Spreading of AKD and ASA on wet paper during the curing phase of paper sizing

Internal sizing is the process through which paper is provided with a controlled level of hydrophobicity to control wetting and penetration of aqueous fluids. The most common hydrophobic (or sizing) agents used for internal sizing are alkyl ketene dimer (AKD) and alkenyl succinic acid anhydride (ASA). It has been proposed that AKD and ASA molecules spread on a water film covering the fibre surface via "film expansion", which was believed to be very fast.. We report in this study on the development of a new method to quantify the spreading velocity of sizing agents on a water surface covering cellulose fibres in wet paper samples. The emphasis of this new method is on differentiating the surface spreading, where the driving force is the film pressure, from the chromatographic effect, where the driving force comes from a mobile phase. This study found that the spreading of sizing agents in this situation cannot be described as film expansion. Our results suggest that sizing agent films on a water surface are uneven and that there are also strong molecular interactions between the sizing agent molecules and the substrate covered by the water.