Decomposing hydroperoxide inhibitors

When the operating temperature exceeds ca 93°C, the catalytic effects of metals become an important factor in promoting oil oxidation. Inhibitors that reduce this catalytic effect usually react with the surfaces of the metals to form protective coatings (see Metal surface treatments). Typical metal deactivators are the zinc dithiophosphates which also decompose hydroperoxides at temperatures above 93°C. Other metal deactivators include triazole and thiodiazole derivatives. Some copper salts intentionally put into lubricants counteract or reduce the catalytic effect of metals. 

Antioxidants have two kinds of mechanisms of actions inhibiting free radical chain reactions and decomposing hydroperoxide. Free radical inhibitors are primary antioxidants, including amines and phenols hydroperoxide decomposers are known as secondary antioxidants, including phosphites and thioesters, which are usually used with primary antioxidants. 

The inhibitors of oxidation reaction of fats are substances which reduce the oxidation rate, regardless of the mechanism of their action. These compounds include antioxidants, synergists, chelating agents and compounds decomposing hydroperoxides by nonradical reactions. Also agents stabiHsing hydroperoxides may reduce the reaction rate because they inhibit the formation of free radicals. 

The research allows one to draw several conclusions. First, these thiocarbamides function as combined-action antioxidants, and some of them strongly inhibit hydrocarbon oxidation. Second, as shown in Fig. 9.8, some of the traditional inhibitors we have synthesized after their reaction with peroxide radicals are unable to catalytically decompose hydroperoxides. Third, in the process of hydrocarbon oxidation, peroxide radicals are earlier oxidation products than hydroperoxides, hence these antioxidants will be consumed following the reaction with peroxide radicals. 

Compounds of variable-valence metals decompose hydroperoxides to form free radicals, which accelerates oxidation. This catalyzed oxidation can be retarded by the introduction of a complex-forming agent, which forms a complex with the metal and is inactive toward hydroperoxide. Diamines, hydroxy acids, and other bifrinctional compounds that form stable complexes with metals are used as inhibitors of this type. 

Some compounds retard oxidation entering simultaneously into several reactions. For example, they react with both alkyl and peroxyl radicals (anthracene, methyl-enequinone), decompose hydroperoxides, and terminate chains in the reaction with RO 2 (metal carbamates and thiophosphates). Such compounds are inhibitors of com-... 

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. 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

The combined introduction of an inhibitor, which terminates chains, and a substance, which decomposes hydroperoxides, is widely used for the more efficient retardation of oxidation processes in polyolefins, resins, lubricants, and other materials. Various phenols, bisphenols, and aromatic amines are applied as an acceptor of RO 2, and aryl phosphites, esters of thiopropionic acid, dialkyl dithiopropionates and thiophosphates of zinc and nickel, and other similar compounds are introduced to... 

Organic peroxides and hydroperoxides decompose in part by a self-induced radical chain mechanism whereby radicals released in spontaneous decomposition attack other molecules of the peroxide.The attacking radical combines with one part of the peroxide molecule and simultaneously releases another radical. The net result is the wastage of a molecule of peroxide since the number of primary radicals available for initiation is unchanged. The velocity constant ka we require refers to the spontaneous decomposition only and not to the total decomposition rate which includes the contribution of the chain, or induced, decomposition. Induced decomposition usually is indicated by deviation of the decomposition process from first-order kinetics and by a dependence of the rate on the solvent, especially when it consists of a polymerizable monomer. The constant kd may be separately evaluated through kinetic measurements carried out in the presence of inhibitors which destroy the radical chain carriers. The aliphatic azo-bis-nitriles offer a real advantage over benzoyl peroxide in that they are not susceptible to induced decomposition. 

Figure 1 The theoretical plot of induction time of oxidation determined for wr — 0, (zero rate of initiation according to reaction 1 of Scheme 1) on composition of the mixture of inhibitors InH (chain-breaking antioxidant) and D (peroxide decomposer) having the total sum of concentrations 0.01 mol/l. The curve 2 below is the plot of induction times for the same values of parameters as for line 1 but w, = 5 x 10-8 mol/l. The initial concentration of hydroperoxides was 0.001 mol/l.

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 ... 

An antioxidant ties up the peroxy radicals so that they are incapable of propagating the reaction chain or to decompose the hydroperoxides in such a manner that carbonyl groups and additional free radicals are not formed. The former, which are called chain-breaking antioxidants, free-radical scavengers, or inhibitors. are usually hindered phenols or amines. The latter, called peroxide decomposers, are generally sulfur compounds or... 

Iron(III) weso-tetraphenylporphyrin chloride [Fe(TPP)Cl] will induce the autoxidation of cyclohexene at atmospheric pressure and room temperature via a free radical chain process.210 The iron-bridged dimer [Fe(TPP)]2 0 is apparently the catalytic species since it is formed rapidly from Fe(TPP)Cl after the 2-3 hr induction period. In a separate study, cyclohexene hydroperoxide was found to be catalytically decomposed by Fe(TPP)Cl to cyclohexanol, cyclohexanone, and cyclohexene oxide in yields comparable to those obtained in the direct autoxidation of cyclohexene. However, [Fe(TPP)] 20 is not formed in the hydroperoxide reaction. Furthermore, the catalytic decomposition of the hydroperoxide by Fe(TPP)Cl did not initiate the autoxidation of cyclohexene since the autoxidation still had a 2-3 hr induction period. Inhibitors such as 4-tert-butylcatechol quenched the autoxidation but had no effect on the decom-... 

All commercial samples of cumene tested by us contained considerable amounts of cumene hydroperoxide. Furthermore, a simple distillation, even in a thirty-plate column, is not sufficient to remove its effect. The hydroperoxide probably decomposes at the boiling point of cumene into other lower boiling inhibitors. The inhibitors can be effectively removed by chromatographing through silica gel or clay. 

It has been established that the decomposition of cumene hydroperoxide in the presence of thiolsulfinate occurs primarily via a polar process (1). However, a homolytic process may be involved in the conversion of the thiolsulfinate to the active peroxide decomposer. This was probed by adding the radical inhibitors /2-naphthol and 2,6-di-tert-butyl-4-methylphenol (see Figure 5). The inhibitors totally suppressed the decomposition of hydroperoxide by thiolsulfinate. In contrast to /3-naphthol and 2,6-di-terf-butyl-4-methylphenol, the addition of cyclo-hexanol had no significant effect. Similarly, the addition of methanol only reduced the decomposition of hydroperoxide by the thiolsulfinate by 1% after 186 hr. 

Antioxidant systans combining two or more materials are generally used. The most effective mixture will combine a free radical inhibitor with a peroxide decomposer. The free radical inhibitor retards the initiation of reaction chains, but some hydroperoxide is nevertheless formed. A peroxide decomposer available to react with the hydroperoxide prevents it from decomposing with free radicals. The nature of the resin will influence the kind and amount of antioxidant used. 

Inhibition studies showed that the consumption of inhibitor was half that of hydroperoxide indicating that no molecular decomposition occurred. Furthermore, the superposition of decomposition curves with or without inhibitor gave evidence that the process did not occur via a free radical mechanism but rather each hydroperoxide molecule was catalytically decomposed to give one free radical. 

To inhibit the oxidation process, certain chemical compounds (antioxidants or stabilisers) are added to the polyethylene, see Sect. 5.2.2. The so-called chain-breaking-donors or primary antioxidants, also called inhibitors, react with the chemical radicals. They thus intermpt the reaction chain. The so-called hydroperoxide decomposer or secondary antioxidants react with the hydroperoxide before it can disintegrate into radicals. Thus they prevent the start of new reaction chains. 

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