Recent research has proven that the in-service aging of silicone rubber (SIR) sheds and sealants of non-ceramic insulators (NCI), which are widely used in high voltage transmission lines, can result from various environmental and electrical factors. In particular, salt, UV, acid, leakage currents and corona discharge – among other factors – can degrade NCIs. Such insulators are mainly built from polydimethylsiloxane (PDMS): a silicon-based polymer with hydrophobic methyl groups. At present, the existing plethora of literature has highlighted that polymer aging results in alterations of the room temperature vulcanized (RTV) SIR surface hydrophobicity. Consequently, catastrophic mechanical and electrical failures of the transmission lines employing NCIs can occur in service. Even worse, in coastal environments, salt accelerates the deterioration of SIRs in NCIs by decreasing their hydrophobicity. Therefore, it is imperative that the resistance of the SIRs to extreme environmental aging be improved so as to avert catastrophic failures of NCIs and minimize maintenance costs.
To this effect, University of Denver scientists: Dr. Monika Bleszynski and Professor Maciej Kumosa from the NSF I/UCRCenter for Novel High Voltage/Temperature Materials and Structures (HVT Center) demonstrated a new and highly transformative approach to improve the resistance of SIRs to extreme environmental aging by embedding titanium dioxide micro-particles. In particular, they addressed the ultra-aggressive aging environments, such as hypochlorous acid or electrolyzed aqueous salts, among which the aforementioned rubbers insulating systems may be exposed to. Their work is currently published in the research journal, Composites Science and Technology.
In brief, the research method employed entailed conducting molecular dynamics simulations so as to determine the combined effects of a titanium dioxide and different concentrations of hydrophobic PDMS methyl groups on surface hydrophobicity of a titanium dioxide/RTV composite. Next, they predicted the effects of both titanium dioxide and silica on the diffusivities of low voltage aqueous salt components in the PDMS and related them to unique interfacial interactions between the particles and the methyl groups of the PDMS. Lastly, titanium dioxide/RTV SIR was subjected independently to hypochlorous acid and electrolyzed low voltage aqueous salt.
The authors observed that the rutile titanium dioxide reoriented methyl groups away from the particles reducing the diffusivities of water and hypochlorous acid. This effect was seen to shield the PDMS network against environmental chain scissions. Conversely, silica was seen to attract the groups and thereby accelerating acid and water migrations and thus enhancing damage to the network. All in all, the titanium dioxide micro-particles were observed to greatly increase the contact angle, reduce the surface energy and most importantly, improve the hydrophobicity of the composite.
In summary, the Bleszynski-Kumosa study demonstrated the application of titanium dioxide microparticles to enhance the hydrophobicity of RTV SIRs. Specifically, the titanium dioxide was noted to have the capability to offset the negative effect of the reduced concentrations of methyl groups thereby improving the hydrophobicity of the titanium dioxide/RTV composite as a whole. Altogether, using this technique, aging of the rubber was effectively reduced by about 50% in highly oxidative environments.
Monika Bleszynski, Maciej Kumosa. Aging resistant TiO2/silicone rubber composites. Composites Science and Technology, volume 164 (2018) page 74–81.Go To Composites Science and Technology