Termination of the induction period

Figure 8 compares the accumulation and disappearance of the intermediate products with the induction period observed in the second order kinetic analysis of the thiophenol-sulfur reaction. Inspection of Figure 8 shows that the HSa-H and ArS rH species reach their maximum concentration at a point near the termination of the induction period. 

The relative effects of phosphonates can be evaluated by comparing the time at which the induction period is terminated at different dosages of a chemical. In Fig. 17, the time of termination of the induction period is plotted as a function of dosage.Among the acids, the most efficient retarder is DTPMP which requires 0.05% for an induction period of 21 hours, compared to periods of 13 and 10 hours respectively for acids ATMP and HEDP at the same dosage. Among the salts, Na DTMP is the most effective retarder. 

The dinitrobenzenes display the characteristics of inhibitors for the more reactive vinyl acetate chain radicals. Two radicals are terminated during the induction period by each molecule of dinitrobenzene, indicating disappearance of inhibitor radicals by a disproportionation reaction. 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

To investigate the copolymerization of trioxane with dioxolane and to determine r1 by the excess method, a molar ratio of trioxane to dioxolane of 100 1.8 was used. All polymerizations were run in methylene dichloride at 30°C. with SnCl as initiator. To reduce the influence of formaldehyde production at the beginning of copolymerization, dioxolane was added to the solution of trioxane and initiator only at the end of the induction period—i.e., at the appearance of the first insoluble polyoxy-methylene. After various reaction times polymerizations were terminated by adding tributylamine. Monomer conversions were determined by gas chromatography, the liquid phase being injected directly. When conversions were small, isolation and analysis of the copolymer yielded more accurate results. 

The causes of the induction period and of its termination have been the subject of much debate. Hypotheses have been reviewed in a collaborative paper (T25). The main ones are as follows ... 

Regarding the chain termination by reactions of HO2, the lack of dependence of Pe on vessel diameter suggests that reaction (x) rather than a surface reaction (v) is the major contributor. This also accounts for the thermal nature of the ignitions at the lower sensitizer limits, where the NO and NO2 concentrations are also low. Near the upper sensitizer limit the NO concentration at the end of the induction period will be comparatively high. This favours the removal of HO2 by reaction (xxxvii) rather than (x), and at the same time favours reaction (xxxiv) rather than (x) as the chain terminating step. With the replacement of quadratic by mainly linear termination, the nature of the ignition changes from purely thermal to nearly isothermal. 

Makedonov Yu.V., Margolin A.L., Rapoport N.Ya. and al. (1986)." On the causes of changes in the effective rate constants continue and chain termination in the induction period of oxidation of isotropic and oriented isotactic polypropylene". Vysoconwlek. Soedin. V.28 A. No 7. PP.1380-1385. (In Russian)... 

The enumerated experiments give evidence of a complex mechanism of the action of antioxidants. We might think that the latter not only terminate the reaction chains, but can also initiate oxidation. If an antioxidant only terminates the chains, then its introduction into the polymer cannot lead to a reduction of the induction period. If it possesses the ability to terminate and initiate the chain, on the other hand, then its addition under certain conditions can lead to a reduction of the induction period. 

As was also shown for polyamides, a break is detected on the curve (Fig. 83). This indicates that the dependence of the induction period on the initial inhibitor concentration is not directly proportional, but is more complex in character the inhibitor is apparently consumed not only for chain termination, but also for side processes (volatilization, oxidation, initiation). An analogous picture is also observed in the stabilization of polyformaldehyde by phenols (Fig. 84). 

PP correlates well with the oxygen uptake and the CL intensity is assumed to be directly proportional to the rate of oxidation [578]. The intensity of emitted light is proportional to the rate of termination for the reactions involved in the Russell mechanism. Chemiluminescence reveals a difficulty in the definition of the induction period and the steady state of oxidation as the sensitivity of the CL analysis is increased, the apparent induction time and the limiting rate decrease. 

Mixtures of inhibitors often possess a combined action. For example, when phenol and sulfide are introduced into the oxidized hydrocarbon, the first one retards by chain termination in the reaction with RO 2, and the second one decreases the rate of degenerate chain branching decomposing hydroperoxide. When two inhibitors enhance the retardation action of each other, we deal with synergism. When their retardation action is simply summated (for example, the induction period under the action of a mixture is equal to the sum of the induction periods under the action of each individual inhibitor), we have their additive retardation action. If the retardation action of a mixture is smaller than the sum of the retardation actions of each inhibitor, we have antagonism of inhibitors. 

Characteristic features of a—time curves for reactions of solids are discussed with reference to Fig. 1, a generalized reduced-time plot in which time values have been scaled to t0.s = 1.00 when a = 0.5. A is an initial reaction, sometimes associated with the decomposition of impurities or unstable superficial material. B is the induction period, usually regarded as being terminated by the development of stable nuclei (often completed at a low value of a). C is the acceleratory period of growth of such nuclei, perhaps accompanied by further nucleation, and which extends to the... 

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

Compounds of transition metals (Mn, Cu, Fe, Co, Ce) are well known as catalysts for the oxidation of hydrocarbons and aldehydes (see Chapter 10). They accelerate oxidation by destroying hydroperoxides and initiating the formation of free radicals. Salts and complexes containing transition metals in a lower-valence state react rapidly with peroxyl radicals and so when these compounds are added to a hydrocarbon prior to its oxidation an induction period arises [48]. Chain termination occurs stoichiometrically (f 1) and stops when the metal passes to a higher-valence state due to oxidation. On the addition of an initiator or hydroperoxide, the induction period disappears. 

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 combined addition of two phenols, one of which is sterically hindered, for example, 2,6-bis(l,l-dimethylethyl)phenol, and another is sterically nonhindered also leads to a synergistic effect [35-38]. As found by Mahoney [35], 2,4,6-tris(l,l-dimethylethyl)phenol with a concentration of 10 4 L mol 1 does not virtually inhibit the initiated oxidation of 9,10-dihydroan-thracene (333 K), but /)-methoxyphenol, taken in the same concentration, does inhibit oxidation. The induction period doubles if two phenols are added together in equal concentrations, which indicates that both phenols are involved in chain termination. The mechanism of synergistic action can be explained by the following kinetic scheme [35] ... 

It is seen that the substitution of the part of amine or phenol by quinone prolongs the induction period by two or three times. The mechanism of synergistic action of quinone is the same as in the case of nitroxyl radicals. Quinone reacts with InH with production of semiquinone radicals. The latter rapidly reacts with peroxyl radicals and provokes the additional rapid chain termination [47],... 

For the cluster mechanism, while growth and termination can be similarly explained, the induction period is less obvious. The hydroxide cluster can start to adsorb on the substrate immediately after immersion of the substrate in the deposition solution, yet experiments have shown that film growth often does not occur for some time. While the reason for this is not clear, it may be connected with the... 

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