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.     

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.     

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.     

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.     

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 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.     

Photostabilizing activity of sterically hindered piperidines—i: Peroxide decomposing and hydrogen donating ability

Sterically hindered piperidine (I) and its corresponding nitroxyl have been examined for their ability to decompose hydroperoxides, donate hydrogen and influence the rate of photolysis of t-butyl-hydroperoxide in solution. None of these effects are significant. The formation of an adduct between hydroperoxide and I has been observed.     

COPOLYMERIZATION OF PROPYLENE WITH HINDERED PIPERIDINE MONOMER OVER A HIGH ACTIVITY SUPPORTED ZIEGLER-NATTA CATALYST

Copolymerization of propylene and hindered piperidine monomers was carried out overa high activity supported Ziegler-Natta catalyst, using Al(C_2H_5)_3 as cocatalyst. Factorswhich affect the copolymerization were studied. The copolymers exhibited high light sta-bility without adding extra light stabilizers. A self-stabilized polypropylene was prepared.     

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.     

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.     

Photofragmentation of Dibenzyl Ketone in the Presence of Tetramethylpiperidine Derivatives. Studies on light stabilizer mechanisms [1]

The interaction of selected tetramethylpiperidine derivatives with radicals arising from the Norrish-type I cleavage of dibenzyl ketone under oxygen was studied. Product analyses and kinetic studies showed that the investigated sterically hindered piperidine derivatives have a pronounced effect on both the nature and distribution of the products of photolysis of dibenzyl ketone in the presence of oxygen. Observations indicated that the phenylperacetoxyl radical is formed as an intermediate during irradiation and that it interacts with the additives used. Possible mechanisms of the reactions studied are discussed. The observation that oxidation of an isolated double bond by the radicals formed in dibenzyl ketone photolysis under oxygen is strongly inhibited in the presence of the studied sterically hindered amines is discussed in the light of the results presented. The findings are considered in relation to the problem of polymer stabilization.     

Kinetics and Thermodynamics of Hydrogen Atom Exchange Reactions in Sterically Hindered Hydroxylamine-Nitroxyl Radical Systems

Rate constants and activation energies were determined for the forward and reverse hydrogen exchange reactions in systems involving a sterically hindered hydroxylamine and a nitroxyl radical of dihydroquinoline, tetrahydroquinoline, diphenylamine, imidazoline, pyrrolidine, or piperidine. The equilibrium constants and heats of these reactions were determined, and the dissociation energies of the NO-H bonds in the hydroxylamines were estimated.     

丙烯-马来酸哌啶酯共聚物的合成与表征

<正> 以2,2,6,6-四甲基哌啶为母体的衍生物作为受阻胺光稳定剂,其效率约为镍-螯合物光稳定剂的2—6倍,已引起人们的注意,但是小分子受阻胺易于挥发,从而使其光稳定作用较差。为了克服上述缺点,使受阻胺稳定剂高分子化是近年来展的趋势,本文合成了含有“丙烯”单元的高分子受阻胺光稳定剂——丙烯-马来酸哌啶酯共聚物,力求改进它与丙烯的相容性,提高光稳定效率,本文还对共聚物进行表征。     

Desorption electrospray ionisation mass spectrometry reveals in situ modification of a hindered amine light stabiliser resulting from direct N-OR bond cleavage

Detection and characterisation of structural modifications of a hindered amine light stabiliser (HALS) directly from a polyester-based coil coating have been achieved by desorption electrospray ionisation mass spectrometry (DESI-MS) for the first time. In situ detection is made possible by exposing the coating to an acetone vapour atmosphere prior to analysis. This is a gentle and non-destructive treatment that allows diffusion of analyte to the surface without promoting lateral migration. Using this approach a major structural modification of the HALS TINUVIN?123 (bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate) was discovered where one N-ether piperidine moiety (N-OC(8)H(17)) is converted to a secondary piperidine (N-H). With the use of 2-dimensional DESI-MS imaging the modification was observed to arise during high curing temperatures (ca. 260 °C) and under simulated physiological conditions (80 °C, full solar spectrum). It is proposed that the secondary piperidine derivative is a result of a highly reactive aminyl radical intermediate produced by N-O homolytic bond cleavage. The nature of the bond cleavage is also suggested by ESR spin-trapping experiments employing α-phenyl-N-tert-butyl nitrone (PBN) in toluene at 80 °C. The presence of a secondary piperidine derivative in situ and the implication of N-OR competing with NO-R bond cleavage suggest an alternative pathway for generation of the nitroxyl radical-an essential requirement in anti-oxidant activity that has not previously been described for the N-ether sub-class of HALS.     

Synergism of hindered amine light stabilizers and UV-absorbers

The synergism of hindered amine light stabilizers (HALS) and UV-absorbers is very important in the practical photo-stabilization of polymers. As very little is known, however, the relationship between the composition of the mixture of the two kinds of light stabilizer and its photo-stabilizing efficiency was investigated in four common polymers. A high level of synergism was observed in polypropylene, high density polyethylene and ABS resin. A moderate synergistic effect was obtained in polystyrene. The maximum efficiency was observed at mixing ratios of hindered amine light stabilizer to UV-absorber of about 75:25 in polypropylene and high density polyethylene, 90:10 in ABS resin and 80:20 in crystal polystyrene.The synergism is due to different stabilizing mechanisms of hindered amine light stabilizer and UV-absorber, and is explained by the diffusion of very effective hindered amine light stabilizer from the polymer bulk, which is protected by the UV-absorber, towards the surface layer where photo-oxidation proceeds.     

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.     

Rayleigh relaxation of pyrazine,pyridine and piperidine molecules adsorbed on the roughened Ag electrode by light beating technique

The light beating technique (intensity correlation algorithm) was employed to analyze the Rayleigh scattering from the roughened Ag electrode in very dilute (10^?4 M) pyrazine, pyridine and piperidine aqueous solutions containing KC1 (0.1 M). The relaxation time is longer when the applied voltages are between ?0.4 V and ?0.8 V (vs. SCE) where the Raman effect also shows greater surface enhancement. Also observed was that for the piperidine case the relaxation time reaches its maximum at the more negative applied voltage. The origin of the relaxation is attributed mainly to the desorption process of the pyrazine, pyridine and piperidine molecules off the roughened Ag electrode. An electrostatic model was also proposed for the interpretation of these experimental observations.     

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

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 (PO₂^?). Strong synergism is also observed with TiO₂ (rutile).     

Reactivity of 2-fluoro-5-nitrothiophene towards sodium thiophenoxide and piperidine. Comparison with other 2-halogeno-5-nitrothiophenes

2-Fluoro-5-nitrothiophene reacts with sodium thiophenoxide and piperidine much faster than other 2-halogeno-5-nitrothiophenes. In methanol the reactions with both nucleophiles follow overall second order kinetics, while in benzene the observed second order rate constants of the reaction with piperidine show a linear dependence by the piperidine concentration. Such a dependence, which is mild for the chloro, bromo and iodo derivative, becomes strong for the fluoro compound. Moreover, the reaction of 2-fluoro-5-nitrothiophene with [1-²H]piperidine shows the absence of a primary isotope effect. The results are interpreted within the framework of the two-stage, intermediate-complex mechanism, the first stage (attack of the nucleophile on the substrate) being rate determing for the reactions of 2-fluoro-, -chloro-, -bromo- and -iodo-5-nitrothiophene with thiophenoxide in methanol and of 2-chloro-, -bromo- and -iodo-5-nitrothiophene with piperidine in benzene. In the case of the reaction of 2-fluoro-5-nitrothiophene with piperidine in benzene the data are in agreement with a mechanism in which the rate determining step is the decomposition of the tetrahedral intermediate into products. The intervention of a second amine molecule in the transition state of the rate determining step can be rationalized in terms of bifunctional catalysis. A comparison of reactivity of thiophenoxide and piperidine towards 2-halogeno-5-nitrothiophenes (Hal = F, Cl, Br, I) indicates a greater sensitivity of the reaction with piperidine than that with thiophenoxide to the change of the leaving group.     

Stabilization of low density polyethylene with sterically hindered derivatives of 4-hydroxybenzoic acid

In comparison with other commercial light stabilizers, sterically hindered 4-hydroxybenzoates are found to possess only a weak power, if they are used for low density polyethylene (LDPE). It is known that these benzoates exhibit significant synergistic effects together with 2-hydroxybenzophenones and hindered amine light stabilizers in light stabilization of high-density and linear low-density polyethylene. The light stabilizing efficiencies of the above mentioned stabilizers and mixtures of them in different weight ratios are determined by weathering in a Xenotest 150 unit. Only small synergistic effects of about 15 −30% are found for light stabilizing LDPE. The effects can be explained by the stabilizing efficiency of sterically hindered 4-hydroxybenzoates during processing. Combining of light stabilizing structure components like 2-hydroxy-benzophenone resp. 2, 2, 6, 6-tetramethylpiperidine with sterically hindered 4-hydroxybenzoates in one stabilizer molecule only, from the first mentioned combination, results a powerful light stabilizer. Its efficiency is the sum of the powers of the single structure components.