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While increasing a 500 Hz voltage from 300 V to 2000 V, the charging and ac loss currents of a water-treed XLPE are observed to increase more with voltage than those of a non-degraded XLPE. The fundamental and the third harmonic of the ac loss current have been numerically analyzed based on a model of water-treed insulation. In the model, a nonlinear voltage dependence of the conductance of a water-filled channel region is assumed. Magnitudes and phase angles of the fundamental and 3rd harmonic components of the ac loss current calculated from the model are compared with the observed ones. It was found that the length of water-filled channel region of a water tree grows with increasing voltage, and that this length approaches that of the water-treed region estimated by the pulsed electroacoustic (PEA) method.

The effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated. The obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another. In all cases, an increase in ramp rate results in an increase in the measured breakdown strength, as does a reduction in sample thickness. Irrespective of the test parameters, the short-term performance of a polyethylene blend containing a designed microstructure is always improved compared to that of the base low density polyethylene alone. Impulse testing leads to an identical conclusion. The effect of the blending method has been shown to have a negligible effect on the breakdown strength, despite consequent changes in nucleation density, implying that it is the local lamellar texture that is responsible for the enhanced properties of the blend.

This study investigates fluid absorption behavior of an unirradiated and a heavily crosslinked ultra-high molecular weight polyethylene (UHMWPE) exposed to bovine serum with pressures varying from 0.1 to 10 MPa. The time-response of serum absorption is biphasic, with an initial 'soak-in' phase (

It is well known that water treeing in polyethylene (PE) is sensitive to frequency, but there is not yet a reliable analytic model describing this empirical observation. We show from existing published data that the length L of water trees grown in the laboratory under one given ionic solution varies linearly when plotted as log L vs. log N, where N is the number of field cycles. This relation is nearly independent of the applied field, suggesting a fatigue-like process. Ion concentration has only a slight influence on watertree length, especially after long aging. It was observed also that field aging and thermal aging reduce the watertree length. Some authors have suggested that there is an upper frequency limit above which water treeing would be independent of frequency. We have observed that the relationship log L vs. log N deviates from its linear behavior between 30 and 445 kHz. More work is needed to define more precisely the upper frequency limit. This means that accelerated aging for watertree degradation could be done at high frequency and results could then be translated reliably to power frequency. Finally, the fact that all experimental data obtained with one given ion obey the same relation regardless of types of the test cells indicates that results obtained with one given cell can be compared directly to those obtained with any other cell. Practical considerations also are discussed briefly.

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress.

Small-amplitude oscillatory measurements, creep and recoil experiments, capillary extrusion flow and shrinkage measurements have been performed to elucidate the effect of block and random chlorinated polyethylene (CPE) on the rheological properties of a ternary high density polyethylene (HDPE)/poly(vinyl chloride) (PVC)/CPE system. It is observed that the storage modulus, the complex viscosity and the steady state viscosity at low shear rates decrease when a small amount of CPE is incorporated to 50/50 (wt.) HDPE/PVC binary blend. However, at high shear rates, in experiments performed in extrusion flow, the trend is reversed, and the incorporation of CPE to the binary blend increases viscosity. The high melt elasticity of HDPE is severely reduced when this polymer is mixed with PVC, but when CPE is included as a third component, elastic recovery is considerably increased. All these rheological results, which are independent of type (block or random) of CPE used, are explained considering the morphological ch anges produced by CPE and during extrusion flow., J.J. Pena (1) A. Santamaria (*) INTRODUCTION As is well known, poly(vinyl chloride) (PVC)/ polyethylene (PE) blends are immiscible, giving rise to a phase-separated system with a very low degree [...]

Lining of underground pipeline networks with deformed/reformed high-density polyethylene (HDPE) liners is a trenchless technique frequently specified for rehabilitation. According to the current design practice, a liner can be structurally designed as a flexible pipe, rigid pipe, or composite pipe, depending on installation conditions. One of the most important design considerations of a confined liner is its buckling under a uniform pressure similar to that exerted by ground water seeping through the deteriorated host pipe. Because long-term testing of a lining system is both time-consuming and expensive, it becomes extremely important to establish a design criteria for long-term conditions more quickly and economically. In the reported study, over 200 short-, medium- and long-term tests were performed to examine the effects of the diameter, thickness, and deformities on the buckling characteristics of a HDPE liner pipe. Based on the findings of the study, some recommendations were proposed for the design and testing of plastic liners under short- and long-term loading conditions.

Problems associated with premature failure of total knee replacements (TKR's) include: wear, creep, and oxidation of ultrahigh-molecular-weight polyethylene (UHMWPe) as well as adverse tissue reactions to polyethylene wear debris. These problems are associated in part with the mechanical behavior of UHMWPe. In TKR's, contact stress analyses have been performed on the UHMWPe tibial component; however, most have employed simplified material properties and not accounted for joint kinematics. A nonlinear viscoelastic rolling model was developed for TKR's to predict the contact stress and rolling friction for varying rolling speed, conformity, applied load, and tibial plateau thickness. Results indicated that the contact stress increased and rolling friction decreased with increasing rolling speed. Effects of conformity, applied load, and tibial plateau thickness were consistent with previous models. At large rolling speeds, predicted peak contact stresses were almost twice their static value, resulting in a compound fatigue problem in UHMWPe components due to normal cyclic loading, moving point of contact, and velocity-dependent stresses.

This paper describes torsional ring shear tests on interfaces comprised of high-density polyethylene (HDPE) geomembranes/nonwoven geotextiles and a drainage geocomposite. Four textured geomembranes with three different manufacturing techniques are utilized to investigate the effect of geomembrane texturing on interface shear resistance. In addition, the effects of geotextile fiber type, fabric style, polymer composition, calendering, and mass per unit area on textured HDPE geomembrane interface strengths are investigated. The textured HDPE geomembrane/nonwoven geotextile and drainage geocomposite interfaces exhibited a large post-peak strength loss. This strength loss is attributed to pulling out or tearing of filaments from the nonwoven geotextile and orienting them parallel to shear, and polishing of the texturing on the geomembrane. At high normal stresses, the strength loss can be caused by damage to or removal of the texturing on the geomembrane surface.

In recent years, the development of high performance Spectra (R) fibers has received increasing attention in industry. Because of the outstanding capability to absorb energy, these polyethylene fibers are frequently used in damage tolerant composite materials to protect equipment, apparatus, and/or people against blast and debris of explosion or projectiles. Since the fiber is thermally stable, it is possible to design composite structures that are suitable for use in applications where prolonged exposure to flame is anticipated. In this paper, we report the findings obtained in a recent flammability evaluation of a Spectra composite subjected to a torch flame, and to hot metal balls. The Spectra composite structures that will be discussed were found to withstand these exposures without burning and the smoke generation and toxic gas emission were far below the limits set in the standard for current aircraft interior materials.

The ultra-high strength/modulus, extended chain polyethylene fiber (Spectra(R) fiber) composite has shown great potential as a lightweight armor material with its extraordinary capability of absorbing the kinetic energy of projectiles. But the relatively low melting point of this fiber (approximately 145 degrees C) has caused concerns regarding the effect of temperature rise during the impact/penetration process on the performance as armor material. In this article, an analysis of temperature rise in projectile and the fiber composite during the impact/penetration process is described. Combining the simulation of impact deformation by finite element analysis and the simulation of temperature rise by a finite difference approximation of the related dynamic equations, the temperature rise caused by the projectile/composite interaction was estimated. Results show that there is a significant temperature rise at the projectile/composite interface due to the friction but that the short length of time involved in the process and the low thermal conductivity of Spectra fiber composite keep the temperature rise in a very small region (in the order of 0.001 cm) around the interface during the impact/penetration process. Consequently, the volume that is affected by the temperature rise is very small, in the order of total 0.001 cm cubed around the projectile, and this is too small to generate any detectable effect on the armor performance. After the projectile is stopped, however, substantial heating of fiber composite can occur under specific conditions as the result of heat flow from the hot projectile embedded in the composite. This heating of fibers, however, is a postmortem effect and hence inconsequential to the ballistic performance of the composite armor.

Breakdown tests were conducted on a variety of polyethylenes and polypropylenes. High density polyethylene (HDPE) and polypropylene random copolymers (PPR) exhibited the highest breakdown strengths among the examined polyolefins. Model cables insulated with HDPE and PPR were manufactured, and the impulse breakdown strength of the HDPE cables made under some specific manufacturing conditions was 1.6x larger than that of conventional crosslinked polyethylene (XLPE). However, improvements were not observed in the impulse breakdown strength of PPR. The ac breakdown strength of HDPE and PPR cables also did not improve. The reason of the improvement in impulse breakdown strength of HDPE is due to the increased crystallinity of the insulating material, brought about by the heat annealing applied to the cable in a manufacturing process.

A Weibull statistical analysis of breakdown voltages of thin polyethylene-insulated power cable slices is performed on large populations. Computation of confidence intervals implies that the statistically correct description is a three-parameter Weibull distribution, i.e. with a non-zero location parameter. It is shown that a data set described by a two-parameter Weibull distribution contains additional statistical dispersion factors which may or may not yield information on the insulation itself. In other words, a zero-location parameter always results from inhomogeneities in the sampling. These may be due to uncontrolled experimental parameters or to defects of various origins; but aging is also a source of dispersion of the electrical properties of dielectrics. To discriminate between the two, we proceed by comparative testing. When obtained under carefully controlled experimental conditions, the location parameter value can be considered a true quality factor of the system under test. The possible physical meaning of the so-called threshold may be envisaged once the breakdown mechanism at work under the conditions of the test is known. The statistical analysis of data collected in routine breakdown tests provides a very sensitive tool to investigate small changes in electrical insulation when performed on extensive data sets.

In order to test the feasibility of applying ultrahigh molecular weight polyethylene (UHMW PE) as the electrical insulation at cryogenic temperatures, mechanical and dielectric assessment was carried out. In particular, the stress cracking resistance and treeing resistance of UHMW PE at 77 K were tested in comparison with those of crosslinked polyethylene (XLPE) and low density polyethylene (LDPE). The test result and theoretic analysis show that mechanical forces play a dominant role both in stress cracking of LDPE and XLPE, and in electrical treeing at low temperature. UHMW PE with extreme stress cracking resistance is recommended as an appropriate candidate for electrical insulation at low temperature.

Commercial carbon-black filled medium density polyethylene pipes were tested for failure with static water as the internal agent and circulating air as the external agent at temperatures of 80 to 105 C. Results show that the pipes display Stage I failure at high circumferential stresses and Stage III failure at low levels. Mass average molar mass degradation rate was 10% and oxidation involved only the amorphous state. In addition, antioxidant concentration decreased rapidly at the beginning and was double at the pipewall center than at the near-inner and outer-wall of the concealed pipe.

Using an improved pulsed electroacoustic (PEA) system which is capable of measuring space charge distribution at minimum intervals of 25 [micro]s, the charge distribution inside water-treed polyethylene (PE) was examined. From the results, the frequency dependencies of conductivity and permittivity were examined for water trees grown in PE. The conductivity of the water-treed region is found to be [is greater than][10.sup.10]x higher than that of non-treed regions, while the permittivity increase is only slight.

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a dc stress, were measured using the pulsed electroacoustic (PEA) method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV ac for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables.

In order to clarify partial discharge (PD) mechanism in an electrical tree, we investigated PD pulse sequence and light emission from a single PD in an artificially-simulated tree channel during the tree propagation in low-density polyethylene (LDPE). For positive PD, the light emission was observed near the needle-tip and there was a positive correlation between the PD magnitude and the length of luminous region. The pulse-sequence analysis showed that PD took place at a constant voltage increments. These results suggest that PD starts at the needle-tip and the PD magnitude is determined by the PD length. On the other hand, for negative PD, there was no correlation between the PD magnitude and the length of luminous region. The PD pulse sequence can be explained by a simple PD model, since the voltage increment is related to the magnitude of preceding PD. These results suggest that there is a difference between positive and negative PD characteristics and their mechanisms in an artificial channel during tree propagation.

A HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Today's commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

In order to improve the resistance to water treeing of standard crosslinked polyethylene (XLPE) insulation used in medium voltage power cables, a number of materials were selected and studied that represent different water tree growth prevention strategies: (a) using materials with crystallinity and morphology different from that of XLPE, or (b) using additives modifying the interaction between water and materials. Two laboratory procedures were adopted that assess the water tree resistance performance of the materials through different criteria: one based on divergent field geometry (CNRS) and another based on uniform field geometry (Pirelli). Both tests are shown to be complementary in their prediction of the performance of the selected water tree resistant materials. The work reveals a specific additive that greatly increases the water tree resistance performance of the insulation in different polymeric matrices.

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.

Loss current through non-penetrated water treed polyethylene increases with exposure time at 1000 V, 500 Hz. The increment is discussed by taking the capacitance of the water treed sheet sample to be the Maxwell-Wagner type capacitor composed of two layers: the capacitance both for the non-treed region and the unfilled channel region is connected in series to the parallel combination of the capacitance and the conductance for the water-filled channel region. By assuming a non-ohmic dependence of the conductance, the fundamental and 3rd harmonic components of the loss current have been calculated numerically, based on the equivalent circuit. The numerical results give a good agreement with the observed changes of the magnitudes of fundamental and 3rd harmonic components [I.sub.1] and [I.sub.3], and their phase angles [[Theta].sub.1] and [[Theta].sub.3], respectively, for the exposure time. It has been concluded that the water-filled channel region in the water treed region grows with exposure time. It is also concluded that the conductivity of the water-filled channel region increases slightly with its growth.

EIICHI MIYAGAWA [2] AKIRA TANAKA [*] [l] HIROMITSU UNO [1] MITSUNOBU KITAMURA [1] KOH-HEI NITTA [3] INTRODUCTION The effect of the morphology on photo-degradation was investigated for low-density polyethylene materials. [...]

The accumulation of space charge in planar crosslinked polyethylene (XLPE) samples under dc electric fields at room temperature was investigated using the pulsed electroacoustic (PEA) method. Three different organic semiconductor (semicon) materials containing carbon black at concentrations ~ 30%wt, and vacuum-evaporated gold, were used as electrodes. Three different conditioning procedures were investigated. Unconditioned samples developed heterocharge with density increasing from zero at the electrodes, to a maximum at roughly one-third of the thickness, and then falling to zero around the center. They also developed homocharge close to the electrodes. The choice of electrode material had little effect on the heterocharge profile in unconditioned samples. Conditioning by holding at 80 [degrees] C for four days, at rotary pump pressure or at atmospheric pressure, suppressed the accumulation of heterocharge. Homocharge accumulation close to the electrodes in samples with semicon electrodes was affected little by this conditioning, but was reduced considerably in samples with gold electrodes. Conditioning by holding at room temperature for seven days at rotary pump pressure had little effect. The heterocharge originates in inhomogeneous polarization due to a spatially inhomogeneous distribution of permanent dipole molecules, probably acetophenone. The dipole concentration decreases in going from the electrodes towards the center of the samples, as a result of diffusion of the molecules in the opposite direction. The homocharge originates in electronic charge injection/extraction across the electrode/XLPE interfaces.

The space charge distribution in low-density polyethylene (LDPE) was measured with the pulsed electroacoustic (PEA) method. We used three types of LDPE: LDPE-L and LDPE-H were prepared by the high pressure process, and m-LDPE was polymerized with a metallocene catalyst. Space charge in LDPE strongly depended upon the electrode material. Semiconductive electrodes enhanced carrier injection into LDPE and, as a result, space charge. The density, polymerization process, applied field, temperature and so on also affected the space charge behavior. This space charge behavior was compared with the results of dc current measurements.

The growth of water trees in low-density polyethylene (PE) has been studied. Trees were grown in solutions containing a fixed anion, [Cl.sup.-], but different cations with valances of +1 ([Li.sup.+]), +2 ([Mn.sup.2+]) and +3 ([Fe.sup.3+]). An ion-specific effect on tree growth and tree appearance has been found. Generally, these results have been shown to be correlated with either the ion-water interactions or the interionic attractions. However, for the specific case of adding small amounts of 3+ cations into univalent ion solutions, such a correlation was not found: it is likely that the hydrogen ions, introduced by the multivalent ions through their electrostatic interaction with water molecules, play an important role. The abnormal transport behavior of such hydrogen ions in the solution and probably in the polymer, may expand the overall region affected by treeing but reduce the visibility of the individual trees themselves. A similar process may account for the particularly translucent appearance of trees formed with pure Fe[Cl.sub.3] solutions. This argument is further demonstrated through tree growth experiments using KCl solution and its mixtures with small amounts of HCl. It was found that this additive accelerated tree growth. It is argued that small amounts of KOH might be expected to show an effect similar to HCl because of the similar transport behavior of [OH.sup.-] ions and [H.sup.+] ions respectively and such enhanced tree growth was demonstrated, using small quantities of KOH added to KCl solutions.

Extruded polyethylene is used as the bulk insulation for ac HV cables because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short-term characteristics and long-term performance of low-density polyethylene (LDPE) insulation under dc electric stress. The results are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well characterized polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-molded under laboratory conditions and subjected to ramp-to-failure and electrical aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of [Eta] in the inverse power model was estimated. The residual life of aged samples also was examined.

In the present paper, the effect of the addition of fine particles 1%wt BaTi[O.sub.3] to plain low-density polyethylene (LDPE) on the short-term dc breakdown strength of LDPE is investigated. The Weibull plots are used to analyze the breakdown test results. This indicates that the addition of Bati[O.sub.3] to LDPE has reduced the short-term dc breakdown strength of the doped material by ~16% and increased the dispersion of the breakdown data. The results also indicate that the Weibull exponent b has been reduced significantly for the doped material. Earlier measurements on the effects of the incorporation of BaTi[O.sub.3] in LDPE on dc current transients and space charge formation in LDPE are presented and correlated with the present results. Results from a variety of measurements using different techniques such as X-ray diffraction, differential scanning calorimeter (DSC), scanning electron microscopy (SEM), and optical microscopy are used in order to establish the effects of the incorporation of the additive in the polymer on its structure and morphology. The multiple role of BaTi[O.sub.3] additive in modifying the dc breakdown strength of LDPE is discussed in general terms of the existing breakdown theories in solid dielectrics. The combined effects of the additive on the different factors which influence the breakdown process in solids such as charge carrier mobility, charge carrier injection at the electrodes, space charge formation, polymer structure and morphology is explained and discussed. The present work introduces some novel elements in explaining breakdown processes in polymeric solid dielectrics by using an additive of known effects to provide an insight into the breakdown in LDPE. Moreover, an attempt is made to integrate results from different measurements using a multitude of techniques to yield a meaningful picture of the breakdown mechanism of LDPE.

Interfacial properties such as space charge accumulation and breakdown characteristics in crosslinked polyethylene (XLPE)/ethylene propylene diene terpolymer (EPDM) laminates were investigated. Homocharge is observed in EPDM containing 1,4-hexadiene while heterocharge is observed in EPDM containing 5-ethylene-2-norbornene. Interfacial charge develops when the EPDM is laminated with XLPE. The polarity of this interfacial charge reverses at less than a few tens of hours heat treatment at 80 [degrees] C. dc breakdown voltage shows a sequential change of an increase, a decrease and a slight increase as a function of heat treatment time, which holds true for both interfacial and volume breakdown voltages. A maximum breakdown voltage is observed at 20 to 24 h heat treatment. After long heat treatment, silicone grease used in the interface shows lower interfacial breakdown voltage than silicone oil for the dc case, which was attributed to the additives in the grease and the molecular weight of silicone molecules in oil and grease. Details of the results are given and their origins discussed.

Current transients in radiation induced conductivity (RIC) of low-density polyethylene (LDPE) exposed to 17 ns bremsstrahlung pulses were studied. Three models of the RIC relaxation, to 30 [micro]s after the pulse, have been tested numerically. Strong influence of the measuring circuit on the experimental results was taken into consideration. Verification shows that the RIC relaxation immediately after the excitation consists of the combination of at least two exponential components with markedly different relaxation times. This supports our thesis, that the charge transport in LDPE during the 30 [micro]s period after the RIC excitation by means of bremsstrahlung is occurring in the conduction band as a shallow-trap-limited, nondispersive current.

THERMOLUMINESCENCE IN THERMALLY QUENCHED ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE. Scott [Lewis.sup.*] and M. S. Jahan, The University of Memphis, Memphis, Tennessee. Ultra-high molecular weight polyethylene (UHMWPE) used in total hip [...]

A METHODOLOGY FOR THE EVALUATION OF THE TENSILE CREEP BEHAVIOR OF ULTRA-HIGH-MOLECULAR-WEIGHT POLYETHYLENE. Eric Walker, Krishy Gopalapillai, and Gladius Lewis, The University of Memphis, Memphis, Tennessee. Ultra-high molecular-weight polyethylene (UHMWPE) [...]

MORPHOLOGY AND MECHANICAL PROPERTIES OF OPEN POROUS ULTRA HIGH MOLECULAR POLYETHYLENE (UHMWPE) FOR BIOMEDICAL APPLICATIONS. T. A. Venkatesh and F. Shutov, Tennessee Technological University, Cookeville, Tennessee. Ultra High Molecular Weight [...]

A. SANTAMARIA [*] The effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features [...]