Effect of heat treatment on some physical and mechanical properties of birch plywood

The plywood consumption in various applications is increasing, especially in construction. In service, plywood can be exposed to a range of outdoor conditions, sometimes at elevated temperatures. This requires an understanding of the structural behavior and knowledge of the effects of the elevated temperatures on the properties of plywood. Therefore, the objective of this research was to study the effect of exposure time on physical and mechanical properties of plywood at elevated temperatures. The plywood samples were tested after exposure to nine different temperatures (50 °C, 75 °C, 100 °C, 125 °C, 150 °C, 175 °C, 200 °C, 225 °C and 250 °C) and ten exposure time levels (0.5, 1, 1.5, 2, 2.5, 3, 6, 8, 10 and 12 h) at each temperature. Additionally, a set of control samples was tested at room temperature. The findings of this study indicated that exposure of plywood panels to elevated temperature caused significant degradation of their bending and bonding strengths, whereas the hydrophobicity of the panels improved. Three stages of plywood degradation could be distinguished. Below 175 °C, there is a slow decline in strength; the decisive change already occurs at 175 °C; but as temperature increases beyond 175 °C, a rapid decrease in strength is observed. Plywood samples lost 98.1% of their initial strength after 1.5 h of exposure at 250 °C, and lost 98.5% after 6 h of exposure at a temperature of 225 °C. Statistical regression-based models were also developed and evaluated for predicting the strength loss of plywood panels as a function of weight loss. As the weight losses of panels increased, the losses in bending and bonding strength increased too. The decrease in bending and bonding strength only becomes significant when the weight loss exceeds 5%. When the weight losses increased above 15%, plywood samples almost lost their strength.