Research On Photo-induced Discoloration Of Dyed Veneer Irradiated By Three Different Light Sources

This paper mainly researches on the influence factors of photo-induced discoloration on surface of dyed veneer. Birch and poplar veneer are used as wood samples as well as dye stuffs Acid Red GR and Acid Blue V. Law of photo-discoloration is also discussed following the International Commission on Illumination CIE (1976) standard. Three kinds of light sources namely xenon light, natural light and ultraviolet light are utilized in order to analyzes the influence factors of discoloration including wood species, pretreatment, dye structure, dye concentration, irradiation time and wavelength. The molecular structure change on dyed veneer surface is also detected by Scanning electron microscopy. Finally, FTIR is applied to monitor the functional groups according to the relative obsorbtion characteristic of dyed veneer during irradiation, which would explain chemical structure changes during photo-degradation. Moreover, spectral reflectance curve in combination of K/S diffuse reflection spectra curve is used to evaluate the light fastness of dyed veneer, main research achievements are list as follows.1) Dyed veneer experience great yellowing process while exposed to the Xeon light. Among the influence factors of dyed veneer discoloration, the effects of pretreatment, kinds of dye, irradiation time is significant, but the effects of wood spicies and dye concentration is low. Bleaching pretreatment using H2O2 before dyeing procedure could improve the color of dyed veneer on the one hand, on the other hand it would decrease the light fastness of dyed wood and cause greater discoloration (ΔE*BV0.5= 35.9,ΔE*B'V0.5= 50.7), because function groups of carbonyl, phenolic hydroxyl are destroyed during bleaching treatment. Photo-discoloration of Acid Blue V and Acid Red GR dyed veneer behaves very different due to their different dye structures,ΔE*of Acid Blue V dyed veneer (ΔE*BV1.5= 41.1) is bigger thanΔE* of Acid Red GR dyed veneer(ΔE*BGR1.5= 21.5). 2) The discoloration law of 100 days sunlight irradiation on dyed veneer surface is almost the same as that under the irradiation of Xenon. SPSS analysis obtained the linear relationshipΔE* N=-1.933+ 0.876ΔE*X, on condition that T=t,in which T(/d) represents irradiation time of sunlight, t (/h) represents irradiation time of Xenon in this research. Aggregation of dye molecules can be observed in the vessel wall by using scanning electron microscopy (SEM). Due to photo-irradiation, pit structures are slightly destroyed, enlargement of pit apertures as well as loss of pit domes is observed.3)The studies have shown that all the 5 wavelength (254nm,313nm,340nm,365nm,420nm) radiation source can induce the degradation of dyed venee. However, wavelength of irradiation source hold negative ratio with degree of discoloration, dyed veneer under the irradiation of 254nm UV light produce the most significant discoloration.4) As irradiation time prolong, the spectral reflectance curves exhibit a downward trend, and gradually move to longer wavelength reflection, which is called red shift. It is a feasible method to evaluate the fastness of dyed veneer through spectral reflectance curve.5) FTIR spectroscopy is used to find out the relationship between chemical structure changes and photo-discoloration on dyed veneer surface during irradiation. Function groups at the region of 800-1800 cm-1 have been changed severely, which mainly come from degration of lignin. Chemical structure of the carbonyl functional group (at the peak of 1727cm-1) content get an obvious increase, carbon skeleton of benzene (at the peak of 1592 cm-1,1504 cm-1), phenol hydroxyl group (at the peak of 1234 cm-1), methyl and methylene groups(at the peak of 1423 cm-1,1458 cm-1) decreased as the degradation of phenoxy through free radical reactions. Degradation of the chemical structure on dyed veneer is closely related to discoloration.In conclusion, this research analyzed the influenced factors for discoloration of dyed veneer, revealed the chemical structure changes on veneer surface during photo degradation. Combining with CIE (1976) L* a* b* standard, it is confirmed that spectral reflectance curve is feasible to evaluate the lightfastness of dyed veneer. This study poesses academic and practical value for improving the value of dyed wood, expanding its application scope, it may also induce high efficiency utilization of plantations.