A new method to determine the water retention of coating colours and its impact on mottling of coated papers, 2010 TAPPI Advanced Coating Fundamentals Symposium

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According to Darcy’s law the control of liquid phase viscosity of coating formulations is of utmost importance to reduce penetration of water and fines from coating colours into the base paper in blade and film press coating processes. Therefore a lab method was set up to measure liquid viscosity.

Viscosity of two phase systems like paper coating formulations is a combination of liquid phase viscosity and flocculation effects, the latter being caused by interaction of ionic components with the hydration layer of pigment particles. As the contribution of these two mechanisms to increase coating colour viscosity is currently not well known, only a poor correlation was found between the low shear viscosity of the coating colour and its liquid phase viscosity. Static water retention measurements also correlated poorly with liquid phase viscosity; better correlations where achieved with “fast” measurements of coating colour dewatering as e.g. the KCL Clara device. Different thickeners were tested in coating formulations using latex and/or starch as binder. Types of thickeners were identified which provide high liquid phase viscosity and at the same time a low high shear viscosity of the coating colour which is always limited at high speed blade coaters.

Mill trials with selected thickeners to raise the liquid phase viscosity of the topcoat formulation proved the lab results and led to a significant improvement of mottling.