Photo-antioxidants

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

Note 1 Antioxidants acting under specified conditions may be referred to as thermal antioxidants, photo-antioxidants, mechano-antioxidants, etc. 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

Antioxidant, Anti-Flex-Crack and Photo-Antioxidant Activities... 

The strength of the acid HX plays a decisive role in the fate of HAS. Salts of weak acids, like carboxylic acids formed in oxidized PO or used as processing aids, benzoic acid or carbonic acid are evidently not dangerous from the point of view of HAS depletion [228]. This was also envisaged in mechanistic studies dealing with the conversion of Af-methylated HAS into the salt of formic acid and secondary HAS 122, not depleting the photo-antioxidant effect. 

Usually, commercial polymers contain a number of additives (thermal and photo antioxidants, plasticizers, etc.) that can be suitably extracted from tire polymer bulk and analyzed separately. Polymers may contain low-mass oligomers, both linear and cyclic, and e identification of these species can be achieved by analyzing the intact polymer, or by extracting the oligomers from fhe polymer bulk. 

Chain breaking - donor antioxidants such as hindered phenols are ineffective under conditions of photo-oxidation due to their rapid consumption in the higher rates of initiation present compared to thermal oxidation, and to photodegredant activity of their derived molecules such as quinones and quinone methides. Although they have limited photo-antioxidant ability when used alone, they can be protected from photoljnic destruction by, for example, UV absorbers, and for this reason they can produce good synergistic results with a variety of co-stabilisers. 

Several approaches to stabilisation can be used. Light stabilisers will be discussed under four broad categories light-absorbing fillers organic UV absorbing compounds excited state quenchers and photo-antioxidants. 

Many sulfur-containing PD antioxidants, especially the metal dithiolates, are very effective long-term thermal stabilizers (also melt and photo antioxidants see later). This versatility in antioxidant action is the direct consequence of their highly effective PD activity (54,105,107,136-140). Moreover, the effectiveness of dithiolate antioxidants, within any one series depends strongly on their solubility in the polymer. For example, in the case of nickel dialkyldithiophosphate (NiDRP, AO 23, Table 3) series, the lower molecular weight members of the series are less soluble, and less effective than the higher members (see Fig. 3) (141). The zinc dialkyl dithiocarbamate (ZnDRC, AO 21) series too show similar behavior (142). 

Hindered amine light stabilizers have attracted enormous scientific and commercial interest due to their efficiency as photo-antioxidants. The stabilization process in the presence of HALS involves the interference of secondary >NH or tertiary (>NCH3) groups with hydroperoxides POOH generated by oxidation chain mechanism of polymer alkyl radicals P. HALS-derived nitroxides >NO, formed through scavenging peroxyls POO by hydroxylamine species >NOH, are the key intermediates in the HALS stabilization mechanism because they are able to quench alkyl radicals arising in the initiation step of oxidation [100, 101]. The involvement of HALS in the stabilization process can be attributed to their ability to remain an... 

Dobashi, Y., Kondou, J., Ohkatsu, Y. Photo-antioxidant abilities of 2-hydroxybenzoyl compounds. Pol5nn. Degrad. Stab. 89, 140-144 (2005)... 

R2NCSS)2M Dialkyl dithiocarbamates M = Fe, thermo-antioxidant, photo-prooxidant M = Zn, thermo-antioxidant, weak photo-antioxidant M = Ni, thermo-antioxidant, photo-antioxidant... 

Many dithiocarbamate complexes (MDRCs) are very sensitive to sunlight and so long as the sulphur ligand remains intact, they are photo-antioxidants but they invert to photo- and thermo-prooxidants after photolysis. ... 

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