Mechanism of AKD migration studied on glass surfaces

Alkyl ketene dimers are widely used in sizing treatment of paper products. It is known that a good sizing effect can only be developed after the sized paper is cured at elevated temperatures. It is generally accepted that AKD re-distributes on the surface of the fibres during curing. The mechanism that dominates the re-distribution of AKD during curing is not fully understood and contradictory opinions can be found in the literature. There have been speculations in the literature that AKD undergoes a spontaneous ‘flow-like’ spreading during curing, leading to the development of an adequate sizing effect of paper. Other studies have suggested that AKD melt does not undergo a spontaneous spreading on the surface of smooth cellulose films. Some authors stressed that the vaporisation/deposition of AKD also plays a significant role in AKD sizing. The re-distribution of AKD in the curing process of papermaking is complicated and may involve the spreading of AKD melt, which is driven by the surface tension and/or the capillary forces, and the evaporation/re-deposition of AKD vapour. In this work, we examined the mechanism of AKD re-distribution on a smooth hydrophilic surface. By excluding the influence of porosity, this study allows us to focus on the effect of interfacial energetics on the possible spreading of AKD and the development of a sizing effect. Glass was used to provide the model for this study. AFM and XPS were used to monitor the spreading of AKD on the glass surface before and after curing. The Wilhelmy method was used to monitor the development of a sizing effect on the glass surfaces. Our results show that AKD wax does not undergo a macroscopic ‘flow-like’ spreading on the glass surface at a temperature well above the AKD melting point within the time scale of the experiment. A sizing effect developed more rapidly on areas of the glass surface which were covered with AKD than on areas that were initially not covered by AKD, since in the latter case sizing develops purely via exposure of the glass surface to AKD vapour. This implies that the distribution of AKD on the glass surface is likely to be very uneven on a microscopic scale. However, such an uneven distribution of AKD does not affect the development of a good macroscopic sizing effect on the glass surface.