Properties Of Pressurized-steam-treated Wood-A Theoretical Research

Research on wood heat treatment is currently active in the field of wood physical modification. By means of heating wood up to around 200℃in inert atmosphere, significant improvement on dimensional stability and biological durability can be obtained. Besides, when used, heat-treated wood will not release any toxic substance to the environment, making it an environmentally friendly product compared to chemically modified wood.In this paper, emphasis is put on the comparison of physical, mechanical and machining properties of heat-treated wood treated in atmospheric steam and pressurized steam, respectively. Chemical and instrument analysis have been performed to explore the underlying reasons for the property difference. Samples of Mongolian pine and Mongolian oak were chosen for the experiments, and results as follows were obtained:At the same treatment temperature, pressurized steam exerted more influence on properties of wood than atmospheric steam. Samples treated in pressurized steam were inferior to those treated in atmospheric steam when mechanical properties, such as Modulus of Elasticity (MOE), Modulus of Rupture (MOR), compressive strength parallel to grain, dynamic fracture toughness and hardness, are considered. However, most of these differences were not statistically significant. Pressurized-steam-treated samples performed better in experiments on hygroscopicity and dimensional stability. Lower equilibrium moisture content (EMC) and less swelling in both radial and tangential directions were found compared to atmospheric-steam-treated samples. Machining experiments showed that, when samples treated in pressurized steam were milled, temperature of the blade was lower. The roughness of the processed surface of both kinds of heat-treated samples was statistically the same. However, samples treated in pressurized steam intended to emit more fine sawdust, asking for better dust collecting system and respiratory protection device.Research on the change of wood structure was performed through chemical and instrument analysis. It was found that the main chemical components of samples treated in different pressure experienced different reaction and degradation. Hemicelluloses of samples treated in pressurized steam degraded more compared to samples treated in atmospheric steam. Meanwhile, the portion of lignin and extractives in the former was higher than in the latter. Those above were supported by the outcome of Fourier transform infrared spectroscopy (FTIR) analysis. Furthermore, the structure of lignin was found changed after heat treatment, although the exact pattern of the change needed further exploration. From the results of chemical and instrument analysis, we concluded that the lower strength of pressurized-steam-treated samples was attributed to more intense degradation of its main chemical components, especially the carbohydrates. Such degradation also decreased the number of free hydroxyl groups in the wood, making it absorb less moisture. The lower strength and hygroscopicity, in turn, were responsible for less energy consumption and finer dust emission of pressurized-steam-treated wood during machining process.Conclusions mentioned above showed that the benefit of pressurized steam heat treatment lied in better dimensional stability, less energy consumption during machining process and guaranteed surface quality of its products. Although pressurized steam made the strength of heat-treated wood even lower, in most cases, it was not statistically significant. What should be pointed out here is that, by means of thermogravimetry (TG)-FTIR analysis, we found that the degradation of wood components was limited if the treatment temperature was lower than 200℃. What's more, factors such as less hygroscopicity of wood and higher crystallinity of cellulose made positive contribution to the strength of wood. That's why some strength values like MOE and compressive strength parallel to grain of heat-treated samples were higher than the control ones. So, it can be concluded that pressurized-steam-treated wood has the potential of full scale application.