Plywood is commonly produced from rotary-cut veneers that are cross-laminated after adhesive spreading, then cold-pressed and finally hot-pressed. However, there is a less known way to manufacture plywood without any external adhesives where phenomenon called auto-adhesion is applied. The method is known also as self-bonding or binderless bonding of wood. Auto-adhesion requires higher temperature, pressure and moisture content of the veneers compared to ordinary plywood production. The phenomenon itself is already known and successfully applied since 1930s at hardboard production, yet with a significantly smaller fraction size. During last decade, the topic was studied in Sweden and Japan with several wood species. This method enables plywood manufacture without external adhesives and the process could be fully free from fossil resources. At Aalto University the self-bonding of wood is being studied since 2010 and both Norway spruce (Picea abies, L.) and birch (Betula pendula, L.) have resulted successful adhesion. According to these studies as well as the previous studies completed in Sweden and Japan the self-bonded plywood suffers from delamination when exposed to moist conditions. This problem occurs also in the bond line achieved through linear friction welding which is another bond type representing auto-adhesion. According to the studies completed at Aalto University a certain kind of thermal treatment enhances greatly the bond stability in moist conditions. Most probably this enhancement relates to lessened equilibrium moisture content and increased hydrophobicity as well as improved dimensional stability caused by lessened internal stresses. However, the process is not fully controlled nor understood and it needs improvements in controlling the internal vapour pressure within the lay-up to hinder blow ups as well as optimise the process pressure and temperature as well as the initial moisture content of the veneers.
Keywords: auto-adhesion, binderless bonding, veneer, plywood, self-bonding
Authors
Ruponen J.
Department of Forest Products Department, Aalto University School of Chemical Technology, AALTO
Rautkari L.
Department of Forest Products Department, Aalto University School of Chemical Technology, AALTO
Hughes M.
Department of Forest Products Department, Aalto University School of Chemical Technology, AALTO
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