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.     

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.     

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.     

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.     

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 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.     

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.     

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: 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.     

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.     

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.     

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.     

Thermal oxidation of clay-nanoreinforced polypropylene

The thermo-oxidation process at low temperatures for a montmorillonite-nanoreinforced polypropylene (PP) was studied. Experimental aging kinetic data at 100, 80 and 60 °C have been obtained and compared with a computational simulation in which a kinetic model based on the closed loop approach was used. As a result, it has been found that the montmorillonite role is not limited to a role of inert filler in the polymer matrix but induces a slight catalytic effect leading to induction period reduction. This effect has been well simulated by increasing initial hydroperoxyde concentration. The consequences of kinetic control by oxygen diffusion have also been investigated by using micro ATR-FTIR mapping to assess concentration profiles of the oxidation products across the sample thickness. It has been found that the oxidized layer thickness is close to 17 μm for the pure polypropylene whereas it is around 10 μm for the nanocomposite at 100 °C. These profile variations have been attributed to differences in oxygen diffusion coefficient values. Simulations based on the kinetic model including diffusion-reaction coupling describe these profiles well.     

Photo-stabilising action of metal chelates in polypropylene—Part II: Photolysis versus photo-sensitised oxidation under monochromatic irradiation

The photo-stabilising action of three metal chelates in unprocessed and processed polypropylene is examined using normal and second order derivative ultraviolet and infra-red spectroscopic techniques and hydroperoxide analysis. The effects of photolysis with 254 nm light versus photo-sensitised oxidation with 365 nm light are compared. For each exposure condition the rate of carbonyl formation in the polymer is compared with the rate of decomposition of the metal complex. On photolysis, carbonyl growth commences well before the complete destruction of the complexes and none offers protection to the polymer. In fact, all three chelates behave as photo-sensitisers, indicating that stabiliser photolysis products are photo-active. On photo-sensitised oxidation, while the initial hydroperoxide concentration appears to control the onset of carbonyl growth in the polymer, the rate of decomposition of the complexes shows no dependence on hydroperoxide concentration. Solution experiments indicate that there are no dark reactions with hydroperoxides apart from one of the nickel chelates (Cyasorb UV 1084) at high concentrations (～ 10^?2m) only. Essentially, the metal chelates operate by scavenging macroalkyl radical species (P·) and not alkoxy (PO·) and hydroxy radicals (·OH) during photo-oxidation. They also inhibit hydroperoxide formation during processing and one of the nickel chelates (UV 1084) gives products during the early stages of photo-oxidation which appear to operate as effective stabilisers.     

Thermal oxidation of ester-modified isotactic polypropylene

The relationship between the thermal oxidation of isotactic PP samples modified by esters miscible and immiscible with the polymer and the structure of these samples has been studied. An analysis of kinetic features of oxygen uptake, buildup of oxidation products, and changes in the mechanical properties and structural parameters of PP in the course of oxidation has shown that the effect of a modifier on the kinetics of thermal oxidation of the polymer depends on compatibility of an additive and a polymer matrix. The addition of ester that is partially miscible with PP accelerates oxidation. In the case of an immiscible ester, the effect is quite the reverse. This phenomenon is rationalized by the fact that the phase state of the system determines changes in the initial structure of the polymer matrix and, hence, manifestation of structural effects in the reaction kinetics and the participation of additives in chain reactions of cooxidation with PP.     

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.