Kinetic Consideration of Photofading of Fluorescent Brightening Agent

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Fluorescent brightening agents, 1, 2-bis 5-methylbenzoxazolypethylene and its derivatives, are dimerized by uv light both in solution and in polymer film. With increase in concentration, the rate of the fading of the dye increases to a certain limit and then decreases. This tendency is more pronounced in polymer film, such as cellulose diacetate and polypropylene, than in ethanol and benzene solutions.In this paper, behaviors of fading of the dye in solution and polymer film are discussed from the chemical kinetic point of view.If diffusion of the dye is assumed to be restricted strongly in polymer film, rate of fading of the dye molecule will decrease with increasing distance from the irradiating surface. A rate law of fading under restricted diffusion is constructed and solved by assuming several initial conditions.The calculations, however, show that restricted diffusion has no effect on the overall decay of the dye (Table 2).Another rate law (equation (10)), constructed by assuming the retardation effect, can not afford a reasonable explanation of the observed decay curves.On the other hand, the experimental results in solution and polymer film appear to be accommodated by considering the simultanious trans-cis photoisomerization and photodimerization, if steady state approximation for concentration of cis-isomer is assumed (Table 4). The decay in cellulose diacetate film is explained by assuming that the photoreaction proceeds in two phases: the one is discrete dispersion and the other assembly phase, and that considerable amount of the dye molecule is restrained in the former phase.The kinetic parameters in solution and in polymer film are compared and discussed (Table 3).