고분자 절연재료의 산화유도시간 측정에 의한 산화안정성 평가

조영신, 송영욱, 김상욱

서울시립대학교 화학공학과Evaluation of Oxidative Stability by OIT of Polymer Insulators

Young-Shin Cho, Young-Wook Song, Sang-Wook Kim

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

Introduction

Many kinds of polymeric materials have been used for the electrical insulation under various service conditions. Polyethylenes(XLPE, LDPE), rubbers(EPDM, silicon), epoxy resins are the most commonly used polymeric insulators under high voltage and serious environmental conditions. Electric components are operated under complex conditions of high temperature, high electric field, mechanical, chemical and environmental stresses, etc. Polymeric materials are more rapidly deteriorated under high temperature and dense oxygen state[1,2]. The oxidative stability of the polymeric insulation is very important for the utilization of electric component. To evaluate the reliability of the polymeric materials as an electrical insulator under the heavy oxygen and high temperature, the oxidation induction time(OIT) of polymer materials were measured and compared with each other.

Experiment

The experimental materials used for the OIT measurement of epoxy resin are shown in Fig. 1. Epoxy resin DGEBA/MDA/SN system was prepared according to the literature[3].

DGEBA

MDA                       SN

Fig. 1. Basic materials for epoxy resin DGEBA/MDA/SN system.

XLPE and EPDM was sampled from the power distribution cable of LG CN/CV 22.9 kV cable and Pyung-Il EPDM insulator. The OIT measurements were performed at 180℃ using a Solomat TA DSC 4000 Calorimeter. Each sample was heated until the specific temperature for oxidation reaction at the heating rate of 20 ℃/min and N2 ambient gas flowing rate of 100 ml/min, then the ambient N2 gas was replaced by O2 gas flowing at the same rate. OIT was detected from the DSC baseline-shift. OMT is the most rapidly oxidizing time in the activated oxidation process.

Results and Discussion

Fig. 2. DSC curve of XLPE under oxygen at 180℃.

Oxidation process is initiated from the surface of the sample which contacts with the oxygen fluid directly. When the surface is oxidized or oxidation reaction is induced, the reaction process is accelerated and shows the exothermic peak on the DSC curve. The peak time is the most rapidly oxidizing time(OMT). Fig. 2 shows the DSC curve of XLPE at 180℃ under O2 environmental condition. From the baseline shift of DSC curve, the measured initial oxidizing time was 2155 secs. For the simple chemical structure of polyethylene, it may be thought that the oxidation mechanism and the oxidation by-products are not complex. The OMT was 4870 secs. The curve is smooth and it is easy to detect the baseline shift time and peak time on the DSC curve for the polyethylene.

Fig. 3. DSC curve of EPDM under oxygen at 180℃.

It is well known that the ethylene propylene diene terpolymers (EPDM) has a superior resistance to oxygen or ozone attacks[4]. But the system is deteriorated by oxidation at high temperature. Fig. 3 shows the oxidation process of EPDM rubber under oxygen measured by isothermal DSC test. The purpose of the OIT measurements is to characterize the oxidizing deterioration stability. It shows the OIT around 2200 secs and OMT at 11700 secs but they are not clear. The measured values depend on the sample thickness and oxygen flow rate. The broad investigation should be carried out to get a reliable results.

Fig. 4. DSC curve of DGEBA/MDA/SN system under oxygen at 180℃.

Fig. 4 shows the oxidation process of epoxy resin DGEBA/MDA/SN system. It is demonstrated that the epoxy film is susceptible to be oxidized and degraded on a aluminum surface at the elevated temperature and oxygen presence. It is known that the amine groups in the epoxy resin are oxidized to nitriles and the polymeric backbone is thermally degraded by the catalysis of metal contact[5].

The measured OMT values at various material conditions can be plotted on the Eyring compensation coordinate to obtain the thermodynamic parameters of □H, □S and □G by using the following equation.

[eqn: t`` ∼ h over kT exp right [ {Delta G^＇} over kT left ]` ∼ h over kT exp right [ -DeltaS^＇ over k left ] exp left [ DeltaH^＇ over kT right ]]

The ageing behavior of polymeric insulation used in the electrical insulation of cable, transformer, circuit breaker, etc. is affected not only by material properties, but also by environmental conditions and the by-products from ageing are various. It is very difficult to evaluate and chase the oxidizing stability of the polymeric materials by using only DSC analysis. The DSC test results should be associated with FT-IR, UV absorption spectrophotometry, density measurements, and so on for the support of DSC measurements. The tests are now on going and the results will be shown in the presentation.

References

1. Dorner G., Lang R.W., "Influence of various stabilizer systems on the ageing behavior of PE-MD - I. Hot-water ageing of compression molded plaques", Polymer Degradation & Stability, 62(3), 421(1998).

2. Allen N.S., Palmer S.J., Marshall G.P., Gardette J.L., "Environmental oxidation processes in yellow gas pipe - Implications for electro-welding", Polymer Degradation & Stability, 56(3), 265(1997).

3. J.Y. Lee, Y.W. Song, M.J. Shim, S.W. Kim, "Cure rate of DGEBA/MDA/GN/HQ system by differential scanning calorimetry analysis", Materials Chemistry and Physics, 50, 256(1997).

4. S.W. Kim, C.H. Lee, "EPDM compounds for electrical insulation application", Polymer Science and  Technology, 9(5), 389(1998).

5. Gu-XH, Xue-G, "Studies of copper-catalyzed degradation of epoxy coating and a method for improving the durability of an epoxy/copper interface after thermal aging", Angewandte Makromolekulare Chemie, 251, 131(1997).