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.     

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.     

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.     

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.     

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.     

Stark effects on the phosphorescence spectra of p-benzoquinone

Stark effects on the phosphorescence spectra of p-benzoquinone have been studied in naphthalene crystals at temperatures between 1.7 and 30 K. The experimental data establish that the band splitting of 24 cm^?1 arises from a double minimum potential in the T₁ state.     

Stabilising effect of monomers,homopolymers and copolymers of vinyl derivatives of sterically hindered amines on the photo-oxidation of polypropylene

Vinyl derivatives of sterically hindered amines, N-(2,2,6,6-tetramethyl-4-piperidyl)methacrylamide, 2,2,6,6-tetramethyl-4-piperidyl acrylate and methacrylate and 1,2,2,6,6-pentamethyl-4-piperidyl acrylate and methacrylate were prepared. Monomers, homopolymers and copolymers with dodecyl methacrylate (DMA) and octadecyl acrylate (ODA) were tested as stabilisers of the photo-oxidation of polypropylene film. The copolymers exhibited higher efficiency and lower extractability. The photostabilising efficiency increased in the order: homopolymer < monomer < copolymer with DMA < copolymer with ODA.     

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.     

Study of the residual linewidth of the level-anti-crossing signal in p-benzoquinone

The linewidth of the level-anti-crossing (LAC) signal in p-benzoquinone has been examined using optical detection of magnetic resonance between states in the avoided crossing region. The results are used to separate the homogeneous (due to hyperfine coupling) from the inhomogeneous (due to disorder) contribution to the LAC linewidth.     

Vibrational and electronic excitation in p-benzoquinone by electron impact

Vibrational and electronic excitation by electron impact in p-benzoquinone was studied using a trochoidal electron spectrometer. Two distinct patterns of vibrational excitation were observed. First, low quanta of a few selected vibrations are specifically excited at incident electron energies corresponding to shape resonances. Some resonances excite mainly the CO stretch, others the CH stretch vibration, and this selectivity is used in the discussion of the assignment of the resonances. A second pattern is an unspecific excitation of a quasi-continuum where no structure due to individual vibrational levels can be discerned. This feature peaks at threshold, large amounts of vibrational energy can be deposited in the molecule, and the excitation also proceeds via shape resonances. Electronic excitation spectra in the valence and Rydberg regions are also presented and discussed. A band observed at 4.37 eV with low residual energies has been tentatively assigned to the second π — π* triplet state ³B_3g.     

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.     

Effect on thermal and combustion behaviors of montmorillonite intercalation nickel compounds in polypropylene/IFR system

A new montmorillonite intercalation nickel compound (MINC) was devised and synthesized. MINC was modified by cetyl trimethyl ammonium bromide to obtain organic MINC (OMINC). The results of X‐ray diffraction show that the layer spacing was expanded by nickel compound and cetyl trimethyl ammonium bromide, indicating that OMINC was prepared successfully. OMINC was further incorporated into polypropylene (PP)/intumescent flame retardant (IFR) system for preparing PP/IFR/OMINC nanocomposites via melt blending. In thermogravimetric analysis, PP/IFR/OMINC nanocomposites exhibit an enhanced thermal behavior and residue amount. Vertical burning test (UL‐94) and limited oxygen index results show that PP/IFR/OMINC nanocomposites have excellent flame retardance, i.e. the limited oxygen index value at 29.5 and UL‐94 V0 level for PP/IFR/4 wt% OMINC nanocomposites. According to cone calorimeter testing, the addition of OMINC brings an efficient reduction of flammability parameters, such as peak heat release rate, total heat release, and smoke production rate. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

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.     

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.