| Abstract: | CONCLUSION. The deterioration of wood materials upon weathering involves a very complex reaction sequence. Penetration of UV light into wood does not traverse deeper than 75 m. Nonetheless, wood surface reacitons initiated or accelerated by light can be observed by discoloration, loss of brightness, and change in surface texture after artificial UV light irradiation or long-term solar irradiation . Free-radical species are readily generated in wood by light. These radicals rapidly interact with oxygen to produce hydroperoxide impurities that are decomposed easily to produce chromophoric groups such as carbonyl and carboxyl groups. The use of singlet oxygen generators, such as rose bengal and methylene blue, as well as singlet oxygen quenchers, such as 1,4-diazo-bicyclo [2.2.2] octane and triethylamine, suggests the participation of singlet oxygen as an effective intermediate in photooxidation reactions at the wood surface. The presence of water in wood also influences the rate of free-radical formation. When moisture content in wood is increased from 0 to 6.3%, more free radicals are formed. Beyond this stage, the rate of radical decay increases. IR studies reveal that carbonyl groups are generated in cellulose and lignin. Water-soluble fractions of degraded wood exhibit characteristics of phenolic absorptions due to the loss of lignin. ESC A studies show that oxidized wood surfaces contain higher oxygen contents than unexposed wood surfaces. A general mechanistic scheme, able to account for the weathering, or more commonly photooxidation, of wood surfaces, is illustrated in Fig. 26. Free-radical chain reactions in the presence of oxygen and light are responsible for the discoloration and deterioration of wood surfaces. |
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