Study On Properties And Mechanism Of Larch Wood Modified By Vacuum Heat Treatment

Larch is one of the main plantation species in northern China.It has the advantages of hard texture,high mechanical strength,beautiful texture and strong natural decay resistance.But it has the disadvantages of easy cracking,poor dimensional stability and high resin content.Therefore,it is necessary to improve its performance by modification.Thermal modification of wood(TMW)is an effective and environmental friendly way to improve wood dimensional stability and durability.Several new thermal modification methods have been developed and one of the new TMW processes is vacuum heat tretment.However,there has been relatively few research on vacuum heat treatment,especially the thermal mechanism is not clear.Therefore,in this study,in order to investigate the influence of vacuum heat treatment on the physical properties,mechanical properties and color changes of larch(Larix kaempferi)wood,specimens were vacuum heat-treated at various temperatures(160°C?180°C?200°C?220°C and 240°C)and times(2h,4h,6h,8h and 10h).In addition,the heat transfer uniformity and the composition and content of volatile gas during the vacuum heat treatment were analyzed and the heat and mass transfer was preliminarily explored.The chemical composition,chemical structure and microstructure of wood during the vacuum heat treatment were studied,and the mechanism of the dimensional stability,mechanical properties and color change under vacuum heat treatment was revealed.In this study,the vacuum modification of plantation wood was carried out.On the one hand,it provided theoretical basis and technical support for the vacuum heat treatment process optimization and performance control of wood.On the other hand,it provided a new way for modification and high value utilization of plantation wood.The main conclusions of this study are as follows:(1)With the increase of heat treatment temperature and time,the mass loss rate,the anti-swelling and anti-shrinking rate of heat-treated wood increased gradually.However,the oven-dry density and equilibrium moisture content decreased with the increase of heat treatment intensity.The effect of heat treatment temperature on the properties of wood was greater than that of heat treatment time.200°C was the critical temperature for the heat-treated process in this study.(2)Compared with untreated samples,at the temperature of 160°C,the bending strength of treated samples that treatment time was 2h and the elastic modulus of treated samples that treatment time were 2h and 4h increased.The bending strength and elastic modulus of other heat-treated wood decreased gradually with the increase of treatment intensity.The longitudinal elastic modulus and hardness of the cell wall of vacuum heat treated wood were higher than that of untreated wood,and reached the maximum at 200°C.The elastic modulus increased by 17.34%(160°C)?54.57%(200°C)and 38.92%(240°C).The hardness increased by 8.63%(160°C)?45.69%(200°C)and 32.55%(240°C).(3)Considering the effects of vacuum heat treatment parameters on the mass loss,oven-dry density,ASE and MOR of heat treated wood,the further optimum heat treatment condition is treatment temperature of 200°C for 6h.(4)The lightness value L* reduced with the increase of heat treatment intnesity.Its variation range was 8.24%~73.16%.The index a* and b* increased with increasing treatment intensity.The variation range was 34.13%~128.45% and 15.13%~39.60%,respectively.The overall color difference ?E* increased gradually as the heat treatment intensity increased,indicating that the difference between the color of the heat treated wood and the untreated wood was gradually increased.(5)The effect of heat treatment temperature on the heat absorption per unit mass and the average heating rate was very significant.Thickness,end distance and side distance had no significant effect on heat absorption per unit mass,average heating rate and maximum heating rate.The heat conduction was uniform within the test size of wood in this study.(6)The gases released from the wood at different vacuum heat treatment temperatures were collected by the derivatization method,the water-soluble method and the Tenax TA adsorption method.The heat treatment temperature did not change the types of volatile organic compounds.The types of volatile organic compounds were aldehydes,ketones,organic acids,organic alcohols,aromatic compounds,alkanes and terpenes.With the increase of heat treatment temperature,the species of volatiles increased gradually from 48 for 160°C samples to 86 for 220°C samples,and then decreased to 68 for 240°C samples.The total concentration of aldehydes,ketones,organic acids and organic alcohols increased with the increase of heat treatment temperature.(7)The mechanism of the increase of dimensional stability for vacuum heat-treated larch wood: The degradation of highly hygroscopic polysaccharides and the increase of lignin contents in the cell wall of heat treated wood reduced the hygroscopicity of treated wood.The degradation of wood polysaccharide and lignin decreased hydroxyl,carbonyl and other groups with a high degree of hygroscopicity.The increased crystallinity of heat treated wood resulted in the decrease in moisture absorption area.(8)The mechanism of the changes of mechanical properties for vacuum heat-treated larch wood: Hemicellulose considered as a binding agent in the cell wall degraded during heat treatment,which resulted in the decrease of the mechanical strength.Heat induced the cellulose molecular chain fracture and the decrease of polymerization degree.Because of the crosslinking reaction of lignin and the increase of the cellulose crystallinity,the bending strength and elastic modulus of the treated wood were increased at the treatment temperature of 160°C.With the increase of treatment temperature time,the effect of the degradation of wood chemical composition was greater than that of crosslinking reaction and the increase of crystallinity,which made the mechanical properties of wood decrease gradually.(9)The mechanism of the changes of color for vacuum heat-treated larch wood: During heat treatment,the degradation of hemicellulose generated a large number of colored extractives,which made the wood color darkened.The relative content of lignin increased,while the lignin structure contained a large number of chromogenic and auxochromous groups.Thus,the change of lignin content caused the change of wood color.In the chemical structure of heat treated wood,the quinone type structure increased,which made the wood color change.