Correlation between mechanical and electrical properties for assessing the degradation of ethylene propylene rubber cables used in nuclear power plants

The correlation among the indenter modulus, the elongation-at-break, and the breakdown-voltage strength for the ethylene propylene rubber (EPR) cables subjected to long-term thermal aging and high-temperature water submerging conditions were investigated in this work. The mechanical and electrical limits of EPR insulation material were evaluated by using an indenter modulus, with non-destructive advantage, to monitor the cable condition and to assess its remaining life. Results show that it is plausible to use indenter modulus to monitor the EPR cables for the condition approaching the end of cable service life. Testing parameters on indenter modulus, including indenter diameter, penetration speed, and penetration depth, were studied for monitoring cable degradation condition. Finally, this paper discusses the cable acceptance criteria under thermal and moisture-related aging environments. Results show that the breakdown-voltage strength is a better acceptance criterion for assessing the cable aging condition in a wet environment. While, if the cable is located in a high temperature environment, the elongation-at-break is a better criterion.     

Investigation on water treeing behaviors of thermally aged XLPE cable insulation

The power cable insulation is in permanence subjected to thermal aging during its operating service. Thermal aging may influence not only the electrical, physicochemical and other properties of the XLPE cable insulation, but also the initiation and propagation of water tree inside it. Our research on the influence of thermal degradation to the water treeing behavior of XLPE cable insulation shows that thermal oxidation is the most influential to the initiation and growth of water treeing from the surface of XLPE cable insulation among all the probable factors caused during thermal aging.     

Comparison of antioxidants for combined radiation and thermal aging and superposition of radiation and thermal aging for EPR and XLPE

Thermal and combined thermal and radiation aging of low voltage EPR and XLPE cable insulation with Agerite MA antioxidant and with the ZMTI/Aminox antioxidant system was examined to compare the relative effectiveness of the antioxidant and polymer systems. All provided significant stability with no clear choice of any particular combination being superior to the others. A comparison of degradation from thermal and radiation aging with degradation from combined thermal/radiation aging showed that the damage from the individual aging effects was superposable. This indicates that synergistic effects have little importance for the EPRs and XLPEs tested under the aging conditions observed.     

Lifetime predictions of EPR materials using the Wear-out approach

The Wear-out approach for lifetime prediction, based on cumulative damage concepts, is applied to several ethylene propylene rubber (EPR) cable insulation materials. EPR materials typically follow “induction-time” behavior in which their material properties change very slowly until just before failure, precluding the use of such time-dependent properties to predict failure. In the Wear-out approach, a material that has been aged at its ambient aging temperature T_a or at a low accelerated aging temperature is subsequently aged at a higher “Wear-out” temperature T_w in order to cause the material to reach its “failure” condition. In the simplest case, which involves the same chemical processes underlying degradation at T_a and T_w, a linear relationship is predicted between the time spent at T_a and the time required at T_w to complete the degradation. Data consistent with this expectation are presented for one of the EPR insulation materials. When the degradation chemistry at the two temperatures is different, a linear relationship between the time spent at T_a and the time required at T_w to complete the degradation is not generally expected. Even so, the Wear-out results for a second EPR material, which has evidence of changing chemistry, are reasonably linear and therefore useful from a predictive point-of-view. The Wear-out approach can therefore be used to transform non-predictive time-dependent material property results into predictive lifetime estimates. As a final example, the Wear-out approach is applied to an EPR insulation that had been aged in a nuclear power plant environment (∼51 °C) for times up to 23 years to show its likely viability for the hundreds of years predicted at this aging temperature from accelerated aging tests on EPR insulation materials.     

Electrical–thermal aging characteristic research of polymer materials by infrared spectroscopy,

Traditional insulation paper (pressboard) used in power transformers has weaknesses such as poor thermostability, low breakdown voltage, and high permittivity, which leads to its degradation or even breakdown over time. For this paper, in order to judge if they could be used as new insulation materials for transformers, polycarbonate and polyester films were selected for electrical–thermal aging tests in temperatures of 110°C and 130°C in comparison with the insulation paper. Several infrared spectral tests were carried out on the three materials under a scanning electron microscope to analyze their electrical–thermal aging characteristics, mechanical behaviors, and degrees of polymerization. The functional group whose absorption peak intensity decreased drastically with aging was referred to as the characteristic functional group, and its peak absorption intensity was used to reflect its aging level. This paper found that the polycarbonate had a better aging resistance than both the insulation paper and PET. Copyright © 2014 John Wiley & Sons, Ltd.     

Long-term performance of poly(vinyl chloride) cables. Part 1: Mechanical and electrical performances

Cables insulated with plasticized poly(vinyl chloride) were aged in air at temperatures between 80 °C and 180 °C and their conditions were assessed by indenter modulus measurements, tensile testing, infrared (IR) spectroscopy and differential scanning calorimetry (DSC). Electrical testing of oven-aged cable samples was performed in order to relate the electrical functionality during a high-energy line break (HELB) to the mechanical properties and to establish a lifetime criterion. The mechanical data taken at room temperature after ageing could be superimposed with regard to ageing time and temperature. The ageing-temperature shift factor showed an Arrhenius temperature dependence. The jacketing material showed an immediate increase in stiffness (indenter modulus and Young's modulus) and a decrease in the strain at break on ageing; these changes were dominated by loss of plasticizer by migration which was confirmed by IR spectroscopy and DSC. The core insulation showed smaller changes in these mechanical parameters; the loss of plasticizer by migration was greatly retarded by the closed environment, according to data obtained by IR spectroscopy and DSC, and the changes in the mechanical parameters were due to chemical degradation (dehydrochlorination). A comparison of data obtained from this study and data from other studies indicates that extrapolation of data for the jacketing insulation can be performed according to the Arrhenius equation even down to service temperatures (20-50 °C). The low-temperature deterioration of the jacketing is, according to this scheme, dominated by loss of plasticizer by migration.     

Ageing Monitoring of Plastics Used in Nuclear Power Plants by DSC

Degradation of polymeric materials used in nuclear power plants (NPP), especially polymeric cable insulation materials, in the course of their service can be monitored by measuring their properties by DSC, mainly oxidative induction time — OIT. The studied materials were in-laboratory aged by applying main stressors that act in NPP — ionising radiation and temperature. The dependence of OIT on radiation and thermal degradation of polymeric material was determined. The OIT values have been compared to elongation at break as a property that directly reflects the functionality of the studied material. The comparison of monitored OIT of real cable samples taken from NPP with dependencies on how the OIT values change with the elongation at break, makes possible to establish the extent of cable degradation. This method can be considered as a suitable and effective technique for lifetime assessment not only of cable insulations but also of many other plastics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nanostructural evidence of mechanical aging and performance loss in ballistic fibers

It is understood that the ballistic resistance of aromatic polyamide fibers is related to the fiber's ultimate tensile strength, strain‐to‐failure, and Young's modulus. Ideal high‐performance ballistic materials maximize these properties while minimizing material density. Equally important is long‐term mechanical and chemical stability: the fibers should not exhibit performance loss over their lifetime. However, less is known quantitatively about their modes of degradation, and experimental methods to quantify the aging and degradation in these fibers are critical. Multiple variations of next generation high‐performance fibers have been investigated under chemical and mechanical accelerated aging conditions. Performance losses have been empirically correlated to chemical degradation of the polymer chain and nanostructural changes in the fiber morphology through X‐ray photoelectron spectroscopy (XPS). Here, we introduce positron annihilation lifetime spectroscopy measurements as a sensitive method to quantify the early onset of damage in the flexed fibers as quantified through changes in the nanoscale void structure in the material. Published 2017.^? J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1711–1717     

A kinetic model for predicting oxidative degradation rates in combined radiation-thermal environments

The complex degradation behavior of a poly(vinyl chloride) (PVC) cable jacketing material in combined radiation/temperature/air aging environments is experimentally separated into two dominant radiation dose-rate effect mechanisms. The first, operative at high dose rates, involves diffusion-limited oxidation, which leads to heterogeneously oxidized samples. The second, important at low dose rates, involves thermally-induced breakdown of intermediate peroxides. In the homogeneous degradation regime, a theoretical kinetic model is derived which, based on experimental evidence, assumes unimolecular termination kinetics and rate-determining, hydroperoxide-mediated branching reactions. Dependent upon the ratio of particular rate constants, the model predicts that dose-rate effects will either continue to increase or eventually disappear as the dose rate is lowered. Theoretical analysis of sequential (radiation followed by temperature exposures) aging experiments allows a time–temperature–dose rate shifting procedure to be developed. Using this procedure, higher temperature combined environment results can be shifted to a lower reference temperature, thereby extending the lower temperature results to lower (and experimentally inaccessible) dose rates. By applying this procedure to experimental PVC data, evidence in support of the theoretical model is obtained. In addition, model predictions are shown to agree with 12-year real-time aging results.     

Morphological study of aging phenomena in XLPE by TEM technique

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The TEM technique has been used to study the XLPE morphology of unaged, laboratory-aged, and field-aged cable samples. A suitable image contrast enhancing and a stabilization of radiation damage were both successfully achieved by staining the sections with ruthenium tetroxide (RuO₄), as individual XLPE lamellae were neatly and reproducibly resolved. Image analysis was used to help in the determination of any aging-induced morphological changes. XLPE samples from an unaged cable (D1) has been exposed to a low-energy electron beam, in an attempt to simulate certain conditions thought to occur in electric field-stressed dielectrics. © 1996 John Wiley & Sons, Inc.     

Lifetime predictions for semi-crystalline cable insulation materials: I. Mechanical properties and oxygen consumption measurements on EPR materials

Long-term accelerated aging studies (up to 7 years of aging) were conducted on four typical EPR materials used as cable insulation in nuclear power plant safety applications with the goal of establishing lifetime estimates at typical aging conditions of ∼50 °C. The four materials showed slow to moderate changes in mechanical properties (tensile elongation) until just before failure where abrupt changes occurred (so-called “induction-time” behavior). Time-temperature superposition was applied to derive shift factors and probe for Arrhenius behavior. Three of the materials showed reasonable time-temperature superposition with the empirically derived shift factors yielding an approximate Arrhenius dependence on temperature. Since the elongation results for the fourth material could not be successfully superposed, consistency with Arrhenius assumptions was impossible. For this material the early part of the mechanical degradation appeared to have an Arrhenius activation energy E_a of ∼100 kJ/mol (24 kcal/mol) whereas the post-induction degradation data had an E_a of ∼128 kJ/mol. Oxygen consumption measurements were used to confirm the 100 kJ/mol E_a found from early-time elongation results and to show that the chemistry responsible before the induction time is likely to remain unchanged down to 50 °C. Reasonable extrapolations of the induction-time results indicated 50 °C lifetimes exceeding 300 years for all four materials.     

Research on mechanical and dielectric properties of XLPE cable under accelerated electrical‐thermal aging

Research into the electrical‐thermal aging properties of cross‐linked polyethylene (XLPE) cable has great significance, because of its wide application. This study conducted accelerated electrical‐thermal aging tests on 10‐kV XLPE cable in order to assess the cable's mechanical and dielectric properties. After being aged by applying 34.8‐kV AC voltage at the four temperatures of 90, 103, 114, and 135°C, the cable samples were taken out in five stages according to the aging time and cut into slices. The slices were conducted experiments to test the breaking elongation, tensile strength, gel content, breakdown voltage, and frequency spectrums of the dielectric constant and dielectric loss. The results demonstrate that the mechanical strength and gel content of XLPE vary greatly under different aging temperatures, a finding that is associated with the crystallization characteristics of the material. The breakdown voltage shows a slight decreasing trend with aging time. The dielectric constant decreases with aging time in high‐frequency areas (10³–10⁶ Hz), while the dielectric loss factor increases with aging time at low frequencies (10^?2–0 Hz). These two parameters can be used to characterize the degree of aging in cable. Copyright © 2016 John Wiley & Sons, Ltd.     

Moisture–absorption,dielectric relaxation,and thermal conductivity studies of polymer composites

In the search for new packaging materials for the electrical/electronics industry, three types of polymer composites have been studied. Silicone/boron nitride powders, polyurethane/alumina powders, and polyurethane/carbon fibers have all been synthesized to study the moisture–absorption kinetics, thermal conductivities, and the dielectric loss spectra under various levels of humidity. The water uptake data indicate that water molecules are absorbed not only by the polymer matrix, but also by the interfaces introduced by the fillers. For all materials, the dielectric relaxation spectroscopy shows the presence of a peak in the 175–200 K range, which is largely due to absorbed water. The silicone/boron nitride samples absorbed the least amount of moisture. Incorporating this result with the thermal conductivity data of the three types of polymer composites, it is concluded that silicone polymers embedded with boron nitride can best serve as the coating for the electronic devices that require heat dissipation and moisture resistance, in addition to electrical insulation. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2259–2265, 1998     

Electron spin resonance spectral study of PVC and XLPE insulation materials and their life time analysis

Electron spin resonance (ESR) study is carried out to characterize thermal endurance of insulating materials used in power cable industry. The presented work provides ESR investigation and evaluation of widely used cable insulation materials, namely polyvinyl chloride (PVC) and cross-linked polyethylene (XLPE). The results confirm the fact that PVC is rapidly degrades than XLPE. The study also indicates that colorants and cable's manufacturing processes enhance the thermal resistance of the PVC. It also verifies the powerfulness and the importance of the ESR-testing of insulation materials compared to other tests assumed by International Electrotechnical Commission (IEC) Standard 216-procedure, e.g. weight loss (WL), electric strength (ES) or tensile strength (TS). The estimated thermal endurance parameters by ESR-method show that the other standard methods overestimate these parameters and produce less accurate thermal life time curves of cable insulation materials.     

Condition monitoring methods applied to degradation of chlorosulfonated polyethylene cable jacketing materials

Three promising polymer material condition monitoring (CM) methods were applied to eight commercial chlorosulfonated polyethylene cable jacket materials aged under both elevated temperature and high-energy radiation conditions. The CM methods examined, cross-sectional modulus profiling, solvent uptake and NMR T₂ relaxation time measurements of solvent-swelled samples, are closely related since they are all strongly influenced by the changes in overall crosslink density of the materials. Each approach was found to correlate well with ultimate tensile elongation measurements, the most widely used method for following degradation of elastomeric materials. In addition approximately universal failure criteria were found to be applicable for the modulus profiling and solvent uptake measurements, independent of the CSPE material examined and its degradation environment. For an arbitrarily assumed elongation “failure” criterion of 50% absolute, the CSPE materials typically reached “failure” when the modulus increased to ∼35 MPa and the uptake factor in p-xylene decreased to ∼1.6.     

Thermal degradation of PVC cable insulation studied by simultaneous TG-FTIR and TG-EGA methods

Thermogravimetry (TG/DTG) coupled with evolved gas analysis (MS detection) of volatiles was used to characterize the thermal behavior of commercial PVC cable insulation material during heating in the range 20-800°C in air and nitrogen, respectively. In addition, simultaneous TG/FTIR was used to elucidate chemical processes that caused the thermal degradation of the sample. A good agreement between results of the methods was found. The thermal degradation of the sample took place in three temperature ranges, namely 200-340, 360-530 and 530-770°C. The degradation of PVC backbone started in the range 200-340°C accompanied by the release of HCl, H₂O, CO₂ and benzene. The non-isothermal kinetics of thermal degradation of the PVC cable insulation in the temperature range 200-340°C was determined from TG results measured at heating rates of 1.5, 5, 10, 15 and 20 K min^-1 in nitrogen and air, respectively. The activation energy values of the thermal degradation process in the range 200-340°C of the PVC cable insulation sample were determined from TG results by ASTM method. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nano-free volume characterization by positron annihilation lifetime technique in flame-retardant poly (vinyl chloride) after thermal treatment

The flammability tests are performed on flame-retardant poly (vinyl chloride) (FRPVC) material that has been used in cable insulation and jacketing construction for multi-purpose reactor (MPR) at Atomic Energy Authority of Egypt, as well as carbon-black FRPVC (CB-FRPVC) material produced by Egyptian Electrical Cable Company (EECC).The temperature variation of thermal conductivity, thermal expansion coefficients, and nano-size free volumes by means of positron annihilation lifetime (PAL) technique are determined. Correlation of positron annihilation and thermal conductivity has been discussed in terms of phonons as the main heat carriers.     

The hydrolytic effect of moisture and hygrothermal aging on poly(butylene succinate)/organo-montmorillonite nanocomposites

The study of hydrolysis on biodegradable poly(butylene succinate) (PBS) is essential to predict the materials properties in a humid environment. In this study, PBS nanocomposites were exposed to different conditions of relative humidity (RH) and temperature. The moisture uptake increased with organo-montmorillonite (OMMT) loading and the RH of the testing environment. The exposure of PBS and the nanocomposites to a humid environment caused changes in the mechanical properties. The hydrolytic degradation becomes more pronounced upon hygrothermal aging at high temperature, whereby premature failure occurred. PBS nanocomposites were found to exhibit a better hydrolytic stability than neat PBS. The degradation was evaluated through Fourier transform infrared (FTIR) spectroscopy and gel permeation chromatography (GPC). A drastic reduction in the molecular weight of PBS has revealed the occurrence of degradation after exposure to moisture and heat. This has led to an alteration of the thermal behavior as investigated using differential scanning calorimetry (DSC).     

Thermal lifetime of cellulose insulation material evaluated by an activation energy based method

A rapid method, based on a logarithmic degradation model of insulation material, is proposed to reduce the test duration in lifetime assessment of cellulose paper insulating materials. This method proposes the determination of the activation energy from a non-isothermal measurement made by differential scanning calorimetry or another thermal analysis technique and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak at ca. 300 °C is proposed for evaluation of the activation energy of degradation. For comparison, the thermal aging of Kraft cellulose paper for power transformer insulation was performed according to the general standard IEC 60216-1/2001 at three different temperatures: 155, 135 and 115 °C, and subsequently, the lifetimes at different service temperatures were estimated. The experimental data proved to have good agreement between the applied methods, the differences being <10 % in terms of the estimated lifetime across the range of service temperatures. The novel proposed method is effective in terms of both energy and manpower costs as compared to the current method: a factor of around 10 in the case of reducing the aging time, a factor of 3 for the time needed for measurements, and a factor of 10 for the reduction of power intake.     

Temperature and humidity aging of poly(p-phenylene-2,6-benzobisoxazole) fibers: Chemical and physical characterization

In recent years, poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers have become prominent in high strength applications such as body armor, ropes and cables, and recreational equipment. The objectives of this study were to expose woven PBO body armor panels to elevated temperature and moisture, and to analyze the chemical, morphological and mechanical changes in PBO yarns extracted from the panels. A 30% decrease in yarn tensile strength, which was correlated to changes in the infrared peak absorbance of key functional groups in the PBO structure, was observed during the 26 week elevated temperature/elevated moisture aging period. Substantial changes in chemical structure were observed via infrared spectroscopy, as well as changes in polymer morphology using microscopy and neutron scattering. When the panels were removed to an ultra-dry environment for storage for 47 weeks, no further decreases in tensile strength degradation were observed. In a follow-on study, fibers were sealed in argon-filled glass tubes and exposed to elevated temperature; less than a 4% decrease in tensile strength was observed after 30 weeks, demonstrating that moisture is a key factor in the degradation of these fibers.