DGEBA/MDA/SN/Natural Zeolite계의 절연파괴거동에 미치는 온도의 영향

김유정, 이홍기*,김상욱

화학공학과 서울시립대학교, *화학공학과 우석대학교

Effects of Temperature on Dielectric Breakdown Behavior

of DGEBA/MDA/SN/Natural Zeolite System

Y.J. Kim, H.K. Lee*, S.W. Kim

Dept. of Chem. Eng., The Univ. of Seoul, Dept. of Chem. Eng., Woosuk Univ.*

1. Introduction

This study is about the insulating material, epoxy composite. Epoxy resins have attracted considerable attention as electrical insulating material because of excellent electrical properties as well as a variety of other physical, chemical and mechanical properties. The insulation systems of the high voltage power machines and cables, in most of the situations, are composited from different insulating materials and fillers. The epoxy molding resins, especially, have been used in the insulation of heavy electric apparatuses with other inorganic fillers such as mica, aluminum oxide, talc, silica, etc. and with many kinds of additives. For technical and economic reasons, inorganic fillers are added in varying amounts to endow the required special properties[1, 2].

But some of the epoxy systems are too brittle because of its high density of cross-link. To modify the brittleness of epoxy, a new reactive additive SN(succinonitrile) as a chain extender, which made the conventional epoxy resin system DGEBA/MDA soft, was introduced[3]. The electrical tree growth is the main cause of the electric failure in thick insulator by the long-term deterioration[4]. The dehydrated natural zeolite, which is clinoptilolite type, was loaded to the epoxy system as an inorganic filler to restrain the electrical tree growth under high voltage. The dielectric breakdown strength (DBS) increased with the increase of zeolite content. We investigated the dielectric breakdown strength of cured specimens of DGEBA/MDA/SN/natural zeolite system with the temperature variation. The DBS over a wide range of temperature is presented here. The DBS of DGEBA/ MDA/SN/natural zeolite system was measured by using needle-plane electrode geometry. The DBS decreased, abruptly, around Tg. At the high temperature, the deterioration by electrical stress was activated. The diameter of puncture at the high temperature was larger than that at the room temperature. Also the puncture of the specimen of DGEBA/MDA/SN/natural zeolite system at various contents and constant high voltage was observed by optical microscope.

2. Experiment

2.1 Materials

The epoxy resin used in this study was diglycidyl ether of bisphenol A(Epon 828 grade) of Shell Co. and its EEW, MW and viscosity were 188 g/mol, 385 g/mol and 11,000-14,000 cps, respectively. 30 phr(parts per one hundred parts of resin by weight) of MDA(4,4'-methylenedianiline) was used for curing the resin. Succinonitrile(10 phr) as a new reactive additive was supplied from Fluka Chemie AG. Natural zeolite, which is crystalline aluminosilicate mineral of clinoptilolite type and has the characteristics of low density and large porosity, from Kampo area in Korea[5] was used in various contents after sieving with #325 mesh and drying in 120℃ vacuum oven for 5 hrs. The average diameter of zeolite particle was 20.03  □. Hard steel needle with tip curvature radius 3 □ and tip angle 30°was used as high voltage electrode. The aluminum layer was used as counter plane electrode.

2.2 DSC analysis

To detect Tg, we conducted the DSC analysis of cured specimen. The cured specimen was made as the following. DGEBA and SN(10 phr) were well mixed with dried natural zeolite(0, 20, 40, 60 and 80 phr) at 80℃ for 1hr, then MDA(30 phr) was added to the mixture. The mixture was cured at 150℃ for 1hr after curing at 80℃ for 1.5 hr. The weighed sample(5~7 mg) was tested by DSC(SEIKO I DSC200). The temperature rising rate was 10 ℃/min and nitrogen gas was flowed into the furnace at 80 ml/min. Tg was detected from the base-line shift on DSC curve.

2.3 Specimen preparation and test.

The ultrasonically washed needle electrode with methyl alcohol and acetone were set in the mold with 1 mm electrode gap and the mold was preheated at 80℃. The mixture of DGEBA, SN and various contents of the dried zeolite was stirred and degassed at 80℃ for 1 hr to make zeolite evenly dispersed. And then the curing agent MDA was added to the mixture. The mixture was degassed and stirred until gel state. It was poured into the preheated mold at 80℃. The overall curing condition was lst cure at 80℃ for 1.5 hr and then 2nd cure at 150℃ for 1.0 hr. The cured system was slowly cooled to the room temperature inside oven to prevent the formation of the microvoids and cracks by shrinking stress and sliced to 8mm(w)×50mm(H)×35mm(L). The specimen was immersed in silicone oil at a constant temperature for 20 mins to suppress surface flashover. High voltage of 60 Hz was applied to the specimen until breakdown at the voltage rising rate of 500 V/sec and the voltage was measured as dielectric breakdown voltage. The dielectric breakdown step was detected by the breakdown sound or the flashover at the tip of needle electrode. Under the specific voltage, the electrical tree phenomena were observed by using image analyser (Image Pro plus).

3. Results and Discussion

3.1 DSC analysis

Fig. 1 shows the dynamic DSC curves for DGEBA/MDA/SN with zeolite contents(0, 20, 40, 60 and 80 phr) at the heating rate of 10 ℃/min. All diagrams do not show the exothermic curve above Tg and it means that the system was completely cured. With the increment of zeolite content, Tg increased by 16℃. The dielectric breakdown strength and mechanical properties strongly influenced by the thermal stability. The epoxy composite is vulnerable to the electrical stress above Tg, because it is a characteristic temperature determining the motion of end chain segments. The dielectric breakdown strength decreases at Tg. So that the increase in Tg of composite by adding zeolite may broaden the range of tolerance limit temperature to the electrical stress of this system.

3.2 The effect of temperature on dielectric breakdown strength

Fig. 2 shows the dielectric breakdown strength(DBS) with various temperature in the specimen of DGEBA/MDA/SN/zeolite(40 phr) at the scanning rate of 500 V/sec until breakdown was measured. Rapid decrement of DBS at the temperature lower than the glass transition temperature(Tg: 109℃) was observed. The dielectric breakdown characteristics depend on the temperature where the movement of the molecular chains become active and the electron diffusion region from the needle electrode is enlarged. When the temperature increases, the electrons discharged from the needle tip are trapped deeply inside and forms space charge region widely and electrical stress becomes relaxed. If the electric field at the tip of the space charge region reaches tree initiation voltage, pits occur and grow. Tree initiates when a pit grows and propagates toward the counter-electrode and then dielectric breakdown occurs. So the abrupt decrement of DBS was observed at the temperature lower than Tg[4,5].

3.3 Dielectric breakdown behaviour

Fig. 3 shows the dielectric breakdown puncture in the specimen of DGEBA/ MDA/SN/zeolite(40 phr) composite at 170℃. In this figure, we can see that the neighborhood of puncture and surface of the puncture became carbonized by the heat originated from the flash over. Fig. 4 shows also the dielectric breakdown puncture in the specimen of DGEBA/MDA/SN/zeolite(5 phr) composite at the room temperature. In this specimen, the carbonized region is less than the Fig. 3 and the diameter of the puncture is also less than the Fig. 3. In the comparison of these figure, we can deduce the following facts. At high temperature, the deterioration of specimen is accelerated by the heat of surroundings and the heat originated by the collisions between the hot electrons emitted from the tip of needle electrode and the chains of molecular bond. At the high temperature, the molecular chain will be more mobile so that the specimen become more soften and vulnerable to the electrical

Fig. 1 Dynamic DSC curves at the heating rate of 10 ℃/min in the DGEBA/MDA/SN system filled with various zeolite contents

Fig. 2 The dielectric breakdown strength with various temperature of DGEBA/ MDA/SN/zeolite(40 phr) composite

Fig. 3 Dielectric breakdown puncture of DGEBA/MDA/SN/zeolite(40 phr) at 150 ℃

Fig. 4 Dielectric breakdown puncture of DGEBA/MDA/SN/zeolite(5 phr) at the room temperature

stress than at the room temperature. By this reason, the puncture occurred very easily and become larger than at the room temperature. Cross-linked epoxy resin is brittle under the temperature lower than Tg, so that fan type crack was occurred by impact waves from discharged energy. But higher temperature(170℃) than Tg(109℃), crack did not occur and growing tree around the main breakdown channel was not observed. It can be inferred that the impact resistance reinforced DGEBA/MDA/ SN/zeolite composite could absorb the impact waves more effectively at higher temperature than Tg.

4. Conclusions

From the study on the effect of temperature on dielectric breakdown behavior of DGEBA/MDA/SN/zeolite system, the following conclusions were obtained. In the specimen of DGEBA/MDA/SN/zeolite system, the dielectric breakdown strength increased as the content of zeolite increased. In the case of temperature effect, around Tg, the dielectric breakdown strength decreased by the activated movement of molecular chain. Tg increased with the content of zeolite. The diameter of the puncture in high temperature was larger than in the room temperature. The dielectric breakdown phenomena was more vigorous in the high ambient temperature.

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