Photo-stabilising performance of a hindered piperidine compound in polypropylene film: Anti-oxidant/light stabiliser effects

The effect of various primary and secondary anti-oxidants and light stabiliser systems on the photo-stabilising performance of a hindered piperidine compound bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. None of the anti-oxidants or light stabilisers gave an additive effect with the hindered piperidine compound. In many cases antagonistic effects were observed. The effects observed are discussed in relation to our current understanding of the mechanistic behaviour of these hindered piperidine systems.     

Stabiliser interactions with hindered piperidine compounds in polypropylene: Influence of processing

The effect of two hindered piperidine compounds on the photo-stability and light stabilising performance of a 2-hydroxy-benzophenone and a 2-hydroxybenzotriazole stabiliser in polypropylene film has been examined. The piperidine compounds—a stable N-oxy radical and a hindered amine—inhibit the photolysis of both the benzophenone and the benzotriazole stabilisers, the amine being more effective. Processing reduces the protective efficiency of the piperidine compounds. The light stability of the polymer was, however, variable, both antagonism and synergism being observed. The processing operation had a dramatic effect on the light stabilising performance of the hindered amine systems whereas it had virtually no effect on the performance of the N-oxy radical systems. The inhibition process is attributed to a regeneration process involving the N-oxy radical and the effects of processing are attributed to the formation of hydroperoxides.     

Photostabilising action of hindered piperidine compounds in polypropylene

Hindered amine compounds derived from piperidine are a relatively new addition to the range of commercial ultra-violet stabilisers for polymers (bulk and fibres). In comparison with other types of stabiliser, the hindered piperidines have proved to be by far the most effective systems. They do not appear to operate by mechanisms of optical screening or by excited state quenching. Essentially, their stabilising effectiveness depends upon their ability to form a stable nitroxyl radical, which then scavenges alkyl radicals produced during photo-oxidation. In polyolefins they also inhibit the photo-reactions of carbonyl chromophores and react with hydroperoxides.This paper reports on the multifunctional behaviour of these commercially important stabiliser systems and their interactions with antioxidants.     

Photo-stabilisation of polypropylene by a hindered piperidine compound: Influence of hydroperoxide and carbonyl groups

The influence of prior thermally generated hydroperoxides and added benzophenone on the photo-stabilising action of a hindered piperidine compound, bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. The hindered piperidine compound was found to effectively inhibit both the hydroperoxide and carbonyl group sensitised photo-oxidation of the polymer. Using ESR spectroscopy, evidence is presented to show that the hindered piperidine compound reacts stoichiometrically with the hydroperoxide groups generated by thermal oxidation to give a stable nitroxyl radical. The photo-stabilising effects observed are discussed in relation to our understanding of the ultraviolet anti-oxidant action of the hindered piperidine systems.     

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

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 antioxidants and hindered piperidine compounds in the photostabilisation of polypropylene: Influence of processing history

The effect of various antioxidants on the photostabilising performance of hindered piperidine compounds in polypropylene has been examined using infra-red, ultra-violet, and ESR spectroscopic techniques. Processing history is shown to play a dominant role in controlling the photostabilising performance of these systems. The destruction of the nitroxyl radical during processing is inhibited by the presence of an antioxidant, and prooxidant activity is assessed by the melt flow index. Antioxidants appear to compete effectively with the nitroxyl radicals for macroradicals. The results are discussed in terms of the importance of antioxidant oxidation versus the formation and destruction of hydroperoxides during processing.     

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.     

Thermal and photo-sensitised oxidation of polypropylene: Influence of hindered piperidine compounds

The effect of two hindered piperidine compounds on the photo-sensitised oxidation of polypropylene by anthracene, anthraquinone, benzhydrol and benzophenone has been examined using infra-red, ultraviolet absorption luminescence and flash photolysis techniques. The compounds—a stable N-oxy radical and a hindered amine—are found to inhibit, to different extents, the photo-sensitised oxidation of the polymer. Neither of the compounds, however, inhibits photo-decomposition of the sensitisers, indicating that mechanisms involving excited state quenching and/or sensitiser regeneration are not operative in stabilisation. On flash photolysis of benzophenone, anthraquinone and benzhydrol, the presence of the N-oxy radical inhibits transient absorption due to the ketyl, semi-quinone and triphenyl methyl radicals. The N-oxy radical also inhibits the thermal reduction of anthraquinone in polypropylene, again indicating that excited state quenching in photo-stabilisation is unimportant. The results are discussed on the basis of a radical trapping mechanism.     

The photo-oxidation of polyolefins containing a hindered piperidine compound

The rate of oxygen absorption by unstabilised PE and PP, as well as by the same polymers stabilised with bis-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770), has been determined during photo-oxidation at several temperatures and uv light intensities. In addition, the rate of stabiliser conversion has been measured under various irradiation conditions, including outdoor exposure. The activation energy of the conversion of Tinuvin 770 appears to be much less than that of the photo-oxidation of PE. When stabilised with 0·1% Tinuvin 770, PE and PP differ with respect to the dependence on light intensity of their rates of oxygen uptake. The consequences of these results for the interpretation of accelerated ageing tests are discussed.     

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.     

ESR study of polypropylene photostabilisation by sterically hindered piperidine derivatives

ESR spectra of sterically hindered piperidine derivatives embedded in polypropylene are superpositions of signals arising from radicals of different mobility. Analysis of the spectra at different temperatures gives information on the mobility and distribution of radicals in solution and in polymer media. At the temperature of irradiation, radicals are homogeneously distributed also in a polymer matrix.During irradiation the concentration of radicals first increases then, after reaching a maximum, decreases. This intermediate character is the result of simultaneous formation and reaction of nitroxyl radicals with radicals arising from polypropylene photo-oxidation.Maximum radical concentration is reached at higher dosage, and subsequent decrease in concentration is slower when hydroxy-benzophenone is applied together with the piperidine derivative. In case of joint application of the latter with a nickel complex, on the other hand, no radical formation can be detected in the polymer before, or in the course of irradiation.     

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.     

Photo-stabilising action of metal chelates in polypropylene—Part IV: Influence of metal stearates in an exchange mechanism

The effect of calcium and nickel stearates on the photo-stabilising action and photo-decomposition of one calcium and two nickel metal chelates in polypropylene has been examined using infra-red and ultraviolet derivative spectroscopy. Whereas the presence of calcium stearate antagonises the photo-stabilising action of one nickel chelate, Irgastab 2002, it strongly synergises with the other, Cyasorb UV 1084. The presence of nickel stearate synergises with the calcium chelate, Irganox 1425, in unprocessed polymer whereas, in processed polymer, it exhibits antagonism. Rates of photo-decomposition of the metal chelates are also affected by metal stearates. The data suggests that, with these nickel chelates, there might be some degree of metal exchange with certain metal stearates.     

Photo-stabilising action of metal chelate stabilisers in polypropylene: Part VII—Additive interactions

The interactions of three metal chelates (nickel (II) 2,2′-thiobis (4-tertoctyl-phenolato), n-butylamine (Cyasorb UV 1084) and the nickel and calcium derivatives of bis(ethyl-3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate (Irgastab 2002 and Irganox 1425)) with other commercial additives, in the stabilisation of polypropylene film, are examined using normal and second-order derivative ultraviolet and infra-red spectroscopic techniques. It is shown that the observed behaviour is dependent on the particular additive, the processing history of the polymer and the metal chelate concerned, and is therefore highly complex. All three chelates show antagonism with a hindered piperidine, bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770), in unprocessed polymer, whereas one of the chelates behaves synergistically with this additive in processed samples. The chelates also show antagonism with a primary antioxidant, tris((3-(3′,5′-di-tert-butyl-4-hydroxybenzyl)-2′'-aceto-ethyl)) isocyanurate (Goodrite 3125), whereas, with a secondary antioxidant, distearyl pent-aerithrityl diphosphite (Weston 618), their behaviour is synergistic in both unprocessed and processed polymer. Two of the chelates show synergism with 2-hydroxy-4-n-octoxybenzophenone (Cyasorb UV 531), the remaining chelate showing antagonism. Mixtures containing a three-additive system show a more complex behaviour. The results are explained in terms of hydroperoxide decomposition, inhibition, stabiliser dispersion, ultraviolet stability and additive compatibility.     

Photo-stabilising action of metal chelate stabilisers in polypropylene: Part VIII—Pigment interactions

The interaction of four metal chelates—Cyasorb UV 1084, Irganox 1425, Irgastab 2002 and Sanduvor NPU—with five commercial pigments, anatase and rutile titanium dioxide, cadmium yellow, Vynamon Blue B and Monolite Yellow G, is examined using infra-red spectroscopy. Individually, four pigments offer some stabilisation to processed polypropylene. Their order of stabilisation is Monolite Yellow G > Vynamon Blue B > cadmium yellow ～ rutile titanium dioxide. Anatase titanium dioxide showed mild sensitising action. When mixed with the metal chelates their order of stabilisation is variable. The effect of a commercial antioxidant, Irganox 1010, is also examined. It is found that the antioxidant tends to suppress any synergistic effects that may be present between the chelate and the pigment.     

Photostabilization of commercial polypropylene by hindered piperidine compounds: An ESR and luminescence study

The photostabilization of commercial polypropylene by a hindered piperidine stabilizer, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate (I) and by a model N-oxy radical compound, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy (II) is examined using ESR and luminescence spectroscopy. ESR spectroscopy shows that I operates through the formation of a stable nitroxyl radical in the polymer. On the other hand, II disappears rapidly during the early stages of photo-oxidation but continues to act as an effective stabilizer. A low steady-state equilibrium concentration of nitroxyl radicals is believed to be responsible for the high photostabilizing efficiency of I. Both compounds also inhibit the photolysis of the luminescent α,β-unsaturated impurity groups present in the polymer; possible mechanisms are discussed.     

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

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.     

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

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.     

Inhibition of the anthraquinone photosensitised oxidation of polypropylene by a hindered piperidine stabiliser: A spectroscopic study

A hindered piperidine stabiliser, bis{2,2,6,6-tetramethyl-4-piperidinyl}-sebacate, has been found to inhibit the photo-sensitising effect of anthraquinone in polypropylene. On flash photolysis in n-hexane solution transient formation due to the semi-quinone radical (AH·) at 370 nm and subsequent formation of the hydroquinone (9,10-dihydroxyanthracene (AH₂) were found to be effectively inhibited by the corresponding nitroxyl radical but not by the amine. The absence of the hydroquinone photoproduct was confirmed using fluorescence spectroscopy. With the aid of phosphorescence spectroscopy it is concluded that the nitroxyl radical is the effective stabilising species and operates by a radical scavenging mechanism to form the hydroxylamine, and not by excited state quenching.     

Photo-stabilising action of metal chelates in polypropylene—Part III: Thermal antioxidant action and its relationship to photo-stabilisation

The thermal antioxidative behaviour of three metal chelates in polypropylene film has been examined using hydroperoxide analysis and infra-red spectroscopy. All three chelates inhibit both hydroperoxide and carbonyl group formation during oxidation. Their thermal antioxidant efficiencies follow closely their photo-stabilising effects, suggesting that similar mechanisms of protection are involved. The rates of photo-decomposition of the metal chelates measured by second order derivative absorption spectroscopy were found to be independent of initial hydroperoxide concentration, indicating that they are ineffective macroalkoxy and hydroxy radical traps. The results suggest that metal chelates compete effectively with oxygen for macroalkyl radicals thermally and photochemically.