Wetting mechanism of alkyl ketene dimers on cellulose films

The wetting mechanism of a commercial Alkyl ketene dimers (AKD) wax on smooth cellulose films was investigated by following the contact angle of sessile drops for differing periods of time ranging from 1 s to 24 h. The advancing and receding contact angles formed by droplets of AKD melt and water over other model surfaces such as glass, cellulose acetate films and against air AKD-grafted surfaces were also measured. The objectives of the study were to elucidate whether or not AKD melt can spontaneously spread on cellulose, and to identify the mechanism and driving forces responsible for the dynamic wetting behavior. When an AKD droplet is deposited on a cellulose surface, the contact angle at the three-phase line follows two sequential kinetics. In the first, the contact angle rapidly decreases to an apparent equilibrium contact angle (theta(AE)). In the second, theta(AE) slowly decreases over periods of hours. The first mechanism is dictated by the balance of the interfacial forces with the viscous forces. The second is caused by an equilibrium shift driven by the hydrolysis of AKD vapor molecules physisorbed on the cellulose film. AKD spreading on cellulose was never observed. Similar AKD wetting was observed on cellulose acetate and glass. A certain surface specificity exists as its chemical composition determines the amount and the ratio of chemisorption/physisorption of AKD vapor present on the surface. The non-spreadability of AKD on cellulose raises serious doubts about the generally assumed mechanism of internal paper sizing. AKD wetting during papermaking is also analyzed. (C) 1998 Elsevier Science B.V. All rights reserved.