is the oxidation of a polymer surface in the presence of oxygen or ozone that is facilitated radiant energy such as UV or artificial light. This proces is often the the most significant component of weather degradation of polymers. Photo-oxidation results in chemical changes and normally a reduction in the polymer's molecular weight. Consequently, the material becomes more brittle coinciding with a reduction in tensile strength, impact strength, elongation at break. Often discoloration and loss of gloss accompany photo-oxidaiton. High temperature and stress concentration are significant factors to increase reaction rate of photo-oxidation.
General mechanism of polymer photo-oxidation
In the photo-oxidation reaction including chain scission, cross linking and secondary oxidative reactions, the following step can be considered:
- Initial step: Free radical formation can be formed by light absorption.
- Chain Propagation step: The free radical reacts with oxygen to produce polymer peroxy radicals (POO•) and generate polymer hydroperoxide (POOH) and new polymer alkyl radical (P•).
- Chain Branching: The formation of polymer oxy radicals (PO•) and hydroxy radicals (HO•) can be formed by photolysis.
- Termination step: Cross linking is a result of the reaction of different free radicals with each other.
where PH = Polymer
P• = Polymer alkyl radical
PO• = Polymer oxy radical (Polymer alkoxy radical)
POO• = Polymer peroxy radical (Polymer alkylperoxy radical)
POOH = Polymer hydroperoxide
HO• = hydroxy radical
Aldehydes, ketones and carboxylic acids either along or at the end of polymer chains are generated by oxygenated species in photolysis of photo-oxidation. The major need to occur initiation of photo-oxidation reactions is the existence of chromophoric groups in the macromolecules. Generally photo-oxidation, however, can occur simultaneously with thermal degradation and one of these can accelerate another.
Adding pigment light absorbers and photostabilizer known as UV absorbers are the ways to protect photo-oxidation in polymers. Antioxidants are another prominent method to inhibit the formation of hydroperoxides in photo-oxidation process.
Effects of dyes/pigments
Dyes and pigments are used in polymer materials to give color changing properties. These additives can change the stability against degradation. Such as Cu-phthalocyanine dye can stabilize against degradation and as well as accelerate degradation in other situations. Cu-phthalocyanine incorporated into polycarbonate (PC) results in an acceleration of the degradation when subjected to photochemical aging. This is based on the idea that PC containing Cu-phthalocyanine where specific interactions among the excited states of PC and Cu-phthalocyanine take place and enhance the formation of relative species in PC. The excited Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in the PC, which increase the formation of free radicals that act as the starting points for the sequential photo-oxidation reactions that lead to the degradation of the PC. Also electron transfer sensitization is another possible mechanism where the excited Cu-phthalocyanine abstracts electrons from PC to form Cu-Ph radical anion and PC radical cations. These species in presence of oxygen can cause oxidation of the aromatic ring.
Protection against photo-oxidation
Poly(ethylene-naphthalate) (PEN) can be protected by applying zinc oxide coating, this film acts a protective layer where it reduces the diffusion of oxygen with PEN. Zinc oxide can also be used on polycarbonate (PC) where often used in outdoor applications. Again the zinc oxide can protect PC form solar radiation and decrease the oxidation rate and photo-yellowing of PC.
- Grassie, N & Scott, G 1985, Polymer Degradation Stabilisation, Press Syndicate of University of Cambridge, England
- Schnabel, W 1981, Polymer Degradation: Principles and Pratical Applications, Macmillan Publishing Co., Inc, New York
- Photo-Oxidation, accessed 22/04/08, http://www.ozonelab.com/glossary/p/photo-oxidation.htm