Troubleshooting guide for paper chemistry

Air that is present in papermaking stock or white water can cause various kinds of problems, including pin-holes, circular marks on the paper, reduced dewatering rates, unsightly foam, deposit problems related to foam buildup, and loss of solid material due to overflow of foam from tanks and chests.

The three main classes of air that are of concern to papermakers are dissolved air, entrained air, and large bubbles. Dissolved air behaves as part of the water phase, except that it can come out of solution as tiny bubbles (entrained air) if the pressure is reduced. Entrained air consists of bubbles that are small enough (say less than 1 mm) to move along with the fibers. Large bubbles have sufficient buoyancy to rise to the surface; however they still can be a problem if the bubbles are persistent and build up as visible foam or froth.

Though our main concern in this website is in regards to chemical factors, it is important to consider mechanical factors that can be contributing to air problems. Leaky seals on pumps are a leading cause of unnecessary air entrainment. Also it is important to realize that the paper machine environment contains all of the key elements needed to create foam – water, air, lots of agitation and splashing, and various substances that can act as stabilizers. On a basic, traditional paper machine the main mechanism of air release is the coalescence, creaming, and breakage of bubbles in the white water tray and white water silo areas, just after the paper has been formed. Often the breakage of surface foam is promoted by spray showers. It is important also to make sure that the showers within air-padded headboxes are working effectively. Many modern paper machines, especially those producing fine and specialty paper grades, contain deaerating equipment. These involve application of vacuum and some type of fluid motion to separate dissolved and entrained air from the water phase.

The most common short-term solution to air problems is to add a chemical defoamer product. A wide variety of chemical formulations has been found to be effective to promote coalescence of air bubbles within papermaking stock and white water or to break bubbles at the surface of water. Some common ingredients of these formulations include water-insoluble surfactants, oils, water, and hydrophobic particles. An essential feature is that the liquid phase has to have a low viscosity and a tendency to spread rapidly on bubble surfaces. Different defoamer products may be needed in different paper machine environments. In particular, many defoamers have an optimum temperature range. Over-use of defoamers should be avoided due to cost and in order to minimize deposit problems. Defoamers often are added in dilute solution either at a fan pump, just before hydrocyclone cleaners, and as surface sprays.

A long-term answer to air problems in a paper mill also should include consideration of chemical factors that tend to stabilize foams. Sometimes these factors can be minimized. Surface active materials tend to stabilize foams by lowering the interfacial tension between water and air. Likely sources of surface active materials include deinking agents, black-liquor carryover (in unbleached kraft operations), excessive rosin soap size or saponified rosin acid emulsion size, and in the formulations of various wet-end additives, including some biocides. Water-soluble, high-molecular-mass polymers such as cationic starch and wet-strength resins can be expected to stabilize foam bubbles if the additives are used at levels beyond what can be efficiently retained on the fiber surfaces. Alternative solutions include either reducing the amounts of such additives or carefully managing the balance of positively and negatively charged additives to improve the retention of all of the polymeric materials on fiber surfaces.

Some other measures that can be considered to reduce various possible root causes of foam include repair of pump seals, repair and adjustment of pulp-washing equipment, and use of foam-control agents and channel-blocking polymers to improve displacement efficiency during pulp washing operations. Bubbles coming out of the headbox sometimes can be reduced by decreasing the level of turbulence in the headbox, adjusting the impingement of the jet to minimize splashing, and adjusting hydrofoils to prevent excessive action on a Fourdrinier table.

Increased problems with dissolved and entrained air (in addition to some deposit problems) usually can be expected if furnish that contains calcium carbonate filler is exposed to acidic conditions below a pH value of about 6.5. Just as in the case of ant-acid tablets, the reaction between calcium carbonate and acid leads to the release of carbon dioxide gas. The high solubility of CO2 means that a high proportion will tend to remain dissolved in the water. Some of this elevated level of dissolved air will tend to come out of solution as entrained air when the jet of furnish emerges from the headbox, greatly lowering the pressure. The tiny bubbles can act just like fiber fines in impeding dewatering from the stock.

References

Avery-Edwards, D. J., Elms, R., and Buckingham, A., “Silicone Antifoams for Nonwoven Applications,” Tappi J. 77 (8): 35 (1994).

Helle, T. K., Meinander, P. O., Nykanen, R. J., Molander, K. S., and Paulapuro, H. V., “Air Removal Mill Trials Using Pomp Deaerator,” TAPPI J. 82 (6): 146 (1999).

Lorz, R. H., “Air Content, Retention, and Drainage: Important Parameters in Paper/Board Production,” Pulp Paper Can. 88 (10): T361 (1987).

Matula, J., and Kukkamaki, E., “How to Deal with Difficult Passengers,” Pulp Paper Europe 1 (10): 121 (1997).

Matula, J. P., and Kukkamaki, E., “New Findings of Entrained Air and Dissolved Gases in PM Wet End: Mill Case Study,” TAPPI J 83 (4) no page (2000).

May, O. W., and Buckman, S. J., “Practical Effects of Air in Papermaking,” Tappi 58 (2): 90 (1975).

Rauch, R., and Sangl, R., “Latest Findings on Entrained Air and Dissolved Gases in Pulp Suspensions,” Proc. TAPPI 2000 Papermakers Conf., 159 (2000).

Wortley, B., “Choosing the Right Weapon in the War on Foam,” PIMA Mag. 71 (3): 36 (1989).

Apparent Density (High, Low , Variable)

HIGH apparent density of paper (basis weight divided by caliper) is a common problem, especially in cases where paper is heavily calendered. Another way to describe this kind of problem is “low caliper at a given basis weight and smoothness.” In a typical paper machine operation there is likely to be a close relationship between apparent density and smoothness. Some common causes of high apparent density (at a given smoothness) include (a) over-REFINING of the fibers, (b) fibers that are naturally flexible and conformable, and (c) high filler levels, especially in the case of clay filler. Some strategies for dealing with high apparent density have little to do with wet-end chemistry. For instance, installation of soft calendering equipment [see references] may help to preserve caliper while meeting a certain smoothness target. Another strategy is to add a percentage of fiber having a bulky nature such as chemithermomechanical pulp (CTMP) [see Moberg 1986]. The scalenohedral or “rosette” form of precipitated calcium carbonate is inherently bulky, so one can expect a lower apparent density when rosette PCC is used in place of either clay or ground calcium carbonate filler [Bown 1996]. However, even in the case or rosette PCC, increasing filler levels above about 10% are likely to increase the apparent density at a given smoothness level.

In cases where papermakers choose to back off on refining or wet-press nip loading to avoid densifying the paper, it is very likely that strength properties such as tensile strength and internal bond strength will suffer. Part of the solution, then, may be to use a more effective program of dry-strength agents [Howard, Jowsay, 1987; Linke, 1968; Robinson 1980].

Some practical things to look for if the apparent density of paper is unexpectedly high include (a) basis weight and moisture of the paper, (b) whether the smoothness is correct, and (c) whether it is feasible to decrease the wet-press loading or calendering nip pressure.

LOW apparent density is less often a problem for papermakers. Low apparent density may be caused by stiff fibers, insufficient refining, or insufficient calendering. Paper can be made denser by reversing many of the suggestions just listed. For instance, one might increase the degree of refining, use kraft fibers in place of mechanical fibers, or use clay filler in place of other fillers. One also can increase the wet-press load, increase the calendering load, or increase the moisture going into the calender stack. Refining will tend to increase the bond strength within the paper, and a high level of clay will tend to weaken the paper, so the papermaker has some flexibility in achieving the desired paper property targets.

VARIABLE apparent density or caliper of paper can have various causes, often associated with machine-directional streaks. Possible root causes can include a non-uniform jet from the slice adjustments or headbox flow problems, wake effects on a Fourdrinier table, partial filling of press felts or forming fabrics with pitch-like materials, uneven loading of wet-press nips, uneven drying profiles, and crown problems or localized heating of calender rolls. Some wet-end chemical factors to watch out for include variations in filler content, freeness variations, and drainage rate variations, possibly caused by changes in cationic demand of the system.

References:

Bown, R., “Physical and Chemical Aspects of the Use of Fillers in Paper,” in Roberts, J. C., Ed., Paper Chemistry, 2nd Ed., Blackie Academic and Professional, London, 1996, Ch. 11, p. 194. [Fillers versus apparent density]

Howard, R. C., and Jowsey, C. J., “The Effect of Cationic Starch on the Tensile Strength of Paper,” Proc. 1987 Paper Physics Conf., 217 (1987).

Linke, W. F., “Retention and Bonding of Synthetic Dry Strength Resins,” Tappi 51 (11): 59A (1968).

Moberg, K., “Wet End Chemicals for Reducing Board Density,” Southern Pulp Paper 49 (4): 9 (1986). [CTMP fiber for increased bulk]

Robinson, J. V., “Fiber Bonding,” in J. P. Casey, Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. II, Wiley-Interscience, New York, 1980, Ch. 7, p. 915.

Smook, G. A., Handbook for Pulp and Paper Technologists, Angus Wilde Pub., Vancouver, 1992, ISBN 0-9694628-1-6. [Calendering practices]

Vreeland, J. H., Ellis, E. R., and Jewett, K. B., “Substrata Thermal Molding. Part 1. A Breakthrough in the Understanding and Practice of the Hot Calendering of Paper,” Tappi J. 72 (11): 139 (1989); and “Part 2. Putting Theory into Practice,” Tappi J. 72 (12): 201 (1989)

Changes in the basis weight of paper over time can be due to problems with the process control system, with consistency control, with the stock valve, or changes related to wet-end chemicals. Among the mechanical factors to consider are (a) a variable level of stock in the machine chest, especially if there is a run-out at the end of the production of a particular grade that requires a certain furnish or dye addition to the thick stock, (b) a malfunction of a dilution valve in the consistency control area, and (c) variations in the vacuum applied at the flat boxes or couch roll. Many more possibility are listed in the CPPA manual titled “Operating Difficulties on Fine, Kraft, and Specialty Paper Machines.” Since many of the possible causes of basis weight variations lie in the thick-stock area of the machine, and since most modern paper machines rely on basis weight measurements at the reel, there is a process control delay, and there is a danger of overshooting if the gain constants are set too high or if the stock valve adjustments are too coarse.

A variable efficiency of the retention aid program may cause changes in basis weight, though these will tend to be short-term if the control system is able to correct for them. Things to check include (a) the concentration, viscosity, and flow rate of retention aids, (b) an variations in pressure of dilution water that carries retention aid to the paper machine, and (c) the possibility that variations in cationic demand of the system are sufficient to affect the efficiency of retention aids. Variations due to retention efficiency can be minimized by such practices as aiming for a higher level of first-pass retention (reducing the fines content of the white water), online control of white-water solids, and online control of cationic demand.

References

Beck, M. W., “The Importance of Paper Machine Process Control and Wet End Stability,” Proc. TAPPI 1997 Engineering Papermakers Conf., 593 (1997).

Gess, J. M., and Wilson, P. H., Troubleshooting the Papermaking Process, TAPPI Press, Atlanta, 2001, ISBN 1-930657-58-7, TP R298, www.tappi.org.

Rantala, T., Nokelainen, J., and Artama, M., “Wet End Management by Controlling Consistencies and Charge,” TAPPI 99 Proc., 1189 (1999).

Breaks of the paper web (Dry-end, Size press, Wet-end)

BREAKS of the paper web can have various causes, many of which have little to do with wet-end additives. For instance, the drives of the paper machine may be unstable or poorly adjusted. Some detective work may be needed to determine why the paper web is breaking. The location at which the web is breaking may be your first clue:

DRY-END BREAKS can result from (a) weak points or holes in the paper, (b) insufficient ability to stretch, relative to the draw applied to the paper, (c) air-handling and fluttering issues, or (d) adhesion of the paper web to tacky surfaces.

Weak points may result from variations in retention or drainage. The ability of the paper web to stretch to some degree without breaking is expected to be a function of the paper’s dryness [Seth et al. 1982], so local over-drying may produce a less stretchable (“brittle”) area of the paper web; this may happen, for instance, if there is a momentarily low basis weight. Alternatively, a sudden increase in the amount of fines retained in paper is likely to weaken the paper and slow the rate of dewatering. A wetter web may have insufficient tensile strength to make it through parts of the paper machine system.

Holes may be the result of biological slime becoming incorporated into the paper. Slime can be controlled by a biocide program. The paper machine system is especially vulnerable to slime in the headbox area. Since the headbox comes after the pressure screens, any slime that gets entrained into the flow is likely to end up in the paper. Biocide treatments often involve a combination of an oxidizing agent such as chlorine dioxide and one or more antibacterial agent [Edwards, 1996; Hoekstra, 1991].

Excessive adhesion of the web to dryer can surfaces sometimes results from deposition of tacky substances [Douek et al. 1997]. Tacky substances may become transferred from the paper surface and build up on dryer cans. Subsequently, the paper may adhere excessively to the dryer can surface and become torn. Papermakers often address this type of problem with the installation of doctor blades to minimize build-up of anything onto the can surfaces. Wet-end chemical approaches include the use of talc [Allen et al. 1993] and other detackifying agents. A more effective use of retention aids also is likely to keep tacky materials better incorporated into the paper so that it doesn’t deposit as much onto dryer cans.

SIZE-PRESS BREAKS can result from holes or weak areas in the sheet, but they also are often related to internal sizing issues. The problem can be especially vexing on paper machines that have traditional “pond”-type size presses (not blade-metered, rod-metered, or gate-roll film-presses). Application of starch to the surface of paper has the potential to temporarily weaken the paper, especially if there is insufficient hydrophobic nature imparted by internal sizing agents. Click on the appropriate links to get information about optimization of internal sizing with rosin, AKD, or ASA wet-end sizes. Rosin sizing problems may be related to dosage of either the rosin or an aluminum compound, pH, or addition points. AKD sizing problems can be due to insufficient dosage, poor retention, or inadequate drying. Since AKD is relatively slow-curing, any wet spots (perhaps due to drops of condensate landing on the paper web) are likely to result in unsized areas. Likewise, streaks of higher moisture may fail to become well sized. Sometimes AKD sizing issues can be minimized by over-drying the paper before the size press, though such practices tend to embrittled the resulting paper. ASA sizing is less susceptible to cure problems because of higher reactivity. However, poor sizing efficiency can result if there is significant decomposition of the size before the paper is dried. Premature decomposition of ASA can be minimized by using the emulsion immediately after its preparation, acidifying the emulsion by adding alum or adipic acid to the starch solution to lower the pH, adding the ASA after the hydrocyclone cleaners, and maintaining high first-pass retention.

WET-END BREAKS

Breaks of the wet web of paper after the couch roll or in the wet-press section can have a variety of causes. Many causes are likely to be related to wet-web mechanical properties such as wet-web tensile strength and stretch. Excellent analyses of wet-web strength issues have been published [see references that follow]. In brief, it has been shown that the ability of a wet web to resist breakage is a function of both tensile strength and stretch, and that both of these variables are affected by moisture content. That means that any variations in moisture content after the forming section are likely to cause large variations in wet-web strength properties. Since it is impractical to adjust draws to compensate for very rapid changes in stretch of the wet-web, large variations easily can result in a web break.

A common way to increase wet-web tensile strength is to increase the softwood content of the furnish. For example, some papermakers adopt a practice of increasing softwood and/or reducing filler content when they thread up a fussy paper machine after a break. For sake of completeness, it might also be mentioned that wet-web tensile strength and toughness are favored by high surface tension of the white water. Surfactants and other materials that tend to lubricate the contacts between fibers tend to weaken the wet web.

Other factors that may cause wet-end breaks are closely related to those mentioned in the case of dry-end breaks. Deposition of tacky materials onto press felts and transfer rolls in the wet-press section can result in excessive adhesion of the paper in these areas.

References:

Allen, L. H., Cavanagh, W. A., Holton, J. E., and Williams, G. R., “New Understanding of Talc Addition May Help Improve Control of Pitch,” Pulp Paper 67 (13): 89 (1993).

Braitberg, L. D., “Controlling Pitch Accumulations in Paper Mill Systems,” Tappi 49 (11): 18A (1966).

Douek, M., Guo, X-Y., and Ing, J., “An Overview of the Chemical Nature of Deposits/Stickies in Mills Using Recycled Fiber,” Proc. TAPPI 1997 Recycling Symp., 313 (1997).

Edwards, J. C., “Biocides – Bug Killers that Enhance Pulpmaking and Papermaking Processes,” TAPPI J. 79 (7): 71 (1996).

Hoekstra, P. M., “Fundamentals of Slime Control,” TAPPI 1991 Chemical Processing Aids Short Course Notes, 55 (1991).

Pikulik, I. I., “Wet-Web Properties and their Effect on Picking and Machine Runnability,” Pulp Paper Canada 98 (1): 161 (1997).

Seth, R. S., “The Effect of Fiber Length and Coarseness on the Tensile Strength of Wet Webs: A Statistical Geometry Explanation,” Tappi J. 78 (3): 99 (1995).

Seth, R. S., Barbe, M. C., Willimans, J. C. R., and Page, D. H., “The Strength of Wet Webs: a New Approach,” Tappi 65 (3): 135 (1982).

LOW BRIGHTNESS may be due to inadequate bleaching, the presence of certain metal ions such as iron or manganese, problems with the size-press application, and various other factors. The recommended strategies for increasing brightness depend on the paper grade, type of furnish, and circumstances under which the condition was discovered. If the low brightness represents a change, then it makes sense to fix whatever went wrong to cause the lower brightness.

One of the first things to check is the brightness of the main fiber constituent in the furnish. There may be deficiencies in bleaching. Also, test the brightness of the broke; it is possible that the level of broke in the paper happens to be high and that the broke has a lower brightness than the supplied pulp. Though it is possible to use chelating agents or polyphosphates to counteract the effects of transition metal ions such as iron and manganese, a better approach usually is to try to remove such ions by treating the water before it is brought into the paper mill. Transition metals also can interfere with peroxide bleaching; in that case it is usual to treat the furnish with silicate to tie up the metal ions. In the case of recycled office waste paper, low brightness sometimes can be associated with inadequate removal of inks. In the short term such problems may be attacked with higher levels of deinking chemicals – though such chemicals are then likely to be transferred to a greater extent to the paper machine, where they may interfere with sizing. In principle it is possible to increase brightness by using bright filler that scatters light effectively. Precipitated calcium carbonate is often a good choice. For papers that must to manufactured under acidic conditions, one can use calcined clay, other high-brightness clay products, or even aluminum trihydrate (ATH). Titanium dioxide is very effective, but usually it is not cost-justified if the only objective is to increase brightness. Some other possible sources of low brightness include (a) humic acids or transition metal compounds in the incoming water, (b) a failure of the filler addition system, (c) excessive addition of dyes, and (d) biological decay. In the case of high-yield and unbleached kraft pulps the brightness can be expected to decrease significantly with increasing pH, especially above the neutral point. The effect of pH has been attributed to changes in the light-absorbing characteristics of lignin and its byproducts in the furnish. This effect may be compensated by bleaching with peroxide or hydrosulfite (dithionite) where this makes sense.

VARIABLE BRIGHTNESS is likely to be a much more serious problem than a persistent condition of either high or low brightness. Some likely causes of variability in brightness include variations in first-pass retention, variations in broke content, variation in contaminate levels, variations in pulp yield, or variations in bleaching. It is recommended to look for any periodicity in the brightness variations. Sometimes brightness variations can follow the batch-wise preparation cycles of a starch (possibly subject to biological decay), retention aid (possibly related to swings in first-pass retention), or biocide (often added intermittently and also affecting retention efficiency in certain cases). If the problem is new, then one needs to ask about recent changes in the process or the types and qualities of the materials or fresh water. Efforts to control retention to a steady value, adjustment in biocide type or addition cycles, and attention to dye additives often will resolve problems with brightness variations.

BRIGHTNESS TOO HIGH is seldom considered a serious problem by papermakers, especially if the condition is steady. High brightness easily can be “solved” by addition of a small amount of inexpensive black dye. Alternatively, papermakers can reduce bleaching chemical concentrations or select less expensive fiber or filler types. Lower-brightness clays are typically less expensive that high-brightness clays.

References:

Bristow, J. A., “ISO Brightness – A More Complete Definition,” TAPPI J. 82 (10): 54 (1999).

Scott, W. E., Abbott, J. C., and Trosset, S., Properties of Paper. Introduction, TAPPI Press, Atlanta, 1995.

Color (Off-shade, Metamerism, Variable)

OFF-SHADE PAPER

Color may be the first thing that a buyer or user of paper products notices when looking at a sample of your product. The human eye is very sensitive to small differences in color, especially when comparing two pieces of paper placed side by side. As a consequence, complaints often result when printers attempt to compile books or magazines from paper that comes from different batches if the color is not precisely controlled.

In modern papermaking operations it is most common to continuously add three dyes and to control the product color by adjusting the flows. The amounts of blue, yellow, and red dyes are adjusted periodically to keep the paper within specified ranges for color. Though color control can be greatly enhanced by online monitoring, it is always important to calibrate the online equipment with offline tests. Online color test results can be affected by changes in opacity and by the fact that the hues of certain dyes are changed by the hot temperatures of the paper at the dry end where such measurements are made.

Though many color problems require little more than an adjustment in the dosages of the dyes, often it is better to address the root causes of color variation. For example, certain biocide treatments can gradually destroy dye molecules; this can be minimized by making sure that the residual chlorine dioxide or hypochlorite is below about 1 ppm at the point where dye is added. Color tends to be more stable in systems where first-pass retention also has a stable value. Variations in the BRIGHTNESS or color of the incoming furnish are likely causes of color variations in the product.

METAMERISM is a word describing the situation in which a sample of paper may exactly match a standard sample under one condition of illumination, but it fails to match the standard under a different type of illumination. In the case of white paper, the most common cause of such a problem is a difference in fluorescent whitening effects of the two paper samples being compared. A simple way to judge the fluorescent whitening effects of paper is to observe them under ultraviolet light. The comparison can be made quantitative by using a brightness meter that is equipped with a filter to either block or permit passage of ultraviolet light in the incident beam. The difference of the “unfiltered” minus the “filtered” brightness gives a rough measure that can be used to control this parameter. If the fluorescent effect in the current product is lower than the standard, then the recommended solution is to add fluorescent whitening agents (FWA) or increase its dosage. If the fluorescent effect is too high, then the problem is likely to be more difficult or costly to resolve. Highly charged cationic polymers such as poly-diallyldimethylammonium chloride (poly-DADMAC) tend to “quench” the FWA. However, such an approach is likely to affect retention, sizing, and the hues of other dyes, etc. About the only other way to get rid of unwanted fluorescent whitening (especially in waste paper) is to bleach the pulp.

In the case of colored papers, metamerism often results when the hues of the dyes used in manufacture are different from those that were used when making the standard for that paper grade. A plot of diffuse reflectance versus wavelength ought to agree with that of the standard paper over the visible range of the light spectrum. Fortunately it is not necessary to use the exact same dye products. Dye suppliers usually have software that allows a quick selection of suitable dyes to match a given standard.

VARIABLE COLOR is likely to be a much more serious issue than a steady error in color relative to a standard. Some likely causes of variability in color include variations in first-pass retention, variations in broke content, or variations in pulp quality or bleaching. It is recommended to look for any periodicity in the color variations. Sometimes color variations can follow the batch-wise preparation cycles of a starch (possibly subject to biological decay), retention aid (possibly related to swings in first-pass retention), or biocide (also affecting retention efficiency in certain cases). Also, if the problem is new, one needs to ask about recent changes in the process or the types and qualities of the materials. Efforts to control retention to a steady value, appropriate biocide use, and attention to dye additives often will resolve problems with color variations.

References:

Jay, S. L., “Color Control for the Paper Producer in the 90’s,” Proc. 1991 Papermakers Conf., 93.

Lips, H. A., “Dyeing,” in Casey, J. P., Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. 3, Ch. 19, 1627 (1981).

Newton, R. J., “Continuous Dyeing: Theory and Practice,” Paper Technol. Ind. 24 (4): 140 (1983).

Curl can be defined as either a curled condition of paper or a tendency of paper to adopt a non-flat shape when exposed to changes in humidity or temperature during normal use. Curl problems can cause jamming in various kinds of converting and printing equipment, interfere with the stacking of paper and board products, and make the appearance of the product unacceptable.

There are many factors that affect the curl properties of paper. Many such problems can be traced back to non-uniformities in either the structure of processing of the paper. Structural differences may be inherent in the way the paper was produced, as in the case of Fourdrinier sheets that tend to have fine material washed out of the wire side of the sheet.

Some practical measures to correct curl problems include (a) adjusting the relative temperatures of top and bottom dryer cans (see later), (b) reducing the refining level, (c) increasing the short-fiber content of the sheet to reduce the degree of fiber alignment, and (d) taking a variety of steps to minimize the structural or chemical nature of the two sides of the paper.

Diagonal curl, a problem that often has the most serious consequences, it often due to a tendency of fibers to be aligned at a skewed angle relative to the machine direction. Such problems can arise due to excessive flow through a headbox or a poorly adjusted headbox and jet impingement. A related approach is to minimize jet-to-wire speed difference and produce a relatively “square” sheet having a minimum alignment of fibers in general. The down-side is that such a sheet may be more streaky or mottled, compared to a sheet formed with a moderate level of rush or drag.

Curl problems tend to be amplified as the inter-fiber bonding is increased. By contrast, a weakly bonded sheet, composed of relatively straight fibers with small bonded areas between them, is expected to show much reduced curl tendencies. Based on these principles there may be a slight advantage to decreasing the level of refining and compensating for strength effects by increasing the dosage or effectiveness of dry-strength additives such as cationic starch.

Adjustments of the temperatures of top and bottom dryer cans can be used to control the final curl condition of paper in the machine direction. Paper tends to curl most in the direction of the side or surface from which moisture migrates last.

Curl problems during use can be expected if the moisture content of the paper does not correspond to the equilibrium humidity condition that will be present when the paper is converted or printed. Since many of these operations have poor or non-existent air conditioning, and since paper is shipped to different locations, there is no way to completely prevent such issues from occurring. On the other hand, the moisture content of paper used in copy machines and laser printers should be about 4.2 to 4.8 (if basis weight is 75 gsm or less). Higher moisture contents can result in curl away from the printed side. In offset printing, excessive one side wetting can produce curl away from the printed side initially and toward the print after drying.

References

Green, C., “Solving Curl Problems: The Basics,” Solutions! 2001 (11): 40 (2001).

Deposits (Inorganic, Organic, Pitch, Stickies)

DEPOSITS first need to be analyzed to determine their composition. The composition often holds the key to solving deposit problems.

INORGANIC DEPOSITS are those that are composed mainly of material that becomes converted to ash when the deposit is incinerated at high temperature. In a typical procedure a sample of the deposit is placed on a tared “ash-free” filter paper, weighed, and then incinerated under standardized conditions, i.e. 900 oC and 30 minutes. X-ray fluorescence analysis can be used to determine the elemental composition of the metals in the ash.

Most inorganic deposits result from the combination of a divalent or multivalent cation with a divalent or multivalent anion, forming an insoluble precipitate. That means that once the main identity of the inorganic deposit is known, it can be attacked from either side; in principle, one can take measures to reduce the concentration of either the cation or the anion. Alternatively, scale control additives are available from chemical suppliers to control various forms of inorganic deposit.

Calcium carbonate deposits fizz when exposed to concentrated hydrochloric acid. Since the solubility of calcium carbonate decreases with increasing temperature, deposits may form on heat-exchanging equipment. Surprisingly, there seldom are calcium carbonate deposits in paper machine systems that use calcium carbonate filler; it appears likely that any excess calcium or carbonate ions, above the solubility product, simply adsorb onto the surfaces of the calcium carbonate particles already present.

Calcium oxalate deposits can be a serious problem in systems where the oxalate ion is formed during oxidative bleaching of kraft pulp. Should these deposits occur on the paper machine, it is likely that better pulp washing could solve the problem, or at least prevent the problem from reaching the paper machine. Though the problem might be attacked by changing either the bleaching chemistry or water hardness, it is more common for papermakers to use a scale-control additive [Richardson, Hipolit, 1990; May, 1991]. Another approach is to add an optimum amount of magnesium ion to change the deposition characteristics of the precipitate [LeFevre, Moran, 1996; Froass et al. 1997].

Barium sulfate deposits occur when the barium divalent cation is released from wood, especially in the case of unbleached kraft. The release occurs especially when the pulp first encounters acidic papermaking conditions, converting the insoluble barium hydroxide in the wood to soluble barium ions. Most paper machine systems have a relatively high content of sulfate ions due to the use of such additives as sulfuric acid and aluminum sulfate. The solubility product of barium sulfate is low, so it is common that the formation of barium sulfate is thermodynamically favored in paper mill systems. However, the ions tend to remain in supersaturated condition until they encounter intense hydrodynamic shear, which nucleates precipitation. Barium sulfate problem sometimes occur downstream of alum addition, since alum lowers the pH and also supplies additional sulfate ions. One way to minimize barium sulfate scale is to use polyaluminum chloride (PAC) in place of the alum. Another way is to acidify the pulp at an earlier, less vulnerable point in the process, perhaps in the first chest after the main refiners. Yet another way is to add scale-control chemicals.

Aluminum hydroxide deposits sometimes occur immediately downstream of the point of addition for aluminum sulfate (papermaker’s alum). Factors that make such deposits more likely include (a) pH values above about 6, (b) inadequate hydrodynamic shear at the point of addition of the alum, and (c) excessive concentration of the alum at the point of addition. Though pre-dilution of alum usually is a good idea, it is not a good idea to dilute alum with alkaline water or to store diluted alum before its use. Some paper machine systems and furnishes may be susceptible to aluminum hydroxide deposits, and in such cases it may be possible to replace the alum with either poly-aluminum chloride (PAC) or highly charged cationic polymers such as polyamines.

ORGANIC DEPOSITS

The main components of organic deposits often can be judged from infra-red light absorption spectra. The best approach to overcoming an organic deposit problem is likely to be different, depending on whether the main binding agent in the deposit can be classed as pitch, stickies, slime bacteria or fungi, or a papermaking additive such as a defoamer, retention polymer, or sizing agent, etc. Keep in mind that the most problematic substance(s) in a deposit may not be those that are present in the highest concentrations. For instance, many deposits on paper machine equipment may be composed mainly of cellulose fibers.

Though direct analysis of deposited material with infrared (IR) spectrophotometry can go a long way to identifying key components of an organic deposit, usually it is worth considering other procedures. Let’s start with some procedures that can make IR analysis itself more effective. The first of these is to combine IR analysis with microscopy. Ordinary light microscopy often can reveal obvious components such as fibers. But what about other “grains” or “flakes” in a deposit? Many IR spectrophotometers now include the option of microscopic analysis. As in any IR work, the strategy is to find a spectrum of pure, known material that can account for the light absorption maxima in the spectrum. You can start by obtaining spectra of various known materials that are in the input streams to the papermaking operation. In addition, absorption peaks can be compared to the wavelength maxima recorded in tables or “libraries” of IR data.

We’ll come back to the subject of microscopy, but let’s assume that we’ve already done what we can with observations of the as-received sample. One of the next important steps can be to perform an extraction. Depending on the choice of solvent, the extraction procedure can divide the sample into two or more components that differ according to their water-loving or hating nature. Relative to the initial sample, each fraction is likely to be either purified or concentrated, making subsequent analysis less difficult. To take one example, 20g of the sample may be placed in 50ml of methylethylketone (MEK) and some water in a separatory flask. After shaking the flask for several minutes, the organic phase is allowed to rise. The water-soluble portion and any fine solids can be withdrawn from the lower portion. The organic-soluble portion can be concentrated by evaporation of the MEK. The most common IR methods applied to organic-soluble, concentrated samples obtained in this way involve incorporation of the material into a pellet composed mainly of potassium bromide (KBr).

Much more detailed information about the molecular nature of an organic-soluble extract usually can be obtained by injecting some of it, in solution form, into a chromatography column. In particular it is worth considering the gas-liquid chromatography (GC) method. The method works because different molecular substances have differing affinities (partition coefficients) for the packing materials used in GC columns. The sample is injected into a carrier stream of solvent. The elution time can be compared relative to the elution times for various substances that might be suspected to be in the deposit. As an add-on, many leading laboratories that service the paper industry combine GC analysis with mass spectroscopy (MS). The MS procedure usually starts by ionizing the material with a flame or plasma. The combined GC-MS analysis can reveal the exact molecular structure of ionic breakdown products associated with each of the elution peaks obtained from the GC procedure. With modern computer analysis it is possible, in most cases, to make exact identifications of the components based on the molecular masses of the observed ionic byproducts.

Other methods to consider once you have concentrated the organic-soluble fraction of a deposit include nuclear magnetic resonance (NMR, to confirm the chemical structure), high performance liquid chromatography (HPLC, another method based on elution times), and thermogravimetric analysis (TGA, to determine melting points and glass-transition temperatures). All of these are well described in standard analytical chemistry textbooks.

It’s unlikely that you have GC-MS capability in a particular paper plant (though your chemical supplier certainly has access to one), so lets get back to some methods that can be applied “right on the spot.” Many of these fall within the category of “wet chemistry,” i.e. tests that can be done with little more than a test tube and a few reagents. Some of the most widely used spot methods involve (a) color-forming reactions, (b) stains that have affinity for various substances, such as oily materials, (c) acid to reveal the likely presence of calcium carbonate by generation of CO2 bubbles, (d) iodine solution to reveal the likely presence of starch due to a black complex with the helical amylose chains, and (e) tests for slime bacteria such as the adenozenetriphosphate (ATP) test.

Every paper mill ought to have several microscopes and people should be encouraged to use them. One of them should be a low-power stereo microscope. Another of them ought to be equipped with options such as polarized light and phase-contrast illumination. Polarized light has the unique ability to reveal crystalline material that has different refractive indices according to the orientation. One such material consists of uncooked starch granules. Such particles will show up with a characteristic “maltese cross” pattern when viewed with polarized, transmitted light. It is also a good idea to have a calibrated scale on the eyepiece of one of the scopes so the user can estimate sizes. Especially if you are on the chemical sales end of things, then you also ought to consider having a camera attachment in order to make your points more persuasively to others in the plant.

PITCH

Pitch is such an important category of organic deposits that it rates its own page of information. But first, let’s be clear on what we mean by the word. A deposit will be considered to be pitch-related if the key binding agent is derived from wood. The most common ingredients of wood pitch are resin acids (from softwood), fatty acids (from all kinds of wood), triglyceride fats, and various unsaponifiable materials such as beta-sitosterol. The tacky properties of pitch can change greatly, depending on whether it has become air-oxidized, polymerized, when the temperature changes, or when it is mixed with other materials. More details about pitch are given elsewhere in this guide.

STICKIES

Though the word is used to cover a variety of problems, most usually stickies are understood to involve adhesive materials coming from the reuse of waste paper pulp. Go to the page labeled “stickies” if it seems that this is what you are dealing with.

The main culprit in many problems with stickies is the polyvinylacetate (PVA) and other binders in the “pressure-sensitive” labels that have become so common in mail and throughout our society over the past couple of decades. The problem with stickies is that they cling together and tend to build up into globs or strings. They can adhere to papermaking equipment, they can fill felts, and they can make spots in the product. The best ways to deal with stickies include avoiding them (by selecting the kind of pulp source), removing them during deinking of the wastepaper (easier said than done), or adding enough talc to the system to overcome their tackiness. For more information about stickies, please click on this link to information about stickies.

References

Allen, L. H., Cavanagh, W. A., Holton, J. E., and Williams, G. R., “New Understanding of Talc Addition May Help Improve Control of Pitch,” Pulp Paper 67 (13): 89 (1993).

Anon., “A Primer on Pitch Problems,” Tappi 62 (4): 20 (1979).

Colman, I., Duckworth, S., and Stork, G., “Selection of Products for Deposit Prevention,” Proc. 1996 Intl. Paper Coating and Chem. Symp., 221 (1996).

Conners, T. E., and Banerjee, S., Surface Analysis of Paper, CRC Press, Boca Raton, 1995.

Doshi, M. R., “Properties and Control of Stickies,” Prog. Paper Recycling 1 (1): 54 (1991).

Douek, M., Guo, X.-Y., and Ing, J., “An Overview of the Chemical Nature of Deposits/Stickies in Mills Using Recycled Fiber,” Proc. TAPPI 1997 Recycling Symp., 313 (1997).

Dreisbach, D. D., and Michalopoulos, D. L., “Understanding the Behavior of Pitch in Pulp and Paper Mills,” Tappi J. 7 (9): 19 (1989).

Dunlop-Jones, N., and Allen, L. H., “The Influences of Washing, Defoamers, and Dispersants on Pitch Deposition from Unbleached Kraft Pulps,” J. Pulp Paper Sci. 15 (6): J235 (1989).

Fogarty, T. J., “Cost-Effective, Common Sense Approach to Stickies Control,” Tappi J. 76 (3): 161 (1993).

Froass, W. C., Francis, R. C., Dence, C. W., and LeFevre, G., “The Interactions of Calcium, Magnesium and Silicate Ions under Alkaline Conditions,” J. Pulp Paper Sci. 3 (7): J318 (1997).

Gill, R. I. S., “Chemical Control of Deposits – Scopes and Limitations,” Paper Technol. 37 (6): 23 (1996).

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

Kowalski, A., Bouchard, D., Allen, L., Larin, Y., and Vadas, O., “Pitch Expert. A Problem-Solving System for Kraft Mills,” AI Mag. Fall 1993, 81.

Le Fevre, G., and Moran, J. R., “Silicate Chemistry Key to Solving Mill Scale Problems,” TAPPI J. 79 (11): 77 (1996).

Petander, L., Ahlskog, T., and Juppo, A. J., “Strategies to Reduce AKD Deposits on Paper Machines,” Paperi Puu 80 (2): 100 (1998).

Rende, D. S., “Wire Deposit Control and Barrier Chemistry,” PIMA 76 (2): 32 (1994).

Richardson, P. F., and Hipolit, K. J., “Inorganics and their Impact on Deposit Formation in Neutral and Alkaline Papermaking Systems,” TAPPI 1990 Neutral/Alkaline Papermaking Short Course Notes, 05 (1990).

Rudie, A., “Barium Sulfate Scale in the Fiberline,” TAPPI J. 83 (10): 50 (2000).

Venditti, R. A., Chang, H. M., and Jameel, H., “Overview of Stickies Research at North Carolina State University,” PaperAge 1999 (11): 18 (1999).

Wilhelm, D., K., Makis, S. P., and Banerjee, S., “Signature of Recalcitrant Stickies in Recycled Newsprint Mills,” TAPPI J. 8 (12): 63 (1999).

Dewatering (Too Slow, Too Fast)

The most common problems related to dewatering involve slow or variable drainage and response to vacuum. Slow dewatering may involve “slow initial drainage” on the forming table, low solids after vacuum application, low solids after wet-pressing, or just a consequent lower operating speed of the paper machine. If you have that kind of problem, then you should consider yourself fortunate; there are ways to accelerate dewatering, as will be described. Variable dewatering can be much more of a headache. Until the root causes of variability are identified and overcome, the results can include web breaks, roll-winding difficulties, and nonuniform product performance. And if you are really lucky, your problem is that dewatering is too fast. If that’s your problem, then you probably can select among various options such as increased refining (for more strength), increased levels of hardwood or mechanical pulps (which often can improve formation uniformity), lower hydrofoil angles or fewer hydrofoils, lower vacuum levels (to reduce pumping energy and drive energy), or reduced wet-press nip pressure (which also is expected to increase the life expectancy of the felts). Choose carefully, since it’s likely that some of these options will be contrary to what you need for your product goals.

SLOW INITIAL DRAINAGE

Drainage rates are highly dependent on the design and set-up of forming equipment, but here let’s concentrate on issues related to the furnish and chemical additives. Some of the most likely causes of slow or decreased initial dewatering rates include high or increased fiber fines levels (often due to refining), fibrillation of the fibers, high basis weight, entrained air, and (less commonly) very high levels of water-soluble polymers. This page, below, will address all of these possibilities. Please go to the following link if you want to learn about applications of drainage chemical programs.

A fiber length analysis or “classification” of fiber length fractions can help settle the question of whether the fines content has changed. Another clue (but not proof) can consist of a higher than usual solids content in the white water. High white water solids, though, can have two basic causes. Sometimes it does mean that the furnish is over-refined or more precisely that the fibers were not strong enough to “take” the amount or intensity of energy applied during refining. Variations in pulp quality, yield, or bleaching can make the fibers more or less susceptible to cutting, even if the refining energy per ton of fiber is kept constant. Alternatively, increased fines might be due to an unexpectedly high content of mechanical fiber, hardwood, or certain kinds of recycled fibers. But be careful; a high fines content may just mean that your retention aid system is not working well (see more, a little later).

So let’s start by assuming that your main problem is due to “high fines content.” Before you attempt to solve your problem by dumping all of the fines to wastewater treatment, consider the following points: First, it’s often found that some moderate level of fines in the product, say 10 to 40%, depending on the furnish type, yields a maximum in paper tensile strength; this has been attributed to a better sheet structure. Second, due to their higher surface area per unit mass, fines are likely to be carrying a disproportionate amount of chemical additives such as sizing agents; dumping such fines will waste chemicals. Third, different kinds of fines can have very different affects. Refining tends to produce ribbon-like fragments that come from delamination of the fiber wall; these ribbons have a very high specific surface area. They are great for inter-fiber bonding, but they can be terrible for drainage. This kind of fines can be decreased by cutting back on the amount of refining or by using a finer plate to decrease the energy imparted per impact (i.e., intensity of refining). Most deinking processes tend to remove a lot of fines, along with the toner, other ink, and fillers. Recycled mixed office waste fibers need to be handled gently, using low intensity refining; they tend to be somewhat brittle due to the processes that occur during drying, and any new fines created during refining will add to the amount already present.

Let’s assume, for the moment, that the papermaking process can be modeled as “simple filtration.” This is never completely true, but it helps explain why fines are so harmful to drainage, especially at high basis weights. One can envision an initial layer of fibers forming adjacent to the forming screen and the rest of the dilute furnish being filtered by this initial layer, which then becomes thicker. Any fines in the furnish will tend to be trapped at “choke points” in the structure of the fiber mat. As a consequence, the resistance to drainage increases with increasing fines content, even beyond what would be expected due to the higher surface area per unit mass. One way to overcome this kind of effect is to change the conditions of the forming process so that the “filtration model” no longer applies. Instead, one aims to establish conditions that are more consistent with a “thickening” mechanism of dewatering. For example, a higher level of “action” from increased hydrofoil angles or optimized spacing can continually redisperse the fiber mat. Conditions consistent with thickening generally hurt the retention of fines.

A moderate treatment with an effective retention aid system can help to overcome some of the worst adverse effects of fines on drainage. This can be understood in terms of the filtration model described in the previous paragraph. One of the likely effects of retention aid treatment is attachment of fine materials onto fibers. That keeps the fines from being able to follow drainage channels to “choke points.” In general, retention aid treatments tend to make the solids adhere together in a more porous structure, a factor that can help drainage. But be careful; increased retention aid addition usually makes the paper less uniform, i.e. poor formation. The results can be deceiving in terms of dewatering. A floccy sheet usually drains quickly in the initial hydrofoil section of a Fourdrinier paper machine, but it is likely to respond poorly to vacuum in later parts of the forming section. Vacuum will tend to pull air through the thin parts of the web, leaving the fiber flocs wet. You might want to click over to the subject of formation problems if this is a major issue at the facility with which you are dealing.

If you think that maybe you are over-refining, it makes sense to consider backing off on the refining energy and instead relying more on the use of cationic starch or other chemical strategies to achieve dry strength. It is unlikely that dry strength additives ever can take the place of refining, but there often is room to achieve a better balance in terms of (a) less damage to the fibers, (b) better drainage, and (c) less reduction in caliper at a given basis weight.

Entrained air in stock can slow drainage considerably, though there is a lack of good published information obtained from commercial-scale paper machines. The definition of entrained air is that it consists of small bubbles that are carried along by fibers and other solids in the flowing furnish. Small air bubbles can have an effect similar to fiber fines, as mentioned above. That is, they can block drainage channels in the paper web. Entrained air consists partly of tiny bubbles formed by mechanical agitation in such unit operations as hydrocyclone cleaners, and pumps, especially if there are leaky seals. But another part consists of dissolved air that gets released as bubbles when the pressure is suddenly released at the headbox slice. Typical levels of entrained air in various papermaking furnishes are in the range from zero to about eight percent. That’s a huge amount, especially if you consider that fibers usually constitute less than 1% of the volume in headbox stock. Furthermore, a typical soda drink is likely to have only about 4% air content by volume after the pressure is released.

The short-term solution to entrained air may be to optimize the defoamer system. Higher dosages are sometimes undesirable due to potential deposit problems and difficulties with hydrophobic sizing agents. Defoamers generally should be added before the hydrocyclones, since that operation has the potential to entrain air. Defoamer in the system also will tend to make the bubbles coalesce so that they are too big to be drawn down with the current in the white water silo to be circulated back to the fan pump.

It has been shown that more stable drainage can be achieved if the entrained air content is automatically controlled. This is done by on-line monitoring of the air content and control of the defoamer flow. One of the attractions of this approach is that it is often possible to run with a rather high set-point of air content. The average dosage of the defoamer can be reduced, relative to manual operation, while still making a quality product at an acceptable speed.

For completeness it also should be noted that air can be reduced to low levels by means of a deaerating system, commonly known as a Deculator, after one of the early inventors. The traditional deaerating system consists of a vacuum chamber at the outlet of a set of primary hydrocyclone cleaners. Recently, a new device known as a “pomp” has been developed to remove air from white water that is being circulated back to a fan pump. The advantage of this approach is that it avoids the flow instabilities caused by variable air content in the fan pump system.

In principle, a very high level of high-mass, soluble polymers ought to increase the viscosity of water enough to impede dewatering. In practice this is seldom a big concern. But let’s consider, for a moment, the kind of polymer solution that is often used as a forming medium for wet-laid nonwoven fabrics. It is common in such applications to use a very high dosage of high-mass anionic acrylamide copolymer. A papermaker would call such a material a “retention aid.” But in the wet-lay application the dosage is so high, relative to the surface area of the synthetic fibers, that most of the polymer remains in solution. The adverse impact on drainage can be very obvious, depending on the dosages. There is some evidence that anionic retention aids can have a similar effect during certain papermaking operations, though it is difficult to isolate the effect from the impact of the same polymers on formation uniformity – something else that can affect dewatering both positively and negatively. Papermakers who use excessive levels of starch or wet-strength resins will often experience poor dewatering, but that is more likely due to increased stabilization of foam, rather than an increase in solution viscosity.

Drainage – Too Fast

Experienced papermakers usually count “too fast dewatering” as a blessing, since excess dewatering capacity often can be traded for something else that is desirable. For instance, in some cases it is practical to pump more white water to the headbox, lowering the forming consistency, and improving formation uniformity. In other cases excess drainage capacity makes it possible to refine more, achieving a higher tensile strength or internal bond strength of the paper. In other cases excess drainage capacity makes it possible to use a larger proportion of lower-grade fiber, which may have a lower freeness or higher fines content.

If the dewatering rate still is too high, some other strategies to consider include (a) speeding up the paper machine, if possible, (b) decreasing the intensity of dewatering elements such as hydrofoils and vacuum boxes, (c) or omit or reduce the amounts of drainage-promoting chemicals (see earlier discussion).

References

Allen, L. H., and Yaraskavitch, I. M., “Effects of Retention and Drainage Aids on Paper Machine Drainage: A Review,” Tappi J. 74 (7): 79 (1991).

Britt, K. W., and Unbehend, J. E., “Water Removal During Sheet Formation,” Tappi 63 (4): 67 (1980).

Farinato, R. S., Huang, S. Y., and Hawkins, P., “Polyelectrolyte-Assisted Dewatering,” in R. S. Farinato and P. L. Dubin, Eds., Colloid-Polymer Interactions, Wiley-Interscience, New York, 1999, Ch. 1, p. 3.

Raisanen, K. O., Paulapuro, N., and Karrila, S. J., “The Effects of Retention Aids, Drainage Conditions, and Pretreatment of Slurry on High-Vacuum Dewatering: A Laboratory Study,” Tappi J. 78 (4): 140 (1995).

Sampson, W. W., “The Interdependence of Sheet Structure and Drainage,” Paper Technol. Ind. 38 (8): 45 (1997).

Wegner, T. H., “Effect of Pulping Liquor on Drainage Aid Performance with Recycled Fiber,” Tappi J. 70 (1): 100 (1987).

Wildfong, V. J., Genco, J. M., Shands, J. A., and Bousfield, D. W., “Filtration Mechanics of Sheet Forming. Part 1. Apparatus for Determination of Constant-Pressure Filtration Resistance,” J. Pulp Paper Sci. 26 (7): 50 (2000).

Dirt in paper can be defined as visible specks of off-color material. Papermakers have various ways of quantifying dirt levels. These include TAPPI method T437 that is based on visual observation and various tests based on scanning devices (e.g. TAPPI Method T563). See the list of references at the end of this section.

Dirt can come from many different sources. The first step in solving a dirt problem is to try to identify the material. Light microscopy is a good place to begin. Often the chemical nature of a dirt speck can be narrowed down by finding out whether the specks are soluble in acid, base, or organic solvents. Melting point tests or scratch tests for hardness might be considered.

Many mill sites and most chemical vendors have access to analytical tools that are especially well suited to the identification of chemical components in dirt specks. In particular, some infra-red (IR) spectrometers are able to microscopically focus on a small spot of interest. IR spectra provide the user with information about the kinds of chemical groups that either (a) may be present or (b) are definitely absent from the discolored material in the paper. The reason for the word “may” is that many IR absorbance peaks overlap. An experienced spectroscopist may be needed to interpret a complicated spectra resulting from a mixture of various materials.

Some dirt problems can be traced back to their source, based on some preliminary knowledge of the composition of the material. For instance, bits of plastic, rust, or scale deposits may be becoming worn away from an exposed surface somewhere in the paper machine system. Alternatively, the dirt may consist of plastic, wood resin, bark, or even sand coming in with wood chips and passing through faulty screens in the pulping system. If a papermaker runs out of other folks to try to blame, also it is worth considering whether wet-end additives are involved.

Some practical steps to reduce dirt in a paper machine system include (a) inspecting screening equipment and possibly using a finer screen, (b) inspecting the hydrocylone cleaner system and possibly increasing the rejects flows, (c) inspecting and repairing any pump or agitator packings that may be coming apart, (d) shutting the system down for a clean-up (“boilout”), and (e) reducing the dirt content of incoming pulp, especially in the case of recycled pulp or pulp that contains bark. If the dirt is on the surface of the sheet it makes sense to inspect the surfaces of felts and rolls, cleaning them as necessary. Dirt also may fall from dryer hoods or dirty ventilation air.

Even though this website is mainly involved with chemical additives, additives seldom can solve dirt problems. Exceptions include (a) cases where the “dirt” consists of aggregates or scale formed from the additives themselves, (b) cases where additives tend to decompose after addition to the wet end, e.g. sizing agents, and (c) cases where bacterial slime problems can be managed by avoiding excess, unretained starch and treating with appropriate biocides. Canister-type filters near to the containers for chemical additives are invaluable for preventing sediment, grit, paint coatings and other potential dirt from reaching the paper. It is a good idea to visually inspect such filters if the chemical additive system in question is considered as a possible source of the observed dirt material.

When using recycled pulps, dirt problems should be mainly addressed during various deinking, screening, floatation, or washing operations. Such operations lie outside of the scope of this website. To a moderate extent, dirt problems related to sticky materials from recycled pulp sometimes can be reduced in severity by adding (a) finely divided talc mineral or certain organic polymers designed to detackify the wetted surfaces, (b) spray application of barrier chemicals (very dilute solutions of highly cationic polymers) to the forming fabric and sometimes to wet-press fabrics, (c) detergent or even solvent treatment of wet-press felts, and (d) use of doctor blades to remove tacky or sticky materials from dryer can surfaces.

References

Anon., “Identification of Specks and Spots in Paper,” TAPPI Useful Method UM 589, 1984.

Rosenberger, R. R., “Putting the New Dirt Count Method into Perspective: A Discussion of TAPPI Method T-563,” Prog. Paper Recycling 6 (1): 9 (1996).

Soderhjelm, L., “Dirt and Shives in Pulp, International Standardization,” Paper Technol. Ind. 37 (10): 51 (1996).

Zeyer, C., Heitmann, J. A., Venditti, R., and Joyce, T. W., “Image Analysis with an Optical Scanner,” Prog. Paper Recycling 3 (3): 29 (1994).

Drainage (Too Slow, Too Fast)