Photo-oxidation Of Polyethylene Research Articles

Limits of UV-light acceleration on the photooxidation of low-density polyethylene

Lifetime prediction of polymers requires accelerated ageing based on an increase in UV light intensity and temperature. However, the representativeness of artificial ageing is still an issue. The relevance of highly accelerated UV-induced weathering raises the question of reciprocity failure depending on the polymer. The influences of irradiance and temperature on the photooxidation kinetics of polyethylene were studied to determine the extent to which increasing light intensity leads to accelerated photooxidation of polyethylene. Polyethylene films were subjected to accelerated photooxidative experiments at various UV light intensities and different temperatures, and the amounts of carbonylated photoproducts were monitored by IR spectroscopy. The results show reciprocity failure for polyethylene photooxidation and the limits of acceleration by UV light.

Use of fluorescent probes in supporting functional group analysis resulting from polymer ageing

Based on the determination of the solvatochromic parameters of a series of polymers with various functional groups containing oxygen atoms, a method was proposed which allowed for discriminating the oxidation products formed by exposure of polymers to photo- and/or thermo-ageing. The polymers used to determine the solvatochromic parameters contained polar groups such as chain-pendant ester (EVA, PMMA, PVAc), chain ester (PLA), alcohol (EVOH, PVA), chain-pendant acid (PAA), ketone (PK), ether (PEO). The solvatochromic parameters π∗, α and β were determined following procedures reported in the literature, and it was shown that the most apolar polymer PE, displays the lowest π∗ value; the presence of acetate groups in PE makes the π∗ value increase. Polyesters (PLA, PMMA, PVAc) have similar π∗ values. Then, alcohols and ether groups (EVOH, PVA, PEO) have higher π∗ values. Finally, the highest π∗ values are obtained for PAA and PK. Concerning the β value, the highest value was obtained for (PE, EVOH, PEO), which have the highest capacity to accept a proton, and are close to polyethylene in terms of chemical structure. On the contrary, the polymers with the lowest β value are the ones which have acid/ester/ketone functions (PK, PAA, PLA), reflecting their lowest capacity to accept a proton. This method was successfully applied to discriminate the oxidation products formed in thermo- and photo-oxidation of polyethylene which are known to be slightly different due to the Norrish reactions that do not occur in thermooxidation.

Photooxidative Destruction of Composites Based on Polyethylene and Montmorillonites Modified with Transition-Metal Salts

Metal-modified montmorillonites containing Ni2+, Co2+, Fe3+, Mn2+, and Cu2+ions are synthesized by interacting layered aluminosilicates Na+-montmorillonite and Cloisite 25A with transition-metal salts. The organomodified Cloisite 25A is saturated with ions of heavy metals Ni, Co, and Fe, retaining the organic modifier. The composition and structure of metal-modified aluminosilicates are studied by X-ray powder diffraction, X-ray fluorescence analysis, and FTIR spectroscopy. Montmorillonites of different degrees of intercalation are obtained depending on the particular transition-metal ion and silicate type. It is shown that metal-modified aluminosilicates being involved in the composition of LDPE-based composites considerably increase the rate of photooxidation of the polyethylene matrix. FTIR studies of the initial and irradiated films suggest that the photooxidation of polyethylene is accompanied by the formation of diverse oxygen-containing and unsaturated groups. According to the calculated carbonyl and vinyl indices, it is demonstrated that the efficiency of oxidation activation under UV irradiation depends on the particular transition metal and increases in the sequence Na < Co < Ni < Cu < Mn < Fe. The introduction of an iron-containing filler decreases the time of polyethylene matrix photooxidation by five to seven times. The irradiation of polyethylene films containing Fe-MMT and Mn-MMT for 250 h leads to the full loss of physicomechanical properties and destruction of the polymer. The activity of Cloisite 25A modified with transition-metal salts in sensitizing photochemical processes is higher than that of the fillers obtained by modification of Na+‑MMT; in this case, there is no destruction of the samples.

The Effect of Cobalt Ions on Thermooxidation and Photooxidation of Polyethylene Containing Modified Montmorillonite

Nanocatalysts obtained by applying cobalt salts to modified montmorillonite significantly accelerate the thermal oxidation and photooxidation of polyethylene. It is important that even short-term irradiation accelerates the thermal oxidation at 23°C. This makes it possible to obtain polymer materials with a regulated lifetime.

Acceleratory and inhibitory effects of uniaxial tensile stress on the photo-oxidation of polyethylene: Dependence of stress, time duration and temperature

The photo-oxidation behavior of high density polyethylene (HDPE) subjected to uniaxial tensile stress is investigated at various temperatures, tensile stress levels and time durations. The effect of stress on the photo-oxidation rate of HDPE is found to be non-monotonous. Tensile stress is found to play distinctly different roles, namely showing acceleratory or inhibitory effect, with the elapse of aging time and at different temperatures. Stress-induced aging inhibition is prone to occur at low temperatures and high stresses within an appropriate aging duration. With elevating temperature, however, this inhibition effect disappears gradually and the stress is found to accelerate the aging process. Microstructural analysis reveals that the effect of stress is closely related to the interplay between the changes in the surface morphology as well as amorphous aggregation structure of materials and the photochemical degradation during aging, which is induced by the variation of molecular orientation and packing state in amorphous region, stress transfer efficiency, thermodynamic and kinetic aspects of photochemical reaction process.

Photo-oxidative degradation of poly-3-hydroxybutyrate and polyethylene based films

The photo-oxidative degradation of films based on polyethylene and poly-3-hydroxybutyrate (PHB) is studied. Application of infrared spectroscopy, differential scanning calorimetry, mechanical strength analysis, and oxygen uptake measurements to studying photoinduced aging shows that the rates of oxidative and photo-oxidative degradation increase with the concentration of PHB in the mixture. It is found that PHB acts as an initiator of the photo-oxidation of polyethylene in the mixture. The greater the surface area of the interphase boundary in the polymer mixture, the more intense oxidation occurs at the initial stage.

Influence of MWNT on Polypropylene and Polyethylene Photooxidation

AbstractSummary: Nanocomposites containing multiwall carbon nanotubes (MWNT) were prepared by melt compounding with polypropylene and polyethylene. The samples were UV‐light irradiated in artificial accelerated conditions representative of solar irradiation (λ &gt; 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by FT‐IR spectroscopy.The behaviour of nanocomposites containing MWNT depends from the type of polymer and presence or absence of antioxidant. The presence of MWNT affect photooxidation of the two polymers working as an UV‐screener on the other hand the increase of thermal oxidation provoked by conversion of photon energy into thermal partially balance the decrease of positive effects on oxidation rate. The negative effects can be minimized by the presence of antioxidants that show interesting synergistic effects with MWNT.

Effect of Clay Modification on Photooxidation of Polyethylene/Clay Nanocomposites

In this paper, effect of clay modification and the resultant clay dispersion and orientation structure on the photooxidation of polyethylene has been investigated upon ultraviolet exposure using the technique of infrared spectroscopy. Polyethylene/clay nanocomposites have been prepared using different clay modification. Both the pristine MMT clay and CTAB ammonium intercalated clay were modified via grafting reaction of silane with hydroxyl groups on the clay edge. The grafting modification leads to decreased photooxidation rate compared with that of neat PE, while the pristine MMT and ammoniums accelerate the photooxidation rate of the corresponding composites. Effect of the dispersion and orientation structure of clay in polymer matrix has also been discussed. In addition, the influence of photooxidation behavior on crystallization and morphology of the composites was studied by means of DSC and SEM.

Effect of Structure and Morphology on Photo-Oxidation Degradation of Polyethylene

Photo-oxidation of polyethylene (PE) has been studied thoroughly these years owing to its high output and various applications, which makes knowledge about its degradation of vital practical importance. Polyethylene products often suffer from service environment factors including ultraviolet, heat or chemicals, which may deteriorate their mechanical properties. The degradation behavior varies with the chain structure and morphology of polyethylene, e.g., branches, linear segments length and crystallization. Natural degradation of three kinds of polyethylene (HDPE, LDPE and LLDPE) has been examined in samples using different annealing conditions in this work. The extent of degradation, described as carbonyl index, was evaluated by Fourier transform infrared spectroscopy (FTIR). Crystallinity measurements were made using differential scanning calorimetry(DSC). The investigation indicates that the branch structure plays a dominant role in the photo-oxidation of polyethylene. Annealing will change the crystallinity of polyethylene before aging, but do not show obvious dependence on the oxidation.

Combining Theory and Experiment to Study the Photooxidation of Polyethylene and Polypropylene

A methodology, which combines experimental evidence and theoretical calculations, able to give a molecular interpretation and a quantification of the photooxidation of polyolefins is here derived. The methodology involves the analysis of experimentally recorded infrared spectra in combination with density functional theory calculations. Environmental effects are taken into account by exploiting the polarizable continuum model. A test against polyethylene and polypropylene samples UV-irradiated under the conditions used for the photocoating is reported. The results obtained with the combined theoretical/experimental methodology are in good agreement with previous findings obtained by exploiting more complex purely experimental approaches.

Wavelength sensitivity of photo-oxidation of polyethylene

The wavelength sensitivity of low-density polyethylene (LDPE) films exposed to filtered xenon arc radiation was determined by the sharp cut filter technique based on the changes in IR absorption and tensile properties. The spectral sensitivity curves in terms of both changes in IR absorption of carbonyl and elongation at break had a similar trend, and the spectral region, responsible for PE photo-oxdation, was found to be in a rather wide range of approximately 260–420 nm. The activation spectra of PE at two different irradiation times based on the changes in IR absorption were also obtained. From the activation spectra it was shown that the most effective spectral region for causing PE photo-oxidation is in the range 330–360 nm. The results are discussed from the point of view of photo-oxidation mechanism.

Infrared spectroscopy studies of the photooxidation of a polyethylene nonwoven fabric

The photooxidation of a nonwoven polyethylene fabric was investigated using Fourier transform infrared photoacoustic spectroscopy. The infrared spectroscopic data indicated that hydroperoxide, alcohol, aldehyde, ketone, carboxylic acid, and anhydride groups were formed as the products of the photooxidation of the polyethylene fabric and that the relative amount of carboxyl among all carbonyls increased as the photooxidation progressed. The distribution of the photooxidation products was inhomogeneous between the two surfaces of the fabric. The highest degree of photooxidation occurred on the surface of the fabric facing the UV radiation source, whereas the lowest degree of photooxidation was on the back surface of the fabric. We also found that the polyethylene nonwoven fabric showed a slower photooxidation rate than the polypropylene nonwoven fabric. The UV radiation at 254 nm caused photooxidation of polyethylene. No photooxidation was observed in the fabric exposed to the UV radiation at 350 nm under the same conditions.

Degradation of polymer blends. Part VI: Photo-oxidation of polyethylene containing SIS triblock copolymers

Two aspects of the problem have been investigated: the initiation of the degradation of the polyethylene phase by the very rapidly oxidized unsaturated block, and the deterioration of the mechanical properties of the blend which finally leads to the spontaneous fragmentation of the film. The first aspect involves the identification and the quantitative determination of the functional CO and OH groups formed in each phase as a function of the irradiation time. The molecular weight of the polyethylene phase was found to decay and to be accompanied by a narrowing of the molecular weight distribution which indicates that its degradation is initiated homogeneously. Concerning the second aspect, two types of spontaneous fragmentation of the films have been identified and related to the morphology of the blend. The first involves initiation and growth of cracks parallel to the direction of extrusion in the SIS phase when SIS is dispersed as a continuous fibrillar structure. The second concerns blends with a low SIS content dispersed as small spheres. Cracks are then initiated at the heavily degraded SIS sites but propagate isotropically in all directions through the polyethylene phase. Such isotropic fragmentation is also observed when low molecular weight photo-initiators are used instead of the unsaturated copolymer.

Re-evaluation of the stabilization mechanisms of various light stabilizer classes

The mechanisms of stabilization of polymers by UV absorbers, Ni stabilizers and hydroxybenzoates are re-examined. This is done in the light of recent findings concerning hindered amine light stabilizers (HALS) stabilization mechanisms as well as photooxidation of polyethylene (PE) and polypropylene (PP). In polyethylene, photoinitiation is thought to originate from excited polymer-oxygen complexes. In polypropylene, after a short initial period, during which such complexes may also be important, photoinitiation is caused mainly by photolysis of tertiary hydroperoxides. Furthermore, in polypropylene, oxidation chains are usually rather long whereas, in polyethylene, oxidation chains are very short or even non-existent. UV absorption by various UV absorber classes and a Ni stabilizer is demonstrated for polypropylene and polyethylene. It is also shown that typical light stabilizers such as UV absorbers, Ni stabilizers, hydroxybenzoates and HALS can protect polyethylene and polypropylene from thermal oxidation in circulating air ovens. However, it is also found that only part of these light stabilizers is efficiently inhibiting formation of trans-vinylene groups thought to be formed in the initiation reaction in polyethylene. The same stabilizers that are efficient in reducing this initiation reaction show good performance in protecting polyethylene from photooxidation. Light stabilizers protecting mainly or exclusively by free radical scavenging are efficient with PP only. Hydroxybenzoates and some Ni stabilizers fall into this category. The work on stabilization mechanisms is illustrated with data from practical stabilization.

Current trends in mode of action of Hindered Amine Light Stabilizers

The stabilization mechanisms of Hindered Amine Light Stabilizers (HALS) involving mainly various oxidation products of HALS do not account for the most important experimental facts. Part of these facts pertains to the dependence of HALS performance on their concentration. In polypropylene the performance increases linearly with HALS concentration as long as this concentration is not too high. In high density polyethylene the performance is a linear function of the square root of HALS concentration. Another part of the experimental facts concerns the decrease of the concentration of the light stabilizers on UV exposure of HALS stabilized polyolefins. The decrease is first-order in both PP and PE-HD. However, this decrease, independent of initial HALS concentration in PE-HD, decreases with increasing HALS concentration in PP. The role of the oxidation products of HALS in polyolefin stabilization is discussed in detail. It is found that there is a fundamental difference between the tertiary hydroxylamine ethers mainly formed in polypropylene and the secondary hydroxylamine ethers formed in polyethylene. The latter are postulated to be much more reactive than the former under photooxidative conditions because they contain an α-hydrogen atom prone to attack by peroxy radicals. Various mechanisms are proposed for this reaction. They account for the products found in polymers as well as in model systems. New stabilization mechanisms are proposed to explain the experimental facts. They are based on modified photooxidation mechanisms of unstabilized polymers. Initiation of photooxidation of polyolefins by excited polymeroxygen complexes is of prime importance. This type of initiation remains essential throughout polyethylene photooxidation during which hydroperoxides do not build up. It usually becomes rapidly negligible in polypropylene in comparison with initiation by tertiary hydroperoxides. Initiation of photooxidation in HALS stabilized polyethylene involves HALS-oxygen complexes throughout the useful lifetime of the polymer. Initiation of photooxidation of HALS stabilized PP is attributed to photolysis of tertiary hydroperoxides as long as the concentration of HALS is not too high. The HALS stabilization mechanisms proposed involve quenching of excited polymer-oxygen complexes. Formation of complexes between peroxy radicals and secondary HALS is another important aspect of these mechanisms. The peroxy radical/HALS complex can either react photolytically on absorption of UV light or react with a second peroxy radical. The latter leads to a termination reaction between two peroxy radicals as it is observed in the absence of HALS. The formal kinetics based on the mechanisms envisaged explain the experimental results obtained with HALS stabilized PP and PE-HD. It is postulated that the mechanisms discussed are valid, at least in part, for many polymers other than polyolefins.

Mechanisms and kinetics of photostabilization of polyolefins with N-methylated HALS

The present work concerns the mechanisms and kinetics of photostabilization with N-methyl HALS. It is found that the efficiency of this HALS type in polypropylene and polyethylene is often comparable to that of the corresponding unsubstituted HALS. This is attributed to very fast transformation of the N-methyl HALS into the formic acid salt of the corresponding NH-HALS. Such a transformation has been found previously and ascribed to free radical attack of the labile hydrogen atoms of the N-methyl group. Although free radical attack can account for corresponding findings on oven aging of polyethylene, it does not explain the findings on photooxidation of polyethylene. As a matter of fact, the rate of transformation of the N-methyl HALS is hardly affected by the presence of phenolic antioxidants. Furthermore, a benzoate-type UV stabilizer protecting mainly by free radical scavenging does not affect significantly the rate of transformation. To account for the effect observed with solid polyethylene, a photooxidative mechanism is envisaged. It is based on the formation of an exciplex oxygen-polymer. This exciplex is able to migrate in the polymer and is rather long lived. It is only when it involves a hydrogen atom that is sufficiently labile that there is reaction. The hydrogen atoms of the N-methyl group of HALS are reactive enough, so that reaction and transformation of the methyl group results, as found experimentally. Mechanism and kinetics of the reaction are discussed.

Some aspects of polyethylene photooxidation

AbstractThe 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.

Findings pertaining to polyethylene photooxidation

AbstractThe kinetics and mechanisms of photolysis of the hydroperoxides formed on thermal oxidation of polyethylene have been investigated. The difficulties to explain the results by the generally accepted decomposition mechanism based on homolytic splitting of the hydroperoxy bond are discussed. New mechanisms of hydroperoxide photolysis are proposed. The intermolecular reactions considered account for the main products found experimentally, ketones and trans‐vinylene groups. They are also in agreement with the kinetics of product formation. The intramolecular photolysis reactions envisaged seem to involve a small amount only of the hydroperoxides present in polyethylene after thermal oxidation. However, they affect directly the mechanical properties of the polymer because they involve main‐chain scission.

Effects of temperature and the partial pressure of oxygen on the rate of photooxidation of polyethylene

Using a novel apparatus, the uptake of oxygen by high density polyethylene (HDPE) and low density polyethylene (LDPE) when exposed to ultraviolet (u.v.) irradiation, was measured in a series of experiments that are intended to test the effects of u.v. stabilizer, temperature and the partial pressure of oxygen on the rate of polymer photooxidation. It is shown that the rate of photooxidation approximately doubles for every 10°C increase in temperature. The kinetics analyses of the oxygen uptake curves, at different initial partial pressures of oxygen, indicate that at low oxygen pressures the photooxidative process is controlled by the available oxygen. As the partial pressure of oxygen is increased, a critical value is reached above which the incipient stage of photooxidation is limited by the initial number of reactive (or oxidizable) centres in the polymer. This behaviour is formally quantified and related to the currently accepted mechanism of polymer photooxidation.

Contribution to polyethylene photooxidation

Etude du mecanisme de photolyse des hydroperoxydes formes lors de l'oxydation thermooxydante du polyethylene