| Wood color is an important index for evaluating the value and surface properties of wood products.In the process of high value utilization of artificial fast-growing forests,color control is an important technological link.Conventional color-matching mechanism is to attach colored substances(such as dyes)into wood chromogenic system.The study of wood induced discoloration found that regulations of wood chromogenic system under specific conditions can also achieve color regulation.The method of controlling wood color by regulating wood chromogenic system provides a new idea and method for the wood color optimization,which lays a theoretical foundation for the in-depth study of wood color regulation and has important scientific significance for the high-value utilization of artificial fast-growing forests.In this paper,Eucalyptus grandis*E.urophylla was used as color-matching material,Pterocarpus Macarocarpus Kurz was used as color-matching target material,thermal medium color-matching,metal ion coordination reaction(grafted metal ions),Maillard reaction(grafted amino and carbonyl groups)were used to coordinate and control wood conjugated chromogenic structure.The aim of directional regulation of wood color is to reveal the mechanism of directional regulation and lay a theoretical foundation for the application of directional regulation technology of wood color.The main conclusions are as follows:(1)In the conjugated chromogenic structure of Pterocarpus grandis,lignin is mainly composed of guaiacyl G and p-hydroxyphenyl H units,with higher degree of condensation,molecular weight and content of conjugated carbonyl groups;the extract has a significant effect on the color of Pterocarpus Macarocarpus Kurz.The lignin of Eucalyptus grandis is mainly composed of syringyl S and guaiaceous G structure units,with small molecular weight and high methoxyl content;the exrtract content is low and has negligible effect on the color of Eucalyptus grandis wood.(2)The color structure of lignin in Eucalyptus grandis tends to be Pterocarpus grandis lignin due to thermal medium coloring process:the basic structural units of syringyl S decrease,the proportion of guaiaceous lignin increases;the structures of lignin non-conjugated carbonyl and unsaturated structure increases;and the molecular weight of lignin increases due to the condensation reaction of lignin.The optimum color matching process is to treat wood at 130℃ of saturated steam.(3)Iron(Ⅲ)ions were introduced into wood chromogenic system during the coordination reaction of metal ions.Two kinds of colored substances were formed during the reaction:red-brown iron oxide(Fe2O3)monomers and Fe(Ⅲ)ion complexes.The optimum process is to treat wood with 0.05 mol/L Fe(Ⅲ)ion solution at 100℃.(4)Amino and carbonyl groups were grafted into wood chromogenic system to increase the proportion of conjugated carbonyl groups in wood chromogenic structure.Amino and hemicellulose formed N-C and N-0 bonds.The proportion of G-structore units increased,which resulted in the increase of conjugation degree and absorbance of wood.The optimum process is to pretreat the treated wood with 0.5 mol/L glycine aqueous solution at 100℃ in water bath and then heat air at 190℃.(5)In the process of synergistic color matching,besides the product formation of individual reaction,Fe(Ⅲ)ion and Maillard reaction product(MRP)form a coloured macromolecule complex with Fe(Ⅲ)ion as the coordination center.As a result of the superposition effect,more conjugate systems were formed,which increased the absorbance,darkened the color and decreased the bar value slightly.The process of increasing a*value depending on the content of Fe2O3.The process closest to the target wood was to coordinate wood flour with Fe(Ⅲ)ions,and then Maillard reaction with glycine(Fe(Ⅲ)+MR). |
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