Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use

RETENTION AID

Composition: The words "retention aid" most often refer to cationic or anionic acrylamide copolymers having molecular mass values in the range of 5 to 20 million grams per mole. These are known to be effective for improving the retention of fine particles during formation of paper. However, it is worth noting that many additional or alternative materials also can be used. Since some of these additives have to be used in various combinations, one may speak of a "retention aid program." The following sequences are used commonly: (a) a high-charge cationic polymer followed by a very-high mass cationic polymer (usually cPAM or PEI copolymer), (b) a high-charge cationic polymer followed by a very-high-mass anionic polymer (usually aPAM), (c) a very high-mass cationic polymer followed by an anionic microparticle such as colloidal silica, bentonite, or a micropolymer, or (d) options involving polyethylene oxide (PEO).

Function: To maintain adequate efficiency, drainage, and cleanliness of a paper machine; to achieve adequate Z-directional uniformity; to retain additives, including fillers.

Strategies for Use: Retention aids can have a profound effect on the efficiency of the process and on the quality of the product. For instance, an overdose of retention aid is likely to produce a highly flocced sheet that is unacceptable to the customer. So the most general rule is to add only the amount of retention aid that is needed to optimize runnability and avoid excessive problems of two-sidedness of the sheet structure. Relatively high retention is desirable, however, when using ASA size or when using rosin emulsion size under neutral or alkaline pH, since those additives will tend to gradually decompose if they are not retained in the first pass. Additional strategies are mentioned under the headings for several of the additives that are often used in retention aid programs.

Cautions: Consult the MSDS for each additive. Be aware that many different patents exist that can affect one's right to use different combinations of additives.

Data illustrating the importance of retention aids for the achievement of a uniform Z-directional distribution of filler. Note that this work was done on a pilot paper machine. The higher shear of a commercial Fourdrinier machine tends to wash fillers from the wire side of paper.

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, m_hubbe@ncsu.edu .