POLYMER COATING OF PAPER USING DRY SURFACE TREATMENT - COATING STRUCTURE AND PERFORMANCE

摘要

The conditions and time scale of coating materials with the dry surface treatment (DST) differ significantly from theones applied in conventional extrusion, solvent-based and dispersion coating. In DST, the coating and surfacefinishing are combined in steps of applying layers of dry coating particles onto a substrate, thereafter densifying andsmoothening the surface layer during a thermomechanical fixing phase. First, an electric field is used to directcharged 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 andaffinity to the substrate surface in order to provide an adequate and homogeneous coating. The dwell time in theheated roll nip can be adjusted to give the polymeric particles sufficient time to sinter and homogenise throughoutthe layer by diffusion mechanisms. The layering, softening and immobilisation of the coating should beaccomplished within milliseconds, where governing factors for example are particle size, thermal conductivitywithin 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 coveringability is an even distribution of particles of a fine particle size, since large particles require longer time to flowsufficiently and level out. The heat treatment is ideally used for controlling the rheological behaviour of the polymerin order to achieve an adequate film formation and allow polymer penetration into the porous surface of the paperand thereby anchor the coating layer mechanically to the substrate and seal it. We studied the effects of thethermomechanical properties of the polymer and fixation conditions on the formed structure and performance of thecoatings prepared in a laboratory scale dry surface treatment unit. The results of these experiments are presented anddiscussed. A main conclusion from the work is that DST appears to be a promising technique for producing coatedstructures.