Mechanisms of antioxidant action: Auto-synergistic behaviour of sulphur-containing phenols

The powerful antioxidant activity of 3,5-di-tert-butyl-4-hydroxybenzyl sulphides has been shown to be due to the generation of a Lewis acid catalyst which may destroy > 3 × 10⁵ moles of hydroperoxide per mole of sulphur compound. The same catalytic species formed from a variety of benzylic sulphides and is believed to be an inorganic sulphur acid (probably S0₃). Dibenzyl monosulphide itself is not initially an antioxidant and one function of the hindered phenolic group in the auto-synergists is believed to be the removal of free radicals formed during the early stages of the reaction.     

Mechanisms of antioxidant action: polymer grafted antioxidants

It is shown that the antioxidant 3,5-di-tert-butyl-4-hydroxybenzyl acrylate, may be grafted to the surface of polymer artefacts. The antioxidant activity of the resulting “efficiently dispersed” antioxidant is very much higher than that of low molecular weight antioxidants containing the same functional group. Even after extraction with an effective solvent for the antioxidant-monomer and derived polymer, the oxidative stability of the polymer films is very much greater than that of polymer containing the antioxidant-monomer or antioxidant-polymer incorporated by a conventional melt technique without extraction. Optimum activity is believed to be associated with the formation of frequent short chain grafts in the surface of the polymer and is achieved by the use of a polymerization photo-activator such as benzophenone which also terminates kinetic chains.Surface grafted antioxidants are also found to be effective u.v. stabilizers for polypropylene in contrast to the normal ineffectiveness of chain terminating antioxidants incorporated into the polymer by conventional techniques.     

Mechanisms of antioxidant action: Mechanochemical addition of thiol antioxidants to ABS

It is shown that thiol-containing antioxidants and stabilisers can be chemically reacted with ABS in high yields and in substantial concentrations under conditions of high temperature and high shear during processing. The resulting antioxidant concentrates are shown to be very effective thermal and photo-oxidative stabilisers for ABS when used as masterbatch additives for unstabilised ABS. Synergistic combinations of a bound antioxidant (BHBM (I(a))) and a bound uv stabiliser (EBHPT (II)) confer a high level of uv stability on ABS when incorporated into ABS. All the antioxidants show a high resistance to removal from the polymer by solvent extraction. The effect of the mechano-chemical process on the efficiency of adduct formation and on the effectiveness of the stabilisers so formed is discussed.     

Mechanisms of antioxidant action: Mechanochemically bound antioxidants to polyethylene and polypropylene

It is shown that a thiolamide antioxidant, 4-mercaptoacetamido diphenylamine (MADA) can be reacted with both polypropylene and polyethylene in a mechanochemical process. The resulting bound antioxidant concentrates (MADA-B) can be used for the thermal and photostabilisation of the parent polymers. The adduct is very stable to solvent extraction and is also very effective as a high temperature antioxidant for polypropylene subjected to water leaching.The uv stabilising mechanism of MADA-B is believed to be related to the regenerative process involved in the mechanism of stabilisation by hindered nitroxyl radicals.     

Mechanisms of antioxidant action: The role of mercaptobenzthiazole and its derivatives as antioxidants

Mercaptobenzthiazole (MBT) and its zinc complex (ZMBT) are compared as antioxidants for paraffin oil at 140°. The latter is much more effective than the former at comparable molar concentrations. Autoxidation studies at lower temperatures indicate that MBT is superior to ZMBT both as a hydroperoxide decomposer and as a chain-breaking antioxidant. However, products formed from them by reaction with hydroperoxides are much more effective than either MBT or ZMBT. Both are oxidised to bis-2-benzthiazole disulphide (MBTS) are benthiazole sulphinic acid (BTS) which decomposes to give benzthiazole (BT) and sulphur dioxide. In addition, MBT gives benzthiazole sulphonic acid (BTSO) and ZMBT gives the very stable zinc benzthiazole sulphimate (ZBTS) which also behaves as a reservoir for sulphur oxides. The difference in behaviour at 140° appears to be associated with the relative instability of BTSO compared with ZBTS.     

Mechanisms of antioxidant action: Sulphur compounds as inhibitors of mechanochemical degradation of rubber

It is shown that the stability of sulphur vulcanised natural rubber to mechanodegradation (fatigue) is associated with the formation of labile oxidation products. The addition of sulphides and their oxidation products to a non-sulphur vulcanisate has a similar effect. Stress relaxation shows that the initial rate of chain scission under stress is directly related to the fatigue resistance of the vulcanisate. However, the oxidation process also produces peroxidolytic antioxidants (PD-C). It is concluded that the fatigue resistance of sulphur vulcanisates is due to a combination of facile restructurisation and an oxidation chain-breaking mechanism.     

Mechanisms of antioxidant action: Formation of antioxidant adducts with rubbers through sulphur by a mechanochemical procedure

It is shown that antioxidants containing a thiol group can be mechanochemically reacted with both natural rubber (NR) and styrene-butadiene-rubber (SBR). Oxygen inhibits the reaction and the optimum temperature is in the region of 70°C. Increase in the antioxidant concentration in the rubber leads to increasing levels of adduct formation. Evidence is presented to show that although a substantial part of the adduct is formed during mechanochemical treatment, about 20% of the total binding occurs during the vulcanisation reaction.     

Mechanisms of antioxidant action: Auto-synergistic antioxidants based on thiol rubber adducts

The mechanism of by-product formation in the reaction of a thiol antioxidant, 3,5-di-tertbutyl-4-hydroxybenzyl mercaptan (BHBM), with rubber latices is shown to involve two processes, viz dimerization of the intermediate thiyl radical to give disulphide (BHBDS) and the reaction of the intermediate quinonemethide with water to give the aldehyde and with BHBM to give the monosulphide (BHBMS). The two sulphides themselves react with rubber in a slower reaction.The bound antioxidants obtained with natural rubber and nitrile-butadiene rubber are very much more effective than the present generation of commercial heat ageing antioxidants under conditions where the antioxidants can be lost from the rubbers by volatilization or solvent or detergent leaching. This is partly due to their resistance to removal from the polymers but there is also evidence that the auto-synergistic antioxidant bound to the rubber is more effective than single function antioxidants, due to the presence of the sulphide linkage.     

Mechanisms of antioxidant action: autosynergistic antioxidants containing chain-breaking and peroxidolytic functions

Antioxidant activities, both in model substrate oxidation and in polypropylene under technological conditions, have been examined for a series of alkyl (3,5-di-tert-butyl-4-hydroxybenzyl) sulphides (I). They were found to be intrinsically much more active than typical commercial chain-breaking antioxidants in a closed system. This higher activity is attributed to an additional antioxidant function resulting from the presence of the sulphur atom. The latter is the precursor for a highly active peroxidolytic species which leads to autosynergism. In a thermal-oxidative (air-oven) test I (R, C₂—C₁₈) increases in activity with decreasing volatility but effectiveness as melt stabilizer during the processing of polypropylene and as u.v. stabilizer appears to depend more on the physical state of the additive in the polymer than on its volatility.     

Mechanisms of antioxidant action: Activity of sulphur-linked bound antioxidants in natural rubber

A number of phenolic antioxidants containing a sulphide linkage have been found to react with natural rubber latex in the presence of a redox radical generator. They are highly effective under conditions of solvent extraction but the extent of adduct formation is only about 50%.     

Mechanisms of antioxidant action: Effects of rubberbound antioxidants in rubber modified polypropylene

The effect of thermal and photo-oxidation on rubber (ethylene-propylenediene terpolymer) modified polypropylene is examined and it is concluded that, although the rate of photo-oxidation increases with the amount of modifier, the rate of loss of impact resistance decreases. Antioxidants and uv stabilisers improve the oxidation resistance of the impact modified polypropylene but conventional stabilisers are removed by solvent leaching. Antioxidants bound to the EPDM through sulphur show a similar stabilising performance to commercial stabilisers before extraction but are much more effective after extraction.     

Mechanisms of antioxidant action: The behaviour of a bound antioxidant as an antiozonant

A rubber reactive antioxidant, 4-(mercapto acetamido)diphenylamine (I, MADA) was found to be more effective as an antiozonant when prereacted with the rubber chain (MADA-B) than when present as a conventional compounding ingredient (MADA-A) at the same molar concentration. In bound form after extraction MADA-B is as effective as an unextracted commercial antiozonant, iso-propylaminodiphenylamine (IPPD), at the same molar concentration. This result casts some doubt on the currently accepted view that antiozonants must migrate to the surface of the rubber in order to exert their protective effect.     

Mechanisms of antioxidant action: Autosynergistic polymer-bound antioxidants as heat and light stabilizers for ABS

An antioxidant (3,5-di-tert-butyl-4-hydroxybenzyl mercaptan) and a u.v. stabilizer (4-benzoyl-3-hydroxyphenyl-O-ethylthioglycollate) containing the thiol group have been reacted with ABS latex to give polymers with exceptionally high heat and light stability. Both chemical and physical phenomena contribute to the high activity of the bound antioxidants. The concentration of the molecularly dispersed antioxidants in the most oxidatively vulnerable part of the polyblend (the polybutadiene segment) has an important role in determining stabilizer efficiency and, particularly under high temperature conditions, the immobilization of the antioxidants by chemical bonding leads to much lower rates of physical loss. The autosynergistic hindered phenol is primarily responsible for the thermal stabilizing activity of the combination but it also powerfully augments u.v. stabilizing effectiveness of the u.v. stabilizer.     

Mechanisms of antioxidant action: Technological effectiveness of sulphur based bound antioxidants in NR and SBR

A technological study is presented of the performance of concentrates (master-batches) of two thiol-based bound antioxidants ((I) and (II)) under conditions of solvent extraction which leads to rapid loss of conventional antioxidants from the rubbers. It is shown that at the concentrations used (0·25-2% in the rubber), the antioxidant concentrates give a performance superior to an effective bis-phenol. Some potential applications of the procedure are outlined.     

Mechanisms of antioxidant action: rubber bound antioxidants based on nitrones—I. Non-sulphur vulcanizates

The antioxidant activities of a number of N-phenyl nitrones containing phenolic functions were compared with N-methyl nitrones and conventional antioxidants in peroxide and TMTD sulphur-less vulcanizates. The N-phenyl nitrones were found to be effectively bound during vulcanization whereas the N-methyl nitrones were not. Unhindered phenols reacted to a higher degree and were relatively more effective than the typical hindered phenol antioxidant structures. A conventional bisphenol was effective in the sulphurless vulcanizate after solvent extraction but this is considered to be due to the formation of an insoluble zinc salt. Synergism was observed in the TMTD sulphurless vulcanizate both before and after extraction.     

Mechanisms of antioxidant action: Synergism between a 2-hydroxybenzophenone uv stabiliser and autosynergistic antioxidants in polypropylene

Synergism between a uv stabiliser (2-hydroxy-4-octoxybenzophenone (I)) and phenolic antioxidants has been studied during the photo-oxidation of polypropylene. It has been found that hindered phenols containing benzylic sulphur (II) are more effective synergists at the same molar concentration than conventional hindered phenols and that optimum synergism occurs at high molar ratios (I/II). The autosynergistic phenols (II) are shown to protect the uv stabiliser against the effects of hydroperoxides under photo-oxidative conditions by catalytically destroying them and scavenging radicals formed from them. The ‘uv absorber’ (I) also appears to deactivate excited species formed in the photo-decomposition of oxidation products of (II).     

Mechanisms of antioxidant action: The synthesis of bound antioxidant concentrates in nitrile rubber

A phenolic thiol antioxidant and two thiol amides have been reacted with nitrile-butadiene rubber both in the latex and during melt processing to give masterbatch concentrates. BHBM (I) reacts to give higher bound antioxidant levels at 20% masterbatch concentration than the amides in the latex but MADA (II) gives the highest level of binding by the mechanochemical procedure (80% masterbatch concentration). The primary initiation process is mechanochemical but secondary peroxide initiated adduct formation occurs during the later stages.     

Surface and bulk modification of ethylene-propylene-diene terpolymer elastomer: Adhesion to polyurethane and mechanical properties

Effect of adding millable polyurethane as adhesion promoter to the ethylene-propylene-diene terpolymer as well as to the terpolymer modified by grafting of maleic anhydride on the adhesion between the terpolymer and polyurethane coating was studied. The results of peel test and water contact angle measurements showed that surface and bulk modification of terpolymer cause a significant improvement in its adhesion properties. The effect of the composition of the polymer blends on the loss tangent was studied to evaluate the extent of polymer compatibility and damping characteristics. Results demonstrated that modification of terpolymer with maleic anhydride can improve its compatibility with polyurethane, which was further confirmed by the results of the mechanical and morphological study.     

Mechanisms of antioxidant action. The role of nitroxyl radicals as chain-breaking acceptor antioxidants in the mechanodegradation of rubber

It is shown that 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl (HTMPO) becomes partially attached to natural rubber during mechano-oxidation in a shearing mixer at ambient temperatures. It is concluded that the main mechanism responsible for the antidegradant activity of HTMPO under shearing conditions is direct oxidation of mechanochemically produced macroalkyl radicals to species containing conjugated unsaturation.     

Mechanisms of antioxidant action: Auto-synergistic behaviour of nitrones

The antioxidant activities of a number of nitrones have been studied in model systems initiated by both alkoxy radical generators (hydroperoxides) and an alkylperoxy radical generator (azo-bis-isobutyronitrile in the presence of oxygen). Simple nitrones are ineffective in the presence of the latter but are highly effective in the presence of the former. An auto-synergistic mechanism has been shown to be involved due to the ability of nitrones to destroy hydroperoxides in a stoichiometric reaction and at the same time to scavenge alkoxy radicals formed by homolysis of hydroperoxides. Nitrones containing additional conventional phenolic antioxidant functions are effective in an alkylperoxy radical initiated system but do not show the usual structure-activity relationships.