Antioxidants induction period

Antioxidant Induction period at 150 °C (h) Days to craze formation... 

From Loliger etal. (1996). Activity values expressed as antioxidant index (AI) = induction period of oil + antioxidant/induction period of control oil. 

The shape of the curves is of some interest. An induction period is noted during which no oxygen uptake, i.e. oxidation, is observed. At the end of this induction period, when in effect antioxidant moieties have been consumed, oxygen uptake is rapid unless some retardation mechanism is at work. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Chain-breaking antioxidant DLTP Carbon black Copper powder Induction period at 140°C (h)... 

IV, CCR and oxidation stability are three strictly co-related parameters. As a general rale, the reduction of IV (on the same feedstock) dramatically improves the oxidation stability. On the contrary the distillation step removes the main part of naturally occurring antioxidants. For this reason, even after hydrogenation the Rancimat induction time (as measured according to the EN 14112 standard) of the hydrogenated sample does not fulfill the EN 14214 requirement for oxidation stabihty (6 hours at 110°C), 4 hours being the measured induction period. 

The observation of an unusual effect occurring as a consequence of preannealing on the induction time of oxidation of PP stabilized with Irganox 1010 (Figures 19 and 20) needs to be taken into account when searching for correlations between induction periods from chemiluminescence and oxygen uptake for oxidation of PP stabilized with different types of antioxidants. 

Oxygen absorption methods have limited sensitivity and require high levels of oxidation as the endpoint for induction periods (Frankel 1993). In foods, antioxidant... 

This reaction is slow due to a high activation energy (see Chapter 15). However, at elevated temperatures and sufficiently high concentrations of antioxidant and hydroperoxide, this reaction becomes fast and, hence, can accelerate the rate of oxidation. As a result, the rate of initiation increases v = vi0 + enkn[ROOH] [InH] (en is the probability of the appearance of active radicals in the bulk). From the other side, this reaction shortens the induction period (T0=/[InH]0/vl0). 

The mechanisms responsible for inhibited oxidation depend on the experimental conditions and particular properties of RH and antioxidant (see earlier). Let us assume that hydroperoxide is relatively stable, so that it virtually does not decompose during the induction period (kdr -c 1). Actually, this means that the rate of ROOH formation is much higher than the rate of its decomposition, / 2[RH] [RO]2 ] 3> d[ROOH]. For each of the mechanisms of inhibited autoxidation, there is a relationship between the amounts of the inhibitor consumed and hydroperoxide produced (see Tablel4.2). For example, for mechanism V with key reactions (2), (7), (—7), and (8), we can get (by dividing the oxidation rate v into the rate of inhibitor consumption) the following equation ... 

For initiated oxidation, the inhibitory criterion could be defined as the ratio v0/v or (v0/ v — v/v0), where v0 and v are the rates of initiated oxidation in the absence and presence of the fixed concentration of an inhibitor, respectively. Another criterion could be defined as the ratio of the inhibition coefficient of the combined action of a few antioxidants / to the sum of the inhibition coefficients of individual antioxidants when the conditions of oxidation are fixed (fx = IfiXi where f, and x, are the inhibition coefficient and molar fraction of z th antioxidant terminating the chain). It should, however, be noted that synergism during initiated oxidation seldom takes place and is typical of autoxidation, where the main source of radicals is formed hydroperoxide. It is virtually impossible to measure the initial rate in the presence of inhibitors in such experiments. Hence, inhibitory effects of individual inhibitors and their mixtures are usually evaluated from the duration of retardation (induction period), which equals the span of time elapsed from the onset of experiment to the moment of consumption of a certain amount of oxygen or attainment of a certain, well-measurable rate of oxidation. Then three aforementioned cases of autoxidation response to inhibitors can be described by the following inequalities (r is the induction period of a mixture of antioxidants). 

The induction period is measured experimentally at the constant sum of concentrations of two antioxidants, namely, Co = [S]o + [InH]0 = const. Theoretically this problem was analyzed in [9] for different mechanisms of chain termination by the peroxyl radical acceptor InH (see Chapter 14). It was supposed that antioxidant S breaks ROOH catalytically and, hence, is not consumed. The induction period was defined as t = (/[InH /v, where vV2 is the rate of InH consumption at its concentration equal to 0.5[InH]o. The results of calculations are presented in Table 18.1. 

The rare example of synergistic action of a binary mixture of 1-naphthyl-A-phcnylaminc and phenol (1-naphthol, 2-(l,l-dimethylethyl)hydroquinone) on the initiated oxidation of cholesterol esters was evidenced by Vardanyan [34]. The mixture of two antioxidants was proved to terminate more chains than both inhibitors can do separately ( > /[xj). For example, 1-naphtol in a concentration of 5 x 10 5 mol L-1 creates the induction period t=170s, 1 -naphthyl-A-phenylamine in a concentration of 1.0 x 10-4 mol L 1 creates the induction period t = 400s, and together both antioxidants create the induction period r = 770 s (oxidation of ester of pelargonic acid cholesterol at 7= 348 K with AIBN as initiator). Hence, the ratio fs/ZfjXi was found equal to 2.78. The formation of an efficient intermediate inhibitor as a result of interaction of intermediate free radicals formed from phenol and amine was postulated. This inhibitor was proved to be produced by the interaction of oxidation products of phenol and amine. 

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... 

We observed a more complicated behavior in the study of retarding action of amines (IV-benzyl-IV -phenyl-l,4-benzenediamine and 4-hydroxyphenyl-2-naphtalenamine) on fuel T-6 oxidation catalyzed by the copper powder [13]. Both antioxidants appeared to retard the autoxidation of T-6 very effectively. They stop chain oxidation during the induction period in concentrations equal to 5 x 10 5mol L 1 and higher. The induction period was found to be the longer, the higher the concentration of the antioxidant and lower the amount of the copper powder introduced in T-6. 

One particular phenomenon, often shown with oxidising reactions in polymers, is an induction period for ageing, whereby relatively little change is seen during this period but then the rate of degradation increases abruptly. It is easy to see how this can happen in materials protected by antioxidants in that with time the protective additives will be consumed. 

Oxidising reactions in polymers often have an induction period for ageing (see Section 4.12.1) during which relatively little change is seen, but after which the rate of degradation increases abruptly. In materials protected by antioxidants the induction time ends when the antioxidants have been consumed and oxidation begins in earnest. 

Various aromatic secondary amines, substituted phenols, and pyrazoli-dones (3) that function as traps for the propagating peroxy radicals gave dead-stop induction periods when used at a concentration of 50 p.p.m. An indication of the ease of oxidation of chloroprene is that 50 p.p.m. of 2,6-di-ferf-butyl-4-methylphenol gave an induction period of only 15 minutes, while the same concentration of antioxidant prevented n-hexadecane from oxidizing for 2 hours at 160°C. 

During the induction periods caused by adding antioxidants, a small contraction in volume occurred because of the formation of dimers of chloroprene (14). This reaction occurs during the oxidation but was most easily studied by dilatometry in the absence of oxygen. A few values of the initial rate of dimerization of chloroprene, inhibited against polymerization with 2,2,6,6-tetramethylpiperidine-l-oxyl, are given in Table III. Their dependence on temperature is given by... 

The inhibition of hydrocarbon autoxidation by zinc dialkyl dithiophosphates was first studied by Kennerly and Patterson (13) and later by Larson (14). In both cases the induction period preceding oxidation of a mineral oil at 155 °C. increased appreciably by adding a zinc dialkyl dithiophosphate. In particular, Larson (14) observed that zinc salts containing secondary alkyl groups were more efficient antioxidants than those containing primary groups. In these papers the inhibition mechanism was discussed only in terms of peroxide decomposition. 

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. 

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