첨가제가 도입된 에폭시계에서 전기적 트리의 프랙탈성에 대한

첨가제의 효과

안현수, 이홍기*, 양영석**, 심미자***, 김상욱

서울시립대학교 화학공학과, 우석대학교 화학공학과*,

한국원자력 연구소**, 서울시립대학교 생명과학과***

Effect of Additives on Fractal Characteristics of Electrical Tree

in Additives Introduced Epoxy Resin System

H. S. An, H. K. Lee*, Y. S. Yang**, M. J. Shim***, S. W. Kim

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

Korea Atomic Energy Research Institute**, Dept. of Life Sci., Seoul City Univ.***

1. Introduction

Epoxy resin system the good properties like high adhesive strength, dimensional stability, chemical resistance, high mechanical strength, etc. and has been used in the fields of automobile, adhesive, sealant, and so on. Also, epoxy resin system has been used in the forms of potting and encapsulating compounds because it has excellent electrical insulating properties.1)

However, insulating materials such as polymers had a breakdown or a deterioration in using for a long time. These long term deterioration and breakdown of electrical insulating materials subjected to ac voltages can be usually associated with treeing phenomena. Electrical trees are produced by partial discharges in small defects located at the areas of high voltage stress concentration because there are defects such as cracks, conducting particles or cavities in insulating materials. These generated trees propagate continuously and then breakdown occures in insulating materials.2,3) Thus, to develop or to design good insulating materials, many investigations on characteristics of electrical tree propagation were carried out. And many researchers have investigated electrical tree quantitatively with fractal definition of self similarity.4-6)

In this study, GN(glutaronitrile) was introduced to DGEBA(diglycidyl ether of bisphenol A)/MDA(4,4'-methylene dianiline) system for the purpose of improving toughness. GN contents dependence of fractal characteristics in DGEBA/MDA/GN system were investigated by using needle/plane electrodes.

2. Experiment

2.1 Materials

DGEBA was used as epoxy resin. MDA was employed as curing agent and had the properties of heat resistance because of the aromatic amine group. GN was used as a reactive additive and its role was to improve the impact strength by extending the chain length. Metal needle electrodes having a radius of curvature of 5 □ were used in this study.

2.2 Procedure

The needle was fixed and the distance between needle tip and plane electrode was 1 □. DGEBA/MDA(30 phr) system and various GN contents were mixed and then cured at 80 ℃ for 1.5 hr before 150℃ for 1.0 hr. The cured system was thermally annealed to room temperature at the rate of -1 ℃/min so as to reduce the voids produced by different thermal expansion coefficients between metal needle electrode and epoxy resin system. Cured systems were cut and ground to the size of 30×30×1 □ and voltage of 10 kV was applied to those system in terms of 10 sec. The length and the shape of electrical tree were investigated through optical microscope after the voltage source was switched off. The fractal dimension was measured by scale covering method. The application of voltage was performed in silicon oil so as to suppress surface discharge.

Fig. . Electrical tree propagation of DGEBA/MDA/GN system with

different GN contents.

3. Results and Discussion

Fig. 1 shows characteristics of electrical tree propagation in DGEBA/MDA/GN system with various GN contents when the voltage of 10 kV was applied to epoxy resin system. In the GN contents of 5 phr, the tree propagation rate was very slow and in that of 0 phr, tree was not produced. When the GN contents were 10, 15 and 20 phr, time to breakdown were 20, 35 and 143 sec, respectively. As GN contents increased, tree propagation rate increased. In particular, it increased sharply in the GN contents of 15 phr.

Fig. 2 shows the fractal dimension of DGEBA/MDA/GN(20 phr) system as a function of applied time. When the applied time were 10 sec and 20 sec, fractal dimensions were 1.30 and 1.35, respectively. Thus, the occurred because the fractal dimension increased with the increment of applied time. These result might be that branched tree increased as applied time increased.

Fig. 2. Fractal dimension of DGEBA/MDA/GN(20 phr) with the different

applied time by scale covering method.

Fig. 3. Fractal dimension of DGEBA/MDA system with various

GN contents as a function of voltage applied time.

Fig. 3 shows fractal dimensions of DGEBA/MDA system with the different GN contents as a function of applied time. In all GN contents, fractal dimension increased in the initial step and in the GN content of 10 phr, fractal dimension was saturated to 1.56∼1.72 as the applied time increased. These assumed that trees propagated from linear to branch type in the beginning step and then were saturated. As GN contents increased, the fractal dimension increased.

References

1. H. Lee, □Handbook of epoxy resins□, McGraw-Hill, 1-5, New-York(1980).

2. R. J. Densley, IEEE Trans. Electr. Insul., EI-14, 148(1979).

3. P. P. Bundenstein, Ibid., EI-15, 225(1980).

4. I. Arima and T. Watanabe, ibid., EI-16, 543(1981).

5. J. C. Fothergill, L. A. Dissado and P. J. Sweeney, IEEE Trans. on Dielectr. & Electr. Insul., 1, 474(1994).