The Effect Of Conjugated Structure Change On Color Of Eucalyptus Urophylla ×Eucalytus Grandis During Heat Treatment

Eucalyptus urophylla ×Eucalyptus grandis wood is a kind of ideal tree species which is a short-period and fast-growing industrial material. However, uneven color of Eucalyptus urophylla ×Eucalyptus grandis wood affect its appearance and economy value significantly. Heat treatment can darken the color of wood and improve its utilization value. In this thesis, temperature, moisture content and heat treatment duration were manipulated and eucalyptus wood extractive samples were treated in different heating conditions. Chroma indexes and compound structures change of eucalyptus extractive were measured. Structure changes of several polyphenol compounds and its relationship with color after heat treatment were analyzed. Their chemical mechanism of conjugated chromophoric groups’ formation was also studied. The experimental results showed that:(1) Polar solvents extraction has better effects on discoloration of Eucalyptus urophylla ×Eucalyptus grandis wood. Hot water extractives showed greatest influence on wood color. After heat treatment, the amount of conjugated structure increased and energy of conjugated system decreased. Because of its absorption band extended to visible range, Eucalyptus urophylla×Eucalyptus grandis wood flour color darkened, which showed a uniform appearance and improved its utilization value.(2) Alcoholic extractives of Eucalyptus urophylla ×Eucalyptus grandis wood mainly includes: tridecanal, dodecanal, dihydrochalcone,2-tridecanone, heptadecanone, quercetin, ellagic acid, cinnamic acid and so on. Those compositions contain chromophoric groups and auxochromic groups, which were oxidized after heat treatment and lead to wood discoloration.(3) After heat treatment, the L of polyphenol compounds decreased, while a*, b* and C* increased. The total color change (AE*) was greater after treatment at 140℃, and the most significant factor of which was oxygen. During heat treatment, hydrolysis reaction and oxygen reaction occurred. Discoloration of polyphenol compounds was attributed to formation of saccharides, polyhydric alcohols, phenolic hydroxyl groups, double structures such as quinoid structures, carboxylic acid, conjugated carbonyl, polymerized quinine and so on. Those substances can lead to the darkness of polyphenol compounds and further cause the discoloration of wood. Among 4 kinds of polyphenol compounds tested in this thesis, dihydrochalcone has greater effect on discoloration of Eucalyptus urophylla ×Eucalyptus grandis wood.