基于FTIR和UV-Vis光谱表征蒸汽介质高温热改性对橡胶木颜色的影响

The Characterization of Effects of Thermal Modification on Rubber Wood Color Change Based on FTIR and UV-Vis Analysis

以水蒸汽作为保护气,将橡胶木分别置于155℃处理2、6 h,185℃处理2、6 h,215℃处理2 h,测定了热改性后的橡胶木颜色,并通过紫外-可见光谱(UV-Vis)和傅里叶变换红外光谱(FTIR)对木材热改性前后的化学结构进行了表征。结果表明,高温热改性处理使橡胶木的颜色从浅黄白色变为橙色然后转向深棕褐色,随着处理程度加深,明度值L*持续降低。热改性程度较浅时色饱和度C*略有增加,木材颜色趋于鲜艳,而随着处理程度加深色饱和度C*大幅降低,颜色趋于黯淡。木材颜色变化的总色差ΔE*可反映热改性的程度,改性处理越剧烈,总色差ΔE*越大,处理温度比处理时间对木材颜色变化的影响更为显著。UV-Vis光谱显示,热改性材在350～550nm波段的吸收显著增强,说明有大量发色基团和助色基团生成。结合FTIR光谱,可以推测热改性过程木质素的相对含量升高,紫丁香基团上的甲氧基脱落,木素上苯环氧键的含量稳步降低,从侧面印证可能发生了生成醌的反应。同时,FTIR光谱中有迹象显示可能在155℃处理2 h时半纤维素的乙酰基就开始大量脱落。

The color changes of thermally modified rubber wood, which was treated at 155 ℃ and 185 ℃ for 2 and 6 h, and 215 ℃ for 2 h respectively, were analyzed using the CIE L^*a^*b^*color space. Changes in chemical structure of the wood samples were analyzed by UV-visible spectrophotometer (UV-Vis) and Fourier transforms infrared spectroscopy (FTIR). The results showed that the color of rubber wood became orange and then approached dark brown, and the light value L^* decreased steadily. The parameter C^* increased at the beginning of thermally modified treatment and then went down rapidly. The parameter ΔE^* showed a positive correlation with the modification degree, and treatment temperature played a more important role in the change of wood color compared with treatment time. A significant increase emerged in the 350-550 nm in UV-Vis spectra of heat-treated rubber wood above 185 ℃, indicating the generation of chromophoric groups. According to FTIR spectra, the content of lignin increased as hemicellulose and cellulose degraded at high temperature;and the loss of syringyl units or breaking of aliphatic side-chains could be inferred from a shift of maximum absorption from 1507 cm^-1 to approximately 1511 cm^-1 due to decrease of methoxyl groups. The cleavage of acetyl groups was considered to be detected in FTIR spectra in the samples treated at 155 ℃ for 2h.