Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: This polyelectrolyte is formed by the reaction between adipic acid and diethylene triamine, and subsequent derivatization of the resulting copolymer with epichlorohydrin. Readers with a strong background in chemistry will recognize that the initial polymerization during the production of PAAE creates a chain very similar to that of Nylon. What's different is that the chain contains secondary amine groups. Some of these react with the epichlorohydrin, and some of them remain in the product, helping to make it highly cationic and soluble under the conditions of use. The reaction with epichlorohydrin creates highly reactive aminochlorohydrin and azetidinium chloride (ring) groups along the chain. These groups appear to be capable of reacting with either the carboxyl groups at a cellulose surface, or amine groups on adjacent molecules of the additive during drying of paper. The groups added to the molecular backbone by reaction with epichlorohydrin are also highly cationic.
Function: Wet-strength agent especially well suited to neutral and alkaline papermaking; also sometimes used in formulation of cationic rosin size emulsions
Strategies for Use: The first consideration is shelf life. Storage for too long or too warm will cause PAAE to react with itself sufficiently that it looses its activity. Another thing to recognize from the beginning is that long exposure to hot papermaking furnish will reduce the performance of PAAE resin as a wet-strength agent. These factors would suggest that the best addition point would be close to the headbox of the paper machine. This would be true except for one other consideration; strength agents often are most effective when they are present on the fibers, rather than on fines. Since fines have surface areas at least three times that of fibers (on a mass basis), the fines are likely to adsorb a disproportionate amount of the additive. This effect is further amplified if one adds the resin in the thin stock - the part of the paper machine system where the relative concentration of fines in the furnish is at its highest. For that reason, strength resins often are added to the thick stock just before it is diluted at a fan pump. The performance of PAAE resin is strongly affected be the anionicity of the fiber surfaces. For this reason it can be easier to develop high levels of wet strength when using high-yield fibers. Another strategy to increase the anionic character of the fibers is to add the PAAE under alkaline papermaking conditions where the carboxyl groups on the fibers are mostly dissociated.
Cautions: Epichlorohydrin monomer may be hazardous, and suppliers of PAAE products have been able to reduce its presence to very low levels. Consult the MSDS.
|Structure of an azetidinium group, a source of cationic charge and reactivity on PAAE wet-strength resins|
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 .