Drainage Chemical Programs

The rate at which water is released from a wet paper web can be increased by chemical treatment. Such chemical treatments are believed to function in one or more of the following ways: (a) by retaining fiber fines onto the surfaces of long fibers so that they are not able to move into positions where they block drainage channels in the wet web; (b) by increasing first-pass retention of fines in general; (c) by causing fibrils at the fiber surface to lie down onto the fiber surface, creating a more streamlined path for water to flow past them; (d) by creating a more bulky, porous structure of the wet paper web; and (e) possibly decreasing the water retention in the fiber walls or in polymeric gels consisting of hemicellulose or additives at the fiber surface.

The most traditional chemical additive used by papermakers for drainage promotion is aluminum sulfate or "papermakers' alum." Alum is usually most effective for dewatering of systems having wet-end pH values in the range 4.5 to 6.5 (acidic to pseudo-neutral). Since alum also has other functions in setting rosin size, in neutralization of excess cationic demand, and in retention, it is best to determine the optimum addition amount on a case-by-case basis.

Drainage also can be increased in most paper furnish types by addition of positively charged (cationic) polymers having a high charge density. Popular chemicals for such use include poly-ethyleneimine (PEI) products, polyamines, and poly-diallyldimethylammonium chloride (DADMAC) of moderate molecular mass. In such applications it is common (but not universal) to find that the maximum in dewatering is achieved near to the point where the net electrical charge on the fiber surfaces has been neutralized by the additive.

In some systems, especially on fast paper machines and low basis weights, drainage rates can be improved by judicious use of very-high-mass acrylamide retention aids (flocculants). However, it can be important to avoid over-use of retention aids, since nonuniform formation may adversely affect vacuum dewatering.

Microparticle (or nanoparticle) programs can be used in cases where a more aggressive approach to dewatering is appropriate. Such systems are provided by various chemical vendors. Most microparticle programs involve the steps of (a) optional adjustment or control of the system charge with high-charge cationic polymers, (b) addition of a high-mass polymer such as cationic starch or an acrylamide retention aid, and (c) final addition of colloidal-sized material such as silica, montmorillonite (bentonite), or organic polymers designed to perform the same functions. Microparticle programs tend to have their best effects when the amount of colloidal material is in a certain ratio to the amount of positively charged polymer used. Charge titrations can be useful for obtaining and maintaining optimum addition rates for dewatering.

