Polymer Coating of Paper Using Dry Surface Treatment - Coating Structure and Performance, 2003 European PLACE Conference Proceedings

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The conditions and time scale of coating materials with the dry surface treatment (DST) differ significantly from the ones applied in conventional extrusion, solvent-based and dispersion coating. In DST, the coating and surface finishing are combined in steps of applying layers of dry coating particles onto a substrate, thereafter densifying and smoothening the surface layer during a thermomechanical fixing phase. First, an electric field is used to direct charged coating particles onto the surface, where the deposited layer attaches mainly electrostatically. Secondly, during the heating and compression phase, the polymer layer should exhibit sufficient flowability, deformability and affinity to the substrate surface in order to provide an adequate and homogeneous coating. The dwell time in the heated roll nip can be adjusted to give the polymeric particles sufficient time to sinter and homogenise throughout the layer by diffusion mechanisms. The layering, softening and immobilisation of the coating should be accomplished within milliseconds, where governing factors for example are particle size, thermal conductivity within the layer, and other polymer characteristics such as softening temperature and rheological behaviour.

At low coat weights, a crucial requirement for obtaining a uniform and smooth coating with a sufficient covering ability is an even distribution of particles of a fine particle size, since large particles require longer time to flow sufficiently and level out. The heat treatment is ideally used for controlling the rheological behaviour of the polymer in order to achieve an adequate film formation and allow polymer penetration into the porous surface of the paper and thereby anchor the coating layer mechanically to the substrate and seal it. We studied the effects of the thermomechanical properties of the polymer and fixation conditions on the formed structure and performance of the coatings prepared in a laboratory scale dry surface treatment unit. The results of these experiments are presented and discussed. A main conclusion from the work is that DST appears to be a promising technique for producing coated structures.