The Mechanism Of Light-induced Discoloration In Surface Decoration Of Wooden Materials

Nowadays, the surface decoration of wooden materials have become basic demand for high-quality, high-grade life in human settlement. Most methods can be used for the surface decoration of the wooden materials. And the veneered decoration and the coated decoration are the most important in these methods. However, the problem of damaging induced by light can be neglected neither in veneered decoration nor in coated decoration. In this work, the light-induced discoloration is investigated in basswood single board, decorative paper (wood-veined paper and impregnated paper) and PVC sticker (dark and light color). The spectrophotometer with the CIE chromaticity of 1976 is used to study the law of the change of color. While, we investigate the change of the surface-chemistry structure and the scientific essence of the producing of color, with surface-analysis technology such as x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FT-IR), electron spin resonance spectroscopy (ESR), fluorescence display and so on. Furthermore, the ultraviolet spectrophotometer is also used to measure the transmission spectra of the typical paint films. In this work, we hope to provide basic and scientific theory to avoid damaging induced by light. The main results are shown as follow:(1) The general characters of the light-induced discoloration are observed in basswood single board, decorative paper and PVC sticker when they are radiated by natural and ultraviolet light. After radiating, we observed basswood single board, decorative paper and PVC sticker by ultraviolet and find that the lightness index decrease, the yellow and blue index increase, the red and green index change little, chromatic difference become big and visual display become yellow. In addition, the color induced by the long-time ultraviolet radiation change more strongly. While the effect induced by natural light is smaller than induced by ultraviolet. The same light-induced discoloration phenomenon is observed in basswood single board, decorative paper and PVC sticker. The change of the chromatic difference is observed mainly in the early 20 hours, and then become steady. Furthermore, the rough index of the morphology increased (excepted PVC).(2) Different decoration materials show different changeable effect of color under the same radiation. From basswood single board, decorative paper to PVC sticker, the change of the color decrease. The curves, which show chromatic difference with the time of radiation, are different (the curve are bed-model and chair-model). The damaging by light of wood-veined paper and impregnated paper are not observed in this experiment. It is because that the most of titanium dioxide is brought to wood-veined paper and impregnated paper.(3) The change of carbon valence and free radical concentration can be observed in the obvious light-induced discoloration materials. And both of them are changing with color. Part of low valence carbon atoms turn to high valence carbon atoms, the content of oxygen increased. At the same time, free radical concentration increase. This can be shown by oxidative degradation of high polymer and rupture of long molecular chain, which produce oxygenic function comprised by hydroxy group, ketone group and aldehyde group. In addition, the change shown above is drastic at the beginning of the reaction, and then turn to steady.(4) Object color-perception comes from the spectra of the overall reflected lights. The overall reflected lights are comprised not only by the reflex after selective absorption, but also by dispersion and diffraction. Therefore, the traditional view must be changed. The traditional view is that the change of color is not just because the light selective absorption change induced by the chemical property change of materials.(5) The absolute flat surface is not observed in microcosm. When porosity can reach the diffraction condition of light, the total reflected light should include the effects of diffraction. The ingredient of the long wave increases, the red-green-axis and yellow-blue-axis color index become higher, as the porosity become bigger. Furthermore, Rayleigh scattering also makes the ingredient of long wavelengths increase in reflected lights. Therefore, selective absorption, scattering and diffraction play different roles to the reason of colors change in different materials. This is physical explanation to the color change of the "scattering and diffraction"(6) According to analysis above, we conclude the light-induced discoloration of the typical surface decoration materials, which we chose, as follow:The light-induced discoloration of basswood single board comes from lignin and oxidative degradation. With the biology configuration of wooden materials changed (cell wall and cell cavity), diffraction influent the change of color. However, scattering factor can not be observed in our experiments.According to the light-induced discoloration of impregnated paper, on the one hand it comes from the chromophoric group change of high polymer materials, which make selective absorption-character changed; on the other hand it comes from the change of asymmetry-effect of surface organize induced by thermal motion of molecule after absorbing light radiation. This means that Rayleigh scattering can change reflected lights.The reason of the light-induced discoloration of PVC sticker is from physical factor, which can be observed by the relative steady of carbon valence. Although the dramatic thermal motion of molecule change the asymmetry-effect of surface organize and the distribute of fuel molecule; the influence of diffraction can not be neglected.In conclusion, the reason and law of the light-induced discoloration are synthesized. This will provide theoretical basis for the scientific protection and optimized selection of the decoration materials. In addition, it provide a very important significance to improve production durability and the quality of human settlement.