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: 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: 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: Synergism between tin stabilizers and polymer-bound antioxidants in the u.v. stabilization of PVC

It is shown that a conventional tin stabilizer (dibutyltin maleate) (DBTM) forms an effective synergistic u.v. stabilizing combination when used in conjunction with a bound phenolic antioxidant (BHBM) and a bound u.v. stabilizer (EBHPT). These additives become bound to the polymer through sulphur during processing in the presence of DBTM and it is suggested that the sulphide of the adduct makes its own contribution to u.v. stabilization by acting as a peroxide decomposing (PD-C) antioxidant during photo-oxidation. However, the 2-hydroxybenzophenone function appears to be an essential part of the system and is probably involved in quenching of excited states of the intermediate sulphoxides which precede the formation of the PD-C antioxidant.     

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.     

Influence of metal ions on antioxidant behaviour in polypropylene

The effect of Cr(III), Fe(III) and Cu(II) ions, at trace levels, on the effectiveness of four commercial antioxidant systems, viz. Topanol, Santowhite, Hostanox 03 and Irganox 1425, in polypropylene (PP) films is examined using infra-red and second-order derivative uv spectroscopy. The results show that at these levels the metal ions have a minor effect on the unstabilised polymer. However, the ions can have a beneficial or adverse effect on antioxidant activity, particularly under oven ageing conditions. The observed effect depends on the metal/antioxidant system, and is attributed to an indirect interaction between the components which can delay or accelerate antioxidant decomposition in the polymer. An exception to this behaviour is the metal complex Irganox 1425, which interacts strongly with copper ions in the polymer matrix. The antioxidant is partially transformed by copper ions during the compression moulding stage. The beneficial effect present in some cases is in direct contrast with what has generally been observed in studies using unstabilised polymers.     

The effect of thermal processing on PVC—VII. Reactive antioxidants as thermal stabilizers

A benzyl thiol (BHBM) containing a hindered phenolic antioxidant group has been compared as a thermal stabilizer for PVC with a typical tin maleate stabilizer (DBTM). It has been shown that BHBM both adds to the developing double bond in PVC during processing and destroys hydroperoxides. It is thus an effective stabilizer but it is partially destroyed by side-reactions. In combination with DBTM, BHBM shows powerful synergistic effects which are optimal at a molar ratio of DBTM: BHBM ? 4:1. The reasons for the side-reactions and the synergism are rationalized on the basis of the known reactions of BHBM.     

Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.     

AAPH-mediated antioxidant reactions of secoisolariciresinol and SDG

Secoisolariciresinol (SECO ) is the major lignan found in flaxseed (Linum usitatissimum L.) and is present in a polymer that contains secoisolariciresinol diglucoside (SDG ). SECO, SDG and the polymer are known to have a number of health benefits, including reduction of serum cholesterol levels, delay in the onset of type II diabetes and decreased formation of breast, prostate and colon cancers. The health benefits of SECO and SDG may be partially attributed to their antioxidant properties. To better understand their antioxidant properties, SECO and SDG were oxidized using 2,2'-azobis(2-amidinopropane), an in vitro model of radical scavenging. The major lignan radical-scavenging oxidation products and their formation over time were determined. SDG was converted to four major products, which were the result of a phenoxyl radical intermediate. One of these products, a dimer of SDG, decomposed under the reaction conditions to form two of the other major products, and . SECO was converted to five major products, two of which were also the result of a phenoxyl radical intermediate. The remaining products were the result of an unexpected alkoxyl radical intermediate. The phenol oxidation products were stable under the reaction conditions, whereas two of the alcohol oxidation products decomposed. In general, only one phenol group on the lignans was oxidized, suggesting that the number of phenols per molecule may not predict radical scavenging antioxidant ability of lignans. Finally, SECO is a superior antioxidant to SDG, and it may be that the additional alcohol oxidation pathway contributes to its greater antioxidant ability.     

Mechanisms of antioxidant action: The effect of thermal processing on the antioxidant activity of sulphur compounds

The nickel dithiophosphates and their corresponding disulphides are shown to be more effective uv stabilisers for LDPE than the closely related nickel xanthates and disulphides (dixanthogens). In general, the nickel complexes in both series are more effective uv stabilisers than the disulphides.Evidence is presented to show that the better performance of the dithiophosphates than the xanthates is associated with their higher solubility in the polymer. For the same reason, nickel dithiolates containing long alkyl substituents are more effective uv stabilisers than those containing shorter alkyl substituents.Thermal processing adversely affects the photo-stabilising performance of nickel dithiolates whereas the performance of the less efficient disulphides is enhanced under the same conditions. In the former case, the uv stability of the polymer is shown to be directly proportional to the amount of the nickel complex left after processing. In the case of the disulphides, on the other hand, the evidence suggests that increasing the severity of their processing increases the extent of oxidation to products which are more effective uv stabilisers than the parent disulphides.     

Mechanisms of antioxidant action: The catalytic rôle of iodine in the stabilisation of polypropylene

It is shown that I₂ is an effective melt stabiliser for polypropylene at 190°C in a closed mixer but that it is much less effective in an open mixer. The formation of HI and unsaturation in the polymer show that a catalytic antioxidant mechanism (CB-A/CB-D) is involved in the antioxidant activity of I₂ in a closed mixer similar to that occurring in the stabilisation of polyolefins with galvinoxyl and nitroxyl radicals.The processed polymer shows slightly greater uv stability than the control with no additive but is essentially similar to the control in thermal oxidative stability. The volatility of HI in an air oven probably accounts for the fact that it has no CB-D activity under these conditions.     

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: Behaviour of a hindered piperidine and related oxidation products during processing and photo-oxidation of polypropylene

Evidence is presented to show that a commercial hindered piperidine (I(A)) is not an effective melt stabiliser for polypropylene whereas related nitroxyl radicals (II(A)) and (II(B)) and hydroxylamine (III(B)) are highly effective. These results are explained on the basis of an oxidative transformation of the piperidine to the nitroxyl during processing and the involvement of the latter in a cyclical regenerative process in which the nitroxyl acts as a chain-breaking acceptor (CB-A) antioxidant and the derived hydroxylamine as a chain-breaking donor (CB-D) antioxidant.The same CB-A/CB-D cycle operates during photo-oxidation of polymer films containing each of the additives. The nitroxyl radical concentration reaches a stationary concentration in the polymer, irrespective of whether it is added as nitroxyl or as parent amine. The derived hydroxylamines are substantially more effective than the nitroxyl radicals as ultraviolet stabilisers.     

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.     

Photo-stabilising action of ortho-hydroxy aromatic compounds in polypropylene film: UV absorption versus radical scavenging

The relative importance of uv absorption and radical scavenging in the photo-stabilisation of polypropylene film by various concentrations of 4-methoxy and 4-n-octoxy substituted 2-hydroxybenzophenones and 2(2′-hydroxy-3′-t-butyl-5′-methyl phenyl)-5-chlorobenzotriazole is examined using infra-red and second-order derivative uv absorption spectroscopy and hydroperoxide analysis. Under both photolysis with polychromatic light wavelengths greater than 300 nm and photo-sensitised oxidation with monochromatic light of mainly 365 nm, the additives were considerably more effective photo-stabilisers when present in the polymer than when used as screens. Only under exposure to far uv light of 254 nm wavelength at 0·5 and 1·0% w/w of additives was any evidence of screening important. At 0·01% w/w concentration both the ortho-hydroxybenzophenones sensitised the photo-oxidation of the polymer and this correlated with higher concentration levels of hydroperoxide in the polymer films, confirming the pro-oxidant behaviour of these additives. Enhanced photo-protection by long n-alkyl groups in the 4-position of ortho-hydroxybenzophenones under photo-sensitised oxidation is associated with the lower mobility of the n-alkyl radical and cage effects in the polymer encouraging radical recombination. These conclusions are confirmed by solution photolysis experiments in the absence and presence of cumene hydroperoxide. The results clearly show that whilst reversible proton transfer may be important for protecting the additive itself, a mechanism involving uv absorption is of little or no importance in photo-stabilisation.     

Some mechanistic insights in the behaviour of thiol containing antioxidant polymers in lignin oxidation processes

Modified polyethylene glycol polymers are described in which thiol containing groups have been added via a simple esterification step. The resultant thiol end-capped polyethers retain the antioxidant behaviours of the monomeric thiols, but mitigate essentially completely the undesirable properties of the monomers, including offensive odour, volatility, and insolubility in aqueous solvents. This concept substantially enhances the number of potential practical applications of the thiol containing polymers as compared to the monomeric substances. A class of polyethylene glycols substituted with thiolactic acid or thioglycolic acid is described with potential in the pulp and paper industry, for the inhibition of radical induced oxidative processes including brightness reversion in lignin containing papers. Several techniques are used to elucidate aspects of the mechanism by which such thiols are effective: a novel photoacoustic technique demonstrates the scavenging effects of the thiol end capped polyethylene glycols towards alkoxy radicals. ESR studies demonstrate the ability of these thiol polymers to quench both the exogenous “phenoxy” radicals present in CTMP and TMP papers as well as the enhanced radical signal resulting from irradiation of these papers. The strong uv absorbance of these relatively persistent phenoxy radicals may decrease the optimal initial brightness of papers made from mechanical pulps.     

Polyolefin photo-stabilisation mechanisms. Reactions of tetramethylpiperidine derivatives in model systems

The relative rate of cyclohexyl radical scavenging by oxygen or 4-oxo-2,2,6,6-tetramethylpiperidino-N-oxyl has been measured at 25°C, together with the relative rates of cyclohexyl peroxy radical attack on cyclohexane or substituted hydroxylamines derived from 2,2,6,6-tetramethylpiperidine. These competitive processes are important in the stabilisation of polymers against photo-oxidative destruction by piperidine derivatives. Peroxy radical scavenging by the substituted hydroxylamine appears to be considerably more important than alkyl radical scavenging by the nitroxide, although both processes are essential for prolonged ultra violet (uv) stabilisation of a polymer. However, a comparison of experimental photo-protection with that predicted by the measured rate constant ratios shows that other processes are needed to account entirely for the observed stabilisation. Other factors which may be involved in piperidine photo-protection of polymers include thermal decomposition of the substituted hydroxylamine to reform nitroxide in polar(oxidised) zones and especially the association of nitroxides with hydroperoxide groups (the dominant photo-initiator in degrading polyolefins).     

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: Stabilisation of polypropylene during processing and on exposure to light

The most important initiators for the photo-oxidation of processed polypropylene are thermally formed hydroperoxides. Peroxide decomposing antioxidants (ZnDEC and NiDEC) effectively control hydroperoxide formation, and hence oxidation, during processing. A chain breaking antioxidant (1076) is effective and a typical uv absorber (HOBP) completely ineffective. The uv stabilising activity of these compounds is NiDEC > HOBP > 1076 > ZnDEC; the first two show a strong concentration dependence whereas the last two do not. The two dithiocarbamates are relatively more effective at high temperatures and they synergise effectively with the uv absorber. The synergism obtained with ZnDEC is higher than that with NiDEC.     

Phytochemical and antioxidant properties of some Cassia species

In this study, the antioxidant activity of aqueous and ethanol extracts of four plants from the genus Cassia were evaluated by various antioxidant assays, including ferric reducing antioxidant power (FRAP), DPPH free radical scavenging, metal chelating activity, phosphomolybdenum reducing power, hydrogen peroxide radical scavenging, hydroxyl radical scavenging, deoxyribose degradation and β-carotene bleaching assay. The various antioxidant activities were compared to standard antioxidant such as ascorbic acid. All the extracts showed antioxidant activity in the tested methods. Among the four species, Cassia auriculata has been found to possess highest activity in most of the tested models. In addition to the antioxidant activity, the total phenolics and flavonoids were measured in the extracts. The ethanolic extract exhibited highest phenolics and flavonoid contents and had also shown potent antioxidant activity in comparison to the aqueous extracts. The possible antioxidant mechanism of the ethanol extract can be due to its hydrogen or electron donating and direct free radical scavenging properties. Hence, the ethanol extract represents a source of potential antioxidants that could be used in pharmaceutical industries.