Naoshi Hirai

Dielectric properties of epoxy/clay nanocomposites - effects of curing agent and clay dispersion method

Effects of the differences in the curing agent and filler dispersion method on the dielectric properties were examined for epoxy/clay nanocomposites. Irrespective of the clay dispersion method, relative permittivity and electrical conductivity are higher in the samples cured with the amine. Moreover, negative heterocharge accumulates in the vicinity of the anode in the amine-cured samples, whereas positive homocharge accumulates in the acid anhydride-cured samples. From the results of UV photon absorption and PL measurement, the bandgap or the energy at which the photon absorption increases drastically is smaller in the amine-cured samples than in the acid anhydride-cured samples. Ion migration can occur easily in the amine-cured samples whose electrical conductivity and relative permittivity are higher than the acid anhydride-cured samples. The curing agent gives the strongest effect, while the existence of clay affects secondly and the filler dispersion method has the weakest effect.

Surface change of polyamide nanocomposite caused by partial discharges

We have investigated partial discharge (PD) degradation for conventional polyamide-6. without nanoscale fillers (nanofillers) and polyamide-6 nanocomposites with 2 weight (wt) %, 4 wt% and 5 wt% addition. Such materials were subjected to partial discharge under the IEC (b) electrode configuration for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness was far smaller in specimens with nanofillers than those without nanofillers, and that the 2 wt% addition was sufficient for improvement. This result indicates that polyamide-6 nanocomposite is more resistive to PDs than polyamide-6 without nanofillers.

Electrical Conduction and Breakdown Properties of Several Biodegradable Polymers

In order to respond to soaring public concern about environmental protection, various biodegradable polymers have been developed. The present paper reports the electrical conduction and breakdown properties of various biodegradable polymers such as poly-L-lactic acid (PLLA), polyethylene terephthalate succinate (PETS), polycaprolactone butylene succinate (PCL-BS), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and polyhydroxybutyrate/valerate (PHB/V) in comparison to those of low-density polyethylene (LDPE). While the permittivity and conductivity of PLLA and PETS are comparable to LDPE, those of PCL-BS and PBS are much higher. The conductivity is also higher in PBSA. This is because PLLA and PETS are in the glass state at room temperature, while PCL-BS, PBS, and PBSA are in the rubber state. Furthermore, PLLA and PETS show a strong temperature dependence of the conductivity, which is divided into two or three regions, and they also show thermally stimulated polarization or depolarization current around their respective glass transition temperatures. In contrast to the large difference in conductivity among different kinds of samples, all the polymers tested have almost similar impulse breakdown strength at room temperature. As for dc or ac breakdown strength, PLLA and PETS show a relatively higher strength than PCL-BS and PBS.

Effects of crosslinking byproducts on space charge formation in crosslinked polyethylene

Space charge is formed in cables insulated with crosslinked polyethylene. It has not been clear whether the crosslinking byproducts or the crosslinked polymer morphology is responsible for the space charge formation. In order to clarify this point additive-free noncrosslinked low-density polyethylene, additive-free crosslinked polyethylene, and degassed crosslinked polyethylene were soaked in the crosslinking byproducts and the space charge distribution was measured after DC voltage application. Samples tested are divided into two categories. The first category is a soaked single-layered sheet and the second category is a two-layered specimen consisting of a soaked sheet and a nonsoaked sheet. As a result, the following conclusions were obtained. (1) Cumyl alcohol is responsible for homocharge layers in front of the electrodes in both low-density and crosslinked polyethylene sheets. (2) Acetophenone is responsible for heterocharge formation in crosslinked polyethylene, presumably as a synergistic effect with water. (3) alpha -methylstyrene has no effect on space charge formation in low-density polyethylene, while it assists charge trapping in crosslinked polyethylene. (4) Charge trapping occurs easier in degassed crosslinked polyethylene than in low-density polyethylene, probably because of carbonyl groups induced by crosslinking

Chemical group in crosslinking byproducts responsible for charge trapping in polyethylene

It is known that space charge is formed in cables insulated with crosslinked polyethylene. It is probable that the crosslinking byproduct or the crosslinked polymer morphology is responsible for the space charge. In order to examine the former possibility, an additive-free low-density polyethylene sheet was soaked in various liquid chemicals including the crosslinking byproducts and space charge distribution inside the sheet was measured under DC voltage application. As a result, space charge appears in the vicinity of the injecting electrode only in the case that the sheet was soaked in the liquids with at least one hydroxyl group in their structures. Therefore, it is concluded that cumyl alcohol, which is the only byproduct with a hydroxyl group in its structure, is responsible for the formation of space charge in polyethylene on the assumptions that chemical functions of a given molecule act separately from each other and that other contributions such as the crosslinked polymer morphology are negligible.

Effects of byproducts of crosslinking agent on space charge formation in polyethylene-comparison between acetophenone and /spl alpha/-methylstyrene

We have examined the effects of acetophenone and /spl alpha/-methylstyrene on the space charge evolution in PE. The samples tested are additive-free LDPE sheets. The important conclusions obtained are as follows: (1) Positive charges are easy to migrate from the byproduct-existing region to the non-existing region while negative charges are difficult. (2) Positive charge migration is easier in the LDPE sheet soaked into /spl alpha/-methylstyrene than in the one soaked into acetophenone, which is probably due to the difference in permeation velocity of the two byproducts. (3) The values of mobility and conductivity calculated from the space charge profiles agree with the assumption that ionic carriers are responsible for the conduction in the byproduct-existing region.

Electrical conduction properties of several biodegradable polymers

Electrical conduction properties of Ave biodegradable polymers, polycaprolactone, polybutylene succinate adipate, polybutylene succinate, polybutylene adipate terephthalate, and poly-L-lactic acid, are discussed in relation to their measured values of glass transition temperature, complex permittivity, and breakdown strength. Conductivity is high in the polymers with low glass transition temperatures, and is in good proportion to the dielectric loss factor of each sample. Breakdown strength is low in such polymers. These results are reasonable, since molecular motion is active, and dipolar orientation and carrier transport are activated, when the polymer is in the rubber state.

Dielectric properties of biodegradable polylactic acid and starch ester

In order to examine the applicability of biodegradable polymers to the fields of electrical insulation, several dielectric properties of two typical biodegradable polymers, polylactic acid (PLA) and starch ester (SE), are examined. A fairly larger amount of space charge is accumulated in both polymers in comparison to low-density polyethylene (LDPE). This seems partly due to the presence of hydroxyl and carbonyl groups in these polymers. Permittivity and conductivity are higher in SE than PLA that has the values close to those of LDPE. The dielectric breakdown strength is lower in SE and is higher in PLA than LDPE. As for the resistance to photodegradation by ultraviolet photons, SE is stronger than PLA, although the two are much inferior to LDPE.

Precise location of the excessive temperature points in polymer insulated cables

This paper demonstrates that it is possible to locate very precisely a point showing a high temperature in a polymer insulated cable. The method is based on frequency domain reflectometry and inverse fast Fourier transform. The cables tested are a coaxial communication cable with a length of 32 m insulated by low density polyethylene and a flat in-house electric cord with a length of 21 m insulated by polyvinyl chloride. The cable or cord was heated at different positions for different lengths. The ratio between the powers of electromagnetic waves incident to and reflected from the cable was measured using a network analyzer in a frequency range from one to several hundred MHz or 1.5 GHz. The spectra obtained by the measurements were then analyzed by inverse Fourier transform. As a result, the position exhibiting a temperature higher than the adjacent points can be located with a spatial resolution as short as 2.5 cm. It was also confirmed that the sensitivity or spatial resolution can be improved by an increase of the highest measurement frequency.

Effects of byproducts of dicumyl peroxide on space charge formation in polyethylene

Crosslinked polyethylene (XLPE) cables are widely used for ac transmission. However, for dc transmission, they are still under feasibility study due to severe concern about easy accumulation of space charge into them. It is considered that byproducts of a well used crosslinking agent (dicumyl peroxide) (DCP), generate space charge into insulated cables. Acetophenone, /spl alpha/-methylstyrene, and cumylalcohol are the byproducts, of DCP. There have been many investigations on the effect of acetophenone on the charge distribution in polyethylene (PE). In the present paper, effect of cumylalcohol. on the space charge formation in PE has been investigated. Furthermore, effects of all the byproducts of DCT, acetophenone, a-methylstyrene, and cumylalcohol, on the space charge formation in PE have been compared by referring to our former reports about acetophenone and /spl alpha/-methylstyrene on the space charge formation in PE.