Thermal studies of ink solvent and toner behaviour on coated paper: modelled in various printing methods using ink-coating component mixtures and laboratory scale print tests

This study is divided into three parts, each of which concerns the fixing of a given "ink" type on coated or uncoated paper substrate, namely, the adhesion of electrophotographic toner, setting and drying of offset inks and drying of water-based inkjet inks analysed with thermal methods. In the first part, dealing with electrophotography, thermal fusing is the final stage in the printing process, which determines largely the print quality properties such as fixing strength, gloss and density of the image. Melting starts from the surface of the toner particle and toner particles start to cohere together and subsequently the melted toner film adheres to the paper surface. The thermal properties of toner are a compromise between being soft enough to support the fusion, but hard enough not to be fused onto the photoreceptor within the development system or to the adjacent sheet (blocking) in storage. The results of this study show adhesion depends on viscosity and melting temperature of the toner. In the second part, setting and drying of offset inks and thermal behaviour of model ink and paper coating colour compound mixtures were studied. The thermal study reveals how certain model compounds, representing in this case ink and typical coating components, interact in increasingly complex mixtures. Thermogravimetric analysis (TgA) and differential scanning calorimetry (DSC) are techniques used in the analysis. To visualise the thermal data, a combination of the TgA and DSC measurements into a single comprehensible observation was achieved using a representation specially devised for the specific case of material evaporation expressed in relation to the respective observed specific evaporation energy, whereby the specific gravimetric change (TgA) and specific energy (DSC) ratios are calculated. A technique was also devised to differentiate between the effects of the extensional "length" of an offset ink and its film split tack behaviour, related to the blanket-paper ink film separation on a press. To study this phenomenon, two separation measurements were used: (a) that of constant force (Printack), and thus varying acceleration, giving sensitivity biased toward ink length, and (b) that of constant extensional strain rate (Hencky strain rate), giving sensitivity biased toward ink-on-paper tack and viscosity (ISIT). The third part of this study targets the identification of interactions between the components of water-based inkjet printing ink and those of the paper coating colour, and specifically their effects on print quality. The methods used to study these interactions are once again thermogravimetry (TgA) and differential scanning calorimetry (DSC).