Challenges set for coating powders applied to dry surface treatment of paper

Powder-formed coating materials applicable for dry surface treatment (DST) of paper and board have been under study the recent few years. DST involves the application of the powdery coating components onto the paper surface by electrostatic deposition and thermomechanical smoothening and fixation of the deposited layer in a heated roll nip. Two types of coating materials have been considered; pigment coatings and polymer coatings.

Drying of conventional pigment-latex slurries to powdery materials was primarily used in order to compare the effects of the treatments on the coating layer. The performance of DST-papers was studied in relation to the structure of the coating particles and to the viscoelastic properties of the polymeric binders. Particle aggregation was recognized to be a dominant factor influencing the structure and hence the characteristics of the DST-papers. To avoid the drying-related aggregation, formation of the binder polymer in water-free conditions was targeted. Within this approach, the polymerization of binder particles in the presence of the pigments in supercritical carbon dioxide was examined and found in principle to be a practical route for the manufacture of dry-processed hybrid particles.

For the polymer coating, the experimental grades of polyethylene were obtained directly from the polymerization reactor and ground to finer sizes. A fine particle size and sufficiently high melt flow improved the sealing of the paper surface and thus the performance of polymer coated DST-papers against their extrusion-coated counterparts.