Wet End
Chemistry
The combination of production, environment, and product quality issues
places greater demand on retention/drainage aids to perform well
New Papermaking Equipment, Furnishes Offer Fiber, Filler Retention Challenge
BY JOHN PELLETIER
and STEVE KUHN
In papermaking, formation of the sheet is a critical yet elusive quality parameter that has always presented a challenge to the papermaker. Formation can be loosely defined by the size and distribution of fiber "flocs" and is actually a measure of the randomness of fiber and filler distribution. Small "soft" flocs of even distribution yield
better formation value than large "hard" flocs that tend to give a spotty or streaky appearance to the sheet.
Traditionally, equipment manufacturers have attempted to mechanically improve sheet formation by use of headbox
rectifier rolls, dandy rolls, or through increased headbox dilution and hydraulic turbulence. With today's fast machine speeds and increased turbulences, retention of fiber and fines on the forming table often suffers. Retention aid suppliers attempt to improve the situation chemically by reducing floc size and improving drainage, with the hope of yielding better formation along with increased retention and distribution of filler and fines.
The most recent web-forming techniques, such as high-turbulence headboxes, top-wire formers, and true twin-wire formers, have made it even more difficult to maintain acceptable retention. Further, environmental and economic aspects have placed additional burdens on retention systems through water system closure, increased filler loading, and the increased use of
recycled fibers, all of which lead to
higher amounts of detrimental substances within the machine system. All
of the factors today add up to a challenge to develop new methods of fiber and filler retention, while maintaining or enhancing the formation of the paper being produced.
SINGLE-COMPONENT SYSTEMS. In the not too distant past, the retention aids of choice were single-component products based on alum, polyacrylamide, and/or starch. These products tend to agglomerate fibers and fines and produce large flocs, thereby reducing the specific surface area of the fiber mass Figure 1).
This reduction of surface area allows for increased drainage in the forming section of the fourdrinier. However, the large flocs formed hold more water than small soft flocs. The water in the large flocs does not press out as easily, and press section performance suffers.
Polyacrylamides tend to function primarily by the bridging mechanism, as described by previous publications.1,2 Polyacrylamides tend to form large flocs that are agglomerates of fines or filler that typically are not an even distribution of the filler and fines. If these flocs are subjected to a high degree of shear (e.g., a centrifugal screen) they can be broken and do not reform readily. Polyacrylamide efficiency can be greatly reduced, sometimes by more than 50% when subjected to shear (Figure 2).3
In laboratory experiments, it is often observed that polyacrylamides show as much as a 30% improvement, in terms
of drainage, over a modified polyethyleneimine. Practical experience has shown, however, that the drainage provided by the polyacrylamide on the paper machine wire is only slightly
better when looking at the wet-line
position.
In addition, when using the modified polyethyleneimine, the most significant improvement is in the after press section moisture. Modified polyethyleneimine has shown 2% reductions of moisture over the polyacrylamide program (Figure 3).3
This can be explained by changing the water retention of the stock suspension. Polyacrylamides tend to form large flocs with a large amount of open area between flocs, thus facilitating dewatering on the wire. The modified polyethyleneimine does a portion of this and also is capable of charge neutralization of the fines, filler, and fiber, which can collapse the floc structure onto itself. This reduces the surface area and allows for hydrogen bonding and Van der Waals forces to function, forcing water out of the fiber matrix.
Pressing of a sheet with a small
average floc size is more efficient as less water is held within the flocs. In addition to improved drainage and reduced press moisture, there is a marked improvement in filler distribution (Figure 4).3 Therefore, choice of the polyacrylamide and the addition point selection is critical for polyacrylamides to continue to be a useful retention agent under today's more demanding conditions.
DUAL-COMPONENT SYSTEMS. The two-pronged approach of fixation and retention with the use of a cationic donor in combination with anionic or cationic polyacrylamide has become increasingly popular. The cationic donors, typically polyamine, polyDADMAC, or dicyandiamide, are used to help control and
neutralize anionic trash (Table 1).4 These products have sufficient charge for neutralization but lack molecular weight.
For proper removal of detrimental anionic materials from the machine
circuit, polymers with excess charge are necessary to neutralize these anionic materials. In addition, molecular weight is important for subsequent attachment of these materials to the fiber surfaces.
Products such as polyethyleneimine and modified polyethyleneimine have the highest charge of any cationic
materials currently used. Moreover, their higher molecular weights, combined with their higher charge, often give a unique advantage in addressing anionic "trash."
Traditional approaches with cationic donors have shown their ability to work in conjunction with a polyacrylamide. Polyethyleneimine has shown to be a cationic donor, which not only addresses the charge requirements, but also provides the molecular weight, yielding the ability to further reduce polyacrylamide dosages (Figure 5).5 Pre-treatment or "pre-fixation" of fillers and fines prior to the thin stock loop aids in achieving their random distribution in a sheet.
In addition to better gross overall
retention, it is possible using scanning electron microscopy to see better filler distribution.
MULTI-COMPONENT SYSTEMS. One recent advance for certain paper grades has been multi-component or microparticle retention aid systems. The
primary benefit of this approach is the enhanced drainage attained and from that, the potential for improved formation at good retention levels.
When balanced, these programs can work well, but they have been shown to be quite sensitive to changes in conditions. Charge variation and the dependence on the right amount of shear make these
systems difficult to control. Complexity can also be a problem in that some systems can have as many as four components, such as starch, polyacrylamide, silica or bentonite, and polyamine or polyDADMAC.
With the modified polyethyleneimine and polyacrylamide two-component approach, it is possible to reduce the aggressive nature of the polyacrylamide and use both the charge and molecular weight of the modified polyethyleneimine to achieve similar results
to the more complex microparticle
systems. For example, in a lightweight
free-sheet grade at a basis weight of 25 lb/3,300 ft2 (60% hardwood kraft/25% softwood/15% broke with a filler loading of 20%-a combination of titanium dioxide, precipitated calcium carbonate, and ground calcium carbonate), a
scanning electron micrograph shows equal performance of the PEI/PAM
mixture vs the microparticle system of polyacrylamide, polyamine, starch, and silica.
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