On the refractive index contrast modification in paper and printing applications : studies with fillers and coating pigments

In papermaking, fillers are traditionally added to the fibrous stock to extend expensive fibre, to improve the paper surface uniformity enabling beneficial liquid interactions during printing, and to improve the light scattering coefficient of paper. It is advantageous, therefore, to introduce as much filler as possible. However, fillers also have negative effects on paper properties, such as weakening the paper by preventing the formation of fibre-fibre bonds, which limit their use. Accordingly, fillers must be optimised to achieve the best functionality at controlled dose. One objective of the work is to improve paper optical properties, which could later promote more efficient use of materials in papermaking. Light scattering potential of fillers can be controlled by changes in the particle size distribution (PSD), specific surface area (SSA), shape, surface chemistry and refractive index (RI). Refractive index has received the least attention of these approaches. Therefore, novel fillers displaying increased effective refractive indices were developed. The results suggest that the methods studied can improve the light scattering effects. Since there are difficulties deconvoluting the boundary refractive index contrast from particle size in complex paper composites, which were highlighted in the first part, a second objective of the work was to study the complementary situation of reducing RI contrast by introducing liquid into a porous paper coating. The liquid distribution was then monitored by reflectance measurements. Knowledge of absorption behaviour of liquids into paper is essential for the print quality and controllability of printing processes. A relationship could be established, normalised for porosity, describing the reflectance change as a function of saturation level. In conclusion, the local change of refractive index boundary in a porous medium can be monitored indirectly via light scattering, or vice versa modified to affect scattering, and the opportunities for enhancing the function of filler-containing porous media can be identified. The combination of the publications and complementary results bound together in this work suggest that the methods studied provide potentially important new means to utilise RI contrast modification in papermaking and printing applications