Stabilization of polymers against photodegradation

Exposure of polymers to sunlight or to artificial light sources results in a more or less rapid deterioration of their physical and mechanical properties as a consequence of either primary and secondary photochemical processes or of subsequent photo-initiated thermal reactions. 

Primary photochemical processes lead to decomposition of the molecule in less than 10 12 s, through a dissociative excited state or through excitation above the dissociative limit of an upper state  

Secondary photochemical processes are chemical reactions involving excited species (in either the triplet or singlet state) produced by electronic excitation of the molecule  

Photo-initiated thermal reactions can follow either primary or secondary photochemical processes. They usually involve free radicals produced by photodissociation or other unstable intermediates  

Stabilization of polymers against photodegradation can be obtained by inhibiting or preventing one or several steps of the over-all photochemical reaction. This result is usually achieved by the use of suitable additives. 

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Stabilization of Polymers Against Photodegradation - chain branching ... 

And so, the work on mechanisms of autooxidation at the British Rubber Producers Association, the early work on the synthesis and reaction of stable free radicals, the recognition of the rale of stable free radicals in polymer stabilization, the discovery of stable triacetonamine-N-oxyl, and the search for practical candidates for commercialization, have led to the development of hindered amine stabilizers, a new class of polymer stabilizers. They are effective in many polymers against photodegradation and also are effective against thermooxidation in some polymers. The structures of the current commercially available products for polymer stabilization may be seen in Figure 7. These compounds are effective in meeting the stabilizer requirements in many commercial polymers however, others are under development to satisfy requirements not being met by them. 

Most hindered amine light stabilizers have evolved from the discovery (4) that compounds containing a 2,2,6,6-tetramethylpiperidine moiety can stabilize polymers against photodegradation and this moiety has been incorporated into HALS of various types (Figure 1). Much has been published on the mechanism of action of HALS and the literature in this area has recently been critically reviewed (5,6). While the complete mechanism of action has not been fully elucidated, the high performance of HALS is generally attributed to the ability of their oxidation products to... 

Hill, C.A.S., etin, N.S., Quinney, R.F., Derbyshire, H. and Ewen, R.J. (2001). An investigation of the potential for chemical modification and subsequent polymeric grafting as a means of protecting wood against photodegradation. Polymer Degradation and Stability, 72(1), 133-139. 

Already carbon black is recognized for its ability to stabilize polymers against degradation by UV irradiation. However, basically three different major classes of light stabilizers can limit and slow down the photodegradation of polymers ... 

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