The Mechanism Of Thermal Induced Discoloratio Of Black Locus

Black Locus(Robinia pseudoacacia L),is originally planted in the USA.After it is planted in China, it is popular and welcomed because its good adaptivity,quick growth and wide usage. In China it is widely planted in North China, northwest and northeast. With good shock strength, Black Locus is used to produce furniture, floor, particleboard, fiberboard, plywood, etc. Because of the thermal discoloration, the utilization of Black Locus is limited. Through the heat treatment, Black Locus wood can get beautiful color and more economic value. To get the relation between the lignin structure changes and the heat discoloration of Black Locus, and the relation between the extracts and the heat discoloration, the colors and chemical changes of Black Locus before and after the thermal treatment were studied. To find the mechanism of the thermal discoloration of Black Locus, the milled wood lignin and Brauns lignin from Black Locus before and after the thermal treatment were studied by the IR, UV and H-NMR. The main results are as follows.1.The L﹡value of the water extracted Black Locus decreased, the L﹡value of the acetone extracted Black Locus increased. After the water and actone extracted, the a﹡value increased, and the latter increased more. After the water and acetone extracted, the b﹡value decreased, and the former decreased more. Theâ–³E﹡of water extracted wood was bigger than the acetone extracted wood.After the water and acetone extraxed, the E﹡of heat-treated Black Locus still changed obviously, which indicated that the residual lignin was one of the reason of the heat discoloration of Black Locus.2.Under 120℃thermal treatment, the MWL absorption peaks of conjugated C=O(1660 cm^-1) and unconjugated C=O(1595cm^-1) increase obviously in the IR spectra; because of the new conjugated structures, the molar absorptivity increased in the area of 300-350nm and 380-500nm, and the color of the lignin becomes darker. In dry and O2 condition, the lignin is oxidized and produces new conjugated C=O, quinonoid structure(1660cm^-1, 1595cm^-1)and unconjugated C=O structures(1730 cm^-1),but had smaller changes compared to the steam thermal treatment. After the Black Locus lignin treated at the 140℃and 160℃, theαunconjugated C=O(1660 cm^-1)andβunconjugated C=O (1730 cm^-1) decrease firstly, and increase latter with time process. At the same time there was slightly demethylation. Under 140℃and 160℃thermal treatment, both the guaiacyl and syringyl decrease with the time process.3.After the Black Locus Brauns lignin treated at 120℃, the molar absorptivity increases and the R absorption band becomes wider in the UV spectra which cause the absorption in the visible light region and the darker color of the lignin. The weak acid condition can cause the discoloration extent.It can be found from the FTIR spectra among the treated samples that there happened the substitution reactions in the benzene ring which may result in the condensation of the lignin unit, the phenolic hydroxyl group increased after thermal treatment because of the broken of theα-o-4 and/orβ-o-4 structure and the conjugated C=O group increased, which could be the important reason for the darkening of lignin.In the ¹³C-NMR spectra, the phenolic guaiacl and syringle units increased after the thermal treatment, and the condensation happened in the C5 and side chain which causes the the elongation of conjugation system, the extra absorption in the visible spectrum and the discoloration of lignin and wood.