에폭시 수지 복합체의 절연파괴 현상에 미치는

제올라이트 입자의 영향

김유정, 조영신, 김상욱

서울시립대학교 화학공학과

Effects of Zeolite Particles on the Dielectric Breakdown Phenomena of Epoxy Resin Composite

Y.J. Kim, Y.S. Cho and S.W. Kim

Dept. of Chem. Eng., The Univ. of Seoul, Seoul 130-743

Introduction

The epoxy resin systems have been used as insulating materials in the field of high voltage apparatus such as large generators and motors because of their good combination of mechanical, thermal and electrical properties[1,3]. During the last 20 years, this society developed into the industrial community at the rapid rate. As the development accelerated, the demand for the supply of electricity with high stability and reliability increased. Electric failure by the breakdown of electrical insulator is one of the cause of the electricity shortage. Therefore the demands for good insulating materials were expanded, and many researchers have studied for developing the more reliable and stable insulation composites by introducing the inorganic fillers such as glass fiber, mica, silica, aluminium oxide, etc[2,3]. But some of epoxy systems are too brittle because of its high density of cross link. To reinforce the brittleness of epoxy, a new reactive additive SN(succino- nitrile) as the chain extender which made the conventional epoxy resin system flexible[4] was introduced into the conventional epoxy resin system DGEBA/MDA. Dehydrated zeolites was added to the system and the effect of zeolite on the dielectric and electrical insulation properties of the composite such as relative permittivity, dissipation factor, and the dielectric breakdown strength were investigated. Zeolite is crystalline aluminosilicate minerals and have the characteristics of low density and large porosity. The material has been used in many fields of catalyst, molecular sieve, ion exchange mineral, and so forth[5]. The electrical tree growth is the main cause of electric failure in thick insulator by the long-term deterioration[1,6] and that is observed by optical microscope. The dielectric breakdown strength of DGEBA/MDA/SN filled with various contents of zeolite system was measured by needle and plane electrode.

Experimental

□Materials

The epoxy resin used in this study was DGEBA(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. MDA(30 phr(parts per one hundread resin by weight)) was used as a curing agent. SN(10 phr) as a new reactive additive was supplied from Fluka Chemie AG. Natural zeolite(clinoptilolite type) from Kampo area in Korea[7,8] was used after sieving with 325 mesh under and drying in 120℃ vacuum oven for five hours under the vacuum state. The average diameter of zeolite particles was 17□. Hard steel needle with tip curvature radius 3 □ and tip angle 30°was used as high voltage electrode. The aluminum layer was used as a counter plane electrode.

□Specimen preparation and test

The ultrasonically washed needle electrodes with methyl alcohol and acetone were set in the mold with 1 mm electrode gap and the mold was preheated at 80℃ in vacuum oven. The mixture of DGEBA, SN and various amount of 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 in succession until gel state. It was poured into the pre-heated mold at 80℃. The overall curing condition was first cure at 80℃ for 1.5 hr and then second cure at 150℃ for 1.0 hr. The cured system was slowly cooled to the room temperature inside oven and sliced to 8 mm(W)×50 mm(H)×35 mm(L). The specimen was immersed into the electrical insulating silicone oil to prevent surface flashover under high electrical stress. High voltage of 60 Hz was applied to the specimen until dielectric breakdown at the voltage rising rate of 500 V/sec and the dielectric breakdown voltage was measured as dielectric breakdown voltage. The dielectric breakdown step was detected by the breakdown sound or the flash over at the tip of needle electrode. Under the specific voltage the electrical tree phenomena were observed by using an image analyzer. In order to investigate the relative permittivity, dissipation factor and volume resistivity of the system, the samples were made in the form of thin plate, 80□×1 mm. The test was carried out at 26℃, and RH 71%.

Results & Discussion

Fig. 1 shows the effect of zeolite content on the dielectric breakdown strength and the time to the puncture of DGEBA/MDA/SN composite system filled with zeolite. The dielectric breakdown strength(DBS) is the supplied electric field when dielectric breakdown occurs. As the content of zeolite increased to 10 phr the DBS increased,

Fig. 1 Effects of various zeolite contents on the dielectric breakdown

strength(■) and the time which is tacken to reach the puncture(●).

sharply. Thereafter, DBS gradually increased. Due to the barrier effect of zeolite, the tree propagation time in the filler loaded epoxy resin increased and dielectric breakdown strength increased. It may be thought that zeolite particle which is inorganic crystalline mineral is more hard than the network bonding of epoxy that the tree growth is stopped or make a detour in front of the zeolite particle. Also the time which is taken to reach the puncture at 15 kV/mm shows well the delay of the tree propagation time by zeolite particles. Tree initiation under high electrical stress is followed by the growth of fine channels driven by partial discharge activity between two electrodes[9]. It grows to be the puncture of epoxy resin.

Fig. 2 shows the tree pattern at the filler loaded epoxy resin. It is observed that the tree channel passes through the interface of zeolite and epoxy resin. The interface is so weaker than other region that the interface is vulnerable to electrical stress. The zeolite content of this specimen was 2 phr in the basis of DGEBA. And the electrical stress of 10kV was applied for 85 min.

Fig. 3 shows the effects of zeolite contents on the dissipation factor and the relative permittivity of the system. As zeolite content increased, both the dissipation factor and the relative permittivity increased. The test was performed at 1 kV and 60 Hz. At high content of zeolite, the values converged to the definite value of 4.47 and 0.059, respectively. The dissipation factor is the ratio of the true in-phase power to the reactive power, measured with voltage and current 90 degrees out of phase. This is an indication of the energy lost within the material trying to realign the molecules every time the current(voltage) changes direction in alternating current. So the zeolite particles are thought to be the material that disturbs the electrical current in the epoxy resin systems.

Fig. 2 Electrical tree growth of DGEBA/MDA/SN/zeolite(2phr) specimen.

Fig. 3 Relative permittivity and dissipation factor vs. the content of zeolite in DGEBA/MDA/SN system.

Conclusion

In this study, the electrical insulating properties of DGEBA/MDA/SN/zeolite system was investigated. From the experimental results, following conclusions were obtained. As zeolite content increased the dielectric breakdown strength and the time to the puncture increased due to the barrier effect of inorganic zeolite filler. The electrical tree propagated along the interface between the zeolite and epoxy resin toward counter plane electrode. The relative permittivity and the dissipation factor with increasing zeolite content was increased to the convergence of 4.47 and 0.059, respectively.

References

1. D.A. Bolon, IEEE Electrical Insulation Magazine, 11, 10(1995).

2. H.S. An, M.J. Shim and S.W. Kim, Kor. J. Mater. Res., 7, 733(1997).

3. H.G. Kim and H.S. Kim, Kor. J. Material Res., 7, 650(1997).

4. M.J. Shim and S.W. Kim, Materials Chemistry & Physics, 47, 198(1997).

5. D.W. Breck, "Zeolite Molecular Sieves", John Wiley & Sons, New York, 4(1974).

6. Y.S. Cho, H.K. Lee, M.J. Shim and S.W. Kim, J. Kor. Ind. & Eng. Chemistry, 3, 171(1997).

7. J.Y. Lee, H.K. Lee, J.S. Lee, M.J. Shim, and S.W. Kim, J. Kor. Inst. of Surf. Eng., 26, 285(1993).

8. J.Y. Lee, H.K. Lee, J.S. Lee, M.J. Shim, and S.W. Kim, Proceed. of IUMRS-ICA-'95, Seoul, 711(1995).

9. J.V. Champion and S.J. Dodd, J. Physics D-Applied Physics, 28, 398(1995).