The Influence Of Heating Rate On Properties Of Heat-treated Wood

Wood heat treatment is widely used in property modification,which can improve the durability and stability of wood.The heating rate has a significant impact on the product properties and reaction mechanism of heat-treated wood,and also plays an important role in production practice,but related research is very limited at home and abroad.In this paper,Mongolian oak and hemlock boards were heated to 200℃ for 2 h at the heating rate of 5℃/h,10℃/h,15℃/h,20℃/h and 25℃/h respectively by superheated steam under atmospheric pressure.The main mechanical properties and acoustic vibration characteristics of heat-treated oak and hemlock at different heating rates were tested,and the changes of moisture content and radial and tangential dimensions of wood under different environmental humidity were studied.at the same time,in connection with the chemical functional group concentration,chemical component content,cell wall microstructure and other factors,the internal mechanism of the effect of heating rate on the properties of heat-treated wood was analyzed,and the following conclusions were obtained.(1)With the decrease of heating rate,the static MOE of oak and hemlock increased at first and then decreased,and reached the maximum at 15℃/h.After heat treatment,the MOR of both oak and hemlock decreased,and the MOR of oak was higher than that of coniferous hemlock.(2)After heat treatment,the hygroscopicity of oak and hemlock decreased gradually with the decrease of heating rate,and was closely related to the change of hemicellulose concentration.While the hygroscopicity decreased,the dimensional stability of oak and hemlock was improved.(3)At different heating rates,the acoustic radiation quality constant and sound conversion efficiency of heat-treated oak and hemlock increased,while the acoustic impedance and loss tangent decreased in different degrees.The results show that the appropriate heating rate can effectively improve the vibration properties of wood.(4)The heating rate has a significant effect on the chemical composition and microstructure of the cell wall of heat-treated wood.The content of hemicellulose in cell wall decreases with the decrease of heating rate,which is the main factor affecting the increase of hygroscopicity and strength of heat-treated wood.The content of lignin increased with the decrease of heating rate,and the content of benzene alcohol extract decreased at first and then increased with the further decrease of heating rate.Cellulose has good thermal stability,and the content of cellulose does not change obviously with the decrease of heating rate,but the crystallinity increases at first and then decreases with the decrease of heating rate,and its change trend is highly similar to that of MOE,while the microfibril angle decreases gradually with the decrease of heating rate.In summary,the heating rate is an effective process parameter to adjust the performance of heat treated oak and hemlock,but the relationship between the change of test material properties and the heating rate is not linear,and the heating rate of wood heat treatment should be determined according to the main performance requirements of wood heat treatment,combined with comprehensive factors such as production efficiency and safety.