Antioxidant combined effect

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

Bonithon-Kopp, C. et al.. Combined effects of lipid peroxidation and antioxidant status on carotid atherosclerosis in a population aged 59-71 years The EVA (Etude sur le Vieillisement Arteriel) study. Am. J. Clin. Nutr, 65, 121, 1997. 

Hence, for both of the oxidants, N02 and ONOO-, a marked synergism in cell protection by the antioxidant combination of P-CAR with vitamins E and C was observed for both in vivo and in vitro experiments, although the synergistic effect was more pronounced in protection from N02 . 

Since carbon black has many stable free radicals, it may be added to polymers such as polyolefins to retard free radical by attracting and absorbing other free radicals. It is customary to add small amounts of other antioxidants to enhance the stabilization by a synergistic effect whereby many antioxidant combinations are more stable than using only one antioxidant. 

Another approach to achieving an effective colour suppression in polymers containing tocopherol is through the use of small concentrations of non-phosphorous-containing compounds, such as polyhydric alcohols [33]. The effect of polyhydric alcohols on colour reduction shows that increasing the concentration of alcohols (e.g. trimethylolpropane, TMP) in the antioxidant combination leads to greater reduction in colour, concomitant with... 

Figures 6, 7, and 8 show the stabilizing efficiency of the combinations consisting of the above absorbers and the antioxidant 2,4,6-tri-fert-butyl-phenol, on Fade-ometer exposure. The total additive concentration in each polystyrene sample was 0.25%. The data show that the rate of discoloration of the polymer was inhibited to a greater extent with the ultraviolet absorber-antioxidant combinations than with the absorber alone. Only a small amount (0.075%) of the antioxidant was required for increased effectiveness of the combination.

Use of both primary and secondary antioxidants usually provides a synergistic effect, where the combined effect of two or more stabilisers is greater than the sum of the effects of the individual stabilisers. It is common practice to include both a phosphite, such as tris(t-butylphenyl)phosphite and a hindered phenol, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyljpropionate to provide improved heat stabilisation in polyolefin formulations. 

The stabilization does not occur through a single process but a combination of all the above proposed processes and their combined effect is known as synergism when the cooperative action is greater than their individual effects [177] taken independently. The mechanism of synergism is unknown. The synergism [178] between UV absorbers (hydroxybenzotriazoles) and antioxidant [tris(3,5-di-with free radicals or oxidation products (hydroperoxide or peroxide) which may be formed even in the presence of ultra-violet stabilizers. 

It has been known that various antioxidants reduce VSCs in raw meats. Nam et al. 40) showed that addition of antioxidants such as tocopherols, gallic acid and sesamol reduced the production of some VSCs in raw pork homogenates and patties. Addition of ascobic acid at 0.1% (wt/wt) or sesamol + a-tocopherol each at 0.01% level to ground beef before irradiation effectively reduced lipid oxidation and VSCs 41). Patterson and Stevenson (25) found that dietary supplementation of a-tocopherol and ascorbic acid to hens reduced the yield of total volatiles. Dietary vitamin E added to turkey diets reduced production of MT, DMS, CS2 DMDS and some hydrocarbons and aldehydes of raw turkey meat 42). However, Lee et al. 43) found antioxidant combinations (sesamol+a-tocopherol and gallate+a-tocopherol) had very little effect on the development of off-odor and the formation of VSCs due to irradiation in raw turkey meat. 

Due to the free-radical character of both the y-radiation induced transformation and oxidation in the post-irradiation phase, free-radical scavenging stabilizers - phenols and amines - effectively protect PO [11,226,227]. This was confirmed by excellent antioxidant/antiradiant effects of DP A, PNA and PD in particular in the post-y-irradiation phase of PO and elastomers [11]. Oligomeric DHQ 21 or its combination with 9b efficiently stabilized EPDM or X-LPE used as insulation materials for electric cables exposed to irradiation doses up to 2MGy at a dose rate 300 Gy h 1 [242]. 

Stabilizing activity is improved in mixtures of aromatic amines varying in their structures. This is due to different contributions to antioxidant, antifatigue or antiozonant effects. Combinations of DPA 9a with PD 11c, d, PNA with PD, ternary mixtures PD/DPA/PNA, PD lib with DHQ 16b or 16c, oligomeric 21 with 11 or 16 (R = NHCftHj), condensate 22 with lib, PT 20 with 9 or lib are examples of combinations effective in rubber protection. Mixtures containing... 

Table 15.12 shows a combined effect of density and antioxidants. The lifetime of the least and the most durable deck boards in the table differs by 420 times. In the real world this would correspond to 1 month and 35 years, respectively ... 

Chen HW, Tsai CW, Yang JJ, Liu CT, Kuo WW, Lri CK. The combined effects of garlic oil and fish oU on the hepatic antioxidant and drug-metabolizing enzymes of rats. Br J Nutr 2003 89 189-200. 

Polymers, during fabrication or storage or in service, may be exposed, sometimes for long periods, to the separate or combined effects of moderate or high temperatures, ultraviolet radiation, and air or other potential oxidants. Under these environmental conditions, polymers are susceptible to thermal, UV, and/or oxidative degradative reactions initiated, in most cases, by the generation of free radicals. Polymer stabilization, therefore, involves incorporation of antioxidants and thermal and UV stabilizers to minimize, if not avoid, such degradative reactions. 

In addition to needing an appropriate interphase design in order to achieve the desired mechanical properties, the long-term endurance of molded parts depends on the prudent choice of stabilizer packages (antioxidants, UV stabilizers, metal deactivators, and so forth) to withstand elevated ambient temperatures and/ or exposure to the combined effects of moisture and sunlight. Chapter 4 addresses the effects of environmental conditions as well as the need to attain effective stabilizer packages for flame-retardant polypropylene in order to enhance weath-erability and prolonged service lifetime of a molded part. 

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