Review Articles

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.

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.

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

The pulp and paper industry is highly energy-intensive. In mills that use chemical pulping, roughly half of the higher heating value of the cellulosic material used to manufacture the product typically is incinerated to generate steam and electricity that is needed to run the processes. Additional energy, much of it non-renewable, needs to be purchased. This review considers publications describing steps that pulp and paper facilities can take to operate more efficiently. Savings can be achieved, for instance, by minimizing unnecessary losses in exergy, which can be defined as the energy content relative to a standard ambient condition. Throughout the long series of unit operations comprising the conversion of wood material to sheets of paper, there are large opportunities to more closely approach a hypothetical ideal performance by following established best-practices.

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: 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., 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.

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.

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.

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.

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.

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.

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.

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 pKa 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., 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.

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.

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 systems 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.

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.

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.

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.

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.

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.

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.

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.

Hubbe, M. A. (2017). “Hybrid filler (cellulose/noncellulose) reinforced nanocomposites,” in: Handbook of Nanocellulose and Cellulose Nanocomposites, Vol. 1, H. Kargarzadeh, I. Ahmad, S. Thomas, and A. Dufresne (eds.), Wiley-VCH, Ch. 8, pp. 273-299. DOI: 10.1002/9783527689972.ch8

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 books 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 water-based dispersions or emulsions, depending on the end-uses.

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.

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.

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., 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., 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. (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). “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.

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.

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.

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 is also 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.

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.

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.

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., 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.5.3.1955-202

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 Brønsted-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., 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.

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.

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., 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.

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.

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., 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 is 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 hydrodynamic surface area of fibers. More recent studies have demonstrated a critical role of finely divided 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.

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 suspensions 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 contain hydrophobic groups, that they 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 inconsistencies 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:
• reduced usage of fresh water
• larger proportions of recycled fibres
• continuing pressure to increase both product quality and rates of production.
This 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 mill, 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.

White water can contain many soluble and finely divided components, some of
which have the potential to detract from the quality of the paper or add to operating costs. For example, white water composition can sometimes be responsible for:
• an unexpected decrease in the brightness of paper.
• problems related to the water-repellent properties of paper.
• unexpected changes in a paper’s strength.
• slime holes and spots in paper.
• the rate of felt-filling and related deposits.
• the efficiency of retention aids.
White water components can come from:
• Pulpwood; the mechanical and chemical pulping processes and fines.
• Recovered paper; recycling can introduce a wide range of materials into white water systems – such as the latex binders and various dispersants from recycled coated grades.
• The chemicals that are added during the papermaking process, eg: the
fillers in printing grades; wet end additives and sizing agents.

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 PM system can affect both process efficiency and paper quality, contributing to, or being responsible for, problems such as
• low brightness, reduced strength and sizing difficulties.
• fabric abrasion, scale and deposit formation, clogging of spray nozzles, and corrosion.
Many of these effects can be understood in terms of concepts such as solubility
products, precipitation, and chelation. The feature presents common situations from paper mills – problems and counter-strategies – and offers insights into the
effects of fresh water components such as:
• Mineral-based suspended solids, i.e. abrasive materials and the coloured
materials and clays, which may affect optical properties.
• Dissolved inorganic solids such as hardness ions, including Ca2+ and Mg2+; “salts,” and quantities related to pH; coloured ions – especially the chelated forms of iron, manganese, and copper ions.

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 hydrophilic 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 hydrophilic character, and have a sufficiently high molecular mass. Though it is possible to achieve significant strength gains by optimal usage of individual polyelectrolytes, greater strength gains can be achieved by sequential addition of oppositely charged polyelectrolytes. Superior strength can be achieved by in-situ formation of polyelectrolyte com-plexes, 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.

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 trans esterification 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.

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). “Mechanistic aspects of microparticle systems,” Tappi J. 4(11), 23-28.

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.

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). “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.

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. (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.

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.

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.

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.

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. (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.