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

2-Mercaptothiazolines (II, R = H, CH₃, C₂H₅ and C₆H₁₃) 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: 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: 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 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: Transformations of 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl (HTMPO) in polypropylene during processing

When 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: 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 nature of the redox behaviour of thiodipropionate esters in polypropylene

The chemical reactions involved in the antioxidant behaviour of the thiodipropionate esters have been investigated and three distinct mechanisms have been identified. The first is a radical trapping process involving the sulphenic acid formed by breakdown of the sulphinyldipropionate ester which is an oxidation product of the thiodipropionate. The second is a radical generating reaction resulting from a redox reaction between hydroperoxides and the sulphenic acid. The third is a Lewis acid catalysed destruction of hydroperoxide by a further oxidation product of the sulphenic acid. Sulphur dioxide satisfies some of the requirements of this catalytic species but it seems likely that other products are also involved. Hydroperoxide deactivation by complexing with sulphoxide does not appear to be involved.     

Mechanisms of antioxidant action: Time-controlled stabilisation of polypropylene by transition metal dithiocarbamates

Combinations of iron and nickel dithiocarbamates approach the ‘ideal’ delayed action photo-activators for polypropylene. They give an induction period to photo-oxidation which can be controlled by the amount of the nickel complex and a rapid rate of photo-oxidation at the end of the induction period which is primarily determined by the iron concentration. Combinations of other photo-activators and uv stabilisers show a less ideal behaviour and do not permit such accurate time control.     

Acetylation of 6-alkyl-5-ethoxycarbonyl-2-hydroxy-4-methyl-dihydropyrimidines and their 2-mercapto analogs

A number of 6-alkyl- and 6-aryl-5-ethoxycarbonyl-2-hydroxy-4-methyldihydropyrimidines and their 2-mercapto analogs and the acetyl derivatives of these compounds have been synthesized. In each case, two isomeric acetyl derivatives were obtained, probably the 1- and 3-acetyl compounds.     

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: 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: Effect of processing on the behaviour of hindered phenols in polypropylene

It is shown that in the case of two hindered phenol antioxidants (I R = H and II) products are formed which are considerably more effective than the phenols themselves. Both dimeric products and polymer-bound antioxidants contribute to this effect but the former is considered to be more important under the conditions examined.     

Mechanisms of antioxidant action: Chemical binding of a thiol antioxidant into polypropylene by means of uv irradiation

3,5-Di-t-butyl-4-hydroxybenzyl mercaptan (BHBM) became chemically combined to polypropylene to the extent of 68% of the antioxidant present when films containing BHBM were exposed to uv irradiation. In addition to the radical coupling reaction of the thiyl radical derived from BHBM with the polymer, a second technologically less useful process also occurs, leading to the formation of a derived stilbenequinone. The latter absorbs strongly both in the visible and in the near uv and consequently slows down the subsequent loss of the bound antioxidant from the polymer.     

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.     

Mechanisms of antioxidant action: the role of 2-hydroxybenzophenones in photo-oxidizing media

A study of the photo-stability of 2-hydroxy-4-octyloxybenzophenone (HOBP) in the presence of possible photo-sensitizers present in polymers shows that this u.v. stabilizer is readily destroyed by them in the presence of light. HOBP is destroyed by carbonyl compounds and therefore appears to interfere with their photolysis at least partly by a sacrificial quenching process.     

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.     

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.     

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: 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: The involvement of nitroxyl radicals in the antifatigue action of secondary amines

Isopropylaminodiphenylamine (IPPD, I) and a hindered bispiperidine (IIa) are effective antifatigue agents in vulcanized rubber. The former is also a powerful thermal antioxidant and antiozonant whereas the latter is not. An investigation of the mechanism of antifatigue action shows that both I and IIa are converted to the corresponding nitroxyl radical during dynamic flexing of the rubber and that the nitroxyl is continuously regenerated in a cyclical mechanism involving the corresponding hydroxylamines.