The photodegradation of some naturally occurring polymers

This review contains a discussion of how chemical and spectroscopic methods have been used to unravel the complex photodegradation processes that occur when wool and paper (made from high yield pulps) are exposed to light. Both materials yellow when exposed to UV light and are bleached by visible light. In addition, both materials are bleached when irradiated in solutions of reducing agents. The photoyellowing of wool is,in the main, associated with tryptophyl residues and the degradation of the oxidation products of this amino acid. Tyrosine also contributes to the yellowing process, whereas cystine offers some protection and retards degradation. Degradation involves both singlet oxygen and radical species. Many of the degradation products are fluorescent and fluorescence spectroscopy can be used to gain an idea of the extent of degradation. Fluorescence microscopy shows that the tips of wool fibres (i.e. the weathered region) are more highly fluorescent than the roots. The degradation of lignin (an important constituent of paper made from high yield pulp) is due to photochemical reactions of carbonyl groups (including quinones), phenols, stilbenic derivatives, etc. Fluorescence microscopy shows that the paper exhibits a photochromic effect and that bleaching the paper with reducing agents leads to an increase in the fluorescence emitted at 400 nm. The most effective way of retarding the photoyellowing process is to apply a UV screen plus a reducing agent.