Analysis of the effect of porosity and pigment shape on the stiffness of coating layers

Abstract

The elastic modulus of paper coating layers made of mineral pigments and containing 9% by weight of a synthetic binder was measured at 23°C. The mineral pigments used comprised clay, ground natural calcium carbonate and different forms of precipitated CaCO₃ (aragonite, rhombic and scalenohedral). The pigments used differed in shape from isometric to plate-like and needle-like particles. From the measured values of the modulus, the corresponding values of void-free coatings were evaluated using a model proposed by Bert which also accounts for the effect of stress concentrations due to the presence of the voids. The calculated modulus values of the void-free coatings were compared with predictions from composite theory, e.g. the Halpin-Tsai equations. In general, a reasonable agreement was noted indicating that paper coatings of this type can be regarded as composite materials and can be analysed with the methods used within that field.