Effect of Thermo-Mechanical Modification on Mechanical Properties and Water Resistance of Plantation-Grown Vietnamese Acacia and Rubberwood

Low density and poor mechanical performance often limit utilization of sawn wood from fast-growing plantation forests. Thermo-mechanical modification of timber (TMTM®) is one innovation for improving the utilization rate of light-weight wood species. The objective of this study was to determine the effects of thermo-mechanical modification and subsequent thermal modification on dry density, modulus of elasticity (MOE), compression strength, Brinell hardness and swelling behavior in immersion test on two fast-grown Vietnamese species, acacia (Acacia mangium) and rubberwood (Hevea brasiliensis). Test boards were modified in an industrial kiln where tangential thickness compression of 14% and 12% were aimed to acacia and rubberwood, either with or without subsequent thermal modification at 190 °C. Dry density, MOE, Brinell hardness, compression strength, and dimensional changes in water immersion of specimens were measured from the modified and reference materials, the latter ones being kiln dried at 50 °C. The results showed that changes in the mechanical properties were more evident for rubberwood than for acacia. In rubberwood, the MOE and compression strength of wood thermo-mechanically modified with or without thermal modification were higher than those of kiln-dried reference specimens throughout the thickness profile. In acacia similar differences between the modified and reference specimens were observed only in the surface layer. Density and Brinell hardness of thermo-mechanically modified rubberwood were higher than those of reference specimens, but after thermal modification they did not differ from (acacia) or were lower (rubberwood) than those of thermo-mechanically modified materials. Subsequent thermal modification increased the water resistance of thermo-mechanically modified specimens