RETENTION: THE KEY TO EFFICIENT PAPERMAKING
Terrence M. Gallagher
Nalco Chemical Company, Naperville, IL , USA
The demand for retention aids has grown as papermakers seek to make alkaline rather than acid paper, incorporate higher levels of fillers, increase the extent of closure of the white water system, operate the paper machines at higher speeds, and increase the use of recycled fibers. All of these factors create an environment where retention is more difficult to achieve without the help of a chemically assisted retention program. Retention aid programs can provide significant cost/operating advantages for virtually all paper manufacturers, and for producers of paperboard as well. An effective retention program is the key to optimum sizing efficiency, good opacity, overall machine runnability, and reduced furnish costs through better utilization of fillers or other additives.
A well-designed retention aid program uses the mechanisms of both coagulation and flocculation to achieve higher retention. The latest generation of high performance retention programs also utilizes microparticle flocculation. The selection of the optimum program must consider many variables including chemistry, molecular weight, system charge, application points, shear, chemical additives, etc. By considering all of these variables, an optimum program can be custom designed for a mill’s unique requirements.
Good retention is important to the efficiency of the papermaking operation due to the large influence it an have on furnish and production costs, as well as on the quality of the finished sheet. Low retention can lead to many problems, including:
• Poor runnability
• Increased deposits
• Sheet defects
• Higher additive costs
• More downtime for wash-ups
• Higher sewer losses
Retention aids can improve the overall runnability of the machine, allow increases in speed through better drainage, reduce deposits and sheet breaks caused by high levels of fillers and fines circulating at the wet end, and reduce furnish costs through better use of fillers or other additives. A good retention program can help the papermaker achieve desired sheet qualities by optimizing the retention of expensive additives such as titanium dioxide, wet and dry strength additives, and alkaline size.
This paper reviews the mechanisms of retention, describes the types of chemicals that are effective retention aids and shows how these products can be applied to achieve optimum results.
It is generally accepted that the retention on a paper machine is largely a filtration process, whereby furnish solids are captured by filtration through the forming fabric and the fiber mat. Retention produced through filtration, or mechanical entrapment, is affected by many operating variables including basis weight, furnish composition, machine speed, drainage forces, type of forming fabric, and the design of the forming section. High basis weight, low ash sheets produced on slow speed single-wire machines may have reasonably high retention even without a retention aid program because these factors tend to increase the filtration efficiency. But trends in papermaking are toward lower basis weights, higher filler levels, faster machine speeds and increasing use of twin-wire formers — which reduce filtration efficiency and result in lower retention. Filler loadings of 30% in alkaline papermaking are now attainable due to chemically assisted retention.
Chemically assisted retention on a paper machine uses the processes of coagulation and flocculation (and microparticle flocculation) to increase the effective size of the particles in the fines fraction. This enables more fines to be retained with the sheet. The fibers used for making paper develop an anionic surface charge in water as a result of the dissociation of carboxyl and sulfonic acid groups. These raw materials include the fibers, fines, and fillers, as well as most of the dissolved and colloidal material liberated from the wood during the pulping and bleaching processes.
Cations may be attached by electrostatic and adsorptive forces (van der Waals forces) to these surfaces. The anionic charge on the surface is partially neutralized by these cations causing the net potential energy of the system to drop rapidly in this region as shown in Figure 1. The zeta potential is determined by the magnitude of the anionic surface charge minus the magnitude of the anionic surface charge minus the magnitude of the cationic charge in this layer of attached counter ions. Beyond the layer of firmly attached cations exists a second layer which contains a high concentration of cations attracted by the zeta potential. These cations, however, are not attached to the particle. The edge of this diffuse layer is that point where the net potential is finally reduced to zero. This description of charge at various points beyond the surface is called the Electric Double Layer model.
In a system with a high zeta potential, a large repulsive force exists which resists the tendency of any two particles to collide as they jostle about in the solution. The addition of a coagulant uniformly neutralizes the charge, reducing the zeta potential. As their diffuse layers shrink, particles are able to approach closer to each other until finally the attractive van der Waals force overpowers the repulsive force and coagulation occurs. Usually this occurs when the zeta potential is reduced almost to zero which is the reason for naming this the charge neutralization mechanism . To retain these fillers and fines in the sheet, two things must be done: first, reduce the repellent forces between the particles (coagulation) and create cationic patches on the particles. Then combine or form a “bridge” between these patches (flocculation) to produce discrete agglomerates that are large enough to be entrapped in the forming web.
Coagulation is the initial step in the retention process. During coagulation, the electrostatic sphere of charge that surrounds the small furnish particles and keeps them well separated is neutralized by a cationic source (coagulant). Reducing the extent of the repelling forces allows the particles to come closer together. Effective coagulation is reached when the distance that separates the fines is sufficiently small that a high molecular weight polymer (flocculant) can span between the particles to form a “bridge”, producing agglomerates that can be retained by filtration through the forming web. This coagulation process is depicted in Figure 1.
The papermaker can choose between two types of coagulants: inorganic or organic polymers. Alum is the most common inorganic coagulant for acid papermaking systems. Alum can function as an effective coagulant at pH 4.0 to 5.5, because it can carry a strong cationic charge within this pH region. At higher pH (greater than about 5.5), alum is only weakly cationic, and becomes much less effective. (Refer to Figure 3.) To review all of the roles that alum plays in retention and drainage is beyond the scope of this paper. Many good articles have been published on this subject, and references 2 and 3 are offered for further reading.
Organic coagulant polymers are designed to performover a broad pH range. They have been used effectively in systems ranging from pH 4.0 to 8.5. Organic coagulants are highly cationic, low molecular weight (LMW) polymers. Most organic coagulants are polymers having a molecular weight of about 20,000 to 200,000. These polymers are supplied as water solutions which may be fed directly to the stock line or with in-line water dilution. Typically, coagulants are added back in the system — to the pulper, blend chest, machine chest, white water silo, or at the inlet of the machine screen. They are always added before the flocculant.
Before coagulation the large sphere of electrostatic charge prevents a bridge from forming between the polymer coated particles.
With good coagulation the charge sphere is reduced, allowing the high molecular weight (HMW) polymer to combine the particles into larger agglomerates.
Flocculation is the second step in the retention process. HMW polymeric flocculants are added to the furnish after coagulation to bridge the neutralized particles and hold them in the sheet. Flocculants are usually acrylamide-based polymers with a molecular weight from about 500,000 to tens of millions. Unlike coagulants, which are always cationic, flocculants can be cationic, anionic, or nonionic. These high molecular weight retention aids are available as dry powders and liquid emulsions. Thus, a variety of flocculant products are available to accommodate the unique needs of each paper machine.
How do HMW polymers work in the papermaking system? Although scientists disagree about the exact mechanisms, most agree that the HMW polymer attaches to the surface of a filler or fine and then extends into the liquid where it attaches to long fibers or other filler particles, forming agglomerates. The degree of extension, or length of the polymer in solution, greatly affects how well fillers and fines are retained via bridging mechanisms. Figure 4 shows how polymers can bridge between particles to form agglomerates. For simplicity, the electrostatic charge spheres have been omitted and the system is assumed to be well neutralized.
The mechanisms for attachment of a HMW polymer to the particle surface are not completely understood. Two mechanisms are probably most important: hydrogen bonding and ion pairing. Acrylamide polymers have a multitude of hydrogen bonding sites that enables the polymers to attach to the surfaces of fillers and fines. Cationic polyacrylamides can become attached to oppositely charged particles through ion pairing.
FACTORS AFFECTING THE PERFORMANCE OF HMW POLYMERS
Proper application of HMW polymers is critical to their success as retention aids. Two of the most important factors are polymer makeup and feed point.
High molecular weight retention aids are available as dry powders and in liquid emulsion form. Both must be properly diluted and mixed before they can be fed to the paper machine, but makeup techniques and product concentration requirements differ for the two forms. Dry polymers need to be dissolved in water, then mixed with good agitation to uncoil the polymers. See Figure 5. The polymer concentration is selected based on viscosity and pumping requirements.
For optimum results, the product concentration and type of mixing should be carefully controlled when using liquid emulsion polymers. Typically, a 0.5 to 2% solution should be prepared and mixed for a minimum of 15 minutes. The concentration is important because liquid emulsion products contain activators in the proper quantity for the polymer to become fully dispersed at these solution strengths. The 15-minute mixing time helps ensure that the molecule will extend fully, a condition that provides all the product’s potential activity. The product should then be diluted to 0.1% or less prior to application. This dilution is needed for adequate distribution of the retention aid in the paper furnish.
The type of mixing is also important. Automatic makeup systems are designed to impart just the right amount of shear to develop optimum product efficiency. Too little shear will not allow the product to reach its full potential; too much shear can cause polymer degradation and a loss in efficiency.
The application point for HMW polymers should be carefully selected. Two variables that will affect the activity of HMW polymers are contact time and shear rate. Good agitation is needed at the polymer feed point to distribute the polymer evenly throughout the stock. Excessive mixing after floc formation should be avoided, however, to prevent disruption of the relatively fragile flocs. If HMW polymers are subjected to too much shear or are allowed to mix with the stock for too long and the polymer and surface are of opposite charge, “compression” or “reconformation” can occur. During compression, the polymer is forced down onto the particle surface. Because less of the polymer extends into the stock, the probability of successful adhesion and bridging on collision with other particles is reduced. Passing the polymer-treated stock through a refiner, fan pump, screen, or other high shear region of the paper machine system will force compression to occur and should be avoided. For best results, HMW retention aids should be fed after the fan pump and just prior to the head box.
The following example illustrates the detrimental effect of excessive shear on polymer performance. A U.S. mill producing business papers typically uses 1.3 to 1.5 pounds per ton of a liquid anionic polyacrylamide for retention. When the retention aid is added just ahead of the machine screen, first pass ash retention is at 49%. By feeding the retention aid after the machine screen, and thus avoiding this high shear environment, this mill achieved 61% first pass ash retention, a gain of 12 percentage points.
An effective retention program should satisfy the requirements for both charge neutralization and particle agglomeration. A single additive will usually not provide optimum retention because the chemical and physical properties needed for coagulation and flocculation are different. The most common retention programs for paper made at acid pH use alum (a coagulant), along with a high molecular weight flocculant. In general, these programs have been very effective. Some paper machines, however, place exceptionally high demands on the retention program and require the use of an additional synthetic coagulant, or dual-polymer program. This is most prevalent under alkaline papermaking conditions where the effectiveness of alum as a retention aid is greatly reduced. Retention aids react in a paper machine system by adsorbing onto the surfaces of colloidal fines and fillers, so any changes in the amount of fines or fillers in the wet end system will affect the need for retention aid. The concentration of dissolved ions and interfering anionic substances in the furnish will also influence the type of retention program that will work best. Some of the variables that should be considered include:
• type of fiber
• pulp cleanliness
• wet end pH
• alum addition
• type and amount of fillers
• cationic starch
• strength additives
• sizing system
• extent of closure
• broke usage
• coated broke
For mills producing coated papers, the use of coated broke can increase the need for good coagulation and flocculation. In some of these mills, the broke system contributes a substantial portion of the sheet ash, or it may be the only source of filler. Ash from broke systems can be more difficult to retain than filler pigments.
Typically, the particle size of coating grade pigments is smaller than filler clays (73 to 92% less than 2 microns versus 55 to 60% less than 2 microns) . Dispersing agents added to coatings increase the electrostatic charge on the pigment particle, and thus increase the demand for a cationic coagulant in the wet end. The amount of dispersant is usually higher in coating formulations with calcium carbonate or titanium dioxide than with clay pigment .
Starch binders present additional challenges to the retention program when coated broke is recycled to the wet end of the paper machine. Larger quantities of dispersing agents are used in coating where starch, rather than protein, is present as the coating adhesive . Oxidized starch is a good pigment dispersant, but is undesirable in the wet end because it is detrimental to retention .
In systems like these, and others, an alum-flocculant program may not be able to provide the desired high level of retention and sheet quality. So much HMW polymer may be needed that the system becomes overflocculated, causing formation to suffer. Excessive use of alum to maintain retention can create imbalance in wet end chemistry. In closed systems, high levels of alum may cause sulfate buildup and excess acidity. This can lead to problems in alum-rosin sizing, increase the potential for alumina deposits, and affect paper machine runnability. Polymeric coagulants can reduce the amount of alum needed, reducing the deposit potential and corrosivity of the system. A dual-polymer program, consisting of a LMW polymeric coagulant fed before a HMW anionic or cationic flocculant, can be used to achieve good retention while reducing the possibility of overflocculation and poor sheet formation without disrupting the wet end chemical balance.
Colloidal silica is a highly anionic particle approximately 4 millimicrons in size. Refer to Figure 6.
It significantly enhances small particle retention by forming strong ionic bonds with cationic additives or coagulants, absorbed on the surface of furnish components. Refer to Figure 7. The silica forms “microflocs”.
The exact mechanism is not known. It is believed the difference between microflocculation and conventional flocculation lies in the ability of colloidal silica to reflocculate after shear. Once the conventional flocs are disrupted, they do not reform. Colloidal silica aids in the reformation of bridging. Reflocculation on a microscale imparts high permeability to the sheet, which is favorable in both cases for dewatering, press-dryness, and the ability to be drie . These micro-flocs are better distributed throughout the sheet and, therefore, give better filler distribution. Mills can take advantage of this improved dewatering capacity by diluting the stock consistency to improve formation or by increasing production. Improved retention and filler distribution can enhance opacity, porosity, brightness, and reduce two-sidedness. Good formation can also press and dry easier due to better surface contact. The microparticle is typically fed after the screens while the retention aid is fed prior to the screens. This program allows for greater first pass and ash retention without overflocculation, which is typically associated with traditional flocculant programs.
When selecting a retention program, many variables must be considered. Coagulant selection is based on the polymer’s ability to satisfy the cationic demand of the system. Lab screening of various available coagulants can be conducted on machine furnish to determine which polymer chemistry will best neutralize the system. Flocculant selection can be similarly determined through the use of the Britt Jar and various drainage tests to determine which flocculant, along with silica, gives the best retention and drainage characteristics. There are numerous flocculants available to the papermaker today. These differ in chemistry, molecular weight, and charge density. A product can be chosen which best fits the wet end chemistry and requirements of the mill. In no way can one flocculant or coagulant be expected to perform optimally in all systems. Every mill is unique and an analysis should be carried out to determine which polymers will function optimally.
Typically, the coagulant will be fed back in the system (machine chest, white water silo, etc.), flocculant before the screens, and colloidal silica after the screens. To optimize the program, the papermaker must be flexible in moving feed points and adjusting dosages. He must gain control of the wet end chemistry and the charge effects additives have on the system. At this point, ash loadings can be pushed to their maximum .
APPLICATIONS FOR RETENTION AIDS
Most mills producing printing and writing papers have recognized the benefits that a good retention aid program can provide, and use retention aids regularly for improved runnability, higher opacity, and cost reductions. But retention aids also offer many benefits in other grades of paper and paperboard. One recycled boxboard mill, for example, established new production records and increased profitability by more than 20% after adopting a retention and drainage program. Drainage improvements resulting from the polymer treatment allowed the mill to increase machine speed by more than 25 fpm. Higher fiber and fines retention reduced foaming, slime spots, and machine deposits. Also, since fewer solids were exiting the system, less polymer was needed in the save-all.
Retention aid programs can also provide significant cost/operating advantages for tissue and towel manufacturers. A dual-polymer program helped one tissue mill increase first pass retention by 18%. As more fiber and fines were retained in the sheet, wet strength retention efficiency increased, allowing a significant reduction of wet strength chemical usage. More fines in the sheet also helped provide a better Yankee coating. As a result, crepe blade life improved. Sewer cost was cut by 50%, while save-all solids were cut by 40%. Since the machine showers use save-all water, fewer fines and wet strength resins in the showers reduced felt filling and shower plugging. Retention aids have not been widely used in newsprint manufacture because many retention aids are ineffective in a newsprint furnish, or may be only marginally cost effective . But new quality standards that are emerging in the newsprint market — higher opacity, brightness, smoothness — are now causing some newsprint mills to consider adding filler clays to the sheet,  and this is changing the need for retention aids. Newsprint manufacturers who are trying filler clays are finding retention aids are very important to help them achieve sheet ash targets.
Successful machine trials to retain fines and filler clays in newsprint have been run, resulting in improved opacity, strength, higher machine speeds, and good ash retention. Dual-polymer programs, consisting of a LMW organic coagulant fed before a HMW anionic or cationic flocculant, have worked well in this application. In alkaline papermaking, one microparticle retention program has allowed filler loadings to 28% while increasing machine speeds. By moving the cationic flocculant before the screens and adding the microparticle after the screens, ash retention and drainage were drastically improved via the formation of “micro-flocs”. These microparticle programs are now utilized in over 20% of the alkaline mills in the U.S. today.12
A good retention aid program can provide significant benefits for nearly any paper or paperboard mill. Some typical achievements have been:
1. Improved overall runnability of the paper machine.
2. Increases in speed through better drainage.
3. Reduced deposits and sheet breaks caused by high levels of fillers and fines circulating at the wet end.
4. Furnish cost savings through better utilization of additives.
But while these cost savings benefits of retention aids are certainly important, one should not overlook the beneficial effect that a retention program can have on the quality of the sheet. Retention aids are widely used to reduce the two-sidedness of filled printing grades caused by improper ash distribution in the z-direction. A more uniform distribution of fillers, reduced sheet two-sidedness, higher opacity, improved sizing, and better formation are all potential benefits of an effective retention program. Multicomponent programs (dual-polymer, microparticle) allow the papermaker more control over the unique requirements of his system. This control allows the papermaker to respond to changes in his system through retention program adjustment. It allows him to take advantage of the economic and quality benefits of alkaline papermaking, while maximizing the return, of the retention program .
1. Cantrell, J. A., Control of Detrimental Substances, Western Section CPPA Conference, Vancouver, Canada, April 1989.
2. Arnson, T. R., The Chemistry of Aluminum Salts in Papermaking, TAPPI, 65 (3) (March 1982), pp. 125-130.
3. Wortley, B. H., Papermakers’ Alum, TAPPI Retention and Drainage Seminar Notes, 1979, pp. 21-27.
4. Pulp and Paper Chemistry and Technology, Ed. J. P. Casey, Vol. IV, John Wiley and Sons, New York, 1983, p. 2032.
5. Mosher, R. and Davis D., Industrial and Specialty Papers, Vol. I, Chemical Publishing Company, Inc., New York, 1968, p. 130.
16. Mosher and Davis, p. 130.
17. Starch: Chemistry and Technology, Ed. R. L. Whistler et al., Harcourt Brace Jovanich, Orlando, 1984, p. 564.
18. Lindstrom, T. L., Microparticle Technique — The New Megatrend, Svensk Paperstiding, No. 1, 1987, pp. 24-25.
19. Roop, M. J., The Development of An Alkaline Fine Paper Retention Strategy — A Continuous Process, TAPPI 1989 Papermakers Conference.
10. Pelton, R. H., Allen, L. H., Nugent, H. M., A Survey of Potential Retention Aids for Newsprint Manufacture, Pulp and Paper Canada, 81 (1), (1980), pp. 9-15.
11. Koppelman, M. H. and Miglionni, I. K., Fillers in Uncoated Mechanical Fiber Furnishes, TAPPI Papermakers Conference Proceedings, 1985, pp. 125-126.
12. Confidential Nalco Marketing Report (November 1990).
13. Gallagher, T. M., High Performance Alkaline Retention Programs, TAPPI Alkaline/Neutral Papermaking Short Course, 1990.
Originally presented at the 1990 TAPPI Neutral/Alkaline Papermaking Short Course, Orlando, Florida, October 16-18, 1990, @TAPPI 1990.
Reproduced by permission of NALCO ITALIANA S.P.A. VIALE DELL'ESPERANTO 71 • 00144 ROMA, ITALIA