Characterization of water absorbed epoxy insulating coating material used in ZnO varistors by dielectric measurements

Transcription

1 Materials Letters 60 (2006) Characterization of water absorbed epoxy insulating coating material used in ZnO varistors by dielectric measurements Shengtao Li a, Lei Liang a, Jianying Li a, Nian-jie Liu b, Mohammad A. Alim c, * a State Key Laboratory of Electrical Insulation and Power Equipment, Xi an Jiaotong University, Xi an , People s Republic of China b Shaan Xi Weihua Electronic Package Materials Company, Xianyang , People s Republic of China c Department of Electrical Engineering, Alabama A and M University, P.O. Box 297, Normal, Alabama 35762, U.S.A. Received 15 April 2005; accepted 1 August 2005 Available online 26 August 2005 Abstract The water (moisture) absorption behavior of epoxy insulating coating material used in ZnO varistors is characterized by dielectric measurements. It is observed that the water absorption process in the epoxy insulating layer matches with Fick s second law in the initial stage, and then the water absorption approaches to a saturated value. The hardener plays an important role in determining saturated value of the water absorption process of the coating material. Further it is noticed that under the same experimental conditions the relative permittivity e r, and the dissipation factor (tan d) increase in the frequency range 30 Hzf 30 khz for dry and wet (water treated) samples. The increasing rate of dissipation factor of the samples cured with the anhydride hardener is higher in the low frequency range than it is in the high frequency range. It is suggested that an ionic conductance caused by water/moisture inside the samples that may have led to the obtained experimental results. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO varistors; Dielectric measurements; Relative permittivity; Dissipation factor; Epoxy insulation coating 1. Introduction The epoxy resin powder coating material is widely used for various electrical insulations [1]. This is because of the flexible processing methods of the powder form to hardened (or cured) structure. The hardened structure of the epoxy insulating layer provides superior electrical, mechanical, and chemical properties that are often necessary in coating various types of devices from degradation, particularly in the ambient conditions for ZnO varistors which is used in circuit protection from temporary over voltages (TOVs) or lightning surges. ZnO varistors are widely used as a transient voltage surge protector in the electrical and electronic systems. Upon using epoxy resin coating * Corresponding author. addresses: sli@mail.xjtu.edu.cn (S. Li), lijy@mail.xjtu.edu.cn (J. Li), mohammad.alim@ .aamu.edu (M.A. Alim). material on to the surfaces of the ZnO varistors firm confidence is achieved from non-intrusion of the ambient gases and particles including moisture ingress when used inside the surge protection assembly (also known as housing). The ZnO varistors referred herein are highly non-linear (non-ohmic) low-voltage semiconducting ceramics small in size similar to that of the button, and achieve stable working condition when coated with the powdered epoxy resin through the processes of fluidization, melting, and hardening. The water absorption frequently limits overall insulation properties of the epoxy insulating layer. This is primarily due to the easy access of the polar water molecules penetrating through the surfaces of the epoxy layers. It is already reported [1] that the ZnO varistors with the epoxy insulating coating degrade or deteriorate under the combined exposure to high temperatures and high humidity. This degradation is exhibited via decreased varistor voltage (lower voltage gradient at breakdown) and increased X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.matlet

2 S. Li et al. / Materials Letters 60 (2006) leakage current which in turn associates with the water absorption of the epoxy insulating layers. Some earlier studies [2] have shown that the properties of the epoxy matrix composites are influenced by the absorption of water including surrounding environmental condition with gases. Epoxy resin absorbs moisture (water molecules) from the atmosphere and reaches to the equilibrium condition with respect to the ambient in a short time and then this process allows further to penetrate at the surface of the ZnO varistor through the diffusion process. In addition, water can be absorbed by the capillary action along any nonvisible minute size cracks or tubing process that may be present on to the surfaces of the insulating layer or along the filler matrix interface. Several models [3 6] have been proposed to explain the water absorption mechanism and the effects of some parameters such as the interfacial phase between the fillers and the matrix, polar radicals, and free space. The dielectric measurements of the materials are expected to be sensitive to the amount of polar molecules contained in the materials. These measurements are helpful not only in further understanding of the behavior of water diffusing in the surfaces of the epoxy insulating coating material but also in characterizing the water absorption process in to the surfaces of the epoxy coating layer. In this investigation four types of epoxy resin powders with different hardeners are applied on to the surfaces of the ZnO varistors to obtain various epoxy insulating layers. These epoxy layers were adopted with various processes and then treated at various conditions of the moisture absorption and then measurements were conducted to delineate relative permittivity e r and the dissipation factor tan d. 2. Experimental Four types of epoxy resin powders with different hardeners and processes were used in this experiment that are defined as A, B, C, and D, respectively. Samples A (PCE-210, Nippon Pelnox Corporation) and B (CP-930, Daejoo Electronic Materials Company Limited) are commercial epoxy resin products used with hardeners. These hardeners were hydroxybenzene and anhydride, respectively. The samples C and D are prepared by mixing these epoxy resins with the hardeners and other additives in minute amount of silica particles, antimony trioxoide, titanium dioxide, pigment, and then extruding at 115 -C and pulverizing. Thus, sample C contains hydoxybenzene hardener constituting inorganic and organic components equal in amount by weight. The ratio of hardener to epoxy resin by weight for this sample was 0.35 (or 35: 100). Similarly sample D contains anhydride hardener constituting inorganic and organic components equal in amount by weight. The ratio of hardener to epoxy resin by weight for this sample was 0.22 (or 22 :100). Each of these samples was easy to use possessing smooth surfaces on the ZnO varistors. The epoxy resin powders were put into a disc die and cured for 1 h at 150 -C. The final specimens turned to round discs having diameter of 50 mm and thickness of 1 mm. The quality of the materials was excellent and presumed to be bubble free. There is high level confidence in the reproducibility of these samples when made in an identical fashion using the same epoxy resin powder. The samples were at 80 -C in a heating cabinet for 24 h, and then the samples were put into a hermetical container with some water already placed on the bottom. The container was heated in an oven with a fixed heating and cooling rate of 1 -C/min. Then the temperature was maintained at 80 -C for different durations such as 10, 20, 60, and 240 min. Each duration is designated as the moisturizing time. The water used in this experiment is likely to contain trace contaminants that may be ionized and, thus, not necessarily de-ionized or distilled water. The experimental processes of moisturizing and measurement interval with respect to time are shown in Fig. 1. This illustration provides duration of both moisturizing time followed by the measurement time. Water absorption is defined and calculated using the following relation: M ¼ m m 0 m 0 100%; ð1þ where M is the water absorption in percent, m 0 is the mass of the dried sample and m is the mass of the water treated sample. The minimum observed increment of mass is about 0.1 mg for a sample. The relative permittivity e r and the dissipation factor (loss tangent) tan d of the samples were measured using a transformer bridge (model TR-10C, Japan), covering the frequency range 30 Hz f 100 khz. The results reported herein are reproducible and repeated with multiple samples keeping identical geometry. Temperature ( C) 100 moisturizing measuring moisturizing measuring Time (min) Fig. 1. Process of moisturizing and measurement interval.

3 116 S. Li et al. / Materials Letters 60 (2006) Results and discussion 3.1. Relation between water absorption and moisturizing time Fig. 2 shows the relation between the water absorption and the square root of time (t 1/2 ) of the samples. This representation indicates that all the samples are saturated with the moisture content when exposed above 20 min. Until this time is achieved the moisture content kept increasing with time from the initial immersion of the sample. This plot further suggests that the water absorption of the samples A, B, and C exhibit identical trend while the sample D possesses much higher saturated value of the water absorption. Overall this behavior indicates that the first three samples are identical while the fourth sample possesses poor behavior. It is reported in the literature [3 5,7 9] that two factors mainly determine the water absorption property of the epoxy insulating layer. First is the combination of epoxy resin and hardener, and the second is the combination of filler and organic hardener producer. The moisture absorbability property of epoxy insulating layer is mainly determined by its water absorbability and water permeability. Thus, the difference of water absorption property is mainly caused by the composition of the epoxy powders. Various hardener materials can alter the structure of the cured epoxy resin. Further change may occur through the free space or polar radicals, which is considered to be the basic reasons for water absorption in hardened epoxy resin. In many cases water absorption [2 5,8] obeys Fick s law, and the diffusion process of water is primarily driven by the concentration gradient between the environment and the material. This process leads to continuous absorption of water on to the epoxy insulating layer surfaces until the saturation is reached. Two diffusion theories are suggested [6] concerning the water absorption process in epoxy composites. The first is the classical single free phase model of absorption in which water molecules are not combined with the matrix. The second is the Langmuir two-phase model which considers a free diffusion phase and a secondary combined phase which does not involve diffusion. The mass of the moisturized samples achieves initial mass of the untreated dry samples following desorption (drying) process. Water absorption (%) t 1/2 (s 1/2 ) sample A sample B sample C sample D Fig. 2. Time dependence of water absorption of epoxy insulating resin samples. The material is not damaged by the presence of water during the entire experimental process. Thus, it can be presumed that the water absorption property of four epoxy resin materials exhibits single phase absorption. This implies that the water diffuses into the epoxy matrix and was weakly bound to the epoxy by the hydrogen bonding. Since samples were made into the shape of round discs it is reasonable to consider that the diffusion occurs in one direction of normal to the plane. According to the experimental process and the curves shape of Fig. 2, it is suggested that the water absorption process of the four samples obey Fick s second law, which is expressed as M t M V ¼ 4 l =2 D t 1=2 ; p sample A sample B sample C sample D sample A sample B sample C sample D Fig. 3. Frequency dependence of the relative permittivity e r, and dissipation factor tan d for hardened dried epoxy insulating resin samples. where M t is the water absorption after the processing time t, M V is stable water absorption, D is the diffusion coefficient, and l is the thickness of the sample Relation between dielectric response and water absorption of the epoxy insulating layer The frequency dependence of e r and tan d for dry epoxy insulating layer at room temperature (20 -C) in the frequency ð2þ

4 S. Li et al. / Materials Letters 60 (2006) range 30 Hzf 100 khz is shown in Fig. 3. It is observed from Fig. 3 that e r slowly decreases with the increasing frequency. Samples A and D possess smaller value as well as lower decreasing rate with increasing frequency compared to the samples B and C. Samples B and C exhibited higher values and slightly higher decreasing rate with increasing frequency. Fig. 3 shows that tan d decreases for every sample and reaches to a minimum value and then increases with increasing frequency. The minimum of tan d lies within 10 3 Hz and 10 4 Hz. Examining carefully, these four curves indicate that sample D has different minimum at about 10 3 Hz as it had higher water absorption behavior shown in Fig. 2. Thus, samples A, B, and C got the minimum value of tan d in the vicinity of 10 4 Hz. Overall the dielectric loss is higher for sample D which is consistent with the higher amount of absorption of moisture reflecting to be reasonably conductive. Since samples A, B, and C exhibited overall near identical dielectric response, the behavior of samples B and D are presented in Figs. 4 and 5, respectively. In Fig. 4 both the dielectric response and the loss tangent indicate consistent increment with the exposure of the moisturizing time. The tan d curve indicated large increment rate for various moisturized samples in the low frequency range than the high frequency range. The high Fig. 5. Frequency dependence of the relative permittivity e r and dissipation factor tan d for the water absorbed sample D Fig. 4. Frequency dependence of the relative permittivity e r and dissipation factor tan d for the water absorbed sample B. frequency range is demarcated with the lowest value of the tan d curve. According to dielectric physics, [10 12] the loss in low frequency range is caused by electrical conductance current, which can be described as tand ¼ r 2pf e 0 e r ; where f is the measurement frequency, r is the electrical conductivity, e o is the permittivity of vacuum, and e r is the relative permittivity. When water is introduced in the epoxy insulating layer, more H +, (OH), and other impurity ions contribute to the terminal conductance. This leads to higher value of r. Since water usually shows a very high value of e r compared to that of pure epoxy insulating layer the introduction of water will cause the increase of e r. This is a parallel effect of terminal capacitance between two species (epoxy and water) which are, in fact, side by side contributing under the same electrode. In the low frequency region the increase of r is much higher than that of e r resulting to increased tan d. In the high frequency region, since the polarization of water molecules are greatly reduced due to the increase in applied frequency. Thus, the contribution of water to the value of e r and tan d was reduced as well. ð3þ

5 118 S. Li et al. / Materials Letters 60 (2006) Hz 30Hz 3kHz 3kHz Water absorption (%) Water absorption (%) 30Hz 30Hz 3kHz 3kHz Hz 30Hz 3kHz 3kHz 30Hz 30Hz 3kHz 3kHz Water absorption (%) Water absorption (%) (c) (d) Fig. 6. Variation of e r and tan d for sample B as a function of water absorption in the frequency range 30 Hzf 30 khz. Variation of e r and tan d for sample D as a function of water absorption in the frequency range 30 Hzf 30 khz. (c) Variation of e r and tan d for sample A as a function of water absorption in the frequency range 30 Hzf 30 khz. (d) Variation of e r and tan d for sample C as a function of water absorption in the frequency range 30 Hzf 30 khz. The dependence of e r and tan d on the absorption of water is considered both at high and low frequencies. The results are shown in Fig. 6 incorporating two extreme measurement values of frequency such as 30 Hz and 30 khz. The behavior of e r and tan d of sample B and sample D as a function of water absorption is shown in Fig. 6 and, respectively. For sample D both e r and tan d increased drastically above water absorption 1.7%. This behavior indicates additional effect may be visible as impurity content is not ruled out in the regular water used in the absorption process. Subsequent investigation concerning the influence of the impurities in the regular water is continuing. The insert in Fig. 6 shows the comparison between the two samples B and D using Fig. 6. The same situation happened for samples A and C suggesting that the effect of water molecules on the dielectric parameters for these pair of sample may have linear-like influence on the dielectric measurements. 4. Conclusions The epoxy insulating samples investigated exhibited water absorption process via single phase absorption for each sample. Water absorption of these samples obeys Fick s second law at the initial stage, and then approaches to a saturated state after a certain period of moisturizing process. The relative permittivity and the dissipation factor increase after the water absorption. At low frequencies the amount of increase is larger than at high frequencies. This increment is attributed to the conductance due to the presence of the impurity ions in the regular water. The water absorption dependence of e r and tan d shows nearly identical type behavior for the samples A, B, and C while a large enhancement in these parameters are observed for the sample D. Furthermore, sample D absorbed a large quantity of water for the same period of exposure and exhibited increased conductance. References [1] J. Li, F. Liu, S. Li, Polymer Materials Science and Engineering (in Chinese), vol. 15, 1999, p [2] J. Li, S. Li, M.A. Alim, G. Chen, Act. Passive Electron. Compon. 26 (2003) 235. [3] M. Ezoe, M. Tada, M. Nakanisi, Proceedings of the 5th Int. Conf. on Prop. and Appl. of Dielect. Mat., May 25 30, 1997, p [4] T. Kumazawa, M. Oshi, M. Todoki, T. Watanabe, Proc. of the 5th Int. Conf. on Prop. and Appl. of Dielect. Mat., May 25 30, 1997, p [5] H.C. Kärner, A. Schütz, Proc. of the 4th Int. Conf. on Prop. and Appl. of Dielect. Mat., July 3 8, 1994, p [6] M.G. McMaster, D.S. Soane, IEEE Trans. Components Hybrids Manuf. Technol. 12 (1989) 373.

6 S. Li et al. / Materials Letters 60 (2006) [7] S. Yamanaka, H. Kawamura, M. Ezoe, M. Tade, 11th Int. Symp. on Hi. Volt. Engr. Symp., London, UK, August 1999, Conf. Publ. No.467, 24, 1999, p. 67. [8] J. Zhou, J.P. Lucas, Polymer 40 (1999) [9] A. Fukuda, H. Mitsui, Y. Inoue, K. Goto, Proc. of the 5th Int. Conf. on Prop. and Appl. of Dielect. Mat., May 25 30, 1997, p. 58. [10] R. Coelho, Physics of Dielectrics for the Engineer, Elsevier Scientific Publishing Company, Amsterdam, [11] I. Bunget, M. Popescu, Physics of Solid Dielectrics, Elsevier Science Publishing Company, Amsterdam, [12] J.C. Anderson, Dielectrics, Reinhold Publishing Corporation, New York, 1964.