Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Though the word "polyacrylamide" is more accurately applied in the case of the homopolymer, papermakers tend to use the term when talking about the most popular varieties of cationic and anionic retention aids. These are actually copolymers of acrylamide (main component) and charged co-monomers (usually minor component). Acrylamide products used as retention aids typically have molecular masses in the range 5 to 20 million grams per mole. Acrylamide products used as dry-strength agents typically have molecular masses in the hundreds of thousands. The cationic copolymers usually have no more than 5% of cationic groups due to FDA limits for paper products that may contact food. The anionic copolymers also can be prepared by an alternative route, partial hydrolysis of the amide groups after synthesis of the polyacrylamide homopolymer. Retention aids are commonly produced and delivered to the mill site in the form of a water-in-oil emulsion. The emulsion needs to be inverted with good agitation and a dilution ratio of approximately 100 to 1. Dry-strength agents often are delivered as aqueous solutions that can be diluted with non-specialized equipment.
Function: Retention aids, dry-strength agents.
Strategies for Use: In their role as retention aids, acrylamide copolymers are added very late to the process, typically either right before or after the pressure screens. The pressure screens always degrade the molecular mass, hurting efficiency of retaining fine particles during paper formation, but the paper is more uniform. In their role as dry-strength agents the most critical aspect of acrylamide copolymer use is to balance the colloidal charge. Typically an oppositely charged material has to be added to the furnish either before or after the strength agent. For control of pitch it is often recommended to treat the offending stream of thick-stock with just enough polymer so that filtrate from that stream has a low turbidity. Micro-polymers (see also microparticles) usually are added last to the process, after a very-high-mass retention aid of opposite charge. One of the most critical factors to the success of a microparticle program is that the furnish should have the same net colloidal charge as the retention aid at that point (usually by adding a charge scavenger).
Cautions: These materials can be very slippery when spilled. The emulsions of very-high-mass retention aids are particularly hazardous in this regard. Spills should be first wiped up with dry absorbent (think "cat litter").
|Critical step in make-down of retention aid solution in the mill from a water-in-oil emulsion|
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, email@example.com .