WOOD PHOTODISCOLOURATION OF ABIES-GRANDIS UNDER SOLAR LIGHT EXPOSURE

In the present study, the effect of the solar-type irradiation was investigated by focusing mainly on fir (Abies grandis) discolouration. The CIE-LAB system was used for colour change measurements (table II, fig 3) and spectrometric methods such as infrared (IR) and ultraviolet visible (UV), were utilized to determine the mechanism. All the IR absorption bands were assigned to the different chemical groups of the wood surface (table IV). The absorption changes occurring during the solar-type exposure showed a decrease in the hydroxyl groups, an increase in the carbonyls and also desaromatization (table VII, fig 8). These changes were oxygen-dependent (fig 9). By cutting the UV or visible light emitted by the lamp with filters, it was possible to show that UV wavelengths were more important in the IR spectral modification (fig 10). Also, lignin photodegradation was similar in spectral evolution of yellowing fir, mainly due to the UV wavelengths (fig 13). At the beginning of light exposure, the yellowing rate seemed to be independent of the gaseous atmosphere. Lignin seemed to be responsible for the light absorption of wood and for discolouration (fig 15), and the main source of radicals. These gaiacoxy radicals appear to form coloured quinonoid structures with oxygen, and colourless products without it (fig 19).