Ethylene Propylene Diene Monomer Insulators Research Articles

Ablation resistance of ethylene propylene diene monomer insulation materials reinforced with liquid hyperbranched polycarbosilane coated aramid fibers

As a type of filler with excellent performance, fibers can effectively enhance the mechanical properties and ablation resistance of the EPDM insulation layer inside a solid rocket motor. However, an excessive amount of short-cut fibers can reduce the mechanical performance of the insulation layer. Therefore, in this work, a novel EPDM insulation material was reinforced by aramid fiber (AF) coated with liquid hyperbranched polycarbosilane (VHPCS) to improve the mechanical properties and ablation resistance of obtained composites by enhancing the interfacial properties between EPDM and AF as well as protecting the AF framework by in-situ transformation into a ceramic structure during ablation. The test results demonstrated a significant improvement in the ablation performance of the AF insulation materials coated with VHPCS. The linear ablation rate and mass ablation rate decreased by 22.67 and 11.67%, respectively. Furthermore, compared with the composites without VHPCS, the elongation at break was improved while maintaining the original tensile strength (60.60%). XRD and XPS analysis confirmed the presence of SiC formation in the carbon layer during ablation. Thermal analysis indicated that VHPCS increased the mass residual rate of the EPDM composites. Coating AF with VHPCS-modified EPDM increased the content of SiC structures on the carbon layer surface, effectively protecting the carbon layer structure and enhancing the ablation performance of the ceramicizable insulation.

Effects of Thermal and Electric Field Distribution on Tree Growth in EPDM for HVDC Cable Accessories

In this article, electrical tree growth characteristics of ethylene propylene diene monomer (EPDM) are studied under combined dc and ac voltages with different temperature gradients (Ts). The temperature difference between needle tip and ground electrodes varies from −90 °C to 90 °C, forming a ΔT within EPDM insulation. Variance is used as a new method to indicate the overall length of the electrical tree. Experimental results show that ΔT can accelerate the growth of electrical tree. Electrical tree tends to grow toward the ground electrode under negative ΔT, while it distributes around needle electrode under positive ΔT. Specific electric field distribution, determined by dc conductivity, is employed to explain the treeing process under different ΔTs. It is concluded that temperature-dependent conductivity results in weaker electric field near needle electrode and a quicker decreasing speed of electric field as it goes ahead to ground electrode under positive ΔT, thus leading to the electrical trees being more concentrated to the needle tip.

Effects of MWCNTs on Char Layer Structure and Physicochemical Reaction in Ethylene Propylene Diene Monomer Insulators.

As one of the most promising ablative fillers, multi-walled carbon nanotubes (MWCNTs) have been used to improve the ablative resistance of Ethylene–Propylene–Diene Monomer (EPDM) insulators by facilitating the carbothermal reduction reaction of silica. However, the contribution of MWCNTs to char layer structure of the insulators was unclear. In this work, the effects of MWCNTs on char layer structure and ablative resistance were investigated in different EPDM-based insulators with and without silica. The results showed that adding only 3 phr MWCNTs can reduce the linear ablation rate of EPDM-based insulators without silica by 31.7%, while 6 phr MWCNTs are required to obtain similar results in EPDM-based insulators with silica. The char layer morphology of the two insulators gradually evolved into a dense porous structure as MWCNTs content increased, but their formation mechanisms were different. The XRD and Raman spectrum showed that different physicochemical reactions occurred around MWCNTs under different charring components. The proposed ablation mechanism was further verified by designing alternating multilayer distribution of MWCNTs and silica. This work can guide the construction of desirable char layer structure for increasing the ablative resistance of EPDM-based insulators.

Synergistic effect of polyphosphazene nanotubes and graphene on enhancing ablative resistance of ethylene propylene diene monomer insulation composites

AbstractMany advanced fillers were used in ethylene propylene diene monomer (EPDM) insulation composites to enhance the ablative resistance by strengthening char layer. However, the structures of fillers and char layer were usually neglected. In this work, a new type of cross‐linked network‐like polyphosphazene nanotubes (PNTs) was added in EPDM composites in combination with graphene. The results showed that the ablation performance of EPDM composites was improved by 21.6% with the synergistic effect of PNTs and graphene. Moreover, the ablation mechanism was studied by analyzing ablated char layer using X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and Raman spectroscopy. PNTs served as skeleton to build the cross‐linked network char layer and graphene played a role in enhancing the properties of char layer. The findings of this work emphasize the synergistic effect of structures and properties of fillers when performing ablative modification, which provides a new idea to enhance the ablation properties of EPDM composites.

Thermal insulation performance and ablation resistance of ethylene propylene diene monomer insulation materials reinforced with polydopamine-coated carbon nanotubes

Filler reinforced ethylene propylene diene monomer (EPDM) rubber was widely used as insulation material for solid rocket motors (SRMs). Carbon nanotubes (CNTs) were employed fillers owing to their excellent physicochemical properties. However, the addition of CNTs to EPDM reduces its thermal insulation performance. To solve this problem, polydopamine-coated CNTs (CNTs @ PDA) were prepared by in-situ polymerization and mixed with EPDM through mechanical compounding. The effects of CNTs @ PDA on the thermal properties and ablation resistance of the resulting CNTs @ PDA/EPDM were investigated. The results indicate that the addition of CNTs @ PDA can reduce the thermal conductivity of the insulation material and the maximum back temperature during oxyacetylene ablation test. The ablation rate of the insulation material decreased as the amount of PDA used to coat on the CNTs increased, compared with the uncoated CNTs (10-CNTs) formulation, the PDA-2 formulation reduced the linear ablation rate by 62 % to 0.01 mm/s. Furthermore, the micro-morphology of the char layer was observed by SEM, combined with the compression resistance of the char layer to explain the effect on the ablation performance of the insulation material. The mechanical performance test results also reveal that the CNTs @ PDA formulation has higher tensile strength.

Erosion resistance of ethylene propylene diene monomer insulations reinforced with precoated multi-walled carbon nanotubes

Condensed particles in the internal flow field of a solid rocket motor can cause serious erosion to thermal insulations under overload flight condition. Multi-walled carbon nanotubes (MWCNTs) can improve erosion resistance of insulations but at the cost of reduced thermal insulation performance. In this study, MWCNTs were precoated with pyrolytic carbon via chemical vapor deposition to reduce their thermal conductivity. Further, ethylene propylene diene monomer insulations reinforced with different contents of MWCNTs and precoated MWCNTs were prepared. Moreover, their ablation performances under simulated overload ablation conditions were tested and compared. Experimental results show that precoated MWCNTs can not only improve thermal insulation performance but also significantly increase particle erosion resistance of ethylene propylene diene monomer insulation materials. Charring ablation rate of the formula with 10phr precoated MWCNTs is 44.3% lower than that of basic formula and 41.6% lower than that of the formula with 3phr MWCNTs which has the same pure MWCNTs content. The char layers of MWCNTs reinforced formulas are more complete than basic formula, indicating that MWCNTs can improve char layer structure under particle erosion condition. The precoated MWCNTs can also promote the CVD reaction of pyrolysis gas in char layers during ablation, decreasing the porosity of char layers, which can increase the particle erosion resistance of insulation.

Mechanism Characterization and Nondestructive Inspection Method of Thermal Degradation Faults in EPDM Cable Termination

There is a significant positive proportional correlation between the condition of ethylene propylene diene monomer (EPDM) insulation and partial discharge(PD) characteristics in electric multiple units (EMU) cable termination. Especially for the cable terminations with long service life, the aging effect of EPDM insulation material is significant due to long-term high-current load. In this paper, electrical deterioration and PD development of EPDM insulation under long-term thermal degradation is conducted. During the test process, the phase-resolved partial discharge (PRPD) analysis, partial discharge inception voltage (PDIV), physicochemical properties, and flashover voltage of EPDM samples with different thermal degradation are measured. The relationship between the thermal degradation extent of EPDM insulation and PD characteristics is also proposed. The results show that electrical deterioration of the aged sample can be divided into four gradations by PD amplitude and pulse repetition rate. In the four gradations, the shape of the PRPD spectrum in every gradation changes significantly with different thermal degradations. Surface morphologies of EPDM insulation show different features such as gully areas and white particles. The results of the Fourier transform infrared spectroscopy(FTIR) test indicate that the increase of oxygen-containing groups and free radicals are the key reasons for the rapid PD development process of cable insulation under different electrical gradations and thermal-oxygen degradations.

Ablation Model Based on Porous Charring Layer Under Alumina Erosion Condition

The porous medium with a carbide layer and the mechanical denudation accompanied by the reaction process make the study of an ablation model more complex. The chemical vapor deposition and thermal chemical ablation processes of pyrolysis gas and fuel gas in the porous structure of a carbonized layer and the erosion process of a fuel gas and particles on a carbonized layer were studied. A thermal–mechanical coupled ablation model of ethylene propylene diene monomer (EPDM) based on the complex physicochemical process of the porous structure of a carbonized layer was established. An Arrhenius formula was used to calculate the carbon deposition and consumption caused by thermochemical reactions and the porosity evolution in the carbon layer. Based on the theory of elastic–plastic indentation fracture, a particle erosion model of the carbon layer was established. The coupling of particle erosion and chemical erosion was completed and applied to the EPDM insulation prediction. The numerical calculation of an EPDM insulation material showed that the maximum error between the experimental results and the numerical results is less than 12%, and the accuracy of the particle erosion model is acceptable. Therefore, the model proposed can be used to predict the erosion rate of an EPDM in a solid rocket motor.

Effect of multi-walled carbon nanotubes on thermal stability and ablation properties of EPDM insulation materials for solid rocket motors

Multi-walled carbon nanotubes (MWCNTs) have excellent physicochemical properties and can be used as fillers to improve the performance of insulation materials. However, there is currently no proper understanding of the mechanism by which MWCNTs improve the ablation performance of insulation materials. Here, we prepare two formulations of ethylene propylene diene monomer (EPDM) insulation materials with and without MWCNTs and study their thermal stability and ablation characteristics via thermogravimetric analysis, high temperature char layer residue experiments, oxyacetylene ablation tests, and other analytical methods along with scanning electron microscopy and energy dispersive X-ray spectroscopy. The intrinsic mechanism by which MWCNTs improve the ablation performance of insulation materials is revealed from the perspectives of char layer formation, consumption, and micro-morphology. The addition of MWCNTs increased the carbon residue rate after thermal decomposition and can suppress the consumption reaction of the char layer under high-temperature conditions. Microscopic morphological analyses show that the insulation material containing MWCNTs has a denser char layer surface and a network-like structure inside, which improve the ability of the char layer to resist erosion by gas flow. The combination of these factors improves the ablation performance of EPDM. Our findings regarding the MWCNT reinforcement mechanism can guide the design of new insulation materials.

EPDM-based heat-shielding materials modified by hybrid elastomers of silicone or polyphosphazene

The erosion resistances of ethylene propylene diene monomer (EPDM) insulations are often inadequate for advanced solid rocket motor (SRM) applications. EPDM modification by blending secondary matrixes is a feasible approach to improve the ablative properties of EPDM insulations. The addition of flexible inorganic hybrid rubbers as a secondary matrix, such as silicones and polyphosphazenes, may impart EPDM insulations with better ablative performance. The blends of EPDM/hybrid rubbers represent the state-of-the-art heat-shielding materials for SRM. In the present work, methyl-phenyl silicone/EPDM and poly(diaryloxyphosphazene)/EPDM insulation systems with various blending ratios of secondary matrixes have been prepared. The ablative properties of the insulations were examined by oxy-acetylene ablation tests, and the results showed that these properties could be enhanced accordingly by blending with hybrid rubbers under appropriate proportions. The unique charred layers resulting from the hybrid rubbers contributed to their excellent ablation properties. For example, the silicone/EPDM insulations exhibited a more significant improvement of ablation resistance properties. With a 1:1 blending ratio of silicone/EPDM, the linear ablation rate was 0.06 mm s−1 after 20 s of oxy-acetylene ablation. The enhancement in the ablative resistance was attributed to the charred layers with bunches of embedded compact microtubes with a length of 2–3 mm, which consisted of silicon carbide, silicon dioxide, and Si–O–C ceramics.

Migrating simulation of novel high-energy SMX-based propellants based on molecular dynamics

Three 2,3-bis(hydroxymethyl)-2,3-dinitro-1,4-butanediol tetranitrate (SMX)–based propellants were firstly reported, then the specific impulses of SMX-based propellants were calculated by the Energy Calculation Star program. Meanwhile, the migration property of the plasticizers and SMX was investigated by molecular dynamic method, and the main results as follows: the theoretical specific impulses of three SMX-based propellants all overpass 280 s, which suggests that they have the potential to be high-energy propellants. The migrating property of plasticizers in SMX-based propellants and ethylene propylene diene monomer (EPDM) insulation all decrease in the order Bu-NENA> BTTN> TMETN. Meanwhile, the plasticizers much easier migrate in EPDM insulation than in SMX-based propellants, and TMETN is significantly more difficult to migrate than the other. The glass transition temperatures (Tg) of GAP/Bu-NENA/Al/SMX, GAP/BTTN/Al/SMX, and GAP/TMETN/Al/SMX systems are 282.3 K, 278.1 K, and 287.6 K, respectively. Due to lower Tg of EPDM, the EPDM/plasticizer systems have no obvious glass transition between 233 and 323 K. The SMX is almost more difficult to migrate than plasticizers in SMX-based propellants while temperature is above 273 K, whereas it is contrary under 273 K.

New mathematical models for predicting the lifetime of EPDM insulators: Effect of elongation at break on the kinetic degradation of EPDM insulators subjected to thermo-oxidation

Ethylene propylene diene monomer (EPDM) is an important polymer extensively exploited in plasturgy. However, relatively few studies have been carried out to predict the lifetime of EPDM in different climatic conditions particularly, thermo-oxidation. Based on this realization, the aim of the present work was to develop mathematical models for predicting the lifetime of EPDM elastomers, used for insulation of electric cables. The kinetic degradation of EPDM insulators, by monitoring change in a physical property (elongation at break test “ℇr”), was studied by following its thermo-oxidative aging (70, 90, 110 and 130 ° C in air circulating oven). The multiple linear regression analysis (MLRA), solved by the Cholesky method, was the mathematical approach developed in the modeling of the kinetic degradation. In this study, we used two insulators materials when the first insulator contained an amorphous EPDM and the second contained a semi-crystalline EPDM. The results showed that the polynomial models developed to predict elongation at break were reliable for both insulators under thermo-oxidation. The half-life times predicted by the mathematical models was found to be statistically significant (p< 0.05). In conclusion, the mathematical models developed in our study could be used confidently to predict the lifetime of EPDM elastomers.

Rate-dependent compressive behavior of EPDM insulation: Experimental and constitutive analysis

To fully characterize the compressive behavior of ethylene propylene diene monomer (EPDM), quasi-static and dynamic compression experiments were conducted for EPDM with a universal testing machine and split Hopkinson bar (SHPB), respectively. Dynamic experimental data employing the two-wave method were used to obtain the stress–strain curves, and the dynamic results were validated. The polymer was compressed under different strain rates varying from 10−4 to 103. The results indicate that the mechanical properties of EPDM insulation are obviously dependent on the strain rate. An improved Zhu–Wang–Tang (ZWT) nonlinear visco-hyperelastic constitutive model, in which the standard elastic component was replaced by the Mooney–Rivlin hyperelastic model, was developed to predict the mechanical behaviors of EPDM under quasi-static and dynamic impact loading. The model parameters were determined by fitting the model to the experimentally obtained curves by means of the least squares method. The stress–strain curves obtained from the improved ZWT model show good agreement with the experimental curves. This research will further improve the design of the internal insulation structure in solid rocket motors.

Impact of weathering on flashover performance of nonceramic insulators

Nonceramic (composite) insulators that were in service on 69-230 kV transmission lines for periods up to 25 years were evaluated in the laboratory with the objective of assessing future performance of similar devices in the field. These insulators had housings made from different formulations of silicone rubber and ethylene propylene diene monomer (EPDM) rubber. They were removed from relatively clean locations where weathering was probably the most dominant aging factor. Some EPDM insulators had flashed over during service. Using a combination of scanning electron microscopy (SEM), electrical surface resistance under wet conditions, and a theoretical model for contamination flashover, it has been shown that it is possible to predict the flashover performance in service, based on the thickness of the degraded surface. A threshold value has been established beyond which flashover is likely to occur in the field. This information can be used for planning insulator replacement or maintenance.

In-service aging and degradation of 345 kV EPDM transmission line insulators in a coastal environment

Aging and degradation of 345 kV ethylene propylene diene monomer (EPDM) transmission line insulators removed from service is presented. These suspension type insulators were installed in a New Hampshire coastal area in 1995 and were removed in 2000 after unexplained outages in that structure. The purpose of this paper is to better understand the aging and degradation mechanisms of EPDM insulators in service. The insulators showed severe chalking and discoloration and partial loss of hydrophobicity on the side facing the sun. The surface structural changes were studied in detail using advanced surface analysis techniques, such as attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). For the first time, the significant differences in surface properties between the chalked/discolored (white) and the other surfaces (dark) were studied quantitatively. The Fourier transform infrared (FTIR) absorption spectra showed a significant decomposition of the CH groups of the white surface, elucidating the effect of photo-oxidation on the EPDM polymer. The SEM micrographs showed the cracking of the surfaces. The XPS spectra showed the formation of various polar carboxyl groups and the presence of high surface energy compounds, such as silica, and silicates. This study provided valuable basic information on the changes in the surface properties of EPDM insulators during service in a coastal environment.

Leakage current and flashover of field-aged polymeric insulators

In this paper results from leakage current (LC) measurements under steam fog and voltage withstand tests under rain conditions obtained on various field aged polymeric insulators are reported. Included in this work are several silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) rubber insulators of ac and dc design that have been energized at suitably high ac and dc voltages under coastal conditions for a period of more than nine years. Their performances are compared to two sets of identical insulators, namely one that was outdoors exposed without being energized and one that was stored indoors. The LC of each insulator was studied three times, namely as received from the field, after it was gently washed to remove soluble pollution, and finally after a voltage withstand test. In between the measurements, the insulators were left to recover for a period of a week. The results show that the LC of the SIR insulators were generally lower than those of the EPDM. In both cases, the LC of the insulators that were stored indoors were significantly lower than those of the outdoor exposed insulators. The voltage withstand tests revealed an almost linear relation between the flashover voltage (FOV) and the arcing distance of the insulators. In general, for similar SIR and EPDM insulators the FOV of the SIR ones were higher than those of EPDM insulators.

Comparison of oxidation induction time measurements with values derived from oxidation induction temperature measurements for EPDM and XLPE polymers

AbstractOxidative stability and retained operational utility of polymers used as insulation for electrical cables, such as ethylene propylene diene monomer (EPDM) and crosslinked polyethylene (XLPE), may be assessed by oxidation induction time (OIT) analysis. OIT is measured directly with a differential scanning calorimeter. Using a simplified kinetics model, Gimzewski demonstrated that it is possible to calculate the OIT from measured values of oxidation induction temperature and the activation energy for petroleum lubricants. In the present research, directly measured OITs are compared with OITs calculated from measured oxidation induction temperatures and activation energies for EPDM and XLPE cable insulation. Good agreement between the two methods was demonstrated for these materials.

Electrical performance of non-ceramic insulators in artificial contamination tests. Role of resting time

The paper presents the results of an investigation on the electrical performance of artificially contaminated non-ceramic insulators as a function of resting time. Resting time is defined as the time interval between the application of contamination and start of testing. New (unaged), full scale, 69 kV non-ceramic insulators using different types of silicone rubber and ethylene propylene diene monomer (EPDM) rubber as weathersheds were studied. Their electrical performance was assessed using the clean fog technique. The applied voltage was kept constant throughout the test, while contamination severity and resting time were varied as parameters. The transfer of hydrophobicity through the contamination layer was demonstrated by flashover tests. This was also visually depicted by analyzing samples in a scanning electron microscope. The results show that resting time exerts little influence on the electrical performance of EPDM insulators. However, for silicone rubber insulators, resting time is shown to drastically improve its electrical performance. Experimental evidence indicates that the transfer or recovery process of hydrophobicity in silicone rubber insulators occurs as a progressive superposition of silicone oil layers with time. Thus, the net effect of resting such insulators before tests can be thought of as a gradual reduction of the effective contamination layer thickness. Such a reduction alters the way in which the contaminant layer interacts with external stressing agents, which could impact the insulator's electrical performance in service. In this work, an attempt has been made to identify and quantify the electrically significant changes introduced by a reduction in the effective contamination thickness. It is in the light of this reduced effective contamination layer that we explain how a seemingly wettable silicone rubber insulator is still able to hold the applied voltage without flashover.

Surface structural changes of naturally aged silicone and EPDM composite insulators

In a long-term outdoor test with high direct and alternating voltages, silicone and ethylene propylene diene monomer (EPDM) rubber composite insulators have, at the beginning, shown a performance superior to that of glass and porcelain insulators. In the long-term test, however, the silicone rubber composite insulator has, in spite of the aging of both insulator types, kept its good performance, while the performance of the EPDM rubber composite insulator was drastically deteriorated. In order to get a better insight into the results obtained, the wettability and the surface structural changes of the insulators were studied by the drop deposition method using a goniometer and by advanced techniques such as scanning electron microscopy, electron spectroscopy for chemical analysis, Fourier-transform infrared spectroscopy, and secondary ion mass spectrometry. The results show that the differences in performance are related to the differences in the surface structural changes and in the dynamic ability of the surface to compensate the aging. Silicone rubber insulators have higher repellency than the EPDM insulators, especially when aged. The reason seems to be low silicone polymer diffusion from the bulk which covers the insulator surface, embeds the pollutants, and keeps the surface hydrophobic. Porcelain insulators with silicone elastomer coatings show lower water repellency than insulators with massive silicone rubber sheds. When aged, EPDM insulators do not, however, seen to possess the same dynamic recovery of their surface, thus becoming hydrophilic.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Application of low-voltage Scanning Electron Microscopy to studies of polymer near-surface diffusion and hydrophobicity

The study of the surface characteristics of a polymeric insulator - such as silicone rubber (SR) and ethylene propylene diene monomer (EPDM) -- is necessary to understand the hydrophobicity recovery mechanism that governs the ability of insulators to withstand electrical stresses during high voltage service. It has been shown that hydrophobicity of a polymeric insulator surface could be reduced by electrical discharges during outdoor service, causing the surface to become wettable, and thus allow water films to form, and ultimately leading to power outages. It was also reported that the recovery process could be due to the diffusion of low molecular weight (LMW) polymer chains from the bulk of the material to the surface. Because the LMW chains are in fluidic form (oil), they are able to diffuse through the material due to the concentration gradient of these species between the bulk and the surface. The exact mechanism of hydrophobicity recovery in polymeric insulators is still not well understood. In this study, an investigation was conducted to examine the effects of near surface molecular diffusion in the hydrophobicity recovery process of SR and EPDM insulators. Different analytical techniques were used to examine the surface morphology and surface charging effects of these elastomeric materials. Low voltage scanning electron microscopy (LVSEM) was used to measure the second cross-over point (E2) of the material surfaces, and electron spectroscopy for chemical analysis (ESCA) was also used to identify the surface chemical composition.