Near infrared spectroscopic investigation of the thermal degradation of wood
Introduction
Wood is a natural composite polymeric material consisting of a cellulose network bonded together by rigid fixed polyosic lingo-polysaccharide amorphous matrix chains. The mechanical and chemical properties of wood are depicted by certain parameters like, density, stiffness and strength, which are closely related to the ultra-fine cell wall structure (less than 1 Å) [1], [2]. Wood is considered to be a durable material, which withstands weathering without losing its structural properties. However, there are some non-biological parameters as humidity, temperature, solar irradiation, ozone content and pollutants [3], that may be responsible for the degradation of wood. The rate of degradation depends upon the type of wood, i.e., softwood or hardwood. For architectural and industrial purposes, wood undergoes through different processes, where several chemical and mechanical changes lead to causing the degradation of the wood [4], [5], [6], [7], [8], [9], [10]. Generally the degradation of wood is caused by light irradiation or by heat treatment [5], [6], [11], [12]. The effect of heat on color change of the different species of wood has been reported previously [13]. More recently Esteves and Pereira published a review on wood modification by heat treatment [14]. Thermal degradation of wood is highly dependent on its constituents [9], [15]. The thermal stability of different wood species has been compared recently [16], [17]. The complex structure of wood and the interaction between its components makes it difficult to differentiate the degradation of each component (cellulose, holocellulose, hemicelluloses and lignin) on heat treatment. The degradation of wood generally begins with decomposition of hemicelluloses followed by an early stage decomposition of lignin and then depolymerization of cellulose. Further degradation involves oxidation of wood to volatile compounds (CO2, CO, H2O, etc.) at higher temperatures (380–470 °C) [11], [18], [19]. The thermal decomposition of crystallites cellulose in wood has been reported [20], [21]. Gartote et al. studied the deacetylation of hemicelluloses during the hydrothermal processing of Eucalyptus wood [22].
Variation in the color of wood is caused by changes in the chemical composition, which result from degradation or weathering [1], [3], [18], [23], [24]. Upon heat treatment the decrease in the brightness and increase in the color difference of wood arise due to the decrease in the content of hemicelluloses [15], [24], [25], [26], [27], [28]. A change in the color of wood arising from heat treatment is closely related to the change in physicochemical structure. These kinds of changes have been characterized by infrared (IR) spectroscopy [2], [3], [4], [5], [6], [7], [8], [25], [26], [29], [30], [31], [32], [33]. In addition thermal degradation of the wood is generally studied by conventional methods namely thermogravimetry, differential scanning calorimetery (DSC) and thermal volatization analysis. These conventional methods are very time consuming and expensive. Therefore, there is a need to develop a non-destructive, rapid and accurate method for analyzing the degradation of wood. This method could be used to monitor the quality of wood in industrial processes.
Though infrared spectroscopy provides a solution to the conventional methods, it introduces some limitation to sample preparation. But, near infrared (NIR) spectroscopy requires very little sample preparation and it is a non-destructive method for analyzing the chemical composition. Since wood species have chemical composition consisting of C–H, O–H, N–H, C–C and C–O bonds, they exhibit unique absorption in the NIR region in the form of overtones and combinations of the fundamental bands. NIR spectroscopy has shown its significant potential in analyzing the changes in mechanical, physical and chemical properties of wood upon heating [1], [24], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]. More recently Gaspar et al. reported the application of near infrared spectroscopy for the evaluation of glue lines of timber before and after ageing [37].
In the present study we investigate the use of diffuse reflectance NIR spectroscopy for the non-destructive analysis of the degraded biomass of Shisham (Dalbergia sissoo) wood upon heat treatment. The wood samples were heated at different temperatures on a hot stage plate. Wood degradation was monitored by the changes in the color through measuring the CIELAB color parameters. Different steps of degradation of the heated wood sample were determined by DSC analysis. The color change and the degradation steps of wood by DSC were analyzed and correlated with NIR spectra. The change in the color of the degraded wood is easily understood by analyzing the diffuse reflectance NIR spectra.
Section snippets
Materials and methods
The wood samples of Shisham (D. sissoo) were taken from a timber factory in the powder form. For the NIR spectral measurement the wood powder was further ground to get almost uniform particle size using a grinder with a rotational speed of 3000 rpm. 2 mm thick circular pellets of 25 mm diameter were made by applying a pressure of 16 N/m2 in a hydraulic pressure machine. This wood pellets was used for thermal, color coordinates and NIR spectroscopic studies. The wood pellets were heated in air on a
DSC analysis
The degradation of wood determined by DSC analysis gives information about the interaction between the constituents of wood and the modification of their chemical structure upon heat treatment [9]. The thermal degradation of wood occurs in different ways in its crystalline and amorphous domains. These domains determine the phase and isophase transition. Transition in the amorphous domain occurs at the moderate temperature range of 50–80 °C while the transition in the crystalline domain occurs
Conclusions
The analysis of the CIELAB color parameters of the wood sample upon heat treatment shows that lightness (L*) value increases up to 90 °C and then decreases on further increasing the temperature. The redness (a*) and yellowness (b*) increase to attains maximum value on increasing the temperature to 210 °C. Then both the parameter starts decreasing on further increases the temperature. The overall color change (ΔE*) increases on increasing the temperature and is related with the rate of formation
Acknowledgement
The authors thank the Director, National Physical Laboratory, New Delhi, India for giving permission to publish this paper.
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