Study On Colorfastness To Light Of Wood And Modified Wood

Colorfastness to light is the resistance of a material to change in its color characteristics asa result of exposure of the material to sunlight or an artificial light source. With the broadapplication in outdoor, the fact that wood can be easily discolored by sunlight irradiation hasbeen paid much attention by the publics.Until now,investigation on wood photo-discolorationhas been developing step by step in China..In this paper, accelerated ageing test was introducedto study the photo-discoloration of wood and modified wood.Meantime,the CIEL*a*b*(1976)was used to measure the discoloration after photo-irradiation,by which the mechinisms ofphoto-discoloration of wood and modified wood were discussed.Twelve wood species such as Sapelli,Okan,Teak and Kosso were introduced to theaccelerated ageing test in order to analyze the differences between the photo-discoloration ofdifferent wood.Furthermore,Fourier Transform Infrared Spectroscopy was applied to test thespectrums of wood surface after photo-discoloration.And the mechnisms ofphoto-discoloration of wood was discussed by changes of the assignments peaks of woodsurface functional groups in FTIR spectrums. In addition, colorfastness to light affected byextraction and heat-treatment were discussed by comparing the differences ofphoto-discoloration before and after treatments.The main conclusions are listed below:1) All the twelve wood species in the experiment were easily discolored after xenonirradiation,especially during the first40hours irradiation. The increase of color changes tendedto be mild with the extension of irradiation time. After photo-irradiation,the suface of Poplarhad the most obvious color change (△E*=24.24)while the minimum color changes occurredon the surface of Teak(△E*=3.85).The photo-discoloration of the twelve wood species areclassified by two types according to the anaylsis of spetral reflectance curve.Wood specieswhose L*value decreased,a*and b*value increased after irradiation were classified as type1.The other wood species whose L*,a*and b*value all decreased were classified as type2. 2) Detected from the FTIR spectrums,the lignin decay and the formation of unconjugatedcarbonyl and conjugated carbonyl of sapelli, teak,kosso and purple heart increased sharply after100hours xenon irradiation.But there is only increase on lignin decay and formation ofunconjugated carbonyl detected in FTIR spectrum of okan. Besides,there is correlationbetween unconjugated carbonyl formation and color change in sapelli,okan and teak.And thecorrelation between lignin decay and color change only exist in okan,teak andkosso.Meantime,there is correlation between conjugated carbonyl and color changes in teak.3) Benzene-ethanol extraction for48hours affected the photo-discoloration ofteak,sapelli,purple heart and okan.And the purple heart discolored quickly when the extractivesexist.But the extractives can protect the fast color changes of sapelli and teak.Inaddition,although the photo-discoloration of okan were not affected by the extractives, the finalcolor changes of okan after irradiation was different with and without extractives.4) Heat-treatment affected the photo-discoloration of larch and poplar in the same way.The heat-treatment larch and poplar L*values decreased,a*values decreased firstly followed aslight increase,and b*values increased during the irradiation. Most importantly, thephoto-stability of heat-treatment wood was higher than the photo-stability of wood withouttreatment.