Mini-Encyclopedia of Papermaking Wet-End Chemistry

CATIONIC STARCH

Composition: The repeating unit of starch is glucose, having a carbohydrate monomer composition of C6H12O6. In the polymer each unit has three -OH groups, and the units are linked together with flexible alpha-1-4 glycocidic bonds. Cationic starch is produced by treating a slurry of partially swollen granules of starch with a reactive compound. An example of such a reagant is epoxypropyltrimethylammonium chloride. This reagant contains a quaternary nitrogen, yielding a positive charge that is independent of pH. The reagent usually attaches to the starch at the C6 position, the most accessible of the -OH groups. The typical level of derivatization is one to two charged groups per hundred glucose units. Because the reaction is usually carried out in a slurry, it is expected that the distribution of charged groups will be highly non-uniform. Also, there is reason to believe that treatment of potato starch or dent corn starch will result in preferential cationization of the linear amylose chains. The branched amylopectin chains of starch tend to be more crystalline in the solid starch granule, and therefore less accessible to treatments. Cationic starch is usually delivered in a dry powder form (10 to 20% moisture content).
Functions: Dry strength, emulsification of sizing agents, part of many retention and drainage programs
Strategies for Use: In the US cationic starch is the most popular dry-strength additive. About half the people who have an opinion will tell you that higher strength can be achieved by adding the starch to the thick-stock. The other half (of those who will tell you what they think) will advocate addition to the thin stock, i.e. later in the process. In either case you have to be a bit careful with the dosage. Depending on the furnish, the maximum practical amount of cationic starch may be between 10 and 30 lb/ton. The problem with adding too much is that it will exceed the adsorption capacity of the surface, based on either the surface area or the limited extent of negative charge of the surfaces of fibers and other solid surfaces in the furnish. Excess starch beyond what adheres to the fibers in likely to cause foam, high biological oxygen demand (BOD) levels in the effluent, and poor retention and drainage. The performance of cationic starch as a strength agent sometimes can be improved by raising the pH; this will tend to make the fibers slightly more anionic and better capable of interacting with the starch. If the furnish contains a very high level of anionic trash, then the performance of cationic starch as a strength agent can be improved by preteating the furnish with a highly charged cationic material such as alum, PAC, or polyamines, etc. Another strategy is to use a combination of cationic starch (first additive) and a microparticle such as colloidal silica or bentonite (second additive). Patents in these areas are held by Eka Chemicals, Nalco, and Ciba Specialty Chemicals, among others.
Cautions: Spills of cationic starch can be slippery. They should be cleaned up promptly with warm or hot water