Physical-mechanical properties and bonding quality of heat treated poplar (I-214 clone) and ceiba plywood

The present paper investigates the physical and mechanical modifications of both poplar (I-214 clone) and ceiba veneers and plywood after heat treatments of different intensities (5 and 7 % of dry mass losses). Plywood panels were glued before and after heat treatment with urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) resins. In order to assess the effects of treatments on both the wood and the glues, the dry mass, the density, the bending strength, the Young's modulus and the bonding quality were measured before and after heat treatment. Different treatments were compared as well. Results showed that the loss in cell wall polymers due to the heat treatment caused a significant reduction of the equilibrium moisture content of samples. From a mechanical point of view, the treatment resulted in an important reduction of strength and in a small reduction of stiffness. Bonding quality and mechanical properties were widely affected by the heat treatment. The different intensities of treatments applied (the treatment range was up to 5% and 7% of dry mass loss) did not show significant differences for most of the features assessed. The mechanical performance and the bonding quality of treated samples suggested that veneers should be glued after heat treatment. Apparent cohesive wood failure revealed that different degradations affect wood and glues with a prominent effect on glues for UF resins and a prominent effect on the wood for MUF resins.