The effect of high temperature superheated steam drying on the mechanical and physical properties of rubberwood

Abstract

The effects of superheated steam treatment on the physical and mechanical properties of rubberwood (Hevea brasiliensis) were investigated. The wood boards were conventional pre-dried from a manufacturer to an average 10+2% moisture content, prior to the superheated steam treatments in this study. The treatments were carried out at three different superheated steam temperatures (140, 150 and 160°C) and three different times (1, 2 and 3 h) under atmospheric pressure and in the presence of air. The results indicated that the mechanical properties were improved as the percentage increase relative to the control samples of each condition. The greatest values for both hardness and compression parallel-to-grain were found in the samples treated at 160°C for 3 h and 140°C for 1 h respectively. In addition, impact strength and tensile strength perpendicular-to-grain of each treatment condition were increased but there are no significant difference (p<0.05) between treated and untreated samples. The results of adsorption properties show decreased equilibrium moisture content for all heat-treated cases throughout the hygroscopic range. The Hailwood-Horrobin model was used to analyze the sorption isotherms and determine monolayer and polylayer moisture contents for untreated and heat-treated rubberwood. The monolayer moisture content clearly decreased with treatment temperature and duration, whereas the reduction in polylayer moisture was relatively smaller. Moreover, the least density of water adsorption sites was found in wood after treatment at 160°C for 3 h, indicating this as the cause for reduced equilibrium adsorption. At higher temperatures and for a long time, the treated samples became darker as indicated by a decrease of brightness (L*), and the greatest total color changes (AE*) were observed in samples after treatment at 160°C for 3 h. The standard methods of ASTM D-1413 and ASTM D3345-74 were used to evaluate the decay resistance from white-rot decay fungus and subterranean termite of rubberwood samples, respectively. Results revealed that superheated steam treatment significant increases the resistance of rubberwood samples. Treated sample at 160°C for 3 h yielded the best performance with the lowest weight loss on both fungal and termite test. Furthermore, FTIR spectroscopy was used to investigate the change in cell-wall components. It was found that high superheated steam temperature caused the number of accessible hydroxyl groups to decrease and the relative cellulose crystallinity to increase. Also the SEM micrographs of wood surface showed that the starch particles decreased consistently with increasing in treatment temperature.