Electrical Engineering in Japan, Vol. 131, No. 3, 2000 Translated from Denki Gakkai Ronbunshi, Vol. 118-A, No. 11, November 1998, pp. 1255 1263 Thermal Aging, Water Absorption, and Their Multiple Effects on Tracking Resistance of Epoxy for Outdoor Use SEIJI KUMAGAI, WANG XINSHENG, and NOBORU YOSHIMURA Department of Electrical and Electronic Engineering, Akita University, Japan SUMMARY The correlation between tracking resistances of outdoor polymer insulating materials and ambient environmental stresses has been studied. In this study, the tracking resistance variation of a cycloaliphatic epoxy resin filled with high concentration silica powder is investigated as a function of thermal aging and water absorption. Experimental works show that the tracking resistance of this epoxy system is decreased considerably by thermal aging and water absorption. The decrease due to water absorption is found to be an apparent phenomenon because on complete drying treatment, the tracking resistance recovers to its initial state. The combined effect of thermal aging and water absorption makes the tracking resistance decrease to a greater extent than each stress individually. In addition, the tracking resistance of epoxy that is aged by thermal treatment and further water absorption-drying treatment cannot recover to its pre-water-absorption state. Mechanisms of the tracking resistance variations of this epoxy system resulting from thermal aging and water absorption are discussed. 2000 Scripta Technica, Electr Eng Jpn, 131(3): 9 18, 2000 Tracking resistances of polymer insulating materials can be varied by ambient environmental stresses such as UV radiation, heat, acid rain, and radiation [1]. With a view to prevention of dielectric breakdown accidents, the impact of aging derived from these environmental stresses on the tracking resistances should be carefully examined. In order to ensure long-term reliability of polymer insulating materials outdoors, much information about the tracking behavior is desired. Epoxies are widely used polymeric materials for either indoor or outdoor insulation. A large number of epoxy systems, the result of variations in chemical compositions of epoxy resins, hardeners, fillers, and the like, have been utilized. For the field of outdoor insulation, which requires epoxies having high resistances to the environment and tracking, cycloaliphatic epoxy filled with silica powder of high concentration is a suitable system [2]. Study of the tracking behavior of this epoxy system and the correlation with environmental stresses will be helpful in designing outdoor insulation systems and improving their reliability. Up to now, the effect of acid rain and UV radiation on the tracking resistance of this system has been investigated in depth [3 5]. In this study, heat and water absorption are employed as sources of environmental stress, and their overlapping effect is also investigated. As a result, it is shown that both stresses decrease the tracking resistance of the epoxy considerably. When they overlap the epoxy, the tracking resistance declines to a greater extent. Mechanisms for reduction of the tracking resistance are proposed. 2. Experimental Key words: Outdoor insulation; epoxy; tracking; tracking resistance; thermal aging and water absorption. 1. Introduction 2.1 Sample The sample prepared for this study is diglycidyl ether-type cycloaliphatic epoxy resin. Quartz silica powder of 200 parts per hundred by weight (pph) is filled. The molecular structure of this type of cycloaliphatic epoxy resin before curing treatment is presented in Fig. 1. The sample shape is a slab (120 u 50 u 3.9 mm 3 ). The sample of this shape is directly subjected to heat treatment, water absorption treatment, and tracking test. Before testing, the sample is desiccated in air for over 1 week at room temperature. In the following, this epoxy system is simply called epoxy. 9 2000 Scripta Technica
Fig. 1. Molecular structure of diglycidyl ether-type cycloaliphatic epoxy resin. 2.2 Heat treatment and water absorbing-drying treatment An electrical oven (Advantec, FV-430) is used to carry out heat treatment. The sample is kept in it (air ambient) at the desired temperature for 100 h. Water absorption is implemented by immersing the sample in distilled water at 95 to 97 C. The sample is dried in the above electrical oven maintained at 100 C. 2.3 Tracking resistance evaluation The tracking resistance is evaluated according to IEC Publication 587. The details of this test method are described in Refs. 3 to 5. The applied voltage is ac 4.5 kv. When the test time exceeds 6 h (i.e., the sample survives the test), it is stopped. In this case, the time to tracking failure is defined to be 6 h. Tracking resistance of epoxy is quantified by the average time to failure of four to seven specimens. 3. Results and Discussion 3.1 Tracking resistance of thermally aged epoxy 3.1.1 Tracking resistance evaluation Figure 2 shows the relation between heat treatment temperature and time to tracking failure. The average time to failure is indicated by the solid circles. When epoxy is aged at 20 to 170 C, both epoxy which allows tracking and survives the tracking test appear. When tracking is allowed, the time to tracking failure ranges between 30 and 180 min. In this temperature range of heat treatment, significant variation in the tracking resistance does not appear. However, epoxy aged at 200 C permits tracking to develop in 5 to 40 min. That at 250 C requires just 1 min. Epoxy aged above 200 C has lost its tracking resistance, implying that the surface of thermally aged epoxy has changed into the transferable molecular structure to conductive carbon. Analysis of the surface structure of thermally aged epoxy Fig. 2. Relation between heat treatment temperature and time to tracking failure of epoxy. will be done to reveal the mechanism of decrease in the tracking resistance. 3.1.2 Visible spectroscopy Color at the surface provides important information in analyzing the surface molecular structure. If the composition of elements is identified to a certain extent, the molecular structure can be estimated from the surface color [6]. It is found from visual observation that the surface color of epoxy turns gray, reddish brown, brown, and blackish brown with an elevation of heat treatment temperature. A color analyzer (Juki, JP7100) is employed to quantify the change in the surface color. Visible rays (wavelength: 400 to 700 nm) emitted from the light source are absorbed at the epoxy surface, and then some reflective rays return to the sensor, enabling measurement of light intensity. Reflective visible (RV) spectra (wavelength versus reflective light intensity) of epoxies aged at different temperatures are presented in Fig. 3. Those of standard colors (white, blue, green, red, and black) whose data are installed in the color analyzer are also presented. Unaged epoxy displays gray, so that an intermediate spectrum between white and black appears. With an increase in heat treatment temperature, the reflectance in the green and blue region decreases, and then that in the red region decreases. This tendency indicates that the color at the epoxy surface becomes black with a development of thermal aging. According to the tracking test result that the decrease in tracking resistance begins at 170 C, the transfer in color from red to black via brown corresponds to the tracking resistance decline. 3.1.3 Attenuated total reflection Fourier transform infrared spectroscopy Chemical bonds at a depth of 1 to 10 Pm can be identified by using attenuated total reflection Fourier 10
Fig. 4. ATR-FTIR spectra of thermally aged epoxies. Fig. 3. RV spectra of epoxies aged at different temperatures and standard colors. transform infrared spectroscopy (ATR-FTIR). A Jeol JIR- 6500 FTIR spectrometer with an ATR attachment of KRS-5 crystal prism is used. Contact area and pressure between the sample and the KRS-5 prism is maintained constant. Figure 4 shows the ATR-FTIR spectra of epoxies aged at various temperatures. IR absorptions at 2927 and 2854 cm 1 (band a) appearing in the spectrum of the unaged are caused by C H bonds outside of cyclohexanes. The absorption at 1450 cm 1 (band c) results from C H bonds inside of cyclohexanes. The absorptions of C=O and C O bonds appear at 1726 and 1159 cm 1, respectively. The large absorption at 1200 to 800 cm 1 (band e) and that below 500 cm 1 are due to Si O bonds in quartz silica. With an elevation of heat treatment temperature, ATR-FTIR spectra become unclear and all of the absorptions weaken. This is attributed to an increase in the surface roughness, which is shown in the following SEM observation. However, it is found that C H bonds both inside and outside of cyclohexanes and C=O bonds disappear at 200 and 250 C, respectively. Only C O bonds are detected at the surface of epoxy aged at 250 C. The loss of C H bonds is consistent with the decrease in tracking resistance. 3.1.4 Scanning electron microscope observation and energy-dispersive X-ray analysis The surface morphology of thermally aged epoxy and the surface composition are analyzed by employing a scanning electron microscope and an energy-dispersive X-ray analyzer (SEM-EDX). The SEM-EDX used is a combination of Hitachi S-4500 and Horiba EMAX-5770W equipment. The accelerating voltage of SEM-EDX is 20 kv, corresponding to an analyzing depth of about 1 Pm. SEM photographs of the surfaces of epoxies aged at 200 and 250 C are shown in Fig. 5. It is found that heat treatment at high temperature exposes the filler of silica and reduces the epoxy resin at the surface. In addition, many pores appear at the surface of the epoxy after aging at 250 C. The composition of elements is detected by EDX to be mostly carbon, oxygen, and silicon. Oxygen is contained in both epoxy resin and silica molecules, so that the compositional ratio of silica molecules (SiO 2 ), carbon atoms (C), and oxygen atoms (O) is presented in Fig. 6. It is seen that carbon decreases and silica increases when aged above 170 C. The atomic composition of carbon at the unaged surface is about 65%. This decreases to about 60% and further to 35% with the development of thermal aging. The ratio of carbon and oxygen in the epoxy resin is about 4.2 to 1. After being aged at 200 and 250 C, this changes to 3.8 to 1 and 2.8 to 1. Thus, it is clear that carbon leaves the surface more preferentially than oxygen. Even though aged at 250 C 11
(causing carbon to disappear), the atomic composition of carbon is about 35% and more carbon atoms exist at the surface than oxygen atoms. Fig. 6. Compositional ratio of silica molecules, carbon atoms, and oxygen atoms at the surfaces of thermally aged epoxies. 3.1.5 Mechanism of the tracking resistance decline induced by thermal aging Fig. 5. SEM photographs of thermally aged epoxy surfaces. Visual spectroscopy, ATR-FTIR, and SEM-EDX could provide much information on the molecular structure transfer at the epoxy surface. ATR-FTIR indicates that C H bonds are broken at 200 C and C=O bonds at 250 C, while C O bonds are not broken below 250 C. It is shown by SEM-EDX that, even after heat treatment at 200 and 250 C, more carbon exists than oxygen. Therefore, it is suggested that, with heat treatment, hydrogen gradually disappears and C C and C O bonds increase in place of C H bonds. Visible spectroscopy reveals that the color at the surface turns red, brown, and black with the development of thermal aging. It is reported that oxidized carbon, which is composed of carbon and oxygen and has a cyclic structure of single and double bonds, displays a red color [7]. The RV spectrum of epoxy aged at 200 C contains a component of red color. In addition, C=O and C O bonds are detected at its surface by ATR-FTIR. Hence, the molecular structure at the surface after aging at 200 C is estimated to be similar to that of red carbon as shown in Fig. 7. When aged at 250 C, C=O bonds disappear and the surface has a black color, suggesting that the thermally aged surface contains many C C and C O bonds and assumes a cyclic structure. This molecular structure is also presented in Fig. 7. The development of tracking on unaged epoxy requires breakage of C H, C=O, and C O bonds and, moreover, the formation of conductive carbon. For epoxy aged at 250 C, only scission of C O bonds is responsible for tracking because C H and C=O bonds have been broken by thermal aging. This can transfer readily into a planar carbon tetragonal structure inducing electrical conduction even if the heat 12
where A, w, and w i are the water absorbance, the weight after water absorption or drying, and the initial weight, respectively. Figure 8 present the water absorbance versus water absorption time and that versus drying time after water absorption for 120 h. It is found that the water absorbance increases with water absorption time. The water absorbance of epoxy is about 1.0 and 2.5% after 48 and 120 h, respectively. In the drying process, the water absorbance decreases considerably after 50 h. However, below 0.5%, the water absorbance does not decrease readily. From these results, it is determined that the tracking resistance of epoxy absorbing water is assessed at about 1.0% water absorbance (48 h) and 2.5% (120 h). Tracking resistance after complete drying for 200 h is also assessed. 3.2.2 Tracking resistance of epoxy that absorbs water and after removal of water The relation between the tracking resistance and water absorption time is shown in Fig. 9. It is clear that the (1) Fig. 7. Estimated molecular structures of thermally aged epoxy surfaces. energy provided from surface discharges is small. Hence, the tracking resistance of epoxy aged at high temperature is considerably reduced. 3.2 Effect of water absorption on the tracking resistance of epoxy 3.2.1 Water absorbing-drying characteristic of epoxy The tracking resistance of epoxy that absorbs water and dries completely is evaluated. In order to understand the water absorbing-drying characteristic, the relation between the amount of water absorbed in epoxy and the absorption or drying time is obtained. The amount of water absorbed is quantified by water absorbance: Fig. 8. Water absorption-drying characteristic of epoxy. 13
tracking resistance decreases with water absorption time (water absorbance). All of the samples allow tracking to develop within 20 min when they absorb water for 120 h, implying that water absorption phenomenon decreases the tracking resistance of epoxy seriously. Figure 10 presents the tracking resistance of epoxy after complete removal of water as a function of previous water absorption time. It is observed that the tracking resistance recovers to the level before water absorption. Hence, it is suggested that the tracking resistance decrease caused by water absorption is temporary, and that the behavior of absorbed water in epoxy is closely related to it. 3.2.3 Behavior of absorbed water and tracking resistance decline A difference between absorbing and unabsorbing epoxy appears during the development of tracking. When epoxy absorbs water, tracking is initiated with cracking, a larger explosion, and a stronger rupture sound (see Fig. 11). Two mechanisms responsible for the decreased tracking resistance are suggested. One is mechanical chain scission induced by great expanding force of absorbed water when boiling. Another mechanism is that carrier ions in contaminated electrolytes migrate into the bulk through absorbed water and cause internal discharges. Internal discharges that occur in oxygen-poor conditions suppress the evolution of CO and CO 2. Owing to mechanical chain scission and internal discharges, the decreased tracking resistance of epoxy can be explained. On the other hand, absorbed water is suspected to have an effect which is similar to that of crystalline water in alumina trihydrate filler [1]. Oxygen in crystalline water promotes the gasification of carbon, not the formation of conductive carbon. To verify this effect, the compositional ratio of carbon and oxygen in epoxy resin at different water absorption levels is theoretically calculated based on the molecular structure shown in Fig. 1. This Fig. 10. Tracking resistance of epoxy after water absorption and further complete drying as a function of water absorption time. result is presented in Table 1. For untreated epoxy, the ratio of carbon and oxygen (C : O) is 2 : 1, indicating that more carbon exists. The ratio at 3% absorbance, which exceeds the highest value in this study (2.5%), is 1.65 : 1. Even if all oxygen can react with carbon to form CO gas at this level, excess carbon unable to gasify exists. Hence, it is shown that absorbed water cannot reduce the tracking resistance considerably. The large decrease in the tracking resistance due to water absorption is suggested to result from mechanical chain scission and internal discharges. 3.3 Tracking resistance of epoxy stressed by both thermal aging and water 3.3.1 Water absorbing-drying characteristic of thermally aged epoxy The water absorbing-drying characteristic of epoxy aged at 140 to 250 C for 100 h is evaluated. Figure 12(a) Fig. 9. Relation between water absorption time and tracking resistance of epoxy. Fig. 11. Appearance tracking failure of water-absorbed epoxy. 14
Table 1. Atomic ratio of carbon and oxygen excluding silica filler Absorbance (%) C : O ratio 0 2.00 : 1.00 1 1.88 : 1.00 2 1.75 : 1.00 3 1.65 : 1.00 presents the water absorbance versus water absorption time and Fig. 12(b) that versus drying time after water absorption for 120 h. A significant dependence of water absorbing-drying characteristic on heat treatment temperature is not observed. In this study, heat treatment is carried out at high temperature in a short time, so that thermal aging takes place only in the surface region, and the bulk part governing the water absorption and drying characteristic is not aged. From this result, we determine the tracking resistance of thermally aged epoxy that absorbs water for 48 and 120 h, which times are similar to those of non-thermally aged samples. The tracking resistance after complete removal of absorbed water by drying for 200 h at 100 C is also evaluated in order to verify whether recovery of the tracking resistance caused by the complete removal of absorbed water occurs in thermally aged epoxy or not. 3.3.2 Tracking resistance of thermally aged epoxy absorbing water and after complete removal of absorbed water The relation between the time to failure of epoxy previously aged at 140, 200, and 250 C and the water absorption time is shown in Fig. 13. When epoxy is previously aged at high temperature, the extent of the tracking resistance decrease due to absorbed water increases. Figure 14 presents the tracking resistance of thermally aged epoxy after complete removal of water as a function of previous water absorption time. It is observed that, with heat treatment temperature and water absorption time, the recovery ability of the tracking resistance resulting from the removal of absorbed water declines. Hence, the process of water penetration and removal tends to decrease the tracking resistance of thermally aged epoxy. 3.3.3 Mechanism of the tracking resistance decline for epoxy stressed by thermal aging and water absorption Fig. 12. Water absorbing-drying characteristics of epoxies aged at different temperatures. The tracking resistance of water-absorbed epoxy decreases with previous heat treatment. This is a result of the two mechanisms mentioned above. The effects of thermal aging, making the chemical structure at the surface change into a planar carbon tetragonal structure, and that of absorbed water, inducing mechanical chain scission and internal discharges, overlap in epoxy and decrease the tracking resistance considerably. An SEM photograph of epoxy surface after being aged at 250 C and further water absorption for 120 h followed by drying for 200 h is shown in Fig. 15. It is seen that, after these treatments, pores appear at the surface and cracks may be formed inside as a result of thermal decomposition and water penetration and removal. It is reported that the adhesiveness between the epoxy resin and silica filler weakens after water absorption by boiling, implying that the cracks between the resin and filler also increase. If electrolyte flows onto and covers the surface in this state, the electrolyte penetrates inside readily through pores and cracks and induces internal discharges. Therefore, the tracking resistance of thermally aged epoxy could not recover to the state before water absorption even though complete water removal was carried out. 15
Fig. 13. Relation between water absorption time and time to tracking failure of epoxies aged at different temperatures. Fig. 14. Tracking resistance of thermally aged epoxy after water absorption and further complete drying as a function of water absorption time. 16
(4) The tracking resistance of thermally aged epoxy that absorbs water is also low. However, it does not recover to the level before water absorption even after complete removal of water. Acknowledgments The authors acknowledge Mr. M. Kudo of Akita Industry Technology Center for help in chemical analysis. The authors also thank Dr. M. Suzuki, Dr. K. Mitobe, and Mr. T. Sato, Akita University, for encouragement and support in this study. REFERENCES Fig. 15. SEM photograph of epoxy surface stressed by heat treatment at 250 C for 100 h, water absorption for 120 h, and drying for 200 h. 4. Conclusions The effects of thermal aging and water absorption on the tracking resistance of cycloaliphatic epoxy resin with 200 pph silica filler are investigated. The results obtained are summarized as follows. (1) On thermal aging at 200 to 250 C for 100 h in air ambient, the tracking resistance of cycloaliphatic epoxy resin filled with silica powder decreases considerably. (2) The tracking resistance of cycloaliphatic epoxy resin filled with silica powder absorbing water is very low. However, loss of the tracking resistance due to water absorption is temporary, and the tracking resistance can recover to the initial level after complete removal of absorbed water. (3) The combined effect of thermal aging and water absorption makes the tracking resistance decrease to a greater extent than each stress individually. 1. Yoshimura N, Kumagai S, Du B. Research in Japan on the tracking phenomenon of electrical insulating materials. IEEE Electr Insul Mag 1997;13:8 19. 2. Effects of additives and fillers on solid insulating materials. Tech Rep IEE Japan Part II No. 342, 1990. (in Japanese) 3. Kamosawa T, Yoshimura N, Nishida M, Noto F, Masui M. Influence of ultraviolet rays on tracking deterioration of epoxy resin. Trans IEE Jpn 1988;108-A:397 404. (in Japanese) 4. Kumagai S, Wang X, Yoshimura N. Effect of UV-rays on the tracking resistance of outdoor polymer insulating materials. Trans IEE Jpn 1997;117-A:289 298. (in Japanese) 5. Kumagai S, Wang X, Yoshimura N. Surface degradation phenomena of outdoor polymer insulating materials due to acid rain. Trans IEE Jpn 1996;116-A: 1121 1128. 6. The Institute of Analytical Chemistry of Japan. Handbook for polymer analysis. Asakura; 1985. (in Japanese) 7. Shibata Y, Kimura K. Encyclopedia of inorganic chemistry. Maruzen; 1965. (in Japanese) 8. Yoshida Y, Higashihara T, Nomura T, Usui E, Hibi S. The effect of silane agent on tensile and bonding properties of silica filled epoxy after water absorption by boiling. Trans IEE Jpn 1997;117-A:1004 1012. (in Japanese) 17
AUTHORS (from left to right) Seiji Kumagai (student member) received his B.E. and M.E. degrees from Akita University in 1995 and 1997. He is currently studying for his Ph.D. degree. His research interests include performance, aging, and evaluation of polymeric materials for outdoor HV insulation. Wang Xinsheng (member) received his B.E. degree from Xi an Jiantong University in 1981, his M.E. degree from Wuhan Hydraulic and Electrical Engineering University in 1986, and his Ph.D. degree from Xi an Jiantong University in 1992. He is currently a lecturer at Akita University. His interests include electrical insulation in HV engineering and electrical insulating materials. Noboru Yoshimura (member) received his B.E. and M.E. degrees from Akita University in 1967 and 1969, and his D.Eng. degree from Nagoya University in 1975. He joined Akita University in 1969. He became a professor in 1983 and Dean of the Mining College of Akita University in 1995. Since 1998, he has been First Dean, Faculty of Engineering and Resource Science, Akita University. From 1978 to 1979, he was a visiting scholar at Clarkson University (USA). His research interests include organic dielectric materials, dielectric and semiconductive ceramics. 18