Oxidation protection against

The methods by which antioxidants function were outlined in Section 9.2. Subsequent sections however amply demonstrated that the behaviour of an antioxidant in an accelerated-sulphur cured diene rubber vulcanizate can be very different from its behaviour in a simple hydrocarbon like petrol or polyethylene or even a peroxide-cured natural rubber. Antioxidant combinations which in some circumstances behave synergistically may in other cases be antagonistic to each other. As was demonstrated in Fig. 9.1 an increase in the antioxidant loading may decrease the degree of pro- 

Whilst it is not to be expected that the rubber technologist will only select an antioxidant after consideration of the detailed molecular structure expected in the vulcanizate he is to prepare, it is clearly inadvisable for him to use an antioxidant in a formulation without attempting some evaluation such as is given by stress-relaxation experiments or by using his experience with other similar rubber compounds. It is almost certainly the case that the selection of antioxidants for use in polyethylene and polypropylene (and which are usually incorporated by the polymer manufacturer) has in the past only been made after a much more thorough investigation than is usually the case with rubber compounds. 

If an antioxidant is volatile or easily leached out of a vulcanizate it will only have a limited period of usefulness in a rubber. This has led to considerable interest, commencing in the mid-l%0s, in the development of antioxidants bound to the rubber. These are sometimes referred to as network bound antioxidants although generally they may be bound to the rubber molecule without a network having been formed. 

Whilst still largely at an experimental stage several methods of producing network bound antioxidants have been described. These include  

The active antioxidant component may be built in at either A or A and can be either amino or phenolic in nature. 

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Molten tin wets and adheres readily to clean iron, steel, copper, and copper-base alloys, and the coating is bright. It provides protection against oxidation of the coated metal and aids in subsequent fabrication because it is ductile and solderable. Tin coatings can be appHed to most metals by electro deposition (see Electroplating). 

On a fresh surface the metal has a steely lustre but rapidly tarnishes in air as a result of surface formation of oxide and carbonate species. For protection against oxidation the metal is usually stored in a light mineral oil. When made finely divided, eg, on being cut, it can be strongly pyrophoric, and, for this reason is used, as the ferro-alloy mischmetal, in lighter flints and ordnance. Cerium reacts steadily with water, readily dissolves in mineral acids, and is also attacked by alkafl it reacts with most nonmetals on heating. 

Aluminizing-a process for impregnating a metal s surface with aluminum to provide protection against oxidation and corrosion. 

SE Service typical of petrol engines in cars and some trucks in 1972-1979. Oils designed for this service provide more protection against oxidation. 

Propyl gallate is an antioxidant. It protects against oxidation by hydrogen peroxide and oxygen free radicals in a catalytic manner similar to superoxide dismutase. 

Silver tarnishes rapidly. Thus a passivation layer is required to keep its reflectance at its value when freshly deposited. It can be done sandwiching the Ag layer between two quarterwave Si02 layers (Fig. 14) the reflectance is not significantly altered (except in the ultraviolet, in a region where silver is not good), but now the Ag layer is protected against oxidation. One can... 

Hydroxy- and amino carboxylic acids can be dimerized in good to moderate yields, when the substituents are not in the a- or P-position and when they are additionally protected against oxidation by acylation (Table 2, No. 17-19). 2-Alkenoic acids cannot be dimerized but lead to more or less extensive passivation of the anode due to the formation of polymer films [136]. 3- and 4-Alkenoic acids give moderate yields when they are neutraUzed with BU3N or EtjN [136]. 3-Alkenoic acids with the structure... 

The OH groups must be protected against oxidation during tho nitration step and the methyl ether (S) is a convenient starting material. Treatment with HBr at the end of the synthesis, removes the methyl groups. 

Enrichment of processed food with plant material or plant extracts rich in polyphenols has two aspects in relation to human nutrition and human health. Food protected against oxidation has better keeping quality and will stay healthy longer since formation of toxic oxidation products, like cholesterol oxides, is being prevented (Britt et al., 1998). The other aspect is the beneficial effects of the intake of polyphenols on human health. Both of these aspects are, however, related to the availability of the phenolic substances. 

Protection against oxidative damage of erythrocyte membrane by the flavonoid quercetin and its relation to iron chelating activity, FEBS Letters, 416, 123-9. 

Lowe, G.M. et al.. Lycopene and beta-carotene protect against oxidative damage in HT29 cells at low concentrations but rapidly lose this capacity at higher doses. Free Rad. Res., 30, 141, 1999. 

Parathasarathy, S., Khoo, J.C., Miller, E., Barnett, J., Witztum, J.L. and Steinberg, D. (1990). Low density lipoprotein rich in oleic add is protective against oxidative modification implica-tiorrs for dietary prevention of atherosderosis. Proc. Nad Acad. Sci. USA 87, 3894-3898. 

Gower, J.D., HeaUng, G., FuUer, B.J., Simpkin, S. and Green, C.J. (1989a). Protection against oxidative damage in cold-stored rabbit kidneys by desferrioxamine and indomethacin. Cryobiology 26, 309-317. 

HOCl is one of the oxidants that activated phagocytes have in their arsenal. Hu et al. (1993) have shown that plasma albumin sulphydryl and ascorbic acid both protect against oxidant injury from HOCl. Neutrophils, interestingly, contain about 25 times more ascorbic acid than plasma. This su ests an endogenous self-protection role. This work is important because of the biological emphasis of reactive chlorine by the neutrophil. 

The additional protection given to nylon by antioxidants has already been mentioned. Since the need is to protect against oxidation by free radicals, antioxidants are essentially of two types peroxide decomposers and radical scavengers. Reviews of these products are available [409,410,413] these should be consulted for details of the mechanisms involved. Peroxide decomposer types include compounds of manganese (II) or copper(I) and copper(II) complexes, such as azomethine bridge derivatives of the type represented by 10.160, of which numerous water-soluble or water-insoluble variants are possible [409]. These products have a catalytic action and are therefore used in very small amounts. 

III. Glutathione reductase (EC 1.6.4.2) It is a flavoprotein that catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG) to glutathione (GSH). This enzyme is essential for the GSH redox cycle which maintains adequate levels of reduced cellular GSH. A high GSH/GSSG ratio is essential for protection against oxidative stress. 

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