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

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 rôle 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.     

Decomposition of polypropylene hydroperoxide by hindered amines

The hindered amine-induced decomposition of polypropylene hydroperoxide was studied in the solid state and in the presence of a liquid solvent and the polymer was compared with model hydroperoxides. The high reactivity of the macrohydroperoxides appears to be related to the adjacent, hydrogen-bonded hydroperoxide groups that occur in the polymer. The hindered amines are converted to nitroxides in the reaction via hydroxylamine intermediates. Amine-induced decomposition of polypropylene hydroperoxide is faster in the absence of a liquid solvent for the amine than in the presence of the solvent, probably because of the strong amine-hydroperoxide association that occurs in the solid state. The decomposition process in the solid state is sufficiently rapid for the reaction to contribute to the effectiveness of hindered amines in the light stabilization of polymers.     

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.     

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.     

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.     

Destruction of carbonyl and hydroperoxide groups in oxidised polypropylene: Effect on photo-oxidation

The effect of destroying carbonyl and hydroperoxide groups in oxidised polypropylene on the subsequent rate of photo-oxidation has been examined using infra-red spectroscopy. The results show that carbonyl groups dominate the rate of photo-oxidation in severely oxidised polymer. In mildly oxidised polymer hydroperoxide groups control the rate, but to a much smaller extent. Destruction of the photo-active carbonyl and hydroperoxide groups in the unoxidised and oxidised polymers by prior photolysis in an inert atmosphere gave rise to some interesting and complex effects on subsequent photo-oxidation. The results indicate that although carbonyl and hydroperoxide groups may control the rates of photo-oxidation of thermally oxidised/processed polymer, their importance as primary photo-initiators is highly questionable. Oxygen-polymer charge transfer complexes appear to be the more likely photo-initiators.     

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.     

Photo-stabilisation of commercial polypropylene by piperidine compounds: The rôle of stable free radicals

The photo-stabilisation of commercial polypropylene by a hindered piperidine stabiliser, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate, a model piperidine compound, 2,2,6,6-tetramethyl-4-piperidinol, and its corresponding N-oxy derivative is examined using luminescence and ESR spectroscopy. All three compounds inhibit the photolysis of the luminescent α,β-unsaturated carbonyl impurity groups present in the polymer. ESR spectroscopy shows that the hindered piperidine stabiliser operates by the same mechanism as that of the stable hindered piperidino-N-oxy radical compound. Possible mechanisms whereby the piperidine compounds inhibit the photolysis of the α,β-unsaturated carbonyl groups are also discussed.     

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.     

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.     

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.     

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.     

Effective 2,6-substitution of piperidine nitroxyl radical by carbonyl compound

Nitroxyl radicals (nitroxides) with unpaired electron are widely used as antioxidants, contrast agents, and spin probes. Although piperidine nitroxyl radicals have many applications, these are mainly tetramethylpiperidine compounds, and only a few reports consider the substitution of N-O surround as a reaction site, such as 2,2,6,6-tetrasubstituted piperidine nitroxyl radicals. Our results revealed that the 2,6-position of the 2,2,6,6-tetramethylpiperidin-4-one compound was substituted by cyclohexyl groups to produce 2,2,6,6-tetrasubstituted piperidin-4-one derivatives under mild reaction conditions. An interesting result was obtained by using ¹⁵N-labeled NH₄Cl instead of ¹⁴NH₄Cl: it gave ¹⁵N-labeled 2,2,6,6-tetrasubstituted piperidin-4-one-1-oxyls with a high ¹⁵N content. In conclusion, the new method for the synthesis of nitroxyl radicals readily yields 2,2,6,6-tetrasubstituted piperidin-4-one under mild conditions.     

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.     

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.     

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.     

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

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.     

Catalytic thermal oxidation of phenolic antioxidants by hindered piperidine compounds

The interactions of two hindered piperidine compounds, bis(2,2,6,6,-tetramethyl-4-piperidinyl) sebacate and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy, with various phenolic antioxidants, during the thermal processing of polypropylene have been examined using ultraviolet-visible absorption, infra-red and ESR techniques. Evidence is presented to show that the stable N-oxy radical generated during thermal processing catalytically oxidises phenolic antioxidants to their corresponding quinone forms. The implications of this interaction, particularly with regard to its effect on light stability, are discussed.     

Effect of hindered piperidine compounds on the thermal reduction of p-benzoquinone in polypropylene and its effect on photo-oxidation

The effect of a hindered piperidine compound and stable nitroxyl radical on the thermal reduction of p-benzoquinone in polypropylene has been examined using ESR, uv-visible and fluorescence spectroscopy. Thermal reduction to hydroquinone was inhibited by both compounds. With the hindered amine the nitroxyl radical was regenerated through the formation of the hydroxylamine whereas, with the nitroxyl radical, there was quantitative conversion to the hydroxylamine and no nitroxyl radical regeneration. Both inhibition processes were found to antagonise the photo-stabilising action of the hindered piperidine compounds due to the regeneration of the quinone.     

Photostabilizing effectiveness of new HALS produced by isocyanation of hindered piperidine derivatives

Twelve new hindered-amine light stabilisers (HALS) were prepared by isocyanation of the hindered piperidine derivatives 2,2,6,6-tetramethyl-4-piperidinol, 1,2,2,6,6-pentamethyl-4-piperidinol and 4-amino-2,2,6,6-tetramethylpiperidine, by using dibutyltin dilaurate as catalyst. Test results show that these new HALS are effective light stabilisers, some of them comparable to a commercial HALS, Tinuvin-770. Reactions of these hindered piperidines with isocyanates proceed at temperatures varying from room temperature to 80 °C and take less than 30 min to complete. The reactions are quantitative and no toxic byproducts are produced.