Comparison of the biodegradability of various polyethylene films containing pro-oxidant additives

The biodegradability of high density polyethylene films (HDPE), low density polyethylene films (LDPE) and linear low density polyethylene films (LLDPE) with a balanced content of antioxidants and pro-oxidants (manganese + iron or manganese + iron + cobalt) was studied. Abiotic pre-treatment consisting of photooxidation and thermal oxidation corresponding to about three years of outdoor weathering (including 3-4 months of exposure to daylight) was monitored by FTIR and SEC measurements. The oxidized samples were then inoculated with the strain Rhodococcus rhodochrous in mineral medium, and incubated up to 180 days. The metabolic activity of the bacteria was assessed by measuring adenosine triphosphate content (ATP) and the viability of the cells. Complementary experiments were performed by ¹H NMR spectroscopy to monitor the biodegradation of soluble molecules excreted from the polymer in the incubation medium. Finally SEM was used to visualize the formation of a biofilm at the surface of the polymer. Three samples among the 12 tested were investigated in compost and soil environments. The results show that the main factor controlling the biodegradability of the polyethylene films is the nature of the pro-oxidant additive and to a lesser extent that of the matrix. Except for the samples containing very high content of cobalt additive, the various polymer films were used as substrates by the bacteria.     

聚烯烃（聚乙烯）低温热氧化与降解

综述了近年来聚烯烃特别是聚乙烯的低温热氧化机理，动力学以及氧化降解研究现状及发展，指出现今低温热氧化与降解研究意义。     

Identification and quantification of polyethylene glycol types in polyethylene glycol methyl ether and polyethylene glycol vinyl ether

Quantitative determination of polyethylene glycol (PEG) impurities in two monofunctional polyglycol types, PEG methyl ether (M-PEG) and PEG vinyl ether (V-PEG), has been carried out by reversed-phase liquid chromatography with evaporative light scattering detection (ELSD). In addition to optimizing the resolution between PEG and monofunctional PEG peaks, the major focus has been to determine the molecular weights of PEG impurities in M-PEG and V-PEG of diverse molecular weights. The latter is achieved by examining liquid chromatography–mass spectrometry (LC–MS) mass spectra of both monofunctional PEG and PEG in several cases, and matching peak retention times with those of available PEG standards for all M-PEG and V-PEG sample types. This information is helpful in selecting the appropriate PEG standard to determine PEG content in each sample type. ELSD response factors for various PEG standards have also been compared. It has been found that PEG standards with molecular weights from 1000 Da to 8000 Da show responses that are within 10% of each other. However, a low molecular weight PEG such as PEG 400, provides approximately 30% less response compared to its higher molecular weight counterparts.     

Study of the degradation mechanisms of polyethylene during reprocessing

The present work aims at evaluating the behaviour of various grades of high and low density polyethylene subjected to continuous injection moulding operations. It is based on a wide range of experimental techniques that allowed the establishment of relationships between the observed property changes and the structural modifications developed in the polymers. The study confirmed the coexistence of two distinct degradation mechanisms, crosslinking and chain scission, that occur due to thermo-oxidative reactions arising during the different injection moulding stages. The relative importance of these two mechanisms depends on the material structure and on the processing conditions used. It was also possible to confirm that the polyethylenes with lower molecular mass show low sensitivity to degradation during reprocessing. Furthermore, the more branched polymers (as is the case of LDPE) also proved to be less sensitive to thermo-degradation phenomena. Overall, it was concluded that the primary recycling of polyethylene, if performed under well controlled conditions, will lead to only minor material property losses.     

Radiolytic unsaturation decay in polyethylene. Part III—the effect of certain chain transfer agents

Small amounts of certain halogenated compounds are found to have, at most, only a slight enhancing effect on the radiolytic decay rates of added poly-unsaturated compounds in polyethylene, but significantly increase the elastic modulus at 433 K (melt modulus) obtained thereby. Experiments with model chlorine-containing additives suggest that this increase is due to a more random distribution of polymer and monomer mediated crosslinks in the polymer, that it does not result from a significant increase in crosslinking and that it is mediated by chlorine atoms, in a similar manner to radiolytic hydrogen atoms, through facilitation of long range free radical migration. Although low molecular weight chloro-paraffins inhibit radiolytically induced growth of melt modulus in monomer containing polyethylenes, even very small additions of chlorinated polyethylenes, which form a separate phase, increase the melt modulus. This again indicates that the active species is the chlorine radical.     

Radiolytic unsaturation decay in polyethylene. Part I—general review and analysis with additional new work

Previously published mechanisms for radiolytic vinyl decay in polyethylene lead to first-order, second-order and modified first-order relations. Such equations are fitted using regression analysis to the results of unsaturation measurements on irradiated polyethylenes carried out by the present author (and colleagues) or made available to him by another. It is found that the modified first-order relation best represents the radiolytic decay of vinyl, vinylidene and added allyl unsaturation as triallylisocyanurate or triallylcyanurate in polyethylene at room temperature.     

Radiolytic unsaturation decay in polyethylene. Part II—the effect of irradiation temperature, thermal history and orientation

The decay rate of vinyl unsaturation in high-density polyethylenes irradiated at temperatures from about 310 to 450 K, changes significantly in the melting range up to the crystalline melting point as does free radical mobility and the polymer crystallinity. However, orienting the polymer, or slow cooling or quenching from the melt, prior to irradiation, do not alter the decay process or its rate, although they do alter the rate of increase of insoluble gel and of elastic modulus in the molten state. It is suggested that, below 340 K, the marked deviations from a first-order decay result from the limited mobility of polymeric free radicals in the crystalline phase and from scavenging, by vinyl groups, segregated into the amorphous phase, of radiolytic hydrogen atoms (H). In the melting range, the mobility of polymeric free radicals increases as the crystallinity decreases, reducing the importance of scavenging, so vinyl decay approximates more closely to a first-order relation. In the melt, the vinyl decay relation is not changed qualitatively by H atom scavenging, but the effective vinyl concentration is lower, so the decay rate drops sharply.     

Crosslinking kinetics of polyethylene with small amount of peroxide and its influence on the subsequent crystallization behaviors

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 ° C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov's model. The curing rate k_2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity X_c due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.     

The influence of micro- and nanoparticles on model atomically flat surfaces on crystallization of polyethylene

Crystallization of high density polyethylene (PE) from the melt on model atomically flat solid surfaces decorated with micro- and nanoparticles of gold or NaCl of different size and densities is investigated. The morphology of the contact layer of PE after its detachment from the support is studied using atomic force microscopy (AFM). It is shown that the nucleating and ordering effect of the solid on PE crystallization depends to a large extend on the nanostructure of its surface, in particular on the size of the atomically flat domains and on the presence of nanoscopic obstacles. The minimum size of the flat domain which can significantly influence the PE crystallization is estimated to be of the order of 150 nm.     

Influence of branch content on the microstructure of blends of linear and octene-branched polyethylene: a MD simulation study

Microstructure of linear low-density polyethylene (LLDPE) with linear high-density polyethylene (HDPE) is investigated by molecular dynamics (MD) simulation. The branch content of LLDPE was varied from 10 to 60 branches/1000 C. An equimolar mixture of each of the two polymers is simulated in the NVT ensemble at 500 K and at average experimental densities of the two polymer melts. Initially, chains of LLDPE and HDPE were completely mixed and evolution of conformations with time was monitored. The blends were found to microphase separate when the branch content of LLDPE exceeded 30 branches/1000 C. At 40 branches/1000 C, the two polymers displayed partial microphase separation and complete microphase separation was observed at 60 branches/1000 C. Substantial short-range order and mismatch of molecular conformations is observed in the microphase-separated systems.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 29: Experimental kinetics and mechanisms of γ-lactone formation

The experimental kinetics for γ-lactone formation shows more complexity than that for acids. Nonetheless, it can be concluded to the existence of a constant rate of formation from the beginning of the experiments with polyethylene melts. There is an additional term contributing to γ-lactone formation in the initial stages that is cubic in processing time. In the advanced stages of processing, in the high temperature range (170-200 °C), the concentration of γ-lactones increases linearly with the processing time.There are many mechanisms susceptible to give γ-lactones on polyethylene melt processing. Some of them are based on decomposition of intermediates formed directly on chain propagation. This is so for the α,γ-keto-hydroperoxides in 4-position to hydroxyl groups. Since decomposition of these intermediates is very fast, the reaction might account for a constant rate of γ-lactone formation from the beginning of polyethylene processing. Decomposition of the α,δ-keto-hydroperoxides formed on intramolecular reactions on chain propagation is not so fast as that of the α,γ-keto-hydroperoxides. Nonetheless, it might account for part of the delayed formation of γ-lactones. The same is valid for the mechanisms based on peroxidation of aldehydes and γ-hydroxy trans-vinylene groups that involve intermediates that are formed on polyethylene peroxidation. They might be important for explaining the cubic term as well as γ-lactone formation in the advanced stages of polyethylene processing.     

Comparison of the migration behavior of nanoparticles based on polyethylene glycol and silica using micellar electrokinetic chromatography

Nanoparticles, spherical particles with diameters less than 100 nm, are promising theranostic devices for noninvasive diagnosis and therapy. In this study, nanoparticles composed of polyethylene glycol and silica were prepared, and their migration behavior was examined using capillary electrophoresis. The effects of the sodium dodecyl sulfate concentration in the electrolyte, the nanoparticle size, and the encapsulated molecule on the migration were examined. The addition of sodium dodecyl sulfate into the electrolyte had a significant effect on the electrophoretic mobility of polyethylene glycol nanoparticles, but a small effect on that of silica nanoparticles. As for the size effect, the mobility became a little faster for smaller nanoparticle sizes for both polyethylene glycol and silica nanoparticles. The encapsulated molecule affected the mobility of the nanoparticles through interactions between the encapsulated molecules and sodium dodecyl sulfate. We propose that the large effect of sodium dodecyl sulfate on the migration of the polyethylene glycol nanoparticles was due to the large spaces within the nanoparticles. These results indicate that nanoparticle migration is mainly determined by the nanoparticle components.     

Crystallization of irradiated polyethylene

This study compares the effects of radiation dose on the isothermal and non-isothermal crystallization of LLDPE, LDPE and HDPE by differential scanning calorimetry (DSC). It includes qualitative comparison of the non-isothermal data and quantitative calculations of Avrami parameters for crystallization rate and nucleation mode. The isothermal crystallization allowed the observation of the changes in the crystallization rate, related to the decrease in the crystallization temperature caused by the crosslinking of the polymer. It was also observed by the non-isothermal crystallization, the development of crystallites of very different sizes in the polymer.     

Comparison of physical-mechanical features of polyethylene based polymers employed as sealants in solar cells

In this article, we compared the sealing strength, flexibility, thermal stability, and moisture-barrier properties of two polymeric films, both based on linear low density poly ethylene (LLDPE), which can be successfully used as sealant materials in electrical devices. One example is the dye-sensitized solar cell (DSSC), a low-cost third generation device, based on a photo-electrochemical system. Characterization of LLDPE modified with maleic anhydride (PE-MAH) or with ionomeric ethylene-acrylic acid co-polymer (EMAA) was carried out. Results highlighted as PE-MAH exhibits better adhesion features toward glass, higher thermal stability, and lower wet ability even at high temperatures (80?°C) compared to EMAA. Sealant’s features have been correlated to their chemical and structural composition.     

Photooxidative behaviour of polyethylene/polyamide-6 blends

The photochemical behaviour of several polyethylene/polyamide-6 blends was studied under conditions of artificial accelerated weathering. Particular attention was paid to five different compositions ranging from pure polyethylene to pure polyamide with blends of PE/PA-6 of various compositions: 75/25, 50/50 and 25/75 wt/wt%. Analysis by infrared spectroscopy of the chemical modifications caused by photooxidation showed that exposing the polyethylene/polyamide-6 blends to UV-light irradiation led to the formation of oxidation photoproducts in both polymer phases. In agreement with both the mechanical and spectroscopic analyses, the photooxidation rate of the blends was observed to be much higher than that of the homopolymers. The results given in this paper suggest that photooxidation of the PE/PA blends starts in the polyamide phase and that the subsequent photooxidation of the polyethylene phase may be initiated by the radicals coming from the oxidation of PA.     

Acquired biodegradability of polyethylenes containing pro-oxidant additives

Biodegrability of high density polyethylene film (HDPE) and low density polyethylene film (LDPE) both containing a balance of antioxidants and pro-oxidants was studied with defined microbial strains particularly with Rhodococcus rhodochrous and Nocardia asteroides in mineral medium. After an abiotic pre-treatment consisting of photooxidation and thermo-oxidation corresponding to about 3 years of outdoor weathering the samples were inoculated, incubated up to 200 days and during the period their metabolic activities were followed by measuring adenosine triphosphate content. Simultaneously the cultures were also monitored by optical microscopy and FTIR spectroscopy. The first initial phase of fast growth caused by the presence of low molecular extractable compounds was followed by a long period of stabilized metabolic activity suggesting that microorganisms continued to gain energy from the substrate but evidently at a much slower rate. Complementary analysis performed at the end of incubation revealed that during the experiment time biodegradation processes probably affected surface layer of materials only.     

Comparison of chain structures between high-speed extrusion coating polyethylene resins by preparative temperature rising elution fractionation and cross-fractionation

Two polyethylene(PE) resins(samples A and B) are synthesized as high-speed extrusion coatings with similar minimum coating thickness and neck-in performance but different maximum coating speeds. Both samples are separated into seven fractions using preparative temperature rising elution fractionation. The microstructures of the original samples and their fractions are studied by high-temperature gel permeation chromatography, Fourier transform infrared spectroscopy, 13 C nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and successive self-nucleation/annealing thermal fractionation. Compared with sample B, sample A has a broader MWD, more LCB contents, and less SCB contents. Moreover, sample A contains slightly more 30 °C and 50 °C fractions with lower molecular weights, and more fractions at 75 °C and 85 °C with higher molecular weight. The chain structure and its distribution in the two PE resins are studied in detail, and the relationship between the chain structure and resin properties is also discussed.     

Anisotropic and heterogeneous thermal expansion of polyethylene foam blocks: Effect of thermal treatments

Crosslinked closed cell polyethylene foams produced in blocks by compression moulding present an anisotropic and heterogeneous thermal expansion behaviour when the temperature is increased. This paper analyses the main reason for this particular behaviour and presents a way to reduce it by using thermal treatments.In order to perform this analysis, an experimental study on the cellular structure, lamellar distribution and thermal expansion is presented as a function of two kinds of thermal treatments. The experimental results have showed that the main factor controlling the foams thermal expansion is an anisotropic and heterogeneous cellular structure of the original foams. It has been also proved that an adequate thermal treatment allows homogenising the foams thermal expansion.     

Photochemical stabilization of linear low-density polyethylene/clay nanocomposites: Towards durable nanocomposites

This article reports a study of the chemical modifications of LLDPE/nanoblend nanocomposites exposed to UV light in conditions of artificially accelerated ageing and natural weathering. Analysis by infrared spectroscopy of the chemical modifications produced by photoageing shows that the presence of an organo-clay leads to the decrease of the oxidation induction time of the polymer (LLDPE), which results in lower durability of the nanocomposites. Protection against photooxidation was tested with different kinds of UV stabilizers and with a metal deactivator. It is shown that the metal deactivator is very efficient in stabilizing the nanocomposite since it totally cancels the prodegradant effect of the organo-clay. This confirms the role played by iron impurities in natural clays. The use of a metal deactivator offers a new insight into the stabilization strategy for nanocomposites.     

Migration of phenolic antioxidants from linear and branched polyethylene

Plaques of linear polyethylene (LPE) and branched polyethylene (BPE) were exposed to oxygen-free media (nitrogen or water) at 75, 90 and 95 °C. The polymers were stabilized with one of the following three bifunctional phenolic antioxidants: Santonox R, Irganox 1081 or Lowinox 22M46. The initial concentration of antioxidant in the plaques was ∼0.09 wt.%. After ageing, the oxidation induction time profiles obtained by differential scanning calorimetry often became very flat, which indicated that migration was controlled by the boundary loss process. The unexpected higher migration rate from LPE than from BPE was due to the dominance of the boundary loss process. It is proposed that the low boundary loss rate in BPE was due to the presence of a thin liquid-like (oligomeric) surface layer which developed during ageing of this polymer. A qualitative relationship was found between the boundary loss rate to water and the polarity of the antioxidant. The antioxidant diffusivities in LPE and BPE were approximately equal, a finding which, in view of the morphological analysis estimating the geometrical impedance factor, indicated that the constraining effect of the crystals on the non-crystalline fraction was not sensed by the antioxidant molecules. It is suggested that the large molecular size and the low segmental flexibility of the antioxidant molecules inhibited their ability to penetrate the interfacial component.