| Summary: | 博士 === 國立臺灣大學 === 森林學研究所 === 88 === The effects of chemical modifications on the durability
of wood were evaluated in this study. After acetylation,
butyrylation or hexanoylation, the amounts of the hydroxyl
groups reduced and then the hygroscopicity of treated wood decreased. Acylation of wood components increased the moisture excluding efficiency of wood and retarded the absorption of water. With the same amount of hydroxyl group substitution, among three acylation treatments the higher moisture excluding efficiency of hexanoylated wood was achieved because its substituted functional group had the greater molecular weight. The improvement in dimensional stability of modified wood was due to the bulking effect of wood after chemical modification. Among the chemical modification treatments, the glyoxal treatment, the isopropyl glycidyl ether treatment, butyric anhydride treatment, and hexanoic anhydride treatment exhibited the superior dimensional stability. The thermostabilities of chemical modified wood were examined by thermogravimetry. The
cleavage of grafted ether bonds or ester bonds of China fir
treated with glyoxal, butylene oxide, succinic anhydride,
maleic anhydride, and phthalic anhydride, respectively, ccurred at between 150 ℃ to 300 ℃. On the other hand, the thermostability of China fir treated with isopropyl glycidyl
ether, glycidyl methacrylate, acetic anhydride, butyric anhydride, and hexanoic anhydride, respectively, was the
same as that of the untreated specimen. The thermostability
of maple treated with the same chemical reagents, especially with isopropyl glycidyl ether, was ameliorated by improving the thermostability of hemicelluloses and lignin after treatment. The structure of the cellulose was deteriorated after the decay test with rot fungi, which resulted in the decrease of the maximum decomposition temperature of wood. Among the etherification treatments, the glycidyl methacrylate treatment, the butylene oxide treatment, or the isopropyl glycidyl ether treatment can meliorate the decay resistance of wood. Among the esterification treatments, the weight loss percentages of wood treated with phthalic anhydride, acetic anhydride, butyric anhydride, and hexanoic anhydride, respectively, were greatly reduced after decay test. Based on the thermogravimetry analysis, the maximum decomposition temperatures of wood treated with isopropyl glycidyl ether, acetic anhydride, butyric anhydride, and hexanoic anhydride, respectively, were
unchanged after decay test. It demonstrated that the degree of polymerization of cellulose of these treated woods didn''t decrease after decay test. Due to the photochemical oxidation, the structure of lignin was decomposed after exposure to the ultraviolet light. The destruction of cellulose resulted in the decrement of crystallinity with the irradiation time. It couldn’t effectively inhibit the photoyellowing of the wood by coating with clear coatings. To illustrate the photoyellowing and the chemical structure alterations of wood beneath the clear coatings, the irradiated free films and irradiated wood specimens underneath the free films were examined. The results revealed that some ultraviolet light penetrated the free films reached the interface between wood and coating film. The addition of light stabilizers into coating could slow down the
photodegradation of wood underneath by reducing more amount of the penetrating ultraviolet light. After the lightfastness testing, the color difference and yellowness index of butyrylated wood were significantly reduced, indicating that butyrylation inhibited the photoyellowing of wood. Results of the IR analysis, ESR analysis, and the diffuse reflectance spectrum demonstrated that the inhibition of photodiscoloration of butyrylated wood was caused by reducing the formation of the phenoxyl radicals after irradiation, and then significantly decreased the colored chromophoric derivatives such as quinoid compounds formed on the wood surface. It was also proven that the change in the chemical structure of butyrylated milled wood
lignin after irradiation was less than that of untreated milled
wood lignin. Hence, the lightfastness of wood can be improved
by butyrylation. According to the above results, it is feasible
that, in order to satisfy the specific requirements for wood
utilization, the durability of wood products can be prolonged
by treating wood with proper chemical modification.
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