Mitsugu Ishioka

Effects of Oxidation on Electrical Conduction and Breakdown of Low-Density Polyethylene Films with Different Densities

In this study, the effect of blending on physical and electrical properties of low-density polyethylene (LDPE) was investigated. Two kinds of LDPEs whose densities are evaluated to be 0.9179 g/cm3 and 0.9192 g/cm3, respectively, were used and blended according to different blend ratios. The LDPE with a blend ratio of 50 wt% had the lowest impulse breakdown strength, FBImp, at 30°C. However, the LDPE with a blend ratio of 50 wt% also had the highest FBImp at 90°C among all specimens. The DC breakdown strength, FBDC, decreased with the increase of the blend ratio at 30°C but increased at 60°C and 90°C. However, the FBDC did not depend on the blend. The current densities for all specimens were almost the same at 30°C, but decreased with a blend ratio up to 75 wt% at 90°C. By analyzing X-ray diffraction (XRD) patterns, we found that the crystal size in the (020) plane increased with a blend ratio up to 50 wt%, and the LDPE with a blend ratio of 50 wt% had the largest crystal size in the (020) plane among all specimens. It was found that the FBImp was strongly related to the crystal size in the (020) plane.

Effects of manufacturing technology on electrical breakdown and morphology of thin films of low density polyethylene blended with polypropylene copolymer

Extruded films prepared from blends of low-density polyethylene (LDPE) and random copolymer of ethylene and propylene (EP) with the T-die method were studied with respect to electrical properties and morphology. Comparisons with data on blown films are made. These blends are of interest as improved LDPE for making XLPE for insulated power cable. In the high temperature region (90/spl deg/C), a specimen with a slightly higher EP content had higher impulse breakdown strength than that with a lower EP content, but no improvement of DC breakdown strength by blending could be found. The improvement of impulse breakdown strength (90/spl deg/C) is explained in terms of morphological changes by blending such as the orientation of chains in a film and the size of spherulites on the assumption of the thermal breakdown. In comparison, a T-die film had higher impulse breakdown strength than that of a blown film for the same composition. The impulse breakdown strength also increased with the use of the higher density LDPE. In the current versus electric field characteristics at 30/spl deg/C, the blend polymer with EP content of 5-10% showed a transition from LDPE behavior at low field region to EP behavior at high field region. However, no appreciable difference in current behavior among the specimens was observed at 90/spl deg/C, which suggests an incompatibility between the two materials that exists at 30/spl deg/C but not at 90/spl deg/C.

Decay of space charge in LDPE and its blend polymer

In this paper, the decay characteristics of space charge under the short circuit for low-density polyethylene (LDPE) and blend polymer of LDPE and polypropylene copolymer were investigated by the pulsed electro-acoustic method. The decay processes for both, LDPE and the blend polymer, have two time constants which suggest the existence of two kinds of traps. The activation energy was estimated from the relationship between decay time constant and temperature. Two activation energies corresponding to the two time constants increase by blending polypropylene copolymer with LDPE. It suggests the deepening of trap depth by blending which is consistent with the previous work.

Space charge behaviors near the interface between different low-density polyethylenes

We investigated the space charge behaviors near the interface between different low-density polyethylenes (LDPE) by using the PEA method. Charge carriers were mainly injected from the semicon electrode in a specimen of Al/LDPE/LDPE/LDPE/semicon and they moved through the interface to the counter Al electrode. Charge carriers moving from LDPE of a lower density to LDPE of a higher one were accumulated near the interface to form space charge, while there was no space charge accumulation for carriers moving in the opposite direction. It suggests that charge carriers are more mobile in LDPE of a lower density. The mobilities of charge carriers were also estimated from the change in space charge profile with time and positive carriers were more mobile in LDPE than negative ones. These space charge behaviors were discussed compared with the results of DC conduction.

Breakdown and space charge of LDPE films prepared using metallocene catalyst

Polyethylene (PE) has been widely used as electrical insulation for power cables because of its excellent electrical and mechanical properties. Low density PE (LDPE) prepared using a high pressure process has been widely used as insulating polymer of power cables. In this paper, the authors have investigated the performance of new LDPE prepared using metallocene catalyst (m-LDPE) as electrical insulating material. m-LDPE has narrower composition and molecular-weight distributions than LDPE. High-field electrical properties of m-LDPE at 90/spl deg/C were compared with those of the other LDPEs. In addition, the prestress effect and the effect of space charge on breakdown strength were discussed.

Effects of electrodes on space charge in low-density polyethylene

The effects of electrode on space charge in low-density polyethylene (LDPE) were measured with the pulsed electro-acoustic (PEA) method. The thickness of a specimen was about 100 /spl mu/m. The electrode materials used were aluminum and semiconductive (SC) polymer. Charge carriers were mainly injected from the SC electrode in a specimen of A1/LDPE/SC and they moved to the counter A1 electrode. The evaporated A1 electrode injected much less carriers than the A1 plate electrode mechanically contacted. We also used a fluorinated ethylene propylene copolymer (FEP) film as a blocking layer.

Charge transport and space charge formation in low-density polyethylene

In the low-density polyethylene prepared by the high-pressure process (LDPE) and in that polymerized by using the metallocene catalyst (m-LDPE), homo space charges with same polarity as the semiconducting electrode were observed. This indicates that charges are easily injected from the semiconducting electrode independent of its polarity. The space charge in LDPEs with the density of 0.9185 g/cm/sup 3/ was broadly distributed and that in m-LDPE with that of 0.9227 g/cm/sup 3/ was accumulated near the semiconducting electrode. These suggest that the density and structure of polyethylene affect the charge transport and space charge phenomena. These effects were discussed by using computer simulation.

Effect of blend and extrusion on breakdown for low-density polyethylene

There are many reports that the technique of polymer blending is a useful method to obtain a new insulating material. We have used two kinds of low-density polyethylene (LDPE) with different densities, 0.9174 and 0.9180 g/cm/sup 3/ and their blends with different blend ratios. There are two XRD peaks due to the different crystallinity, [110] and [020] peaks, respectively. The crystal size estimated from the [020] peak depends upon the blend ratio. We assume that F/sub Blmp/ of LDPE with a blend ratio of 50 wt% is associated with the crystal size estimated from the [020] peak. We also prepare specimens with different mixture methods in order to investigate the relationship between morphology and blending in detail.

Effects of Copolymerization on Space Charge in Polypropylene

The DSC curve, the DC current and the space charge distribution have been measured on polypropylene (H-PP), block copolymer (B-PP) and random copolymer (Z-PP) copolymerized with Ziegler catalyst, and M-PP copolymerized with metallocene catalyst. The copolymers have 3.0~4.5 wt% of ethylene. B-PP with 4.5 wt% of ethylene had similar DSC curve and space charge distribution to H-PP. The magnitude of DC current is in the order of M-PP>Z-PP>B-PP>H-PP. Space charge in PP and PP copolymers depended on copolymerization, electrode, temperature and so on. The effects of copolymerization on morphology, DC current and space charge distribution of PP and PP copolymers have been also discussed

Effects of drawing on space charge behavior of LDPE film

In this paper, the effects of drawing on space charge behavior of LDPE have been investigated by the pulsed electro-acoustic method at 20/spl deg/C with the electrode system of aluminium/sample/semiconductive layer. Two kinds of LDPE films were used, one was an original LDPE film without drawing and the other was LDPE film with drawing process. The LDPE film after drawing had more space charge than the LDPE film without drawing under a DC field of 50 MVm/sup -1/. The decay of positive space charge during short-circuiting has two time constants. The drawn film has longer time constants than the original film. This result suggests the modification of traps by drawing.