Mild Heat Treatment Of Mongolian Oak Wood: Property Evaluation And Mechanism Investigation

Wood heat treatment is a widely applied wood modification technology.The temperature of conventional heat treatment is generally between 160℃and 240℃.The existing work shows that treatments under this temperature level can also adjust wood properties,but relevant studies are unsufficient,and there is no systematic understanding on the characteristics of the treated wood and the mechanism hehind it.In this study,Mongolian oak wood was treated by atmospheric steam at 110℃,130℃or 150℃,and the properties of the teated samples were compared with those conventionally dried or thermally modified at 180℃.The investigated properties are mechanical and hygroscopic hehavior,dynamic viscoelasticity and acoustic vibration properties.The property modification mechanism of the mild heat treatment was investigated by studying chemical functional group concentration,chemical component contents,cell wall ultrastructure and mesoscopic pore structure.The main conclusions are listed as follows:(1)After the mild thermal treatmen,the lightness(L~*)of wood decreased,the red green color index(a~*)and yellow blue color index(b~*)increased,and the appearance turned red and yellow,the color similar to the naturally aged wood.However,the L~*,a~*,b~*values of the conventionally heat-treated wood all decreased significantly,leaving them coffee-like appearance.Mild thermal treatment improved the rigidity and bending strength of wood.The maximum increase of modulus of elasticity(MOE)was 32.3%(150℃,2h),the maximum increase of dynamic elastic modulus was 19.3%(150℃,2h),and the maximum increase of modulus of rupture(MOR)was 11.5%(110℃,22h).In contrast,the rigidity of conventionally heat-treated wood was improved,but the MOR decreased by as much as 21.3%.(2)Mild thermal treatment reduced the moisture adsorption of wood,and the moisture adsorption of wood decreased more significantly with the increase of treatment temperature.Compared with conventional heat treatment,the moisture adsorption of wood decreased more.The fitting data of H-H model show that the single-layer molecular adsorption of mild thermal treatment wood decreases slightly,but the multi-layer molecular adsorption decreases significantly.The single-layer molecular adsorption and multi-layer molecular adsorption of conventional thermal modification wood are significantly reduced.(3)Similar to conventional thermal modification,mild thermal treatment can also improve the acoustic properties of wood.The acoustic indexes such as loss factor,specific dynamic elastic modulus,acoustic radiation quality constant and acoustic conversion efficiency of the treated wood are better than those of the control wood,and the mild thermal treatment at 130℃or 150℃can basically achieve the property improvement similar to conventional thermal modification.(4)The chemical composition of the mildly heat-treated wood changed little.FTIR spectroscopy showed that the concentration of main functional groups did not change significantly.Wet chemical analysis showed that the change range of holocellulose and lignin content was less than 5%.In contrast,the acetyl group concentration of conventionally heat-treated wood decreased,the benzene ring structure of lignin increased.The content of holocellulose and lignin decreased by 10.1%and 11.3%,respectively.(5)The crystallinity of 130℃-treated wood and 150℃-treated wood increased by 8.9 and8.8%,repectively.The crystallite width increased by 4.0%and 3.7%,and the microfibril angle decreased by 7.6%and 24.4%.The microstructure of the conventionally heat-treated wood also changed,the crystallinity and crystal width increased by 5.4%and 3.1%,respectively,and the microfibril angle decreased by 21.2%.The change of average pore size of mildly heat-treated wood was small as a whole,which proves the retention of its mechanical strength.The mesoporous specific surface area of the conventionally heat-treated wood decreased and the average pore diameter increased significantly,which resulted from the degradation of hemicellulose,resulting in the shrinkage of cell wall,cell breakage and cracks,and loss of mechanical strength as a result.The above research results show that mild thermal treatment can improve the main mechanical properties,moisture adsorption and acoustic vibration properties of wood.The temperature has a significant effect on the modification degree.The main physical and mechanical properties of 110℃-treated wood are closer to the control wood,while 130℃-or150℃-treated wood can obtain similar performance improvement to the conventionally heat-treated wood without deteriorated mechanical properties.Different from conventional thermal modification,the the property modificaiton mechanism of mild thermal treatment may not be mainly attributed to the change of wood cell wall components,but rather the combined effects of the decrease of wood moisture content,the change of cell wall microstructure and the change of chemical components.The performance characteristics and modification mechanism of the mildly heat-treated Mongolian oak wood can be summarized as follows:...