Fluorescent Whitening Agents

Abstract

Ultraviolet absorption and fluorescence spectroscopy have been used to determine the nature and extent of photo-decomposition of fluorescent whitening agents in polymer films and in surface-whitened wool. Irradiation of the coumarin (I) in polymer films at 365 nm gives rise to a mixture of photodimers which rapidly revert to I on irradiation at wavelengths less than 320 nm. Irradiation of the stilbene (II) at 365 nm also gives a photodimer which absorbs strongly around 350 nm, but this reverts only slowly to II on irradiation.

Fluorescence studies indicate that there is an increase in the degree of aggregation of I in polymer films with increasing whitener concentration, and that the kinetics of whitener photodecomposition change from first-order at low whitener concentration (0.0025%) to second-order at the higher concentration (2.5%) used in surface-whitening. The coumarin (I) is more stable to light in poly(vinyl acetate) film cast from formic acid than in films cast from less polar solvents, probably because of differences in the degree of whitener aggregation. The photostability of I in poly(vinyl acetate) film is also improved if the film is heated for short periods or stored at room temperature for longer periods.