Hydrophobicity effect of silicone housed composite insulators
The intrinsic hydrophobic property in combination with very good uv, tracking and erosion resistances of well designed silicone rubber formulations make this kind of rubber superior to all other electrometric and duromeric polymeric housing materials. Further on, a phenomenon well known as “Hydrophobicity transfer” to different kinds of pollution layers offer great benefits regarding the electrical insulation capability even under heavy pollution conditions. The ability to transfer the hydrophobic effect to pollution layers located on insulator surfaces can be regarded as an “intelligent” or “advanced” property and can only be observed for silicone rubbers. Coastal or marine environments are characterized by increased ESDD (Equivalent salt deposition density) values due to salt pollution. Especially in the Mediterranean countries a combination of salt pollution from the sea and dusty and sandy pollution can be observed on insulator surfaces. The latter kind of pollution results in increased NSDD (Non Soluble deposit density) values of the pollution layer. Silicone housed composite insulators show outstanding service experiences in environments where high ESDD and high NSDD values are existing. This positive service behaviour is based on the above mentioned transfer of the hydrophobic effect to pollution layers.
Hydrophobicity transfer mechanism
In the bulk of Silicone rubber there exists some quantity of highly mobile low molecular weight chains. It is a homologous compound of PDMS (Polydimethyl Siloxane), defined as LMW. Smaller units n=1 ~20 (or even higher) are called LMWS. Migration process of Silicone LMWS are mainly responsible for the hydrophobicity of PDMS surface and the hydrophobicity transfer to pollution layers. Due to the coating of the deposited pollutants by LMWS, the pollution layer behaves like a Silicone surface and becomes hydrophobic.
The intrinsic hydrophobic behavior of Silicone is caused by the orientation of – CH3 groups towards the Silicone/air interface and by the adhesion of LMWS on the surface. A generation mechanism of LMWS was found to work in Silicone where the LMW content is in an equilibrium state with PDMS backbone. This means that PDMS bulk material produces as much LMWS as needed for the external hydrophobicity transfer to pollution layer. This equilibrium reaction is also responsible for the excellent function of the hydrophobicity transfer to heavy polluted Silicone surfaces even after decades.
Considering this generation mechanism, there is no lifetime limitation for hydrophobicity transfer effect. Because of the hydrophobicity transfer behavior there is no need to wash the Composite Insulator having specially formulated Silicone rubber housing.