Mechanisms Of Antioxidant Action Research Articles

Mechanisms of antioxidant action: Antidegradant activities of phenols and quinones derived from phenolic sulphides in a peroxide vulcanisate

Sulphur-free phenols and quinones are formed during fatiguing of peroxide-vulcanised natural rubber containing phenolic sulphides (I). Only phenols are formed during vulcanisation and it is shown that the derived quinones are the main secondary antifatigue agents. However, none of these, including galvinoxyl, is as effective as the parent sulphides. The evidence suggests that quinonoid products trap macroalkyl radicals, becoming chemically attached to the polymer in the process. The high activity of the phenolic sulphides is primarily due to sulphur-containing oxidation products.

Mechanisms of antioxidant action: Phenolic sulphides as antidegradants for rubbers

It is shown that autosynergistic phenolic sulphides are effective non-discolouring antidegradants for vulcanised natural rubber. They are in general more effective in non-sulphur vulcanisates (peroxide and thiuram) than in the conventional polysulphide rubbers. There is evidence that the processing and vulcanisation operations may play some part in the transformation of the antidegradants to more active products.

Mechanisms of antioxidant action: Nitrosamines as heat and light stabilisers for polypropylene

N-Nitrosamines are shown to be very effective melt stabilisers for polypropylene and one of these, derived from the hindered amine light stabiliser, Tinuvin 770, is almost twice as effective as the latter. The mechanism is believed to involve the formation of two kinds of nitroxyl radical, the first derived from the amine and the second formed by the reaction of nitric oxide with alkyl, and possibly alkoxyl, radicals. Nitrosamines which do not dissociate to NO are neither melt stabilisers nor uv stabilisers.

Mechanisms of antioxidant action: C–nitroso compounds as UV stabilizers for polypropylene

AbstractIt is shown that nitroso‐tert‐alkanes are highly active UV stabilizers which become at least partially chemically attached to polypropylene during processing. The aromatic nitroso compounds are less effective as light stabilizers but are more effective than their alkyl analogues as thermal oxidative stabilizers. Mechanisms are proposed to account for the antioxidant activity of the nitroso compounds and their ability to cause crosslinking of the polymer during processing.

Mechanisms of antioxidant action: Cupric stearate as a catalytic antioxidant

Cupric stearate is an effective processing stabiliser for polypropylene and is shown to act by a redox mechanism in which alkyl and alkylperoxyl radicals are alternatively oxidised and reduced. This process resembles the antioxidant mechanism of stable phenoxyl and nitroxyl radicals.

Mechanisms of antioxidant action: The behaviour of 4-alkyl-2-mercaptothiazolines during thermal and photo-oxidation of polypropylene

2-Mercaptothiazolines (II, R = H, CH 3, C 2H 5 and C 6H 13) are compared as antioxidants and uv stabilisers for polypropylene. All the homologues are effective melt stabilisers and moderate thermal antioxidants in an air oven at 140°C. They are also effective uv stabilisers and become more effective as the alkyl chain length increases. The higher homologues are highly concentration dependent, due to their higher equilibrium concentration in the polymer. The effectiveness of 4-hexylmercaptothiazoline increases markedly with the severity of the processing operation.

Mechanisms of antioxidant action. Part 4. The decomposition of 1-methyl-1-phenylethyl hydroperoxide by zinc dialkyldithiocarbamates and zinc xanthates

Four zinc dithiocarbamates and two zinc xanthates have been used as promoters for the decomposition of 1-methyl-1-phenylethyl hydroperoxide (cumene hydroperoxide). The distribution of products formed from the hydroperoxide is independent of the promoter. This, together with other evidence in the literature, suggests that the dithiocarbamates and xanthates decompose the hydroperoxide via the formation of sulphur trioxide–sulphuric acid. This is in direct contrast to the results obtained in previous work using zinc O,O′-dialkyl(aryl)phosphorodithioates (zinc DDPs) as promoters where the hydroperoxide decomposition catalyst was shown to be the corresponding acid O,O′-dialkyl(aryl) hydrogen phosphorodithioate.

Mechanisms of Antioxidant Action: Complementary Chain-Breaking Mechanisms in the Mechanostabilization of Rubbers

Abstract It is clear that aliphatic nitroxyls cannot function effectively in the catalytic CB-A/CB-D cycle (Scheme 1) during mechanodegradation in a sulfur vulcanizate, although they have been shown to be effective mechano-chemical inhibitors in rubbers before vulcanization. Aliphatic nitroxyls appear to be more effective when they contain an additional CB-D function.

Mechanisms of antioxidant action: Stabilization of polymers by spin traps

Mechanisms of antioxidant action: Stabilization of polymers by spin traps

Mechanisms of Antioxidant Action: Aromatic Nitroxyl Radicals and Their Derived Hydroxylamines as Antifatigue Agents for Natural Rubber

Abstract Diarylnitroxyl radicals and their reduction products, the diaryl hydroxylamines are effective anti-fatigue agents for rubber, confirming the previously proposed mechanism of action of the diaryamincs. The nitroxyls are reduced by sulfur species formed during the vulcanization reaction to a mixture of hydroxylamines and parent amines, and these species act as reservoirs for the nitroxyl radical during fatiguing. Removal of the reservoir of reduced species by solvent extraction removes the antifatigue activity.

Mechanisms of antioxidant action: Transformations of 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinoxyl (HTMPO) in polypropylene during processing

AbstractWhen 4‐hydroxy‐2,2,6,6‐tetramethyl piperidinoxyl (HTMPO) is processed in polypropylene in a closed mixer, almost 50% is converted to other products during the first few minutes whilst the applied torque in the mixer is high. There is associated formation of unsaturation and this fact, in conjunction with the almost complete regeneration of nitroxyl within five minutes, suggests that the corresponding hydroxylamine (HTMPOH), which can be qualitatively identified, is the major transformation product. A study of the UV stability of PP films fabricated from polymer processed for varying times shows that UV stability is related to the quantity of the redox couple (HTMPO + HTMPOH) remaining in the polymer. This is considerably reduced by severe processing. The redox capable has almost no thermal antioxidant (oven aging) activity.

Mechanisms of antioxidant action: effect of processing conditions on the stabilizing effectiveness of metal xanthates and related dixanthogen in polypropylene

The effects of processing conditions, on the thermal and photo-oxidation of polypropylene containing different metal octyl xanthates (MOX) and the related dixanthogen (OX) have been examined. It is concluded that in the presence of a limited amount of oxygen the metal xanthates (e.g. nickel and cobalt) are better u.v. stabilizers than the dixanthogen, and their greater effectiveness is atrributed to a combination of complementary mechanisms, i.e., peroxide decomposition, u.v. absorption, u.v. screening and radical scavenging. Under severe processing conditions (excess oxygen), the activity of the metal xanthates is reduced and their effectiveness becomes very similar to that of the corresponding dixanthogen in both melt and u.v. stabilization. The main mechanism of action in this case is peroxide decomposition.

Mechanisms of antioxidant action: The reaction of (3,5-di-tert.butyl-4-hydroxyphenyl)methane thiol with tert.butyl hydroperoxide

The radical initiated addition of (3,5-di-tert. butyl-4-hydroxyphenyl)methane thiol (BHBM, I, R = H) to olefins and its behaviour as an antioxidant are found to be complementary processes. In a system initiated by tert. butyl hydroperoxide (TBH), the adduct process is found to predominate for [BHBM]/[TBH] > 1 whereas radical trapping/peroxidolytic antioxidant reactions occur at molar ratios <1. The antioxidant species have been shown to be sulphur acids, eliminated from the oxidation products of I together with the corresponding sulphonic acid which is stable under the conditions of the reaction.

Mechanisms of antioxidant action. Part 3. The decomposition of 1-methyl-1-phenylethyl hydroperoxide by OO′-dialkyl(aryl)phosphorodithioate complexes of cobalt, nickel, and copper

OO′-dialkyl(aryl)phosphorodithioates of cobalt(II), nickel(II), and copper(I) have been used as promoters for the decomposition of 1-methyl-1-phenylethyl hydroperoxide (cumene hydroperoxide). The results have been compared with previous work using zinc bis-[OO′-dialkyl(aryl)phosphorodithioates] as promoters for cumene hydroperoxide decomposition. The comparison shows that the product distribution and activation parameters for the reactions promoted by the cobalt, nickel, and copper complexes are identical with the product distribution and activation parameters for the reactions promoted by the corresponding zinc complexes. On this evidence it is proposed that the metal complex promoters form the corresponding OO′-dialkyl(aryl) hydrogen phosphorodithioate and it is this acid that is the catalyst for hydroperoxide decomposition. The order of the reaction with respect to the hydroperoxide depends upon the promoter and is second order for all the zinc complexes but first order for all the nickel complexes. This can be explained by assuming that there are two mechanisms for the formation of the acid catalyst. The relative importance of the two mechanisms depends upon the experimental conditions used and this provides an explanation for the three-stage reaction observed in hydroperoxide decompositions carried out at low temperatures in the presence of relatively high concentrations of OO′-dialkyl(aryl)phosphorodithioate promoters.

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: Effect of benzophenone-substituted xanthates on the stability of polypropylene

Stabilisation of polypropylene by benzophenone-substituted xanthates is examined. These systems afford greater melt stabilisation of polypropylene than the corresponding controls, i.e. benzophenone and alkyl xanthates. In contrast to the pro-oxidant behaviour of benzophenone during photo-oxidation of polypropylene, benzophenone-substituted metal xanthates show a three-step behaviour: an initial rapid photo-oxidation, followed by a secondary inhibition period and a third autoacceleration step. The first step is shown to be mainly due to the benzophenone substituent while the length of the inhibition period is directly related to the amount of the sulphur component. During this period xanthates therefore lead to complete inhibition of photo-oxidation both with regard to chemical oxidation and physical change.

Mechanisms of antioxidant action: The effect of galvinoxyl on the thermal oxidative stability of processed polypropylene

The effectiveness of galvinoxyl (G·(I)) as a thermal antioxidant in polypropylene at 140°C is dependent on the prior processing treatment of the polymer. A characteristic fluctuation of antioxidant activity with time of processing is shown to be associated with the extent of conversion of galvinoxyl to hydrogalvinoxyl (GH) in the polymer. It is concluded that the main stabilisation mechanism involved is deactivation of alkylperoxyl radicals (CB-D) by GH but secondary processes (CB-D and CB-A) involving G· may also make a contribution.

Mechanisms of antioxidant action: Evidence for a regenerative cycle during the melt stabilisation of polypropylene by galvinoxyl

Evidence is presented to show that galvinoxyl (G·), a ‘stable’ aryloxyl radical, is converted to the corresponding phenol(GH) in polypropylene during processing. The redox couple (G·/GH) is a catalytic CB-A/CB-D antioxidant and is capable of deactivating 50 kinetic chains, thus effectively protecting polypropylene against oxidative chain scission for up to 20 min at 200°C in a closed mixer. A reciprocal alternation of the concentration of G· and GH is accounted for by changing conditions in the mixer and G· is slowly and irreversibly destroyed by alkylperoxyl radicals.

Mechanism of antioxidant action. Transformation involved in the antioxidant function of nickel dithiolates—2

The initial transformation products from nickel dialkyl dithiophosphates and nickel alkyl xanthates have been identified by spectrophotometric and kinetic methods. A common mechanism of action is involved for both nickel complexes and the distribution of products from complexes during their reaction with hydroperoxides suggests a greater contribution from an ionic mechanism in the case of dithiophosphate. Nickel dialkyl dithiophosphate is more stable to u.v. light than nickel alkyl xanthate and its reaction with tert. butyl hydroperoxide in the presence and absence of light is 7–8 times faster than that of the xanthate.

Mechanisms of antioxidant action. Effect of the polymer medium on the antioxidant efficiency of the dithiolates

Peroxide decomposing antioxidants (e.g. nickel dithiophosphates and thiophosphoryl disulphides) control hydroperoxide formation during processing and on exposure to light. However, these additives are more efficient u.v. stabilizers in polypropylene (PP) than in low density polyethylene (LDPE). It is suggested that this difference results from the more rapid formation of hydroperoxides in the more oxidisable substrate under normal processing conditions. In contrast, nickel xanthates are completely destroyed in PP under the same processing conditions and the transformation products obtained in this case are less effective u.v. stabilizers than the original xanthates. Nickel dialkyl dithiophosphates stabilise both LDPE and PP very effectively, while nickel alkyl xanthates are much less effective u.v. stabilisers in both matrices. However, the difference between the efficiencies of the two dithiolates is much less in the case of LDPE. The nickel dithiophosphates and xanthates effectively synergise with the commercial u.v. absorber Cyasorb u.v. 531 (HOBP) but they show antagonism towards a typical chain breaking antioxidant, Irganox 1076, during u.v. exposure. They are however synergists under thermal oxidative conditions.