The Mechanism And Effect On The Properties Of Heat-treated Okan Wood

Wood is a kind of superior reproducible biological material. Whereas it has some effect factors of its utilization. There are many physical and chemical methods to improve the properties of wood in this time. However these methods will do harm to our healthy, because of its chemical substance. The heat treatment technology is used in this study, which is pollution-free, enviornment-friendly and non-chemical. The test material is okan (Cylicodiscus spp))wood, which is the imported tree species in recent years. The effect and mechnism of heat treatment on the properties of okan wood was studied in this thesis. The optimal technology of okan wood heat treatment was determined, the dimensional stability of okan wood was enhanced, the color and the color stability of okan wood was improved. The mechanical strength loss of wood was controlled in allowable range. This study favors the extended application and the improved adding values of okan wood, furthermore it has great significance to deepen the wood heat treatment research, extend the using field of the heat treated wood, and add the contents of wood science.The full factorial method was used to design each trial unit in this research. There are two groups of okan heartwood and sapwood, which size is 500mm(longitudinal direction)×125mm (radial direction)×25mm(tangential direction), were heat treated at temperatures of 160℃, 180℃, 200℃, 220℃, for duration of 2h, 4h, 6h, 8h, in a high tempetature drying machine, with its lower than 2 percent of oxygen. The effect and mechanism of heat treatment on the properties of okan wood were studied, through analyzing the colour, physical property, mechanical property, and color stability of heat-treated wood. The variance analysis method was used to analyze the physical and mechanical properties of heat-treated okan wood. The optimal technology was determined based on the effects of temperature, duration and the interaction of these two factors to the properties of okan wood. Seven ways were used to explore the mechnism of the property changes of okan wood, which are FTIR, thermal analysis, X-ray diffraction, photoelectron energy spectrum, CP/MAS 13C-NMR, Ultra-visible spectrum, GC-MS respectively. The samples for mechnism analysis are okan woodparticle and the extractioves of okan wood powder.The main research results are summarized as follows:1. Heat treatment can significantly reduce the moisture content and water absorption of okan wood. The equilibrium moisture content of control heartwood is 10.39%, the equilibrium moisture content range of heat-treated heartwood is 4.36%~8.06%, the moisture excluding efficiency is 22.43%~58.04%. The equilibrium moisture content of control sapwood is 10.57%, the equilibrium moisture content range of heat-treated sapwood is 4.53%~8.38%, the moisture excluding efficiency is 20.72%~57.14%. The water absorption rate of control heartwood is 51.43%, the water absorption rate of heat-treated heartwood is 38.98%~50.09%, the maximum decrease rate is 24.21%. The water absorption rate of control sapwood is 54.95%, the water absorption rate of heat-treated sapwood is 39.09%~52.85%, the maximum decrease rate is 28.86%.2. The dimensional stability of okan wood is substantially improved through heat treatment. The air-dried volum shrinkage ratio of heartwood is 10.93%, that of heat-treated heartwood is 6.18%~10.55%, the maximum air-dried volumetric anti-shrinkage efficiency of heat-treated heartwood is 43.66%. The air-dried volum shrinkage ratio of sapwood is 11.23%, that of heat-treated sapwood is 6.78%~11.01%, the maximum air-dried volumetric anti-shrinkage efficiency of heat-treated sapwood is 39.63%. The oven-dried volume shrinkage ratio of heartwood is 14.35%, that of heat-treated heartwood is 9.31%~13.94%, the maximum oven-dried volumetric anti-shrinkage efficiency of heat-treated heartwood is 35.67%. The oven-dried volume shrinkage ratio of sapwood is 14.69%, that of heat-treated sapwood is 9.88%~14.06%, the maximum oven-dried volumetric anti-shrinkage efficiency of heat-treated sapwood is 39.19%. The air-dried volum swelling ratio of heartwood is 11.05%, that of heat treatment heartwood is 6.37%~10.98%, the maximum air-dried volumetric anti-swelling efficiency of heat-treated heartwood is 42.35%. The air-dried volum swelling ratio of sapwood is 11.27%, that of heat treatment sapwood is 6.86%~10.96%, the maximum air-dried volumetric anti-swelling efficiency of heat-treated sapwood is 39.13%. The water saturated volum swelling ratio of heartwood is 13.53%, that of heat-treated heartwood is 9.56%~13.06%, the maximum water saturated volumetric anti-swelling efficiency of heat-treated heartwood is 40.35%. The water saturated volum swelling ratio of sapwood is 14.28%, that of heat-treated heartwood is 9.74%~13.86%, the maximum water saturated volumetric anti-swelling efficiency of heat-treated sapwood is 40.17%.3. Heat treatment darkens the color of okan wood. Compared with the same sample, the color variance of heat-treated heartwood and sapwood is as follows: the L* value reduced, the a* increased at first and then decreased, b* reduced, C* reduced, Ag* reduce slightly,△E increased. The heartwood total color variance△E is 5.51~35.52, the sapwood total color variance△E is 10.59~39.04. The color of heat-treated heartwood and sapwood became more and more dark, compared to the color of the sample before heat treatment with the temperature went up from 160℃to 220℃and the duration extended from 2h to 8h. The color variance of the sapwood is more obvious than that of heartwood.4. Heat treatment can increase the color stability of okan wood. The total color variance of the same sample is as follows after 100 hours xenon irradiation: The△E of heartwood is 3.15~20.15, The△E of heat-treated heartwood is 0.97~15.58. The△E of heat-treated heartwood is smaller than that of the heartwood, which indicates the color stability of heat-treated heartwood is better than that of the heartwood. The△E of sapwood is 5.6~23.79, The△E of heat-treated sapwood is 2.28~14.45. The△E of heat-treated sapwood is smaller than that of the sapwood, which indicates the color stability of heat-treated sapwood is better than that of the sapwood.5. Heat treatment reduces the oven-dried density of okan wood. The oven-dried density of okan heartwood and sapwood decreased slightly after heat treatment. The oven-dried density of control heartwood is 0.867g/cm~3, the oven-dried density range of heat-treated heartwood 0.865g/cm~3~0.765g/cm~3. The decrease range is 0.23%~11.76%. The oven-dried density of control sapwood is 0.872g/cm~3, the d oven-dried ensity range of heat-treated sapwood 0.868g/cm~3~0.775g/cm~3. The decrease range is 0.46%~11.12%.6. Heat treatment can reduce the strength of okan wood. Low temperature and short duration heat treatment had no significant effect on the strength of wood. The strength of wood reduced obviously, with the temperature(160℃~ 220℃)increasing and the duration(2h~8h) prolonging. The strength decrease rate of heat-treated heartwood is 7.14%~48.21%. The strength decrease rate of heat-treated sapwood is 10.33%~42.42%. Low temperature(160℃)and short duration heat treatment can increase the modulus of elasticity of wood. The modulus of elasticity of wood reduced, with the temperature increasing and the duration prolonging. The modulus of elasticity of the heartwood is 17.1 GPa, the change ratio is -6.43%~5.26%. The modulus of elasticity of the sapwood is 16.85 GPa, the change ratio is -11.04.43%~9.61%. The hardness of heat-treated wood is increased at first and then reduced. The surface hardness of the heartwood is 0.36KN/mm2, the maximum hardness increase rate of heartwood is 8.33%, the maximum decrease rate of heartwood is 25%. The surface hardness of the sapwood is 0.31KN/mm2, the maximum hardness increase rate of sapwood is 12.9%, the maximum decrease rate of sapwood is 16.13%.7. The mechanism of the improved dimensional stability of heat-treated okan wood. The infrared spectrum analysis shows that the feature absorption peak of hemi-cellulose fades, hemi-cellulose degrades. X-ray diffraction experiments shows that the relative degree of crystallinity of cellulose increases by 14.16%~26.02%. Thermal gravimetric and thermal differential analysis shows that the pyrolysis graph of heat-treated wood hystersis that of control samples, hemi-cellulose decomposes in heat treatment progress. Photoeletron energy spectrum analysis shows that the carbon-carbon bond increases, hydroxyl group, acetyl group, and ether bond group diminishes, the hemi-cellulose pyrolysis, lignin decomposes and condensates. The CP/AMS13C-NMR analysis shows that the feature absoption peak of hemi-cellulose drops off, hemi-cellulose decomposes, the hydroxyl group in cellulose reduces, the degree of crystallinity of cellulose increases. These changes improves the dimensional stability of heat-treated okan wood.8. Photoelectron energy spectrum and the CP/AMS13C-NMR analysis shows that lignin decomposes and polymerizes, which darkens the color of okan wood. The ultraviolet spectrum analysis of ethanol extractioves shows that lignin decomposes and condensates, the conjugated system prolongs, color radicals increases, the absorption peak in visible light intensifies, which leads to the color of wood darken. The GC-MS analysis of acetone extractives shows that there are multi-phenols in extractives, the substance including color radicals conjugated to aromatic nucleus increases, furthermore, there are much low molecular substance after heat treatment, which increases the extractives content and darkens the wood color. The content of water soluble extractives decreases, the content of organic solvent extracrives increases in heat-treated wood.9. The mechanism of reduced strength of heat-treated okan wood. The infrared spectrum analysis shows that the feature absorption peak of hemi-cellulose fades, the feature absorption peak of lignin changes, hemi-cellulose degrades, lignin decomposes and polymerizes. Theramal analysis shows that hemi-cellulose and cellulose pyrolyzes. Photoelectron energy spectrum ananlysis shows that the hydroxyl, acetyl and ether bond group diminishes, regerated carbonyl group increases, hemi-cellulose pyrolysis and lignin decomposes and polymerizes. The CP/AMS13C-NMR analysis shows that the feature absoption peak of hemi-cellulose drops off, hemi-cellulose decomposes, the feature absoption peak of lignin changes, lignin decomposes and polymerizes. These changes reduces the strength of heat-treated okan wood.