Visualisation, Modelling and Image Analysis of Coated Paper Microstructure: Particle Shape - Microstructure Interrelations

Most physical and functional properties of pigment coated papers are influenced by coating layer structure at a microscopic level. For example, number and size of micro voids contribute to macroscopic properties such as brightness,light scattering efficiency, opacity and the printability of paper. In this paper high resolution Focused Ion Beam (FIB)sectioning and imaging is combined with a novel advanced image analysis technique, to produce new information about the microscopic properties of paper coatings. These observations are compared with computer-generated numerical model structures.Distinctly different coating structures were created using different morphology coating pigments in an attempt to interrelate the particle shapes and the corresponding microscopic pore structures. Three paper coatings were prepared containing a narrow particle size distribution ground calciumcarbonate (GCC), a precipitated calciumcarbonate (PCC) and kaolin respectively as the sole pigments. After being applied to a woodfree basepaper, the coatings were characterised using mercury porosimetry, reflectometry and standard paper tests. The pore structure was then where possible filled with a varnish and the pore space within the coating layer imaged by means of a FIB sectioning technique.An image analysis technique called "Maximal Inscribed Sphere" (MIS) algorithm was used to characterise the experimental 2-D sections. The technique partitions a void space of porous material into an unambiguously defined collection of individual pores and throats connecting them. The results show differences in the pore structure as created by the different pigment particle geometries. A semi-quantitative agreement with the numerical model structures was observed.The approach produces useful new information concerning the microscopic properties of paper coatings including distributions of pore size, pore surface area, pore connectivity, pore surface-to-volume ratio, throat-to-surface area ratios well as fractal structural parameters.This fundamental knowledge can be used to improve our mechanistic understanding of the link between pigments,coating structure and paper/ printing performance.