BEHAVIOUR OF NANO SIO 2 FILLED SILICONE RUBBER NANOCOMPOSITESS

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 12, December 2018, pp , Article ID: IJMET_09_12_0600 Available online at aeme.com/ijmet/issues.asp?jtype=ijmet&vtype= =9&IType=12 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed STUDIES ON THE SURFACE TRACKING BEHAVIOUR OF NANO SIO 2 FILLED SILICONE RUBBER NANOCOMPOSITESS S. Mohamed Ghouse, K. Vijayarekha School of Electrical and Electronics Engineering, SASTRA Deemed to be University Thanjavur, India. ABSTRACT Outdoor power system insulators are subjected to various stresses like mechanical, electrical, thermal which cause degradation of the insulator. By adding nanofillers into the conventional insulators, the mechanical and electrical properties of the insulators can be enhanced. This paper presents the preparation of silicone rubber nanocomposites using Silica nanoparticles with different filler concentration. The prepared nanocomposite samples were subjected to surface erosion studies using the inclined plane test in according to IEC under ac voltage. The mechanical properties of the samples were analyzed. To study the uniform dispersion of nanoparticles Scanning Electron Microscopy Analysis (SEM) were carried out. The contact angle measurement was done to check whether the sample is hydrophobic after the addition of nanoparticles. X-Ray Powder Diffraction (XRD) analysis was carried out to study the chemical changes in the material. Key words: Silicone Rubber Nanocomposites, Inclined Plane Test, Contact Angle Measurement, SEM analysis, XRD analysis. Cite this Article: S. Mohamed Ghouse and K. Vijayarekha, Studies On The Surface Tracking Behaviour of Nano SIO 2 Filled Silicone Rubber Nanocomposites, International Journal of Mechanical Engineering and Technology, 9(12), 2018, pp et/issues.asp?jtype=ijmet&vtype=9&itype e=12 1. INTRODUCTION Silicone rubber is an elastomer having flexible siloxane backbone comprising of silicone and oxygen chain. The word elastomer describes that vulcanized polymeric materials that can be deformed and when the stress is released it returns to the original shape. Silicone rubber has a wide energy gap and very narrow conduction bands that makes it an excellent insulating material. The ceramic and glass insulators were used for many years is now slowly being IJMET/index.asp 575 editor@iaeme.com

2 S. Mohamed Ghouse and K. Vijayarekha replaced by silicone rubber insulators because of better dielectric strength, low weight, easy handling and maintenance [1]-[6]. Tracking is continuous degradation of the surface of the insulator by local discharges. The local discharges form conducting paths or partial conducting paths. To enhance the tracking resistance of the insulators fillers are being dispersed into the insulators. Uniform dispersion and particle size of the fillers play a critical role in the polymer nanocomposites [7]-[10]. The main drawback in nano silica is agglomeration which is nothing but particles are held together by Vander Waal forces and polar bonds. Agglomeration is due to the large surface area of the particles. Silica being polar particles has a much greater tendency to agglomerate. Silica particles agglomerate not only because of Vander Waal forces but also due to the dipoledipole forces [9]-[12]. This force can be reduced by the use of surfactants or silane coupling agents. The common surfactants used are Triton X-100 and ethanol. In this work, ethanol has been used as the surfactant. The advantage of using ethanol is that it changes the hydrophilic silica to hydrophobic that makes compatible with the hydrophobic polymer chain. It also reduces the surfaces energy and inter-particle attraction [13]. This reduces the agglomeration and uniform dispersion of the nanoparticles into the silicone rubber. IEC standard provides the procedure to conduct erosion studies on solid insulators [14]. In this work using the inclined plane test, tracking and erosion studies were carried out. The leakage current waveforms were recorded, and the surface erosion behavior of nanocomposite was studied. Generally, the silicone rubber is hydrophobic. In order to study the hydrophobicity change due to the addition of nanofillers contact angle measurement was carried out. SEM analysis was used to ensure the uniform dispersion of nanoparticles in the polymer matrix. XRD analysis was carried out to study the chemical change in the material after surface erosion. 2. METHODOLOGY 2.1. Material Preparation The silicone rubber sheets were prepared using two-roll mills. Two roll mills use two rollers. These rollers rotate in opposite directions. The first roller speed is about 16 rpm, and back roller rpm is about 18 rpm. The roller temperature is around º C. The rubber when it is between the rollers has a temperature of around º C. The operator can set the time for how long the rubber has to be crushed between the rollers. The spacing between the two rollers can be adjusted by the operator. Dicumyl peroxide curing agent was added with the silicone rubber for proper curing. Mixing of nano silica with the surfactant can be done in many ways. In this work, the nano-silica and the surfactant were mixed using magnetic stirrer. The stirrer is a laboratory device which use a rotating magnetic field and a stir bar to produces a stirring action in the liquid. The rotating fields are produced by rotating magnet or a set of stationary magnets placed below the vessel with the liquid. The stir bar is a magnetic bar placed in the vessel containing the nano-silica and ethanol. The required amount of nanoparticles is mixed with 100 ml of ethanol. The magnetic stirrer is made to rotate at about 300 rpm for 5 to 10 minutes. When the rubber is flattened between the rollers, nano silica mixed with the ethanol was added uniformly to ensure uniform dispersion of nanoparticles. Silica Nanofillers with 2.5% by weight, 5% by weight and 10% by weight were added into the base silicone rubber, and nanocomposites were prepared. In this work, Peroxide cure was carried out. In the pre-curing phase, the rubber was cured at 180 º C for about 30 minutes, and in post-curing phase, the rubber is cured at a temperature of nearly 200 º C for about 5 hours. Then the rubber sheet was cut into the required shape. IJMET/index.asp 576 editor@iaeme.com

3 Studies On The Surface Tracking Behaviour of Nano SIO 2 Filled Silicone Rubber Nanocomposites Samples with and without fillers were prepared. The samples used were 11.5 cm X 6 cm X 2 mm Experimentation using Inclined Plane Test Inclined plane test deals with the measurement of tracking resistance of insulating material under severe polluting conditions. The specimen under test is kept at 45 degrees inclination, and tracking is carried out in the presence of liquid contaminant such as ammonium chloride. There are two types of inclined plane test. They are Constant voltage tracking method and Step-wise voltage tracking method [14]. The second method has been used to measure the tracking resistance of the sample with and without nano-silica. The test samples are cleaned and mounted on to the test kit carefully to avoid contamination. Power frequency high voltage AC supply up to 5 kv is used to carry out the test using a high voltage test transformer of 300 kv, 300 kva rating. The test setup and electrode configuration are specially designed according to IEC The contaminant used in this study is 0.1 % mass ammonium chloride solution (NH₄Cl). The filter paper of 8 layers is placed below a top electrode or high electrode which will act as a reservoir for the contaminant. The test specimen is inclined with an angle of 45 in the specimen holder. Initially, the contaminant flow rate should be checked from the top electrode should reach the bottom electrode properly. The initial test voltage is chosen as 2.5 kv, and this voltage is high voltage electrode or top electrode, and the ground electrode or bottom electrode is appropriately grounded. The test voltage is gradually increased in steps of 0.25 kv until the partial discharge is observed. Then the voltage was maintained at 4.5 kv as prescribed in the standard [14]. Figure 1 shows the schematic diagram of the experimental setup. The high voltage terminal was connected to the top electrode of the setup, and the bottom electrode was connected to a current limiting resistor. The ammonium chloride solution was allowed to flow through the surface of the sample using a flow control mechanism. The leakage current was measured using a DSO. Figure 2 shows the photograph of the experimental setup. During tracking the magnitude of the leakage current was monitored and the end point was selected as specified in the standard. When the leakage current magnitude reached 60 ma the experimentation was stopped. The eroded region of the test samples was cut properly for XRD analysis. Figure 1 Schematic Diagram of Inclined Plane Test IJMET/index.asp 577 editor@iaeme.com

4 S. Mohamed Ghouse and K. Vijayarekha 3. RESULTS AND DISCUSSIONS Figure 2 Photograph of Inclined Plane Test 3.1. Analysis of Mechanical Properties Since the silica comes under the reinforcing fillers, it will improve the mechanical properties of the rubber. Tensile strength, hardness and elongation at break were measured, and the results showed that there was an improvement in these properties. Table 1 shows the mechanical properties of the nanocomposite samples. Table1 Mechanical Properties of Silicone Rubber Samples Filler Concentration (%) Tensile Strength Elongation at break (Kg / cm2) (%) Hardness A B C D It was found that the tensile strength was maximum for the samples with 10% filler concentration. The elongation at break was maximum for the samples with 2.5 % filler concentration. Hardness was maximum for 5% filler concentration and 10% filler concentration samples. With higher filler concentration the tensile strength and hardness were found to be maximum, but the elongation at break was decreased with higher filler concentration. IJMET/index.asp 578 editor@iaeme.com

5 Studies On The Surface Tracking Behaviour of Nano SIO 2 Filled Silicone Rubber Nanocomposites 3.2. Scanning Electron Microscope (SEM) Analysis SEM is used to find whether the nano silica particle is dispersed uniformly on the surface of the sample. The SEM images of nano-filled samples are shown in Figure 3. The images indicate that there are some changes in the morphology of the silicone rubber sample. From the SEM images, we can observe that for 2.5 filler concentration samples the dispersion of the fillers was uniform. No agglomeration of the nanoparticles was observed. But in the case of 5% and 10% filler concentration samples, the agglomeration was found in a few places. Hence the samples prepared using two-roll mill technique; lower percentage filler concentration was effective when compared to higher filler concentration. Figure 3 Scanning Electron Microscope images of a) 2.5% Nano-Silica b) 5% Nano-Silica c) 10% Nano-Silica filled silicone rubber 3.3. Leakage current analysis Initial leakage current starts flowing due to the conductive path offered by the contaminant. The leakage current partially evaporates the moisture content in the contaminant due to the heat developed, and thus the dry band is formed. When the dry band is formed, we can observe the discontinuity in the waveform of the leakage current. This is because dry band offers high resistivity. The voltage across the dry band increases, and it leads to arcing. Due to the continuous flow of the contaminant, the surface gets wetted again and again, and the discharges at the surface of the test sample get repeated. Due to continuous dry band formation and arcing the temperature on the surface gets increased and thus leads to the carbonization of the test specimen. The erosion starts near the bottom electrode. As the time progress, the track length increases and a bright light spot was observed near the ground electrode. The experiment was stopped when the track length reaches half the distance between the upper and the lower electrode or when the leakage current reaches 60 ma. By the addition of nanosilica particles, the tracking time of the sample got increased, and the track length got decreased. This improves the tracking performance of the samples with nano-silica. By increasing the filler property the tracking time is increased. The voltage and current waveforms at a) before dry band b) initiation of dry band c) after partial tracking respectively is shown in Figure 4. These waveforms belong to the sample without nano-silica. The voltage and current waveforms of the specimen with nano silica is shown in figure 5. IJMET/index.asp 579 editor@iaeme.com

6 S. Mohamed Ghouse and K. Vijayarekha a) Initial Voltage and Current Waveform b) Voltage and Current Waveform during dry band formation c) After Partial Tracking Figure 4 Voltage and Current Waveforms of Sample without Nano Silica a) initial condition b) during the initiation of dry band c) after partial tracking IJMET/index.asp 580 editor@iaeme.com

7 Studies On The Surface Tracking Behaviour of Nano SIO 2 Filled Silicone Rubber Nanocomposites a ) Initial Voltage and Current Waveform (b) Voltage and Current Waveform during dry band formation (c) Voltage and Current Waveform after Partial Tracking Figure 5 Voltage and Current Waveforms of Sample with Nano Silica a) during initial condition b) during the initiation of the dry band, c) after partial tracking. From figure 4 & 5 it was observed that due to the flow of contaminant, the leakage current is produced. Due to the temperature of the leakage current, the dry band is formed, and during dry band formation the leakage current is interrupted to zero, and there is a drop in the voltage. From table 3 it was observed that as the filler concentration increases the tracking time increases. The length of the tracked region was also decreased for the higher filler concentration samples. IJMET/index.asp 581 editor@iaeme.com

8 S. Mohamed Ghouse and K. Vijayarekha Samples Filler Concentration in (%) Table 2 Results from tracking studies Voltage Applied in kv Time Taken For Tracking in minutes Track Length in mm A B C D Contact Angle Measurement A solid material is said to be highly hydrophobic if it resists the flow of water dropped on it. The contact angle is used to measure the hydrophobicity on the surface of the material when a liquid drop makes contact with the solid material. The contact angles were measured for the samples prepared, and the results were tabulated. Table 3 Contact Angle Measurement Sample Without Nano Silica Sample With 2.5 % Nano-Silica Height Width Angle Height Width Angle Mean value Mean value Sample With 5% Nano-Silica Sample With 10 % Nano-Silica Height Width Angle Height Width Angle Mean value Mean value From table 3, it is clear that the contact angle is greater than 90º and hence the samples were found to be hydrophobic. The addition of nano silica into the sample slightly decreases the hydrophobic behavior of the insulator. But this variation does not affect the property of the silicone rubber since by nature silicone rubber is hydrophobic. Figure 7 shows the water droplet placed on the surface of the specimen with and without nano silica respectively. Figure 7 Water Droplet of Specimen a) Without Nano-Silica b) With 2.5% Nano-Silica c) With 5% Nano-Silica d) With 10% Nano-Silica IJMET/index.asp 582 editor@iaeme.com

9 Studies On The Surface Tracking Behaviour of Nano SIO 2 Filled Silicone Rubber Nanocomposites 3.5. XRD Analysis X-Ray Powder Diffraction is an analytical and a non-destructive technique used to find the phase identification of the crystalline structure. The XRD results of the four samples matched the JCPDS database. The material which matched the database was found to be Silicone Dioxide. The peaks in figure 8 correspond to the SiO 2. Figure 8 XRD Analysis of a) Sample Without nano silica b) Sample With 2.5% nano silica c) Sample With 5% nano silica d) Sample With 10% nano silica From figure 8, it was observed that after tracking the white colored powder like substance that formed on the insulator is composed of the only SiO 2. This is because the silicone chain present in the polymer has reacted with the oxygen present in the air and also due to the SiO 2 dispersed in the samples. The major peak in the graph corresponds to 27º approximately. It was observed from the peaks that the eroded sample becomes slightly crystalline. 4. CONCLUSION To disperse the nanoparticles uniformly into the silicone rubber the nanoparticles have to be mixed with the surfactant using magnetic stirrer. Two roll mill technique was used to preparing the rubber sheet. Silica being a reinforcing filler, it has improved the mechanical properties like tensile strength, hardness and elongation at break. By filling the silicone rubber with the nanofillers, the tracking time and tracking resistance were improved. The track length of the sample with silica was found to be less than the sample without nano-silica. The leakage current waveform comprises two regions. The two regions are a dry band region and the other without a dry band. Due to leakage current the heat increases which lead to continuous arcing and complete carbonized black spot occurs.sem images showed that the nanoparticles were uniformly dispersed into the rubber surface. The contact angle slightly decreased due to the addition of nanofillers. From the XRD analysis, it was inferred the white colored residue produced comprises of SiO 2 and the sample slightly become crystalline due to tracking mechanism. ACKNOWLEDGMENTS The authors thank the management of SASTRA Deemed to be University and DST-FIST (Sanction order ref: SR/FST/ETI-338/2013(C) dated 10/09/2014) for their motivation and financial support for creating the facilities at HV lab of SASTRA, with which the experimental works were carried out. IJMET/index.asp 583 editor@iaeme.com

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