Plastic Coating of Plywood Using Extrusion Technique

The target was to study and develop industrially suitable methods to apply thermoplastic coatings on plywood. Typically, wear and climatic resistance for wooden sheets, such as chip board and plywood, is achieved by thermosetting resin coating (e.g. phenolformaldehyde). However, economical and environmental issues are driving towards thermoplastic coatings (e.g. polyethylene) in such products. Several technologies, such as film gluing and laminating, have been introduced, but the costs and processing are limiting factors. This study concentrates on direct coating of plywood with extruded molten plastic film, in other words extrusion coating of plywood. This technique provides fast coating process and cheap raw materials. In the study it was found that extrusion coating of plywood is difficult. The high heat energy of the molten plastic causes moisture evaporation of the top veneer, and the only possible escape direction of the evaporated moisture is through the plastic, which causes bubbling in the plastic coating. Another disadvantage of the plywood is its rigidity, which limits the development of the process. In this work, a new technique was applied for coating the flexible veneers with typical extrusion coating equipment, where the escape of the evaporated moisture was possible through the top veneer to the atmosphere. Another main issue was to reach sufficient adhesion between plastic and plywood. Target was achieved using either monolayer polyethylene (PE-LD) or 2-layer polypropylene/adhesion polymer plastic coatings.