M. Ishioka

Space charge distribution in low-density polyethylene with blocking layer

Charge carriers injected from the electrode and charge carriers generated in the bulk contribute space charge in low density polyethylene (LDPE). However, it is sometimes difficult to separate their contributions. In this paper, a fluorinated ethylene propylene copolymer (FEP) film was used as a blocking layer. It was effective to separate injected carriers and bulk-generated carriers. The electrode materials used were aluminum and semiconductive (SC) polymer. Charge carriers were mainly injected from the SC electrode into a specimen of Al/LDPE/SC and they formed homo space charge. A blocking FEP film suppressed the carrier injection and it gave information about the behavior of bulk-generated carriers. From these results, we discussed the contributions of injected carriers and bulk carriers to space charge in LDPE.

Space charge behavior near LDPE/LDPE interface

We investigated the space charge behavior near the interface between different low-density polyethylene (LDPE). Charge carriers were mainly injected from the semiconductive electrode in a specimen of Al/LDPE/LDPE/semiconductive layer and they moved through the interface to the counter Al electrode. Charge carriers moving from a lower density LDPE to a higher density one were accumulated near the interface to form space charge, while there was no space charge accumulation for carriers moving in the opposite direction. The mobilities of charge carriers were estimated from the change in space charge profile with time. Charge carriers are more mobile in the lower density LDPE than in the higher density one. Positive carriers were more mobile in LDPE than negative ones. The interface between different LDPEs greatly affected space charge behavior and discharging currents.

Space charge dynamics in polypropylene

We investigated the space charge dynamics in polypropylene, M-PP and Z-PP which were polymerized using metallocene and Ziegler catalysts, respectively. Space charge distributions depended upon temperature, polarity and electrode materials. Carriers injected from the semiconducting electrode played an important role in the formation of space charge in PP at high temperatures. The amount of space charge was the largest around 40/spl deg/C for both M-PP and Z-PP. They showed similar space charge dynamics in spite of their difference in morphology. Z-PP showed larger current than M-PP. These results were also compared with those of polyethylene and they were discussed considering the difference in morphology among M-PP, Z-PP and polyethylene.

Space charge dynamics of low-density polyethylene

The space charge behavior in various low-density polyethylene (LDPE) has been investigated. The specimens are LDPE polymerized with high pressure method (h-LDPE), with Ziegler-Natta catalysts (LLDPE) or with methallocene catalysts (m-LDPE). The space charge distribution in a semicon/LDPE/Al specimen was measured by the Pulsed Electro-Acoustic (PEA) method. The space charge distributions were different among three kinds of LDPE, h-LDPE, LLDPE and m-LDPE. They also depended upon the electrode materials. Charge carriers were injected more easily from the semicon electrode than the Al or Au electrode. The formation and decay processes of space charge have been discussed considering the morphology of LDPE. The carrier mobility and the trap depth were also estimated from the space charge dynamics, formation and decay of space charge.

High-field electrical properties of LDPE films prepared using new catalyst

New Low Density Polyethylene (m-LDPE) prepared using metallocene catalyst has a narrower composition distribution and a molecular weight distribution than Linear-Low Density Polyethylene (LLDPE) using Ziegler catalyst. We compared the electrical insulating properties of m-LDPE films and conventional LLDPE films. The high-field current of m-LDPE was found to be lower than that of LLDPE. The difference in current increased with the decrease of temperature. The high-field current was much suppressed in m-LDPE with a low melting point. The impulse breakdown strength of m-LDPE increased with the decrease of temperature. The insulating properties of m-LDPE with a low melting point were improved at 30/spl deg/C. These results were explained by a fact that m-LDPE with a low melting point includes more low-molecular-weight components.

Effects of electrodes on space charge in polypropylene

We investigated the effects of electrodes, i.e. Al plate, semiconducting (SC) layer and evaporated Al, on space charge in polypropylene (PP) by using the pulsed electroacoustic (PEA) method and also on conduction current. Evaporated Al electrodes suppressed the carrier injection and, as a result, space charge compared with Al plate or SC layer (mechanically contacted electrode). The conduction current also revealed that the carrier injection was suppressed by evaporated Al. The space charge formation and the current showed a significant dependence on not only electrode material but also contact condition between metallic electrode and insulator.

Space charge behaviors of various low density polyethylene

We investigated the space charge behavior in low-density polyethylene polymerized by using metallocene catalyst (m-LDPE). The space charge distributions were measured by Pulsed Electro-Acoustic (PEA) method. We used the Al/semiconductive electrode system. The specimens are four types of m-LDPE; mB is polymerized with butene, mH-1, mH-2 and mH-3 are polymerized with hexene as comonomers. The densities of mB, mH-1, mH-2 and mH-3 are 0.923, 0.925, 0.928 and 0.931 g/cm/sup 3/, respectively. We discussed the effects of comonomer, density, polarity and temperature on the space charge behavior. The experimental results revealed the strong dependence of density and temperature on space charge behavior, but the types of comonomer (butene or hexene) did not affect space charge properties so much.

Effect of anti-oxidants on space charge in low-density polyethylene

Polyethylene is widely used as insulating material for power cables. Recently, much attention have been paid to the developments of extruded DC power cables. Many papers have been published on space charge in low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE). However, space charge behavior and charge dynamics are very complicated and sensitive to various factors such as physical/chemical structures of LDPE, additives, interfacial conditions, applied field, temperature, and so on. They have not been well understood yet. More research work on space charge behavior in LDPE and effects of these factors on space charge behavior are required to develop high performance DC cables. In this paper, we investigated the effect of anti-oxidants on space charge behavior and DC current in LDPE.

Space charge and conduction in LDPE-polypropylene copolymer blends

In the previous paper, the electrical breakdown properties of blend polymers of LDPE and polypropylene copolymer were investigated. The impulse breakdown strength in the high temperature region (90/spl deg/C) was improved by blending. In the present paper, the space charge behavior and electrical conduction of the blends were studied with the electrode system of aluminium/sample/semiconductive layer. The specimen with 90 wt.% LDPE and 10 wt.% polypropylene copolymer (B10) had a larger amount of space charge than pure LDPE (PE) at 30/spl deg/C. On the other hand, the DC current of B10 was less than that of PE in this temperature region. It was found that the anode field was related to the current in both PE and B10, which suggests that the carrier injection from the anode is dominant to the electrical conduction. The DC currents of both PE and B10 showed good straight lines on the Schottky plot with the same Schottky coefficients.

Comparison of space charge behavior between polypropylene and polyethylene

In this paper we investigated space charge and electrical conduction in polypropylene (PP), H-PP and M-PP which are homopolymer and copolymer with 3/spl sim/3.5 % ethylene, respectively. Both space charge distribution and charging current were affected by the copolymerization of ethylene which enhanced electrical conduction. The amount of space charge in Al/PP/SC was the largest around 40 /spl deg/C. The charge carriers injected from the semiconducting (SC) electrode became dominant with increasing temperature. Their results were compared with those of low-density polyethylene (LDPE). LDPE showed much higher current than PP. The difference in space charge distribution among H-PP, M-PP and LDPE was explained in terms of mobilities of charge carriers. The field dependence of charging currents was explained by Schottky injection.