perspective.

Hubbe, M. A., and Waetzig, D. (2018). “Charge monitoring and control,” in: Advances in Papermaking Wet End Chemistry Application Technologies, M. A. Hubbe and S. Rosencrance (eds.), TAPPI Press, Atlanta, Chapter 6, pp. 133-152.

The charge properties of papermaking furnish, as obtained from pulping operations, are typically negative, or “anionic.” The fibers, fillers, and other solids in a papermaking system have ionized chemical groups at their surfaces. Ionized groups such as the carboxyl group (-COOH) and the sulfonate group (-SO3) give rise to a negative surface charge on the fiber component of paper furnish at typical values of pH. Dissolved and colloidal substances such as hemicellulose breakdown products, various dispersants, wood pitch, and latex also contribute to the charge properties of fiber suspensions in the paper machine. The charge properties of solids, dissolved polyelectrolytes, and colloidal (very finely divided) materials in papermaking stock affect how the system runs, and ultimately the attributes and performance of the paper product.

Hubbe, M. A., and Dölle, K. (2018). “Drainage strategies and micro- or nanoparticle systems,” in: Advances in Papermaking Wet End Chemistry Application Technologies, M. A. Hubbe and S. Rosencrance (eds.), TAPPI Press, Atlanta, Chapter 8, pp. 185-206.

The removal of water in the forming section is often the rate-limiting process during the manufacture of paper. In such cases, if a means can be found to allow water to separate itself more quickly from the wet web of paper, then the rate of production on the paper machine could be increased. Depending on the details, there can be opportunities also to improve product quality. Vendors of paper machine equipment provide a variety of devices, such as hydrofoils, vacuum boxes, vacuum couch rolls, and extended-nip wet-presses to encourage quicker or greater release of water before the evaporative drying. This chapter will focus on drainage-promoting chemical additives and the manner of their implementation.

Hubbe, M. A., Powell, J. S., and Delozier, G. (2018). “Enzymatic technology for wet-end implementation,” in: Advances in Papermaking Wet End Chemistry Application Technologies, M. A. Hubbe and S. Rosencrance (eds.), TAPPI Press, Atlanta, Chapter 12, pp. 269-286.

Enzymes represent an emerging class of processing aids for papermaking. Enzymes are protein molecules with specific structures, folded to form active sites of precisely arranged amino acids. Chemical reactions are catalyzed within these active sites under otherwise unfavorable conditions. The placement of the amino acids and shape of the active site as well as the overall three-dimensional structure of the enzyme determine the type of material (e.g., cellulose, hemicellulose, triglycerides) that will enter and react within the active site. In recent years there has been much progress in the use of these specialized proteins to facilitate or enhance paper machine operations [Bajpai 1999]. Of particular relevance to conventional papermaking chemistry, enzymes are capable of providing effects similar to those of dewatering aids, charge-adjustment additives, and pitch-control additives. Moreover, certain enzymes also can be used to modify intrinsic fiber properties such as bleachability, surface chemistry, refinability, morphology, and other factors of interest to the producers and consumers of paper and board. Although some of the effects of enzymes are similar to those of some other wet-end additives covered in this textbook, important differences exist and will be highlighted in this chapter. For example, wet-end conditions may have a relatively greater impact on the performance of enzymes than chemical additives. When exposed to a suboptimal environment, the catalytic activity of the enzyme may be significantly reduced or lost entirely. System parameters such as temperature, pH, conductivity, inhibiting substances, dissolved oxygen, and dwell time must be considered to guide the selection and application of an appropriate enzyme. Such subjects will be considered after addressing some of the important effects that can be achieved in the wet end through the use of such enzymes as cellulases, hemicellulases, pectinases, esterases, and oxidoreductases.

Rahmaninia, M., Rohi, M., Hubbe, M. A., Zabihzadeh, S. M., and Ramezani, O. (2018). “The performance of chitosan with bentonite microparticles as wet-end additive system for paper reinforcement,” Carbohydrate Polymers 179(1), 328-332. DOI: 10.1016/j.carbpol.2017.09.036

In this research, the effect of bentonite micro-particles on the performance of chitosan as a new additive system for improving the dry strengths of acidic papermaking was studied. Chitosan, an abundant carbohydrate biopolymer, in 4 dosages (0, 0.75, 1.25 and 2% based on dry weight of pulp) was applied with bentonite in 4 dosages (0, 0.3, 0.6 and 0.9% based on oven-dry weight of pulp). Although the addition of chitosan up to 0.75% (without bentonite) improved tensile index and burst index, but the addition of more chitosan decreased all mechanical properties in comparison with the control sample. The application of bentonite in combination with chitosan had a significant impact on chitosan performance in mechanical properties. The best results were obtained with 0.3% bentonite consumption. Visual formation ranking had a proper correlation with this obtained results. The micro-kjeldahl indirectly confirmed chitosan retention in the treated paper with chitosan/bentonite.

Hubbe, M. A., Pizzi, A., Zhang, H. Y., and Halis, R. (2018). “Critical Links Governing Performance of Self-binding and Natural Binders for Hot-pressed Reconstituted Lignocellulosic Board without Added Formaldehyde:  A Review,”BioResources 13(1), 2049-2115. DOI: 10.15376/biores.13.1.Hubbe

The production of fiberboard, particleboard, and related hot-pressed biomass products can convert small, relatively low-valued pieces of wood into valuable products. There is strong interest in being able to manufacture such products without the addition of formaldehyde, which is a health hazard during both production and use. This article reviews literature describing various challenges that need to be faced in order to achieve satisfactory bonding properties in hot-pressed bio-based board products without the addition of formaldehyde. Bonding mechanisms are examined in the form of a hypothesis, in which the strength development is represented by a chain with four links. Failure of a board is expected to occur at the weakest of these mechanistic links, which include mechanical contact, molecular-scale wetting and contact, various chemical-based linkages, and structural integrity. The most promising technologies for environmentally friendly production of hot-pressed board with use of lignocellulosic materials tend to be those that favor success in the development of at least three of the mechanistic links in the hypothetical chain.

Hubbe, M. A., Tayeb, P., Joyce, M., Tyagi, P., Kehoe, M., Dimic-Misic, K., and Pal, L. (2017). “Rheology of nanocellulose-rich aqueous suspensions: A review,”BioResources 12(4), 9556-9661. DOI: 10.15376/biores.12.1.2143-2233

The flow characteristics of dilute aqueous suspensions of cellulose nanocrystals (CNC), nanofibrillated cellulose (NFC), and related products in dilute aqueous suspensions could be of great importance for many emerging applications. This review article considers publications dealing with the rheology of nanocellulose aqueous suspensions in the absence of matrix materials. In other words, the focus is on systems in which the cellulosic particles themselves – dependent on their morphology and the interactive forces between them – largely govern the observed rheological effects. Substantial progress in understanding rheological phenomena is evident in the large volume of recent publications dealing with such issues including the effects of flow history, stratification of solid and fluid layers during testing, entanglement of nanocellulose particles, and the variation of inter-particle forces by changing the pH or salt concentrations, among other factors. Better quantification of particle shape and particle-to-particle interactions may provide advances in future understanding. Despite the very complex morphology of highly fibrillated cellulosic nanomaterials, progress is being made in understanding their rheology, which supports their usage in applications such as coating, thickening, and 3D printing.

Brockman, A. C., and Hubbe, M. A. (2017). “Charge reversal system with cationized cellulose nanocrystals to promote dewatering of a cellulosic fiber suspension,” Cellulose 24(11), 4820-4830.  DOI 10.1007/s10570-017-1477-5

A surface-modified form of cellulose nanocrystals (CNC) was employed to explore mechanisms related to the release of water from cellulosic fiber suspensions during papermaking. The CNC surface was rendered partly cationic (forming CCNC) by adsorption of poly-(diallyldimethylammonium chloride) (poly-DADMAC), a high charge density cationic polymer. Meanwhile, a suspension of cellulosic fibers and calcium carbonate particles was prepared from recycled copy paper, which was treated sequentially with poly-DADMAC and a very-high-mass anionic acrylamide copolymer (aPAM). Subsequent addition of CCNC strongly promoted water release, whereas ordinary CNC had the opposite effect. The effect of the CCNC was achieved with ten times less poly-DADMAC, as the final additive, compared to when adding the polymer alone. Results were consistent with a model of nanoparticle-enabled bridging, based on an assumption of non-equilibrium or slowly equilibrating processes of adsorption.

Tayeb, A. H., Rojas, O. J., and Hubbe, M. A. (2017). “Lipoxygenase-mediated peroxidation mechanism of lipids studied by ¹³C/¹H NMR and FTIR spectroscopy,” Industrial Crops Prod. 104(1), 253-262. DOI: 10.1016/j.indcrop.2017.04.041

Three unsaturated fatty acids, namely 9-cis,12-cis-linoleic acid, 1,2,3-tri-cis, cis-9,12-octadecadienoyl (glycerol trilinolein) and 1,2,3-tri-cis-9-octadecenoyl (triolein) were selected as models of components of plant extractives to monitor the hydroperoxygenation induced by soybean lipoxygenase (LOX), which was applied as an oxidative catalyst at room temperature. The fatty acids were monitored in colloidal dispersions in relation to their molecular changes using 1H/13C nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and UV spectroscopies. The detection of the hydroperoxy group was limited due to its unstable nature. However, the reduction of protons associated with the diene groups and the substitution of hydroperoxy groups at the allylic positon in conjugated lipids were detected by the induced chemical shift of HOO-bearing 13C and 1H resonances and the oxygen absorption owing to changes in the molecule. Moreover, compared to the two other substrates, no oxygen substitution was observed in triolein, in accordance with its lower level of saturation and the absence of bis-allylic carbon. Our results are of relevance to plant fiber processing, since fatty acids are major constituents of hydrophobic deposits that cause a range of manufacturing challenges.

Tayeb, A. H., Hubbe, M. A., Tayeb, P., Pal, L., and Rojas, O. J. (2017). “Soy proteins as a sustainable solution to strengthen recycled paper and reduce deposition of hydrophobic contaminants in papermaking: A bench and pilot-plant study,”ACS Sustain. Chem. Eng. 5(8), 7211-7219. DOI: 10.1021/acssuschemeng.7b01425

Hydrophobic contaminants (stickies) incorporated with recycled fibers cause severe papermaking processing and product quality problems, which lead to low runnability and increased production cost. Stickies negatively affect paper strength and many other properties. In this work, we propose a sustainable approach by the application of soy protein isolate (SPI), soy flour (SF), and soybean lipoxygenase (LOX) as agents to combat hydrophobic contaminants. Tests at the bench and pilot-plant scales and under conditions similar to industrial operations demonstrated the reduction of associated challenges and the improvement of a paper’s dry strength. The soy agents were applied to aqueous dispersions of lignin-free recycled fibers (dosage levels of 1-2% based on the fiber dry weight), which contained additives typically used in papermaking (fillers, sizing agent, and others). Talc, a common detackifier, was applied in similar systems that were used as reference. The proteins were added under both high and low shear conditions, and their effect in reducing paper tackiness and increasing internal bonding was confirmed. The maximum reduction in tacky particles count was achieved with SF (2% addition level based on fiber) under mild agitation and 30 min contact time. Remarkably, synergistic effects on the fiber electrostatic charges and paper porosity and formation were observed upon the addition of the soy proteins. The obtained results indicate that residual soy products represent an inexpensive, sustainable, and environmentally benign solution to enhance papermaking performance relative to conventional and more expensive agents that are in current use.

Tayeb, A. H., Hubbe, M. A., Zhang, Y. X., and Rojas, O. J. (2017). “Effect of lipoxygenase oxidation on surface deposition of unsaturated fatty acids,”Langmuir 33(18), 4559-4566. DOI: 10.1021/acs.langmuir.7b00908

We studied the interactions of lipid molecules (linoleic acid, glycerol trilinoleate and a complex mixture of wood extractives) with hydrophilic and hydrophobic surfaces (cellulose nanofibrils, CNF, and polyethylene terephthalate, PET, respectively). The effect of lipoxygenase treatment to minimize the affinity of the lipids with the given surface was considered. Application of an electroacoustic sensing technique (QCM) allowed the monitoring of the kinetics of oxidation as well as dynamics of lipid deposition on CNF and PET. The effect of the lipoxygenase enzymes (LOX) was elucidated with regards to their ability to reduce the formation of soiling lipid layers. The results pointed to the fact that the rate of colloidal oxidation depended on the type of lipid substrate. The pre-treatment of the lipids with LOX reduced substantially their affinity to the surfaces, especially PET. Surface plasmon resonance (SPR) sensograms confirmed the effect of oxidation in decreasing the extent of deposition on the hydrophilic CNF. QCM energy dissipation analyses revealed the possible presence of a loosely adsorbed lipid layer on the PET surface. The morphology of the deposits accumulated on the solids was determined by atomic force microscopy and indicated important changes upon lipid treatment with LOX. The results highlighted the benefit of enzyme as a bio-based treatment to reduce hydrophobic interactions, thus providing a viable solution to the control of lipid deposition from aqueous media.

Hubbe, M. A. (2017). “Hybrid filler (cellulose/noncellulose) reinforced nanocomposites,” in: Handbook of Nanocellulose and Cellulose Nanocomposites, Vol. 1, H. Kargarzadeh, I. Ahmad, S.

This chapter considers studies in which two different kinds of solid filler materials are used simultaneously as reinforcements in an effort to improve various performance attributes of a polymeric matrix. As in the case of a hybrid vehicle, one having both an electric and a gasoline engine, the two reinforcing elements ought to provide unique combinations of properties or synergistic effects. The term “hybrid composite” also can be employed when two different materials are combined together in the preparation of a hybrid reinforcing element, which then can be used in the manufacture of a composite. The systems to be discussed in this chapter will be limited to cases in which one of the two reinforcing elements is cellulose-based. In most of the work to be discussed, the non-cellulosic reinforcement is an inorganic material such as a specialized clay product or finely chopped glass fiber.

Hubbe, M. A., Smith, R. D., Zou, X., Katuscak, S., Potthast, A., and Ahn, K. (2017).“Deacidification of acidic books and paper by means of non-aqueous dispersions of alkaline particles: A review focusing on completeness of the reaction,”BioResources 12(2), 4410-4477. DOI: 10.15376/biores.12.2.Acidic_Books_Hubbe

Deacidification refers to chemical treatments meant to slow down the acid hydrolysis and embrittlement of books and paper documents that had been printed on acidic paper. From the early 1800s up to about 1990, papermakers used aluminum sulfate, an acidic compound, in most printing papers. Certain deacidification methods use non-aqueous media to distribute alkaline mineral particles such as MgO within the pages of the treated books. Evidence is considered here as to whether or not the proximity of alkaline particles within such documents is sufficient to neutralize the acidic species present. Because much evidence suggests incomplete neutralization, a second focus concerns what to do next in cases where book already have been treated with a non-aqueous dispersion system. Based on the literature, the neutralization of acidic species within such paper can be completed by partial moistening, by high humidity and pressure, by water condensation, as well as by optional treatments to enhance paper strength and a final drying step.

Hubbe, M. A., Ferrer, A., Tyagi, P., Yin, Y., Salas, C., Pal, L., and Rojas, O. J. (2017). “Nanocellulose in thin films, coatings, and plies for packaging applications: A review,”BioResources 12(1), 2143-2233. DOI: 10.15376/biores.12.1.2143-2233

This review article was prompted by a remarkable growth in the number of scientific publications dealing with the use of nanocellulose (especially nanofibrillated cellulose (NFC), cellulose nanocrystals (CNC), and bacterial cellulose (BC)) to enhance the barrier properties and other performance attributes of new generations of packaging products. Recent research has confirmed and extended what is known about oxygen barrier and water vapor transmission performance, strength properties, and the susceptibility of nanocellulose-based films and coatings to the presence of humidity or moisture. Recent research also points to various promising strategies to prepare ecologically-friendly packaging materials, taking advantage of nanocellulose-based layers, to compete in an arena that has long been dominated by synthetic plastics. Some promising approaches entail usage of multiple layers of different materials or additives such as waxes, high-aspect ratio nano-clays, and surface-active compounds in addition to the nanocellulose material. While various high-end applications may be achieved by chemical derivatization or grafting of the nanocellulose, the current trends in research suggest that high-volume implementation will likely incorporate water-based formulations, which may include

Farhat, W., Venditti, R. A., Hubbe, M., Taha, M., Becquart, F., and Ayoub, A. (2017). “A review of water-resistant hemicellulose-based materials: Processing and applications,”ChemSusChem 10, 305-323. DOI: 10.1002/cssc.201601047

Due to their hydrophilic nature, hemicelluloses may tend to be overlooked as the main ingredient for water-resistant product applications. However, their domains of use can be greatly expanded by chemical derivatization. The present review article considers research in which hydrophobic derivatives of hemicelluloses or combinations of hemicelluloses with hydrophobic materials have shown promise in preparation of films and composites. The review also will cover research publications dealing with isolation methods that have been used to separate the hemicellulose from the biomass, as well as summarizing the most useful pathways that have been used to change the hydrophilic character of hemicelluloses, thus increasing its water resistance and the applications of the targeted water-resistant hemicellulose.

Rohi, M., Ramezani, O., Rahmaninia, M., Zabihzadeh, S. M., and Hubbe, M. A. (2016). “Influence of pulp suspension pH on the performance of chitosan as a strength agent for hardwood CMP paper,” Cellulose Chem. Technol. 50(7-8), 873-878.

Chitosan – a well-known carbohydrate – has found new applications in the papermaking industry. Although the effect of pH on the properties of chemical pulps has been investigated in a number of studies, it has not been reported related to hardwood mechanical pulp, to our knowledge. In the present research, the effect of different pH levels (5.5, 7 and 8.5) on the performance of chitosan as a dry strength additive in paper produced from chemi-mechanical hardwood pulp was studied. The results indicated that the effect of chitosan on the properties of chemi-mechanical paper depended strongly on the pH of the furnish. The addition of chitosan to the stock at acidic pH did not have any specific effect on the strength properties. In contrast, neutral and alkaline pH resulted in improvements in the dry strength properties of paper sheets. The maximum apparent density was observed at pH 8.5, which was attributed to the precipitation of the chitosan on the fiber surface. The results showed that chitosan would be more effective in low dosages. The dependence of chitosan performance on pH in chemi-mechanical pulp was similar to that reported for chemical pulps.

Bildik, A. E., Hubbe, M. A., and Gurboy, K. B. (2016). “Alkyl ketene dimer (AKD) sizing treatment and charge interactions in recycled paper,”Appita J. 69(4), 331-338.

Alkylketene dimer (AKD) sizing dispersions from two commercial sources, in addition to the corresponding laboratory-produced AKD dispersions, were investigated relative to their usage in a recycled office waste furnish. Two main sets of experiments were carried out. One set involved testing the pulp after AKD treatment, with the evaluation of dewatering rates, retention efficiency, and charge. The other set involved brightness and water resistance properties when AKD was added in making handsheets. There was generally a positive but decreasing incremental effect of the sizing treatments (dispersions or associated cationic polyelectrolytes) with increased levels of addition, on drainage rate and retention efficiency. AKD treatment resulted in increased brightness, which was attributed to increased retention of calcium carbonate and of fluorescent whitening agent in the paper. Less sizing agent was required in the recycled furnish compared to the virgin fibre. Results were consistent with the charged character of the emulsified AKD formulations

Monfared, M. H. A., Resalati, H., Ghasemian, A., and Hubbe, M. A. (2016). “Passivation of pressure sensitive adhesive stickies by addition of acrylic fibers to OCC pulp before papermaking,” Tappi J. 15(10), 631-639. DOI: 10.32964/TJ15.10.631

This study investigated the addition of acrylic fiber to Old Corrugated Container (OCC) pulp as a possible means of overcoming adverse effects of water-based pressure sensitive adhesives during manufacture of paper or paperboard. Such adhesives can constitute a main source of stickies, which hurt the efficiency of the papermaking process and make tacky spots in the product. The highest amount of acrylic fiber added to recycle pulps generally resulted in a 77% reduction in accepted pulp microstickies. The addition of acrylic fibres also increased pulp freeness, tear index, burst strength, and breaking length, though there was a reduction in screen yield. Hence, in addition to controlling the adverse effects of stickies, the addition of acrylic fibers resulted in the improvement of the mechanical properties of paper compared with a control sample.

Lenze, C. J., Peksa, C. A., Sun, W. M., Hoeger, I. C., Salas, C., and Hubbe, M. A. (2016). “Intact and broken cellulose nanocrystals as model nanoparticles to promote dewatering and fine-particle retention during papermaking,” Cellulose 23(6), 3951-3962. DOI: 10.1007/s10570-016-1077-9

Cellulose nanocrystals (CNCs), either in intact form or after mechanical shortening, were used as a model nanoparticle for enhancement of dewatering and fine-particle retention during lab-scale papermaking process evaluations. Cryo-crushing, using dry or wet CNCs, was performed to shorten the particles from an initial mean value of 103.1 nm to either 80.4 nm (wet crushed) or 63.4 nm (dry crushed). Papermaking-related tests were performed with the solids from 100% recycled copy paper, which were prepared as a 0.5% solids suspension in dilute Na2SO4 solution and then treated successively with 0.05% of poly-diallyldimethylammonium chloride, 0.05% of very-high-mass cationic acrylamide copolymer, and then various types and dosages of negatively charged nanoparticles. The performance of the CNCs, relative to papermaking goals, was compared to that of two colloidal silica products that are widely used in industry for this purpose. All of the nanoparticles were observed to promote both dewatering and fine-particle retention. The intact CNCs were more effective than the broken CNCs with respect to fine-particle retention. Effects on flocculation of the fiber suspension were detectable, but not large relative to the sensitivity of the test employed. Results are discussed in the light of concepts of polyelectrolyte bridges and the participation of elongated nanoparticles in completing those bridges in such a way as to form shear-sensitive attachments among solids surfaces in the suspension.

Ferrer, A., Pal, L., and Hubbe, M. A. (2016). “Nanocellulose in packaging: Advances in barrier layer technologies,”Industrial Crops and Products 95, 574-582. DOI: 10.1016/j.indcrop.2016.11.012

The review aims at reporting on recent developments in nanocellulose-based materials and their applications in packaging with special focus on oxygen and water vapor barrier characteristics. Nanocellulose materials, including cellulose nanocrystals (CNC), nanofibrillated cellulose (NFC), and bacterial nanocellulose (BNC), have unique properties with the potential to dramatically impact many commercial markets including packaging. In addition to being derived from a renewable resource that is both biodegradable and non-toxic, nanocellulose exhibits extremely high surface area and crystallinity and has tunable surface chemistry. These features give nanocellulose materials great potential to sustainably enhance oxygen and water vapor barrier properties when used as coating, fillers in composites and as self-standing thin films.

Bildik, A. E., Hubbe, M. A., and Gurboy, K B. (2016). “Alkyl ketene dimer (AKD) sizing of paper under simplified treatment conditions,” Tappi J. 15(8), 545-552. DOI: 10.32964//TJ15.8.545

Alkyl Ketene Dimer (AKD) has been widely used by manufacturers of paper and paperboard as a hydrophobic sizing agent. Ordinary sizing with AKD involves a complex series of processes, including emulsification of the waxy AKD material, measures to avoid the agglomeration of the emulsified AKD particles, addition of a stabilized AKD dispersion to papermaking furnish, interactions with various retention aid chemicals to fix the material onto solid surfaces, and various spreading and curing processes that take place during the drying and cooling of the paper product. In the present work, as a means to gain insight into the mechanisms attributable to just the AKD in isolation from the other additives and sub-processes, the AKD wax was dissolved in heptane and applied to filter paper between two aluminum foil layers, followed by evaporation of the solvent and optional heating. Surprisingly, hydrophobic character was obtained regardless of whether or not the treated sheets had been heat-cured. Also, for the first time it was observed that the AKD treatment resulted in a substantial increase in sheet strength, suggesting that the AKD was able to serve as the matrix in an AKD-saturated paper structure. The results add support to past suggestions in the literature that potential covalent interactions cannot account for all of the effects attributable to AKD treatment of paper.

Hubbe, M. A. (2016). “Catalysts inspired by life,” Biofuel Res. J. 11, 430-430. DOI: 10.18331/BRJ2016.3.3.2

Biosynthetic processes take place throughout our world with astonishingly high precision and rapidity to create biological systems, trees, and even such complex products as our own bodies.  Though each of the chemical reactions involved in creating something as complex as a tree is thermodynamically possible, there is nearly zero possibility of creating complex biomaterials without the use of enzymes.  Breaking down those biomaterials is somewhat easier – even fire can accomplish that – but still it is enzymes that make most biodegradation possible throughout the world.  An enzyme acts as a catalyst, accelerating and often helping to direct the path of a reaction that would not otherwise take place fast enough or which might otherwise tend to take a different reaction path from what is needed.

But enzymes are not the only kinds of catalysts.  The present issue of Biofuels Research Journal, for instance, has an article that shows how zeolites can help direct the reactions of vapors of pine wood pyrolysis.  Catalysts made by humans often follow our ancient tradition of alchemy:  selecting or modifying minerals or metal ores in the hopes of obtaining something valuable.  In his book The Alchemy of Air, Thomas Hager describes how the chemical engineer/inventors Fritz Haber and Carl Bosch managed to convert gaseous nitrogen into ammonia.  The key was to use an impure iron wire, along with incredibly high pressure and high temperature.  It is estimated that one-half of the nitrogen atoms presently incorporated into your own body, right now, are a direct result of the Haber-Bosch process.  Yes, nitrogen also can be “fixed” by biological processes, but not at a rate that would support the current human population, and humanity had to discover another catalyst in order to sustain the growth of civilization.

Maybe humanity’s current challenge involves advancing beyond “alchemy” and returning to biology as a main arena for catalysis.  Such an approach is represented by the article dealing with biogenic hydrogen production.  Just as in the case of the inorganic catalysts discovered by Haber and Bosch, it takes a great deal of patience and many unsuccessful attempts in order to come up with high-performing enzymes, which may be regarded as biocatalysts.  Many factors may degrade or inhibit the activity of a catalyst.  As emphasized in the review article “Green biodiesel production,” we can expect catalysts to take center stage as humanity grapples with the challenge of sustainability in this increasingly crowded and often hungry world.  These catalysts will take many forms – from transition metal complexes, to enzymes, to pieces of rusty wire.  But without progress in the field of catalysis there is no way that all of us will be able to survive on this planet.

Hubbe, M. A., Metts, J. R., Hermosilla, D., Blanco, M. A., Yerushalmi, L., Haghighat, F., Lindholm-Lehto, P., Khodaparast, Z., Kamali, M., and Elliott, A. (2016). “Wastewater treatment and reclamation: A review of pulp and paper industry practices and opportunities,”BioResources 11(3), 7953-8091. DOI: 10.15376/biores.11.3.Hubbe

The pulp and paper (P&P) industry worldwide has achieved substantial progress in treating both process water and wastewater, thus limiting the discharge of pollutants to receiving waters. This review covers a variety of wastewater treatment methods, which provide P&P companies with cost-effective ways to limit the release of biological or chemical oxygen demand, toxicity, solids, color, and other indicators of pollutant load. Conventional wastewater treatment systems, often comprising primary clarification followed by activated sludge processes, have been widely implemented in the P&P industry. Higher levels of pollutant removal can be achieved by supplementary treatments, which can include anaerobic biological stages, advanced oxidation processes, bioreactors, and membrane filtration technologies. Improvements in the performance of wastewater treatment operations often can be achieved by effective measurement technologies and by strategic addition of agents including coagulants, flocculants, filter aids, and optimized fungal or bacterial cultures. In addition, P&P mills can implement upstream process changes, including dissolved-air-flotation (DAF) systems, filtration save-alls, and kidney-like operations to purify process waters, thus reducing the load of pollutants and the volume of effluent being discharged to end-of-pipe wastewater treatment plants.

Hubbe, M. A., and Koukoulas, A. A. (2016). “Wet-laid nonwovens manufacture – Chemical approaches using synthetic and cellulosic fibers,”BioResources 11(2), 5500-5552. DOI: 10.15376/biores.11.2.Hubbe

Wet-laid forming, which can be regarded as being analogous to conventional papermaking processes but with use of chopped synthetic or staple fibers, continues to draw attention as an advantageous way to prepare advanced nonwoven textile products. This review of the literature considers scientific advances in the field, with emphasis placed on applications involving cellulosic fibers as a significant component of the product. Some primary challenges with respect to wet-laid processing concern the dispersion of the synthetic fibers in aqueous media and methods for avoiding their subsequent entanglement. Both mechanical and chemical strategies have been employed in order to achieve well-formed sheets of high uniformity and binding among the fibers to meet a variety of end-use specifications. The incorporation of cellulosic fibers has been shown to facilitate fiber dispersion and to impart certain beneficial characteristics and properties to wet-laid fabrics. The contrasting attributes of synthetic and cellulosic fibers contribute to some unique challenges during the processing of their mixtures during wet-laid forming.

Hubbe, M. A., and Gill, R. A. (2016). “Fillers for papermaking: A review of their properties, usage practices, and their mechanistic role,”BioResources 11(1), 2886-2963. DOI: 10.15376/biores.11.1.2886-2963

Issues of cost and product quality have caused papermakers to place increased attention on the use of mineral additives, which are the subject of this review article. Technologists responsible for the production of paper can choose from a broad range of natural and synthetic mineral products, each of which has different characteristic shapes, size distributions, and surface chemical behavior. This article considers methods of characterization, and then discusses the distinguishing features of widely available filler products. The mechanisms by which fillers affect different paper properties is reviewed, as well as procedures for handling fillers in the paper mill and retaining them in the paper. Optical properties of paper and strategies to maintain paper strength at higher filler levels are considered. The goal of this review is to provide background both for engineers working to make their paper products more competitive and for researchers aiming to achieve effects beyond the current state of the art.

Hubbe, M. A. (2015). “Oops, I thought that those books had been deacidified,”BioResources 10(4), 6305-6309. DOI: 10.15376/biores.10.4.6305-6309

Major libraries have been placing increasing reliance upon non-aqueous mass deacidification in an effort to avoid hydrolytic decomposition of the cellulose during storage of bound volumes. Such decomposition is especially a problem when the printing papers used in manufacture of the books have been prepared under acidic conditions, using aluminum sulfate. But there is reason to doubt that the widely used non-aqueous treatments, in which “alkaline reserve” particles are deposited in the void spaces of the paper, can achieve neutralization of acidity throughout the paper structure under the conditions most commonly used for treatment and storage. Anecdotal evidence suggests that alkaline particles such as CaCO3, MgO, Mg(OH)2, or ZnO can be present for long periods of time adjacent to acidic parts of cellulosic fibers without neutralization of the acidity, especially the acidity within the fibers. If these phenomena can be better understood, then there may be an opportunity to use a high-humidity treatment of certain “deacidified” books in order to achieve more pervasive protection against acid-induced degradation.

Silva, D. J., Park, S. W., Hubbe, M. A., and Rojas, O. J. (2015). “Amphoteric polymers to improve paper dry strength,” J. Chem. Eng. Chem. 1(2), 65-79. DOI: 2446941601022015065

The objective of the present work is to evaluate the application of an amphoteric polymer, with acidic and basic groups on the polymer chain, as a paper dry strength additive. The polymer studied here is random terpolymer of high electrostatic charge density, and high molecular weight. The results of our work showed that the balance between the charge densities of the surface and the polymer structures is an important factor to be considered when using this polymer. The highest paper strength value, measured by tensile index, was found at the polyampholyte’s isoelectric point (ca. pH of 7.3) when the charge of the fiber surface was negative and the polymer structure charge was symmetric. This
observation agrees with our dynamic light scattering results which, demonstrated that at the isoelectric point there was a maximum in association among polyampholyte molecules, leading to a maximum in size of molecular aggregates. When adsorbed on an electrically charged surface, the maximum amount of adsorbed polymer, measured by the shift in resonance frequency in a quartz crystal microgravimetric balance, was observed for the same isoelectric point. Better results for paper dry strength were found when the fiber surface and the polymer structures were oppositely charged or at the isoelectric point of the polymer. Less effective addition strategies were found in the case when the fiber surfaces and the polymer structures had same sign of charge.

Hubbe, M. A., Gardner, D. J., and Shen, W. (2015). “Contact angles and wettability of cellulosic surfaces: A review of proposed mechanisms and test strategies,” BioResources 10(4), 8657-8749. DOI: 10.15376/biores.10.4.Hubbe_Gardner_Shen

Contact angle methods are widely used to evaluate the wettability of cellulose-based surfaces and to judge their suitability for different applications. Wettability affects ink receptivity, coating, absorbency, adhesion, and frictional properties. There has been a continuing quest on the part of researchers to quantify the thermodynamic work of adhesion between cellulosic surfaces and various probe liquids and to account for such components of force as the London/van der Waals dispersion force, hydrogen bonding, and acid and base interactions. However, due in part to the rough, porous, and water-swellable nature of cellulosic materials, poor fits between various theories and contact angle data have been observed. Such problems are compounded by inherent weaknesses and challenges of the theoretical approaches that have been employed up to this point. It appears that insufficient consideration has been given to the challenging nature of cellulosic materials from the perspective of attempting to gain accurate information about different contributions to surface free energy. Strong hysteresis effects, with large differences between advancing and receding contact angles, have been overlooked by many researchers in attempting to quantify the work of adhesion. Experimental and conceptual approaches are suggested as potential ways to achieve more reliable and useful results in future wettability studies of cellulosic surfaces.

Hubbe, M. A., Sanchez, M. A., Baosupee, D., and Nazhad, M. (2015). “Breakup of agglomerated clusters of cellulosic fines and CaCO₃ particles exposed to hydrodynamic stress,”J. Dispersion Sci. Technol. 37(6), 836-845. DOI: 10.1080/01932691.2015.1065504

The capacity of fine particles to remain clustered together after having been agglomerated with the help of polyelectrolytes plays an important role in papermaking and in the treatment of wastewater. Tests were carried out with agglomerated suspensions of calcium carbonate and primary cellulosic fines in neutral buffer solution. Agglomeration was induced either by a high-charge cationic polyelectrolyte ( a coagulant) or by sequential treatment with a coagulant and a very-high-mass anionic acrylamide copolymer (a flocculant). Particle size analysis, based on diffraction of laser light, showed that the coagulated suspensions were susceptible to being redispersed by application of hydrodynamic shear. By contrast, flocculated suspensions were only partly broken up by shear. In a flocculated mixture of CaCO3 and cellulosic fines, only the cellulosic fines could be separated from each other, under the conditions employed. The intensity of shear proved to be more critical than its duration. Different flow systems were compared and explained in terms of the relative values and types of hydrodynamic stress experienced by the agglomerated particles in different cases.

Baosupee, D., Massey, A. J., Nazhad, M., and Hubbe, M. A. (2015). “Polyelectrolyte coagulant and flocculant effects on heteroagglomeration of cellulosic fines and CaCO₃ particles,”APPITA J. 63(3), 237-245.

Effects of a high-charge cationic polyelectrolyte (a coagulant), a very-high-mass cationic polyelectrolyte (a flocculant), or the combination of a cationic coagulant followed by an anionic flocculant were evaluated relative to the particle size distributions in suspensions of cellulosic fines and CaCO3 particles. Laser diffraction particle size analysis for the mixed suspension showed the virtual disappearance of signal corresponding to unattached CaCO3 particles upon addition of cationic flocculant. Charge effects related to the coagulant had less influence on the particle size distribution compared to the polymer bridging effects of the flocculant. Microscopic images revealed differences in the structure of agglomerates in the polymer-treated systems. The polymer-induced attachments between CaCO3 and cellulosic fines can be interpreted as an additional stage of heteroagglomeration, supplemental to those attachments that had already been formed between CaCO3 and cellulose surfaces prior to addition of the polyelectrolytes. Resulting structures depended not only on the polyelectrolytes but also on the characteristic shapes of cellulosic fines and the fibrillation of their surfaces.

Hubbe, M. A., Rojas, O. J., and Lucia, L. A. (2015). “Green modification of surface characteristics of cellulosic materials at the molecular or nano scale: A review,”BioResources 10(3), 6095-6229. DOI: 10.15376/biores.10.3.Hubbe

Many current and potential uses of cellulosic materials depend critically on the character of their surfaces. This review of the scientific literature considers both well-established and emerging strategies to change the outermost surfaces of cellulosic fibers or films not only in terms of chemical composition, but also in terms of outcomes such as wettability, friction, and adhesion. A key goal of surface modification has been to improve the performance of cellulosic fibers in the manufacture of composites through chemistries such as esterification that are enabled by the high density of hydroxyl groups at typical cellulosic surfaces. A wide variety of grafting methods, some developed recently, can be used with plant-derived fibers. The costs and environmental consequences of such treatments must be carefully weighed against the potential to achieve similar performances by approaches that use more sustainable methods and materials and involve less energy and processing steps. There is potential to change the practical performances of many cellulosic materials by heating, by enzymatic treatments, by use of surface-active agents, or by adsorption of polyelectrolytes. The lignin, hemicelluloses, and extractives naturally present in plant-based materials also can be expected to play critical roles in emerging strategies to modify the surfaces characteristics of cellulosic fibers with a minimum of adverse environmental impacts.

Hubbe, M. A. (2015). “What next for wood construction/demolition debris?”BioResources 10(1), 6-9. DOI: 10.15376/biores.10.1.6-9

Residents in localities throughout the world voluntarily participate in the routine recycling of household wastes, such as paper, metals, and plastics containers. But when a house in their neighborhood gets built or torn down, most of the debris – including wood waste – gets landfilled. Such a waste of material suggests that there are opportunities to add value to these under-utilized resources. The great variability, as well as contamination, pose major challenges. It is recommended that reclaimed wood be primarily used in the manufacture of durable goods, and then whatever is left over be used for energy (or heat) generation.

Hubbe, M. A. (2014). “A review of ways to adjust papermaking wet-end chemistry: Manipulation of cellulosic colloidal behavior,” Lignocellulose 3(1), 69-107.

This article reviews various adjustments in chemical additives and process conditions that can be used in the course of papermaking to manipulate either the efficiency of the process or the attributes of the resulting paper.  Published studies show that the effects of certain chemical additives to the fiber suspension can be understood based on the forces of interaction between surfaces, i.e. the colloidal forces.  There are opportunities to use such concepts to optimize the efficiency of retention of fine particles and the rate of water release during papermaking.  It is proposed that – for easier understanding – the papermaking process should be viewed as a series of pairwise interactions, for which the outcomes depend on the ionic charges of surfaces, the hydrophobic or hydrophilic character of those surfaces, the balance of charges of dissolved polyelectrolytes, and conditions of hydrodynamic shear inherent in the unit operations of papermaking.

Hubbe, M. A. (2014). “Lignocellulosic biodegradation in composting,” in: Composting for Sustainable Agriculture, Maheshwari, D. K. (ed.), Vol. 3 in Sustainable Development and Biodiversity ser., Springer, Heidelberg, Ch. 3, 43-66. DOI: 10.1007/978-3-319-08004-8_3

Plant-derived material, i.e. lignocellulosic biomass, makes up a major proportion of the initial mass in a typical composting operation.  Such biomass plays some key roles as the mixture is being converted to prepare a useful soil amendment.  For instance, the lignocellulosic component can provide bulking, can help to balance the C:N elemental composition, and serves as the main source of energy for the bacterial processes that go on during composting.  This chapter reviews recent research helping to clarify these roles and to explain the underlying mechanisms.   Recent studies have highlighted the importance of bacterial communities, as well as the succession in the composition of those communities during the different thermal phases of composting.  Progress also has been made in understanding the flows of heat resulting from metabolism, aeration, and chemical changes in the compost mixture.  Advances have been reported in the chemical analysis of compost, revealing details of chemical transformations occurring during the decomposition and stabilization of compost.  The lignin component in a compostable mixture provides chemical building blocks that give rise to humic acids and other substances that resist further biodegradation and allow mature compost to retain water and bind minerals.  Based on the literature one can conclude that composting, especially when lignocellulosic materials are employed under suitable conditions, is an environmentally responsible, relatively mature technology that can be expected to receive increasing research attention in the future.

Hubbe, M. A., Park, J., and Park, S. (2014). “Cellulosic substrates for removal of pollutants from aqueous systems: A review. Part 4. Dissolved petrochemical compounds,”BioResources 9(4), 7782-7925. DOI: 10.15376/biores.9.4.7782-7925

Dissolved petroleum-based compounds, e.g. solvents, pesticides, and chemical reagents such as phenolic compounds, can pose significant hazards to the health of humans and ecosystems when they are released to the environment.  This review article considers research progress related to the biosorption and removal of such contaminants from water using cellulose-derived materials.  The fact that cellulosic materials show promise in removing such sparingly soluble materials from water lends support to a hypothesis that lignocellulosic materials can be broad-spectrum adsorbents.  Also, the hydrophobic character and sorption capabilities can be increased through thermal treatment and the preparation of activated carbons.  As shown in many studies, the efficiency of uptake of various petrochemical products from water also can be increased by chemical treatments of the adsorbent.  It appears that more widespread adoption of biosorption as a means of removing petroleum-based products from water has been limited by concerns about the used, loaded biosorbent.  Disposal or regeneration options that need to be considered more in future research include enzymatic and biological treatments, taking advantage of the fact that the biosorbent material is able to collect, immobilize, and concentrate various contaminants in forms that are suited for a number of packed bed or batch-type degradative treatment systems.

Hubbe, M. A. (2014). “Puzzling aspects of the hydrophobic sizing of paper and its inter-fiber bonding ability,” BioResources 9(4), 5782-5783. DOI: 10.15376/biores.9.4.5782-5783

Internal sizing agents make it possible to prepare water-resistant paper from an aqueous suspension comprising water-loving fibers and an emulsified hydrophobic agent. Why doesn’t the hydrophobic treatment get in the way of inter-fiber bonding? The answer appears to involve the order in which nano-scale events happen during the manufacture of paper. It appears that the inter-fiber bonded areas develop first. Molecular distribution of the hydrophobic agents appears to happen later, especially during the later stages of evaporative drying. The topic seems to be crying out for someone to carry out appropriate experiments to shed more light on the mechanism.

Hubbe, M. A. (2014). “Zipping backwards the other way – Yet another unique aspect of cellulose,”BioResources 9(3), 3759-3760. DOI: 10.15376/biores.9.3.3759-3760

Readers of this journal may be keenly aware of cellulose’s remarkable attributes, such as high stiffness, insolubility in just about everything, resistance to enzymatic attack, dimensional stability in the lengthwise direction, and toughness associated with the alternating crystalline zones and less organized regions. But it you dissolve cellulose and then allow it to recrystallize, the resulting crystals are at the same time radically different, and yet remarkably similar in most respects to the native form. Exactly half of the macromolecules in regenerated cellulose have been reversed 180 degrees in their direction. The behavior of dropped pencils can help explain why this happens.

Zhang, W., Liu, W., Li, H., Hubbe, M. A., Yu, D., Li, G., and Wang, H. (2014). “Improving stability and sizing performance of alkenylsuccinic anhydride (SAS) emulsion by using melamine-modified laponite particles as emulsion stabilizer,”Industrial & Engineering Chemistry Research 53, 12330-12338. DOI: 10.1021/ie501381a

Alkenylsuccinic anhydride (ASA) is commonly applied as oil-in-water (o/w) emulsions in the papermaking industry. Herein Laponite mineral nanoparticles were employed as a stabilizer of the ASA emulsions after being modified with melamine just before emulsion preparation. The emulsion was prepared by homogenizing the mixture of ASA and melamine-modified Laponite aqueous dispersion. The modification of melamine on the Laponite was characterized by infrared spectroscopy and X-ray diffraction, whereas the impacts of the modification on the morphology, wettability and zeta-potential of the Laponite, as well as the interfacial tension between ASA and Laponite aqueous dispersion, were also analyzed. It is found that the adsorption of melamine on Laponite particles neither causes the aggregation nor significantly changes the charge properties of the Laponite particles. However, the adsorption of melamine can significantly increase the wettability of Laponite by the ASA liquid, and adequately lower the apparent interfacial tension between ASA and Laponite aqueous dispersion when the melamine-to-Laponite mass ratio is less than 3%. This results in an improvement in emulsion stability, a reduction in emulsion droplet size and an enhancement in the sizing performance of the ASA emulsion when the emulsion is stabilized by melamine-modified Laponite particles. The ASA emulsion with the smallest droplet size and best sizing performance is produced at a melamine-to-Laponite mass ratio of 3%. By monitoring the variations of the emulsion with time, it is discovered that the modification of Laponite with melamine can restrain the growth of emulsion droplets and the hydrolytic action of ASA substantially, thus decreasing the loss in sizing performance of the ASA emulsion with time. This is particularly important for the wide application of ASA emulsions in the papermaking industry.

Song, X. L., and Hubbe, M. A. (2014). “Cationization of oat beta-D glucan as a dry-strength additives for paper,”Tappi J. 13(7), 57-64. DOI: 10.32964/TJ13.7.57

Oat beta-D glucan was treated with 3-chloro-2-hydroxypropyl-trimethyl ammonium chloride (10%, 20%, 30%, or 50% of beta glucan) to obtain a range of cationic beta-D glucan samples. The derivatization was confirmed by the results of Fourier transform infrared (FTIR) tests and elemental analysis. Addition of 1% cationic beta-D glucan based on the mass of unbleached pine kraft fiber increased burst, tensile, and folding endurance properties of the resulting paper. Similar effects were observed at pH 5 and pH 8.5, showing that the system can be considered robust relative to typical acidic and alkaline papermaking conditions. The strength benefits were also observed in recycled sheets made by reslurrying paper prepared with cationic beta-D glucan, even when the initial drying conditions had been severe. The beneficial results of cationization, which can be explained by a more hydrophilic nature and better retention, hold promise as a means of improving the strength properties of virgin paper. The treatment of the initial paper also can enhance the dry-strength performance when the fibers are recovered and used again.

Tan, H., Liu, W., Yu, D., Li, H., Hubbe, M. A., Gong, B., Zhang, W., and Li, G. (2014). “ASA-in-water emulsions stabilized by laponite nanoparticles modified with tetramethylammonium chloride,”Chem. Eng. Sci. 116, 682-693. DOI: 10.1016/j.ces.2014.06.005

Alkenyl succinic anhydride (ASA) is a widely used paper sizing agent that is applied in the form of oil-in-water (o/w) emulsions in order to impart a water-resistant character to the resulting paper. To obtain stable o/w emulsions of ASA, laponite, a highly hydrophilic synthetic clay, was selected as the stabilizer after it had been modified with tetramethylammonium chloride (TMAC), a quaternary ammonium salt with the shortest possible hydrocarbon groups. It was found that the TMAC moderately neutralized the negative charges of laponite particles, lowered the apparent viscosity, but enhanced the turbidity of laponite aqueous dispersion by enhancing the hydrophobicity of the laponite particles, favoring adsorption of laponite particles on the ASA water interface. Meanwhile, the TMAC significantly decreased the interfacial tension between ASA and water/aqueous laponite dispersion, promoting the formation of an emulsion with small droplets. When the added amount of TMAC reached 1 wt% based on laponite, the as-prepared ASA emulsion had small droplet size, low viscosity and uniform droplet size distribution, and exhibited good creaming/coalescence stability. By using TMAC to modify laponite nanoparticles, the hydrolysis stability and sizing performance of ASA emulsion were also improved.

Hubbe, M. A. (2014). “Prospects for maintaining strength of paper and paperboard products while using less forest resources: A Review,”BioResources 9(1), 1634-1763. DOI: 10.15376/biores.9.1.1634-1763

Paper production requires large amounts of cellulosic fiber, whereas the world’s forested lands and croplands have a finite capacity to supply such resources. To deal with likely future pressure on forest resources, as well as to hold down costs of materials, publications examined in the preparation of this review suggest that the paper industry will need to implement several concurrent strategies. In particular, the industry can be expected to view recycling as a central part of its activities. Basis weights of various paper-based products can be expected to decrease over the coming decades, and more of the fiber content will be replaced with fillers such as calcium carbonate. Such trends will place intense demands upon chemical-based strategies to enhance the bonding within paper and paperboard. Based on the literature, further progress in reducing the amount of new forest resources used to meet a given set of paper product requirements will require a combined approach, taking into account various fiber attributes, nanostructures, novel concepts in bond formation, and advances in the unit operations of papermaking.

Song, X. L., and Hubbe, M. A. (2014). “Enhancement of paper dry strength by carboxymethylated beta-D-glucan from oat as additive,” Holzforschung 68(3), 257-263. DOI: 10.1515/hf-2013-0108

The hypothesis has been tested that a carboxymethyl hemicellulose improves more effectively the dry strength of papers than a native hemicellulose. To that end, beta-D-glucan from oat was treated with an alkaline ethanolic solution of sodium chloroacetate for different times to obtain a range of carboxymethylated beta-D-glucan (CM-glucan) samples. The derivatization was confirmed by Fourier transform infrared spectroscopy and elemental analysis. The physical properties of paper concerning burst and tensile strength as well as folding endurance were essentially improved if CM-glucan was added to unbleached kraft fiber suspension from pine before papermaking. The effects could be maximized by a proper selection of carboxymethylation time and the amount of CM-glucan added to the suspension. The effect was also beneficial in the case of recycled fibers.

Baosupee, D., Massey, A. J., Nazhad, M., and Hubbe, M. A. (2014). “Heteroagglomeration as a mechanism of retaining CaCO₃ particles on the fibrils of cellulosic fines: A study by laser light diffraction and microscopy,” Colloids Surf. A:Physicochem. Eng. Aspects 441, 525-531. DOI: 10.1016/j.colsurfa.2013.10.020

Mutual agglomeration involving contrasting types of particles can be expected to play a major role during the formation of paper. The present work employed laser diffraction particle size analysis, as well as microscopy, to characterize the state of agglomeration between cellulosic fines and precipitated calcium carbonate (PCC) particles. Primary fines from bleached hardwood kraft pulp were compared with fines collected from the same pulp after mechanical refining. Various ratios of cellulose to PCC were studied. Results were consistent with a process of heteroagglomeration occurring mainly between the PCC and slender cellulosic fibrils associated with the cellulosic fine particles. Adhesive attachments were formed between the PCC and cellulosic surfaces in spite of their having the same sign of zeta potential.

Song, X., and Hubbe, M. A. (2014). “TEMPO-mediated oxidation of oat beta-D-glucan and its influences on paper properties,”Carbohydrate Polymers 99, 617-623. DOI: 10.1016/j.carbpol.2013.08.070

An enhanced bonding agent for papermaking was prepared by selective oxidation of a hemicellulose-rich byproduct of oat processing, which will be identified here by its primary component, beta-D-glucan. The beta-D-glucan was treated sequentially with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and sodium hypochlorite, or alternatively just with sodium hydroxide. When added to a slurry of unbleached softwood kraft fibers, in combination with an optimal dosage of aluminum sulfate, the oxidized beta-D-glucan yielded greater increases in tensile strength and folding endurance in comparison to untreated beta-D-glucan. NaOH treatment also improved dry-strength performance of the beta-D-glucan, except for folding endurance. The improvements were attributed to increased charge density of the treated polyelectrolytes, leading to better distribution and retention on fibers prior to sheet formation. Modified beta-D-glucan also enhanced the strength of recycled sheets when the treated paper was repulped and formed into recycled paper with no further chemical addition.

Hubbe, M. A. (2013). “New horizons for use of cellulose-based materials to adsorb pollutants from aqueous solutions,”Lignocellulose 2(2), 386-411.

This article reviews recent research related to biosorption – the use of plant-derived materials to remove various pollutants from aqueous systems. Emphasis is placed on biosorption studies dealing with the removal of heavy metal ions, dyes, and spilled oil from water. Much progress already had been achieve in understanding the factors that affect adsorption capacities, rates of uptake, and possible release back into the water. It has been shown that the performance of cellulose-based sorbent materials often can be improved by physical of chemical modification of the sorbent. There is a critical need for research related to strategies for dealing with the adsorbent materials after their use. In addition to regeneration and re-use of sorbent materials, attention also needs to be paid to the incineration of contaminated sorbents, as well as the biodegradation of sorbent material after uptake of various pollutants.

Peresin, M. S., Zoppe, J. O., Vallejos, M. E., Habibi, Y., Hubbe, M. A., and Rojas, O. J. (2013). “Nano- and microfiber composition reinforced with cellulose nanocrystals,” in: Cellulose Based Composites: New Green Nanomaterials, Hinestroza, J., and Netravali, A. N. (eds.), Wiley-VCH, New York, 77-113.

The integration of reinforcing materials in composites with synthetic polymers has gained increased attention in recent years, especially in the area of nanocomposites. Most of the efforts in this respect have focused on inorganic reinforcing agents and mostly by using the casting method. Recently, the developments based on renewable materials have become increasingly popular because of the need for finding alternatives to nonrenewable fossil carbon resources. Among such materials, readily available cellulose nanocrystals (CNs) have attracted great interest because of their availability, renewability, biodegradability, and excellent mechanical properties. Furthermore, CNs are relatively easy to produce and are much less expensive than most of the particles currently used for reinforcement purposes. An additional advantage of CNs and ‘‘lignocellulosics’’ fillers compared to equivalent inorganics include high specific strength, surface reactivity, and the option of surface modification to give better compatibility with the matrix materials.

Ayoub, A., Venditti, R. A., Pawlak, J. J., Salam, A., and Hubbe, M. A. (2013). “Novel hemicellulose-chitosan biosorbent for water desalination and heavy metal removal,”ACS Sustainable Chem. Eng. 1(9), 1102-1109. DOI: 10.1021/sc300166m

Hemicellulose material is an abundant and relatively under-utilized polymeric material present in lignocellulosic materials. In this research, an alkaline treatment was applied to pinewood (PW), switchgrass (SG), and coastal bermuda grass (CBG) in order to extract hemicelluloses to subsequently produce a novel biosorbent. Alkaline extraction at 75 degrees C recovered 23% of the biomass as a predominantly hemicellulose material with a number average degree of polymerization of similar to 450. These hemicelluloses were grafted with penetic acid (diethylene triamine pentaacetic acid, DTPA) and were then cross-linked to chitosan. The effects of hemicellulose DTPA concentration, reaction time, and temperature of reaction with chitosan on the resulting salt (sodium chloride, NaCl) uptake and weight loss in saline solutions were determined. A maximum salt uptake for the materials was similar to 0.30 g/g of foam biosorbent. The foam biosorbent was characterized by FT-IR spectra, porosity, and dynamic mechanical analysis. Batch adsorption equilibrium results suggest that the adsorption process for salt follows a second-order kinetic model. The hemicellulose-DTPA-chitosan foam biosorbent had uptakes of 2.90, 0.95, and 1.37 mg/g of Pb2+, Cu2+, and Ni2+ ions, respectively, from aqueous medium at initial concentrations of 5000 PPB at pH 5. The cross-linked hemicellulose DTPA chitosan material has good potential for environmental engineering applications.

Hubbe, M. A., Ayoub, A., Daystar, J. S., Venditti, R. A, and Pawlak, J. J. (2013). “Enhanced absorbent products incorporating cellulose and its derivatives: A review,”BioResources 8(4), 6556-6629.  DOI: 10.15376/biores.8.4.6556-6629

Cellulose and some cellulose derivatives can play vital roles in the enhancement of the performance of absorbent products. Cellulose itself, in the form of cellulosic fibers or nano-fibers, can provide structure, bulk, water-holding capacity, and channeling of fluids over a wide dimensional range. Likewise, cellulose derivatives such as carboxymethylcellulose (CMC) have been widely studied as components in superabsorbent polymer (SAP) formulations. The present review focuses on strategies and mechanisms in which inclusion of cellulose – in its various forms – can enhance either the capacity or the rate of aqueous fluid absorption in various potential applications.

Salas, C., Genzer, J., Lucia, L. A., Hubbe, M. A., and Rojas, O. J. (2013). “Water-wettable polypropylene fibers by facile surface treatment based on soy proteins,”ACS Appl.  Mater. Interfaces 5(14), 6541-6548. DOI: 10.1021/am3024788

Modification of the wetting behavior of hydrophobic surfaces is essential in a variety of materials, including textiles and membranes that require control of fluid interactions, adhesion, transport processes, sensing, etc. This investigation examines the enhancement of wettability of an important class of textile materials, viz., polypropylene (PP) fibers, by surface adsorption of different proteins from soybeans, including soy flour, isolate,glycinin, and beta-conglycinin. Detailed investigations of soy adsorption from aqueous solution (pH 7.4, 25 degrees C) on polypropylene thin films is carried out using quartz crystal microbalance (QCM) and surface plasmon resonance (SPR). A significant amount of protein adsorbs onto the PP surfaces primarily due to hydrophobic interactions. We establish that adsorption of a cationic surfactant, dioctadecyldimethylammonium bromide (DODA) onto PP surfaces prior to the protein deposition dramatically enhances its adsorption. The adsorption of proteins from native (PBS buffer, pH 7.4, 25 degrees C) and denatured conditions (PBS buffer, pH 7.4, 95 degrees C) onto DODA-treated PP leads to a high coverage of the proteins on the PP surface as confirmed by a significant improvement in water wettability. A shift in the contact angle from 128 degrees to completely wettable surfaces (approximate to 0 degrees) is observed and confirmed by imaging experiments conducted with fluorescence tags. Furthermore, the results from wicking tests indicate that hydrophobic PP nonwovens absorb a significant amount of water after protein treatment, i.e., the PP-modified surfaces become completely hydrophilic.

Salas, C., Rojas, O. J., Lucia, L. A., Hubbe, M. A., and Genzer, J. (2013). “On the surface interactions of proteins with lignin,” ACS Appl. Mater. Interfaces 5(1), 199-206. DOI: 10.1021/am401065t

Lignins are used often in formulations involving proteins but little is known about the surface interactions between these important biomacromolecules. In this work, we investigate the interactions at the solid-liquid interface of lignin with the two main proteins in soy, glycinin (11S) and beta-conglycinin (7S). The extent of adsorption of 11S and 7S onto lignin films and the degree of hydration of the interfacial layers is quantified via Quartz crystal microgravimetry (QCM) and surface plasmon resonance (SPR). Solution ionic strength and protein denaturation (2-mercaptoethanol and urea) critically affect the adsorption process as protein molecules undergo conformational changes and their hydrophobic or hydrophilic amino acid residues interact with the surrounding medium. In general, the adsorption of the undenatured proteins onto lignin is more extensive compared to that of the denatured biomolecules and a large amount of water is coupled to the adsorbed molecules. The reduction in water contact angle after protein adsorption (by similar to 40 degrees and 35 degrees for undenatured 11S and 7S, respectively) is explained by strong nonspecific interactions between soy proteins and lignin.

Hubbe, M. A. (2013). Ten-year Forecast of Disruptive Technologies
in the Pulp and Paper Supply Chain, Smithers-Pira, Leatherhead, UK

This major study from Smithers Pira identifies and profiles the top 25 disruptive technologies that are likely to impact the global pulp and paper supply chain during the coming years up to 2024. The goal of the work is to anticipate the direction of technology-driven changes related to the use of cellulosic fibres, with an emphasis on pulp and paper products.

Hubbe, M. A., Rojas, O. J., Fingas, M., and Gupta, B. S. (2013). “Cellulosic substrates for removal of pollutants from aqueous systems: A Review. 3. Spilled oil and emulsified organic liquids,”BioResources 8(2), 3038-3097. DOI: 10.15376/biores.8.2.3038-3097

Water-insoluble oils, including crude petroleum and a wide variety of refined organic liquids, can cause major problems if spilled or leaked to aqueous environments. Potential environmental damage may be reduced if the spilled oil is promptly and efficiently removed from the water. This article reviews research that sheds light on the use of cellulose-based materials as sorbents to mitigate effects of oil spills. Encouraging results for oil sorption have been reported when using naturally hydrophobic cellulosic fibers such as unprocessed cotton, kapok, or milkweed seed hair. In addition, a wide assortment of cellulosic materials have been shown to be effective sorbents for hydrocarbon oils, especially in the absence of water, and their performance under water-wet conditions can be enhanced by various pretreatments that render them more hydrophobic. More research is needed on environmentally friendly systems to handle oil-contaminated sorbents after their use; promising approaches include their re-use after regeneration, anaerobic digestion, and incineration, among others. Research also is needed to further develop combined response systems in which biosorption is used along with other spill-response measures, including skimming, demulsification, biodegradation, and the use of booms to limit the spreading of oil slicks.

Jin, H. Y., Lucia, L. A., Rojas, O. J., Hubbe, M. A., and Pawlak, J. J. (2012). “Survey of soy protein flour as a novel dry strength agent for papermaking furnishes,”J. Agric. Food Chem. 60(39), 9828-9833. DOI: 10.1021/jf303023j

A series of experiments were conducted on recycled pulp samples for the novel purpose of determining the efficacy of employing soy protein flour to increase the strength of dry paper. Values of short span compression and tensile strength were the prime criteria for comparison based on industrial considerations. Various conditions were considered to uncover effective schemes for applying the soy proteins under industrial-like papermaking conditions including alkaline versus acidic as well as high or low ionic content papermaking conditions. A hybrid system of starch, a dry strength additive currently used in paper furnishes, and soy protein was considered to study the possible existence of any synergistic chemical effects. Results indicated that a 1 part (by mass) soy protein to 3 parts cationic starch hybrid system resulted in the highest strength increase in comparison to solely either the soy protein or the cationic starch as dry strength additives.

Payne, K. C., Jackson, C. D., Aizpurua, C. E., Rojas, O. J., and Hubbe, M. A. (2012). “Oil spills abatement: Factors affecting oil uptake by cellulosic fibers,” Environ. Sci. Technol. 46(14), 7725-7730. DOI: 10.1021/es3015524

Wood-derived cellulosic fibers prepared in different ways were successfully employed to absorb simulated crude oil, demonstrating their possible use as absorbents in the case of oil spills. When dry fibers were used, the highest sorption capacity (six parts of oil per unit mass of fiber) was shown by bleached softwood kraft fibers, compared to hardwood g bleached kraft and softwood chemithermomechanical pulp(CTMP) fibers. Increased refining of CTMP fibers decreased their oil uptake capacity. When the fibers were soaked in water before exposure to the oil, the ability of the unmodified kraft fibers to sorb oil was markedly reduced, whereas the wet CTMP fibers were generally more effective than the wet kraft fibers. Predeposition of lignin onto the surfaces of the bleached kraft fibers improved their ability to take up oil when wet Superior ability to sorb oil in the wet state was achieved by pretreating the kraft fibers with a hydrophobic sizing agent, alkenylsuccinic anhydride (ASA). Contact angle tests on a model cellulose surface showed that some of the sorption results onto wetted fibers could be attributed to the more hydrophobic nature of the fibers after treatment with either lignin or ASA.

Hubbe, M. A., Sundberg, A., Mocchiutti, P., Ni, Y., and Pelton, R. (2012). “Dissolved and colloidal substances (DCS) and the charge demand of papermaking process waters and suspensions: A review,”BioResources 7(4), 6109-6193. DOI: 10.15376/biores.7.4.6109-6193

Dissolved and colloidal substances (DCS) in the process waters of paper machine systems can interfere with the retention of fine particles, retard the drainage of water from the wet web, and generally hurt the intended functions of various polyelectrolytes that are added to the process. This review considers publications that have attempted to characterize the nature and effects of different DCS fractions, in addition to some of the ways that paper technologists have attempted to overcome related problems. The consequences of DCS in a paper machine system can be traced to their ability to form complexes with various polyelectrolytes. Such tendencies can be understood based on a relatively strong complexing ability of multivalent materials, depending on the macromolecular size and charge density. Continuing research is needed to more fully understand the different contributions to cationic demand in various paper machine systems and to find more efficient means of dealing with DCS.

Carrillo, C. A., Saloni, D., Lucia, L. A., Hubbe, M. A., and Rojas, O. J. (2012). “Capillary flooding of wood with microemulsions from Winsor I systems,”J. Colloid Interface Sci. 381, 171-179. DOI: 10.1016/j.jcis.2012.05.032

A new approach based on microemulsions formulated with at least 85% water and minority components consisting of oil (limonene) and surfactant (anionic and nonionic) is demonstrated for the first time to be effective for flooding wood’s complex capillary structure. The formulation of the microemulsion was based on phase behavior scans of Surfactant-Oil-Water systems (SOWs) and the construction of pseudo-ternary diagrams to localize thermodynamically stable one-phase emulsion systems with different composition, salinity and water-to-oil ratios. Wicking and fluid penetration isotherms followed different kinetic regimes and indicated enhanced performance relative to that of the base fluids (water, oil or surfactant solutions). The key properties of microemulsions to effectively penetrate the solid structure are discussed; microemulsion formulation and resultant viscosity are found to have a determining effect in the extent of fluid uptake. The solubilization of cell wall components is observed after microemulsion impregnation. Thus, the microemulsion can be tuned not only to effectively penetrate the void spaces but also to solubilize hydrophobic and hydrophilic components. The concept proposed in this research is expected to open opportunities in fluid sorption in fiber systems for biomass pretreatment, and delivery of hydrophilic or lipophilic moieties in porous, lignocellulosics.

Salas, C., Rojas, O. J., Lucia, L. A., Hubbe, M. A., and Genzer, J. (2012). “Adsorption of glycinin and beta-conglycinin on silica and cellulose: Surface interactions as a function of denaturation, pH, and electrolytes,” Biomacromolecules 13(2), 387-396. DOI: 10.1021/bm2014153

Soybean proteins have found uses in different nonfood applications due to their, interesting properties. We report on the kinetics and extent of adsorption on silica and cellulose surfaces of glycinin and beta-conglycinin, the main proteins present in spy. Quartz crystal microgravimetry (QCM) experiments indicate that soy protein adsorption is strongly affected by changes in the physicochemical environment. The affinity of glycinin and the mass adsorbed on silica and cellulose increases (by ca. 13 and 80%, respectively) with solution ionic strength (as it increases from 0 to 100 mM NaCl) due to screening of electrostatic interactions. In contrast, beta-conglycinin adsorbs on the same substrates to a lower extent and the addition of electrolyte reduces adsorption (by 25 and 57%, respectively). The addition 01 10 mM 2-mercaptoethanol, a denaturing agent, reduces the adsorption of both proteins with a significant effect for glycinin. This observation is explained by the cleavage of disulfide bonds which allows unfolding of the molecules and promotes dissociation into subunits that favors more compact adsorbed layer structures. In addition, adsorption of glycinin onto cellulose decreases with lowering the pH from neutral to pH 3 due to dissociation of the macromolecules, resulting in flatter adsorbed layers. The respective adsorption isotherms fit a Langmuir model and QCM shifts in energy dissipation and frequency reveal multiple step kinetic processes indicative of changes in adlayer structure.

Hubbe, M. A., Beck, K. R., O’Neal, W. G., and Sharma, Y. C. (2012). “Cellulosic substrates for removal of pollutants from aqueous systems: A review. 2. Dyes,”BioResources 7(2), 2592-2687. DOI: 10.15376/biores.7.2.2592-2687

Dyes used in the coloration of textiles, paper, and other products are highly visible, sometimes toxic, and sometimes resistant to biological breakdown; thus it is important to minimize their release into aqueous environments. This review article considers how biosorption of dyes onto cellulose-related materials has the potential to address such concerns. Numerous publications have described how a variety of biomass-derived substrates can be used to absorb different classes of dyestuff from dilute aqueous solutions. Progress also has been achieved in understanding the thermodynamics, kinetics, and chemical factors that control the uptake of dyes. Important questions remain to be more fully investigated, such as those involving the full life-cycle of cellulosic substrates that are used for the collection of dyes. Also, more work needs to be done in order to establish whether biosorption should be implemented as a separate unit operation, or whether it ought to be integrated with other water treatment technologies, including the enzymatic breakdown of chromophores.

Hamzeh, Y., Najafi, S. M. H., Hubbe, M. A., Salehi, K., and Firouzabadi, M. R. D. (2012). “Recycling potential of unbleached and bleached chemical pulps from juvenile and mature wood of Populus deltoids,” Holzforschung 66, 155-161. DOI: 10.1515/HF.2011.141

The recycling potential of unbleached and bleached pulps of juvenile and mature wood of poplar (namely “eastern cottonwood”, Populus deltoides) has been investigated. First, chemical and morphological characteristics of juvenile wood (JW), transition wood, and mature wood (MW) of the trunk were determined. Then, high yield and low yield pulps were produced separately from JW and MW by kraft pulping (KP), followed by bleaching of the low-yield pulp with a DED sequence. The obtained handsheet papers were subjected to five successive drying and rewetting cycles (as recycling simulation), and the properties of the corresponding pulps were characterized. The results show that JW is inferior with respect to chemical and morphological properties and resulted in low-yield KP. The bleached pulp obtained from JW required more bleaching chemical to achieve the brightness targets. The strength losses of pulps resulting from recycling were more significant in the first recycling cycle, and then the loss rate decreased with further cycles. The comparison between different pulps showed that the JW pulps were more susceptible to the effects of recycling than MW pulps.

Spence, K. L., Venditti, R. A., Rojas, O. J., Pawlak, J. J., and Hubbe, M. A. (2011). “Water vapor barrier properties of coated and filled microfibrillated cellulose composite films,” BioResources 6(4), 4370-4388. DOI: 10.15376/biores.6.4.4370-4388

Microfibrillated celluloses (MFCs) have mechanical properties sufficient for packaging applications but lack water vapor barrier properties in comparison to petroleum-based plastics. These properties can be modified by the use of mineral fillers, added within the film structure, or waxes, as surface coatings. In this investigation it was determined that addition of fillers resulted in films with lower densities but also lower water vapor transmission rates (WVTR). This was hypothesized to be due to decreased water vapor solubility in the films. Associated transport phenomena were analyzed by the Knudsen model for diffusion but due to the limited incorporation of chemical factors in the model and relatively large pore sizes, accurate prediction of pore diameters for filled films was not possible with this model. Modeling the filled-films with Fick’s equation, however, takes into account chemical differences, as observed by the calculated tortuosity values. Interestingly, coating with beeswax, paraffin, and cooked starch resulted in MFC films with water vapor transmission rates lower than those for low density polyethylene. These coatings were modeled with a three-layer model which determined that coatings were more effective in reducing WVTR.

Hubbe, M. A., Wu, N., Rojas, O. J., and Park, S. (2011). “Permeation of a cationic polyelectrolyte into mesoporous silica. Part 3. Using adsorption isotherms to elucidate streaming potential results,” Colloids Surf. A 381(1-3), 1-6. DOI: 10.1016/j.colsurfa.2010.12.052

Parts 1 and 2 of this series showed that the streaming potential of silica gel particles in aqueous media can be profoundly affected by their exposure to solutions of a cationic polyelectrolyte. The extent of the change in streaming potential depended on such variables as pH, salt concentration, polyelectrolyte molecular mass and concentration, pore size, and time. However, questions arose concerning the relationship between the observed changes in streaming potential and the net amount of adsorbed polyelectrolyte. Some preliminary experiments suggested that, compared to adsorption tests, the streaming potential method may be much more sensitive to the permeation of minor amounts of oligomeric impurities into the network of mesopores in the substrate. The present article follows up on these findings, evaluating adsorption isotherms for the same systems that earlier had been examined by the streaming potential method. In contrast to the earlier work, it was possible to interpret the isotherms based on a model in which adsorbate interacts with a set of equivalent, non-interacting adsorption sites. The kinetics of adsorption were time-dependent and diffusion limited. The polymer adsorbed amount was controlled by both the pore size and the surface area. The highest adsorption amount, based on mass of the substrate, was achieved when using silica gel having an intermediate pore size (15 nm) at a relatively high solution concentration of very-low-mass polyelectrolyte. The results could be fit well to a Langmuir model of the adsorption process.

Silva, D. J., Rojas, O. J., Hubbe, M. A., and Park, S. W. (2011). “Enzymatic treatment as a pre-step to remove cellulose films from sensors,” Macromol. Symp. 299/300, 107-112. DOI: 10.1002/masy.200900081

In this work an enzymatic treatment is proposed as a preparative, cleaning protocol to remove cellulose films from resonators and sensors. Quartz crystal and surface plasmon gold sensors, coated with ultrathin films of cellulose are used in studies of molecular (for example, polymer and surfactant) adsorption. The sensors are usually recycled after removal of the film, with limited success, after one of two treatments, either hot acid or ammoniac solutions. In the proposed, improved protocol a mixture of cellulases from Aspergillus species, are used as a pre-treatment to facilitate the release of the cellulose film from the surfaces of the sensors. Two concentrations of NaCl solutions were considered in the enzymatic treatment, 1 and 10 mM, at given enzyme solution concentration, temperature and pH. It was found that after 80 min, the water contact angle after treatment with both salt concentration conditions reached a plateau. The average water contact angle after integration of the enzymatic and ammoniac treatments was found to be low enough, between 6.4 and 7.1 deg to allow reuse the sensors. It is concluded that the use of the ammoniac cleaning solution after the enzymatic treatment is a very convenient, safe and less time consuming way to remove the cellulose films from the sensors to be recycled.

Hubbe, M. A., Hasan, S. H., and Ducoste, J. J. (2011). “Cellulosic substrates for removal of pollutants from aqueous systems: A review. 1. Metals,” BioResources 6(2), 2161-2287. DOI: 10.15376/biores.6.2.2161-2287

Recent years have seen explosive growth in research concerning the use of cellulosic materials, either in their as-recieved state or as modified products, for the removal of heavy metal ions from dilute aqueous solutions. Despite highly promising reports of progress in this area, important questions remain. For instance, it has not been clearly established whether knowledge about the composition and structure of the bioadsorbent raw material is equally important to its availability at its point of use. Various physical and chemical modifications of biomass have been shown to boost the ability of the cellulose-based material to bind various metal ions. Systems of data analysis and mechanistic models are described. There is a continuing need to explain the mechanisms of these approaches and to determine the most effective treatments. Finally, the article probes areas where more research is urgently needed.  For example, life cycle analysis studies are needed, comparing the use of renewable biosorbents vs. conventional means of removing toxic metal ions from water.  

Hubbe, M. A. (2010). “Using cellulosic materials to remove heavy metals from water,”Research Progress in Paper Industry and Biorefinery (4^th ISETPP), S. China Univ. Technol., Nov. 8-10, 1078-1082.

Literature related to bioremediation of toxic metal ions from aqueous solution, using various cellulose-based resources and products, is briefly reviewed. Studies have proven the effectiveness of a very wide range of cellulosic materials, including wood powder, cellulosic fibers, plant materials, and biomass from bacteria and fungi in removing lead, cadmium, chromium, and many other ions. Sorptive capacities often can be enhanced by chemical treatment, derivatization, of by pyrolysis to form activated carbon products. Though the words “ion exchange” are often used to describe the mechanism of uptake, it is clear that chemical complexation – offering a degree of metal-selectivity – is often involved. Bioeremediation offers a way for cellulose-based industries to contribute to cleaning up aqueous environments.

Hubbe, M. A., Nazhad, M., and Sánchez, C. (2010). “Composting as a way to convert cellulosic biomass and organic waste into high-value soil amendments: A review,” BioResources 5(4), 2808-2854. DOI: 10.15376/biores.5.4.2808-2854

Plant-derived cellulosic materials play a critical role when organic wastes are composted to produce a beneficial amendment for topsoil. This review article considers publications dealing with the science of composting, emphasizing ways in which the cellulosic and lignin components of the composted material influence both the process and the product. Cellulose has been described as a main source of energy to drive the biological transformations and the consequent temperature rise and chemical changes that are associated with composting. Lignin can be viewed as a main starting material for the formation of humus, the recalcitrant organic matter that provides the water-holding, ion exchange, and bulking capabilities that can contribute greatly to soil health and productivity. Lignocellulosic materials also contribute to air permeability, bulking, and water retention during the composting process. Critical variables for successful composting include the ratio of carbon to nitrogen, the nature of the cellulosic component, particle size, bed size and format, moisture, pH, aeration, temperature, and time. Composting can help to address solid waste problems and provides a sustainable way to enhance soil fertility.

Baty, J. W., Maitland, C. L., Minter, W., Hubbe, M. A., and Jordan-Mowery, S. K. (2010). “Deacidification for the conservation and preservation of paper-based works: A review,” BioResources 5(3), 1955-2023. DOI: 10.15376/biores.5.3.1955-2023

Embrittlement threatens the useful lifetime of books, maps, manuscripts, and works of art on paper during storage, circulation, and display in libraries, museums, and archives. Past studies have traced much of the embrittlement to the Bronsted-acidic conditions under which printing papers have been made, especially during the period between the mid 1800s to about 1990. This article reviews measures that conservators and collection managers have taken to reduce the acidity of books and other paper-based materials, thereby decreasing the rates of acid-catalyzed hydrolysis and other changes leading to embrittlement. Technical challenges include the selection of an alkaline additive, selecting and implementing a way to distribute this alkaline substance uniformly in the sheet and bound volumes, avoiding excessively high pH conditions, minimizing the rate of loss of physical properties such as resistance to folding, and avoiding any conditions that cause evident damage to the documents one is trying to preserve. Developers have achieved considerable progress, and modern librarians and researchers have many procedures from which to choose as a starting point for further developments.

Hubbe, M. A., Wu, N., Rojas, O. J., and Park, S. (2010). “Permeation of a cationic polyelectrolyte into mesoporous silica. Part 2. Effects of time and pore size on streaming potential,” Colloids and Surfaces A 364(1-3), 7-15. DOI: 10.1016/j.colsurfa.2010.05.005

Streaming potential tests were carried out to determine effects of time and pore size in the adsorption and desorption from aqueous suspensions of cationic polyelectrolytes on silica gel particles. Results in Part 1 of this series showed that the adsorption of cationic polyelectrolytes exposed to mesoporous silica gels can be highly dependent on pH, the polyelectrolyte’s molecular mass, and the solution’s electrical conductivity. Also, the observed changes in streaming potential indicated that the adsorption tended to be relatively slow and incomplete under the conditions of analysis. The present results indicate that the rate of change of streaming potential is proportional to the logarithm of exposure time. The related changes in adsorbed amounts of polyelectrolyte were below the detection limits of typical polyelectrolyte titration procedures. Contrasting charge behaviors were observed on the exterior vs. interior surfaces of silica gel particles as a function of pore size, electrical conductivity, and polyelectrolyte molecular mass. Increasing ionic strength tended to enhance the effect of adsorption of high-mass cationic polymers on the outer surfaces, but produced only a relatively small effect on streaming potential related to their permeation into silica gel (nominal pore sizes of 6 nm or 30 nm). Adsorption of very-low-mass cationic polymer onto the outer surfaces and inside the 6 nm pore size silica gel appeared to be maximized at an intermediate salt level. Finally, electrokinetic tests were used for the first time in a protocol designed to provide evidence of polyelectrolyte desorption from the interiors of mesoporous materials.

Kim, G.-Y., and Hubbe, M. A. (2010). “Engineering of a wet-end additives program relative to process parameters and to the physical and optical properties of filled paper,”Indus. Eng. Chem. Res. 49(12), 5644-5653. DOI: 10.1021/ie100434u

In the manufacture of a paper product, the application of wet-end additives and the adjustments of various conditions can have major effects on the physical and optical properties of the final paper. In this study, we prepared paper handsheets and investigated the effects of many process variables, including the type and amounts of cationic polyacrylamide and colloidal silica, in addition to temperature, duration of mixing, hydrodynamic shear, pH, and variations in electrical conductivity due to salt addition. The most important effects were attributable to variations in the amount, ionic charge, and molecular weight of cationic polyacrylamide, as well as the type and amount of colloidal silica. Many of the observed effects could be explained in terms of fiber flocculation, and the adverse effect of flocculation on the uniformity of the paper, a factor that significantly affected the physical properties. An understanding of the relationships between chemical variables, hydrodynamic shear, and other system variables can be helpful in selecting optimal operating conditions to meet process requirements as well as physical properties of the resulting paper. A statistical analysis was carried out, using normalized coordinates, to show which of the independent variables had significant effects on the response variables.

Hubbe, M. A. (2010). “Factors to consider for enhancing and extending the recyclability of paper,” O Papel 71(4), 40-60.

“Saving a valuable resource – fibers” is the reason that many people give when asked why it is good to recycle paper. But the quality and cost of fibers that can be obtained from post-consumer waste paper depends to a critical extent on how it was manufactured and converted. The recyclability of printing paper has been favorably affected in recent decades by the transition to alkaline papermaking conditions. Conventional dry-strength agents, such as cationic starch, have been found to be very compatible with recycling. In addition, recent work suggests that the lifetime of kraft fibers can be prolonged by refining strategies that emphasize external fibrillation and preservation of bulk. But some other trends are likely to be unfavorable to recycling. There is a danger that some technologies to remove hemicelluloses prior to pulping will yield fibers that are more susceptible to brittle failure, especially when they are recycled. Also, high levels of fillers, wax, wet-strength resins, and some forms of curable inks will continue to pose challenges to paper recycling.

Wu, N., Hubbe, M. A., Rojas, O. J., and Park, S. (2010). “Permeation of a cationic polyelectrolyte into mesoporous silica. Part 1. Factors affecting changes in streaming potential,” Colloids and Surfaces A 364(1-3), 1-6. DOI: 10.1016/j.colsurfa.2010.05.005

A recently developed streaming potential (SP) strategy was used for the first time to investigate factors affecting permeation of the cationic polyelectrolyte poly-(diallyldimethylammonium chloride) from aqueous solution into silica gel particles. Factors affecting cationic polyelectrolyte permeation were considered, including polyelectrolyte dosage, molecular mass, solution pH, and electrical conductivity. Samples were equilibrated for approximately 20 h before testing. The magnitude of change in streaming potential, which was taken as evidence of permeation, increased with increasing polyelectrolyte dosage, with decreasing molecular mass, and with decreasing pH in the range 11 to 3. The pH effect supports a mechanism in which excessively strong electrostatic attraction between the polyelectrolyte and the substrate immobilizes macromolecules at or near the entrances to the pore network, thus inhibiting permeation of like-charged macromolecules. The same mechanism is consistent with observations that permeation increased with increasing electrical conductivity, though the latter observation also could be explained in terms of conformational changes.

Hubbe, M. A. (2010). Ten Year Forecasts of Disruptive Technologies in
Paper and Board to 2020, Pira International.

This major new study from Pira International identifies and profiles the top 25 disruptive technologies that can be expected to affect the global pulp and paper supply chain over the next ten years to 2020. The goal is to anticipate the direction of technology-driven changes related to the usage of cellulosic fibres, with an emphasis on pulp and paper products.

Song, J., Yamaguchi, T., Silva, D. J., Hubbe, M. A., and Rojas, O. J. (2010). “Effect of charge asymmetry on adsorption and phase separation of polyampholytes on silica and cellulose surfaces,”J. Phys. Chem. B 114(2), 719-727. DOI: 10.1021/jp909047t

The relation between the properties of polyampholytes in aqueous solution and their adsorption behaviors on silica and cellulose surfaces was investigated. Four polyampholytes carrying different charge densities but with the same nominal ratio of positive to negative segments and two structurally similar polyelectrolytes (a polyacid and a polybase) were investigated by using quartz crystal microgravimetry using silica-coated and cellulose-coated quartz resonators. Time-resolved mass and rigidity (or viscoelasticity) of the adsorbed layer was determined from the shifts in frequency (Δf) and energy dissipation (ΔD) of the respective resonator. Therefore, elucidation of the dynamics and extent of adsorption, as well as the conformational changes of the adsorbed macromolecules, were possible. The charge properties of the solid surface played a crucial role in the adsorption of the studied polyampholytes, which was explained by the capability of the surface to polarize the polyampholyte at the interface. Under the same experimental conditions, the polyampholytes had a higher nominal charge density phase-separated near the interface, producing a soft, dissipative, and loosely bound layer. In the case of cellulose substrates, where adsorption was limited, electrostatic and polarization effects were concluded to be less significant.

Hubbe, M. A., and Bowden, C. (2009). “Handmade paper: A review of its history, craft, and science,”BioResources 4(4), 1736-1792. DOI: 10.15376/biores.4.4.1736-1792

For over 2000 years the manual craft of papermaking has been practiced all over the world utilizing a variety of techniques.  This review describes the evolution of hand papermaking and its cultural significance. Paper’s evolution has been shaped by the structure and chemical composition of the fibers.  Almost every aspect of modern papermaking technology has been foreshadowed by traditional practices.  Such practices were passed down for many generations within families of papermakers.  The main sources of cellulosic fiber evolved as the ancient craft migrated from its birthplace in China to Korea and Japan, the Islamic world, and then to Europe and America.  Though most paper made today comes from automated, continuous production systems, handmade paper has enjoyed a resurgence, both as a traditional craft and as an art-form.  In addition, traditional papermaking methods can provide insights to help in modern applications involving cellulosic fibers.

Chen, H., Park, A., Heitmann, J. A., and Hubbe, M. A. (2009). “Importance of cellulosic fines relative to the dewatering rates of fiber suspensions,”Ind. Eng. Chem. Res. 48(20), 9106-9112. DOI: 10.1021/ie9006613

When cellulosic fines are present in significant amounts they can have a dominant influence on dewatering. Pulp suspensions drain rapidly if the fines have been removed. In this study, the dependency of gravity dewatering rates on the level and properties of cellulosic fine matter was quantified. Bleached hardwood kraft pulp was used as a source of primary fines (collected before refining) and secondary fines (collected after refining of fines-free fiber suspensions). Fractions of fine matter also were obtained from chemithermomechanical (CTMP) pulp. Size distributions of these fines were characterized using a laser diffraction method. Results were explainable by a mechanism in which unattached fines are able to move relative to adjacent fibers during the dewatering and consolidation of a mat of fibers. Due to such movement, fines end up in locations where they plug drainage channels in the mat. The contribution of the fines to dewatering increased in inverse proportion to particle size and with increasing surface area, as calculated from the light scattering analysis.

Wu, N., Hubbe, M. A., Rojas, O. J., and Park, S. (2009). “Permeation of polyelectrolytes and other solutes into the pore spaces of water-swollen cellulose: A review,”BioResources 4(3), 1222-1262. DOI: 10.15376/biores.4.3.1222-1262

The rate and extent of transport of macromolecules and other solutes into cellulosic materials and fibers have important applications in such fields as papermaking, textiles, medicine, and chromatography. This review considers how diffusion and flow affect permeation into wood, paper, and other lignocellulosic materials. Because pore sizes within such materials can range from nanometers to millimeters, a broad perspective will be used, also considering some publications related to other porous materials. Factors that limit the rate or extent of polymer or other solute transport into pores can involve thermodynamics (affecting the driving motivation for permeation), kinetics (if there is insufficient time for the system to come to equilibrium), and physical barriers. Molecular flow is also affected by the attributes of the solute, such as molecular mass and charge, as well as those of the substrate, such as the pore size, interconnectedness, restricted areas, and surface characteristics. Published articles have helped to clarify which of these factors may have a controlling influence on molecular transport in different situations.

Silva, D. J., Rojas, O. J., Hubbe, M. A., Park, S. W., Yamaguchi, T., and Song, J. (2009). “Polyampholytes: Their use in papermaking and their solution and adsorption behaviors,” O Papel 70(9), 40-50.

A high molecular weight random polyampholyte having both positive and negative charge in the same chain was used in this study as a paper strength additive. Changing the balance between the positive and negative groups in response to different environmental solution conditions was found to affect paper strength properties. In this research the polyampholyte solution and the adsorption behaviors were studied at a molecular level over a large pH range (4.3 up to 8.5) by using up to date techniques, e.g., quartz crystal microbalance with energy dissipation (QCM-D), atomic force microscopy (AFM) and dynamic light scattering (DLS). The results showed that the polymer structure, morphology and the net charge density of the aggregates at solid-liquid interface affected the adsorbed amount and the viscoelastic layer properties. A higher amount of adsorbed polymer mass and more viscoelastic layers were found for pH close to the isoelectric point (pHIEP 7.3). In this condition the best paper strength properties were found. Also, we found that this polyampholyte worked very well for recycled fiber application.

Silva, D. J., Rojas, O. J., Park, S. W., and Hubbe, M. A. (2009). “Evaluation of adsorbed polyampholyte layers by using quartz crystal microbalance,” 10^th Intl. Symp. Process Systems Engineering – PSE2009, de Brito Alves, R. M., do Nascimento, C. A. O, and Chalbaud Biscaia, E. C., Jr., (eds.); Computer-Aided Engineering 27, 1929-1934. DOI: 10.1016/S1570-7946(09)70712-6

A high molecular weight random polyampholyte having both positive and negative charge in the same chain was used in this study as a paper strength additive. Changing the balance between the positive and negative groups in response to different environmental solution conditions was found to affect paper strength properties. In this research the polyampholyte solution and the adsorption behaviors were studied at a molecular level over a large pH range (4.3 up to 8.5) by using up to date techniques, e.g., quartz crystal microbalance with energy dissipation (QCM-D), atomic force microscopy (AFM) and dynamic light scattering (DLS). The results showed that the polymer structure, morphology and the net charge density of the aggregates at solid-liquid interface affected the adsorbed amount and the viscoelastic layer properties. A higher amount of adsorbed polymer mass and more viscoelastic layers were found for pH close to the isoelectric point (pHIEP 7.3). In this condition the best paper strength properties were found. Also, we found that this polyampholyte worked very well for recycled fiber application.

Hubbe, M. A., Nanko, H., and McNeal, M. R. (2009). “Retention aid polymer interactions with cellulosic surfaces and suspensions: A Review,” BioResources 4(2), 850-906. DOI: 10.15376/biores.4.2.850-906

Retention aids can be defined as very-high-mass, water-soluble polymers that are added to cellulosic fiber slurries before the formation of paper in order to improve the efficiency with which fine particles, including cellulosic fines, are retained in the paper product. Optimization of retention aid performance can be a key to achieving efficient and environmentally responsible papermaking objectives. This article reviews various published theories related to retention aid use. Findings related to three main classes of retention aid polymers are considered: cationic acrylamide copolymers (cPAM), anionic acrylamide copolymers (aPAM), and polyethylene oxide (PEO). While many aspects of the interactions of each of these classes of retention aid products can be understood based on colloid chemistry principles, further research is needed in order to more fully bridge the gap between theory and practice.

Hubbe, M. A., and King, K. (2009). Cost-Saving Strategies in Papermaking Chemistry, TAPPI Press, Atlanta, 237 pp.

The goal of this book is to introduce a variety of strategies for cost savings during the manufacture of paper. Our focus will be on strategies that involve chemical additives. Chemicals cost money. It is possible to achieve savings by prudent and well optimized use of additives. The target audience for this handbook includes two groups: 1) those who will read the book on their own, and 2) those who are fortunate enough to participate in a course. As members of the papermaking community we are proud to uphold a TAPPI tradition of providing technical books. Books are a time-proven medium for dissemination of helpful information, allowing the reader to study the material at a self-selected rate, while providing the opportunity to skip directly to the subject matter of most interest and importance. Readers of this handbook should include engineers, scientists, paper mill staff, chemical company technical support representatives, students, and people from other disciplines who are interested in promoting the economic success of papermaking operations.

Lee, S. Y., and Hubbe, M. A. (2009). “Morphologies of synthetic mineral microparticles for papermaking as a function of synthetic conditions,”Colloids Surf. A 339(1-3), 118-125. DOI: 10.1016/j.colsurfa.2009.02.006

Morphological characteristics were determined for a system of Synthetic Mineral Microparticles (SMM), which have been developed to promote drainage of water and retention of fine particles during papermaking. Prior research, as well as our own preliminary research showed that the SMM system can have advantages in both of drainage and retention, compared with montmorillonite (bentonite), which is one of the most popular materials presently used in this kind of application. A partially gelled form of a silica-type microparticle additive is known to perform better than the corresponding sol form, in terms of fine-particle retention during papermaking. For this reason it was of interest to investigate the morphological behavior of SMM as a function of the conditions of synthesis. BET nitrogen adsorption was used to measure the surface area of SMM. The distribution of SMM particle size was investigated in the aqueous state, using a light-scattering technique. The coagulation behavior and morphology of SMM were analyzed using scanning electron microscopy (SEM). It was found that the structural characteristics of SMM particles could be explained in terms of the effects of ionic charges on colloidal stability of primary particles during formation of the SMM.

Hubbe, M. A., Chen, H., and Heitmann, J. A. (2009). “Permeability reduction phenomena in packed beds, fiber mats, and wet webs of paper exposed to flow of liquids and suspensions: A review,”BioResources 4(1), 405-451. DOI: 10.15376/biores.4.1.405-451

Filter media, including those prepared from cellulosic materials, often suffer from permeability loss during continued use. To help understand such issues, one can take advantage of an extensive body of related research in such fields as industrial filtration, water purification, enhanced oil recovery, chromatography, paper manufacture, and the leaching of pollutants from impoundments. Though the mechanisms that appear to govern permeability-loss phenomena depend a lot on the details of various applications, the published research has revealed a number of common features. In particular, flow through a porous bed or fiber mat can be markedly reduced by deposition of particles or colloidal matter in positions that either block or partially restrict fluid flow. Progress has been achieved in the development of mechanistic models, as well as the use of such models in numerical simulations to explain various experimental findings. Further research of this type needs to be applied to cellulosic materials, which tend to be much more elongated in comparison to the bed materials and suspended matter considered most often by most researchers active in research related to permeability loss.

Lee, S. Y., and Hubbe, M. A. (2008). “Application of synthetic mineral microparticles with various metal species,”J. Korea TAPPI 40(5), 1-10.

Hubbe, M. A., and Rojas, O. J. (2008). “Colloidal stability and aggregation of lignocellulosic materials in aqueous suspension: A review,” BioResources 3(4), 1419-1491. DOI: 10.15376/biores.3.4.1419-1491

Aqueous dispersions of lignocellulosic materials are used in such fields as papermaking, pharmaceuticals, and preparation of cellulose-based composites. The present review article considers published literature dealing with the ability of cellulosic particle dispersions (fiber, fines, nanorods, etc.) to either remain well dispersed or to agglomerate in response to changes in the composition of the supporting electrolyte solution. In many respects, the colloidal stability and coagulation of lignocellulosics can be understood in terms of well-known concepts, including effects due to osmotic pressure arising from overlapping electrostatic double layers at the charged surfaces. Details of the morphology and surface properties of lignocellulosic materials give rise to a variety of colloidal behaviors that make them unique. Adjustments in aqueous conditions, including the pH, salt ions (type and valence), polymers (charged or uncharged), and surfactants can be used to control the dispersion stability of cellulose, lignin, or wood-extractive materials to serve a variety of applications.

Hubbe, M. A. (2008). “Accurate charge-related measurements of samples from the wet end: Testing at low electrical conductivity,”Paper Technol. 49(6), 21-26.

Modest changes in testing procedures can improve the trustworthiness of charge-related measurements at the paper machine wet-end. This article makes the case for implementing such changes broadly within our industry. Substantial advantages can be achieved by reducing the salt content of samples of wet-end stock, white water, and similar samples before carrying out tests related to charge. Improved monitoring and control of charge-related quantities in the wet end has the potential to reduce chemical costs and make the paper machine run more uniformly and efficiently. This paper provides modifications to streaming current (SC) titration procedures and fiber pad streaming potential (SP) procedures. The SC method is most often used for cationic demand titration endpoint determination, both in the lab and when using online monitoring equipment. The fiber-pad SP method is most often used for estimating the zeta potential of fiber surfaces, especially for product development and for troubleshooting.

Lee, S. Y., and Hubbe, M. A. (2008). “Polyelectrolyte titrations of synthetic mineral microparticle suspensions to evaluate charge characteristics,”Colloids and Surfaces A, 331, 175-182. DOI: 10.1016/j.colsurfa.2008.08.003

Colloidal charge properties were determined for a system of Synthetic Mineral Microparticles (SMM), which have been developed to promote drainage of water and retention of fine particles during papermaking. Prior research, as well as our own preliminary research showed that the SMM system can have advantages in both of drainage and retention, compared with montmorillonite (bentonite), which is one of the most popular materials presently used in this kind of application. Streaming current titrations employing highly charged polyelectrolytes and were used to evaluate the charge properties of SMM suspensions and to understand the interactions among SMM particles, fibers, fiber fines, and cationic polyacrylamide (cPAM). Polyelectrolyte titrations were carried out under different conditions of pH to predict the charge properties of SMM under conditions that reflect paper manufacturing practices. It was found that pH variation, caused by the change of Al/Si ratio and partial neutralization of aluminum’s acidity, profoundly affected the charge properties of SMM, due to the variation of Al-ions and the influence ionizable groups on the Si-containing particle surface.

Hubbe, M. A., Heitmann, J. A., and Cole, C. A. (2008).  “Water release from fractionated stock suspensions.  2.  Effects of consistency, flocculants, shear, and order of mixing,”TAPPI J. 7(8), 14-19.

The rate of gravity drainage from a papermaking stock suspension was found to be highly dependent on the initial consistency, as well as on the presence of cellulosic fines. The results of testing were fitted to a model based on different linear contributions to drainage resistance due to the fibers and due to each type of fines. Deviations from the model at relatively high consistency were tentatively attributed to flocculation phenomena. By selectively treating either the fines, the fibers, or the combined furnish with cationic polyelectrolytes it was possible to achieve substantially higher rates of dewatering. Results were consistent with several mechanisms, which may possibly act in parallel during ordinary papermaking. Attachment of cellulosic fines to fiber surfaces can prevent those fines from migrating to choke points within a wet web. Agglomeration of fines to each other can reduce their effective surface area. Flocculation of the fibers can make the fiber mat less uniform, thus providing larger channels for water to flow from the mat.

Cole, C. A., Hubbe, M. A., and Heitmann, J. A. (2008). “Water release from fractionated stock suspensions.  1.  Effects of the amounts and types of fiber fines,”TAPPI J. 7(7), 28-32.

Tests with a gravity-based freeness device demonstrated a highly non-linear effect of cellulosic fines on resistance to dewatering. Fines at low to moderate levels had little effect on gravity dewatering, but fines slowed drainage considerably as their level increased beyond a threshold. Fines created by refining (secondary fines) slowed drainage to a much greater extent than those originally present in bleached hardwood kraft pulp (primary fines). The results were consistent with a mechanism in which unattached fines can migrate within a wet web to choke points at which they tend to block the flow of water.

Hamzeh, Y., Ekhtera, M. H., Hubbe, M. A., Izadyar, S., and Pourtahmasi, K. (2008), “Effects of process variables on poly-aluminum chloride (PAC)-rosin sizing performance under neutral papermaking conditions,”Industrial and Engineering Chemistry Research 47, 4302-4307. DOI: 10.1021/ie800107x

The effects of process variables, including poly-aluminum chloride (PAC) and rosin levels and equilibration time on PAC-rosin sizing performance, were investigated for three alternative sizing processes, include conventional, reverse sizing, and premixing under neutral papermaking conditions. The individual and interaction effects of sizing variables were determined. It was found that, in contrast to conventional sizing, the sizing efficiency increased with increasing equilibration time when reverse and premixing processes were used for the sizing treatment. In both processes, increasing equilibration time can lead to potential savings in PAC or rosin. Finally, the optimum conditions for more cost-effective sizing were determined for the three sizing processes.

Bastidas, J. C., Pawlak, J. J., Venditti, R. A., Heitmann, J. A., Hubbe, M. A., and Kadla, J. F. (2008). “A colloidal probe microscopy study of cellulose/gypsum interactions,” Materials Characterization 59(2), 144-150.  DOI: 10.1016/j.matchar.2006.12.007

Drywall, which is made primarily of a calcium sulfate dihydrate (gypsum) core with paper on both sides, is one of the most widely used construction materials. Because board failure often occurs at the gypsum core/paper interface, it has become important to know the exact nature of the gypsum/cellulose bond. This study provides data about the nature of this interaction by means of AFM and Colloidal Probe Microscopy. These methods made it possible to distinguish among the different crystal faces and their respective interactions with cellulose. Measured in air, the adhesive forces between the AFM tip and the different faces varied according to f(010) < f(120) < f(111) at 50 % relative humidity. The differences in adhesive force with the different gypsum crystals can be attributed to the differences in surface chemistry. The information obtained will help guide improvements in the crystal production process to obtain better bonding between the crystal and the paper.

Hubbe, M. A. (2008). “Minimizing the environmental impact of the papermaking process,”Proceedings of the 2^nd IPEC Conference, Tianjin, China, Book A, 37-40.

From an environmental perspective, papermakers have a positive story to tell. The products that we manufacture mainly come from renewable resources. Paper can be recycled. It’s also biodegradable. It also can be incinerated to recover its energy value. However, the process of making paper also can raise some environmental concerns. Decades ago it was common to be able to see environmental impacts of papermaking additives just by looking at a waterway downstream of a paper mill. The color of river water often was a good clue as to what color of paper was being manufactured. Such impacts have been greatly reduced, not only due to implementation of wastewater treatment operations, but also to advances in papermaking chemistry – making it possible to achieve high levels of retention efficiency. In the future we can expect there to be increasing emphasis on the overall environmental impact of each chemical additive that is used. Issues that will be considered will include the chemical’s toxicity, biodegradability, and its tendency (or lack thereof) to become retained on solid surfaces. Certain papermaking additives also have the potential to reduce the need for fibers – or to enable the papermaker to meet specifications at higher filler levels or lower basis weights. Further opportunities lie in the area of automation, making it possible to use chemical additives more efficiently. Looking toward the future, perhaps energy use will emerge as the dominant issue. Evaporation of water is the most energy-demanding part of the papermaking process. Though it is very difficult to demonstrate conclusively, some of the same chemicals that can promote faster dewatering in the forming section also appear to make it easier to squeeze water out of paper in the press section, reducing the amount of evaporation required.

Wu, N., Hubbe, M. A., Rojas, O. J., and Yamaguchi, T. (2008). “Penetration of high-charge cationic polymers into silica gel particles and cellulosic fibers,”Proceedings of the 2^nd IPEC Conference, Tianjin, China, Book B, 626-649.

The gradual penetration of positively charged polymeric additives below the outer surfaces of cellulosic fibers can play an important role in the selection of addition points and optimization of chemical dosages on a paper machine. The present work was undertaken to help understand the role of high and low-molecular-mass fractions of a cationic polyelectrolyte, in determining the charge behavior of the outer surfaces of fibers. Commercial polyelectrolyte samples typically contain a broad range of molecular masses. A dialysis procedure was used to selectively remove the low-mass fraction of poly-diallyldimethylammonium chloride (poly-DADMAC) from aqueous solutions of the polymer. Only cationic polymer samples having a very-low-mass component exhibited penetration into the interior spaces of silica gel, which has a well-defined, narrow pore size distribution. The presence of low-mass components of the polymer also affected the electrical potential associated with the outer surfaces of solids treated with high-mass poly-DADMAC. Preliminary observations showed related behavior in the case of cellulosic fibers. The wide range of time required for measured streaming potential values to decrease to zero, depending on the amount of poly-DADMAC added at time zero, was consistent with a gradual diffusion of the macromolecules below the outer surfaces of the fibers.

Hubbe, M. A., Pawlak, J. J., and Koukoulas, A. A. (2008). “Factors affecting paper’s appearance,” Proceedings of PaperCon ’08, TAPPI Press, Atlanta.

Hubbe, M. A., Rojas, O. J., Lucia, L. A., and Sain, M. (2008). “Cellulosic nanocomposites, A review,”BioResources 3(3), 929-980. DOI: 10.15376/biores.3.3.929-980

Because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties, a great deal of attention has been paid recently to cellulosic nanofibrillar structures as components in nanocomposites. A first major challenge has been to find efficient ways to liberate cellulosic fibrils from different source materials, including wood, agricultural residues, or bacterial cellulose. A second major challenge has involved the lack of compatibility of cellulosic surfaces with a variety of plastic materials. The water-swellable nature of cellulose, especially in its non-crystalline regions, also can be a concern in various composite materials. This review of recent work shows that considerable progress has been achieved in addressing these issues and that there is potential to use cellulosic nano-components in a wide range of high-tech applications.

Hubbe, M. A., Pawlak, J. J., and Koukoulas, A. A. (2008). “Paper’s appearance: A review,” BioResources 3(2), 627-665. DOI: 10.15376/biores.3.2.627-665

This review article highlights progress in understanding the optical properties of paper. Paper’s appearance can be defined in terms of its opacity, brightness, color, fluorescent properties, gloss, and various quantities related to its uniformity. The phenomena that give rise to paper’s optical properties, especially its ability to scatter and absorb visible light, are highly dependent on paper’s structure and its chemical composition. In an effort to engineer low-cost products having relative high opacity and brightness, it is necessary to optimize the material selection and processing conditions. The dimensions of solid materials and void structures within the paper are key factors for optimizing the optical properties. In addition, additives including bleaching agents, mineral particles, dyes, and fluorescent whitening agents can impact paper’s optical properties Paper’s appearance depends, in subtle ways, on the processes of its manufacture.

Hubbe, M. A., Venditti, R. A., and Rojas, O. J. (2007). “What happens to cellulosic fibers during papermaking and recycling? A review,” BioResources 2(4), 739-788. DOI: 10.15376/biores.2.4.739-788

Both reversible and irreversible changes take place as cellulosic fibers are manufactured into paper products one or more times. This review considers both physical and chemical changes. It is proposed that by understanding these changes one can make better use of cellulosic fibers at various stages of their life cycles, achieving a broad range of paper performance characteristics. Some of the changes that occur as a result of recycling are inherent to the fibers themselves. Other changes may result from the presence of various contaminants associated with the fibers as a result of manufacturing processes and uses. The former category includes an expected loss of swelling ability and decreased wet-flexibility, especially after kraft fibers are dried. The latter category includes effects of inks, de-inking agents, stickies, and additives used during previous cycles of papermaking.

Hubbe, M. A., Rojas, O. J., Lucia, L. A., and Jung, T. M. (2007). “Consequences of the nanoporosity of cellulosic fibers on their streaming potential and their interactions with cationic polyelectrolytes,”Cellulose 14(6), 655-671. DOI: 10.1007/s10570-006-9098-4

Electrokinetic tests, based on the streaming potential method, were used to elucidate interactions between cationic polyelectrolytes and cellulosic fibers and to reveal aspects of fibers’ nanoporosity. The fibrillated and nanoporous nature of bleached kraft fibers gave rise to time-dependent changes in streaming potential, following treatment of the wetted fibers with poly-diallyldimethylammonium chloride. Electrokinetic test results were consistent with an expected longer time required for higher-mass polyelectrolytes to diffuse into pore spaces, compared to lower-mass polyelectrolytes. Further evidence of the relative inability of polyelectrolyte molecules to diffuse in to the pores of cellulose was obtained by switching back and forth between high and low ionic strength conditions during repeated measurement of streaming potential, after the fibers had been treated with a moderate amount of cationic polymer. By changing the concentration of sodium sulfate it was possible to switch the sign of streaming potential repeatedly from positive to negative and back again. Such results imply that a continuous path for liquid flow exists either in a fibrillar layer or within the cell walls. The same concepts also helped to explain the dosages of high-charge cationic polymer needed to achieve maximum dewatering rates, as well as the results of retention experiments using positively and negatively charged microcrystalline cellulose particles.

Hubbe, M. A. and Panczyk, M. (2007). “Dewatering of refined, bleached hardwood kraft pulp by gravity, vacuum, and centrifugation with applied pressure.  Part 1.  Physical and ionic effects,” O Papel 68(10), 74-87.

Three types of dewatering tests were performed with refined, bleached hardwood kraft suspensions. A modified water retention value (MWRV) test was used, with pressure applied to damp plugs of fibers during their centrifugation. Refining levels and drying conditions had dramatic and consistent effects on dewatering rates by gravity (freeness tests), with the application of vacuum, and also the MWRV tests. Effects of drying were roughly equivalent to a reversal of refining effects. Though changes in the pH and salt concentration of the aqueous solution had large effects on dewatering by gravity or vacuum, these chemical conditions did not affect MWRV results to a significant extent.

Hubbe, M. A. and Panczyk, M. (2007). “Dewatering of refined, bleached hardwood kraft pulp by gravity, vacuum, and centrifugation with applied pressure.  Part 1.  Physical and ionic effects,” O Papel 68(10), 74-87.

Three types of dewatering tests were performed with refined, bleached hardwood kraft suspensions. A modified water retention value (MWRV) test was used, with pressure applied to damp plugs of fibers during their centrifugation. Refining levels and drying conditions had dramatic and consistent effects on dewatering rates by gravity (freeness tests), with the application of vacuum, and also the MWRV tests. Effects of drying were roughly equivalent to a reversal of refining effects. Though changes in the pH and salt concentration of the aqueous solution had large effects on dewatering by gravity or vacuum, these chemical conditions did not affect MWRV results to a significant extent.

Hubbe, M. A. and Panczyk, M. (2007). “Dewatering of refined, bleached hardwood kraft pulp by gravity, vacuum, and centrifugation with applied pressure.  Part 2.  Effects of wet-end additives,”O Papel 68(10), 88-100.

Several wet-end chemical additives significantly affected dewatering rates according to gravity drainage tests (freeness), the application of vacuum, and a modified water retention value (MWRV) test. Relatively large increases in dewatering rates, including reductions in MWRV, were obtained by addition of high-charge synthetic cationic polymers to the pulp suspensions. Results were consistent with charge neutralization and polyelectrolyte complexation within the fibrillated layers of the fiber surfaces. These mechanisms appear to have caused less water to be held in spaces between the fibers after they had been centrifuged in the presence of applied pressure. Successive treatment with a cationic acrylamide copolymer (cPAM), followed by colloidal silica, a so-called microparticle program, resulted in very pronounced acceleration of gravity- and vacuum-assisted dewatering. Though the microparticle system yielded reduced MWRV results under certain conditions, combinations of cPAM and colloidal silica at high levels increased the amount of water retained in compressed fiber pads after centrifugation.

Hubbe, M. A., and Heitmann, J. A. (2007). “Review of factors affecting the release of water from cellulosic fibers during paper manufacture,”BioResources 2(3), 500-533. DOI: 10.15376/biores.2.3.500-533

The ease with which water becomes released from cellulosic fiber material during the manufacturing of paper can affect both the production rate and the consumption of energy during the manufacturing process. Important theoretical contributions to dewatering phenomena have been based on flow through packed beds of uniformly distributed fibers. Such descriptions are able to explain why resistance to dewatering increases as a function of the wet surface area of fibers. More recent studies have demonstrated a critical role of fine matter. If the fines are unattached to fibers, then they tend to move freely through the fiber mat and plug channels in the paper web during the dewatering process. Dewatering also is affected by the deformability of cellulosic fibers, and by whether the fibers easily slide past each other, thereby forming a dense mat. By emphasizing the role of fine matter, colloidal forces, and conformability of cellulosic materials, one can gain a more realistic understanding of strategies that papermakers use to enhance initial drainage and vacuum-induced dewatering.

Wang, Y., Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, X., and Sezaki, T. (2007). “Charge and Dry-Strength Performance of Polyampholytes.  Part 3.  Streaming Potential Analysis,”Colloids Surf. A 301(1), 33-40. DOI: 10.1016/j.colsurfa.2006.11.052

Results reported in Part 1 of this series showed that paper strength improvements could be optimized by varying pH and the overall content of ionic groups in random terpolymers containing a fixed molar ratio of acidic and basic monomeric groups. Further treatment of kraft fiber slurries with polyaluminum chloride (PAC), after polyampholyte addition, yielded significant strength benefits. The present paper shows how these results can be explained in terms of the streaming potential of glass fibers, which were used as a model substrate. The data suggest that aluminum ions interact both with the anionic carboxyl groups of the polyampholytes and with anionic silanol groups at fiber surfaces. The streaming potential of the treated surfaces could be changed by varying the pH, the overall density of charged groups of the polyampholytes, the ratio of cationic to anionic groups on the polymer, and by post-treatment with poly-aluminum chloride.

Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, Y., Song, J., Sulić, N., and Sezaki, T. (2007). “Charge and the Dry-Strength Performance of Polyampholytes.  Part 2.  Colloidal Effects,” Colloids Surf. A 301(1), 23-32. DOI: 10.1016/j.colsurfa.2006.11.053

Polyampholytes, which are macromolecules that contain both positive and negative ionizable groups, can provide superior strength improvements for paper manufacture, compared to the addition of simple polyelectrolytes. Colloidal effects, which were measured in solution and in fiber suspensions, were consistent with observed bonding effects. The same colloidal effects were found to correlate with the effects of pH and of the density of the ionizable groups on the polyampholytes. Tests were carried out with a series of polyampholytes having a constant ratio of cationic to anionic monomeric groups and molecular mass. Their charge density varied in the ratio 1:2:4:8. The greatest strength gains were obtained at intermediate charge density and under conditions of pH favoring instability of the aqueous polymer mixtures. Colloidal phenomena were elucidated by turbidimetric tests, sediment volumes of treated fiber suspensions, flocculation tendencies of treated fiber suspensions, and zeta potentials of probe particles.

Hubbe, M. A. (2007). “Flocculation and redispersion of cellulosic fiber suspensions: A review of effects of hydrodynamic shear and polyelectrolytes,” BioResources 2(2), 296-331. DOI: 10.15376/biores.2.2.296-331

Cellulosic fibers in aqueous suspensions are subject to flocculation effects that involve two contrasting scales of dimension. The net effect of flocculation determines how uniformly fibers can become formed into a sheet during the manufacture of paper. At a macroscopic level, the highly elongated shape of typical wood-derived fibers in agitated sus-pensions can give rise to frequent inter-fiber collisions and the formation of fiber flocs. At a submicroscopic scale, surfaces of suspended materials can become joined by macromolecular bridges. Although such bridges tend to reduce paper’s uniformity, polyelectrolyte flocculants are used in most paper machine systems to achieve relatively high retention efficiencies of fine particles as paper is being formed. By adjusting the papermaking equipment, judiciously selecting points of addition of chemicals, and by managing chemical dosages, papermakers employ a variety of strategies to achieve favorable combinations of retention and uniformity. This review considers scholarly work that has been directed towards a greater understanding of the underlying mechanisms.

Hubbe, M. A. (2007). “Paper’s resistance to wetting – A review of internal sizing chemicals and their effects,”BioResources 2(1), 106-145. DOI: 10.15376/biores.2.1.106-145

This review considers research related to internal sizing agents. Such chemicals, when added as emulsions or in micellar form to slurries of cellulosic fibers before paper is made, can make the product resist water and other fluids. Significant progress has been achieved to elucidate the modes of action of alkylketene dimer (AKD), alkenylsuccinic anhydride (ASA), rosin products, and other sizing chemicals. Recent findings generally support a traditional view that efficient hydrophobation requires that the sizing chemicals are efficiently retained on fiber surfaces during the papermaking process, that they become well distributed on a molecular scale, and that they need to be chemically anchored. A variety of studies have quantified ways in which internal sizing treatments tend to be inefficient, compared to what is theoretically possible. The inefficient nature of chemical and physical processes associated with internal sizing, as well as competing reactions and some interfering or contributing factors, help to explain apparent inconsis-tencies between the results of some recent studies.

Hubbe, M. A. (2007). “Water and papermaking. 3. Measures to clean up process water,”Paper Technol. 48(3), 23-30.

As was noted in Part 2 of this series, the build-up of dissolved and finely divided materials in the process water (or “white water”) of a paper mill can hurt product quality and also the efficiency of papermaking operations.  Such problems are expected to become increasingly important, given current trends in decreased usage of fresh water, larger proportions of recycled fibers, and continuing pressure to increase both product quality and rates of production.  The article reviews various measures that have been developed to remove contaminants from white water.  The best-known strategies include membrane filtration, the use of coagulating chemicals, biological treatments under anaerobic or aerobic conditions, enzymatic treatments, use of oxidizing agents, and multiple-effect evaporation.  Whichever strategy or combination of strategies is adopted at a given millsite, it is recommended also to take measures to reduce the amount of fresh water employed, thus reducing the volumetric flows that need to be purified.  Effective washing of the incoming pulp, as well as a good retention aid system, can reduce the burden on a kidney system used for white water purification.

Hubbe, M. A. (2007). “Water and papermaking. 2. White water components,” Paper Technol.  48(2), 31-40.

The term “white water” can be defined as the aqueous solution that drains from a wet sheet of paper as it is being formed.  White water’s composition can differ significantly from that of the in-coming fresh water, which was considered in Part 1 of this series.  Soluble, as well as finely suspended components tend to build up in the white water as it is recirculated multiple times.  White water components can come from the wood, from the recycling of paper, and also from chemicals that are intentionally added during the papermaking process.  The composition of white water can affect paper’s appearance and strength, as well as the costs associated with achieving efficient fine-particle retention, sizing, and smooth operation of the paper machine.

Hubbe, M. A. (2007).  “Water and papermaking. 1. Fresh water components,”Paper Technol. 48(1), 18-24. DOI: 10.1093/yiel/18.1.209

The quality of fresh water entering a paper machine system can affect both the efficiency of the papermaking process and the quality of the resulting products.  Many of these effects can be understood in terms of concepts such as solubility products, precipitation, and chelation.  Some common situations involving papermaking chemistry are reviewed, offering insight into the effects of fresh water during papermaking.

Hubbe, M. A., Rojas, O. J., Lee, S. Y., Park, S., and Wang, Y. (2007). “Distinctive electrokinetic behavior of nanoporous silica particles treated with cationic polyelectrolyte,”Colloids and Surfaces A 292(2), 271-278. DOI: 10.1016/j.colsurfa.2006.06.034

In this study we show, for the first time, that the streaming potential of aqueous suspensions of nanoporous silica gel, after treatment with the cationic polyelectrolyte poly-diallyldimethylammonium chloride (poly-DADMAC), can depend very strongly on the concentration of background electrolyte. An increase in the electrical conductivity from 60 to 1000 µS/cm resulted in an approximately 1000-fold increase in the amount of poly-DADMAC that was required to reach an endpoint of zero streaming potential. Results were explained by two contributions to the overall electrokinetic behavior – one due to the outer surfaces and another due to the interior surfaces of nanopore spaces that were inaccessible to the polyelectrolytes. Experiments with cyclical changes in salt content revealed a high degree of reversibility; such observations help to rule out explanations based on salt-induced desorption or enhancement of pore penetration. Supplementary tests with non-porous glass fibers showed no evidence of the distinctive electrokinetic behavior observed in the case of nanoporous particles. Effects of polymer molecular mass and pH, evaluated under similar experimental conditions, agreed with well-established trends.

Hubbe, M. A. (2006). “Does production of the world’s highest-tonnage manufactured item often involve nanotechnology?”Nanotechnol. Perceptions 2(3), 263-266.

Depending on the definition that one chooses to use, it can be claimed that the paper
industry is the world’s largest practitioner of nanotechnology. Alternatively, by using certain
narrower definitions, one might claim that the technology currently in widespread use by
papermakers all fails to qualify as nanotechnology

Hubbe, M. A. (2006). “Bonding between cellulosic fibers in the absence and presence of dry-strength agents – A review,”BioResources 1(2), 281-318. DOI: 10.15376/biores.1.2.281-318

Various water-loving polyelectrolytes, including cationic starch products, are used by papermakers to promote inter-fiber bonding and increase paper’s dry-strength.  Thus, papermakers can meet customer require-ments with a lower net cost of materials, more recycled fibers, or higher mineral content.  In the absence of polymeric additives, key mechanisms governing bond development between cellulosic fibers include capillary action, three-dimensional mixing of macromolecules on facing surfaces, conformability of the materials, and hydrogen bonding.  Dry-strength additives need to adsorb efficiently onto fibers, have a water-loving character, and have a sufficiently high molecular mass.  Though it is possible to achieve significant strength gains by optimal usage of individual polymeric agents, greater strength gains can be achieved by sequential addition of oppositely charged polyelectrolytes.  Superior strength can be achieved by in-situ formation of polyelectrolyte complexes, followed by deposition of those complexes onto fiber surfaces.  Polyampholytes also hold promise as efficient dry-strength additives.  Opportunities for further increases in performance of dry-strength agents may involve fiber surface modification, self-assembled layers, and optimization of the dry film characteristics of dry-strength polymers or systems of polymers.

Wang, Y., Hubbe, M. A., Sezaki, T., Wang, X., Rojas, O. J., and Argyropoulos, D. S. (2006). “The role of polyampholyte charge density on its interactions with cellulose,”Nordic Pulp Paper Res. J. 21(5), 158-165. DOI: 10.3183/npprj-2006-21-05-p638-645

Polyampholytes offer considerable promise as dry-strength additives, but the molecular mechanism involved in their adsorption needs to be better understood.  Amphoteric terpolymers of acrylamide, itaconic acid, and N-[3-(NN’,N’-dimethylamine)propyl]acrylamide (DMAPAA) with a constant ratio of basic to acidic groups (5:4) were prepared by random polymerization.  The basic groups ranged from 2.5 to 20 mole percent in the terpolymers.  Analysis by 1H and 13C nuclear magnetic resonance revealed near-quantitative agreement with the make up stoichiometry.
Streaming potential tests showed significant effects of polyampholyte adsorption, depending on the charge density of the polyampholyte, its level of addition, the pH, and the background electrolyte.  Polyampholytes having higher density of ionizable groups yielded more positive streaming potential at low pH values and more negative streaming potentials at high pH values, compared to polyampholytes of lower charge density.  At the extremes of pH, e.g. pH=3 and pH=11, the effects of a polyampholyte on streaming potential were similar to those of single-charged polyelectrolytes having a matched degree of substitution of charged monomeric groups.  Except for the sample having the lowest density of charges, all of the polyampholyte samples showed a broad maximum in adsorbed amount vs. pH within the range of about pH=5 to pH=9, which is intermediate between the pKa values of the respective charged groups.

Hubbe, M. A., Rojas, O. J., Sulić, N., and Sezaki, T. (2007). “Unique behavior of polyampholytes as dry-strength additives,”Appita J. 60(2), 106-111.

Polyampholytes yielded superior dry strength increases following their addition to slurries of papermaking fibres.  The bi-ionic polymers achieved greater tensile strength, compared to similar polymers having ionic groups of only positive or negative charge.  Dry-strength efficiency increased with increasing charge density of the polyampholyte.  Strength results were consistent with turbidity data, showing that the polyampholytes generally became less soluble at intermediate values of pH.  In contrast to simple polyelectrolytes, the adsorbed amphoteric macromolecules imbibed significant amounts of water of hydration.  Though high levels of polyampholytes added to the furnish tended to reduce the rate of gravity dewatering, such effects tended to be lower than the drainage inhibition caused by single-charge polyelectrolytes.  The effects of polyampholytes were achieved without the adverse effects often associated with refining, e.g. decreased dewatering rates, fibre shortening, or changes in the conformability of the fibres.

Song, J., Rojas, O. J., Hubbe, M. A., Wang, Y., Argyropoulos, D. S., and Wang, X. (2006). “Amphoteric polymers in papermaking,”Proc. 2006 Ibero-American Pulp and Paper Congress, CIADICYP, Santiago, Chile, Oct. 23-27.

Lee, S. Y., Hubbe, M. A., and Saka, H. (2006). “Prospects for Biodiesel as a Byproduct of Wood Pulping – A Review,”BioResources 1(1), 150-171. DOI: 10.15376/biores.1.1.150-171

Effective utilization of byproducts can affect the profitability of kraft pulping to produce cellulosic fibers from wood. This review considers opportunities to use tall oil components, obtained from kraft pulping, as a source of raw material for biodiesel fuel, or as a source of additives for petrodiesel. Considerable progress has been achieved with respect to converting vegetable oils to diesel fuel, and some of what has been learned appears to have potential application for processing of wood-derived fatty acids and related compounds. Alkaline-catalyzed transesterification strategies, while seemingly well adapted for relatively pure vegetable oil source materials, may not be the best fit for the processing of tall oil fractions. The promising strategies to consider include acid-catalyzed esterification, enzymatic processes, hydrogenation, and the use of supercritical methanol.

Hubbe, M. A. (2006). “Sensing the electrokinetic potential of cellulosic fiber surfaces,”BioResources 1(1), 116-149. DOI: 10.15376/biores.1.1.116-149
The charged nature of a cellulosic fiber surface is expected to play major roles in such phenomena as fiber dispersion, flocculation, adhesion, and adsorption of polyelectrolytes. This review focuses on the evaluation of such charges by means of electrokinetic measurements, with emphasis on the fiber-pad streaming potential technique. Results of recent experiments suggest that a continuous network or networks of pores below the outer surface of a kraft fiber can significantly contribute to observed streaming potential data. At present it is not clear whether the main subsurface contributions to the observed electrokinetic effects come from fibrillar layers on the fiber surfaces or from systems of nanopores within the cell walls of fibers. Based on the literature it is possible to suggest two conceptual models to account for the fact that the streaming potential of polymer-treated fibers can change in sign, dependent on the concentration of salt. Additional research is needed to clarify various theoretical and practical points. There may be opportunities to make more effective use of streaming potential tests in the future by carrying out such tests at reduced salt levels.

Song, J., Wang, Y., Hubbe, M. A., Rojas, O. J., Sulić, N., and Sezaki, T. (2006). “Charge and the dry-strength performance of polyampholytes.  Part 1.  Handsheet properties and polymer solution viscosity,”J. Pulp Paper Sci.  32(3) 156-162.

Paper strength was increased by adding random terpolymers having a fixed ratio of basic to acidic monomeric groups to bleached kraft fiber slurries over a wide range of pH. Subsequent treatment of the fiber slurries with polyaluminum chloride (PAC) further increased tensile breaking length. By contrast, PAC tended to reduce the dry strength contribution of a cationic polyelectrolyte having the same mass and cationic monomer content as one of the polyampholytes . The reason for adding the PAC last to the mixture was to evaluate possible effects of fresh, highly cationic aluminum species with the polymers as well as the fiber surfaces. It is proposed that such effects are due to the influence of pH and ionic aluminum species on the molecular conformations, as well as the electrokinetic behavior of solids exposed to the polyampholytes. Results of solution viscosity tests indicated more expanded polyampholyte conformations resulting from PAC addition, especially in those cases where dry strength advantages of PAC addition were observed.

Heermann, M. L., Welter, S. R., and Hubbe, M. A. (2006). “Effects of high treatment levels in a dry-strength additive program based on deposition of polyelectrolyte complexes: How much glue is too much?”Tappi J. 5(6), 9-14.

Large increases in paper’s dry strength were achieved in this study by depositing high levels of polyelectrolyte complexes onto fibers. A previous study, utilizing glass microfibers, showed an almost linear increase in strength with increasing polymer amounts up to 10%, on a dry basis. Such results prompted questions related to the practical upper limits of addition of dry-strength chemicals. Substantial gains in tensile breaking length were achieved in the present work by sequential addition of balanced amounts of positively and negatively charged polymers to bleached kraft fibers. Strength increased strongly with increasing dosages up to 40% net polymer addition. These results contrasted with past work showing that further addition of single-polymer dry-strength additives often becomes ineffective above about 0.2 to 2% addition, in different cases. Efficient retention of the polymers onto the fibers, even up to a level of equal amounts of polymer and fiber, was evident from the basis weight of handsheets prepared from a constant amount of fiber. Practical upper limits of polymer addition for the dual-polymer system were evident from an increasing stickiness of undried paper handsheets, with increasing polymer dosage. Other problems associated with the highest polymer dosages considered included polymer deposition onto the forming fabric, reduced dewatering rates, and reduced opacity of paper produced with 40% or more dry mass of polyelectrolyte added, based on fiber mass. Important factors affecting the results included the ratio of the two kinds of polymers and the manner in which the polyelectrolyte solutions were added.

Sezaki, T., Argyropoulos, D. S., Heitmann, J. A., and Hubbe, M. A. (2006). “Colloidal effects of acrylamide polyampholytes. 2. Adsorption onto cellulosic fibers,”Colloids Surf. A 289(1-3), 89-95. DOI: 10.1016/j.colsurfa.2006.04.010

Colloidal titrations of commercial acrylamide-based terpolymers having both weak-acidic and weak-basic groups were carried out at pH 3 and 11, using a streaming current technique. At these pH values it was found that the polyampholytes could be considered as simple polyelectrolytes, though it was necessary to use a modified titration procedure. The titration endpoint defined by zero streaming current (SC) output deviated from a 1:1 stoichiometry, depending on the salt concentration. The endpoint also depended at the speed of titration, consistent with a relatively slow rate of forming equilibrated poly-ion complexes between polyampholytes and titrants. The adsorption of the amphoteric polyacrylamide copolymers onto bleached hardwood fibers was maximized near to its isoelectric pH, such that the net charge of the polymer was relatively low, but opposite to that of the substrate. Neutral and negatively charged polyampholytes also adsorbed in significant amounts onto the negatively charged fibers, though the amounts were lower than when the net charges were opposite. Addition of salt generally increased adsorption up to a conductivity value of 1000 mS/cm. Adsorption increased slightly with increasing time, until reaching a plateau. The rate of fluid agitation during adsorption did not affect adsorption significantly under the conditions employed.

Sezaki, T., Hubbe, M. A., Heitmann, J. A., Argyropoulos, D. S., and Wang, X. (2006). “Colloidal effects of acrylamide polyampholytes.  Part 1.  Electrokinetic behavior,”Colloids Surf. A 281(1-3), 74-81.  DOI: 10.1016/j.colsurfa.2006.02.019

The colloidal and electrokinetic behavior of three amphoteric acrylamide-based water-soluble terpolymers of high molecular mass was elucidated in terms of their structure and composition, using potentiometric and colloidal titrations, as well as microelectrophoresis, viscometry, and turbidity measurements. Independent variables included polymer composition, pH, and the concentration of salt ions. The electrokinetic properties, titratable charge, and isoelectric pH values of the samples were compared to their monomeric composition, as confirmed by NMR and FTIR analysis. The electrophoretic mobilities of the polyampholytes changed relatively rapidly with pH in the neighborhood of the isoelectric pH values, consistent with an enrichment of excess charges toward the outer parts of the macromolecules. Interactions of the polyampholytes with highly-charged titrants appeared to be less pH-dependent, in the neighborhood of the isoelectric condition, relative to a linear prediction based on the numbers of acidic and basic macromolecular groups. Specific viscosity measurements, in the vicinity of the isoelectric point, were found to increase with increasing salt concentration, which is a typical anti-polyelectrolyte behavior. In a similar manner, salt addition suppressed the development of a turbidity maximum at the isoelectric point.

Hubbe, M. A., Tripattharanan, T., Heitmann, J. A., and Venditti, R. A. (2006). “The ‘Positive Pulse Jar’ (PPJ): A flexible device for retention studies,”Paperi ja Puu 88(1), 39-45. 

A bench-scale apparatus, developed for studies of fine-particle retention, compares effects of contrasting flow conditions and retention chemical strategies, using simulated headbox stock.  The apparatus includes an agitated jar, a forming screen, and a continuous phase of aqueous solution below the screen.  A gear pump provides a constant time-averaged rate of dewatering, and a bellows pump produces a sinusoidal component of flow normal to the screen.  Pulsations of known velocity and displacement distance are used to represent effects due to hydrofoils.  As another alternative, the suspension may be subjected to a uniform time-averaged shear stress during dewatering.  Conditions of flow during dewatering affected not only the efficiency of fine particle retention, but also the distribution of fiber fines and fillers in the thickness direction of the fiber mat.  Retention aid use reduced filtrate turbidity, indicating increased retention efficiency.  Retention also was affected by hydrodynamic shear before dewatering; effects of such shear application were apparent even after subsequent exposure to flow pulsations during dewatering.  The design of the apparatus provides flexibility in evaluating mechanistic questions under quantified conditions of pulsating flow and/or steady shear application to the suspension before or during dewatering.

Hubbe, M. A., Rojas, O. J., and Venditti, R. A. (2006). “Control of tacky deposits on paper machines – A review,”Nordic Pulp Paper Res. J. 21(2), 154-171. DOI: 10.3183/npprj-2006-21-02-p154-171

Wood-derived pitch and tacky materials of synthetic origin in recovered fiber streams often cause serious deposit problems on papermaking equipment. Ideally such materials would be completely removed in processes such as screening, cleaning, washing, or flotation de-inking. In practice, tacky materials that remain in the fiber furnish can build up within paper machine headboxes, forming fabrics, press sections, and dryer sections, reducing production efficiency. Product quality is likely to suffer, especially if deposited material ends up in the sheet. This review considers a variety of chemical additives that papermakers have used to combat deposit problems. The premise of this article is that knowledge of the chemistry and colloidal behavior of existing deposit-control agents can guide us in the selection, usage practices, and further development of strategies for the control of tacky deposits, especially in the case of pitch, adhesive-based stickies, and wax-like deposits.

Hubbe, M. A. (2005). Emerging Technologies in Flocculation, Pira International, Leatherhead, UK, 105 pp.

The tendency of fibers used in paper manufacture to become entangled, often leading to a flocculated structure and appearance of paper products, has a huge but often overlooked economic impact. Recent work suggests that if it were practical to form perfectly uniform paper, the tensile strength might be as much as twice that of conventionally formed paper. That implies that there is untapped potential to achieve equivalent product performance with less material. Even if advances in technology can reach only part way to the ideal goal of “perfectly formed paper,” the potential savings are of the order of magnitude of tens of billions of Euros per year. In addition to materials savings, paper’s uniformity is known to have a major impact on such processes as coating, printing, and the high-speed handling of paper in automated equipment. What can today’s paper technologists do to minimize fiber flocculation and to reap benefits of more uniform paper and paperboard? This report focuses on a variety of concepts for managing flocculation. Some of these concepts are reasonably well established, and others have not yet been convincingly demonstrated. Mechanical and hydrodynamic principles underlie many of the most promising strategies. Others strategies focus on the selection and use of various wet-end chemical additives. Certain chemicals can be used in combination with hydrodynamic shear to produce more uniform paper, often at a higher speed of the paper machine. Today’s paper technologists also have something else working in their favor; various methods have become available for both online and laboratory evaluation of flocculation.The following topic areas are covered in this report:

• The definition and importance of flocculation
• Measuring the consequences of flocculation
• Advances in mechanical and hydrodynamic aspects of flocculation
• Advances in chemical aspects of flocculation
• Advances in flocculation measurements
• Emerging strategies for management of flocculation

Hubbe, M. A. (2005). “Mechanistic Aspects of Microparticle Systems,” Tappi J. 4 (11): 23-28 (2005).

Uses of specific microparticle programs and applications will be considered in the following chapters, but there seem to be some common features with respect to “how these programs work.” This section will consider mechanistic aspects of microparticle programs in general, weighing evidence for and against various concepts. In principle, an understanding of the mechanisms may make it easier to optimize and control additives flows in a microparticle system. Also there may be clues in the mechanisms that can lead to future developments.

Hubbe, M. A. (2005). “Dry-strength development by polyelectrolyte complex deposition onto non-bonding glass fibers,”J. Pulp Paper Sci. 31(4), 159-166.

Sheets formed from glass microfibers had almost zero tensile strength in the absence of polymer treatment, but paper-like dry-strength was achieved if fiber suspensions were treated with combinations of a cationic and an anionic polyelectrolyte before sheet formation. Strength increases generally were maximized when the ratio between the cationic poly-(diallyldimethylammonium chloride) and carboxymethylcellulose was within about 2:3 and 3:2 stoichiometry of charged groups on the macromolecules. Sequential addition of the polyelectrolytes to fiber slurry generally yielded higher strength than pre-mixing the polyelectrolytes before their addition. Significant effects were observed, depending on the ratio of the two polyelectrolytes, optional cationic pretreatment of the fibers, salt concentrations, and the elapsed time between mixing of polyelectrolytes and their addition to a fiber slurry. Time-dependent electrostatic interactions appear to control the deposition of polyelectrolyte complexes, and it was found that the details of chemical addition strategies can have major effects on the results.

Lofton, M. C., Moore, S. M., Hubbe, M. A., and Lee, S. Y. (2005). “Polyelectrolyte complex deposition as a mechanism of paper dry-strength development,”Tappi J. 4(9), 3-7. 

Dual-polymer treatments involving a high-charge cationic polymer followed by anionic carboxymethylcellulose (CMC) increased the strength of handsheets formed from the fiber fraction of recycled xerographic copy paper. The amount of the first additive, poly-diallyldimethylammonium chloride (poly-DADMAC), was varied, whereas the amount of CMC was held constant. Results contrasted with earlier work, in which maximum strength was obtained when the amount of poly-DADMAC was just sufficient to saturate the adsorption capacity of unbleached kraft fibers. Rather, in the case of recycled copy paper, significantly higher tensile strength was obtained when the poly-DADMAC addition exceeded the saturation level by a factor of ten. Tests were performed to evaluate a hypothesis that the strength increase was due to polyelectrolyte complex (PEC) formation in the bulk phase, followed by deposition of PECs onto the fibers. Pre-formed complexes were retained efficiently by the fibers, especially if their surfaces had been pretreated with a saturation level of poly-DADMAC. Surprisingly, such pretreatment increased the retention efficiency of all of the PEC mixtures tested, regardless of which sign of charge was in excess. The results suggested that PEC deposition yielded an additional increase of about 13% in dry strength, beyond what could be achieved by treatments not involving complexation.

Bastidas, J. C., Pawlak, J. J., Venditti, R. A., Heitmann, J. A., Hubbe, M. A., and Kadla, J. F. (2005). “Gypsum-cellulose interactions – A colloidal probe microscopy study,”Proc. 13^th Internat. Symp. Wood Fiber Pulping Chem., 59^th Appita Annual Conf., Vol. 2, New Zealand Forest Council.

Drywall, which is made primarily of a calcium sulfate dihydrate (gypsum) core with paper on both sides, is one of the most widely used construction materials. Because board failure often occurs at the gypsum core/paper interface, it has become important to know the exact nature of the gypsum/cellulose bond and how crystal morphology affects it. This study provides data about the nature of this interaction by means of AFM and Colloidal Probe Microscopy. These methods made it possible to distinguish among the different crystal faces and their respective interactions with cellulose. Measured in air, the adhesive forces between the AFM tip and the different faces varied according to f(010) < f(120) < f(111) at 50 % relative humidity. The differences in adhesive force with the different gypsum crystals face can be attributed to the differences in surface chemistry. The information obtained in this study will help guide improvements in the gypsum wallboard production process to obtain better bonding between the crystal and the paper.

Hubbe, M. A. (2005). “Paper,” in Kirk-Othmer Encyclopedia of Chemical Technology, Online Edition, Wiley, New York. DOI: 10.1002/0471238961.1601160512251405.a01.pub2

McNeal, M. R., Nanko, H., and Hubbe, M. A. (2005). “Imaging of macromolecular events occurring during the manufacture of paper,”Proc. XIII Fundamental Research Symposium, Cambridge.

A novel transmission electron microscopy (TEM) technique, developed to observe the nano-scale interactions of polymeric additives and cellulosic fibrils under idealized laboratory conditions, was applied for the first time in a comprehensive study of the colloidal interactions within a mill producing light-weight coated publication paper. The technique allows the observation of incremental changes in the nano-scale appearance of the papermaking slurry as successive additives are introduced to the system. Such changes include the coagulation of colloidal and dissolved substances present in thermomechanical (TMP) pulp after the addition of a low molecular weight, high charge density polymer, and the subsequent flocculation of the coagulated matter, hydrophobic materials, and fines following the introduction of talc, aluminum sulfate, a high mass cationic polyelectrolyte, and silica nanoparticles. The new results demonstrate that the TEM technique can be applied even in systems as complex as commercial papermaking, leading to a more accurate understanding of what happens on a macromolecular level.

Rojas, O. J., Dedinaite, A., Byrd, M. V., Hubbe, M. A., and Claesson, P. M. (2005). “On the origins of adhesion in papermaking systems,”Proc. XIII Fundamental Research Symposium, Cambridge.

Polyelectrolytes are commonly used as additives to control colloidal stability and adhesive properties of surfaces. This investigation is related to the latter case which is relevant to several papermaking processes such as pretreatment of filler particles with cationic polymers (e.g., polyethyleneimine) to increase deposition on pulp fibers or the development of dry strength. The dry strength of paper is often increased by addition of cationic starch or acrylamides to the fiber furnish, which is subsequently dried. The cationic polymer adsorbs to the negatively charged fibers and mediates an increased fiber-fiber bond. It has been reported that the dry strength of the paper increases with decreasing charge density of the polymer, presumably due to increased polymer-polymer interpenetration and due to increased viscoelastic losses that occur during the rupture of the paper sheet under strain.

Hubbe, M. A. (2005). “Why different charge demand test procedures can give different endpoints for aqueous samples from paper mills,”Proc. Pira 2005 Sci. Tech. Advan. Wet End Chemistry, Lisbon, Portugal, Pira International, Leatherhead, Surrey, UK.

Have you ever encountered situations where two people, both measuring the
same samples taken from a paper machine system, have reported sharply
different results for cationic demand tests? Such situations actually are quite
common. A lot of needless energy can be spent trying to figure out which of the
two sets of data is “correct.” Fortunately, there are some understandable
reasons to expect variations in the details of charge titration tests to yield
significantly different results. By keeping these issues in mind, it is possible to
get beyond questions of the correctness of different procedures. The end goal
should be to improve the reproducibility of the results of charge demand titrations
and to interpret the results with greater confidence.

Hubbe, M. A., and Zhang, M. (2005). “Recovered kraft fibers and wet-end dry-strength polymers,”Proc. TAPPI Practical Papermakers Conf., TAPPI Press, Atlanta.

The drying and recycling of paper profoundly affects the bulk and surface properties of kraft fibers. Recovered kraft fibers tend to be less porous on a sub-microscopic scale, less flexible, and less able to swell with water, compared to never-dried, refined fibers. Recovered fibers also tend to be less able to form inter-fiber bonds. This review considers how changes associated with the drying and recycling of kraft fibers affect their interactions with dry-strength polymers such as cationic starch, copolymers of acrylamide, and high-charge cationic polymers. A key finding of recent research is that the listed changes in fiber properties occur independently of whether or not dry-strength polymers are present. There are two main ways that strength-enhancing additives can compensate for losses in bonding ability. First, dry-strength polymers present in the original paper can contribute to bonding when the same material is recycled. Second, additional wet-end polymers applied during production of the recycled paper can help compensate for deficiencies of bonding ability. Recently a dual-polymer dry-strength program was adjusted to match the ability of recovered kraft fibers to retain cationic polymers. This approach makes it possible to tailor a treatment system to the type of furnish that is being used.

Hubbe, M. A. (2005). “Microparticle programs for drainage and retention,” in Rodriguez, J. M. (ed.), Micro and Nanoparticles in Papermaking, TAPPI Press, Atlanta, Chapter 1, 1-36.

When papermaking technologists use the word “microparticles,” usually they mean certain chemical additive programs that can promote the release of water and help retain fine particles during formation of paper. The prefix “micro” is actually somewhat of a misnomer; some of the most commonly used particles, for which this technology is named, have primary diameters in the nanometer range, 1-5 nm. Microparticles, as well as certain analogous papermaking additives that we will call “micropolymers,” are useful only when used in sequence with certain oppositely charged high-mass polymers, hence the term “microparticle programs” as used above. This present book serves as a witness to explosive growth. Before 1980 the subject of microparticle technology, as we know it today, did not exist. Papermakers were generally unaware of potential uses of such additives as colloidal silica and bentonite, except for their occasional uses in water treatment or for control of paper’s frictional properties. Now there are at least 550 paper machines that have used these two types of very finely divided minerals – in sequence with cationic starch or cationic acrylamide copolymers – to promote drainage and retention. About 300 have been reported to use colloidal silica, and about 250 have been reported to use bentonite. In addition, there are various paper machines that have used, or are currently using, related retention and drainage programs with highly cross-linked anionic polymers [12] or lignin byproducts playing the role of microparticle. The purpose of this introductory chapter is to review the already-extensive literature related to microparticle programs. This includes descriptions of the materials, how they work together mechanistically, and how the papermaker can take advantage of them to increase paper production rates or product quality.

Hubbe, M. A., and Rojas, O. J. (2005). “The paradox of papermaking,”Chem. Eng. Education 39(2), 146-155.

Students and educators in chemical engineering, are you aware of the paper industry and its impact in our society? With retirements and with changing technology there is a continual need for new technical and scientific skills to face the challenging goals of our times. The purpose of this article is to introduce some intriguing aspects of papermaking technology. The paradoxical nature of the papermaking process is sure to capture your interest and imagination.

Welf, E. S., Venditti, R. A., Hubbe, M. A., and Pawlak, J. (2005). “The effects of heating without water removal and drying on the swelling as measured by water retention value and degradation as measured by intrinsic viscosity of cellulose papermaking fibers,”Prog. Paper Recycling 14(3), 1-9.

The effects of heating without water removal and drying of bleached kraft fibers were separately investigated. Water swellability as measured using a water retention value method (WRV) and cellulose degree of polymerization as measured using a viscosity method were used to gauge the effects of such treatments on fibers. The drying of fibers at temperatures above 100°C resulted in significant decreases in WRV, as expected. However, heating fibers without water removal at the same temperatures resulted in a decrease in WRV much less than caused by drying. Drying at high temperatures reduced the cellulose viscosity only slightly, whereas heat treatment without water removal at high temperatures resulted in much greater losses in cellulose viscosity. The results of this study indicate that the time-temperature-humidity history of a fiber during papermaking and paper recycling can produce fibers with very different papermaking qualities.

Hubbe, M. A. (2005). “Acidic and alkaline sizings for printing, writing, and drawing papers,”The Book and Paper Group Annual 23, 139-151.

This review of paper sizing systems describes a recent, quiet revolution with respect to the chemicals used during the manufacture of paper. Before this revolution the primary means of imparting water-resistance to mass-produced paper involved rosin and alum, the latter of which is highly acidic. During the 1980s, and continuing up to today, there has been a dramatic shift to new sizing chemicals that employ an alkaline buffering system. As a side benefit of this change, most printing, writing, and drawing papers now made in the US tend to be brighter and more resistant to embrittlement during storage. Perhaps surprisingly, however, the roots of the revolution may have had little to do with paper’s permanence.

Hubbe, M. A., Moore, S. M., and Lee, S. Y. (2005). “Effects of charge ratios and cationic polymer nature on polyelectrolyte complex deposition onto cellulose,”Indus. Eng. Chem. Res. 44(9), 3068-3074.  DOI: 10.1021/ie048902m

Sequential addition of poly-diallyldimethylammonium chloride, which is highly cationic, followed by anionic carboxymethylcellulose, has been found to promote inter-fiber bonding during the manufacture of paper, with potential benefits to the recycling of fibers. The present results help to confirm a hypothesis that observed strength gains, in cases where the amount of the first additive exceeded the adsorption capacity of the fibers, were due to formation of polyelectrolyte complexes in the solution phase, followed by their deposition onto fiber surfaces. Complex formation and retention of complexes on fiber surfaces occurred efficiently over a wide range of polymer charge ratios, cationic polymer attributes, and other conditions, regardless of whether or not the fibers had been pre-treated to reverse their net charge.

Hubbe, M. A. (2005). Emerging Technologies in Wet End Chemistry, Pira International, Leatherhead, UK, 91 pp.2004

Do today’s papermakers have what it takes to remain competitive in the years ahead? The answer may depend on how well they make use of emerging technologies, applying recent advances in various branches of science. At the wet end of a paper machine, papermakers already use various chemical additives to enhance product end-use performance and to enhance the efficiency of the manufacturing process. This report looks at emerging strategies that can allow future papermakers to leverage their investments in papermaking equipment and to face new challenges. Such challenges include new end-use applications for paper, a need to reduce costs while maintaining quality, efforts to increase production rates, increased paper recycling, reduced fresh-water use, and increasing competition. Tomorrow’s papermakers will face a daunting task of competing not only with each other, often across international boundaries, but also with plastic, electronic media, and a variety of other technology platforms that presently we can only imagine. While this report cannot possibly anticipate all the challenges that papermakers will face in the future, emphasis is placed on the following critical areas of emerging technology:

• Nanotechnology in the wet end
• New wet-end starch technologies
• Process control developments for wet end additives
• Changes in the chemical environment of the wet end
• Efforts to enhance the performance of wet end additives
  In each of these key areas it is possible to discern some promising strategies that can be used by papermaking technologists in the coming years. The present report aims to help with decisions to apply new technologies judiciously and effectively to meet both longer-term and shorter-term goals.

Hubbe, M. A., and Wang, F. (2004). “Charge-related measurements – A reappraisal. Part 2: Fiber-pad streaming potential,”Paper Technol. 45(9), 27-34. 

Continuing a theme introduced in Part 1, the present article addresses concerns raised in 1995 about the use and interpretation of charge-related measurements. The “fiber-pad streaming potential” method is becoming increasingly popular at paper mills and in the research labs of companies involved with papermaking technology. One drawback of the method, as it is commonly practiced, involves a lack of calibration to more fundamental quantities, such as zeta potential. Nevertheless, progress has been achieved with respect to the precision of the method, especially at increased levels of salinity. Also, one can avoid many theoretical difficulties by using the method to determine endpoints of charge titrations. With these considerations in mind, fiber-pad streaming potential tests can reveal important information about the charge-related properties of fiber suspensions.

Hubbe, M. A., and Chen, J. (2004). “Charge-related measurements – A reappraisal. Part 1: Streaming current,”Paper Technol. 45(8), 17-23. 

A 1995 article in this magazine raised concerns about the use and interpretation of two kinds of measurements that are being carried out in paper mills to evaluate the electrical charges at surfaces in fiber slurries. This article relates to the streaming current method, which is widely used for endpoint detection when testing the charge demand of whitewater or filtrate samples from fiber stock. Although there is still some truth in the statement that the “streaming current detector has no established theoretical basis,” subsequent work has helped to define ranges of experimental conditions within which the test gives reliable results. Also, some specific sources of interference have become better understood.

Hubbe, M. A., Chen, J., and Heitmann, J. A. (2004). “Measurement and impact of charge: A practical guide,” Solutions! 87(11), 47-49. 

The electrical charges on the surfaces of fibers and other materials in a papermaking furnish have profound, but subtle effects on both the process and the product. Because “charges” are invisible, they are sometimes overlooked as a source of operational problems and variability. However, the balance of surface charges within a paper machine system can directly affect the performance of retention aid chemicals. Low or variable retention of fine materials during paper formation can lead to other problems. Charge also can impact such things as dewatering rates, sizing efficiency, and deposit control.

About three years ago at NC State University we began a detailed study of one of the most widely used methods for charge determination – the streaming current titration method. One of us (Chen) earned a PhD degree in the process. Details of our research results have appeared or will appear elsewhere (see, for instance Colloids and Surfaces 223: 215, 2003). During this work, and also while putting together and checking the literature review section of the thesis, we have had occasion to think about practical implications of charge measurements. Though the opinions expressed below are our own, we need to acknowledge the substantial research progress by others that helped lead us to the following general conclusions.

Chen, J., Heitmann, J. A., Chang, H.-M., Hubbe, M. A., and Venditti, R. A. (2004). “The effect of paper additives on toner agglomeration during the recycling process,”Prog. Paper Recycling 13(14), 16-23.

One of the principal contaminants in recovered office paper is toner inks. These toner inks can be removed by agglomeration, followed by conventional screening and cleaning techniques. However, it has been found that, for some types of toner, various paper chemicals can interfere with the agglomeration process. In this research two types of toner were investigated. One toner agglomerated well under most conditions, while the other performed less well and was more susceptible to adverse effects by a variety of paper chemical additives. The surface properties of the two toners were examined, and a model system was used to determine how various copy paper additives influenced agglomeration behavior. It is hypothesized that cationic polymers adsorb onto negatively charged toners and reduce their hydrophobicity, which interferes with the agglomeration process. Market pulp, cured toners, and 1-octadecanol were used as a basic model system. The effect of additives such as calcium carbonate, cationic and anionic starches, polyamine, poly-DADMAC, and polyacrylamide were investigated. It was found that the toner that agglomerated well was uncharged, while the more difficult to agglomerate toner was negatively charged. For the anionic toner, cationic hydrophylic polymers had very adverse effects on agglomeration, and it is believed that this is because these polymers are adsorbed onto the toner. It was found that the cationic polymers can be adsorbed onto the agglomerating agent, causing dispersion and failure of the agglomeration process. Several chemicals which would reduce this type of problem were investigated. They helped to confirm the mechanism and also offer possibilities of treatment of the problem in commercial systems.

Hubbe, M. A. (2004). “Filler particle shape vs. paper properties – A review,”Proc. Spring Tech. Conf., TAPPI Press, Atlanta.    

Contrasting shapes and sizes of mineral filler particles provide today’s papermaker with many options to affect paper properties. One can choose between plate-like clay products, irregular-shaped products of grinding, and a diverse assortment of filler shapes that can be achieved by mineral precipitation methods. This review considers the connection between filler morphology and such attributes as apparent density, opacity, strength, and demand for sizing chemicals. Although no one type of filler product will suit every application, published information can help the papermaker deal with a series of compromises. Plate-like particles can be effective for paper products having a high apparent density. More rounded, solid-form particles tend to minimize the demand for sizing chemicals and generally allow more rapid dewatering. Particles with internal voids general offer high light scattering ability, contributing to opacity. Though there is often an inverse relationship between light scattering and strength, it is possible to design fillers that achieve a more favorable balance between these two attributes.

Rojas, O. J., and Hubbe, M. A. (2004). “The dispersion science of papermaking,”J. Dispersion Sci. Technol. 25(6), 713-732.  DOI: 10.1081/DIS-200035485

Paper is formed from a slurry of fibers and much smaller particles that are often called “fines.” Ahead of the paper forming process the slurry is subjected to a series of steps, including treatment with polyionic species and passage through unit operations that impose shear forces on the fluid mixture. These steps alternately disperse the solids apart or re-gather them back together. The overall process is optimized to achieve a highly uniform product, while at the same time achieving high efficiency of retaining fines in the sheet and allowing water to drain relatively quickly from the wet paper as it is being formed. As we approach the 1900-year anniversary of the first detailed account of the papermaking process, it is the goal of this review to explore the scientific principles that underlie the art of papermaking, emphasizing the state of dispersion of the fibrous slurries during various procedural phases of the manufacturing process. Some concepts that arise out of the experience of papermakers have potential applications in other fields.

Chen, J., Hubbe, M. A., Heitmann, J. A., Argyropoulos, D. S., and Rojas, O. J. (2004). “Dependency of polyelectrolyte complex stoichiometry on the order of addition. 2. Aluminum chloride and poly-vinylsulfate,”Colloids Surf. A 246(1-3), 71-79.  DOI: 10.1016/j.colsurfa.2004.07.021

In the first part of this series it was shown that the stoichiometry of complexation between oppositely charged polyelectrolytes became increasingly dependent on the order of addition as the concentrations of monovalent and divalent ions were increased. This study considers the effect of aluminum ions on titrations between solutions of a strong poly-acid and a strong poly-base. In addition, the titratable charge of aluminum ion itself was also investigated. It was found that aluminum ions can interfere with the results of charge titrations, and stoichiometric relationships fail to explain the observed results. The word “interfere” implies an unpredictable effect on titration results. Several factors affect this interference. However, by controlling solution pH and degree of charge neutralization of aluminum ions, the titratable charge of the system can be estimated by streaming current titration. The results are consistent with the presence of aluminum polynuclear species within the ranges of aluminum and base addition where the highest titratable charge was reached.

Hubbe, M. A. (2004). “Pulp and paper chemical additives,” in Encyclopedia of Forest Sciences, Chapter (MS 135), Elsevier Science and Technology.  DOI: 10.1016/B0-12-145160-7/00135-6

Chemical additives can be critical to the economic viability of paper machines. Relative to fibers (see Chapters 2 through 6) other materials are present in paper and paperboard at much lower levels. However, additives allow papermakers to differentiate their products to meet the differing needs of customers. They also use additives to make their operations more efficient. This chapter considers the most widely used papermaking additives, focusing on their composition, their modes of preparation and use, and their impacts on either product attributes or process efficiency. The fact that relatively small amounts of additives can make huge differences makes this a fascinating field of work and study.

Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004). “Effect of idealized flow conditions on retention aid performance. 1. Cationic acrylamide copolymer,”Appita J. 57(5), 404-410.

The performance of a cationic acrylamide copolymer retention aid was evaluated with respect to five contrasting flow conditions applied either before or during the formation of a fibre mat. Different levels of pre-shearing of a simulated fine-paper headbox stock were applied after chemical addition, but before constant-rate dewatering. Dewatering conditions included simple filtration, flow pulsations of known amplitudes and frequencies of normal to the forming screen, and continuous stirring during dewatering either with an impeller or at a uniform time-averaged shear rate. Under all conditions of flow the retention aid reduced the turbidity of the filtrate, consistent with improved retention of fine materials. However, the effectiveness of the polymer was irreversibly decreased by high shear before dewatering. The irreversible loss was observed even when the slurry subsequently was subjected to vigorous flow pulsations during dewatering. Results were consistent with a polymer-bridging mode of retention aid action.

Tripattharanan, T., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004). “Effect of idealized flow conditions on retention aid performance. 2. Polymer bridging, charged patches, and charge neutralization,”Appita J. 57(6), 448-454. 

Retention aids can affect papermaking process efficiency and product quality. The efficiency of these polyelectrolyte treatments may be affected by conditions of flow before and during sheet formation. Four contrasting retention aid systems were compared. Hydrodynamic shear before dewatering decreased the retention efficiency of very-high-mass acrylamide copolymers, consistent with irreversible breakdown of polymeric bridges. Such shear had little effect in the case of a moderately high-mass ethyleneimine copolymer. The relative effectiveness and responses to flow conditions of different polymeric treatments were consistent with concepts of charge neutralization, charged patches, and two types of polymeric bridges. Flow velocity pulsations normal to the plane of the forming screen lowered the retention efficiency for all of the retention aid systems, though not as much as the application of uniform time-averaged shear stress to the suspension during dewatering.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004). “Refining to overcome effects of drying unbleached kraft fibers in the presence or absence of sugar,”Progress in Paper Recycling 13(2), 5-12. 

Previous work has shown that drying of chemical pulps results in lower inter-fiber bonding when the fibers are subsequently redispersed and formed into recycled handsheets. It was also found that the strength loss could be decreased if the initial drying was carried out in the presence of relatively concentrated sugar solutions. The present research was undertaken to determine whether or not the effects of drying and or sugar treatment still remain significant after the fibers are subsequently refined. Unbleached kraft fibers were optionally subjected to oven drying in the presence or absence of 10% dextrose solution and then subjected to various levels of refining in a PFI mill. Although significant differences in fiber flexibility were still apparent after 6000 revolutions of PFI refining, there was no residual effect of either drying history or the presence or absence of sugar on the strength of recycled paper subsequently formed from those fibers. While it is understood that application of additional refining energy to recycled fibers can be expected to produce additional fine material, reducing the freeness of the stock, the present results suggest that such refining can play a critical role in overcoming the adverse effects on strength when kraft fibers are dried.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004). “Effects of sugar addition before drying on the wet-flexibility of redispersed kraft fibers,”J. Pulp Paper Sci. 30(1), 29-34. 

Paper made from recycled, chemically pulped fibers typically has lower strength than paper made from virgin fibers. This was confirmed with respect to tensile and compression strength for an unbleached softwood kraft pulp. It was also confirmed that the addition of sucrose, at high concentration, to virgin pulp before drying could improve the recycled paper strength, compared to a control with no sugar added. The use of glucose was found to be slightly more effective than sucrose. Fibers treated with the sugars were found to have higher flexibility and water retention values than untreated fibers that had been subjected to the same drying conditions.

Hubbe, M. A., Venditti, R. A., Barbour, R. L., and Zhang, M. (2003). “Changes to unbleached kraft fibers due to drying and recycling,” Progress in Paper Recycling 12(3), 11-20.

Drying of unbleached kraft pulp in the laboratory revealed two main stages in its response to increasing temperature of drying. The first stage was characterized by significant decreases in water retention value, capacity to adsorb a cationic polymer, dry strength, and apparent density of handsheets formed after re-slurrying the pulp with no additional treatment. These changes, which were independent of the drying temperature, were attributed to the action of capillary forces in the closure of micro-pores in the cell wall during the initial drying. The second stage was characterized by further significant decreases in all of the same parameters when drying temperatures became as high as 150 to 175 oC. In addition, high-temperature drying also resulted in a loss of molecular mass of the cellulose, as revealed by viscosity tests. Surprisingly, neither cellulose molecular mass nor water retention was affected to a significant extent by the value of pH prior to drying, within a range of 3 to 8. The results suggest that whereas some irreversible changes in fiber properties are unavoidable during conventional papermaking practices, further losses in the bonding ability of unbleached kraft fibers can be caused by over-drying.

Chen, J., Heitmann, J. A., and Hubbe, M. A. (2003). “Dependency of polyelectrolyte complex stoichiometry on the order of addition.  1.  Effect of salt concentration during streaming current titrations with strong poly-acid and poly-base,”Colloids Surf. A 223(1-3), 215-230.  DOI: 10.1016/S0927-7757(03)00222-X

Titrations were carried out between solutions of a strong poly-acid (polyvinylsulfate, potassium salt) and a strong poly-base (poly-diallyldimethylammonium chloride) over a range of salt concentrations. Streaming current analysis of the titration endpoints appeared to show increasing deviations from 1:1 stoichiometry of complexation with increased salt. The results depended on the direction of the titration, such that a stoichiometric excess of the titrant (second additive) was required to achieve a streaming current reading of zero. These symmetrical results, depending on the order of addition, were obtained despite the fact that the plastic surfaces of the streaming current device had a slight negative charge and differing adsorption tendencies for the two kinds of polymer. A qualitative model of molecular events, based on non-equilibrium entrapment of non-complexed polymer segments was found to be inconsistent with results of tests carried out over a range of initial polymer concentration. Results were better described by a qualitative model involving formation of polyelectrolyte complexes (PECs) in solution, in which near-stoichiometric core complexes are stabilized by an excess of the second additive on their surface. Implications of the latter model were compared with the results of turbidimetric tests, aqueous contact angles on polymer-treated plastic surfaces, and microelectrophoresis of PECs. Results of this study have consequences for interpretation of polyelectrolyte titrations, as well as for industrial operations that involve the mixing of oppositely charged polyelectrolytes.

Hubbe, M. A., Jackson, T. L., and Zhang, M. (2003). “Fiber surface saturation as a strategy to optimize dual-polymer dry strength treatment,”Tappi J. 2(11), 7-12. 

The compression strength of unbleached kraft handsheets was maximized when the first component of a dual-polymer treatment was added at a level corresponding to saturation of the fiber surface. The saturation level of poly-diallyldimethylammonium chloride (poly-DADMAC), determined by streaming current analysis, also coincided with a maximum in water retention value (WRV) and a minimum in the light scattering coefficient of the paper. Idealized descriptions of the polymer interactions are proposed to explain the observations.

Hubbe, M. A. (2003). “Selecting lab tests to predict effectiveness of retention and drainage aid programs,”Paper Technol. 44(8), 20-34; Originally published in  Proc. 4th Pira Internat. Conf. Sci. Tech. Advan. Fillers & Pigments for Papermakers, Barcelona, Spain, May 20-21, 2003.

This paper compares laboratory test procedures that predict the performance of chemicals used to enhance retention or dewatering during the manufacture of paper. Key points of difference among the various laboratory methods include the presence or absence of fiber mat formation during the test, the optional application of vacuum, the presence or absence of pressure or velocity pulsations during dewatering, and the use of automation in some test procedures. A well-chosen laboratory test can provide useful information without incurring the high cost and risks associated with having to do full-scale evaluations of many different retention and drainage programs and dosage levels. However, it is important to understand the compromises inherent in different lab-scale tests to guard against premature rejection of specific chemical program options.

Hubbe, M. A. (2002). “Fines management for increased paper machine productivity,”Proc. Sci. Tech. Advan. Wet End Chemistry, Pira International, Leatherhead, UK.

Fiber fines present in papermaking furnish can have an adverse effect on the productivity
of paper machines, especially at high fines levels and in products having high basis
weight. The goals of this study are to compare the effects of different types of fine,
fibrous material on the ease of water removal from paper and also to compare different
strategies for the addition of a drainage-promoting additive. Two contrasting types of
fiber fines were prepared from a southern U.S. bleached hardwood kraft pulp. Primary
fines, consisting mainly of relatively short, “blocky” parenchyma cells, were obtained by
classifying the unrefined pulp with a 100-mesh screen and collecting the fraction that
passed through the screen. Secondary fines, consisting mainly of thin, flexible strands,
were prepared by extensively refining the fraction that had been retained by the screen;
then the refined fraction was classified again, using the same 100-mesh screen.
Consistent with work done by others, fines tended to impede dewatering in a simple
filtration test. Results of tests with a high-mass cationic polymer were consistent with the
existence of at least two important mechanisms to account for the effects of fines on
drainage. The adverse effect of primary fines on drainage could be partly overcome by
adding a flocculant in such a way that the fines became attached to fibers, preventing the
fines from moving through the fiber mat to points where they would obstruct drainage
channels. The adverse effect of secondary fines on drainage could be more effectively
overcome by treating them in such a way as to reduce their effective surface area. These
findings suggest that addition of a flocculant to white water upstream of a fan pump may
promote more effective release of water from paper machine webs in some cases.

Wang, F., and Hubbe, M. A. (2002). “Charge properties of fibers in the paper mill environment. 1. Effect of electrical conductivity,”J. Pulp Paper Science 28(10), 347-353

The electrical conductivity of water used in papermaking tends to increase over time due to water conservation efforts. This study concerns the effect of changes in conductivity on the apparent surface charge of fibers and its measurement with a fiber-pad streaming potential method. Poly-diallyldimethylammonium chloride was added to vary the observed streaming potential from its initial value to zero. The charge of bleached hardwood kraft fibers, determined by this test, increased moderately as the electrical conductivity was increased from 0.5 to 10 mS/cm. The amount of cationic polymer required to reach neutrality increased with increasing contact time and with decreasing molecular mass of the polymer. Results are consistent with the porous nature of kraft fibers and the effects of salt on the effective size of macromolecules in solution.

Hubbe, M. A., and Wang, F. (2002). “Where to add retention aid: Issues of time and shear,” TAPPI J. 1(1), 28-33; also reprinted in O Papel (Brazil). 

Papermakers continually wrestle with the question of whether to add high-mass acrylamide copolymer retention aids before or after pressure screens in the approach flow to a paper machine forming section. Early addition of a retention aid provides more opportunity for breakage of chemically induced fiber flocs, possibly leading to more uniform formation. Early addition also provides more chance for chemicals to become degraded or lost in the porosity of fiber cell walls. Later addition tends to maximize chemical efficiency in terms of first-pass retention. A new method of fiber floc evaluation was applied in the case of headbox-consistency fiber slurries to help understand what happens to chemically induced fiber flocs when they are exposed to increased time and shear. The extent of flocculation was determined by the force required to move a pair of 6-mm probes through a slurry. Supplementary tests of the streaming potential of fibers were used to help explain the separate effects of time and hydrodynamic shear on the state of flocculation.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2002). “Can recycled kraft fibers benefit from chemical addition before they are first dried?,”APPITA J., 55(2), 135-144.

Over 50 chemical treatments of never-dried pulp were compared relative to the strength of paper made subsequently after the fibers had been dried and recycled once. Treatments having the greatest beneficial effect on the compression strength of recycled unbleached kraft paper tended to be polyelectrolytes of relatively high molecular mass. Many of the most effective treatments, either alone or by sequential addition, included both cationic and anionic functional groups on the polymers. Results were consistent with the persistent nature of charged complexes formed by polyelectrolytes at fiber surfaces, and the contribution of such complexes to inter-fiber bonding, even after drying and recycling.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2001). “Effect of chemical pretreatments of never-dried pulp on the strength of recycled linerboard,”Proc. TAPPI Papermakers Conf.  

According to the American Forest and Paper Association (AF&PA) the recovery rate of corrugated boxes used in the US now exceeds 75%. In principle the recycling of boxes saves fiber resources and requires less total energy. However, further progress in old corrugated container (OCC) recycling faces a potential barrier. It is known that recycled kraft fibers have a reduced bonding ability. The approach taken in this study was to pre-treat the never-dried fibers before the first cycle of papermaking. New data have been obtained with never-dried, refined, unbleached kraft pulp. Simple drying, low-shear disintegration, and forming without further refining yielded a loss in compression strength in the range 19 to 26%, depending on the pulp batch. Pretreatment with various chemical agents was able to compensate for some of the strength loss. Two general classes of treatment agent were identified that were able to favorably affect the strength of recycled sheets. Certain low-molecular weight materials such as sucrose appeared to interfere with the mechanism of pore closure during the initial drying. In contrast, certain high-mass polyelectrolytes such as guar gum products, cationic starch, and polyelectrolyte complexes appeared to affect the adhesiveness of the fiber exteriors of the repulped fibers.

Wang, F., and Hubbe, M. A. (2001). “Development and evaluation of an automated streaming potential measurement device,”Colloids and Surfaces A 194, 221-232.  DOI: 10.1016/S0927-7757(01)00802-0

The streaming potential and other colloidal properties of aqueous suspensions of bleached kraft fibers were evaluated by a new laboratory instrument, the Streaming Potential Jar (SPJ). This device provides precise streaming potential data under moderately high electrical conductivity levels of 0.5 to 10 mS/cm. Features of the SPJ include automated operation, rapid acquisition and processing of data, continuous stirring, and applied pressures up to 276 kPa. The SPJ also provides data related to drainage rates and the turbidity of the filtrate. Test results showed a high degree of linearity of the streaming potential signals with applied pressure and very little dependence of the results on the solids levels of the fiber slurries. These results, which are consistent with the Helmholtz-Smoluchowski equation, tend to justify the level of applied pressure used in this work. Changes in streaming potential with increasing pH were consistent with expected dissociation of surface-bound carboxyl groups on the fibers. The absolute magnitudes of the streaming potential values of bleached kraft pulps were strongly affected by increasing concentrations of Na2SO4. However, a high repeatability of measurements was obtained throughout the range of conductivities considered; relative standard deviations of streaming potentials were consistently below 3%. Titrations with poly-(diallyldimethylammonium chloride) yielded curves that had shapes similar to those of parallel tests by micro-electrophoresis; however, the amount of titrant needed to reach the endpoints was about three times higher in the case of the streaming potential tests. The disagreement between the endpoints determined by the two types of test is attributed to a diffusion process of the titrant into the porous fibers.

Hubbe, M. A. (2001). “Method for determining electrokinetic properties of papermaking furnish,”U.S. Patent 6,176,974, Jan. 23.

A method and apparatus for determining electrokinetic properties of a papermaking furnish includes mixing a sample of furnish in a container with a known amount of a charged additive and then measuring the streaming potential of the resultant ionically modified furnish sample. The amount of charged additive added to the container is increased or decreased as repeated streaming potential measurements are made until a desired streaming potential of the modified furnish sample is obtained. The results, which are expressed as the amount of titrant required to achieve the desired streaming potential in a given volume of furnish, may be used by papermakers to adjust process variables and achieve optimum, stable paper quality and machine runnability.

Chen, J., Hubbe, M. A., and Heitmann, J. A. (2001). “Measurement of colloidal charge in the paper mill by streaming current,”Proc. TAPPI Papermakers Conf. 

The colloidal charge of process water within a paper mill can profoundly affect process efficiency and product quality. With increased pressure for productivity and reduced costs the need for accurate and reliable charge measurements has become more urgent. But the paper machine environment presents challenges that may limit the accuracy of such tests. Recently the streaming current method has become the most widely used means of charge analysis. This report presents new data, helping to define the range of sample types and electrical conductivity where it is possible to achieve accurate and reliable results from streaming current titrations. Within the limits of these ranges the results are consistent with a model of polyelectrolyte adsorption onto the plastic probe surfaces of the test instrument. Outside of these limits the results may be unreliable.

Hubbe, M. A. (2001). “Reversibility of polymer-induced fiber flocculation by shear. 2. Multicomponent chemical treatments,”Nordic Pulp and Paper Research Journal 16(4), 369-375.  DOI: 10.3183/npprj-2001-16-04-p369-375

Optical and viscometric tests showed essentially complete dispersal of fiber flocs when polymer-treated chemithermomechanical (CTMP) fiber slurries were exposed to intense shear in a blender. Two types of chemical systems showed evidence that flocs formed again after the shear application were stronger or larger than those in an untreated slurry. Treatments that included poly-diallyldimethylammonium chloride (DADMAC) showed a charge-dependent maximum in the optical test of flocculation that approximately corresponded to charge neutralization. By contrast, treatments involving cationic polyacrylamide (cPAM) following by nano-size anionic materials (microparticles) yielded a net increase in viscometer output after intense shear. In all cases the polyelectrolyte mechanisms holding calcium carbonate filler particles to the fibers or to each other appeared sufficient to withstand intense shear.

Hubbe, M. A. (2000). “Reversibility of polymer-induced fiber flocculation by shear. 1. Experimental methods,”Nordic Pulp and Paper Research Journal 15(5), 545-553.   DOI: 10.3183/npprj-2000-15-05-p545-553

Papermakers desire two seemingly incompatible outcomes. On the one hand, strong agglomeration of fibers and fines can help one to achieve rapid drainage and satisfactory fine-particle retention. On the other hand, papermakers also want uniform distribution of fibers in the sheet. Procedures have been developed in our lab to evaluate effect of different retention and drainage chemical programs under stressed conditions of salt content, fines content, or high levels of charged colloidal matter. In the work described here the same tests were used to compare the reversibility of agglomerative effects of some common classes of retention and drainage programs. Optical and viscometric tests showed increased flocculation following treatment with increased amounts of cationic polyacrylamide. Application of intense hydrodynamic shear caused essentially complete reversal of flocculation. By contrast, treatment with a highly charge density cationic polymer yielded a maximum in flocculation, according to the optical test, at a treatment level corresponding to the point of charge neutralization. Divergent results were obtained when comparing fine-particle retention tests to drainage tests. In general, retention results were consistent with a model in which polymer bridges, i.e. “hard flocs,” between fibers may be irreversibly broken by shear. Meanwhile, bonds formed between fibers and fine particles appeared to remain intact. In contrast, drainage results appeared to be more highly dependent on the electrokinetic properties of the furnish, i.e. factors related to “soft floc” formation.

Wang, F., Tanaka, H., Kitaoka, T., and Hubbe, M. A. (2000). “Distribution characteristics of rosin size and their effect on the internal sizing of paper,”Nordic Pulp and Paper Research Journal 15(5), 80-85. DOI: 10.3183/npprj-2000-15-05-p416-421

It was attempted to systematically elucidate three aspects of acidic and neutral rosin sizing. These were (a) the relationship between sizing efficiency and the retention behavior of rosin sizes at the wet end, (b) the effect of pulp beating and fiber fines on the size distribution and sizing properties of paper, and (c) the distribution characteristics of rosin size on pulp fiber surfaces in internal paper sizing. Pyrolysis-GC and the oxine extraction method were used to determine the retained amounts of rosin size and aluminum in the paper. Scanning electron microscopy (SEM) and SEM-EDXA (energy dispersing X-ray analyzer) were employed to evaluate the distribution of rosin size on the pulp fiber surfaces. Under neutral to alkaline conditions neutral rosin and acid rosin sizes yielded distinctly differing sizing effects. The results depended to a great degree on the chemical stability of rosin particles and the retention efficiency of each type of sizing agent under the wet-end conditions of papermaking. Pulp beating and fiber fines influenced the size distribution and sizing properties of paper. Both pulp beating and the existence of fiber fines were considered as important contributing factors leading to the observed uneven rosin size distribution. Furthermore, the rosin size was unevenly distributed on the fiber surfaces not only for freeze-dried paper, but also for cured paper, and its distribution was similar and correlated to that of aluminum. A continuous rosin size film could not be formed even after drying by heating. It is proposed that an uneven aluminum distribution on fiber surfaces can be a root cause of non-uniform sizing with rosin.

Hubbe, M. A. (2000). “Selecting and interpreting colloidal charge measurements,”Proc. Scientific and Technical Advances in Wet End Chemistry,” PIRA, Barcelona, June 19-20. 

Achieving an optimum balance between negatively and positively charged materials in a fiber slurry can be critical to the profitable operation of a paper machine. There is no consensus, however, regarding what tests to use and how to interpret the results. Papermakers are free to choose among several competing methods, including micro-electrophoresis, colloidal titrations with a color endpoint, streaming current titrations, and fiber-pad streaming potential methods. Each method has its strengths and weaknesses. It is critical to be aware of the potential weaknesses and interferences with each method before selecting it for a given papermaking application such as process control surveys of paper machine operations. An understanding of potential errors also can improve a user’s ability to draw reliable conclusions.

Olsen, W. L., Zhu, H., and Hubbe, M. A. (2000). “Method of improving pulp freeness using cellulase and pectinase enzymes,”U. S. Patent 6,066,233, May 23.

A method for enhancing the freeness of pulp made from secondary fiber is provided by adding an enzymatic mixture comprised of cellulase and pectinase enzymes to the pulp and treating under conditions to cause a reaction to produce an enzymatically treated pulp. The freeness of the enzymatically treated pulp is increased from the initial freeness of the secondary fiber pulp without a loss in brightness.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2000). “Loss of bonding strength due to drying and repulping of kraft fibers: Effects of chemical additives,”Proc. International Symposium on Environmentally Friendly and Emerging Technologies for a Sustainable Pulp and Paper Industry, Taipei, Taiwan, April 25-27.

Chemical pretreatment of never-dried kraft fibers was found to improve the strength of recycled paperboard. Lab tests with guar gum added to unbleached softwood kraft pulp (linerboard furnish) showed that the additive increased the tensile strength and resistance to compression failure. Strength values fell by 25-35% when the same fibers were reslurried and then formed into second generations of paper. However, the strength of the paper made from pre-treated fibers remained significantly higher than the recycled, untreated control. The results suggest a strategy whereby an initial additive or additives to never-dried fibers yield strength benefits that persist over at least one generation of recycled paper. Expected benefits include reduced overall costs for strength-enhancing chemicals, reduced basis weight requirements to achieve product strength requirements, and an ability to use higher levels of waste fibers.

Tests were carried out to shed light on the mechanism of strength loss and the effects of chemicals relative to strength loss when paper is dried and then recycled. The chemical effects on strength appeared to be governed by at least two significant mechanisms. For instance, it was found that pretreatment of the never-dried pulp with a very high level of sucrose yielded an improvement of the tensile strength of recycled paper, compared to untreated fibers that were dried and recycled. This observation is consistent with the known ability of sucrose to penetrate into the fine pores in the walls of kraft fibers. The sucrose-pretreated fibers retained higher levels of water retention value (WRV), indicating a greater degree of swelling when the once-dried fibers were placed back into water. These results with sucrose are consistent with a mechanism in which loss of bonding ability is related to irreversible closure of pores in the fiber cell wall. Contrasting results were obtained when the never-dried fibers were treated with underivatized guar gums. The guar increased the relative strength of both the primary and secondary sheets, but there was no effect on the water-holding ability of the fibers. It is proposed that the guar’s mechanistic role is related to its effects on relative bonded area or shear strength of bonds per unit of bonded area.

Chen, J., Hubbe, M. A., Heitmann, J. A., and Chang, H.-M. (2000). “The effect of paper additives on agglomeration during the recycling process,”Proc. International Symposium on Environmentally Friendly and Emerging Technologies for a Sustainable Pulp and Paper Industry, Taipei, Taiwan, April 25-27.

A major contaminant in mixed office waste paper is toner inks. One method for removal of these contaminants is chemically aided agglomeration followed by conventional screening and cleaning techniques. Most agglomeration agents are oil-like hydrophobic or amphipathic materials. Because of their hydrophobicity they wet the surfaces of toners and other hydrophobic materials in aqueous suspensions. Thus they have the ability to form bridges and aid toner agglomeration. One very effective agglomerating chemical was found to be 1-octadecanol. Unfortunately, for some types of toner, it has been found that various paper chemicals can interfere with the agglomeration process. This study was focused on the effect of various paper additives on agglomeration of different toners.

Hubbe, M. A. (2000). “Wetting and penetration of liquids into paper,”Encyclopedia of Materials Technologies, Elsevier, 6735-6739.  DOI: 10.1016/B0-08-043152-6/01192-X

End-use applications of paper require a wide range of properties with respect to interactions with liquids. At one extreme are tissue papers and toweling. They are designed to imbibe water quickly. At the other extreme are cup-stock and greaseproof papers that are designed to inhibit passage of liquids. It is remarkable the degree to which variations in the processes and chemical additives used in paper can achieve such diverse products. On a microscopic scale paper is porous, directional, and rough. Papermakers and specialists have used two approaches to define and measure wettability and penetration by liquids. On the one hand tests have been carried out involving smooth, flat surfaces or filaments composed of pure materials such as cellulose films or other ingredients that compose paper. On the other hand, the paper industry has developed a wide range of practical, empirical tests that predict aspects of product performance.

Hubbe, M. A. (1999). “Method and apparatus for measuring an electrical property of papermaking furnish,”U.S. Patent 5,936,151, Aug. 10.

An apparatus for determining an electrical characteristic [the endpoint of a streaming potential titration] of a papermaking furnish includes a sample chamber having first and second sections for containing a sample of the papermaking furnish. A fiber-collecting screen having a mesh sufficient to inhibit the passage of suspended fibers separates the first and second sections of the sample chamber. Furnish is urged back and forth through the screen in a plurality of cycles so that as the furnish flows in one direction through the screen, a fiber pad is formed adjacent to the screen and as furnish is move through the screen in a second direction, the fiber pad is expelled from the screen back and redispersed back into the furnish sample. A stirrer or mixer assists in removing the fiber pad from the screen and in redispersing fibers comprising the fiber pad back into the furnish sample. A furnish is urged through the screen, electrodes positioned on opposite sides of the screen produce signal outputs corresponding to an electrical characteristic of the furnish [streaming potential]. A voltmeter receives the electrode outputs and measures voltage across the screen. In one embodiment, furnish is moved through the screen by means of a variable volume chamber in fluid connection with the screen and furnish. In an alternate embodiment, fluid is urged through the screen by reciprocating the screen within the furnish sample. Colloidal charge of the furnish may be determined by moving furnish through the screen in repeated cycles as the furnish sample is titrated with a highly charged additive.

Hubbe, M. A. (1999). “Method and apparatus for measuring an electrical property of papermaking furnish,”U.S. Patent 5,936,151, Aug. 10.

An apparatus for determining an electrical characteristic [the endpoint of a streaming potential titration] of a papermaking furnish includes a sample chamber having first and second sections for containing a sample of the papermaking furnish. A fiber-collecting screen having a mesh sufficient to inhibit the passage of suspended fibers separates the first and second sections of the sample chamber. Furnish is urged back and forth through the screen in a plurality of cycles so that as the furnish flows in one direction through the screen, a fiber pad is formed adjacent to the screen and as furnish is move through the screen in a second direction, the fiber pad is expelled from the screen back and redispersed back into the furnish sample. A stirrer or mixer assists in removing the fiber pad from the screen and in redispersing fibers comprising the fiber pad back into the furnish sample. A furnish is urged through the screen, electrodes positioned on opposite sides of the screen produce signal outputs corresponding to an electrical characteristic of the furnish [streaming potential]. A voltmeter receives the electrode outputs and measures voltage across the screen. In one embodiment, furnish is moved through the screen by means of a variable volume chamber in fluid connection with the screen and furnish. In an alternate embodiment, fluid is urged through the screen by reciprocating the screen within the furnish sample. Colloidal charge of the furnish may be determined by moving furnish through the screen in repeated cycles as the furnish sample is titrated with a highly charged additive.

Hubbe, M. A., Wagle, D. G., and Ruckel, E. R. (1999). “Method for increasing the strength of a paper of paperboard product,”U.S. Patent 5,958,180, Sept. 28.

Processes for increasing the [bonding] strength of cellulosic fibers are carried out by contacting relatively dry cellulosic fibers with an agent in particulate or vapor form comprising a carboxylic acid cyclic anhydride at an elevated temperature for a time sufficient to significantly increase the bonding strength of the fibers. The treated fibers bond more readily to one another and they also hold wet and dry strength aids more strongly. Furthermore, the treatment does not significantly affect the internal chemical structure of the fibers so that paper made from the fibers exhibits overall improved dimensional stability.

Hubbe, M. A. (1999). “Difficult furnishes,”Proc. TAPPI ’99, 1353-1367.

Changes in paper furnish are making it harder to achieve targets of retention, drainage, formation, and strength. This review takes a closer look at components that tend to make furnish difficult. Prime examples are increasing filler levels, dissolved and colloidal anionic materials, high levels of fines, high conductivity, contaminants, and fibers lacking in ability to form bonds. The review also considers chemical additives and strategies which papermakers use to cope with each furnish deficiency and maintain retention, drainage, formation, and strength at desired levels. The chemical additives and strategies can work by affecting the structure, surface area, state of agglomeration, surface properties, or relative bonded area of the components of a paper sheet.

Hubbe, M. A. (1995), “Retention aids,”Proc. TAPPI Short Course on Dyes, Fillers, and Pigments, Chicago, April.

The purpose of this course module is to introduce the subject of retention aids. Students will learn about the basic functions, safe use, and chanical nature of the additives most commonly used as retention aids. Widely known theories of zeta potentail and retention aid mechanisms will be summarized. The course module also describes a popular method for evaluating retention aid effectiveness.

Lisnyansky, K., and Hubbe, M. A. (1991). “System for on-line measurement of color, opacity, and reflectance of a translucent moving web,”U.S. Patent 5,047,652, Sept. 10.

Systems for the on-line optical measurement of properties of a translucent moving web, such as paper or plastic, as it is continually produced, colored or otherwise converted. Measured properties include color, reflectance, and opacity. A backing roll has a cylindrical or approximately cylindrical surface which comprises at least one optical standard. The roll is positioned such that a circumferential portion of the roll surface contacts the back web surface where the web characteristic is to be measured, with the web curving around the circumferential portion. An optical sensing device is positioned so as to view the front web surface backed by the optical standard or standards. In several embodiments, two sets of reflectance data are collected, one with a “white” optical standard backing and the other with a “black” optical standard backing. The backing roll surface and the optical sensing device can be arranged such that the sensing device either alternately of simultaneously views portions of the front web surface backed by each of the optical standards. In other embodiments, the backing roll has a uniform optical standard surface.

Hubbe, M. A. (1988). “How do retention aids work,”Proc. TAPPI Papermakers Conf., 389-398.

This paper presents a literature review of mechanisms that have been proposed to explain the action of polymeric retention aids used in paper manufacture. The available experimental evidence can be used to support a variety of hypotheses. For instance, do retention aids work by binding the fine particles to the fibers, or by flocculating the filler into aggregates that can be filtered by the fiber mat? Does the mechanism depend on charge neutralization, or on polymeric bridges? The effects of complicating factors such as time, shear, and detrimental substances are discussed briefly.

Hubbe, M. A. (1987). “Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow. 3. Forces of adhesion,”Colloids Surf. 25(2-4), 311-324.  DOI: 10.1016/0166-6622(87)80311-6

Colloidal hydrous titania, alumina, and chromium hydroxide spheres were detached from cellulose and glass surfaces by hydrodynamic shear. Independent variables included pH, ionic strength, and the addition of strongly adsorbing ions. The shear stress required for detachment was consistent with a model based on dispersion and electrostatic forces of adhesion. The goodness of the fit of the theory to the data depended on the boundary conditions assumed for the electrostatic forces. In some cases the assumption of constant surface potential was more consistent with the data. In other cases the constant charge assumption yielded a better fit.

Hubbe, M. A. (1987). “Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow. 4. Effects of polyelectrolytes,”Colloids Surf. 25(2-4), 325-339.  DOI: 10.1016/0166-6622(87)80312-8

Pretreatment of cellulose and glass surfaces with cationic polyelectrolytes greatly increased the forces needed to detach titanium hydrous oxide spheres. The force of adhesion was as much as 30 times greater than the highest values obtained in the absence of polymers. The hydrodynamic shear stress required for detachment increased with pretreatment level, molecular mass, and decreasing cationic charge of the polymer. The results are consistent with the presence of polymeric bridging between the solids.

Hubbe, M. A. (1986). “Retention and hydrodynamic shear,”Tappi J. 69(8), 116-117.

Recent articles describe the levels of hydrodynamic shear stress in paper machines. High intensities of shear tend to pull particles of filler pigment and fiber fines from the surfaces of fibers. My purpose here is to compare the levels of shear stress relative to the ability of small particles to remain attached. The results imply that suitable synthetic retention aid systems are more than adequate for the shear stresses in the forming section of the machine. Also, smaller particles are more easily detached than larger particles, under specified conditions of treatment and shear stress.

Hubbe, M. A. (1985). “Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow. 1. Experimental system,”Colloids and Surfaces 16(3-4), 227-248.  DOI: 10.1016/0166-6622(85)80256-0

This article describes an improved technique for studying the detachment of very small particles from solids walls exposed to turbulent shear flow. The technique is useful as an assay of the strength of adhesion between solids immersed in solution. It is also useful in determining the mechanism by which detachment takes place. The experiment is designed so that the particles rest on a window in the outer annular wall of a system of coaxial cylinders. This arrangement permits more rapid counting of particles remaining on precisely the same area throughout the experiment. Results are presented for the detachment of uniform colloidal hydrous oxide spheres from cellulose and glass substrates. Independently controlled variables included the applied shear stress, the size of the particles, the composition of the aqueous solution, and the time of shearing.

Hubbe, M. A. (1985). “Detachment of colloidal hydrous oxide spheres from flat solids exposed to flow. 2. Mechanism of release,”Colloids and Surfaces 16(3-4), 249-270.  DOI: 10.1016/0166-6622(85)80257-2

Experiments show how small particles are detached from a flat window exposed to turbulent shear flow. The onset of detachment is governed by the component of hydrodynamic forces which pulls the particles in a downstream sense. An adhesive torque opposes the applied hydrodynamic torque. In the rate-determining step a released particle rolls from its initial site of attachment. Resistance to rolling is proportional to the product of the net adhesive force and a characteristic length of the region of contact. The kinetics of release indicate that the process is governed by random events. Continued shearing at the same average shear stress results in continued entrainment of particles. The data are consistent with an idealized model of fluctuations in the local hydrodynamic force within the viscous sublayer of turbulent shear flow. Brownian motion does not properly account for the effect of shear stress on the rate of detachment.

Hubbe, M. A. (1984). “Theory of detachment of colloidal particles from flat surfaces exposed to flow,”Colloids and Surfaces 12(1-2), 151-178.  DOI: 10.1016/0166-6622(84)80096-7

Theoretical models are presented for the detachment of colloidal particles from solid surfaces exposed to shear flow. The models are most relevant to cases of hard, spherical particles that are small enough to display Brownian motion. It is concluded that the component of hydrodynamic force acting parallel to a sheared wall is usually much larger than the lifting forces. Thus, in most cases, one can expect the downstream component of force to govern the critical or rate-determining step in the process of entrainment. Alternative limiting modes of incipient motion, e.g. rolling, sliding, and lifting, can be distinguished, based on the dependency of the shear stress required for detachment on the size of particles. Rate laws for detachment and the dependency of rates on the applied shear stress permit one to discriminate between processes limited by viscous flow, Brownian motion, and fluctuations in hydrodynamic forces. Finally, it is proposed that separate geometric models of sphere-wall interaction can be employed in computing long- and short-range forces.

Hubbe, M. A. (1984). “Bonding of filler to cellulose: Effects of cationic retention aids,”Proc. TAPPI 1984 Papermakers Conf., 23.

Experiments reveal that pretreatment of cellulose film with cationic polyelectrolytes greatly increases the strength of bonding between the film and model filler particles. Levels of hydrodynamic shear required for detachment of spherical colloidal oxides were affected by the dosage, density of charge, and molecular mass of various cationic agents to which the cellulose had been exposed. The most tenacious bonding was generally achieved at high dosage of cationic agents having low to moderate density of charge. Very high molecular mass was not needed in order to form strong attachments. Both branched and linear polymers were found to be effective. The results suggest strategies for improving the retention of filler on paper machines.

Hubbe, M. A. (1981). “Adhesion and detachment of biological cells in vitro,”Prog. Surface Sci. 11(2), 65-137. DOI: 10.1016/0079-6816(81)90009-5

Adhesion between biological cells and various surfaces is explained in terms of various models, including coagulation at primary or secondary minima of free energy, macromolecular bridges or matrices, and specialized structures at the surfaces of some cells. These models are used to predict the magnitudes of forces necessary to detach a cell in the limiting cases of peeling and simultaneous separation over finite areas of contact. Diverse experimental assays of cellular adhesiveness are reviewed and the forces applied to individual cells are estimated. A very wide range of forces applied to cells in different assays suggests that different mechanisms of bonding are dominant for different types of cells and surfaces under various conditions of growth and chemical environment. The peeling mode of separation is most consistent with the magnitudes of applied force used experimentally in the detachment of cells.

Hubbe, M. A. (1980). “A polarization resistance corrosivity test with a correction for resistivity,”British Corrosion J. 15(4), 193-197. DOI: 10.1179/000705980798275391

A polarization resistance test that employs three identical sample probes and compensates for the interference of solution resistivity is described. The corrosion test vessel and probes are treated as a conductivity cell for which the cell constant is found using a standard a-c bridge technique. The constant is used to calculate the contribution of ohmic resistance (IR) to the polarization resistance measurements. Data from two- and three-day exposures of mild steel and brass to salt and acid aqueous solutions yield an excellent linear correlation between corrosion rates predicted by the polarization test and measurements of sample weight loss. The procedure offers a potential increase in precision over some other polarization resistance tests, and the apparatus is fairly simple and can be made portable. The procedure is recommended for non-passive alloys and for industrial process water where the corrosion rate is expected to be in the range 0.01-1.0 amperes per square meter and the solution resistivity is below 10,000 ohm-cm.

Hubbe, M. A. (1979). “A modified reporting procedure for polyelectrolyte titrations,”Tappi 62(8), 120-121.

Titration of fibrous slurries with polyelectrolytes to determine the surface charge of the slurry solids has been offered by Halabisky as a means of paper machine wet-end control. Further experience has shown that certain modifications in reporting of the titration results lead to the improved reliability and utility of the original procedure.

Hubbe, M. A., and D. F. Bowers (1978). “Survey of white water corrosivity in 30 north European paper mills,”Paper Trade J. 1978: 53-56 (Nov. 1).

Reuse of paper machine white water has become an almost universal practice within the industry. Energy costs and environmental restraints on effluent discharge have made some form of this practice mandatory. While the closure of white water systems is becoming a worldwide activity, we describe trends in a recent survey of 30 North European papermaking operations.