Enhancing the Properties of Thermoplastic-Bonded Plywood by Treating the Birch Veneers with Citric Acid

Formaldehyde-free plywood can be successfully manufactured using thermoplastic polymers as adhesives. However, such polymers are characterised by low interfacial adhesion to wood. Citric acid was sprayed on to the surface of the wood veneer to improve its interfacial adhesion to low-density polyethylene film. Birch wood veneers, and virgin (LDPE) and recycled (rLDPE) low-density polyethylene film as adhesives were used to manufacture plywood panels. The wood veneer surface was sprayed with 5 %, 15 % and 25 % citric acid at a dosage of 50, 100 and 150 g/m2 before the bonding process. The citric acid treatment was proven to enhance the properties of plywood measured. It was found that the values of water absorption and thickness swelling of plywood decreased (improved), while shear and bending strength increased. The concentration of the citric acid solution significantly affected the properties of plywood, while its dosage did not. Plywood bonded with virgin LDPE film exhibited better properties than that bonded with recycled rLDPE film.