Photo-oxidation antioxidants

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Cells have substantial chemical defenses against the UV photoproducts produced in seawater and intracellular fluids. Many organisms have antioxidants (e.g., carotenoids, ascorbate, tocopherols, anthocyanins, and tridentatols) that quench photo-oxidative reactions.64-67 Cells also have enzymes (e.g., catalase and superoxide dismutase) that can counteract the oxidative nature of peroxides and other radicals.26 Some compounds, such as the UV-absorbing pigment melanin, can act as both optical filter and antioxidant.68 The MAA mycosporine-glycine (Figure 15.3) functions in a similar dual capacity.69 The role of UV-mediated reactions in seawater relative to biological effects is an important current area of study. 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

PospKil J (1981) Photo-oxidation reactions of phenolic antioxidants, In Developments in polymer photochemistry-2, Allen N S (Ed), Applied Science Publishers London, pp 53-133. 

Stabilisation to photo-oxidation can be achieved by the use of suitable UV absorbers in combination (synergistic action) with antioxidants (AH) which are capable of preventing reactions (a) and (b) ... 

Carotenoids are present in edible oils at different levels. These are powerful antioxidants against both autoxidation and photo-oxidation. Therefore, attempts have been made to retain them or recover them, as in the case of palm oil. However, carotenoids may be degraded to colorless products at high temperatures exceeding 150°C. 

Bagheri, R. Chakraborty, K.B. Scott, G. Mechanisms of antioxidant action behavior of a hindered piperidine and related oxidation products during processing and photo-oxidation of polypropylene. Polym. Deg. Stab. 1982, 4 (1), 1-16. 

Since UV absorbers cannot completely prevent the initiation reactions above, we felt that better antioxidants were necessary to obtain improved long-term photo-oxidative stability of polyolefins. We reasoned that stable radicals which scavenge radicals formed in the oxidative process might compensate for the imperfections of the absorbers. On the basis of the assumption that the diaryl nitroxyl radicals, rather than their parent diarylamines, are the active antifatigue species in rubber protection, we first attempted to obtain new stable nitroxyl radicals. 

Moore, D.E. (1976) Antioxidant efficiency of polyhydric phenols in photo-oxidation of ben-zaldehyde, J. Pharm. Sci., 65 1447-1451. 

---