The wetting mechanism of a commercial Alkyl ketene dimers (AKD) wax on smoo
th cellulose films was investigated by following the contact angle of sessi
le drops for differing periods of time ranging from 1 s to 24 h. The advanc
ing and receding contact angles formed by droplets of AKD melt and water ov
er other model surfaces such as glass, cellulose acetate films and against
air AKD-grafted surfaces were also measured. The objectives of the study we
re to elucidate whether or not AKD melt can spontaneously spread on cellulo
se, and to identify the mechanism and driving forces responsible for the dy
namic 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 (t
heta(AE)). In the second, theta(AE) slowly decreases over periods of hours.
The first mechanism is dictated by the balance of the interfacial forces w
ith 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 ob
served on cellulose acetate and glass. A certain surface specificity exists
as its chemical composition determines the amount and the ratio of chemiso
rption/physisorption of AKD vapor present on the surface. The non-spreadabi
lity 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.