Views: 0 Author: Site Editor Publish Time: 2026-03-23 Origin: Site
Hydrophobicity in silicone rubber is not an inherent property of the base polymer but is derived from the presence of low-molecular-weight (LMW) polymer chains that diffuse to the surface. This dynamic mechanism allows the surface to repel water, forming discrete droplets rather than a continuous conductive film.
The loss of hydrophobicity is primarily driven by three synergistic factors:
· Electrical Discharge Activity (Dry Band Arcing): In polluted environments, leakage current flows across the housing surface. As the surface heats up, dry bands form. These dry bands experience micro-arcs (corona discharges) that generate localized temperatures exceeding 300°C. This thermal energy breaks the polymer backbone (Si-O and Si-C bonds) of the LMW chains. The result is a permanent reduction in the material’s ability to recover hydrophobicity, leading to a transition from a hydrophobic (Class HC1-HC2) to a hydrophilic (HC5-HC6) state.
· Environmental Contamination: The accumulation of industrial pollution, salt, or dust physically masks the silicone rubber surface. While silicone rubber exhibits 'hydrophobicity transfer' (where LMW chains migrate through contaminants to render the pollution layer hydrophobic), severe or acidic contaminants (such as NOx and SOx from industrial emissions) can chemically alter the polymer. Acidic hydrolysis cleaves the siloxane backbone, permanently destroying the hydrophobic recovery mechanism.
· Ultraviolet (UV) Radiation: Although silicone rubber generally offers better UV resistance than EPDM, prolonged exposure to high-altitude sunlight causes photo-oxidation. UV radiation initiates cross-linking and chain scission at the surface layer. This creates a brittle, chalky outer layer (often visible as a whitening effect) that inhibits the migration of LMW chains from the bulk to the surface.
Once hydrophobicity is permanently lost, the arrester operates in a wet condition. Continuous water films form on the housing, resulting in sustained leakage current. This increases the thermal stress on the internal Metal-Oxide varistor blocks, potentially leading to thermal runaway if heat dissipation is insufficient.
Unlike the electrical surface phenomenon of hydrophobicity loss, casing fractures represent a mechanical structural failure. Fractures typically occur at the root of the sheds or along the interface between the housing and the fiber-reinforced plastic (FRP) tube.
· Hydrolytic Degradation at Interfaces: The most insidious cause of fracture is moisture ingress. Although silicone rubber is hydrophobic on the surface, water vapor can diffuse through the bulk material over time if the interfacial adhesion with the FRP rod is compromised. When moisture reaches the FRP/ silicone interface or the FRP/ end-fitting interface under mechanical load (e.g., wind, seismic forces, or thermal expansion), hydrolytic degradation occurs. This leads to corrosion of the glass fibers in the FRP core, known as brittle fracture. The swelling stress caused by corrosion products eventually splits the silicone rubber housing from the inside out.
· Thermal-Mechanical Fatigue: Power system operations expose arresters to cyclic loading. Ambient temperature fluctuations combined with resistive heating during overvoltage events create differential thermal expansion between the metallic end-fittings, the FRP core, and the silicone rubber housing. Silicone rubber, despite its high elasticity, suffers from accelerated aging under repetitive strain cycles. Micro-cracks initiate at stress concentration points, typically at the junction where the shed meets the central tube.
· Ozone and Corona Attack: While silicone rubber is resistant to general weathering, high levels of corona discharge—often a result of poor manufacturing quality or sharp edges on end fittings—generate ozone (O3). Ozone attacks the polymer chains at the surface of the shed root. Over 5–10 years of service, this results in deep crazing (fine network cracks) that propagate under wind-induced vibration, eventually leading to the complete detachment of the shed.
Fracture of the sheds reduces the creepage distance, compromising the arrester’s ability to withstand switching and lightning impulses. More critically, if the main housing (the tube) fractures or if the interface seal fails, moisture ingress leads to internal flashover, resulting in a catastrophic short-circuit failure of the arrester.
To extend the operational lifespan of silicone rubber surge arresters, a combination of material selection, manufacturing quality control, and field maintenance is required.
· High-Temperature Vulcanized (HTV) Silicone: Prefer HTV silicone rubber over Liquid Silicone Rubber (LSR) for high-voltage applications. HTV typically contains higher concentrations of ATH (Alumina Trihydrate) filler and stabilizers, offering superior tracking and erosion resistance compared to LSR, which is more suited for medium-voltage indoor applications.
· Optimized Shed Design: Selecting arresters with alternating large and small sheds (alternating diameters) improves self-cleaning capabilities, reducing the accumulation of conductive pollutants that trigger dry band arcing.
· Premium Interface Sealing: Ensure that the interface between the silicone rubber housing and the FRP core is either co-extruded (molded under pressure) or utilizes a diffusion-bonded process to eliminate voids. Micro-voids act as condensation sites that initiate internal partial discharges.
· Hydrophobicity Monitoring: Employ the Static Contact Angle (SCA) method or the Spray Method according to IEC TS 62073 during routine inspections. A shift in hydrophobicity class from HC1-HC2 to HC5-HC6 indicates advanced aging and necessitates replacement.
· Thermographic Inspection: Use infrared thermography during damp conditions. Arresters that have lost hydrophobicity will exhibit a hot spot pattern along the length of the housing due to increased leakage current. Early detection allows for replacement before thermal runaway occurs.
· Pollution Management: In coastal or industrial zones, the application of silicone grease or the installation of anti-pollution coatings can temporarily restore surface hydrophobicity. However, if the underlying silicone rubber has already oxidized, periodic cleaning with non-abrasive, deionized water is preferred to prevent the formation of conductive biofilms.
A significant portion of premature fractures originates from micro-defects introduced during production. Buyers should specify that arresters undergo resistive leakage current measurement at factory acceptance, as well as verification of the mechanical withstand load (SML) to ensure the silicone rubber-to-metal interface can handle long-term cyclic loading without delamination.
The aging of silicone rubber in surge arresters is an inevitable electrochemical and physical process. While hydrophobicity loss is primarily a surface phenomenon driven by electrical discharge and pollution, leading to increased leakage current and thermal risk, mechanical fracture represents a deeper structural failure driven by interface degradation, moisture ingress, and cyclic stress.
