Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
ROSIN SOAP SIZE
Composition: The word "rosin" refers to a series of chemicals isolated from the "tall oil" that is produced during kraft pulping of softwood species. One of the major components of softwoodwood rosin is abietic acid, a partially unsaturated compound with three fused six-membered rings and one carboxyl group. It is common to treat the native rosin with fumaric acid, converting at least some of the abietic acid and related compounds to tricarboxylic species called "fortified rosin." These acids are tacky solids at room temperature. Something has to be done before they can be added effectively to a paper machine. One of these things is to saponify the rosin by addition of base so that it becomes soluble in water. The rosin soap is delivered to the mill as a liquid. It used to be very common to receive rosin soap in the form of a paste, though in the U.S. this product has almost disappeared from the scene.
Function: To resist penetration of aqueous liquids into paper, especially in the case of paper that is prepared in the pH range between about 4 and 4.5.
Strategies for Use: The recommended pH conditions for rosin soap sizing are dictated by the effect of pH on the predominant species of the aluminum ions. Low pH conditions favor the presence of trivalent aluminum, a hydrated form of Al3+. This is the species that appears to be most useful for the retention and "setting" of rosin soap size. The traditional way to accomplish this has been to first mix the rosin soap with the furnish, then add alum. This order of addition is sometimes reversed when the water contains a high level of calcium ions, i.e. water hardness. The reaction product of calcium and rosin soap is more tacky and it is not contribute very much to water resistance. The amount of alum is usually well in excess of the stoichiometric amount needed to react with the rosin. This makes sense when one considers the fact that alum also plays the roles of pH controller, scavenger of excess anionic charge, retention aid, and drainage aid. When the pH is very low (say less than 4), there is increased danger that it builds up in the system. The recirculated alum then has the potential to precipitate rosin soap at its point of addition, before it has become well mixed with the furnish. The resulting rosin-alum precipitate particles then are expected to be too large for the most efficient sizing.
Cautions: Rosin soap does not pose any particular dangers. Consult the MSDS and follow the supplier's recommendations.
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 .