Photodegradation and photo-oxidation of synthetic polymers

Abstract

Most synthetic polymers are susceptible to degradation initiated by ultraviolet and visible light. Of particular interest is the near UV radiation (290–400 nm) in the sunlight at sea level which usually determines the lifetime of polymeric materials in outdoor applications. The mechanisms of the degradation and oxidation reactions are in principle fairly well known: • Extraneous groups and/or impurities in the polymer absorb light quanta and form excited states, initially singlets (short-lived) which are transformed to triplets (long-lived and reactive). • Excited triplet states may cleave the polymer chains and form radical pairs (Norrish Type I reaction) or form pairs of saturated and unsaturated chain ends by hydrogen transfer (Norrish Type II reaction). • The polymer radicals formed add molecular oxygen (in triplet ground state) to peroxy radicals which abstract hydrogen and form hydroperoxide groups. • The hydroperoxide groups absorb UV light or become excited by energy transfer, the weak OO bonds break and pairs of alkoxy and hydroxyl radicals are formed which may react in various ways, e.g. by hydrogen abstraction, chain scission, rearrangement, etc. This means accelerated photodegradation. • Double bonds may add excited oxygen molecules in singlet state. In this reaction the double bond is shifted to an adjacent CC bond and a hydroperoxide group is formed. Some synthetic polymers, e.g. aromatic polyesters and polyamides, have inherent absorption of UV light which give excitation, radical formation, oxygen addition, splitting off of small molecules, chain scission, etc. as previously described. Some of these polymers are autostabilized towards photodegradation by formation of an oxidized surface layer with high absorption of near UV and visible light of short wavelengths which prevents further penetration of light into deeper layers. The photoinitiated reactions are studied by electron spectroscopy for chemical analysis (ESCA), IR and electron spin resonance spectroscopy (ESR) and the reaction products analyzed by gas chromatography and in particular gas chromatography/mass spectrometry for complex structures in the same way as applied in pyrolysis.