Effect of thermal aging on the electrical properties of crosslinked polyethylene

This article reports on the effect of thermal aging on the electrical properties of crosslinked polyethylene (XLPE) used in high voltage cables. It was shown that thermal aging modifies the electrical properties of the polymer. The degradation is accelerated when the temperature is increased. This degradation is due to a thermooxidation, followed by a loss of antioxidant, and a change in color of the material. At 80 and 100°C, the antioxidant is practically efficacious during the aging, whereas at 120 and 140°C, it is consumed after 1500 and 1000 h, respectively, which results in an increase in the dielectric losses and a diminution in the volume resistivity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Service life of crosslinked polyethylene as high voltage cable insulation

This paper provides a summary of four EPRI-sponsored contracts that studied the physico-chemical characteristics and thermal and electrical behavior of unaged and service-aged crosslinked polyethylene (XLPE)—insulated cables, and sections of the insulation obtained from this extruded cable. It has been demonstrated that volatile components generated primarily from the peroxideinduced crosslinking process can be defined and quantitatively measured. The morphology of the aged and unaged XLPE extruded insulation can be defined, and the thermal behavior on a micro (morphology changes) and macro (cable aging) level compared. Changes in microvoid level due to melting and recrystallization during cable operation appear to correlate with the limited amount of ac breakdown strength data obtained, and thermal overload of the cable conductor causes complete melting (and recrystallization on cooling) reducing the microvoid level. A single thermal overload alters the morphology but many thermal overloads are required to drive off the volatiles.     

Properties of nanofillers/crosslinked polyethylene composites for cable insulation

In this work, we report the effect of nanofillers and filler loading on mechanical, physical, dielectric, and thermal properties of the crosslinked polyethylene (XLPE) matrix. XLPE filled with 0.5–2% of zinc oxide (ZnO), aluminium oxide (Al₂O₃), and organoclay (OMMT) nanofillers prepared by melt mixing with a single screw extruder followed by hot press moulding. Nanocomposites were tested as per ASTM standard methods and characterized with tensile test, water absorption, linear rate of burning, dielectric breakdown strength, and thermal stability. Scanning electron microscopy (SEM) was used to examine the surface morphology of the nanocomposites. The results showed that addition of nanofillers improved tensile strength, elongation at break, Young's modulus, burning rate, dielectric breakdown strength, and decomposition temperature. However, water absorption increased with time due to the hydrophilic properties of nanofillers. In general, based on the properties measured Al₂O₃ exhibits the highest properties than those of ZnO and OMMT nanofillers. Addition of 1.5% of Al₂O₃ in XLPE matrix has led to the improvement in tensile strength, elongation at break, Young's modulus, burning rate, and dielectric breakdown strength as compared to the unfilled polymer. J. VINYL ADDIT. TECHNOL., 25:E147–E154, 2019. © 2018 Society of Plastics Engineers     

Effect of soil environment on the photo‐degradation of polyethylene films

Outdoor weathering field trials performed with oxo‐degradable polyethylene (PE) thin films were conducted across temperate, grassland, and subtropical sites around Australia. It was found that a site factor, that was apparently independent of total solar dose and temperature, significantly impacted the rate and extent of photo‐oxidation. Controlled laboratory‐based accelerated aging trials of both PE film with no prodegradant and oxo‐degradable PE films (containing iron stearate) revealed that the rate and extent of PE photo‐oxidation did not correlate with temperature under the film or UV exposure, but was soil dependent. Under accelerated photo‐oxidative conditions, the time to reach embrittlement for a PE film aged over the soil from the temperate site (OM 8.4) was half (24.5 days) the time taken when aged over air (48 days). Further investigation revealed that humic acids and fulvic acids within soil organic matter may contribute to an increased rate of PE photo‐oxidation, possibly through the formation of volatile reactive oxygen species that may form under photo‐oxidative conditions. The presence of water also had a significant impact on the rate of photo‐oxidation. Overall, the impact of soil on PE photo‐oxidation was found to be complex and likely dependent at least in part on soil components that varied between different soil types, consequently influencing their photo‐chemistry. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42558.     

Synergistic effects of tetrabutyl titanate on intumescent flame‐retarded polypropylene

The synergistic mechanism of tetrabutyl titanate (TBT) in the intumescent flame‐retardant polypropylene (PP) composites was investigated in this work. The intumescent flame‐retardant was composed of pentaerythritol (PER) as a carbonizing agent ammonium polyphosphate (APP) as a dehydrating agent and blowing agent. Five different concentrations (1, 1.25, 1.5, 1.75, 2 wt %) of TBT were incorporated into flame retardant formulation to investigate the synergistic mechanism. The thermal degradation and flammability of composites were characterized by thermogravimetric analysis (TGA), limiting oxygen index (LOI), and UL‐94 tests. The morphology and chemical structure of char layer was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and energy dispersive spectrometer (EDS). The results showed that LOI was increased from 27.8 to 32.5%, with the increase of TBT content from 0 to 1.5 wt %. Results from SEM, and FTIR demonstrated that TBT could react with APP and PER to form the stable char layer. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4255–4263, 2013     

Thermal aging performance of glass fiber/polyphenylene sulfide composites in high temperature

In order to use the glass fiber reinforced polyphenylene sulfide composites (GF/PPS) in high temperature environments, thermal aging performance of two kinds of commercial grade PPS composites, reinforced by 40% glass fiber, PPS-G40 HM and 1140L4, in thermal aging temperature of 250°C was compared by tensile strength, oxidized layer, color, crystallization and melting behavior. The results showed that tensile strength of GF/PPS composites is significantly decreased with increasing of aging time below 200 h and the tensile strength of aged PPS-G40 HM is higher than that of aged 1140L4. The thickness of dark color area is increased with increasing of aging time. The thickness of oxidized layer of 1140L4 is thinner than that of PPS-G40 HM. However, the color of oxidized layer of PPS-G40 HM is lighter than that of 1140L4. The recrystallization in thermal aging results in the formation of crystal with higher melting point and increased melting temperature of GF/PPS composites. It is found that addition of epoxy resin can increase the initial mechanical property and improve the thermal aging performance of GF/PPS composites. A novel modified GF/PPS composite with higher thermal aging properties was obtained.     

Shrinkage in extruded moisture crosslinked silane‐grafted polyethylene wire insulation

Shrinkage studies were conducted on silane‐grafted moisture crosslinkable linear low‐density polyethylene (LLDPE) insulation stripped from extrusion‐coated copper conductors. The insulation, which possesses orientation imparted during melt processing, showed remarkable levels of shrinkage when heated above the melting point of the polymer, though the shrinkage can be greatly reduced by moisture crosslinking the insulation below the melting point of the LLDPE. Shrinkage along the direction of orientation was accompanied by swelling in the other dimensions. Differential scanning calorimetry (DSC) revealed several trends, including a decrease in both melting point and degree of crystallinity with increasing crosslinking. In the first heat after annealing, crosslinked samples exhibited a shoulder in the DSC endotherm several degrees below the normal melting point of the LLDPE. In agreement with prior studies in silane‐grafted HDPE, relaxation of orientation by annealing appeared to result in an increase in the enthalpy of melting. The degree of shrinkage was also found to be dependent on the insulation thickness, which is attributed to faster cooling in thinner insulation immediately following extrusion coating. The results highlight the extensive built in stresses that can be frozen into polymer layers in fabricated articles due to melt orientation. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermo‐oxidative degradation of additive free polyethylene. Part I. Analysis of chemical modifications at molecular and macromolecular scales

The effects of thermal oxidation on the molecular and macromolecular structures of additive free PE were investigated between 100 °C and 140 °C in air in order to tentatively establish non‐empirical structure/property relationships. In the first part, the changes in POOH concentration were assessed by three different analytical methods: iodometry, modulated differential scanning calorimetry, and Fourier transform infrared (FTIR) spectrophotometry after SO₂ treatment. All these methods provided very similar results until the end of the induction period, after which iodometry overestimated strongly POOH concentration because it titrates also other chemical species formed at high conversion ratios, namely double bonds. In parallel, the changes in carbonyl group concentration were determined by FTIR spectrophotometry after NH₃ treatment. As the accumulation kinetics of ketones, aldehydes, and carboxylic acids were closely interrelated, the question of their actual formation mechanisms in the current thermal oxidation mechanistic scheme was raised. An alternative reaction pathway was proposed for the bimolecular decomposition of POOH. In the second part, the corresponding changes in weight and number average molecular masses were monitored by high temperature gel permeation chromatography equipped with a triple detection technology. As both quantities decreased dramatically from the beginning of exposure and their ratio M_w/M_n tends toward the asymptotic value of 2 and it was concluded that a “pure” chain scission process operated. Finally, as the number of chain scissions perfectly correlates, the concentration sum of aldehydes and their oxidation products (i.e., carboxylic acids), it was also concluded that these carbonyl groups result exclusively from the β scission of alkoxy radicals. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43287.     

Continuous supercritical decrosslinking extrusion process for recycling of crosslinked polyethylene waste

This work demonstrates that extrusion process can be used as a continuous reactor for the recycling of crosslinked polyethylene (XLPE) via supercritical methanol decrosslinking reaction. The multistage single screw extruder (L/D = 36, D = 40 mm) with methanol injection pump was customized for the continuous supercritical decrosslinking reaction. Reaction temperature was ranged from 360°C to 390°C. The amount of methanol was varied from 0 to 7 mL/min. The extruder provided the crosslinked polymers with the supercritical conditions of methanol during continuous process. The gel content of XLPE decreased with the increase in the reaction temperature and methanol content. Although XLPE experienced supercritical methanol for less than 2 min retention time in the continuous supercritical extruder, it was completely decrosslinked above 390°C at the methanol feeding rate of 7 mL/min. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41442.     

Thermal degradation and aging behavior of polytriazole polyethylene oxide-tetrahydrofuran elastomer based on click-chemistry

Polytriazole polyethylene oxide-tetrahydrofuran (PTPET) elastomer was prepared based on the click-chemistry reaction between the  CCH and  N₃ groups. The PTPET exhibits improved thermal stability with an activation energy of 204 kJ/mol compared with hydroxyl-terminated polyethylene oxide-tetrahydrofuran (PET) elastomer. TG-FTIR and pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) were used to analyze the pyrolysis mechanism and products of the PTPET elastomer. The pyrolytic mechanism of PTPET mainly involves breakage of the polyether chains. The main pyrolytic products were identified. The PTPET underwent for 180 days at 50°C and 60°C. The tensile properties, dynamic mechanical analysis (DMA) and TGA of the aged PTPET were investigated. The tensile stress and strain of the aged PTPET increased until 120 days (at 60°C) or 150 days (at 50°C), which could be attributed to the postcuring of the elastomer. The scission of the PTPET chains due to aging could dominate after aging for 120 days (at 60°C) or for 150 days (at 50°C), resulting in the reduction of mechanical properties.     

1H T2‐NMR monitoring of crosslinked polyolefin aging

This work examines the correlation between the ¹H‐NMR T₂ relaxation constant and the mechanical properties of aged crosslinked polyolefin cable insulation. T₂ experiments on unswollen samples could not differentiate between unaged and highly aged materials; all exhibited ¹H T₂ constants of approximately 0.5 ms. To accentuate the effects of aging, samples were swollen in various solvents. Unaged samples had T₂ values of approximately 15 ms in good solvents. With thermal aging, T₂ values decreased as the ultimate tensile elongation decreased. However, the correlation between T₂ and elongation differed for samples irradiated with high‐energy radiation and for materials aged above versus below the crystalline melting temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2578–2582, 2003     

Assessment of oxidative thermal degradation of crosslinked polyethylene and ethylene propylene rubber cable insulation

This paper presents procedures to monitor oxidation degradation of two commonly used cable insulations, crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR). Since the techniques described require only micro specimens, deterioration of cable insulation can be monitored without destroying the function of the cable. The techniques described are melting point, crystallinity, infrared carbonyl absorbance, gel content, relative hardness, and differential scanning calorimetry (DSC) oxidation induction time. The techniques were applied following accelerated aging over a period in excess of two years. The results from the micro specimens were directly related to embrittlement and the decrease in ultimate elongation. The previous thermal history of XLPE was determined using DSC melting peak analysis.     

Ultraviolet protection of recycled polyethylene terephthalate

Polyethylene terephthalate (PET) is commonly used for food packaging due to its high clarity, high resistance to water vapor, oxygen and carbon dioxide permeation, and good mechanical properties. However, its high transmittance in the ultraviolet‐A (UVA) region leaves food susceptible to UV‐induced degradation reactions. Incorporating post‐consumer recycled (PCR) PET into virgin PET increased UVA absorption 100% when utilizing 100% PCR‐PET, thus, increasing the protection potential of food packaging by reducing UVA‐induced degradation reactions. Comparison of the current data with previous work demonstrated the reproducibility of UVA protection independent of the PC flake source. The thickness‐normalized absorbance at 350 nm was a reliable predictor of the UVA protection potential regardless of composition and manufacture date. Raman, fluorescence, and attenuated total reflectance‐Fourier transform infrared spectroscopy analyses provided support that the UVA absorbing moiety was a quinone derivative formed during degradation reactions that are known to occur during melt processing/re‐processing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45181.     

Photodegradation of thermodegraded polypropylene/high‐impact polystyrene blends: Mechanical properties

The influence of the addition of high‐impact polystyrene (HIPS) on polypropylene (PP) photodegradation was studied with blends obtained by extrusion with and without styrene–butadiene–styrene (SBS) copolymer (10 wt % with respect to the dispersed phase). The concentrations of HIPS ranged from 10 to 30 wt %. The blends and pure materials were exposed for periods of up to 15 weeks of UV irradiation; their mechanical properties (tensile and impact), fracture surface, and melt flow indices were monitored. After 3 weeks of UV exposure, all of the materials presented mechanical properties of the same order of magnitude. However, for times of exposure greater than 3 weeks, an increasing concentration of HIPS resulted in a better photostability of PP. These results were explained in light of morphological observations. This increase of photostability was even greater when SBS was added to the blends. It was more difficult to measure the melt flow index of the binary PP/HIPS blends than that of PP for low concentrations of HIPS; this was most likely due to energy transfer between the blend domains during photodegradation. This phenomenon was not observed for the ternary blends. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Detergent impurity effect on recycled HDPE: Properties after repetitive processing

High density polyethylene (rHDPE) is extruded 1 to 8 times, with and without detergent, to simulate the effects of impurities on the material and on the artificial ageing process. The mechanical properties, thermal stability, rheology, Fourier transform infrared spectroscopy (FTIR), and volatile organic compound (VOC) emissions are measured. According to the results, ageing of rHDPE increases tensile strength, reduces elongation, and enhances side chain branching of the material and thus causes rheological changes. The addition of detergent reduces changes in mechanical properties and rheological behavior but accelerates thermal degradation. VOC and FTIR measurements of the samples with detergent addition show generation of harmful 1,4‐dioxane. The amount of total emission, as well as emissions of important perfumes limonene and 1R‐α‐pinene, decreases during multiple extrusion cycles. Heating of the plastics is found to be a major factor in the VOC emission reduction. Impurities have a notable effect on the artificial ageing results. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43766.     

Thermal aging of undoped polyaniline: Effect of chemical degradation on electrical properties

Aging experiments, with a special emphasis on the atmosphere effect, have been carried out on undoped polyaniline, in its half‐oxidized state, namely emeraldine base (EB). The polymer has been aged under vacuum and in air atmosphere. The chemical degradation has been analyzed by several complementary techniques such as viscosity measurements, FTIR, XPS, and UV‐Vis‐nIR spectroscopy. We show that emeraldine base exhibits two different degradation mechanisms. The first one is intrinsic to the polymer and occurs independently on aging conditions (vacuum or air). It consists of crosslinking via tertiary amine groups created from imine nitrogen via double‐bond breaking. The second mechanism is extrinsic and occurs concomitantly with the first one upon aging in air. It consists of oxygen incorporation in a form of carbonyl groups and chain scission. Both degradation pathways result in a decrease of the electrical conductivity of the polymer due to the lowering of the average conjugation length. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 395–404, 2002     

Thermal aging properties of flame retardant silicone rubber based on melamine cyanurate

In this work, an efficient preparation method for flame retardant silicon rubbers (FRSR) was established with using melamine cyanurate (MCA) as flame retardants. In order to analyze the thermal aging mechanisms and flame retardancy of FRSR, cone calorimetric test (CCT), Fourier transform infrared (FTIR) spectra, scanning electronic microscopy (SEM), and energy dispersive X-ray spectra were performed. Results indicated that the aging time significantly decreases the tensile strength and the elongation at break. SEM images revealed that the porous surface of the aged FRSR provide diffusion path for heat and oxygen, which resulted in a continuously increase in Shore A hardness due to the increase of cross-linking density during the aging process. FTIR spectra further proved that the MCA is well-embedded into samples and cross-linking reaction between free radicals of silicone rubber (SR) and oxygen was occurring during the aging process. Besides, CCT results showed that all FRSR samples before and after thermal aging exhibit excellent flame retardancy with compact and stable char residue layers, effectively hindering the heat transfer and oxygen diffusion. It was expected that our findings could provide important information to fabricate SR with flame retardancy and outstanding long-term thermal-aging resistance.     

Nonlinear influences of process parameters on mechanical properties of physically foamed,fiber-reinforced polypropylene parts

The importance of foam injection molded components in industrial applications increases, above all driven by sustainability concerns. In practice, their applicability almost exclusively depends on their mechanical behavior, which is still difficult to predict based on their microstructure. This work aims to present an approach based upon phenomenological observations. From a processing perspective, the objective is to describe the direct processing-properties-relationship. Therefore, this work focuses on the effects of different processing parameters on selected final mechanical properties of foam injection molded components using glass fiber-reinforced polypropylene. A full factorial, central composite design allows for the detection of nonlinear effects, the application of response surface methodology, and the creation of contour plots. Considering three important process parameters (mold temperature, degree of foaming, delay time) and—for the automotive industry—highly important mechanical properties in bi- and uniaxial bending, the results show a detailed picture of individual dependences, but also two-dimensional interactions between the different process parameters. Improvements of more than 140% in absorbed energy and flexural stiffness were obtained at constant part weight. Modulus and strength were increased by 37 and 44%, respectively.     

Degradation and biodegradation of polyethylene with pro‐oxidant aditives under compost conditions establishing relationships between physicochemical and rheological parameters

In the present work, an analysis is carried out to provide a relationship between the Molecular Weight (M_w) of degraded LDPE films (containing Mn stearate as pro oxidant (MnSt‐LDPE) and changes in viscosity, elongation at break (EB %) and carbonyl index (CI) occurring during thermal degradation in the thermophilic phase of the compost process. The thermal treatment comprised various temperatures (50°C, 60°C, and 70°C) and exposure times, and was characterized through a so‐called Energy‐Time Factor (the product of thermal energy and exposure time). Changes in viscosity, EB %, and CI were correlated to this factor. A modified Mark‐Houwink equation was used to relate the zero shear‐rate viscosity and M_w of the degraded LDPE films. Results indicate that the EB %, M_w and viscosity decrease simultaneously with an increase in the CI as the Energy‐Time Factor augments, allowing the assessment of the variation of these properties with M_w. Calculations of the percentage abiotic degradation (%D) of LDPE films indicate that a M_w of 6 kg mol^?1 corresponds to a maximum abiotic degradation degree of 91.85%, which is henceforth susceptible to biodegradation. The film treated with Energy‐Time Factor of 2.79E+09 J s mol^?1 reached a 74% of biodegradation in 90 days (average time of the composting process). Results exhibit clearly the correlation between abiotic and biotic degradation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42721.     

Effects of crosslinking densities on mechanical properties of nitrile rubber composites in thermal oxidative aging environment

The effects of crosslinking densities on mechanical properties of nitrile rubber (NBR) composites before and after thermal oxidative aging are investigated. Tensile strengths of NBR composites are enhanced slightly at the initial aging stage, attributing to moderate increment of crosslinking densities. Continuous decrease with further aging is followed, resulting from over-crosslinking and uneven distribution of crosslinking densities. The digital image correlation method is explored for large-strain deformation measurement by displacement accumulation and establishing strain model, being promising in practical age detection and measurement of mechanical properties in complex environments. Compression sets are gradually increased with aging because of the destruction of the original crosslinking structures. The molecular chains fracture inhibits elastic recoveries of compressive NBR composites and results in residual deformation in thermo-oxidative and compressive environment after unloading. This study provides new ideas for exploring mechanical properties of rubber-based composites before and after thermal oxidative aging.