Effect of Method of Preparation of Coating Colours on the Rheological Behaviour and Properties of Coating Layers and Coated Papers, 1995 Coating Fundamentals Symposium Proceedings

Due to the interactions between the components of the coating colour, the manner in which the colour is prepared, i. e. the order in which the different components are mixed, can affect the properties and the structure of the suspensions and, in principle, also those of the coating layer. This has been investigated here in some detail using two conventional types of coating colours based on clay with latex as binder. In one case, carboxymethyl cellulose (CMC) was used as the water-soluble polymer and, in the other, poly(viny1 alcohol) (PVA). For both types of colours the order in which the different components were mixed was varied. The power requirement during the preparation of the colours was also measured. In a sense, this kind of measurement provides a characterization of the preparation procedure. Certain orders of mixing the components resulted in a significant increase in the shear viscosity and/or visco-elastic parameters characterizing the colours. This suggests a flocculation or an aggregation of the coating colour which is here discussed in terms of the interaction between the water-soluble polymer (CMC or PVA) and the clay pigment. Important mechanisms in this context may be the formation of entangled networks and bridging. The different coating colours were drawn down on poly-ester film in order to produce uniform coating layers. It was found that the orders of mixing the components that apparently resulted in a flocculation of the colours also increased the porosity of the coating layers and enhanced their light scattering efficiency. When these colours were used to coat a woodfree paper using a high-speed labora-tory coater, a relatively marked increase in the gloss level (after calendering) was noted. This may be due to a higher porosity of the coating layer resulting in a more defor-mable coating structure which in turn promotes the gloss level after calendering. However, the positive effects of changing the preparation procedure gave in this case only a rather limited improvement in the optical properties (brightness and opacity) of the coated paper, both in the uncalendered and in the calendered state. The benefits of modifying the preparation procedure were thus to some extent probably counteracted by or less important than other processes taking place during the coating of the paper.