Mechanical and physical properties of thermally modified plywood and oriented strand board panels

Thermal modification can increase resistance to biological degradation, reduce equilibrium moisture content, and improve the dimensional stability of solid wood. In this study, oriented strand board (OSB) and two types of plywood were thermally modified as a posttreatment at 140°C, 150°C, 160°C, 170°C, and 180°C. Plywood moduli of rupture (MOR) and elasticity (MOE) decreased up to 54 and 22 percent, respectively, at the 180°C treatment, while OSB MOR and MOE decreased up to 25 and 4.3 percent, respectively. Internal bond strength of plywood decreased with increasing temperature, while OSB experienced minimal change. Screw-holding strength of all panels was more adversely affected by increasing temperatures than nail-holding strength, with OSB experiencing 17 and 27 percent maximum reductions in nail- and screw-holding strengths, respectively. Thickness swell performance of all panels improved with increasing temperature, with plywood exhibiting 41 and 77 percent improvements at the 160°C and 180°C treatments, respectively. Mass increase (when subjected to a water soak) of OSB decreased 12 percent at the 150°C treatment, after which it increased. These results suggest that thermal modification posttreatments can improve the thickness swell and water absorption performance of plywood and OSB panels. However, some mechanical properties decreased significantly at treatment temperatures exceeding 160°C. The results provide a technical baseline that may help advance thermal modification technology from primarily solid-wood-only applications toward new, high-volume engineered wood markets. With further research, it may be possible to optimize the treatment technique(s) to ensure that the panels retain sufficient mechanical strength for the desired end-use applications.