Relaxation-Induced Dilatancy in Separable Visco-Elastic Suspensions: Preposing a Novel Rheological Phenomenon, 1997 Advanced Coating Fundamentals Symposium Proceedings

The limitations of runnability during the application of pigmented coating colours onto a porous substrate, such as paper, manifest themselves as defects or non-uniformities in the coating. These may arise from a variety of causes, of which, film splitting and misting, the build-up of dry deposits on applicator systems (e.g. on metered sizepress rolls and at blade tips), scratching of the coating,, or the inability to control coat weight at the desired level, are some examples. Classical rheological analyses, assuming the coating colour to be an homogeneous continuum, fail to account for many of these phenomena. Recognition that coating colours are multiphase dispersions which undergo viscoelastic deformation at high rates of change of applied strain, has led to the hypothesis of a novel rheological phenomenon defined as ‘relaxation-induced dilatancy 1 . The necessary condition for this proposed phenomenon to arise occurs when the application of a pressure pulse leads to partial separation of the fluid medium from the suspended solids during viscoelastic deformation - for example, pulse filtration of a coating colour on paper under a high speed, high pressure application nip. It is suggested that the subsequent elastic recovery results in a transient dilatant structure within the fluid-depleted colour, extending over the short-timescale relaxation of the elastic solids interactions. Results of trials, based on this hypothesis, indicate that formulation development for the future of high speed pigmented coating requires decoupling of the elastic and viscous components of the colour rheology. Energy loss may then be enhanced through more purely viscous systems in order to control blade bleeding, misting,, coat weight profiling etc., whilst water retention under applied pressure pulses should be derived without the traditional allied rise in the elastic component to avoid those problems, such as dry deposits, dusting, dry stalagmites, spits and scratches, which, it is suggested, form as a result of the proposed transient, relaxation-induced, dilatant structures.