Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
ROSIN EMULSION 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 softwood 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." Rosin 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 form an emulsion. The supplier of the rosin accomplishes this by heating the rosin above its melting point and applying hydrodynamic shear as it is poured into a hot aqueous solution in the presence of a stabilizer. The pH may be adjusted so that at least a small amount of rosin on the surface of the droplets is saponified to the carboxylate form. That solution may contain casein or various cationic polyelectrolytes as stabilizers to help the emulsion from coalescing into larger particles.
Function: To resist penetration of aqueous liquids into paper
Strategies for Use: It is most common to use rosin emulsions under mildly acidic conditions, e.g. between pH value of about 4.5 and 6. These are conditions that favor very high retention of alum or PAC on the fibers. Rosin emulsions are most often used in conjunction with a "reverse addition" strategy for alum and rosin. In other words, the soluble aluminum-containing product is added first. This order favors complexation of any dissociated carboxyl groups with aluminum species, an interaction that is favorable to sizing. At the same time, this practice tends to minimize the opportunity for the rosin to react with calcium. Usually it is recommended to have the addition points relatively close in time, but with good mixing of the furnish between the two addition points. There can be exceptions when attempting to reach very high pH values; in such cases it is sometimes recommended to pre-mix the rosin emulsion and a solution of PAC or alum with specialized equipment before it encounters the furnish. High first-pass retention is recommended to minimize the gradual conversion of the free-acid form of rosin to its dissociated (saponified) state, which is more stable under the conditions of use (pH > 4.5). It is worth noting that somewhat different usage strategies apply when one is using rosin soap as a sizing agent. It is also worth noting that the material from rosin emulsion droplets mostly does not interact with alum until the paper is dried. Despite this, it is still important to use an aluminum source (alum or PAC) to help retain the rosin emulsion and so that the rosin molecules anchor and orient themselves on the surface of the fibers when they encounter the high temperatures of the dryers.
Cautions: Rosin emulsion 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 .