Study On Discoloration Components And Photodiscoloration Mechanism Of Teak Wood(Tectona Grandis Linn.fil.)

The market value of teak is enhanced by the change of color from brown to yellow.The purpose of this paper is to explore the reason of discoloration and the mechanism of photodiscoloration of teak through experiments,and to provide experimental and theoretical support for the establishment of wood color control technology.In this paper,the relationship among photodiscoloration of teak and lignin,composition of extracts and chromaticity parameters was studied.The composition and reaction mechanism of the chromogenic substances in teak were discussed through characterizing the discoloration process of typical chromogenic components in teak.The relationship between reaction type of photodiscoloration of teak and chromaticity parameters was preliminarily confirmed through artificial induction the color of teak veneer,which laid a foundation for the theory and technology of directional discoloration and fixation of teak veneer.The main results included as follows:(1)Teak veneer specimens were exposed to xenon lamp light for 80 h.The color changes before and after exposed were analyzed by naked eye and colorimeter.The results showed that teak turned yellow and red as the irradiation time increased,and the chromaticity parameters L*,a*and b*increased in the end.The irradiation for 40 h was considered the key for producing golden yellow teak.(2)The surface morphology of teak veneer was observed by SEM.The results showed that with increasing irradiation time,the teak surface and cell wall pits are abrasion.The surface crystallization characteristics of teak veneer could be analyzed by XRD.The results showed that the teak crystallinity caused by irradiation was 0.81%lower than that of untreated samples,which indicated that the change of crystallinity contributed less to the color of teak.(3)The changes of surface functional groups of teak veneer were analyzed by FT-IR.The results showed that the color changes of teak surface during simulated exposure were closely related to lignin.The degradation of lignin was accompanied by the generation of carbonyl derivatives,which could change the total light-absorbing wavelength of the teak veneer and alter the color towards red and yellow.It is suggested that the degradation of lignin was one of the reasons for the photodiscoloration of teak.(4)The solvent of 2:1(V/V)benzene-ethanol was the best to study the photodiscoloration of teak.Benzene-ethanol extract were analyzed by FT-IR,UV and GC-MS.The results showed that the photooxidation of tannins,flavonoids,terpenoids and volatilization of some volatile organic compounds in extracts,such as anthraquinones,can lead to the discoloration of teak.Water and ethanol have strong extraction ability for small molecular lignin in the extraction process.At the same time,water and ethanol have strong promotion effect on lignin degradation during irradiation.(5)The structure of the monomer model before and after irradiation were analyzed by UV,HPLC and GC-MS.The results showed that gallic acid,quercetin and eugenol monomers were prone to occur photooxidation,polymerization and esterification reactions under acidic,aerobic and light conditions,which led to the formation of new conjugated systems.2-Methylanthraquinone volatilized easily after irradiation.In addition,quercetin and gallic acid produced other and two weak polar substances.Eugenol could be isomerized to isoeugenol,and propylene group on the benzene ring of isoeugenol could be oxidized to aldehyde group to produce vanillin.Furthermore,in the presence of water,the conjugated double bonds in isoeugenol breaked down to produce quinone intermediate.And the side chain promotes the formation of ?,?-dihydroxy structure,which could be further oxidized to derivates containing C=O,?-COOH.It was difficult to change the structure of 2-Methylanthraquinone with irradiation and aerobic environment.However,a small amount of 2-methylanthraquinone may isomerize to anthraquinone or other anthraquinones.(6)Teak veneers could be induced to show different colors by using different kinds of reagents during irradiation,which indicated that teak can be packaged into desired colors by artificial induction.The change direction and degree of chromaticity parameters of teak treated with water and irradiation were most similar to that of the teak veneer treated with HNO3,which means that the reaction type of teak veneer during irradiation was the oxidation reaction after HNO3 treatment.