Irradiation experiment of urushi film with UV-A was carried out as the successive reference experiments of irradiation with UV-C and UV-B in the author's previous report. Summarizing the results of these experiments, monotonous weight decrease of urushi film during irradiation was found in all cases with UV -C, -B and-A, although these was some lapse of time ratio of weight decrease of urushi film in the same amount of irradiance of 10W/m² was 1 (UV-A): 8.1 (UV-B): 14.9 (UV-C). Spectral irradiance of solar UV radiation on the earth was nearly equal to 2 times of irradiance (UV-B) plus 3 times of irradiance (UV-A) in these experiments. Based on this result, deterioration of urushi film with solar radiation on the earth was estimated. At the same time, UV absorption spectra of three kinds of urushi films differing their thickness and FTIR spectra of UV-C irradiated urushi film were measured. By analyzing the abtained data, it was concluded that almost all UV radiation was absorbed by the thin surface layer of urushi film. From these experimental results, it was presumed that the weight loss of urushi film was due to decomposition of urushiol and its polymers in the surface layer.

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The structural changes of material caused by chemical reaction are reflected in the visco elastic properties of those substances. Such a visco elastic property, especially that responsible for relaxation behavior is termed chemical relaxation. Therefore, chemical relaxation is a relatively new discipline encompassing the physical and chemical aspects involved in the elucidation of the mechanisms, at a molecular level, of structural changes of the rubber vulcanizates in nitrogen at high temperature during their degradation. This paper deals with the scission mechanisms of natural rubber vulcanizates by stress relaxation means in nitrogen at high temperature.

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[in Japanese]

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