Khirud B. Chakraborty

The effects of thermal processing on the thermal oxidative and photo-oxidative stability of low density polyethylene

Abstract The effects of thermal processing on the chemical and physical properties of low density polyethylene are strongly dependent on the amount of oxygen present in the mixer. Subsequent thermal and photo-oxidation show very similar characteristics which appear to depend primarily on the amount of hydroperoxide formed during processing. Removal of hydroperoxide by heat treatment in an inert atmosphere, although increasing ketonic carbonyl concentration, markedly decreased the rate of photooxidation, introducing an induction period similar to that of an unprocessed sample. It is concluded that hydroperoxides are the most important initiators during the early stages of photo-oxidation of normally processed polymers, although carbonyl initiation is important in heavily oxidised polymers in the later stages of photo-oxidation.

Mechanisms of antioxidant action: Synergism between antioxidants and ‘U.V. absorbers’

Abstract 2-Hydroxy-4-octyloxy benzophenone (HOBP), a typical ‘u.v. absorber’, synergises with both peroxide decomposing antioxidants (metal dithiocarbamates) and a chain breaking antioxidant (hindered phenol, Irganox 1076) during photo-oxidation of LDPE; the evidence suggests that the function of the ‘u.v. absorber’ is, at least in part, to remove reactive species formed from the antioxidants during irradiation and thus protect the dithiocarbamates from photolysis. By contrast, the peroxide decomposing and chain breaking antioxidants which are effective synergists during thermal oxidation of LDPE are antagonistic during photo-oxidation; it is suggested that this may be due to sensitisation of the photolytic destruction of the dithiocarbamates by oxidation products of the phenol.

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

Abstract 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: Stabilisation of polypropylene during processing and on exposure to light

Abstract 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

Abstract 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: Evidence for a regenerative cycle during the melt stabilisation of polypropylene by galvinoxyl

Abstract 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: Concentration effects of zinc dialkyldithiocarbamates in the uv stabilisation of polypropylene

Abstract The higher molecular weight homologues in the zinc dialkyldithiocarbamate series have been found to be much more effective uv stabilisers in polypropylene than the methyl and ethyl analogues and this is shown to be due to the increased equilibrium solubilities of the former in the polymer. The uv stabilities and uv screening and peroxidolytic activities of the series are very similar at the same molar concentration but the relationship between their uv stabilising activities and concentration is non-linear, suggesting an auto-protective effect. The evidence suggests that this results from their peroxidolytic activity which protects not only the substrate but also the additives themselves.

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

Abstract 2-Mercaptothiazolines (II, R = H, CH 3 , C 2 H 5 and C 6 H 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: The photo-antioxidant mechanisms of the 4-alkyl-2-mercapto thiazolines in polypropylene

Abstract An explanation is given for the favourable effect of oxidative processing on the behaviour of the 4-alkyl-2-mercapto thiazoles (RMTs) as photo-antioxidants in polypropylene (PP). The thiols are shown to be oxidised to the corresponding disulphides, followed by further oxidation through radical intermediates to products which have undergone sulphur loss. The chemistry of the oxidative transformations is believed to involve radical species during the early stages of photo-oxidation.

Mechanisms of antioxidant action: Cupric stearate as a catalytic antioxidant

Abstract 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.