The results of the lignocellulosic compositions are shown in Table 1. Results are calculated as % w/w with respect to oven dried raw material. It seems that the wood chips of Afares oak contain more cellulose and less lignin than Zeen oak, while the Maritime pine present the highest lignin content and the lower ratio of hemicellulose. Amongst all constituents of wood samples, cellulose, hemicelluloses, and lignin, only lignin absorbs relatively strongly in the UV/visible region. Therefore, the light induced degradation of wood is mainly caused by photochemical reactions occurring in lignin. Wood discoloration has been associated with the formation of carbonyl groups and degradation of lignin ; this latter contains especially guaiacoxyl: the guaiacoxyl radical is formed by degradation of the guaiacyl chromophore group .
In this work, artificial accelerated aging was adopted to test surface light resistance of our wood species. Several methods and devices for artificial weathering have been developed to accelerate the testing of wood with the aim of increasing reproducibility. Xenon arc chamber match better to the solar spectrum than other devices with UV fluorescent lamps .
Figure 1 shows the discoloration effect of UV aging on wood samples. Photodegradation of the samples is manifested by an initial color change, followed by roughening and cracking as shown by SEM microscopy. However, the processes of decomposition and crosslinking in wood include a wide variety of interrelated ionic, radical-chain, and molecular reactions. Scission of chemical bonds can take place leading to a production of radicals; this process is claimed to be responsible for yellowing of wood [20, 21]. The color change due to photodegradation is linearly dependent on the appearance of the carbonyl groups on the surface of the material but not the disappearance of the benzene ring of the lignin . The decisive role in wood aging is performed by competitive chemical processes of decomposition and crosslinking of macromolecules in this natural polymer composite.
Figure 2 shows the SEM images of the samples structure before (pictures in left) and after (right pictures) UV irradiation. The action of light leads to formation of microscopic cracks or checks. Cells lose bond strength with adjacent cells near the wood surface because of the degradation of lignin. The observed changes can be then summarized as formation of micro-cracks and destruction of the various layers of the cell wall. Microscopic changes accompany the color changes and chemical changes of wood during degradation.
Spectroscopy was used for studying the chemical changes in wood caused by light irradiation. FTIR spectroscopy is used as a powerful technique nowadays. In case of wood, the fingerprint region is located between 800 and 1800 cm−1. Figure 3 shows the FTIR spectra of the wood samples before and after aging. The overall FTIR spectrum of both aged and crude wood polymers indicates that a number of spectral features appear to be sensitive to irradiation. All bands assigned only to lignin component, such as 1590, 1505, and 1465 cm−1 for the afares and zeen oak woods in Fig. 3a and b, and 1505, 1630, and 1450 cm−1 for maritime pine in Fig. 3c, decrease significantly as a result of the irradiation process. This result indicates that the structure of the lignin of the three wood samples was degraded to a significant extent.
The evolution of the lignin loss is best followed by the band at 1505 cm−1 of the three woods, assigned to the partial decomposition of lignin, the decrease in the intensity of this peak indicated degradation of lignin during the irradiation process. Other authors have investigated the difference FTIR spectra recorded for lignin from irradiated and non-irradiated wood [23, 24]. The decrease observed for this band is significantly larger in the case of maritime pine than in the case of oaks, after 120 h of treatment.
For the softwood, the decrease of the absorbance at 1740 cm−1 assigned to C=O stretching vibration of acetyl or carboxylic acid groups was compensated by an increase in the same groups derived mainly from lignin. The behavior of the band at 1430 cm−1, which is characteristic of crystallized cellulose I, indicates that the amorphous area of the cellulosic component of both woods is more affected by the degradation process. Two great absorption decreases are apparent at 1236 and 1154 cm−1. The first decrease belongs to the asymmetric stretching of ether bond, while the second belongs to the symmetric stretching of ether bond, the aromatic C–H deformation, and to the glucose ring vibration. These absorption decreases indicate the ether splitting and the degradation of cellulose . The observed changes can be then summarized as formation of micro-cracks and destruction of the various layers of the cell wall. The samples were withdrawn from the device after different irradiation times ranging from 30 to 120 h. A material lack is also observed; Fig. 4 shows the wood mass loss, expressed as a percentage loss, against UV time exposure. After the first 30 h of light irradiation by xenon lamp, the mass loss increases sharply, and then the rises stabilize gradually for Maritime pine and zeen oak. On the other hand, the afares oak mass loss seems to increase steadily throughout the entire exposure time.
The mechanical strength was used also as measure of the level of degradation in the wood specimens. We have measured the tensile strength of the wood specimens before and after UV exposure of 120 h. Table 2 shows the mechanical characteristics of the wood samples. Based on the mechanical test results and although the ultimate strengths of the unexposed woods were higher than their aged wood counterparts, their Modulus of elasticity MOE was clearly higher. This behavior can be explained probably by micro-cracks in the irradiated wood samples, which have a much greater influence on the tensile strength than on the modulus [26, 27]. On the other hand, the decrease of MOE after aging indicates that the wood samples loss some of stiffness due to the lowered moisture content of samples involved by the degradation of hemicelluloses and lignin.