Studies On The Mathematical Model Of Heat And Mass Transfer In Alder Birch Wood And The Color Control During The Thermo-vacuum Treatment

After high-temperature heat treatment(HTHT), wood gradually changes from light color to light brown, deep brown, and even black. Thus, heat treatment provides a new technique for changing color of wood. However, it is difficult to control color change to a desirable degree during heat treatment. Therefore, relationships between heat and moisture transfer behaviors, color and chemical changes during heat treatment need to b e systematically studied so as to be able to control color changes. These studies would provide a scientific guidance to optimize heat treatment for obtaining quality treated w ood.Alder birch(Betula alnoides)wood, a common commercial species in Yunnan Pro vince, was selected as the testing material in this study. The mathematical model of he at and moisture transfer in alder birch wood during the thermo-vacuum treatment(TV T)process was established in this study. The regression equations of changes in holoce llulose(△HOLO), cellulose(△CELL), hemicellulose(△HEMI) and lignin content(△LI G)as the function of temperature(t)and time(τ) also were obtained under the condit ions of 160~200℃, 0~4h and absolute pressure of 0.02 MPa. Based on the mathematical model of heat and moisture transfer and the regression equations of △HOLO, △CELL,△HEMI, and △LIG, the control of wood chemical changes was achieved. The regressi on equations of the lightness difference(ΔL*), total color difference(ΔE*), color saturation difference(ΔC*) as the function of t and τ were obtained. Based on the ma thematical model of heat and moisture transfer and the regression equations of ΔL*, ΔE*, ΔC*, the control of color changes was achieved. Based on the above research, the relationship between wood color change and chemical change was analyzed, and the mechanism of wood color change during the TVT process was also analyzed using UV spectroscopy(UV), Fourier transform infrared spectroscopy(FTIR) and X-ray photoelectron spectroscopy(XPS).The conclusions of this study are as follows:(1) The mathematical model of heat and moisture transfer in alder-birch wood d uring the TVT process was established and compared to the experimental value. The e xperimental and model values of heat and moisture transfer in wood were found to be in good agreement, and the determination coefficient(R2) between the experimental an d model values was greater than R2=0.98, while the regression relation were all extrem ely significant. The mathematical model established in this study did not accounted for free water migration, and so it was only suitable for the condition that the initial moist ure content(MC) was below the fiber saturation point(F.S.P).(2)A systematic study also was carried out to determine the effects of different heating temperatures, initial MC, thickness, radiant heat transfer coefficient(hR), and ma ss transfer coefficient(hm) of the wood samples on the mathematical model of heat an d moisture transfer during the TVT process. The higher the heating temperature, the m ore rapidly the temperature of the wood sample rose at different temperatures, and the more rapidly the MC dropped. The initial MC had little influence on the moist wood t emperature rise. However, the temperature of dry wood samples rose faster than the m oist wood. The higher the initial MC was the longer the time it needed for moisture m igration. The thinner the wood thickness, the more rapidly the temperature of the wood sample rose, and the more rapidly the MC dropped. The hR and hm influenced only sl ightly on the increase of wood temperature, but influenced greater at decreasing wood MC. The higher the hR and hm, the more rapidly the MC dropped.(3) Holocellulose, cellulose, hemicellulose, cold-water and hot-water extractives cont ents decreased while lignin and benzene~ethanol extractive increased with increasing he at treatment temperature and the treating time. The regression equations of △HOLO, △CELL, △HEMI, and △LIG as the function of t and τ during the TVT process were est ablished, and determination coefficients of all regression equations were greater than R2=0.86, while the regression relation were all extremely significant.(4) Based on the relationships between mathematical model of heat and moistur e transfer and the regression equations of the wood chemical changes, a model was est ablished to control wood chemical change. The experimental and model values of the wood color changes were in good agreement, where all determination coefficients value s were greater than R2 =0.97, while the regression relation of △HOLO and △LIG were all extremely significant, and that of △CELL�'�△HEMI were all very significant.(5) The lightness values(L*)decreased and the red-green color coordinate(a*)had no obvious changes with increasing heating temperature and treating time. However,the yellow-blue color coordinate(b*) decreased first and then increased, ΔL* decrease d, ΔE* increased, and ΔC* increased with increasing temperature and treating time. Ho wever, the hue difference(ΔH*)and the gloss(Ag*)had no significant change. The r egression equations of L*, ΔL*, ΔE*, and ΔC* as the function of heat treatment tempe rature t and τ during the TVT process were also established, and the determination coe fficients of the all regression equations were greater than R2 =0.78, while the regressio n relation were all extremely significant.(6) Based on the relationships between mathematical model of heat and moistur e transfer and the regression equations of the wood color changes, a model was establi shed to control wood color change. The experimental and model values of the wood c olor changes were in good agreement, where all determination coefficients values were greater than R2 =0.93, while the regression relation were all extremely significant.(7) The relationships between △L*、△E*、△C* and wood chemical changes we re established, where the R2 values were greater than 0.86, while the regression relation were all extremely significant.(8) UV, FTIR and XPS showed that hemicellulose content decreased and relativ e lignin content increased as a result of heat treatment. Thmal degradation of hemicellu losed produced coloring materials such as furfural and hydroxymethyl furfral. Exposure to high temperatures darkened lignin.