Coating layer formation in dry surface treatment of paper substrates.

APPLICATION: This report provides insights into the experimental technique of dry coating and its possible applications.

Dry surface paper coating can potentially eliminate drying after coating and produce a final surface quality equal to that achieved with dry surface treatment process. In the process, powdered coating, exposed to a corona charge and an electric field, is directed and guided onto paper. In a subsequent step, the dry coating powder is anchored to the paper surface by thermomechanical treatment in a nip formed between heated rolls, forming the final coating layer.

In dry surface treatment, coating is applied without wetting the paper substrate, thus eliminating the possibility for fiber swelling and surface roughening. Coating penetration into the paper is minimal, as a distinct boundary forms between the coating layer and paper. Sufficient cohesion and adhesion to the paper surface are needed to obtain conventional coating processes. This work reports on recent experiments involving pigmented coating of paper substrates with a an appropriate coating layer. The greatest challenge is to reduce surface openness and create a structure similar to that obtained with conventional coating methods.

In this study, we investigated factors affecting coating structure. The focus was on the influence of the applied pressure and temperature on fixation of coating powder onto paper. We achieved adequate surface strength and were able to improve paper surface properties. View this paper online at http://www.tappi.org/index.asp?pid=29257

Juha Maijala and Putkisto are with Tampere University of Technology, Paper Machine Automation, Korkeakoulunkatu 3, 33720 Tampere, Finland: Gron is with Metso Paper Technology, Wartsilankatu 100, 04400 Jarvenpaa, Finland. Email Maijala at [e-mail], Putkisto at [e-mail], or Gron at [e-mail].