An approach to the dry-state preparation of coating particles for use in dry surface treatment of paper

Abstract

Desired savings in energy, raw materials and processing in the manufacture of coated papers may lead to the development of processes with reduced usage of water. One proposed process, the dry surface treatment (DST) for pigment-coating of paper has been studied in a laboratory scale. The treatment involves the application of dry, powdery coating materials on the paper surface and fixation of the deposited layer in a thermomechanical compression and smoothening phase in a heated roll nip. Electrostatic deposition of charged particles enables a non-contact coating application. In the previous studies, the DST-papers produced with two or multi-component powdery materials, that were formed by drying coating suspensions, exhibited coating coverage, surface strength and smoothness similar to commercial coated mechanical reference papers at equal coat weights, but a generally larger pore size of the dry-formed coatings gave higher levels of oil absorption, air permeability and somewhat irregular ink setting properties. This study aims to explore the possibilities to prepare the DST coating particles in a dry state. The eventual target is to obtain coating particles in the size range used in conventional pigment coating and adjust the binder properties, content and distribution in a favourable way with regard to the structure of the coating layer. Synthesis of particulate polymers under water-free conditions in supercritical carbon dioxide is described and performed. Hybrid coating particles, consisting of the pigment and binder together, were prepared and evaluated for their structure and the coating applicability.