Antioxidants photo-sensitive

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,). 

It may be anticipated that the antioxidant consumption during use leads to a shorter remaining life-time and that it is necessary to add more. Recycled plastics suffer from the consequences of degradation during processing and first-life application. This leads to the introduction of new functional groups, which in particular for oxidisable polymers such as PE, enhance the sensitivity of the recyclate to thermal- and photo-degradation. With the formation of new pro-oxidative moieties, a substantial part of the stabilisers are simultaneously consumed [14]. 

As with thermal stabilizers, photostabilizers must satisfy basic chemical and physical requirements (see the section titled ""Antioxidant permanence effects of chemical and physical factors ). In addition, they must be photo-stable, i.e., stable to UV-light, to withstand continuous periods of UV-exposure, without being prematurely destroyed or effectively transformed into sensitizing products. There are essentially three classes of compounds that are categorized as photostabilizers/ photoantioxidants UV-absorbers and pigments, peroxide decomposers including nickel complexes, and sterically hindered amine light stabilizers. 

Polybutadiene.—Sensitized photo-oxidation of polybutadiene and butadiene -styrene copolymers has demonstrated the suceptibility of such polymers to attack by singlet oxygen/ " Conversely, quenchers of singlet oxygen act as stabilizers for the photo-oxidative decomposition of polymers containing unsaturation. Rabek and Lala have shown that a number of carotoneoids are effective stabilisers for polybutadiene. Nitroxides and their precursors have been shown to be effective antioxidants. Kelleher and co-workers found that in ABS copolymers deterioration occurs in the butadiene segments. They have also found that carbon black is an effective stabilizer. ... 

Food sample pretreatment may consist of either (a) saponification to quantify the free forms (retinol or xanthophylls may occur free or ester-ified in foods) [95,96] or (b) direct extraction to determine the unaltered A vitamers [84,88]. Alkaline hydrolysis is also an expedient to simplify the vitamin A analysis, since retinol is the only form to be quantified nevertheless, due to its sensitivity to light and oxygen, it is important to prevent photo-oxidation by inclusion of a antioxidant (ascorbic acid, hydroquinone, or pyrogallol). A drawback of hot saponification is the generation of artifacts, such as geometric isomers of retinol and carotenoids [97]. 

The genus Curcuma includes some 30 species, native to India and the southeast of Asia. An extract is produced from the rhizomes, whose main components are curcumin and its derivatives. This extract has well-known anti-inflammatory, antioxidant, wound healing, anti-microbial and tyrosinase inhibitory properties. Its versatility makes it suitable for a number of cosmetic formulations on the basis of its anti-ageing, photo-protective, sensitive skin treatment, skin whitening, antiseptic and insect-repellent properties (Srimal, 1997). 

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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