Alkyl ketene dimer (AKD) sizing of paper under simplified treatment conditions

Alkyl ketene dimer (AKD) has been widely used by manufacturers of paper and paperboard as a hydrophobic sizing agent. Ordinary sizing with AKD involves a complex series of processes, including emulsification of the waxy AKD material; measures to avoid the agglomeration of the emulsified AKD particles; addition of a stabilized AKD dispersion to papermaking furnish; interactions with various retention aid chemicals to fix the material onto solid surfaces; and various spreading and curing processes that take place during the drying and cooling of the paper product. In the present work, as a means to gain insight into the mechanisms attributable only to the AKD in isolation from the other additives and subprocesses, the AKD wax was dissolved in heptane and applied to filter paper between two aluminum foil layers, followed by evaporation of the solvent and optional heating. Surprisingly, hydrophobic character was obtained regardless of whether or not the treated sheets had been heat cured. Also, for the first time, it was observed that the AKD treatment resulted in a substantial increase in sheet strength, suggesting that the AKD was able to serve as the matrix in an AKD-saturated paper structure. The results add support to past suggestions in the literature that potential covalent interactions cannot account for all of the effects attributable to AKD treatment of paper.