Ian Parker

Contact angle measurement and surface energetics of sized and unsized paper

Abstract Determining the surface free energy of paper from contact angle data for various liquids is not straight forward and the results can sometimes be misleading. For insufficiently sized paper, errors may arise from rapid penetration of liquid probes into the sheet, which prevents accurate measurement of contact angles. For well-sized paper, errors may arise from factors associated with the roughness of the sheet. Despite the fact that some of the sources of error are well known, contact angle methods are still favoured for determining the surface free energy of paper. In this study, we focus on some factors associated with the topographical and chemical heterogeneity of paper (particularly sized sheets) that may cause contact angle data to lead to incorrect prediction of surface energetics. Whatman filter paper with and without calendering and AKD sizing treatment was used as a model system for studying the effect of sheet surface heterogeneity. To evaluate the sheet surface roughness at an interfibre pore level, the contact area between a water drop and the sized sheet surface was studied using confocal laser-scanning microscopy (CLSM). For well-sized sheets the actual contact area between the drop and the sheet was much smaller than the apparent contact area. The chemical heterogeneity of the sized filter paper sheets was studied using XPS and IGC. Both techniques again showed that the surfaces of well sized papers still possess acid and base polarities that are capable of interacting with liquid or vapour probes with acid and base functionality. Poor liquid–sheet contact area, however, acts as a factor that limits the number of acid–base adducts formed across the liquid–sheet interface. A possible way of correcting for the effect of sheet surface roughness on the apparent liquid–sheet contact angle using the Cassie–Baxter equation was tested.

Mechanism of AKD migration studied on glass surfaces

Abstract 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.