Alan Parkinson

Thermal and photo-oxidative behaviour of hindered piperidine compounds in polypropylene: Importance of hydroxylamine in stabilisation

Abstract The thermal oxidation behaviour of three hindered piperidine compounds has been examined in polypropylene film by oven ageing in air at 140°C together with its subsequent effect on rate of photo-oxidation. The amine, bis[2,2,6,6-tetramethyl-4-piperidinyl] sebacate, shows a rapid growth in nitroxyl radical concentration due to reaction with hydroperoxides whereas the bis-nitroxyl derivative is relatively stable and decreases in concentration very slowly, giving the hydroxylamine. The ageing process results in an improvement in photo-stability and this is associated with the reaction of the nitroxyl radical with a tertiary radical to give the hydroxylamine and ethylenic unsaturation. The mono nitroxyl compound behaves differently during the early stages of ageing in that there is a rapid decrease in concentration and this is followed by a rapid fall in photo-stability. Only during the latter stages of ageing does hydroxylamine production appear to control photo-stability.

Photo-stabilising action of a p-hydroxybenzoate light stabiliser in polyolefins: Part III—Antioxidant behaviour and additive/pigment interactions in high density polyethylene

Abstract The effect of a number of commercial primary and secondary antioxidants, light stabilisers and pigments on the photo-stabilising action of an n -alkyl substituted p -hydroxybenzoate light stabiliser (Cyasorb® UV 2908) is examined in high density polyethylene. At the concentration levels employed here the p -hydroxybenzoate light stabiliser showed a favourable interaction with all the light stabilisers and pigments studied. Interactions with the primary and secondary antioxidants appeared to be concentration dependent. Synergism with the light stabilisers is associated with the ability of these compounds to destroy hydroperoxides and/or react with alkoxy and hydroxy radicals produced in their photolysis. Synergism with primary and secondary antioxidants is observed at low concentrations of each component (0·05% w/w each) whereas antagonism is observed at higher concentration levels, the latter being associated with unfavourable interactions between intermediate radical products of the additives. With the pigments there is strong synergism and this is associated with the known ability of the pigments to catalyse the photo-decomposition of hydroxperoxides, the resulting alkoxy and hydroxyl radicals being effectively scavenged by the p -hydroxybenzoate stabiliser. Further information on the selectivity of the p -hydroxybenzoate light stabiliser to terminate only the active free radical species in the polymer compared with conventional antioxidant structures is also provided.

Photo-stabilising action of a p-hydroxybenzoate compound in polyolefins—Part I: Thermal and photo-chemical behaviour in polypropylene film

Abstract The photo-stabilising action of a new aliphatic p -hydroxybenzoate light stabiliser, Cyasorb® UV 2908 (American Cyanamid Company), has been examined in polypropylene film, with the aid of a number of related compounds, by both normal and derivative uv absorption, infra-red techniques and hydroperoxide analysis. During processing and oven ageing the stabiliser operates as an effective chain breaking donor, terminating macroalkyl radicals and inhibiting the formation of hydroperoxides. Under both monochromatic (365 nm) and polychromatic ( λ ′s > 300 nm) irradiation conditions the decomposition of the stabiliser shows a direct dependence on initial hydroperoxide concentration in the film, indicating that it operates as an effective light stable alkoxy and hydroxy radical scavenger. Under ‘direct photolysis’ conditions (254 nm light) the stabiliser does not undergo unfavourable dimerisation reactions like other related p -alkyl substituted phenols. Evidence is also presented to show that the presence of the long aliphatic hydrocarbon chain has a powerful protective effect on the molecule and this is associated with a radical recombination process due to the polymer cage.

Interaction between coloured pigments and hindered piperidine/antioxidant combinations in the photo-stabilisation of polypropylene

Abstract The complex interaction between coloured pigments, a primary antioxidant and a hindered piperidine compound in the photo-stabilisation of polypropylene has been examined using infra-red absorption spectros-copy. Alone, the pigments offered little photo-protection, their order of efficiency being copper phthalocyananine > chromic oxide > cadmium yellow. In the presence of an antioxidant only an additive effect was observed whereas, in the presence of a hindered piperidine stabiliser, there was powerful synergism. However, in the presence of both antioxidant and hindered piperidine compound the synergistic effects were considerably reduced with two of the pigments, cadmium yellow and chrome green, whereas, with copper phthalocyanine, the synergism was enhanced even further. The results indicate that additive adsorption is important with the first two inorganic pigments whereas, with the latter, the bulky organic ligands inhibit any such interaction. In fact, copper phthalocyanine and a hindered piperidine stabiliser exhibit a highly favourable interaction for photo-stabilisation.

Ultraviolet derivative absorption spectra of nylon 6,6: Effect of photolysis versus photo-induced oxidation

Abstract The irradiation of nylon 6,6 film with light of wavelengths greater than 250 nm and 300 nm has been studied using uv derivative absorption spectroscopy. Initially, the polymer showed a strong absorption at 290 nm which is associated with an α,β-unsaturated carbonyl species. Changes in the absorption band were found to be dependent on the atmosphere and are discussed in terms of a reversible photolysis versus a photo-induced oxidation process.

Spectroscopic properties and mechanistic action of a new p-hydroxybenzoate light stabiliser: a comparative study in polypropylene and high density polyethylene and anti-oxidant interactions

The photostabilising action of an n-alkyl substituted p-hydroxybenzoate light stabiliser (Cyasorb® UV 2908) is examined in both polypropylene and high density polyethylene using luminescence, normal and derivative uv-visible and infra-red spectrophotometric techniques and hydroperoxide analysis. By comparison with various anti-oxidants it proved to be a very effective light stabiliser in high density polyethylene but not in polypropylene. Whilst its uv absorption spectrum showed it to be a non-absorber in sunlight its luminescence properties were totally different from those of conventional anti-oxidant structures. The photostabilising action of the p-hydroxybenzoate light stabiliser is found to be dependent on the initial concentration of hydroperoxide groups in the polymer indicating that it is an effective alkoxy and hydroxy radical scavenger. In polypropylene, it suppressed hydroperoxide formation during processing but had no effect in high density polyethylene, indicating it to be an effective thermal anti-oxidant in the former polymer. Using methylene blue as a singlet oxygen sensitiser and dimethylanthracene as a chemical trap, the stabiliser proved ineffective in quenching singlet oxygen in the polymer matrix. Its inability to photolyse to give active quinone products, good compatibility and alkoxy and hydroxy radical terminating properties are concluded to be key factors responsible for its light stabilising function in high density polyethylene. The interaction of the light stabiliser with conventional thermal primary anti-oxidants is also examined and discussed.

Photostabilising action of a p-hydroxybenzoate light stabiliser in polyolefins: Part IV—Catalyst effects and additive interactions in linear low density polyethylene

Abstract The photostabilising action of a p -hydroxybenzoate light stabiliser (Cyasorb® UV2908) is examined in linear low density polyethylene (LLDPE). The efficiency of this light stabiliser is shown to be significantly influenced by levels of residual catalyst such as Ti and Al. Deactivation of the catalysts with phosphoric acid in 2-propanol markedly improves both the performance and light stability of the stabiliser. Processing history reduces the efficiency of the stabiliser whereas its light stability increases linearly with processing time and is associated with the ability of the residual catalyst to destroy hydroperoxides during processing. Stabilising efficiency, however, is reduced by a corresponding reduction in stabiliser concentration during processing. The stabiliser synergises effectively during both oven ageing and photo-oxidation with a hindered piperidine light stabiliser due to the complementary function of each additive. Thus, whilst the former terminates primarily alkoxy/hydroxy radicals produced from the decomposition of hydroperoxides, the latter destroys hydroperoxides in a stoichiometric reaction and through the nitroxyl radical intermediate which selectively reacts with macroalkyl (P • ) radicals and through the hydroxylamine which reacts with peroxy radicals (PO 2 • ). Strong synergism is also observed with TiO 2 (rutile).

Photo-stabilising action of a p-hydroxybenzoate compound in polyolefins: Part II—Thermal and photo-chemical behaviour in high density polyethylene film

Abstract The processing and photo-stabilising action of an n -alkyl substituted p -hydroxybenzoate light stabiliser (Cyasorb UV2908) is examined in high density polyethylene using normal and derivative ultraviolet-visible and infra-red spectrophotometry and hydroperoxide analysis. In comparison with two related systems—2,6-di-tert-butyl- p -cresol (Topanol OC) and 4-hydroxy-3,5-di-tert-butylbenzoic acid (Shell Acid)—it proved to be an effective light stabiliser, particularly when compared with its performance in polypropylene. The photo-stabilising action of the p -hydroxybenzoate light stabiliser is found to be dependent on the initial concentration of hydroperoxide groups in the polymer, indicating it to be an effective alkoxy and hydroxy radical scavenger. Unlike its behaviour in polypropylene, it did not suppress hydroperoxide formation during processing, indicating it to be ineffective as a macroalkyl radical scavenger in high density polyethylene. Its inability to photolyse to give active quinone products and its good compatibility and alkoxy and hydroxy radical terminating properties are key factors which are responsible for its light stabilising function in high density polyethylene.

Photo-stabilising action of a polymeric hindered piperidine compound in polypropylene film: Influence of processing

Abstract The effect of a phenolic antioxidant on the photo-stabilising performance of a polymeric hindered piperidine compound in polypropylene has been examined using infra-red and ESR spectroscopic techniques. Processing history is shown to play a dominant role in controlling the photo-stabilising performance of these systems. Whilst the antioxidant gave enhanced performance, in most cases its effect is antagonistic. The ESR results suggest that maximum stabilisation is associated with the conversion of the amine to the substituted hydroxylamine and not the nitroxyl radical.

Flash photolysis and antioxidant activity

Abstract The photo-chemical behaviour of a number of mono- and polyfunctional commercial phenolic antioxidants has been examined using kinetic micro-second flash photolysis. The technique provides useful information on the relationship between antioxidant structure and the efficiency of phenoxy radical production. The kinetics of decay of the phenoxy radicals are also found to be dependent on structure. Mono-functional antioxidants give phenoxy radicals which decay by a second-order process whereas polyfunctional antioxidants give phenoxy radicals that decay by a first-order process. In the former case dimerisation to give bisphenolic coupling products is observed whereas, with the latter, this process is sterically inhibited. The value of flash photolysis as a probe for studying antioxidant activity is discussed.