Surface Resistivity of Silicone Rubber Formulations Tested in Room Ambient Conditions: The case of silicone rubber formulations with and without filler materials

Silicone rubber formulations in the form of thin discs have been studied under room ambient conditions for their surface characteristics. The samples were silicone rubber manufactured in laboratory and those industrially manufactured. The measurements were done using an electrometer high resistance meter, applying dc voltage under normal room ambient conditions. The results show that the silicone rubber samples show higher values of surface resistivity when the dc voltage was applied. Silicone rubber samples manufactured in laboratory seem to exhibit erratic behaviour unlike their corresponding silicone rubbers manufactured in industry; this could be due to manufacturing shortcomings in laboratory and the irregularities in the way the silicone rubber adhered to the concentric ring electrodes. The empirical current traversing the surface of the silicone rubbers does not decay exponentially but rather it decays as an exponential power of the energization time. Key-Words: Silicone Rubber, Surface Resistance, Surface Resistivity, Surface Charging.


Introduction
NCIs are increasingly being used in new power lines, which are installed in polluted environment as well as for replacements of installed ceramic and glass insulators during maintenance. The excellent performance of the polymeric insulators in polluted and wet conditions is attributed to the ability of the material to maintain the hydrophobicity of the surface in the presence of severe contamination and wet conditions (Gubanski et al., 2000;Zhang & Hackam, 1998). This is because of the presence of low molecular weight (LMW) mobile fluid in the bulk of the silicone and EPDM rubbers, which diffuses to the surface and to the above contamination layer. The diffusion process occurs when the outer film of LMW is removed in the presence of dry band arcing and then washed away by the rain.
Polymeric insulators thus have a good surface hydrophobicity which suppresses the onset of leakage current and increase the withstand voltage. The electrical, physical and chemical properties of the surface of the polymer insulator become critical to its continuing satisfactory and reliable service throughout the lifetime of the insulator (Zhang & Hackam, 1998).
Other benefits of employing polymer insulators are their: resistance to weathering, excellent suppression of leakage currents, higher flashover voltage even when wet and polluted, resistance to vandalism, higher puncture strength and the famous excellent hydrophobicity.
There has been extensive research on polymeric outdoor insulation (Chen et (Gorur et al., 2006;Gorur et al., 2001), despite the earlier problems to establish a specific value for which problems in field can occur (Gorur et al., 2001). Surface resistivity and surface resistance values are related by a simple factor of 50, which account for material geometry and electrode configurations of the electrometer probe, used (Keithley, 2003a(Keithley, , 2003b. These means that the values obtained in this study are for those silicone rubber when in room ambient conditions using concentric ring electrodes and energized by a 400 Vdc.

Sample Preparation
Test samples consisted of silicone rubbers, some of which were manufactured in laboratory and some were industrially manufactured. First type of sample of silicone rubber was prepared from a commercially available silicone elastomer. These silicone rubber samples were in two groups; first group (sample S1 and sample S2) were cured at an oven temperature of 120 ˚C for 120 minutes and second group (sample S481 and sample S482) were cured at room temperature (22 ˚C -25 ˚C) for 48 hours, see Table 1. Second type of sample of silicone rubber was a HTV silicone rubber (sample SF1 and sample SF2), and third type of sample of silicone rubber was a HTV silicone rubber (PS) used for insulator shed manufacturing delivered to the laboratory by the manufacturers. These variations gave silicone rubber samples with different characteristics, i.e., silicone rubber with no additives (S1, S2, S481 and S482), silicone rubber with additives (SF1, SF2 and PS) and different curing times and temperatures (see Table 1).

Results
Results from the experiments are presented in Fig.1 for the time variations of surface resistivity in first 50 minutes.

Discussion of the Results
In the plots of Fig.1 silicone rubber samples resistivity values for the duration of 3000 seconds (50 minutes) is presented. Silicone rubber shows varying values of surface resistivity after the first 2 -3 minutes (120 -180 seconds) as seen in Fig.1. Samples S1 and sample S2 shows very different values from each other, since S1 shows a stable behavior but S2 shows an erratic behavior. It is seen that sample S2 had a lot of spikes even after smoothing the data. This could be due to the reason of interference since during that time there was transformer measurements being undertaken in the same laboratory.
Samples S481 and S482 both shows stable but different values. S481 shows pretty stabilized values for the increasing time, whilst S482 shows a higher value but slowly increasing after 1000 seconds. Samples SF1 and sample SF2 both exhibit same curves.
Sample PS exhibited behaviour similar to samples SF1 and SF2 although at a slightly lower values of surface resistivity.
The main reason for the differing values exhibited by samples S1, S2, S481 and S482 could be a contact problem of the electrodes, since the manufacturing process could not guarantee perfect smooth surface finish, and the rubber was very soft such that it could adhere to one part of the electrode and leave a gap on another part, for which case industrially made rubbers (SF1, SF2 and PS) showed some consistent behaviour with one another due to their smooth surface finish and hardness. Another thought reason could be caused by the electrodes affecting charge formation on the materials (Chen et al., 2005) and other reasons as explained elsewhere (Escribe-Fillipini et al., 2001).

Comparison with other Surface Resistance Measurements Results
From measurements it was possible to estimate the average value of surface resistance of the silicone rubber (see Table 1)

Conclusion
One thing noted was that the samples had a tendency of retaining some charges inside them which could be a little explained elsewhere (Chen et al., 2005), although in that paper emphasis was done on ac stress aging; (Dakin, 2006;Jonscher, 1999) which explain about the charging of dielectrics. After energizing the material for roughly 5 minutes, if the energization process was interrupted and resumed later, the material quickly reached the level of the previous value. This tendency implies polarization of the samples by the dc voltage and higher relaxation times of the samples (Dakin, 2006;Jonscher, 1999).
If the energization was done for longer than 50 minutes the resistivity values continued to increase up to the point that the data obtained was not useful since the values became so high that the instrument generated overflow (Keithley, 2003a, 2003b). This has to be explained by the fact that the current measured becomes so low to be measured by our  Engineers.

Acknowledgements
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