Critical concentration antioxidant

This transition may exhibit a critical behavior when, at a certain concentration of inhibitor known as the critical concentration [InH]cr, the dependence of the induction period on [InH] drastically changes, so that di-/d[InH] at [InH] > [InH]cr becomes much higher than dr/d[InH] at [InH] < [InH]cr. In the literature this problem has been treated only with reference to mechanisms II, III, and VIII [61-68], while all the known mechanisms of inhibited oxidation of RH will be envisaged here (see earlier) [69]. The equations for the chain length, critical antioxidant concentration [InH]cr, stationary concentration of hydroperoxide [ROOH]st, and induction period are given in Table 14.3 and Table 14.4. 

Metal catalysis, which is claimed to have an important role in initiating autoxidation, appears to be so complex that in some systems catalysts are converted to inhibitors when their concentrations are increased. The additives examined include the N-butylsalicylaldimino and N-phenylsalicylaldi-mino chelates of cobalt(ll), copper(11), nickeVJl), and zinc as well as a number of 3,5-diisopropylsalicylato metal chelates. Some were autoxidation catalysts, some were inhibitors, and some exhibited catalyst-inhibitor conversion. Reaction mechanisms which account for most of the observed phenomena are proposed. The scope for developing metal chelates as antioxidants and the implications concerning the critical antioxidant concentration are outlined. 

In recent years Emanuel, Neiman, and their respective schools have greatly contributed to the theory of antioxidant action by studying the phenomenon of the critical antioxidant concentration in terms of a degenerate branched chain reaction. The critical antioxidant concentration, a well-established feature of phenolic antioxidants, is one below which autoxidation is autocatalytic and above which it proceeds at a slow and steady rate. Since the theory allowed not only a satisfactory explanation of the critical antioxidant concentration itself but elucidation of many refinements, such as the greater than expected activity of multifunctional phenolic antioxidants (21), we wondered whether catalyst-inhibitor conversion could be fitted into its framework. If degenerate chain branching is assumed to be the result of... 

Dr. Uri I cannot agree entirely with Dr. Bergers view that the equation for the critical antioxidant concentration, which was developed by the schools of N. M. Emanuel and M. B. Neiman, should be revised by introducing x for the following reason. 

Apart from this mathematical aspect I think that the useful concept of critical antioxidant concentration is valid for degenerate chain branching where the effect of the presence of antioxidant on hydroperoxide decomposition is relatively minor but not when it is the predominant initiation reaction. For metals reacting with hydroperoxides the number of radicals formed may even exceed unity. Kolthoff and Medalia [/. Am. Chem. Soc. 71,3777 (1949) ] demonstrated that for the reaction of ferrous ion with hydrogen peroxide as many as six ferrous atoms can be oxidized by one molecule of hydrogen peroxide as a result of this effect. I do not think, therefore, that the critical antioxidant concentration should be applied to those cases in which the so-called antioxidant is the catalyst. 

In actual use, the catalytic oxidative d radatlon of polyolefins Is controlled, to a large extent, by the additives and contaminants present In the polymer. Although the stability of polyethylene Is related Initially to the antioxidant concentration and type, upon aging the critical factor Is the rate of loss of the antioxidant. Figure 4 shows typical examples of effective stabilizer loss, due to migration on aging. The samples, shown... 

As was shown by Yu. A. Shlyapnikov, the introduction into the scheme of polymer oxidation of the hypothesis that 6-active radicals RO2 are ultimately formed in the decomposition of the hydroperoxide makes it possible to determine the conditions, under the observance of which a "critical" antioxidant concentration is possible [71]. 

An analysis analogous to the analysis of equation (19) permits us to calculate the value of the critical antioxidant concentration ... 

We can see from (35) that the steady-state reaction and critical concentration of the antioxidant can occur only in tiie case when d < 1, i.e., when less than one active radical on the average is obtained in the decomposition of the hydroperoxide. If d 1, then at any concentration of the antioxidant the reaction proceeds in a nonsteady-state basis and no critical antioxidant concentration exists. 

The introduction of the sulfide R2S into the pol3rmer in sufficient concentration can make the denominator in the right-hand portion of equation (26) positive even if 6 > 1, which makes it possible for a critical antioxidant concentration to be realized. The correctness of this conclusion was confirmed by the experiments described in [71]. 

If the antioxidant 2,6-di-tert-octyl-4-methylphenol is introduced into polypropylene in various concentrations, then at 200°C and pQ =300mmI the induction period increases linearly from 8 to 65 min, i. e., no critical antioxidant concentration is observed. If, in addition to the indicated phenol, didecyl sulfide is introduced into the polymer, a critical concentration will be distinctly observed, and will decrease with increasing sulfide concentration, in accord with the formula of [34] (Fig. 21). 

The data obtained in the latter work are depicted in Fig. 27. When the diphenylamine concentration is increased, the induction period t slowly rises (curve 1). If diphenylamine is introduced into polypropylene together with polyphenylene, then the dependence of t on the concentration of diphenylamine is depicted by curve 2, i.e., a well-defined intensification effect is observed. Moreover, the appearance of a critical antioxidant concentration is distinctly visible. 

F. Gugumus. Critical antioxidant concentrations in polymer oxidation—II. Experimental proofs . Polymer Degradation and Stability 60(1), 99-117, April (1998). 

Equations for the Rate of Oxidation v, Critical Concentration of Antioxidant [lnH]cr, and Quasistationary Concentration of Formed Hydroperoxide [ROOH]s at Common Action of Antioxidant and Quinone Q or Nitroxyl (>NO ) [4]... 

The effectiveness of the antioxidant action is characterized by the induction period t under fixed oxidation conditions and inhibitor concentration. Another parameter is the critical... 

Dl. Dasgupta, A., Malhotra, D., Levy, H., Marcadis, D., Blackwell, W., and Johnston, D., Decreased total antioxidant capacity but normal lipid hydroperoxide concentrations in sera of critically ill patients. Life Sci. 60,335-340 (1997). 

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