Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Like cellulose, starch is comprised of glucose units linked together by oxygen bridges called glycosides. The main difference is the orientation of the molecules in that linkage (alpha rather than beta). This subtle difference means that starch is more readily soluble in water and more easily digested by bacteria and other living things. Papermakers encounter mainly two forms of starch. The starch added to the wet end is mainly cationic starch. Such starch usually has been derivatized with a quaternary ammonium compound. The degree of substitution is usually about 0.02 to 0.03 on the basis of glucose units (0.2 to 0.35% nitrogen). Because the cationization reaction is carried out with a partially swollen slurry of starch granules, the distribution of cationic groups can be expected to be very nonuniform. Papermakers also may add amphoteric starches that also contain phosphate groups, in addition to the cationic quaternary amine groups. In addition to the wet-end starches, it is very likely that a major part of the starch in a wet-end system comes from recycled broke from the size press, after a coating operation, or from waste paper. Starch from these sources is likely to be uncharged pearl starch or oxidized starch that has been partly degraded to adjust its viscosity to make it suitable for surface applications.
Function: Starch contributes to the stiffness and bonding within a sheet of paper. Cationic starch is added to improve internal bond, tensile strength, and as part of certain retention and drainage programs. It also is widely used for the preparation of sizing agents such as alkenylsuccinic anhydride (ASA).
Strategies for Use: The main thing to bear in mind when using wet-end starch is the fact that each papermaking furnish has a limitation on its ability to retain that starch efficiently. In some cases the critical factor is the surface area of the furnish. In other cases the critical factor is the limited amount of anionic colloidal charge at the fiber surfaces. In yet other cases there is competition between cationic starch and other materials such as wet-strength agents for adsorption sites. As a rule of thumb, papermakers usually are well advised to add about 10 lb/ton of cationic starch and then gradually optimize the system from that point. Addition of colloidal silica downstream of the cationic starch (Eka Nobel technology) can help drainage and retention, and it even can help achieve higher dry-strength. Dewatering aids can sometimes produce a dry-strength improvement because the easier removal of water may permit higher pressures to be applied in compaction of the fibers in the wet press. By contrast, cationic starch by itself is expected to contribute to bonding strength by increasing the effective area of bonding between the fibers.
Cautions: Spilled starch can be slippery. Starch make-down areas also may be subject to bacteria growth if not treated.
PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.
RETURN TO INDEX PAGE OF ENCYCLOPEDIA
This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, firstname.lastname@example.org .