古建木构件化学组分近红外光谱分析

Analyses on chemical composition of ancient wood structural component by using near infrared spectroscopy

在材种鉴定的基础上,通过近红外光谱(NIRS)定性分析了建成约600 a的木造古建筑木构件的化学组分,结合木构件化学组分定量分析,与现代木材相比较,探讨了近红外光谱技术评价木构件老化的可行性。结果表明:近红外二阶导数特征性谱带反映了木材纤维素以及半纤维素和木质素的基团信息,而其差谱反映出木构件与对照样化学组分变化。这些光谱特征与传统的化学组分定量分析的结果非常一致:纤维素和半纤维素相对含量减少而木质素相对含量增加,与各种组分谱带差谱的增减相对应。此外,在5 882,5 587和5 464 cm-1等谱带处反映的纤维素结晶和半结晶区的光谱信息,差谱观察到木构件与现代材落叶松之间化学组分的不变或减少,其结果与X射线衍射(XRD)方法获得的木材结晶度分析结果相一致。通过NIRS定性分析木构件化学组分及结晶度变化,接近于现场检测方法,使用便携式NIRS,在古建筑木构件端头裸露部位获取光谱信息,能够实现现场对木材化学组分的无损定性评价。本实验结果也表明,除了常规的红外光谱(FT-IR)、XRD分析技术,NIRS技术对于木构件老化状况的评价是一种有潜力的无损检测方法。

Based on wood identification using near infrared spectroscopy(NIRS),this study qualitatively investigated the characteristic band variations reflecting chemical compositions of the wood structural component in a historic architecture that was built 600 years ago.Also,this study discussed the possibility of evaluating aging degradation using the NIRS information of qualitative analysis in association with quantitatively wood chemical composition analyses of ancient wood structural components.The results were compared to the present architecture as a control.With the second derivative spectra derived from NIRS in combination with their difference spectra,variations of chemical composition in ancient wood structural component and control one existed significant differences on several characteristic band assignments which related to crystalline and semi-crystalline region of cellulose,hemicellulose and lignin.The chemical compositions were analyzed by the wet chemistry method and the results showed that the relative contents in the ancient wood structural component and control one were 42.3% and 46.3% for α-cellulose,20.24% and 24.36%for hemicellulose and 34.2% and 22.98% for lignin,respectively.The results perfectly reflected the bands in difference spectra assigned to cellulose,hemicellulose and lignin,which showed that the contents of cellulose and hemicellulose decreased while lignin increased.Moreover,the degree of crystalline investigated by the X-ray diffraction(XRD) showed that relative crystallinities in the ancient wood structural component and control one were 42.3%and 46.3%,respectively.There existed a slight difference between the ancient wood structural component and control one for crystallinities,which were closely related to the bands at 5 882,5 587 and 5 464 cm-1 in the NIRS spectra.These spectra showed slight difference between the ancient wood structural component and control one.The results obtained from the difference NIRS spectra were in well agreement with those obtained from the traditional analysis methods in both chemical composition and crystallinity.Information provided by NIRS showed that the aging degradation of the ancient wood structural component mainly determined by the decreases of polysaccharides in wood.The increase in relative content of lignin for the ancient wood structural component contributed to stiffness retention but the wood became brittle.The studies evaluating qualitatively chemical composition of the wood and crystallinity by NIRS were carried out nearly in-field conditions.In practice,it is feasible to collect spectral information using the portable NIRS equipment on naked cross section to form a beam in the historic architecture to meet the needs of non-destructive healthy diagnose.The results also demonstrated that,except for the traditional analysis methods such as the Fourier transform infrared spectroscopy(FT-IR) and XRD,which need sample preparation and are time-consuming,NIRS should be a potential tool in non-destructive evaluation of ageing condition of the ancient wood structural components for wooden historic architectures.