Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
ALKENYLSUCCINIC ANHYDRIDE (ASA)
Composition: The active ingredient of ASA is an oily monomer. For purposes of papermaking, the most important components of this monomer are a five-membered anhydride ring and a linear chain having between 14 and 20 -CH2- groups (often 18). The reactive ring can be at various positions relative to the chain, and most commercial ASA consists of a mixture of these isomers. The product is almost always delivered as a light amber oil that must be kept very dry until emulsification. ASA is added to the furnish in the form of an aqueous emulsion, in which the stabilizer is usually cationic starch or another cationic, hydrophilic polyelectrolyte. Some suppliers also add a very small amount of surfactant to aid in emulsification. Likely contaminants include linear saturated oils, linear mono-unsaturated oils, and the hydrolysis products of ASA (see below).
Function: ASA is a sizing agent designed to increase resistance to water penetration in the case of paper formed under neutral or alkaline conditions. ASA is especially used in cases where full cure is desired before the size press and where it is important to maintain a high frictional coefficient in the paper product. ASA can improve paper machine runnability and preserve paper's dimensional stability by limiting penetration of size-press solution into the sheet. Holding the size-press starch out nearer to the paper surface also can make the surface-applied additives more effective for such purposes as promoting surface strength, reducing dusting, reducing picking of vessel segments during offset printing, and even improving the performance of hydrophobic polyelectrolytes added with the size-press starch solution. Examples include styrene-maleic anhydride (SMA), styrene acrylates (SA), alkylated urethane copolymers, and certain emulsion products having chemistries related to SMA or SA.
Strategies for Use: The key goals in using ASA are (a) avoid hydrolysis, (b) distribute it well in the furnish, and (c) retain it efficiently. Hydrolysis is minimized by preparing the emulsion as late as possible - usually only seconds before the material is added to the thin stock. The cationic starch solution used in preparing the emulsion may be reduced in pH with the addition of such materials as adipic acid or alum, and it is usually cooled to some degree relative to its cooking temperature. A net ratio of about 3 to 5 parts cationic starch per part of ASA oil usually yields the most efficient sizing. To avoid excessive molecular chain cleavage of this starch, only a part of it is passed through the high shear zone of the emulsifier. Microscopic images (or other methods) can show whether one has achieved the desired narrow size distribution of droplets, usually with an average size near or below one micrometer. The recommended point of addition is after the hydrocyclone cleaners. Although the cationic starch sheath around each ASA droplet has some effect in attaching the size to cellulosic materials, a good retention aid system is needed to achieve a relatively high first-pass retention. Otherwise, a lot of the ASA will follow the white water circuit, giving it time to decompose. Deposit problems usually can be minimized by such practices as limiting the dosage (often to within 2.5 lb/ton in the case of virgin bleached kraft furnish), having alum or PAC present somewhere in the system, turning off the ASA flow during wet breaks, and maintaining good retention.
Cautions: See MSDS.
Two reactions of ASA. One is desirable; the other is not.
For ideas about optimization of ASA droplet size, see ESSAY
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 .