Weathered wood, free radical

Sunlight, especially a small portion of UV light, is the principal instigator of weathering reactions. The immediate consequence of the interaction of wood with light is the generation of free radicals at the exposed surface (7, 19). As these labile free radicals terminate and stabilize, chromophoric and auxochromic groups are formed and discoloration and deterioration occur. 

Free Radical Characteristics and Reactions in Weathered Wood. Wood, wood fiber components, and isolated lignin contain certain amounts of free radicals that are detectable by ESR spectroscopy (SS, S9). Unexposed green wood with 69% moisture content (in dark and in vacuo) was found (77a) to contain no free radicals. A trace amount of free radicals may be produced in the presence of oxygen, and most of these free radicals are generated in wood during mechanical preparation (90) as well as in wood exposed to electromagnetic irradiation. ESR studies revealed that wood interacts readily... 

Effect of Water and Moisture on the Formation and Stability of Free Radicals. Water is considered to be a critical element in wood s weatherability. Because water is a polar liquid it readily penetrates and swells the wood cell walls. Water molecules may interact with free radicals generated by light. In order to study the influence... 

Wood apparently does not contain any intrinsic free radicals (26, 5i, 52). However, free radicals were generated by irradiation of wood with fluorescent light at ambient temperatures (52, 53). Rate of free-radical formation was enhanced when moisture content increased from 0 to 6.3%. The ESR and UV absorption studies on the behavior of generated free radicals and their interaction with oxygen molecules to form hydroperoxides revealed that free radicals and singlet oxygen play important roles in the discoloration and deterioration of wood surfaces. The chemistry of weathering and protection was summarized in a technical publication (2). 

Because of the low boiling point of vinyl fluoride ( —72°C), the free radical polymerization with BPO as initiator is carried out at 300 bar and 85 C. The polymer is partially crystalline and is more similar in its properties to poly(ethylene) than to poly(vinyl chloride). Since the melting point of the polymer is 2(XfC, it is processed at temperatures of 210°C. Films of poly(vinyl fluoride) are more stable to weathering than those of either PE or PVC. Consequently, poly(vinyl fluoride) is usually used for coating wood or metals. 

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