Formulation Optimisation for Improved Runnability of High Speed Pigmented Coatings on the Metered Size Press, 1997 Coating Conference Proceedings

The mechanism of sizepress application of pigmented coating colours is known to depend strongly on speed and coat weight in respect of coating pick-up, film split and misting. Promotion of filter-cake formation is now widely accepted as a means of reducing the wet film thickness during contact with the basepaper. However, the effect of formulating to achieve this immobilisation alone can lead to further problems such as dry build-up on the application roll and/or misting from the remaining wet film. The factors controlling runnability are shown to derive from a recognition that coating colours are multiphase dispersions. That these dispersions may undergo visco-elastic deformation at high rates of change of applied strain has led to the hypothesis of a novel rheological phenomenon defined as ‘relaxation-induced dilatancy’, (Gane, Tappi Fundamentals Symposium 1997). The necessary conditions for this proposed phenomenon to arise can occur if the dynamic pressure pulse leads to partial up-concentration during visco-elastic deformation at the application nip. Up-concentration may arise due to pulse filtration of the colour or inertial separation of solid from the fluid medium during extension at the nip exit. It is suggested that the subsequent elastic recovery results in a transient dilatant structure within the fluid-depleted portion of the colour, extending over the short-timescale relaxation of the elastic solid-solid, solid-liquid interactions. The right viscosity characteristics are, therefore, crucial. Most commonly used viscosifyng water retention agents tend to generate additional interactional v&o-elasticity by structuring the coating colour and do not act positively to prevent misting Results show that it is necessary to adopt a formulation and pigment strategy which partially &couples the elastic and viscous components of the colour rheology. This can be achieved by an approach which develops a higher degree of purely viscous flow together with controlled dewatering, thereby creating newly optimised conditions especially suitable for blocky, isometric pigments, such as natural ground calcium carbonate and the relatively low aspect ratio glossing clays. This has been achieved through pigment viscosity modification, control of particle size as well as by innovation in formulation design. Keywords: Runnability, roll-coating, misting, deposits, spitting film-split, patterning sizepress, metering, rheology.