Investigation of surface modification in silicon rubber due to UV irradiation
L.A.J. Tavares, M.E.G. Valerio, Z. S. Macedo.
Laboratory of Materials Processing and Characterization- Physics Department
Federal University of Sergipe
49100-000 São Cristovão, SE
BRAZIL
Abstract: Polymer materials are currently being used as high voltage outdoor insulator structures in distribution and transmission power systems, because of their good dielectric properties, light weight and cost compared to the porcelain or glass insulators. In the present work, tracking phenomenon has been studied in new and aged silicon rubber under electric discharge. Polymeric insulators (15kV, silicon rubber) were aged under UV irradiation (UVA, UVB, UVC) and the changes in the surface were investigated by optical and atomic force microscopy. The measurement of the contact angle helped in the understanding of the hydrophobicity of the material, correlating the changes of the surface microstructure with the aging process.
Key-Words: silicon rubber, tracking, aging, hydrophobicity
1Introduction
Silicon rubber is a well known material that presents good dielectric properties used as high voltage outdoor insulator structure in transmission power systems [1, 2]. There is an ever-increasing interest in power industry world-wide, to understand the degradation process and the performance characteristics of polymer insulating material in severe conditions like UV irradiation. The single largest problem yet to be overcome is the tracking and erosion of the outdoor insulator structure. The aim of this study is to investigate the effects caused by UV irradiation on the dielectric insulator and some of its properties in the surface of the material. For this, visual inspection in optical and atomic force microscopy were done, and measurement of the contact angle.
2Experimental details
All of the insulators studied were destined to be used in a nominal voltage of 15kV, type suspension, and were analyzed in Laboratory of Materials Processing and Characterization (LPCM) in UFS. For each insulator we analyzed a new sample, a naturally aged sample and an artificially aged sample (by UV irradiation with a lamp of mercury vapor - 400W).
2.1 Optical microscopy analysis
The samples were analyzed by Optical Microscopes aiming to describe the progression of the aging.
2.2 Atomic force microscopy
The morphology and the surface characteristic of the silicon new sample were also investigated using AFM in contact mode by the Thermos-Microscopes given us an idea of as roughness is this type of material.
Fig. 1 AFM image (contact mode) of the silicon new sample.
2.3 Contact angle measurement
The contact angle of a drop of distilled water in rest over the surface of an insulator, shown in figure 2, was used as hidrophobicity measurement of the material. The contact angles were measured through optical microscopy images (51.2 gain), with the aid of a software called Geometer’s Sketchpad.
Fig. 2 Water drops scattered in silicon rubber.
3Results and Discussion
3.1Contact angle measurement
It is known that UV irradiation takes the loss of hydrophobicity in polymeric insulators. In regions of high environmental aggressiveness due to irradiation, the polymeric insulators submitted to an electric discharge can be held responsible for the erosion followed by the tracking process. The measurement of the hydrophobicity could be analyzed using the contact angle variation. If the contact angle of the material increases above 90º, it shows that the material is hydrophobic and if the values are less than 90º, it shows that the material is hydrophilic (Crank 1975).
Insulator / New Sample / ArtificiallyAged
Sample / Naturally
Aged
Sample
Silicon
Rubber / 48 2º / 30 4º / 37 5º
Table 1- Results of contact angle measurements.
We can observe the contact angles of all the samples in figure 3.
Knowing that the artificially aged sample was exposed to UV irradiation during a week, while the Naturally Aged Sample was exposed not only to the UV radiation emitted by the sun but also to the inclemency during a month.
Figure 3 – Distilled water bubbles: (a) New Sample, (b) Artificially Aged Sample and (c) Naturally Aged Sample.
3.2 Optical Microscopy
Observing the images obtained from the samples, the formation of mineral charge was noticed after 2 and 4 hours, respectively, of aging through UV irradiation.
Figure 4 – New and Artificially Aged Samples: (a) New Sample, (b) Sample after 2 hours of aging exposed to UV irradiation, (c) Sample after 4 hours of aging exposed to UV irradiation.
Notice that there is a similarity between the naturally aged sample (a) and the artificially aged sample (b), both shown in figure 5.All images in figures 4 and 5 were measured through optical microscopy (51.2 gain).
Figure 5 – Naturally and Artificially Aged Samples: (a) Naturally Aged Sample, (b) Artificially Aged Sample through exposure to UV irradiation.
Figure 5 shows that the aging of the silicon insulator provokes superficial micro erosion through the formation of holes spread uniformly over the whole surface of the material. These wholes seem to be caused by the extraction of mineral charge particles from the surface of the material.
Due to the aggressive incidence of UV irradiation on the surface of the silicon insulator there is a weight-loss resulting in the formation of tracks and inducing to its dehydration or decomposition.
4Conclusion
The contact angle measurement varies with the type of ageing and indicates the degree of severity.
The aging through UV irradiation provoked apparent erosion or tracking on the samples.
The micro erosion in the silicon insulators provoked the outcrop of charge particles on the surface, making them hydrophilic.
Acknowledgments
The authors are grateful to LMPC-UFS, CEFET-SE for financial support.
References:
[1]N. Yoshimura & S. Kumagai & S. Nishimura, “Electrical and Enviroment Aging of Silicone Rubber Used in Outdoor Insulation”, IEEE Transactions on Dieletrics and Electrical Insulator, vol.6, pp.632-650 (1999)
[2]H. Hillborg & U. W. Gedde, “Hidrophobicity Changes in Silicone Rubbers”, IEEE Transactions on Dieletrics and Electrical Insulator, vol.6, pp.703-717 (1999)
[3]Deng Hui and Hackam R2000 IEEE Transactions Dielectrics & Electrical Insulation. 784
[4] Crank J 1975 Mathematics of diffusion (Oxford:Clarendon) 2nd ed.