| As a very important wood modification technology,high temperature heat treatment is one of the effective means to improve the dimensional stability and durability of wood.Chemical components and multi-scale structure of pores in wood undergo changes with the application of heat treatment.Consequently,water adsorption and absorption characteristics of wood are changed.However,the previous studies on the water adsorption and absorption characteristics of heat-treated wood did not show a consistent variation regularity,and the relationships between water adsorption/absorption characteristics and the chemical composition and multi-scale pore structure of wood have not been clarified.Therefore,the heart wood of Chinese fir(Cunninghamia lanceolata [Lamb.] Hook.)was studied in this paper.The wood samples were heat-treated for 2h at 160 ?,180 ?,200 ? and 220 ?,respectively,with steam as medium and protective gas.The variation of water adsorption/absorption properties,chemical components and pore structures was investigated by various characterization methods.By investigating the relationship between the chemical components,pore structure and water adsorption/absorption properties of wood,the mechanism of the effect of heat treatment on the water adsorption/absorption properties of wood was revealed.Major achievements under the experimental conditions of this study were summarized as follows:1.The dynamic water vapor adsorption test results show that the high temperature heat treatment does not change the water absorption isotherm type of wood.Under current humidity condition,no obvious capillary condensation phenomenon was found in each sample according to the shape of the isotherm.The EMC of the samples were 5.21%,3.83%,3.60%,3.42%,3.26% under 30% RH,9.15%,7.44%,6.74%,6.10%,5.57% under 60% RH and 20.21%,17.38%,15.69%,14.36% and 12.20% under 95% RH at 25 ?,significant difference exists in the equilibrium moisture content between the high temperature heat-treated and the untreated wood samples at 30%,60% and 95% relative humidity(P<0.05).The high temperature heat treatment significantly enhanced the hygroscopic hysteresis of the wood and the hysteresis rate increased with the increase of the heat-treated temperature.After a moisture absorption-desorption cycle,the hygroscopic hysteresis of the heat-treated samples is weakened in the subsequent moisture absorption process compared with the first moisture absorption process,while the sorption isotherms for the untreated wood samples were found to be essentially reproducible.2.The heat treatment weakened the water absorption properties of the wood in the initial stage of water absorption.After water absorption for 96 h,the average MC of the untreated sample was 180.28%,while 143.45%,114.51%,77.61% and 62.06% for the heat-treated samples,which were 20.43-65.58% lower than that of the untreated sample.However,the samples were almost reached water absorption equilibrium at the same time.The low-field nuclear magnetic resonance analysis shows that the fiber saturation point(FSP)of the untreated and heat-treated samples is about 30.66%,29.30%,28.89%,25.61% and 22.14%,respectively.The value decreased by 6.45%-29.03% of the heat-treated samples compared with the untreated ones.There are two peaks in the T2 relaxation spectrum of the untreated,160 ? and 180 ? heat-treated wood samples,which are belonged to free water and bound water in the wood.For the 200 ? and 220 ? heat-treated samples,in addition to the peaks corresponding to the above two states of water,a new peak with a short relaxation time of about 20 ms appeared,which belonged to free water.The high temperature heat treatment has little effect on the relaxation time of the bound water which were still about 1 ms,but the relaxation time of the free water for heat-treated wood was got longer,from about 100 ms(untreated)to about 400 ms(HT220).3.High temperature heat treatment had the most obvious effect on hemicellulose of the wood cell wall,and the degradation was mainly the cleavage of xylan acetyl side chain.After treated temperature higher than 180 ?,the degradation of glucomannan gradually increased.The relative crystallinity of each heat-treated samples were higher than that of the untreated sample,but a slightly decrease was found after the treated temperature was higher than 200 ?.The degradation of cell wall components leads to the change of the effective hydroxyl,and the amount decreases with the increase of the treated temperature.The relative content of lignin on the surface of the heat-treated samples was increased,and the relative content of polysaccharides decreased.Compared to the untreated sample,the relative content of holocellulose decreased by 2.52%-14.36%,?-cellulose decreased by 1.72%-11.21% and lignin increased by 3.86%-12.09%.The change of chemical composition of heat-treated wood leads to the increase of hydrophobicity.4.Nitrogen adsorption test analysis showed that the high temperature heat treatment had a significant effect on the morphology of the pores in the cell wall and the slit-like mesopores with parallel walls were significantly increased after heat treatment.Compared with the untreated samples,the BET specific surface area increased by 21.45% and 33.61% for 160 ? and 180 ? heat-treated samples,and decreased by 18.08% and 21.76% for 200 ? and 220 ? heat-treated samples,respectively.The results of mercury intrusion test shows that heat treatment at 160 ?,180 ? and 200 ? mainly affects the pit aperture and small pores on the pit margo,decreases the average pore diameter but increases the pore volume.After heat treatment at 220 ?,the average pore size and pore volume of the axial tracheids of the wood increased significantly.The results of NMR cryoporometry showed that,under the saturated state,the cell wall porosity of the untreated sample was the largest,and the porosity of the heat-treated samples decreased with the increase of the treatment temperature.5.The change in the amount of hydroxyl groups was one of the important factors influencing the hygroscopicity of wood.There are two possible reasons for the hygroscopic hysteresis change of the heat-treated samples.One is that the mechanical relaxation of the wood cell wall components at high relative humidity causes the sorption site to be rearranged.The other one is that lignin cross-linking leads to an increase in cell wall stiffness and thus affects the response of wood pores to ambient humidity.Both the changes in the chemical composition and pore structure have an effect on the water absorption of the wood.In the initial stage of water absorption,the slowing of the transport speed of water along the longitudinal direction of the wood is mainly due to the degradation of the chemical composition of the wood,which increased the hydrophobicity of the wood,while the change of the pore size of the tracheids has no obvious effect.The slower transport of moisture along the transverse direction of the wood is mainly related to the decrease in the average pore size of the pit aperture and small pores on the pit margo.The increased volume of the cell cavity results in an increase in the absorption of free water. |
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