Silane 결합제로 처리된 Al2O3 충전 에폭시 수지

복합재료계의 절연 특성

조영신, 심미자*, 김상욱

서울시립대학교 화학공학과, *생명과학과

Dielectric Breakdown Characteristics of Epoxy Resin Composite System Filled with Al2O3 Treated by Silane-coupling Agent

Young Shin Cho, Mi Ja Shim*, Sang Wook Kim

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

*Dept. of Life Sci., The Univ. of Seoul, Seoul 130-743, Korea

1. Introduction

The epoxy resin system has a good combined properties of electrical, mechanical and thermal properties and has been used for the electrical insulating system in types of molding, impregnating, encapsulating, lamella, etc. To enhance the electrical properties non-reactive fillers are incorporated with polymeric insulator. Inorganic filler filled epoxy composites are used for the complex insulator in high voltage electric power system[1,2]. Some epoxy resin systems are too brittle because of its high cross-linking density. In this study a new reactive additive SN(succinonitrile) was introduced into conventional epoxy resin system DGEBA(diglycidal ether of bisphenol A)/MDA(4,4'-methylene dianiline) to increase its impact strength. Silane treated inorganic filler Al2O3 was mixed with the system[1]. The dielectric breakdown phenomena by electrical treeing deterioration were observed. The effects of elecrode spacing between needle electrode and counter-plane electrode were studied. From all the test results the dielectric breakdown mechanism and the way how to increase electrical treeing resistance were discussed.

2. Experiment

2-1. Materials and Specimen Preparation

The liquid epoxy resin of DGEBA(EPON 828 grade) at room temperature and curing agent MDA were used as the polymeric electrical insulating matrix. A reactive additive SN(5 phr) and various amount of silane coupling agent treated Al2O3 were added. The mixture was degassed and stirred until gel state and then poured into the releasing agent applied and pre-heated mold at 80℃. The ultrasonically washed needle electrodes(hard steel, diameter of 1 □, tip curvature radius of 3 □, tip angle of 30o) with methyl alcohol and acetone were placed into the mold. The gap between needle and plane electrodes was various from 0.3 to 1.8 mm. The overall  curing condition of epoxy resin composite system was 1st cure at 80℃ for 1.5 hr and 2nd cure at 150℃ for 1.0 hr. The thermally cured specimen was cooled to the room temperature inside oven slowly and sliced to the standard specimen size and checked by using microscope. The specimens without defects such as voids and cracks at the tip of needle electrode were selected and used.

2-2. Dielectric Breakdown Test

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 by using high voltage generator and the voltage when the system was broken down was measured as dielectric breakdown voltage. The dielectric breakdown step was detected by the dielectric breakdown sound or the flash over at the tip of needle electrode. Under a specific voltage the electrical tree propagating phenomena were observed by using high-performance image analyzer.

3. Results and Discussion

Fig. 1 and 2 show the effects of electrode gap on the dielectric breakdown voltage and strength of DGEBA/MDA/SN/Al2O3 composite system. The apparent dielectric breakdown voltage(Fig. 1) is the supplied electrical stress when dielectric breakdown occurs. The dielectric breakdown strength(Fig. 2) was measured by the dielectric breakdown voltage divided by electrode gap. As the electrode gap increased the dielectric breakdown voltage increased and the increasing deviation decreased. Due to the barrier effect of the filler, the tree propagation time in the filler-loaded epoxy resin increases and the dielectric breakdown voltage increases. But the dielectric breakdown strength curve is not linear. From this result it is clear that

Fig. 1 Effects of electrode gap on the dielectric breakdown voltage.

Fig. 2 Effects of electrode gap on the dielectric breakdown strength.

doubling up the using voltage of electrical system the thickness of electrical insulating material should be thicker than 2 times. The internal defects such as voids, cracks, free volume or impurities can affect the electrical properties. As the contents of Al2O3 increase, it is thought, the number of voids and amount of impurities increase and the probability of dielectric breakdown strength increases. Like this as the thickness of specimen increased the dielectric breakdown strength decreased and saturated to 30 kV/mm.

Fig. 3 shows the electrical treeing deterioration phenomena from the needle electrode which was inserted to simulate the stress concentration point. Electrical tree was started from the side region of electrode not from the tip. This is because the voids or impurities were placed around the needle electrode. As impurities or defects

Fig. 3 Electrical treeing deterioration phenomena from the needle electrode.

increased the electrical stress was concentrated on them easily and the dielectric deterioration was initiated from the weakest point. Electrical treeing deterioration phenomena are the main causes of dielectric breakdown of high electrical power system[3,4]. To increase the treeing resistance the defects should be reduced. It is suggested that the system should be completely degassed. Also the bonding strength between filler and epoxy resin matrix can affects the properties. In this study the silane-coupling agent was introduced to enhance the bonding strength between organic matrix and inorganic filler. A new way of increasing the bonding strength should be studied. The test results and discussions of the modified epoxy composite system will be discussed in the next presentation.

4. Conclusions

From the test results of the dielectric breakdown characteristics in DGEBA/MDA/SN system at various electrode gap and silane coupling agent treated Al2O3 contents under high electric field, following conclusions were obtained. As the electrode gap or specimen thickness increased the dielectric breakdown voltage increased but the dielectric breakdown strength decreased and saturated to 33 kV/mm. Eclectical tree was not propagated from the tip of needle electrode but from the side of the electrode around which lots of defects were placed. Electrical tree was propagated through blocking fillers toward counter plane electrode.

References

1. M. Ruike, S. Fujita and F. Noto, T. IEE Japan, 115-A, 744 (1995).

2. K.T. Park, G.H. Park, J.B. Wang and J.U. Lee, Proc. 2nd Kor.-Jap. Joint Symp. on EI&DM, 50 (1993).

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

4. Y.S. Cho, M.J. Shim and S.W. Kim, Materials Chemistry & Physics, 52, 94 (1998).