Duncan S. Carr
Feb. 1, 2005
Anionic trash collectors (ATCs), either organic or inorganic, can be critical to raising wet end performance and lowering dosages of alkenyl succinic anhydride (ASA) size. Evidence suggests that greater interaction between nanoparticles and the starch component of ASA emulsion ensures optimum fixation of the ASA on the fiber, whether a long chain polyacrylamide is present or not.
In addition to minimizing ASA dosages, ATCs have also demonstrated an ability to reduce starch and polymer dosages, enabling papermakers to overcome bottlenecks and unscheduled downtime, therefore raising productivity and lowering chemical costs. In fact, a wide range of proprietary ATCs is increasing sizing efficiencies, even at equal fines and ash retention.
Higher efficiency retention and sizing
The importance of first pass retention of high surface area fines and filler in optimizing sizing efficiency is well known and reported. Several chemical systems have been proposed to optimize fines and filler retention, many of which use either an organic or inorganic ATC.
In the early 1990s, with the use of precipitated calcium carbonate (PCC) with alkyl ketene dimer (AKD), caution was needed in the use of ATCs as this could contribute to reversion and fugitivity characteristics.1 Today, ASA has replaced AKD in most North American fine paper mills. The best practices for making and applying ASA emulsion are now well established.2
Different retention systems, despite providing equal fines and ash retention on the same paper machine and grade, can produce very different sizing efficiencies.
Compartmentalizing nano retention and drainage
In a truly optimized microparticle system relying completely on a charge neutralization mechanism, retention, drainage, and formation are optimized simultaneously.3 Not only are retention of fines, filler, and fiber optimized but also is the retention of colloidal material, such as starch and size. This, coupled with the best-formed sheet through optimized formation, lends itself to providing the best sizing at minimum size dosage.
Structured silica nanoparticles are now being used with polyacrylamides. The dosages of these two primary components can be manipulated to provide increased drainage, increased retention, or both. This is distinct from a true microparticle system where both retention and drainage can only be optimized simultaneously. However, in starch-containing nanoparticle systemsparticularly those using wet end starch to emulsify the sizethere is a synergy between the nanoparticle and the polyacrylamide and classic microparticle effects involving nanoparticles and starch. Again, this lends itself to sizing optimization.
The best way to minimize ASA size addition is to optimize distribution and retention of the starch-based emulsion. This takes place when there is optimum charge neutralization between the starch and the nanoparticle. It is possible to have high fines and filler retention yet have a poor distribution of the size emulsion particles and poor colloidal retention of the size.
Papermaking systems can be further optimized by the use of ATCs, which can be either organic or inorganic. Both types tend to be multifunctional. ATCs can help in agglomeration or fixation of wood or white pitch. Their effectiveness in either mechanism is related to their charge density, molecular weight, and the presence of hydrophobic groups.
Polyamines, polyethylene imines (PEIs), polyvinyl amines, and polydadmacs have typically been used as organic ATCs. They impart a permanent cationic charge to the papermaking system, which has the ability to move the charge into an operating area where the classic microparticle effect between the starch and nanoparticle is optimized. At the same time, an ATC can provide a blocking mechanism to maintain the most favorable configuration of the polyacrylamide.4
Alum and polyaluminum chloride (PAC) have typically been used as inorganic ATCs. They provide a temporary yet very significant cationic charge to even alkaline systems.
Due to the temporary nature of the charge provided by the inorganic ATC, the choice of addition point is much more critical. In addition to moving the charge into the most favorable operating window for maximum microparticle effect between the starch and nanoparticle, the aluminum species has a synergy with amphoteric starches independent of the silica interaction.5 The alum also plays an important role in protecting the paper machine from sticky deposits of calcium and magnesium hydrolyzate by forming the less tacky aluminum hydrolyzate.
The mechanism of colloidal retention
Most of the size will be associated with the fines and filler because of their relatively high surface area compared with fiber. It is therefore important to ensure adequate retention of size on the long fiber and that addition point strategies address this issue. Also, the use of an ATC to neutralize some of the charge density on the fines can help redirect some of the size to the long fiber.
With AKD sizing, the generally accepted reaction mechanism is that a covalent ester bond is formed with the fiber after spreading. In reality, this represents perhaps 25 to 35% of the total AKD retained in the sheet, with an absolute upper limit of 50%. The remainder is either hydrolyzed to the ketone, which contributes nothing to sizing, or is retained as unreacted AKD and does contribute to sizing in the presence of bound AKD, although not to the level of the bound AKD.6-8
The size emulsion is simply too small to be retained on its own. Particularly in a non-microparticle system, it requires the fines and filler to assist in its retention by the formation of agglomerates up to perhaps 40 to 50 microns in size. This agglomerate formation is by a charge neutralization mechanism.
The presence of cure promoters in the size will not only help the size to be self retaining but will also help to neutralize the charge on the fines and filler and allow for agglomeration. This applies to both ASA and AKD sizing.
The key to colloidal retention of sub-micron particles is to find a way to retain them in a form that is well dispersed but well adsorbed on fiber and fines. The separate use of an ATC in the wet end can facilitate this charge neutralization of the fine material and agglomerate the fines for high retention. It will also ensure optimal distribution and retention of size emulsion particles on the surfaces of the fines and fiber in the presence of a silica nanoparticle in the wet end.
Alum is typically added as an inorganic ATC in ASA sizing systems as a scavenger for hydrolyzed ASA. A significant number of machines use an alum-APAM retention aid system, which provides a very economic, though less beneficial, way to attain good retention, especially in uncoated fine paper.
Probably the most important factor contributing to sizing efficiency is emulsion retention. However, as stated above, it is not just the retention of fines and filler that are responsible for sizing efficiency. Sizing efficiency can be increased dramatically without changing any machine conditions or changing fines or filler retention. Changing colloidal retention alone may be sufficient, as in the case studies discussed below.
Energy, ASA efficiency gains
In the first example, a gap former produced uncoated fine paper at 3,350 fpm. The reference system used an organic microparticle (OMP)-added post screen with a conventional late addition of ASA emulsified in cornstarch. An inorganic ATC addition of alum was added to the ASA emulsion.
On switching to a system using a cationic polyacrylamide added prescreen and silica nanoparticle post screen, the same tray solids were maintained by adjustment of the dosage of these two components. In addition to a major reduction in steam demand, the sizing efficiency as defined by HST/ASA lb/ton was improved dramatically (Figure 1). The ASA dosage was reduced by 25%.
In addition to the improvement in sizing efficiency, the variability of the sizing was also considerably improved with a significant reduction in the Hercules sizing tester (HST) standard deviation of sample sizes of 200 reels of copy paper made at 3,350 fpm with equal ash content (Table 1).
||System 1: Organic Microparticle
||System 2: Nanoparticle
|Mean HST, Top
|Mean HST, Wire
|Std. Dev., Top
|Std. Dev., Wire
It is further evident that HST numbers are always higher if the polyacrylamide dosages can be reduced at the same ash retention. This would suggest a stronger microparticle effect between the starch and silica nanoparticle providing better sizing efficiency.
Nano advantage to penetrate starch structure
In a second example, a fourdrinier machine fitted with a hybrid top wire former was making uncoated fine paper at 3,600 fpm. The reference system used a cationic polyacrylamide prescreen and silica nanoparticle post screen with a late addition of ASA emulsified in cationic cornstarch. An inorganic ATC was added to the cleaner accepts. Additional cornstarch was added to the thick stock. The sheet ash content was 16%.
On switching to a system using an organic microparticle post screen and cationic polyacrylamide prescreen, the same tray solids and ash retention were maintained. However, the sizing efficiency declined, necessitating a significant increase in ASA dosage (Figure 2).
Toward the end of this trial, the organic microparticle dosage was increased still further, and the cationic polyacrylamide was completely removed. Although the tray solids were maintained, the sizing efficiency was further reduced, necessitating a further increase in ASA size dosage.
Anionic polyacrylamide can adversely affect sizing, particularly if it has a high charge. The agglomeration of size emulsion particles has been proposed as causing poor distribution of the emulsion on fiber.6,7
In both case studies, the organic microparticle was indeed a highly charged, cross-linked anionic polyacrylamide product with around 10 times greater meq/g charge than the anionic silica nanoparticle. It was also about 100 times greater in size when swollen compared with the silica nanoparticle.
One of the major reasons for the success of silica nanoparticle systems using starch is the ability of the silica nanoparticle to penetrate the branched amylopectin structure in the starch. The system is also able to initiate a charge neutralization mechanism, resulting in very strong association of the finer material with the fiber to provide excellent drainage and retention. In an ASA sizing system, this uniform fine floc distribution ensures excellent distribution of the size. It can be optimized, if necessary, by a separate ATC addition.
Conversely, the organic microparticle is simply too big to penetrate the amylopectin structure of the cationic starch. It is distinctly possible that the organic microparticle, rather than penetrating the starch of the ASA emulsion, actually coagulates the emulsion particles due to its high charge density, causing an inferior distribution of the ASA on the fiber.
To verify this, a laboratory simulation of the different systems was conducted by sequential addition of the wet end chemicals to a standard fine paper furnish representing headbox stock. To quantify the amount of emulsion agglomeration, the mixed stock was stained with iodine to "label" the starch and view the fiber under a microscope.
The agglomeration with the organic microparticle was much greater than with the smaller inorganic silica nanoparticle. It was also clear that the optimized starch-nanoparticle microparticle effect produced a much closer proximity of smaller stained particles to the fiber, which would impart greater shear resistance on the paper machine and promote excellent colloidal retention (Figures 3a and b).
Duncan S. Carr, senior papermaking specialist, Eka Chemicals, Marietta, Ga. The author would like to thank Richard Urbantas for his assistance in developing the image analysis techniques reported in this article.
1. A.R. Colasurdo and I. Thorn, "The interactions of alkyl ketene dimers with other wet end additives," TAPPI Journal, 1992, Vol. 75, No. 9, pp. 143-149.
2. R.M. Savolainen, "The effects of temperature, pH, and alkalinity on ASA sizing in alkaline papermaking," Proceedings of the 1996 TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, GA, pp. 289-295.
3. J.G. Penniman and A.G. Makhonin, "Optimizing microparticulate process efficiency," Proceedings of the 1993 TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, pp. 129-135.
4. S. Main and P. Simonson, "Retention aids for high speed paper machines," TAPPI Journal, 1999, Vol. 82, No. 4, pp. 78-84.
5. P. Christian and B. Carre, "The complementarity between two dewatering and retention microparticle systems: cationic starch/anionic colloidal silica and potato starch/aluminum salts," Proceedings of the 1993 TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, pp. 163-170.
6. P. A. Patton, "On the mechanisms of AKD sizing and size reversion," Proceedings of the 1991 TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, pp. 415-423.
7. T. Lindstrom and G. Soderberg, "On the mechanism of sizing with alkyl ketene dimers. Part 3. The role of pH, electolytes, retention aids, extractives, Ca-lignosulfonates and mode of addition on alkyl ketene dimer retention," Nordic Pulp and Paper Journal, 1986, Vol. 1, No. 2, pp. 31-38.
8. T. Lindstrom and G. Soderberg, "On the mechanism of sizing with alkyl ketene dimers. Part 1. Studies on the amount of alkyl ketene dimer required by different pulps," Nordic Pulp and Paper Journal, 1986, Vol. 1, No. 1, pp. 26-33.