Properties And Control Theory For Strength Loss Of Steam Heat-treated Wood

Steam heat treatment has been known for long time as one of a number of effective methods to improve the dimensional stability and durability of wood. The main effect obtained by the heat-treatment of wood is a reduced wood hygroscopicity. Foremost advantages of wood treated in this manner are the wood's increased resistance to different types of biodegradation and an improved dimensional stability. Additionally, the treatment leads to a darkening of the wood color which can also be a beneficial aspect of the treatment.Unfortunately, undesired side effects, in particular the loss of strength and increased brittleness of the heat-treated wood, have prevented the commercial utilization of thermal modification. In order to expand the industrial applicability of heat-treated wood, it is important to study the reasons for the adverse effects and devise methods to control mechanical strength loss and the property changes of heat-treated wood during the heating process.The Random Complete Block Design was developed to arrange each experiment unit in this paper. Three wood specimens of China-Fir heartwood, sapwood and Chinese White Poplar were heat treated at temperatures from 170℃to 230℃for times from 1h to 5hrs in airtight equipment with an atmosphere within comprising less than 2 per cent of oxygen content. Saturated steam was used as a heating medium and a shielding gas. The properties and mechanical strength of the heat-treated wood were analyzed by statistical methods of variance and multiple regression analysis. Mathematical regression models relating to the treatment temperature, treatment time and the properties of the heat-treated wood were established, along with mathematical regression models comparing loss ratio of bending strength or modulus of elasticity and other properties of the heat-treated wood.A method of controlling mechanical strength loss was devised based on results of experiments in this study to provide a mechanism for, and justification of, increasing the utilization of heat-treated wood so as to make better use of the plantation forests of China and other countries where similar problems with timber use may occur.The main research results are summarized as follows:1. Temperature and time are two crucial factors affecting the final quality of heat-treated wood, with temperature having a greater effect than time. Analysis shows 200℃and 2hrs are the critical temperature and time factors for the heat-treated process in this study. When the temperature is below 200℃factors such as the absolute-dry density, the content of holocellulose andα-cellulose, the shrinking ratio and swelling ratio of volumes, wood color were slowly decreased, while the weight loss ratio, decay resistance, hardness, bending strength and modulus of elasticity were slowly increased. However, when the temperature is over 200℃, except for decay resistance which is still increased, the all above mentioned other properties are decreased rapidly.2. Fourier Transform Infrared spectroscopy indicates the intensity of the characteristic absorption peak of holocellulose and hemicelluloses become weaker and weaker with the increase of temperature and lengthening time of the heat treatment. However, the density of the characteristic absorption peak of lignin becomes stronger and stronger. It indicates that the heat treatment decreases the contents of holocellulose andα-cellulose respectively, while enhancing the content of wood lignin. In this study, the loss ratios of holocellulose,α-cellulose and the increased ratio of lignin in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 2.87%～21.40%, 0.33%～35.30%, 0.53%～22.63% 2.91%～22.71%,0.34%～50.32%, 0.47%～37.09%;2.52%～23.72%,0.94%～41.44%, 9.06%～123.64%. There is a highly significant difference at the 0.01 level between the chemical components content of the wood and temperature with time. It is likely that the thermal degradation leads to a decrease in the content of holocellulose andα-cellulose respectively, while the condensation reaction leads to an increase in the lignin content of the heat-treated wood.3. Lignin level is the main factor relating to wood color, with the increase of lignin content of the wood being the essential reason for the treatment making the wood color become darker and darker. The color of heat-treated wood becomes brown and puce with the increasing of temperature and time during the heating process. In this study, the range of variance value ofâ–³C*,â–³E*,â–³H* in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 3.67～-7.73, 6.61～43.46, 0.60～6.02;4.46～-8.31, 11.18～57.49, 1.76～7.11;6.87～-5.14, 13.98～62.00, 3.63～10.46. There is a significant difference at the 0.01 level between the color of the wood and the temperature and time respectively. This data indicates that the heat treatment changes the color of wood significantly and that the desired timber color will be able to be obtained by setting relative and selected temperatures and times in manufacturing operations.4. The thermal degradation of holocellulose andα-cellulose and the disappearance of inorganic and volatile materials are the most likely reason for the decrease of absolute-dry density of the heat-treated wood. In this study, the loss ratios of densities in China-Fir heartwood, sapwood and Chinese White Poplar respectively, are 0.62%～15.07%,0.77%～15.80%,0.52%～13.63%。There is a significant difference at the 0.01 level between the absolute-dry density of the heat-treated wood and the temperature and time respectively.5. Heat treatment enhances the dimensional stability of heat-treated wood significantly. The dimensional stability was improved step by step with the increase of temperature and time during the heat treatment process. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 72.63%, 67.21 % and 70.71 %. There is a significant difference between the volume change of the heat-treated wood and the temperature and time respectively. Heat treatment reduces large numbers of carbonyl while apparently generating significant amounts of new hydrophobic materials, resulting in the hygroscopic property of the wood being reduced substantially, thereby significantly improving the wood's dimensional stability.6. The hardness of treated wood is increased by heat-treating at around 200℃for about 3 hours. It is possible that the water in the amorphous region in the wood disappears causing the production of the new hydrogen bonds between fibres of wood, further improving the hardness of the heat-treated wood. In this study, the highest improving ratios of China-Fir heartwood, sapwood and Chinese White Poplar respectively are 12.67% at 200℃for 1hr, 26.82% at 200℃for 2hrs and 15.82% at 200℃for 3hrs. When temperatures are above 200℃, thermal degradation becomes the main effect during the heat-treatment so that the hardness of the heat-treated wood is decreased rapidly. The loss ratio of the three heat-treated wood species, at 230℃for 5hrs, are 26.07%, 24.36% and 22.09% respectively. There is a highly significant difference at the 0.01 level between hardness of wood and temperature with time.7. Heat treatment enhances the decay resistance of Chinese White Poplar fromâ…£Grade toâ… Grade. The untreated and the heat-treated Chinese White Poplar respectively belong to â…£Grade with a weight loss ratio of 55.746% andâ… Grade with a weight loss ratio of 2.052% at 230℃for 5hrs. The nutrient materials such as polysaccharide and the inorganic materials and the like in the wood that generally provide food for bacteria were likely eliminated so that the bacteria have not enough food to survive. Therefore, the decay resistance of heat-treated wood is improved remarkably. The decay resistance of China-Fir heartwood and sapwood are bothâ… Grade, whether heat-treated or not heat-treated. The aim of the heat-treated of China-Fir is to improve the other properties such as dimensional stability.8. The bending strength and the modulus of elasticity of the China-Fir sapwood and the Chinese White Poplar are improved with treatment below 200℃for around 2hrs. This is likely caused because of the disappearance of the water of the amorphous region in the wood resulting in the production of new hydrogen bonds between fibers of wood, with the lower temperatures not causing thermal degradation in the wood. The highest improving ratios of the bending strength and the modulus of elasticity respectively are 6.38% and 2.84% for the China-Fir sapwood, with 11.28% and 15.80% for the Chinese White Poplar. When the temperature is above 200℃, the thermal degradation of cellulose and hemicellulose is the main reaction in which the fiber chains become shorter and shorter along with the increase of temperature and time during the heat-treated process. Therefore, the mechanical properties of the heat-treated wood are decreased. The loss ratios of the bending strength and the modulus of elasticity of the China-Fir heartwood, sapwood and Chinese White Poplar at 230℃for 5hrs respectively are 49.39% and 21.93%, 49.72% and 22.42%, 54.20% and -2.73%. There is a highly significant difference at the 0.01 level between the bending strength and the modulus of elasticity of the wood and the temperature and time respectively.9. In this study, y means loss ratio of bending strength of heat-treated wood, x₁ means treatment temperature, x₂ means treatment time, so the regression models between bending strength and x₁with x₂ of the China-Fir heartwood, the sapwood and the Chinese White Poplar respectively, are y = 0.558x₁+2.806x₂-99.975 (R²=0.943),y = 0.693x₁+5.566x₂-137.897 (R²=0.909),y =0.961x₁+4.218x₂-183.832 (R²=0.953). If y means the modulus of elasticity, the regression models between the modulus of elasticity and x₁with x₂ of above the three species wood respectively, are y = 0.247x₁+1.235x₂-44.865 (R²=0.874),y = 0.222x₁+3.512x₂-47.676 (R2=0.927),y =0.089x₁+1.544x₂-32.172 (R²=0.777). Using above mathematical regression models, the loss ratios of the bending strength and the modulus of elasticity of heat-treated wood will be predicted under different treatment temperatures and times respectively.10. The best method of controlling the loss of the mechanical strength of the wood is by the controlling of the temperature. Based on the results from the above three wood species experiments the temperature should be controlled at below 200℃so that there is no loss or a minimum loss in the mechanical strength of the wood, with an enhancement of the hardness, bending strength and modulus of elasticity properties of the heat-treated wood within a short time. With the correlative mathematic regression models, the optimum temperature and time used in the heat treatment can be determined to accord with the final use of the heat-treated wood.