Some aspects of polyethylene photooxidation

The hydroperoxides produced by thermal oxidation of LDPE films were used to study their photolysis. Product analysis, kinetics of hydroperoxide decomposition and product formation as well as experiments with model compounds point to new mechanisms of hydroperoxide photolysis. Intermolecular as well as intramolecular decomposition mechanisms are proposed. In polyethylene, these reactions are essentially non-initiating. In addition, it is confirmed that ketones such as those formed by oxidation of polyethylene do not have a significant initiating effect. Reactions of excited charge-transfer complexes polyethylene-oxygen are proposed to account for initiation of photo-oxidation. One of these reactions yields trans-vinylene groups and hydrogen peroxide whose direct decomposition or subsequent photolysis will generate hydroxyl radicals. It is found that this reaction is quenched very efficiently by small amounts of HALS (Hindered Amine Light Stabilizers) and by amines in general. It is postulated that quenching is due to energy transfer from the charge-tranfer complex polymer-oxygen to a charge-transfer complex HALS-oxygen or amine-oxygen. The data available so far support such a mechanism.     

Antagonism between hindered amine light stabilizers and sulfur-containing compounds

Antagonism between hindered amine light stabilizers (HALS) and sulfur-containing compounds has been examined using esr and uv spectroscopic techniques. Evidence is presented to show that formation of nitroxyl from HALS and/or its disappearance is not affected by a sulfur-containing compound itself, but considerably by its acidic transformation product. From the results obtained in this study, a new mechanism of antagonism is proposed.     

The effect of hindered amine light stabilizers on the photooxidative stability of high-density polyethylene

The photoprotective effectiveness of various polymeric and nonpolymeric hindered amine light stabilizers (HALS) was determined by exposing samples of high-density polyethylene (HDPE) containing these additives to ultraviolet (UV) light and measuring the resultant oxygen uptake characteristics. Values of the initial quantum yield for oxygen uptake calculated for these formulations indicate that the higher molecular weight HALS compounds are less effective photostabilizers than the nonpolymeric HALS, and this is partly attributable to their decreased mobility in the polymer matrix. It was further found that the addition of an ultraviolet absorber (UVA) to a formulation containing a polymeric HALS compound enhances its photostability, although this phenomenon may be partly due to synergism between the UVA and the antioxidant, the latter having been added as part of the base stabilization. The antagonism which exists between certain sulfur-containing antioxidants and HALS compounds was also investigated and it was found that the lower molecular weight sulfur-containing antioxidants exhibit the greatest degree of antagonism. The results confirm that the mobility in the polymer matrix of the stabilizer system can serve as an explanation of its effectiveness. The article provides evidence that the technique of oxygen uptake monitoring is a sensitive and rapid method of assessment of polymer photostability in the presence of stabilizer systems.     

Aspects of the thermal and photostabilisation of high styrene-butadiene copolymer (SBC)

The thermal and photo-oxidative stabilisation of high styrene-butadiene copolymer (SBC) with high styrene content (K-Resin) has been studied using a variety of analytical and spectroscopic methods including yellowness, luminescence and FT-IR spectroscopy coupled with hydroperoxide analysis in order to understand the nature and effectiveness of the processes involved. The next stage of the program was to evaluate the effects of various chemical/solvent treatments on the role of metal ions/residual catalysts and hydroperoxides in the thermal and photostabilisation of SBS as well as combinations of phenolic antioxidants and phosphites/phosphonites. Other additives, such as HALS and a metal deactivator, were also added to the combinations of phenolic and phosphite antioxidants in order to study their behaviour and efficiency. The chemical treatments appeared to stabilise SBS against thermal oxidation to a greater or lesser extent. Phosphoric acid treatment via reflux and zinc dithiocarbamate treatments showed better performances than the rest of the treatments, the latter was particularly effective at inhibiting the discolouration. During photo-oxidation, on the other hand, chemical treatments involving phosphoric acid and pre-thermal effects showed the importance of catalyst effects. The addition of phenolic antioxidants, phosphites/phosphonites, metal deactivator and HALS was found to stabilise the SBS against thermal and photo-oxidation. In thermal oxidation, the combination of Irganox^® 1010/Irgafos^® 168 was found to effectively stabilise the polymer when the finalisation of the polymerisation was with adipic acid. When the same antioxidants were used, but with polymer finalised with BHT, strong yellowing was observed and a higher amount of hydroperoxides and oxidation products. Increasing the amount of antioxidants did not increase the stabilisation efficiency. The stabilisation efficiency of Irganox^® 1010 combined with Alkanox^® P-24 was found to be more effective than when it was combined with Irgafos^® 168. The formulations containing Irgafos^® 168/Irganox^® 1010 and Irgafos^® 168/Irganos^® 1330 were more effective in colour protection and retarding the formation of oxidation products than the combinations of Irgafos^® 168/Irganox^® 3114 and Irgafos^® 168/Lowinox^® 1790. The effect of the addition of HALS, such as Tinuvin^® 770, Tinuvin^® 622 and Chimassorb^® 944, and a metal deactivator, such as Irganox^® MD 1024, to the combination of Irgafos^® 168/Irganox^® 1010 was found to be antagonistic. In photo-oxidation, a combination of Irganox^® 1010/Irgafos^® 168 protected the polymer efficiently, when the polymerisation of the polymer was finalised with adipic acid. When the polymerisation was finalised with BHT, a higher amount of hydroperoxides and oxidation products was found. An increase in the amount of antioxidants did not enhance the stability of the polymer. The addition of Alkanox^® P-24 exhibited an opposite effect to that seen in thermal oxidation, as the stabilisation efficiency was less effective than with Irgafos^® 168. The formulation containing Irgafos^® 168/Irganox^® 1010 was found to be the most efficient compared with the other phenolic antioxidants. The addition of Tinuvin^® 770 to the formulation Irgafos^® 168/Irganox^® 1010 was found to have a synergistic effect. The addition of polymeric HALS or Irganox^® MD 1024, a metal deactivator, had an antagonistic effect on the stabilisation of the polymer. Disruption of the excimer sites in the styrenic phase also correlated with stabilisation effects.     

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.     

The oxidation of hindered amine light stabilizers to nitroxy radicals in solution and in polymers

Cyclohexane and three polyolefins (PE, PP and EPM) have been oxidized by γ-irradiation, and the concentration of nitroxy radicals derived from a typical HALS added either before or after irradiation has been studied by ESR spectroscopy. The results indicate that HALS is rapidly oxidized by peroxy radicals under irradiation and that afterwards it can be oxidized more slowly by hydroperoxides. This last reaction is, however, very limited.     

The influence of an N-alkoxy HALS on the decomposition of a brominated fire retardant in polypropylene

This work studies the effect of an N-alkoxy HALS on the thermal decomposition of a brominated phosphate ester fire retardant. We have monitored the fate of the fire retardant in the presence of the N-alkoxy HALS during thermal decomposition using TGA, FTIR, TD-GC-MS, NMR and ESR methods. We have shown that the two additives interact in the condensed phase at temperatures below the onset of polymer decomposition to produce 1,3-dibromo-2,2-bis(bromomethyl)-propane as the main decomposition product. It is believed that this molecule is the key to the fire retardant action of the brominated phosphate ester because it readily decomposes to the effective gas phase flame inhibiting agent, HBr.     

聚乙烯光引发交联过程中的表面光氧化和光稳定

通过测定凝胶含量并利用红外-光声光谱和光电子能谱对光交联聚乙烯表面氧化程度和氧化产物进行了研究.结果表明,聚乙烯光交联过程中随着光照时间的增加,表面光氧化加剧,氧化产物主要是氢过氧化物和含羰基的化合物.考察了预辐照和添加受阻胺型光稳定剂对聚乙烯光交联过程的影响,发现这两种方法都能有效地降低光交联聚乙烯(XLPE)的表面氧化,但有些光稳定剂会降低XLPE的交联度.     

Preparation of degradable polypropylene by an addition of poly(ethylene oxide) microcapsule containing TiO2

In order to prepare a novel photo-degradable polypropylene (PP), an addition of poly(ethylene oxide) (PEO) microcapsule containing TiO₂ to PP was performed. Adsorbed H₂O in the PEO phase and the TiO₂ photocatalytically reacted, and a hydroxyl radical (OH), which initiated the PEO degradation, was produced. The degraded PEO produced an acid and an aldehyde, which were able to facilitate PP degradation. The addition of the PEO/TiO₂ microcapsule brought about the facilitative effect of the PP degradation. In addition, an addition of a hindered amine light stabilizer (HALS) had a potential to suppress the PP degradation initiated by the microcapsule. The suppression effect was rising by the simultaneous addition of a phenolic antioxidant in the early phase of the PP degradation. However, the simultaneous addition showed an antagonism after 4 h degradation. This behavior suggested that the HALS also worked as a neutralizer of the produced acid.     

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.     

Catalysis of radical reactions in mixed micelles of surfactants with hydroperoxides

The peculiarities of the catalytic action of cationic surfactants (CSurf) in combination with hydroperoxides on the generation of radicals and the influence of various factors on this process (transition metal compounds, oxygen, and external magnetic field) were considered. In the oxidized hydrocarbons (RH), hydroperoxides (ROOH), which are the primary amphiphilic products of oxidation, form mixed micelles {mROOH…nCSurf} with CSurf, in which fast decomposition of ROOH into radicals occurs and other polar components (metal compounds, inhibitors, etc.) can concentrate, which significantly affects the rate and mechanism of oxidation. The cationic surfactants immobilized on a solid support retain the ability to catalyze the decomposition of hydroperoxides, forming radicals, and to initiate radical oxidation and polymerization. It was found that acetylcholine, which is the most important neurotransmitter that plays an important role in the neuromuscular and cognitive activity of living beings, like cationic surfactants, catalyzes the radical decomposition of hydroperoxides in organic media, and the yield of radicals in this process decreases in a magnetic field and in the presence of oxygen.     

Oxidation of ultra high molecular weight polyethylene (UHMWPE) part 2: Critical examination of the total luminescence intensity (TLI) method for determining hydroperoxides

The validity of the total luminescence intensity (TLI) method for determining the amount of hydroperoxides in a UHMWPE sample has been assessed. Measurements of hydroperoxides with FTIR before and after a TLI run showed that only about 50% of the hydroperoxides were decomposed at 150 °C. It was also found that this value was not constant with ageing time, which means that the TLI value could not be representative of the total number of hydroperoxides in a sample as a function of ageing time. Thermoluminescence was also found to complicate the measurements and could, if care was not taken, give TLI values that were much too high. In addition it was found that the TLI value is actually connected with the build-up of carbonyls rather than the build-up of hydroperoxides. This last finding is consistent with part one of this study, where it was reported that CL from oxidising UHMWPE is a type of activated CL, where carbonyls are the activating species. From all of these results it is concluded that TLI is not a suitable method for determining hydroperoxides in UHMWPE.     

Mechanisms of antioxidant action: Antioxidant-active products formed from the dialkyl thiodipropionate esters

Some products of the decomposition of dimethylsulphinyldipropionate (DMSD) have been identified and have been found effective as antioxidants. The only products able to destroy hydroperoxides as rapidly as DMSD are sulphur dioxide and methyl-β-sulphinopropionate (MSP). The latter undergoes pyrolysis to give sulphur dioxide at the elevated temperatures during processing of polymers; at lower temperatures, it appears to give rise to an equally effective acid catalyst (probably SO₂ or H₂SO₄) by disproportionation and further oxidation by hydroperoxides. MSP and its further oxidation product, methyl-β-sulphopropionate (MSOP), are auto-synergistic, showing both chain-breaking and peroxide decomposing activity. There is no evidence that MSOP is involved in the inhibition processes.     

Natural weathering test for films of various formulations of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE)

Natural (outdoor) weathering test was performed to investigate the UV stability of thin films (0.06 mm) of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE). The PE films were prepared from various formulations of LLDPE and LDPE resins. Some of these films contained a single high molecular mass HALS only, along with a primary antioxidant (i.e. Irganox 1010) and a secondary antioxidant (i.e. Irgafos 168 or Alkanox TNPP), while others contained HALS and UVA (i.e. Chimassorb 81 or Tinuvin P or Tinuvin 326) along with these antioxidants. The HALS used was either an oligomeric or a synergistic mixture of a high molecular mass (HMM) hindered amine stabilizer and co-additives. The UV stability was investigated by exposing the prepared films at 45° towards south in the direct sunshine up to 365 days. Fifty percent of tensile strength retention was determined for all these exposed films and it was found that the films containing a single HALS gained improved UV stability by about two to 12 fold over the pure films. On the other hand, films that contained a combination of HALS and UVA obtained further improved UV stability over the films containing a single HALS (both have antioxidants). Films containing a single HALS reached 50% TS retention within 205 days, whereas, films containing a combination of HALS and UVA reached 50% TS retention within 590 days, which is about three times further improvement in UV stability.     

Surface and bulk modification of polyolefins by functional aryl nitrenes as highly reactive intermediates

Aromatic azides with hindered amine light stabilizer (HALS) residues or hydrophilic groups such as glucose, sucrose and dextrine residues were synthesized and used for surface modification of polyolefins. By UV‐irradiation nitrenes were formed, which are able to react with polyolefin surfaces. By photochemical immobilization of the carbohydrates hydrophilicity of PE and PP was strongly increased (surface tensions > 44mN/m). Light stability of PP surfaces modified with HALS azides was comparable with PP, stabilised with Tinuvin 770. Bulk modification of ethylene‐propylene and ethylene‐octene copolymers was achieved by grafting nitrenes formed by thermal decomposition of azido benzoic acid. In a circulating air oven up to 1.55 wt% amino benzoic acid residues could be bonded covalently to ethylene‐propylene‐copolymers, less than half of it to ethylene‐octene‐copolymers. Reactive extrusion resulted in grafting yields of more than 50% for both types of copolymers.     

Determination of a polymeric hindered amine light stabilizer in polypropylene formulated with magnesium hydroxide flame retardant by reactive thermal desorption-gas chromatography.

A polymeric hindered amine light stabilizer (HALS), Tinuvin 622 (MW [symbol: see text] 4000), in PP materials formulated with a magnesium hydroxide flame retardant was determined by reactive thermal desorption (RTD) gas chromatography (GC). Two kinds of the HALS components that were formed through the RTD in the presence of tetramethylammonium hydroxide [(CH3)4NOH, TMAH] were clearly observed in the chromatograms of the PP samples, with negligible interference from the other additives and the PP substrate. Here, the coexisting flame retardant was proved to affect significantly the RTD process of the occluded HALS. As a result, the recovery of the HALS components in the RTD-GC chromatograms of the PP samples increased with increase in the content of the flame retardant. This enhancement of the HALS recovery is attributed mainly to the preferential exposure of the HALS on the surface of the ground PP sample through the interaction between the polymeric HALS and the flame retardant in the molten PP during kneading. In spite of such a considerable action of the flame retardant, the observed intensities of the characteristic peaks of HALS by RTD-GC showed a good linear relationship with the HALS content in the PP samples with constant content of the flame retardant (50 phr); this relationship could be used as the calibration line for the determination of the polymeric HALS in the PP materials containing the flame retardant.     

Relationship between Phenolic Compounds,Antioxidant Properties,and the Allergenic Protein Mal d 1 in Different Selenium-Biofortified Apple Cultivars (Malus domestica)

Notable parts of the population in Europe suffer from allergies towards apples. To address this health problem, the analysis of the interactions of relevant allergens with other substances such as phenolic compounds is of particular importance. The aim of this study was to evaluate the correlations between the total phenolic content (TPC), polyphenol oxidase (PPO) activity, antioxidant activity (AOA), and the phenolic compound profile and the content of the allergenic protein Mal d 1 in six apple cultivars. It was found that the PPO activity and the content of individual phenolic compounds had an influence on the Mal d 1 content. With regard to the important constituents, flavan-3-ols and phenolic acids, it was found that apples with a higher content of chlorogenic acid and a low content of procyanidin trimers and/or epicatechin had a lower allergenic potential. This is probably based on the reaction of phenolic compounds (when oxidized by the endogenous PPO) with proteins, thus being able to change the conformation of the (allergenic) proteins, which further corresponds to a loss of antibody recognition. When apples were additionally biofortified with selenium, the composition of the apples, with regard to TPC, phenolic profile, AOA, and PPO, was significantly affected. Consequently, this innovative agronomic practice seems to be promising for reducing the allergenic potential of apples.     

Advances in the determination of hindered amine light stabilizers – A review

Within this paper we discuss analytical strategies for the characterization and quantitation of hindered amine light stabilizers (HALS) an important sub-group of polymer additives. For the determination of monomeric HALS a range of mature and reliable techniques exists, allowing their determination in polymer extracts. If qualitative or semi-quantitative information suffices, certain techniques are capable of sampling directly from the polymer surface with limited or no sample preparation. Different strategies for the determination of complex oligomeric HALS in extracts from polymer samples are discussed. Here, approaches providing only a sum parameter including all HALS oligomers have been distinguished from more sophisticated technologies allowing the determination of single oligomers, their degradation and by-products. Particularly, the latter issue is facing increased interest as it provides important information for polymers aging studies. A tabulated overview provides comprehensive information on different analytical techniques suitable for HALS determination.     

Physico-chemical aspects of polyethylene processing in an open mixer: Part 27: Formal kinetics of aldehyde and carboxylic acid formation in the initial stages

There are many reactions susceptible to yield aldehydes and acids in polyethylene melts. It is β-scission of the alkoxy radicals formed on bimolecular hydroperoxide decomposition that is expected to be one of the main sources of the aldehydes that are formed at increasing rates in the early stages of polyethylene processing. Acid-catalyzed decomposition of allylic hydroperoxides is another source of substantial amounts of aldehydes. Formation and decomposition of α,γ- and α,β-di-hydroperoxides should yield acids. The activation energy estimated for these different processes is very large (about 57 kcal/mol) so that their contribution could be significant in the high temperature range only. This is different for the reaction of aldehydes with hydroperoxides to yield peroxy-hemiacetals. These intermediates can be expected mainly in the low temperature range where hydroperoxides are accumulating. Decomposition of the peroxy-hemiacetals gives acids as one of the main products. Free-radical induced oxidation of aldehydes is likely to yield peracids as far as oxygen addition is competitive with decarbonylation. The main problem is the transformation of the peracids into acids. The reaction with double bonds is expected to yield significantly more acids than thermal decomposition of peracids. If the last occurs, it will be followed mainly by decarboxylation. The overall activation energy for both processes of acid formation is negative (−18 to −20 kcal/mol). It is some combination of the various mechanisms examined that might account for the experimental activation energy for acid formation in the initial stages that is close to 18 kcal/mol.     

Quantitative analysis of an oligomeric hindered amine light stabilizer in polypropylene by matrix-assisted laser desorption/ionization mass spectrometry using a solid sampling technique

A small amount of an oligomeric hindered amine light stabilizer (HALS) (Adekastab LA-68LD) in polypropylene (PP) materials was directly determined by solid sampling matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using an internal standard method. First the matrix reagent (dithranol), 20 mg, and the empirically selected internal standard, angiotensin I (MW = 1296.5), 5 microg, were premixed in the solid state. The matrix mixture was then co-ground with the PP sample containing the HALS in liquid nitrogen using a freezer mill. The powdered sample mixture was spotted on the sample plate, suspended in ion-exchanged water, dried to adhere on the plate, and subjected to MALDI-MS. Three series of the HALS components accompanied by the oxidized species were clearly observed as their molecular ions (M*+)) along with that of the internal standard in the mass spectra. A fairly good linear relationship (R2 = 0.9991) with a relative standard deviation of ca. 11% was observed between the relative peak intensities of the HALS components and the HALS contents ranging from 0.1-2.5 wt%, which could be used as the calibration line to determine the HALS content in PP composites directly by MALDI-MS. The UV-exposed PP composite samples were evaluated by this method to interpret the photostabilizing action of HALS in the PP materials based on the observed change in the relative abundances of the original and oxidized HALS components as a function of UV-exposure time.