THE PHOTODEGRADATION OF SOME NATURALLY-OCCURRING POLYMERS

This review contains a discussion of how chemical and spectroscopic me thods have been used to unravel the complex photodegradation processes that occur when wool and paper (made from high yield pulps) are expos ed to light. Both materials yellow when exposed to UV light and are bl eached by visible light. In addition, both materials are bleached when irradiated in solutions of reducing agents. The photoyellowing of woo l is,in the main, associated with tryptophyl residues and the degradat ion of the oxidation products of this amino acid. Tyrosine also contri butes 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 o f degradation. Fluorescence microscopy shows that the tips of wool fib res (i.e. the weathered region) are more highly fluorescent than the r oots. The degradation of lignin (an important constituent of paper mad e from high yield pulp) is due to photochemical reactions of carbonyl groups (including quinones), phenols, stilbenic derivatives, etc. Fluo rescence microscopy shows that the paper exhibits a photochromic effec t and that bleaching the paper with reducing agents leads to an increa se in the fluorescence emitted at 400 nm. The most effective way of re tarding the photoyellowing process is to apply a UV screen plus a redu cing agent.