Spreading Mechanisms of Sizing Agents (AKD and ASA) on Thin Water Film
Internal sizing is the process through which paper is given a controlled level of hydrophobicity to control wetting and penetration of aqueous fluids into the paper. The hydrophobic agents mostly used for internal sizing are alkyl ketene dimmer (AKD) and alkenyl succinic acid anhydride (ASA). Several mechanisms are involved in the distribution of reactive sizing. One of them is the spreading on water film over the fibre surface.
AKD and ASA molecules are believed to spread very fast on the water film covering the wet fibre. This mechanism was linked with surfactant “freely spreading” over water surface at a rate of 15 to 30 cm/s. However, this assumption is incorrect since spreading of surfactant molecules on water surface is not a two dimensional process, but a three dimensional process involving a 200 micrometers thick water layer. The water film on cellulose fibre surface at the beginning of the drying process has a thickness of only several micrometers.
We use a Wilhelmy balance with a precise amount of sizing agent deposited on a paper strip which is touching the water. When the water rises via capillarity, it touches the sizing agent. The spreading of the sizing agent over the very thin water film proceeds and is recorded. The Wilhelmy balance will detect the reduction of the surface tension when the spreading front reaches the free water surface. The spreading rate can then be quantified. The results already showed that the spreading rate is much slower than 15 to 30 cm/s.