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 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.     

Modelization of photocatalyzed oxidation of polyolefins. II. ZnO and TiO2 photocatalyzed decomposition of tert-butyl hydroperoxide and atactic polypropylene hydroperoxides

Direct evidence of the TiO₂ and ZnO photocatalytic decomposition of tert-butyl hydroperoxide and atactic polypropylene hydroperoxides in solution is reported. Molecular and macromolecular hydroperoxides behaved similarly. Untreated TiO₂ in the rutile form was a far more efficient photocatalyst than ZnO in solution; the photoactivities of both pigments were limited on preferential absorption sites. In the solid state no preferential reaction sites were observed. When added to preoxidized atactic polypropylene both pigments were photocatalysts of the formation of macromolecular hydroperoxides and of their decomposition. The photoreactivities of untreated TiO₂ and ZnO varied less than in solution.     

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.     

Polymer reactions. IV. Thermal decomposition of polypropylene hydroperoxides

The thermal decomposition of polypropylene hydroperoxide (PPH) consists of two consecutive reactions. The initial, faster reaction has rates up to 60 times that of the slower process. The former is largely suppressed by the addition of an excess of 2,6-di-tert-butyl-p-cresol. The course of reaction is the same in either solid state or in solution. The results are consistent with an intramolecular radical-induced mechanism for the initial reaction. This faster reaction consumes about 70–95% of the total hydroperoxides. The decomposition of PPH yields a maximum of about 1.8 radicals. Samples prepared from crystalline and amorphous polypropylenes have identical decomposition kinetics.     

Mechanisms of antioxidant action. Effect of the polymer medium on the antioxidant efficiency of the dithiolates

Peroxide decomposing antioxidants (e.g. nickel dithiophosphates and thiophosphoryl disulphides) control hydroperoxide formation during processing and on exposure to light. However, these additives are more efficient u.v. stabilizers in polypropylene (PP) than in low density polyethylene (LDPE). It is suggested that this difference results from the more rapid formation of hydroperoxides in the more oxidisable substrate under normal processing conditions. In contrast, nickel xanthates are completely destroyed in PP under the same processing conditions and the transformation products obtained in this case are less effective u.v. stabilizers than the original xanthates. Nickel dialkyl dithiophosphates stabilise both LDPE and PP very effectively, while nickel alkyl xanthates are much less effective u.v. stabilisers in both matrices. However, the difference between the efficiencies of the two dithiolates is much less in the case of LDPE. The nickel dithiophosphates and xanthates effectively synergise with the commercial u.v. absorber Cyasorb u.v. 531 (HOBP) but they show antagonism towards a typical chain breaking antioxidant, Irganox 1076, during u.v. exposure. They are however synergists under thermal oxidative conditions.     

Photo-stabilising action of metal chelates in polypropylene—Part I: Excited state quenching versus UV antioxidant action under polychromatic irradiation

The photo-stabilising action of three metal chelates—nickel (II) 2,2′ thiobis (4-tert-octylphenylato) n-butylamine and the nickel and calcium derivatives of bis (ethyl-3,5-di-tert-butyl-4-hydroxybenzyl) phosphonate, in polypropylene—is examined using normal and second order derivative ultraviolet, infra-red and phosphorescence spectroscopic techniques and hydroperoxide analysis. Whilst all three stabilisers quenched the phosphorescence emission of a powerful photo-sensitiser, benzophenone, there was no protective action during photo-sensitised oxidation of the polymer. In the case of anthraquinone, there was no quenching and no protection. Processing history plays a dominant rôle in controlling the photo-stabilising performance of the chelates. Each stabiliser operates differently, being dependent on its relative stability during processing and photo-oxidation and on the formation and destruction of polymeric and antioxidant hydroperoxides during processing. Metal chelates operate by inhibiting polymer hydroperoxide formation during processing and acting as ultraviolet stable chain terminators or by giving products during the early stages of photo-oxidation which are themselves effective stabilisers.     

Synthesis of poly(arylene ethylene) oligomeric hydroperoxides and their use as initiating agents of radical polymerization

The oxidation to hydroperoxide of poly(arylene ethylenes) (PAE) by oxygen carried out in solutions at 80–110°C. The effect of initiating additions and the nature of solvent relative to the content of hydroperoxide groups in oxidized PAE were investigated. The oxidation to hydroperoxides in PAE occurs at the methylene groups, and the synthesized hydroperoxides are secondary peroxides. The decomposition of PAE hydroperoxides in toluene and chlorobenzene at concentrations of 0.006–0.03 mole/l. for hydroperoxide in the presence and absence of N-phenyl-α-naphthylamine (PNA) was studied. The decomposition of one hydroperoxide has been studied in the presence of cobaltous and manganese resinates and of PNA in chlorobenzene at 30–50°C. The addition of PNA to a chlorobenzene solution of PAE hydroperoxide containing cobaltous or manganese resinate accelerates the hydroperoxide decomposition, reduces the activation energy, and changes the reaction order from the second-order to first-order. The synthesized hydroperoxides initiate the radical polymerization of styrene and methyl methacrylate. The initiating activity of one of the synthesized hydroperoxides of PAE for polymerization of styrene (60°C) in the presence and absence of activating addition of manganese resinate was also evaluated.     

Comportement des hydroperoxydes pendant la photo-oxydation d'un copolymère de styrène-butadiène

During the photo-oxidation at 254 nm of a styrene-butadiene copolymer, the quenching by hydroperoxides of singlet excited states of phenyl groups (and to a small extent of excimers) yield their photosensitized decomposition. The created radicals initiate hydroperoxide propagation reaction into the polymeric bulk. The natures of the various phases, through which this chain reaction develops, depend on the amount of available polymeric sites which can be easily oxidized (particularly allylic positions). Moreover, according to their behaviour, hydroperoxides have been placed in two classes: the “active” groups which are located at a stationary concentration in the vicinity of phenyl groups and the “inactive” groups which are outside this active sphere and remain stable after formation because they are not involved in the primary photophysical process.     

Polymer reactions. III. Structure of polypropylene hydroperoxide

Oxidation of polypropylene introduces hydroperoxide groups into the polymer. Infrared spectroscopy has determined that more than 90% of these groups are intramolecularly hydrogen bonded. The sequential lengths and sequence distributions of these neighboring hydroperoxides were estimated from the electronic spectra of the polyenes derived from the polypropylene hydroperoxide by two methods: (1) reduction, acetylation, and pyrolysis, and (2) reduction and dehydration. The results indicate that all the hydroperoxide groups are present in sequences of length two and greater. Intramolecular hydrogen abstraction during oxidation could account for the formation of these neighboring hydroperoxides.     

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.     

Investigations of stabilizing additives. I. A model system for studying radical scavenging activity in solution

In the current study an electron spin resonance model system was developed to compare the thermal stability and radical scavenging activity of stabilizers in solution. High-resolution spectra and the influence of molecular structure on radical stability provided a basis for the interpretation of spin concentration data in the model system. A correlation was established between the radical scavenging activity measured in the model system and actual behavior in irradiated polypropylene formulations measured by radiation-induced degradation of mechanical properties.     

Synthesis of allylic hydroperoxides and EPR spin-trapping studies on the formation of radicals in iron systems as potential initiators of the sensitizing pathway

Many terpenes used as fragrance compounds autoxidize when exposed to air, forming allylic hydroperoxides that have the potential to be skin contact allergens. To trigger the immunotoxicity process that characterizes contact allergy, these hydroperoxides are supposed to bind covalently to proteins in the skin via radical pathways. We investigated the formation of reactive radical intermediates from 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol and 2-hydroperoxylimonene, responsible for the sensitizing potential acquired by autoxidized linalool and limonene. Both compounds were synthesized through new short and reproducible synthetic pathways. The hydroperoxide decomposition catalyzed by Fe(II)/Fe(III) redox systems, playing a key role in degradating peroxides in vivo, was examined by spin-trapping-EPR spectroscopy. Alkoxyl and carbon-centered free radicals derived from the hydroperoxides were successfully trapped by the spin-trap 5,5-dimethyl-1-pyrroline N-oxide, whereas peroxyl radicals were characterized by spin-trapping studies with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide. Using liquid chromatography combined with mass spectrometry, we demonstrated the formation of adducts, via radical mechanisms induced by Fe(II)/Fe(III), between the hydroperoxides and N-acetylhistidine methyl ester, a model amino acid that is prone to radical reactions. Free radicals derived from these hydroperoxides can thus induce amino acid chemical modifications via radical mechanisms. The study of these mechanisms will help to understand the sensitizing potential of hydroperoxides.     

The Mechanism of the Decomposition of Peroxides and Hydroperoxides by Mineral Fillers

The interaction of fillers and pigments with free radical initiators has been studied. Clay minerals have a marked influence on both the rate and the mechanism of the decomposition of peroxides and hydroperoxides. Kaolinite is a particularly effective catalyst and causes rapid decomposition even at room temperature. The reaction of cumene hydroperoxide with kaolinite is first-order in peroxide and the rate constant is proportional to the ratio of clay to hydroperoxide. From a study of the products of the reaction and the influence of solvent on the decomposition, a mechanism involving an intramolecular rearrangement or closely associated ion pairs has been proposed. The application of these results to polymer filler composites is discussed.     

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.     

Energy transfer processes involving ultraviolet stabilizers. Quenching of singlet oxygen

The role of electronically excited singlet (¹Δ_g) oxygen molecules in the photooxidation of polymers has received increased attention in recent years. Little information regarding the interaction of ultraviolet stabilizers with singlet oxygen is known, however. In this study, singlet oxygen was produced by a microwave discharge in a flow system and rate constants were obtained for quenching by ultraviolet stabilizers. It was found that some nickel chelate stabilizers are effective quenchers of singlet oxygen and their quenching behaviors can be correlated with ultraviolet stabilization effectiveness in thin polypropylene and polyethylene films. The best oxygen quenchers of those examined are nickel chelates with sulfur donor ligands.     

Note on EPR study of decomposition of polymeric hydroperoxide with polyamines in styrene

Decomposition of polypropylene hydroperoxide in styrene with triethylenetetramine, diethylenetriamine and ethylenediamine at 23–80 was studied by electron paramagnetic resonance measurements. The hfs parameters for isotropic EPR spectra of aminoalkylnitroxy radicals from these polyamines were determined. The supposed complex formation of polymeric hydroperoxide with polyamine by two hydrogen bondings facilitates its decomposition with styrene.     

Heterogeneous initiators for the synthesis of polymer nanocomposites

Nanocomposites are obtained by the radical polymerization of styrene and methyl methacrylate on the surface of a dispersed filler containing chemisorbed compounds of quaternary ammonium, which catalyze decomposition of cumene hydroperoxide. The heterogeneous catalysts of hydroperoxide decomposition are obtained via the adsorption of cetyltrimethyl ammonium bromide and acetylcholine chloride on sodium montmorillonite, cellulose, and chitosan. The highest rate of the polymerization of both monomers is provided by the cellulose–cetyltrimethyl ammonium bromide catalyst. For a more hydrophilic methyl methacrylate, the rate of radical initiation is significantly lower at the same concentrations of the catalyst and hydroperoxide compared with hydrophobic styrene; however, the rate of polymerization is higher than for styrene because of a higher activity of methyl methacrylate in chain-propagation reactions. Relatively high rates of radical generation upon contact of cellulose–cetyltrimethyl ammonium bromide and cellulose–acetylcholine with hydroperoxides open the possibility to create cellulose-based disinfecting and medical materials.     

“Close loop” mechanistic schemes for hydrocarbon polymer oxidation

Mechanistic schemes of radical oxidation of hydrocarbon polymers in which initiation is only due to unimolecular or bimolecular hydroperoxide decomposition have been studied. The results of their kinetic analysis have been compared with literature data relative to the thermal oxidation of polypropylene in solid state (60-160°C). These data are in remarkably good agreement with the “unimolecular” scheme whose main characteristics are: (1) the quasi-independence of the kinetic behavior with initial conditions (for low initial content of thermolabile structures), and (2) the fact that an arbitrarily defined induction period depends only on the rate constant of unimolecular hydroperoxide decomposition. © 1995 John Wiley & Sons, Inc.     

The determination of small amounts of organic hydroperoxides in the presence of hindered amine light stabilizers and their nitroxyls

A method for the quantitative determination of hydroperoxide (ROOH) in the presence of hindered amine light stabilizers (HALS) and/or their nitroxyl free radical derivatives has been elaborated. The method is based on the quantitative reduction of hydroperoxides by triphenyl phosphine. The resulting compounds (alcohols) are then determined by GLC using the internal standard technique. The method has been tested on the hydroperoxides derived from 2,4-dimethylpentane. Its sensitivity and reproducibility appear to be comparable with other methods for ROOH determination but, unlike the latter, it has the advantage that its results are not influenced by the presence of HALS and/or their nitroxyl radical derivatives in the analyzed medium.