Mechanism of internal sizing by alkyl ketene dimers (AKD): The role of thespreading monolayer precursor and autophobicity

Alkyl ketene dimers (AKD) only partially wet cellulose and silica surfaces. Further spreading of AKD molecules is shown to occur by surface diffusion in the form of an autophobic monolayer precursor, which grows from the foot of the AKD drop/particle. The surface area covered by the monolayer grows linearly with time. The rate of the precursor growth is proportional to the ambient temperature and decreases with increasing AKD melting point. Overa ll, the monolayer diffusion process is shown to be quite slow. The apparent surface diffusion coefficient is of the order 10-(11) m(2)/s. The effect of the monolayer spreading can he seen in the time evolution of the hydrophobicity during storage of AKD sized hand-sheets. There are clear indications that the relatively slow monolayer spreading process plays a k ey role for the build-up of paper hydrophobicity. The rate of this process is, for instance, observed to be compatible with the AKD monolayer spreadin g rates. The temperature dependence of the monolayer spreading and the buil d-up of sheet hydrophobicity are also similar. After long storage times, th e build-up of hydrophobicity of the hand-sheets is expected to level off du e to the fact that the AKD reservoirs in the form of surface retended AKD p articles are limited. This predicted behaviour was observed in the present study. The sizing efficiency was indeed found to revert after prolonged sto rage. This de-sizing effect is attributed to desorption and vapour phase tr ansfer of surface spread sizing agents.