Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
ALKYLKETENE DIMER (AKD)
Composition: This alkaline sizing agent is synthesized from fatty acids. The most common form is a waxy solid material dispersed as small particles in a solution that contains a stabilizer. The stabilizer may be cationic starch or another cationic polyelectrolyte. Recently the Hercules Corp. has patented the use of unsaturated fatty acids to make a liquid form of AKD. Though slightly less efficient as a sizing agent than the waxy form, the unsaturated product does not make the paper as slippery (see below). AKD is much less reactive than ASA, and there is no consensus between researchers as to how much of it ever forms covalent ester bonds with cellulose or starch when paper is dried.
Function: Hydrophobization of paper, especially when made under alkaline conditions. AKD is widely used for liquid containers, ink-jet printing papers, and many other grades of paper and paperboard. AKD is especially favored for products that need to resist water over a long period of time.
Strategies for Use: Because AKD is received at the paper mill as a ready-made, milky emulsion, it can be a very convenient product to use. Also, the lower reactivity of AKD, compared to ASA, means that the papermaker has more flexibility on where to add it. For example, many users of AKD add it to the thick stock; this practice tends to get the AKD to the fiber surfaces. In contrast, adding a sizing agent to the diluted furnish in the thin stock loop can be expected to concentrate more of the size onto the fines fraction. The presence of precipitated calcium carbonate (PCC) in the system can reduce the effectiveness of the sizing agent and also it may cause the sizing to lose some of its effect during storage (sizing reversion). Recent work reported by Bud Brungart of Hercules showed that this effect can be minimized by adding the PCC early in the process. The mechanism by which early addition of the PCC minimizes sizing reversion is not known, but it possibly involves coverage of the PCC by dissolved polymers and colloids from the pulp, dissipation of the surface alkalinity of the PCC, or increasing the bicarbonate content of the process water. Paper made with high levels of AKD is likely to be slippery, and it may cause problems in precision cutting and register during conversion, or in stacking during high-speed xerographic copying. These effects can be minimized by limiting the dosage (perhaps supplementing the sizing effect with surface hydrophobes added to the size-press starch), or by use of alkenylketene dimer (unsaturated form) in place of the more usual AKD.
Cautions: AKD hydrolyzes slowly during storage. Manufacturers instructions should be followed regarding the temperature and time of storage. Also consult the MSDS.
|Possible reactions of AKD sizing agent. In addition, some unreacted AKD is likely to remain in the paper, at least initially.|
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 .