Experimental analysis of hydrophobic characteristics of Silicone rubber (SiR) insulators under different climatic conditions

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1 International Journal of Physical Sciences Vol. 7(47), pp , 16 December, 2012 Available online at DOI: /IJPS ISSN Academic Journals Full Length Research Paper Experimental analysis of hydrophobic characteristics of Silicone rubber (SiR) insulators under different climatic conditions E. A. Al-Ammar 1 * and B. A. Arafa 2 1 Electrical Engineering Department, King Saud University, College of Engineering, P. O. Box 0, Riyadh 11421, Saudi Arabia. 2 Department of UHV Studies and Research Center, Egyptian Electricity Holding Company, Ministry of Electricity and Energy, Egypt. Accepted 29 October, 2012 The electrical performance of polymeric insulators depends on the hydrophobicity of their surfaces. In this study, the contact and / or receding angle of a water droplet at the surface of Silicone rubber insulators is measured under different climatic conditions of Egypt. This paper experimentally investigates the effects of various environmental factors (e.g desert s dust, cement and fertilizer) on the hydrophobicity of Silicone rubber (SiR) insulators. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopic (XPS) analysis were also performed on the tested polymeric insulators. The results report that the SiR composite insulator changes its surface properties from hydrophobic to hydrophilic nature for the aged insulators as environmental condition changes. Key words: Hydrophobicity, contact angle, receding angle, Silicone rubber composite insulator, accelerated aging. INTRODUCTION Electrical insulators form a very important component of high voltage electric power networks such as substations, transmission and distribution lines. In the early days, insulators were made of ceramic and glass materials. But in 1963, composite insulators were developed and its improvements in design and manufacturing in the recent years have made them attractive to utilities. The primary impetus for their increased acceptance by the electric power utilities is their substantial advantage compared to inorganic insulators. These are light weight, easy installation, comparable or better withstand voltage, improved contamination performance, improved resistance to vandalism, improved handling of shock loads and high hydrophobicity, etc (EPRI Technical Brief, 2003). Together with many advantages, the composite insulators are subjected to chemical changes on the *Corresponding author. essam@ksu.edu.sa. surface due to weathering and from dry band arcing, suffer from erosion and tracking which may lead ultimately to failure of the insulator, faulty insulators are difficult to detect and life expectancy is difficult to evaluate (EPRI Technical Brief, 2003). The property of high hydrophobicity of the composite insulators assures them better performance in contaminated environments. Hydrophobicity of a material is its resistance to flow of water on its surface or resistance to the formation of continuous film of water. It can be described using contact angle (θ c ) on the material surface that a liquid drop makes on the solid surface when it comes into contact with it. The hydrophobic materials allow less water surface contact and thus make θ c > 90 as shown in Figure 1(a), whereas materials which are easily wettable allow water to touch a large surface area and hence make θ c < 90 as shown in Figure 1(b) and is known as hydrophilic. Surface is said to be hydrophobic, when θ c > 90, hydrophilic when c < 35 and partially wettable when 35< θ c < 90 (Amin et al., 2007). Two principal mechanisms exist for failure of composite

2 Al-Ammar and Arafa 6163 c c (a) c > 90 (b) c < 90 Figure 1. Schematic diagram of water drop on: (a) Hydrophobic surface (b) Hydrophilic surface. insulators. These are mechanical failure and electrical flashover. The mechanical failure is catastrophic and irreversible. Modern designs have almost overcome this problem and the important challenge in this area is quality control (Karady, 1999; Kumosa et al., 2005). The aging mechanism and the flashover processes are different from the ceramic and polymeric insulators due to different aging characteristics (Karady, 1999; Hackim, 1999). Many investigations are attempted to study the actual hydrophobicity status of the composite insulators, specially the Silicone rubber (SiR) insulators (Sundarajan et al., 2004; Rowland et al., 2007; Khan et al., 2006; Khan, 2009; Khan, 2009). It was also reported (Naito et al., 1996) that nonceramic composite insulators are equipped with a hydrophobic properties on which beads of moisture condenses. In this way, a continuously wetted pollution layer cannot be built up and leakage currents are widely suppressed. At present, different organic insulating materials are used as non-ceramic insulators, which is able to transfer its hydrophobicity to pollution particles. Low molecular chains migrate from the bulk of the Silicone material and cover the pollution particles until a thin layer of Silicones covers them. It was also reported that the electrical performance of polymeric insulators is influenced by the hydrophobicity of their surfaces (Kindersberger and Kuhl, 1989). This paper investigates the effect of climatic conditions (involving different factors such as desert s sand, cement and fertilizer wastes) on the hydrophobic characteristics of SiR insulators by analyzing the experimental results of its hydrophobicity. The hydrophobic nature of these insulators is analyzed by measuring the contact angle of water droplets on the surface. Measurement of contact angle Both the loss and recovery of hydrophobicity can be identified by different electrical and non-electrical techniques, such as the measurement of the contact angle, electron spectroscopy for chemical analysis (ESCA), scanning electron microscopy (SEM) etc. (Sherif and Vlastos, 1985). Recovery of hydrophobicity of surfaces of organic materials is due to the migration of hydrophobic low molecular chains from the bulk of the material to the surface (STRI guide 92-1, 1992; Gorur et al., 1988; Gubanski and Vlastos, 1990). Although, it has been reported that some composite insulators may recover by time (Sherif and Vlastos, 1985; STRI guide 92-1, 1992) whereas (Gubanski and Vlastos, 1990; Hager et al., 1994; Arafa et al., 1995) discussed recovery of hydrophobic characterizes of SiR. Currently, researchers are studying the factors that influence the recovery of Silicones and it is assumed that elevated temperatures accelerate the recovery time. A common spray bottle which produces a fine mist was needed for spraying water on the insulator surface for measuring the contact angle. This spray bottle is filled with deionized water. Other miscellaneous equipments include magnification glass, a lamp and a measuring tape. The cross section area of tested specimen was about 50 to cm 2. The spray bottle was squeezed once or twice a second from a distance of 25 ± 10 cm. This process was continued for 20 to 30 s. Results of the hydrophobicity class were performed after every ten seconds. Now the contact angle ( c ) between the water droplets and the surface must be taken into account. On an inclined surface, two different contact angles are defined, the advancing contact angle ( a ) and the receding contact angle ( r ) as shown in Figure 2. The receding angle (θ r ) is the most important when the wetting properties of an insulator shall be evaluated. The inclination angle of the surface ( I ) affects the r. The equilibrium contact angle corresponds to the lowest energy state for a system. Test specimen and procedure Long rod insulators of SiR shown in Figure 3 are used for the experimental investigation. The technical data of the test specimens are given in Table 1. The SiR insulators exposed to artificial desert sand storm is called Sample A, while the other specimens were exposed to industrial pollution generated from two different types of factory

3 energy state for a system Int. J. Phys. Sci. I r 1. Test Specimen & Procedure θ a θ i water droplet Long rod insulators of SiR shown in figure 3, are used for the experimental Figure 2. Measurement of contact and receding angles. θ a: Advancing angle, investigation. The θ r: technical Receding angle data and of the θ I: Inclination test specimens angle of are the plane. given in Table 1. θ I : Inclination θ a : Advancing angle θ r θ r : Receding angle Figure 3. Test specimen. Table 1. Technical data of test specimens. Specifications (Units) Dimensions Creepage distance (mm) 6 No. of sheds 7 Suspension length (mm) 460 Shed diameter (mm) 65 Core diameter (mm) 25 wastes; the first specimen was exposed to cement pollution called Sample C and the second specimen was exposed to fertilizer pollution called Sample F. of these polluted insulators. The test specimens were individually immersed in the waste solution for a period of eight weeks. And after every week one of the specimens was taken out, dried and the contact angles were measured. EXPERIMENTAL SET-UP AND METHOD For accelerated aging, some of the samples were subjected to sandstorm after cleaning their surfaces and removing traces of pollutants (Gubanski and Vlastos, 1990). The sand grain size was about 250 μm and velocity of the air-storming-sand at the insulator was 20 m/sec. Then, the effect of sand storms on the contact angle of these surfaces was noted, measured and analyzed. The contact and receding angles were measured and recorded after every 5 hour intervals. On the other hand, when an insulator is exposed to industrial pollution and subjected to moisture in humid conditions, the SiR insulator s surface is affected by the chemical composition of the wastes. To analyze this effect, few specimens of SiR insulators were also immersed in wastes of cement factory and fertilizer factory. These wastes were also then dissolved in distilled water to simulate rain or foggy conditions and to evaluate the performance RESULTS AND DISCUSSION Effect of sandstorms on the hydrophobicity Contact angle c was measured 10 times at different points on the surface of tested specimens. The average values of contact angle versus time of exposure to sand storm before and after removing the pollutants are shown in Figures 4 and 5, respectively. Whereas, Figure 6 shows the effect of sand storm on the receding angle characteristics of the composite insulator. Receding angle r has importance in the wetting properties of an insulator which in turn depends on the inclination angle. So, the receding angle was measured at an inclined

4 highest inclination angle at which droplets start to move. Contact angle (Degree) Contact angle(degree) Contact angle(degree) Contact angle (Degree) Al-Ammar and Arafa Time of exposure to artificial sand storm Time of exposure to artificial sand storm Figure 4. Effect of exposure to artificial sand storm on the contact angle (Before removing the pollutants) Figure 4. Effect of exposure to artificial sand storm on the contact angle (Before removing the pollutants) Time of exposure to sandstorm(h) 30 Time of exposure to sand storm (h) Figure 5. Contact angle of SiR insulators exposed to artificial sand storm (After removing the pollutants). plane of 20 which is the highest inclination angle at which droplets start to move. From Figure 4, it can be observed that there is no significant change in the contact angle on the polluted surface. This may be attributed to the migration of low molecular Silicone from the bulk of Silicone rubber to the surface pollution layer in such a way that the contact angle will be affected with the surface roughness to a small degree (Gubanski and Vlastos, 1990). However, Figures 4 and 5 show that water drops were not able to move easily on the roughed surface. So, higher the exposure time to sand storm, lower the contact and/or receding angle of the cleaned surface and vice versa making the surface hydrophilic in nature.

5 Contact angle (Degree) Residing angle (Degree) Receding angle(degree) 6166 Int. J. Phys. Sci Time of exposure to sandstorm(h) Time of exposure to sand storm (h) Figure 6. Effect of exposure to artificial sand storm on the receding angle when i = 20 o (After removing the pollutants) Contact angle(degree) Specimen C Time of immersion (Week) Specimen F Figure 7. Contact angle of SiR specimens immersed in Cement and Fertilizer wastes (Specimen C: SIR specimen immersed in cement wastes and Specimen F: SIR specimen immersed in fertilizer wastes). Effect of cement and fertilizer wastes on the hydrophobicity Figure 7 depicts the measured results of the contact angle of each tested specimen of SiR insulator which was aged under different waste solutions as mentioned above. It is clearly observed that the hydrophobicity of Specimens C and F shows a significant change after exposure to industrial environmental pollution. Figure 7 also shows that the contact angle reduced with the increase in the aging time. This change is higher in case of the one immersed in fertilizer waste solution. The percentage reduction in the contact angle is nearly 28 and 38% for Specimens C and F, respectively after 8 weeks of aging. Scanning electron microscopy (SEM) SEM is used to provide information about the surface

6 Al-Ammar and Arafa 6167 a b Figure 8. SEM micrographs for virgin and the aged samples. (a): SiR (New) 0; (b): SiR (Aged) 0. a magnification of 500, 0, 2000 times. Figure 8 shows SEM results of new as well as samples aged under sand storm conditions. The virgin samples have a smooth, more homogenous and less porous surface while for aged insulators, the surface roughness and porosity has increased with very visible cracks aging as shown in Figure 8(b). Moreover, it may be pointed out that this surface roughness has resulted due to localized degradation in tested samples. X-ray photoelectron spectroscopic (XPS) analysis b Fig. Figure 9: XPS 9. XPS analysis analysis of SiR of insulator SiR insulator (a): New (a): New (b): Aged (b): Aged sample sample. XPS is a surface sensitive technique that probes the composition of the outer 10 nm of the surface. This method is used to obtain qualitative as well as quantitative information on the surface composition of samples. The sample XPS spectrum recorded for each tested specimen of SiR insulator is as shown in Figure 9. These spectrums identify all the major elements such as Oxygen, Carbon, Silicon and Aluminum in the investigated samples. In these samples, the presence of oxygen detected by XPS on the new and aged surfaces is attributed to the availability of oxygen from the additives or from the moisture in the atmosphere or due to oxidation of the rubber during manufacturing. Peaks of Aluminum are also observed in all samples as shown in Figure 9. topography of tested surfaces. Small samples (3 3 mm) were sectioned from each insulator and their surface analysis was obtained using JEOL JSM-6360-A (Japan). Secondary electron imaging (SEI) was performed to study the surface morphology at an accelerating voltage of 20 kv. SEM photographs were captured for analyzing surface condition for all tested insulators at a Conclusion This paper presents the effect of Egyptian climatic conditions on the hydrophobicity of SiR insulators. The following conclusions were made based on the experimental results mentioned above under the diverse desert s environmental conditions simulated:

7 6168 Int. J. Phys. Sci. i. No significant change in the contact angle is observed on the Silicone rubber insulators containing the polluted deposits when they were subjected to simulated desert storm condition, ii. Whereas, when the deposits were removed by cleaning the surface, both the contact angle and receding angle were reduced over the period of exposure to the storm implying to the fact that the surface (SiR) losses it hydrophobicity over this period of exposure to sandstorms. Thus, the surface losses it hydrophobic characteristics and tends to become hydrophilic in nature, iii. The hydrophobicity of specimens immersed in cement and fertilizer wastes show a significant change with respect to unaged insulators. This change is higher in case of the sample immersed in fertilizer waste solution as compared to specimen immersed in cement waste, iv. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopic (XPS) analysis results reports that the insulator changes its surface properties with the aging period and hence losses its hydrophobicity. REFERENCES Amin M, Akbar M, Amin S (2007). Hydrophobicity of SiR used for outdoor insulation. Rev. Adv. Mater. Sci. 16: Arafa BA, Hamed M, Sedeek A, Elgendy O, Awad M, Nosseir A (1995). Aging of polymer insulator surface under the effect of sandstorm 9th ISH Graz. Austria. P EPRI Technical Brief (2003). "Polymer Insulator Survey-7769". Gorur RS, CherneyEA, Hackam R, Orbeck T (1988). The electrical performance of polymeric insulating materials under accelerated aging in a fog chamber. IEEE Trans. Power Del. 3:3. Gubanski SM, Vlastos AE (1990). Wettability of naturally aged silicone and EPDM composite insulators. IEEE Trans. PD. 5(3): Hackim R (1999). Outdoor HV Composite Polymeric Insulators. IEEE Trans. DEI. EI-6(5): Hager M, Hartings R, Ruter F (1994). The dry salt layer (DSL) method a new pollution test for coastal environments Nordic Insulation Symposium. Nord IS. 94(1.2a): Karady GG (1999). Flashover Mechanisms of Non-ceramic insulators. IEEE Trans. DEI. EI-6(1): Khan Y (2009). Degradation of High Voltage Polymeric Insulators in Arid Desert's Simulated Environmental conditions. Am. J. Engg. Appl. Sci. AJEAS-2(2): Khan Y (2009). Hydrophobic Characteristics of EPDM Composite Insulators in Simulated Arid Desert Environment. Int. J. Appl. Sci. Eng. Technol. 5(3): Khan Y, Qureshi MI, Malik NH, Al-Arainy AA (2006). Performance of composite insulators in simulated environmental conditions related to central region of Saudi Arabia. IEEE Int. Conf. Emerg. Technol. (ICET). pp Kindersberger J, Kuhl M (1989). Effect of hydrophobicity on insulator performance, 6th ISH. New Orleans. pp Kumosa L, Kumosa K, Armentrout D (2005). Failure Analysis of nonceramic insulators, Brittle fracture Characteristics. IEEE Elect. Insul. Mag. EI-21(3): Naito K, Izumi K, Takasu K, Matuoka R (1996). Performance of composite insulators under polluted conditions. CIGRE. pp Rowland SM, Xiong Y, Robertson J, Hoffman S (2007). Aging of Silicon Rubber Composite Insulators on 400 kv Transmission Lines. IEEE Trans. Dielectr. Elect. Insulation. EI -14(1): Sherif E, Vlastos AE (1985). Performance of long-rod composite insulators under various environmental conditions. HVDC insulators Symposium Universe of South Calif. STRI guide (1992). Hydrophobicity classification guide. Sundarajan R, Mohammad AN, Chaipanit T, Karcher Z, Liu Z (2004). In Service aging and degradation of 345 kv EPDM transmission line insulators in a coastal environment. IEEE Trans. DEI. EI -11(1):