Antioxidants stabilization mechanism

These considerations lead to the assumption that the practical aspects of the problem lie in the possibility of obtaining PCS-based thermally resistant materials, catalysts for some chemical reactions, antioxidants, stabilizers, photochromic substances, and materials combining valuable mechanical properties with special electrical (particularly semiconductive) properties. 

Schwetlick, K. Habicher, W. D. Antioxidant action mechanisms of hindered amine stabilizers. Polym. Degrad. Stab. 2002, 78, 35-40. 

It might be assumed that, as condensed-phase flame retardants function by modifying the normal thermal degradation processes of polymers, they would also function as thermal stabilizers and that thermal antioxidant stabilizers would show flame-retardant properties. However, these statements are rarely the case, and to understand why, it is necessary to compare the mechanistic aspects of flame retardance as discussed earlier with those of thermal degradation and thermal oxidation as well, briefly alluded earlier, and in the case of the latter, the Bolland and Gee mechanism,17 in Scheme 2.1. 

Pospfsil J (1991) The key role of antioxidant transformation products in the stabilization mechanism - a critical analysis, Polym Degrad Stab 34 85-109. 

Product stability and performance can be affected by exposure to several oxidative sources, including oxygen, free radicals, UV radiation, oxidative enzymes, catabolic oxidation, and chemical oxidation. Many antioxidants are also good UV absorbers due to their conjugated chemical structure. Typical antioxidants found in cosmetic products are flavonoids, polyphenols, carotenoids, thiols, tocopherol (vitamin E) and ascorbic acid (vitamin C) [71,72], According to Black [73], a combination of antioxidants from different classes is more effective than a single antioxidant due to an antioxidant cascade mechanism. 

This process is very important in the HALS stabilization mechanism. It should, however, be considered only as a minority pathway in diarylamines just because of the lower stability of the corresponding diarylnitroxides. This results in the participation of the latter in side reactions leading to antioxidant ineffective species, e.g. benzoquinone (BQ) and nitrobenzene ( 7 ). Transformation of diarylnitroxide into a mixture of diarylamine and N-aryl-1,4-benzoquinone monoimine-N-oxide (4. 8) seems therefore to be a more probable pathway regenerating partly amine antioxidant than the hydroxylamine/nitroxide cyclical process (Scheme 2). 

Due to the high reactivity of ozone with unsaturated hydrocarbons moieties, surface cracking of stressed or flexed NR, BR, NBR, and SBR vulcanizates arises. Rubber goods designed for outdoor applications must therefore be stabilized against both C>2 and 0 attacks. Antioxidant protection mechanisms have been discussed in detail ( 1). Discussions dealing with antiozonant mechanism involve some contradictory experimental observations. 

Denisov, E. T." ATheoretical Approach to the Optimization of Antioxidant Action." Mechanisms of Polymer Degradation and Stabilization, edited by G. Scott. London Elsevier Applied Science, 1990. 

The adherence to the polymer and elimination of diffusion to the environment are both essential however, they should be mobile enough to move to the surface layer. Novel stabilizers, based on steric hindered amines (HALS) are considered to be very efficient, albeit there is lack of agreement regarding their stabilization mechanism. It is assumed that there exists a mutual reaction of antioxidation and hydroperoxide decomposition combined with the destruction of free radicals. These stabilizers are essential for protecting polyolefins and other polymers. 

Based on previous studies, we can assume that the MR has antioxidant properties [10]. In this case MR behavior will allows a stabilization mechanism that is based on interactions with protein molecules which lead the formation of a stiff viscoelastic surface film that can also work as barrier... 

THERMAL ANTIOXIDANTS STABILIZATION OF PP AND MECHANISMS OF ANTIOXIDANT ACTION... 

Formation of hydroperoxides that become potential chain reaction initiators is characteristic of the phenolic antioxidant action mechanism. They decompose at high temperature (100-150 C) and a portion of the antioxidant is consumed by their deactivation. Their decomposition to form radicals is accelerated by light [46]. For this reason phenolic antioxidants are usually not effective at high temperatures and special antioxidants must then be used. These stabilizers combine effective parts of phenolic antioxidants to fragments that function by destroying peroxidic compounds. 

On one hand, the chroman head is capable of donation of a hydrogen atom to free radicals leading to the stabilization of these species. On the other hand, the long lipophilic phytyl tail provides high compatibility and miscibility with UHMWPE. The stabilization mechanism of a-tocopherol in UHMWPE have been studied in depth [20, 86]. The cycle of oxidative stabilization of UHMWPE by means of this antioxidant can be found in Ref [87]. 

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... 

Rychly et al. used infrared and UV spectroscopies to estimate and to compare the action of two antioxidants, Irganox HP 136 and Irganox 1010, by the UV stabilization of polypropylene films [78]. It was observed by the evolution of carbonyl peak at 1730 cm that the high efficiency of Irganox Hp 136 is attained in stabilization of PP exposed to UV radiation, whose activity is quite competitive to Irganox 1010 (Table 3). The stabilization mechanism is mainly based on the scavenging of free radicals formed by UV initiation. It can be observed that the... 

The third chapter mainly concentrates on stabilization of irradiated polyethylene by the introduction of antioxidants (vitamin E). The discussion of this chapter has two parts. The first part delineates the main types of antioxidants, stabilization by Vitamin E, structure and biological function of vitamin E, mechanism of stabilization of vitamin E, methods of incorporation of vitamin E, vitamin E stabilized polyethylenes the second part discusses the analysis of the content of vitamin E, using such instruments as FTIR, UV, HPLC and thermal methods. 

Al-Malaika S (1993) Antioxidants preventive mechanisms in Scott G, ed. Atmospheric Oxidation and Antioxidants, 2nd edn, Elsevier, Amsterdam, Chapter 5. Scott G (1999) Evironmental Stability of Polymers , Polymers and the Environment, Royal Society of Chemistry Paperbacks, Chapter 3. 

Antioxidants/stabilizers are additives that stabilize plastics against aging and degradation during processing and while in practical use. Degradation can be caused by light, heat, mechanical stress, oxidation and hydrolysis. 

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