Physical And Mechanical Properties And Thermal Degradation Characteristics Of Heat-treated Chinese Fir

Chinese fir wood has a wide distribution range and excellent processing performance.However,there are disadvantages such as poor dimensional stability,heat treatment can improve the dimensional stability and durability of wood.The influence of heat treatment on the quality loss and physical and mechanical properties of Chinese fir needs to be quantitatively studied.In this study,the plantation Chinese fir was used as the research object,under nitrogen atmosphere,different heat treatment temperature(160 °C,180 °C,200 °C,220 °C,240 °C)and heat treatment time(1 h,2 h,3 h,4 h)were selected for heat treatment.To study the thermal decomposition of the main chemical components(cellulose,hemicellulose,lignin)during the heat treatment.The main conclusions obtained are as follows:(1)Under the heat treatment conditions of this study,with the increase of heat treatment temperature and time,the equilibrium moisture content,water absorption and total dry density of Chinese fir all decreased,the color of the wood surface gradually darkens,the mass loss rate gradually increased,it is from 0.25% to 11.1%.An empirical prediction model for the mass loss and the heat treatment temperature and time is established.(2)The main mechanical properties of heat-treated fir showed a downward trend with the increase of heat treatment temperature and time.Its static bending strength,elastic modulus,along-grain tensile strength,along-grain tensile elastic modulus and the surface hardness value decreased by5.34%?59.84%,1.83%?21.3%,9.57%?71.75%,6.57%?30.62% and 0.88%?29.7%,respectively.Established an empirical prediction model of heat treatment temperature and time and mechanical properties,as well as a model of the relationship between mass loss and static bending strength,and the overall prediction accuracy is high.(3)During the pyrolysis of fir,cellulose and lignin showed relatively good thermal stability,while the thermal stability of hemicellulose was poor,and it has the fastest degradation rate,which is the main degradation component.Compared with untreated wood,with the increase of heat treatment temperature and time,the relative content of holocellulose decreased from 69.94% to 61.04%;the relative content of cellulose was between 43% and 46%,but the degree of crystallinity increased from4.28% to 12.4%;the relative content of hemicellulose decreased from 24.51% to 12.75%,and the relative content of lignin increased from 35.64% to 39.44%.Infrared analysis indicated that water-absorbing groups such as hydroxyl groups reduced after heat treatment,improved the dimensional stability;SEM observed that the cell wall structure was compressed and deformed,causing cracks and pits to be destroyed.(4)Cellulose,hemicellulose and lignin were separated from fir by chemical method,and the activation energy and the pre-finger factor were calculated based on the thermogravimetric analysis(TG)data.Based on the Arrhenius equation,established a prediction model for the thermal decomposition of Chinese fir,cellulose,hemicellulose and lignin.The independent parallel reaction model is used to predict the mass loss model of Chinese fir during the heat treatment process,and the prediction results are in good agreement with the experimental test results.