Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: The term "dry strength additives" refers to a variety of water-soluble polyelectrolytes. The term "dry strength resin" is most often used in connection with anionic copolymers of acrylamide. Acrylamide polymers, including amphoteric products (with both anionic and cationic groups), are commonly used in Japan. Synthetic dry-strength agents tend to have molecular mass values below one million grams per mole. (Note that polyacrylamide flocculants used as retention aids often have much higher masses and this allows them to bridge between particles in suspension.) In the US it is far more common in North America for papermakers to use cationic starch. Other derivatives of natural products that are useful as dry-strength agents include carboxymethyl cellulose and guar gum derivatives.
Function: To increase the relative bonded area or strength per unit of bonded area between the fibers in a sheet of paper. Properties most likely to be favorably affected include internal bond strength and tensile strength.
Strategies for Use: Some keys to achieving the best performance from dry-strength additives include (a) getting it to retain onto the long-fiber portion of the furnish, (b) adding it to the thin stock (after the fan pump) so that it doesn't have time to disappear into pores in the fiber cell walls, (c) avoiding conditions that tend to hurt the uniformity of formation, and (d) achieving a near-neutral colloidal charge. What is remarkable about this list is how many of the items appear to conflict with each other. For example, one might try to achieve item (a) by adding the polyelectrolyte to the thick stock, but such practices are in conflict with item (b) where one tries to minimize the time of contact with the furnish. Likewise, adding polyelectrolytes very late in the process (item b) is likely to increase the level of fiber flocculation, a conflict with item (c). Charge neutralization (item d) is clearly an excellent way to maximize retention of dry-strength additives but not all charge neutralization strategies will get the dry strength agent where it is most needed (item a). The costs and benefits of dry-strength additives always should be compared with optimization of refining conditions. Often the chemical approach will make it possible to achieve strength goals while maintaining somewhat higher dewatering rates and lower density of the paper.
Cautions: Consult MSDS information.
|Effect of pH on the retention of anionic acrylamide copolymer dry-strength resin, sizing, and alum retention|
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.
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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, firstname.lastname@example.org .