Aged Low-density Polyethylene Research Articles

An innovative microplastic extraction technique: The switchable calcium chloride density separation column tested for biodegradable polymers, polyethylene, and polyamide

Extracting microplastics from complex matrices poses challenges due to the potential impact of harsh chemical treatments on microplastic properties. For fate and hazard assessment reliable techniques are needed to not only quantify the particle number but also to assess the physicochemical properties of environmental microplastics with minimum changes induced by extraction. Here we present the method development for an innovative and non-destructive extraction protocol based on a switchable calcium chloride density separation column. In contrast to commonly reported extraction protocols, the presented technique is suitable for targeted microplastic property analysis (e.g., surface chemistry and texture) by keeping chemical treatments (such as oxidation and enzymatic digestion) to a minimum. By adjusting the temperature we can control the aggregate state of the highly concentrated salt solution, allowing to separate the microplastics from matrix by cutting of purified, solidified samples. Harsh chemical treatments are avoided, as well as obstruction of microplastic extraction by adsorption to matrix components when passing the tap at the bottom of traditional density separation funnels.The use of microplastics that were prelabeled with a fluorescence dye helped to solve difficulties observed during method development by visual inspection before measurement of extraction efficiency: We spiked a blank compost with low-density polyethylene (LDPE) and polyamide (PA). Additionally, UV aged LDPE was used to demonstrate applicability to more hydrophilic, more environmentally relevant microplastics. The obtained initial results show high recovery of both unaged and aged LDPE over 97 wt.-% and an efficient compost removal but a lower and less robust recovery (between 68 and 18 wt.-%) for PA particles that are more challenging to extract due to an unfortunate synergistic combination of smaller particle size and higher density. Method adaptation to other microplastic types may still be necessary. In short:•A low-cost and simple approach without oxidation to extract (pre-aged) microplastics from compost•Method development by visual observation using fluorescent labelled microplastics and method validation by spike-recovery tests

Physics and chemistry-based constitutive modeling of photo-oxidative aging in semi-crystalline polymers

This paper proposes a physio-chemically-based constitutive framework to predict the response of severely photo-oxidatively aged semi-crystalline polymers. Photo-oxidation induced by the exposure to Ultra-Violet (UV) light and oxygen is one of the main and dominant degradation mechanisms affecting the lifespan of polymers. In this work, we first characterize the physio-chemical changes of low-density polyethylene (LDPE) to understand and map the photo-oxidation degradation process. Changes in crystallinity and mass loss are characterized by Differential Scanning Calorimetry (DSC) and Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) experiments, respectively. The evolution of crystallinity and mass loss relative to the initial pristine films over exposure time are considered as indicators of material degradation. Based on these tests, we then propose evolution functions for the material properties in the polymer constitutive equations to incorporate the effects of photo-oxidation on the mechanical responses. Connecting the physio-chemical processes affecting polymer network evolution to the mechanical response of LDPE eliminates the need for defining extra fitting parameters that carry no physical meaning. The developed constitutive framework is validated with respect to a series of in-house uniaxial tensile tests performed on LDPE aged for different UV exposure times. Validation of the proposed constitutive framework confirms the accuracy of DSC and QCM-D as rigorous techniques in the characterization of degradation in LDPE films, and confirms the robustness of the developed framework in predicting the mechanical responses of photo-oxidatively aged LDPE.

Effects of microplastics on soil carbon dioxide emissions and the microbial functional genes involved in organic carbon decomposition in agricultural soil

The accumulation of microplastics (MPs) in agricultural fields can not only disguise soil organic carbon (SOC) storage but also affect the production of carbon dioxide (CO2) by microbial decomposition. However, little is known about the impact of this emerging pollutant on soil CO2 emissions and the functional genes related to SOC degradation. In the present study, a short-term (30-day) microcosm experiment was performed to investigate the effects of virgin and aged low-density polyethylene (LDPE) MPs on soil CO2 emissions. We also measured functional gene abundances related to starch (sga), hemicellulose (abfA, manB and xylA), cellulose (cex) and lignin (lig and mnp) degradation through a high-throughput quantitative-PCR-based chip. Compared with the soils without MPs, low doses (0.01% and 0.1%) of both virgin and aged MPs had negligible effects on SOC decomposition, whereas a high dose (1.0%) of these two MPs significantly (p < 0.05) accelerated the production of CO2 in soils by 15–17%, showing a dose-dependent effect. The presence of MPs did not significantly affect soil dissolved organic carbon or microbial biomass carbon. A higher metabolic quotient at 1.0% MP concentration indicated that the microbes were stressed and needed more substrates and energy during their metabolic process, which could likely explain the increase in CO2 emission induced by this dose of MPs. Exposure to virgin MPs significantly reduced the functional genes related to hemicellulose (abfA and manB) degradation, whereas increasing the aged MPs concentrations significantly decreased the abundances of functional genes encoding starch (sga), hemicellulose (abfA, manB and xylA), and cellulose (cex) hydrolysis. Overall, we conclude that the low dose (<0.1%) of MPs in the soils has a negligible effect on the production of CO2, but this factor should be considered in evaluating the global C budget in future research as this contaminant reaches a certain threshold (1.0%).

Sorption and leaching behaviors between aged MPs and BPA in water: The role of BPA binding modes within plastic matrix

Due to the hydrophobicity and large specific surface area microplastics (MPs) have become the vector for the migration of environmental organic pollutants. Environmental aging process affects the physiochemical structure of MPs and their corresponding environmental behaviors, in which the effect of bisphenol A (BPA) binding mode within plastic matrix on aging behaviors of MPs is not reported. In this work, the structural properties and BPA sorption behaviors of low density polyethylene (LDPE) MPs with BPA additives and polycarbonate (PC) MPs with BPA monomers exposed to three types of artificial accelerated aging processes including UV/H2O, UV/H2O2, and UV/Cl2 systems were comparatively investigated. Virgin LDPE and PC exhibited obvious leakage of BPA additives or monomers. Aged LDPE had stronger sorption ability towards BPA in water environment with no observed leakage of BPA additives. While, aged PC had extremely high leakage of BPA monomers, which is similar to virgin PCs and was proved to be a persistent source of BPA release. The BPA sorption on aged LDPE or leaching from aged PC was influenced by aging processes, water pH, salinity, co-existing estradiol (E2), and water sources. This study reveals the potential ecological and environmental risks of MPs containing toxic additives/monomers during aging processes from a new perspective.

Microplastic pollution alters forest soil microbiome

The impact of microplastic pollution on terrestrial biota is an emerging research area, and this is particularly so for soil biota. In this study, we addressed this knowledge gap by examining the impact of aged low-density polyethylene (LDPE) and polyester fibres (i.e. polyethylene terephthalate, PET) on a forest microbiome composition and activity. We also measured the corresponding physicochemical changes in the soil. We observed that bacteria community composition diverged in PET and LDPE treated soils from that of the control by day 42. These changes occurred at 0.2% and 0.4% (w/w) of PET and at 3% LDPE. Additionally, soil respiration was 8-fold higher in soil that received 3% LDPE compared to other treatments and control. There were no clear patterns linking these biological changes to physicochemical changes measured. Taken together, we concluded that microplastics aging in the environment may have evolutionary consequences for forest soil microbiome and there is immediate implication for climate change if the observed increase in soil respiration is reproducible in multiple ecosystems.

Abiotic and biotic degradations of a LDPE blend in soil of South Brazil landfill

Polyethylene degrades slowly when discarded in the environment and exposed to natural weathering. A solution to this problem is the incorporation of additives to accelerate its biodegradation. In the present study, a biodegradable low-density polyethylene (LDPE) blend was obtained by the mixture of LDPE with a biodegrading additive (8% w/w) and the effect of accelerated weathering on biodegradation process was assessed. Chemical, mechanical, thermal, and morphological properties of non-aged and aged samples were studied. Results showed that aging process caused changes on structural characteristics (insertion of functional groups), morphological (appearance of micro-cracks and increased roughness), mechanical (greater stiffness and loss of plasticity) and thermal (lower degree of crystallinity and thermal stability) properties. The presence of biodegrading additive contributed to aging process, since polar functional groups were inserted into additive exposed to soil underwent greater degradation than their non-aged counterparts. CO2 production of aged LDPE blend demonstrated that accelerated weathering influences biodegradation process. In this work, due to availability of O2 gas, aerobic microorganisms can be the main responsible by the material deterioration, leading to production of microbial biomass, CO2 and H2O. Thus, the results of biodegradation (166 days) obtained in this work are promising, once it was reached a natural soil from South Brazil landfill.

Competitive heavy metal adsorption onto new and aged polyethylene under various drinking water conditions

The study goal was to identify factors that influence copper (Cu), iron (Fe), lead (Pb), manganese (Mn), and zinc (Zn) loading on new and aged low-density polyethylene (LDPE) under various drinking water conditions. The applied aging procedure increased LDPE surface area, hydrophilicity and the number of oxygen containing functional groups. Aged LDPE adsorbed up to a 5 fold greater metals than the new LDPE: Cu > Pb, Zn > Mn. Water pH (5.5 to 10.5) significantly altered LDPE surface metal loading. The organic carbon leached from plastic pipes inhibited Cu adsorption (−43.8%), but other metals were less impacted (−5.7% to −9.1%). The addition of free chlorine and corrosion inhibitor retarded metal adsorption to suspended LDPE materials. Overall, by changing water conditions total metal loadings (i.e., Cu, Mn, Pb and Zn) were altered 20.1 to 35.4%. When Fe was present, Cu (−4.0%) and Pb (−4.5%) loadings were reduced, while lesser impacts were found for Mn and Zn. Cu2+, Pb2+ and Zn2+ hydroxides and oxides were identified as the major metal deposit forms on the LDPE surface by XPS. To better predict metal fate in plastic piping systems, plastic surface characteristics, dissolved organics, water pH, hydraulic conditions and microbial growth should be considered.

Thermal degradation properties of LDPE insulation for new and aged fine wires

The thermal behavior of new and aged low-density polyethylene (LDPE) insulations was investigated using an SDT Q600 thermal analyzer. The activation energy and pyrolysis reaction model were estimated using the non-isothermal and masterplots methods. The thermal degradation processes present different behaviors of the LDPE insulations before and after thermal aging. The thermogravimetric curves shift to the direction of higher temperature, for the aged LDPE insulation. The values of activation energy evaluated using the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods are almost same. However, the activation energy values estimated by the Friedman method were slightly higher than those obtained using the KAS and FWO methods. The suitable pyrolysis reaction models of the new and aged LDPE insulations were attributed to the “Contracting area” (R2) model, which was determined using the generalized masterplots method. In addition, the pre-exponential factor and compensation effect are discussed. Finally, it should be stressed that the aged LDPE insulation generally pyrolyzes more weakly and with more difficulty than the new insulation, i.e., the ignition and flame spread of aged wire in old buildings are not relatively easy.

The Effect of DC Electro-thermal Ageing on Electrical Treeing in Polyethylene

Electrical treeing is among the main mechanisms responsible for electric breakdown in polymeric high voltage cables. In this paper, electrical treeing in electro-thermally aged low density polyethylene (LDPE) was evaluated by means of a real-time microscopic detection system. A wire – plane electrode test object with semiconducting tab was used in the study. The ageing was performed at a constant temperature of 80°C and an applied DC voltage of 10 kV of both polarities and lasted from 100 up to 800 hours. For testing the resistances to electrical treeing, tests were performed by applying 50 Hz AC voltage at a ramping speed of 0.5 kV/s. The obtained experimental results showed that mainly the thermal component of the ageing had an influence on electrical treeing parameters, namely on the initiation voltage as well as the shape and occurrence frequency of the trees. With an increase of the ageing time, the tree inception voltage gradually decreased.

Investigation of the factors that influence lead accumulation onto polyethylene: Implication for potable water plumbing pipes

The influence of polymer aging, water pH, and aqueous Pb concentration on Pb deposition onto low density polyethylene (LDPE) was investigated. LDPE pellets were aged by ozonation at 85 °C. ATR-FTIR and X-ray photoelectron spectroscopy (XPS) analysis of aged LDPE surfaces showed that a variety of polar functional groups (>CO<, >CO, >COO) were formed during aging. These functional groups likely provided better nucleation sites for Pb(OH)2 deposition compared to new LDPE, which did not have these oxygen-containing functional groups. The type and amount of Pb species present on these surfaces were evaluated through XPS. The influence of exposure duration on Pb deposition onto LDPE was modeled using the pseudo-first-order equation. Distribution ratios of 251.5 for aged LDPE and 69.3 for new LDPE showed that Pb precipitates had greater affinity for the surface of aged LDPE compared to new LDPE. Aged LDPE had less Pb surface loading at pH 11 compared to loading at pH 7.8. Pb surface loading for aged LDPE changed linearly with aging duration (from 0.5–7.5 h). Pb surface loading on both new and aged LDPE increased linearly with increasing Pb initial concentration. Greater Pb precipitation rates were found for aged LDPE compared to new LDPE at both tested pH values.

Statistical Analysis of Breakdown Voltages in Virgin and Aged LDPE Using Johnson S&lt;sub&gt;B&lt;/sub&gt; and Weibull Distribution

Polymeric material such as low density polyethylene (LDPE) has been used for decades as insulating material. Any polymeric material will experience degradation after prolonged application of high electrical stresses. Deeper understanding of the long term electrical degradation of the insulating material is necessary to predict the life of high voltage cable. Electroluminescence method (EL) is used to detect the breakdown voltages of thin film LDPE. This method utilizes a Peltier cooled electron multiplying charge coupled device (EMCCD) camera to detect the breakdown of the sample. Statistical distribution of the AC breakdown voltages of 100μm virgin and aged LDPE has been analysed. Comparison for the best fitted distribution was made for Weibull distribution and Johnson SB distribution using Anderson-Darling (A2) goodness-of-fit and Kolmogorov-Smirnov (D) goodness-of-fit (GOF). Johnson SB is rarely used in high voltage engineering application. The probability density function (PDF) and the cumulative density function (CDF) for both distributions are defined in this article. The statistical parameters used are estimated based on Maximum Likelihood Estimation (MLE) for both distributions. Based on the statistical analysis, it is observed that Johnson SB provide better fitting than Weibull distribution with lower fitting error and that 3-parameter Weibull is much better fitting than 2-parameter Weibull distribution for most cases. It is also found that the median breakdown voltage of LDPE samples decreases with increasing aging temperature.

The electro-mechanical effect from charge dynamics on polymeric insulation lifetime

For polymeric material used as electrical insulation, the presence of space charges could be the consequence of material degradations that are thermally activated but increased by the application of an electric field. The dynamics of space charge, therefore, can be potentially used to characterize the material. In this direction, a new aging model in which parameters have clear physical meanings has been developed and applied to the material to extrapolate the lifetime. The kinetic equation has been established based on charge trapping and detrapping of the injected charge from the electrodes. The local electromechanical energy stored in the region surrounding the trap is able to reduce the trap-depth with a value related to the electric field. At a level where the internal electric field exceeds the detrapping field in the material, an electron can be efficiently detrapped and the released energy from detrapping process can cause a weak bond or chain scission i.e. material degradation. The model has been applied to the electro-thermally aged low density polyethylene film samples, showing well fitted result, as well as interesting relationships between parameter estimates and insulation morphology.

An improved model to estimate trapping parameters in polymeric materials and its application on normal and aged low-density polyethylenes

Trapping parameters can be considered as one of the important attributes to describe polymeric materials. In the present paper, a more accurate charge dynamics model has been developed, which takes account of charge dynamics in both volts-on and off stage into simulation. By fitting with measured charge data with the highest R-square value, trapping parameters together with injection barrier of both normal and aged low-density polyethylene samples were estimated using the improved model. The results show that, after long-term ageing process, the injection barriers of both electrons and holes is lowered, overall trap depth is shallower, and trap density becomes much greater. Additionally, the changes in parameters for electrons are more sensitive than those of holes after ageing.

Aging of low‐density polyethylene in natural weather, underground soil aging and sea water: Effect of a starch‐based prodegradant additive

This article reports the results of studies on the degradation of the blend of low‐density polyethylene (LDPE) with a starch based prodegradant additive, named commercially as Polystarch plus H and abbreviated as PSH, under natural weather, underground soil aging, and sea water aging in Dhahran, Saudi Arabia. The work includes measurements of tensile properties, thermal behavior, and scanning electron microscopic (SEM) studies on the unaged and aged LDPE and the LDPE/PSH blend having 25% (w/w) of PSH. Results of the preliminary experiments showed that changes in properties are not prominent at lower loadings of PSH. In the case of the blend, incorporation of the additive PSH caused a reduction in elongation at break, which is accentuated under aging. The effectiveness of the prodegradant additive in the degradation behavior follows the order, natural aging &gt; underground soil aging &gt; sea water aging. It is believed that the prodegradant additive is effective in chain scission in sunlight under natural weathering conditions, in contrast to underground soil aging and sea water. POLYM. ENG. SCI., 53:2389–2397, 2013. © 2013 Society of Plastics Engineers

Influence of oxidation on space charge dynamics in DC aged low-density polyethylene

In this study, low-density polyethylene (LDPE) samples were aged under different oxygen concentration (0, 20%, and 50%). The ageing was implemented with an average external DC electric field of 50 kV/mm at 40 °C for 1000 h. It was found that augmentation of charge accumulation and suppression of charge dissipation were enhanced with increasing of oxygen concentration. The capture capability of trap sites got stronger with higher oxygen contents, which could be attributed to oxidation product newly generated during DC ageing. These trap sites acted as trap centers to capture the injected carrier while it was on the way towards the opposite electrode and decrease the apparent mobility. FTIR absorption spectra showed that new elements between 1680 and 1750 cm −1 corresponding to ketone and aldehyde occurred due to oxidation. The bipolar charge injection model was implemented to simulate space charge transport in aged LDPE samples. It was shown that more carriers were captured and the traps gradually filled up when the oxygen concentration in the ageing test increased. A higher oxygen concentration seemed to result in a deeper average trap depth, which increased the amount of trapped charge because it was not easy to detrap. These simulation result corresponded to the experimental measurement.

Space charge and electroluminescence characteristics of thermally aged LDPE films

Polymeric insulation aging is closely related to the change of its physical and chemical properties. An attempt is tried to correlate the space charge and electroluminescence (EL) behaviors of aged low-density polyethylene (LDPE) samples films. LDPE samples were aged in a thermal aging oven with five different time intervals (120, 240, 360, 480 and 600 h). After thermal aging, their molecular structures were analyzed via the Fourier transform infrared (FTIR) absorption spectra, and due to oxidation reaction, two new peaks appeared at ∼1725 and 3380 μm corresponding to carbonyl and hydroxyl groups, respectively. Then the pulsed electro-acoustic (PEA) method was employed to measure the space charge distribution for each sample, and aged samples showed a higher space charge distribution. EL phenomena from aged LDPE surface were investigated under stepped ac voltages, and aged samples showed a relatively lower EL onset voltage and intensity than that without aging. It is considered that EL occurs due to the radiative recombination of electrons and holes injected from both electrodes. New products due to the oxidation reaction can result in more charge traps, which will reduce the surface contact potential barrier and also enhance the charge accumulation inside, and hence lead to the space charge and EL phenomena observed.

Degradation of abiotically aged LDPE films containing pro-oxidant by bacterial consortium

The degradation of abiotically aged low density polyethylene (LDPE) films containing trace quantities of a representative pro-oxidant (cobalt stearate) was investigated in the presence of well defined enriched microbial strains namely, Bacillus pumilus, Bacillus halodenitrificans and Bacillus cereus in Basal salt medium. The films were initially subjected to an abiotic treatment comprising UV-B irradiation, and subsequently inoculated with the bacterial strains. The degradation in the polymeric chain was monitored by changes in the mechanical, morphological, structural and thermal properties. The abiotic treatment led to the formation of extractable oxygenated compounds as well as unoxidised low molecular weight hydrocarbons, which was confirmed by GC–MS studies. These were utilized by the bacterial consortium in the subsequent biotic phase and led to a mass loss of the polymer (8.4 ± 1.37%), which was also accompanied by an increase in the bacterial count. A decrease in the surface tension of the cell free medium was observed, which indicates that the bacterial consortium produced extracellular surface active molecules in order to enhance the bioavailability of the polymeric fixed carbon. The spectroscopic investigations reveal that the bacteria preferentially consume the oxygenated products leading to a decrease in the Carbonyl Index (CI), which in turn leads to an increase in the initial decomposition temperature as observed in the TGA traces. The morphological investigations reveal a biofilm formation on the surface, which was found to be scattered in certain regions and not uniform on the polymeric surface.

Effects of charge trapping in gamma irradiated and accelerated aged low-density polyethylene

The effects of gamma irradiation in air and accelerated aging of low-density polyethylene (LDPE) were studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation-induced oxidation was observed using IR spectroscopy. The influence of radiation dose and accelerated aging on the intensities and the positions of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements, and found to be strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by oxidation and crosslinking.

Space charge characterization in aged LDPE amalgamated insulation regions from underwater telecommunication systems

The requirement of insertion of optical amplifier units and joints, into long-distance submarine telecommunication systems, results in regions of the polymeric insulation which have to be reinstated, using injection molding techniques. These techniques introduce internal amalgamated interfaces, which subsequently can be the sites of possible electrical insulation failure in the system. This paper reports on space charge measurements, (using the pulsed electroacoustic (PEA) technique), from microtomed thin film samples taken from the interface region of typical system joints which have undergone accelerated aging tests under a dc voltage at room temperature. The space charge distributions obtained for the samples are discussed with reference to the underlying mechanisms of both the bulk low-density polyethylene (LDPE) and the surface (LDPE/electrode interface). The effect of varying the electrode material (i.e. Au and Al) is discussed with reference to previously published data. Finally the effects of long-term aging on space charge accumulation within LDPE are considered briefly.

Conductivity of carbon black-PE composites as a function of temperature and UV aging

We performed a study of mechanisms related to ac electrical conductivity of virgin and aged low-density polyethylene (LDPE) loaded with different concentrations of carbon black (CB). Experimental ac conductivity measurements were carried out at different temperatures on samples with different CB concentrations. Scanning calorimetry (DSC) and thermal expansion measurements were used as auxiliary techniques in this study. We also investigated the aging effect of UV radiation on the electrical properties of the samples. Based on the distribution of the CB in the polyethylene matrix, the ac conductivity is explained in terms of the hopping mechanism considering the material as a system where the energy potential barriers are distributed randomly in the bulk of the material.