Hydrophobicity of composite insulators

What is Hydrophobicity?

Hydrophobicity refers to the ability of a material’s surface to repel water. In composite insulators, which are typically made from silicone rubber, this property ensures that water forms discrete droplets rather than spreading into a continuous film on the surface.

Why is Hydrophobicity Important in Composite Insulators?

 1. Reduction of Leakage Current:

• When water spreads into a continuous film on an insulator’s surface, it can create a conductive path.

• This can lead to the flow of leakage current, causing localized heating, material degradation, or even flashovers.

 2. Improved Performance in Contaminated Environments:

•  Composite insulators often operate in environments where dust, salt, or industrial pollutants settle on their surface.

• When combined with moisture, these contaminants can increase conductivity and cause surface discharges.

•  Hydrophobic surfaces mitigate this risk by preventing the formation of conductive paths.

 3. Enhanced Longevity:

• Continuous wetting and drying cycles, combined with electrical activity, can degrade insulator materials.

• Hydrophobicity helps preserve the integrity of the insulator by minimizing water-related stress.

 4. Reduced Maintenance Needs:

• Traditional ceramic insulators require regular cleaning in areas with high pollution to prevent flashovers.

• Composite insulators with hydrophobic properties can maintain performance with less frequent maintenance.

Why is it Needed in High Voltage Applications?

High-voltage transmission lines demand reliable insulation to ensure uninterrupted power delivery and safety. Hydrophobic composite insulators:

• Offer superior performance under extreme weather conditions.

• Ensure reliability in high-contamination zones, like coastal areas or industrial sites.

• Enhance overall efficiency by reducing energy losses caused by leakage currents or flashovers.

Major Mechanisms Behind Hydrophobicity Recovery

1 Diffusion of Low-Molecular-Weight (LMW) Siloxanes: In silicone rubber used for insulators, a small fraction of oligomeric or low-molecular-weight chains (often referred to as LMW siloxanes) is present in the bulk. These LMW components possess high mobility relative to the crosslinked network.

When a silicone insulator’s surface is exposed to corona discharge, UV, or contamination, the topmost layer may become oxidized or partially eroded. This increases surface energy (making it less hydrophobic). In response, LMW siloxanes from the bulk migrate or diffuse outward. This diffusion is driven by thermodynamic and concentration gradients—the damaged/oxidized layer has less of these nonpolar molecules, so LMW species move toward the interface to equalize concentrations.

Environmental Influences

Temperature: Higher temperatures (e.g., hot climates or during in-service heating) speed up migration.

Humidity: Can be a double-edged sword—moisture might remove newly arrived LMW species but also slightly swell the surface, potentially improving inward–outward mobility of the LMW molecules. 

Electric Field: Under continuous high-voltage stress, electric charges and partial discharges can alter the rate at which LMW siloxanes move and chemically modify the surface.

Practical Impact in HV Insulators: Over time, the outward migration of LMW siloxanes re-establishes a hydrophobic film on the surface. This significantly reduces leakage currents in polluted environments and prevents flashovers.

2. Surface Energy Considerations

The surface energy of a material governs its interaction with water. Silicone rubbers, with their abundance of nonpolar methyl groups, intrinsically exhibit low surface energy (~20–24 mN/m), leading to high contact angles against water.

If the surface is damaged (e.g., by microcracks or contamination), the contact angle typically decreases (surface becomes more hydrophilic).

Recovery: As LMW siloxanes or reoriented methyl groups rebuild a low-energy “coating” on the surface, the water contact angle climbs back toward its original value.

3. Reorientation of Polymer Chains

In addition to the physical migration of LMW species, the polymer matrix itself can undergo chain reorientation at or near the surface. When the outermost layers of the polymer are perturbed—by corona discharge, UV radiation, or chemical contamination—longer polymer chains (e.g., the PDMS backbone) can rearrange themselves such that the lowest surface energy groups are oriented outward.

Factors promoting chain reorientation:

Thermal Activation – Above the glass transition temperature, chains have increased mobility, assisting reorientation.

Molecular Architecture – Crosslink density and chain flexibility influence how effectively chains move.

Surface Oxidation – Oxidative stresses can promote scission and rearrangement, triggering partial breakdown but also reorientation.

Hydrophobic recovery test

Key Methods to Test Hydrophobic Recovery:

1. Static Water Contact Angle Measurement

 • Principle: Measures the angle formed by a water droplet on the surface of the silicone rubber.

 • Procedure:

 1. Contaminate or treat the insulator surface (e.g., exposure to corona, UV light, or artificial pollution).

 2. Allow recovery over a defined period.

 3. Place a water droplet on the surface and measure the contact angle using a goniometer or other imaging tools.

 • Interpretation: A larger contact angle indicates stronger hydrophobicity. Observing changes over time shows recovery behavior.

2. Spray or Wettability Test (HC Classification)

 • Principle: Evaluates the formation of water droplets or a film on the surface.

 • Procedure:

 1. Treat the insulator surface to remove initial hydrophobicity.

 2. Expose the surface to recovery conditions (e.g., rest in air or at elevated temperatures).

 3. Spray the surface with water under controlled conditions.

 4. Assess the hydrophobicity visually or with photographs against HC classification standards (HC1 to HC7).

 • Interpretation: The classification indicates the level of recovery (HC1 = most hydrophobic, HC7 = hydrophilic).

3. FTIR Spectroscopy (Surface Chemistry Analysis)

 • Principle: Examines changes in the chemical structure of the silicone surface to infer hydrophobic recovery.

 • Procedure:

 1. Analyze the surface chemistry before and after contamination or stress using Fourier Transform Infrared Spectroscopy.

 2. Monitor the reformation of low molecular weight siloxane chains responsible for hydrophobicity.

 • Interpretation: The recovery of chemical signatures associated with hydrophobicity confirms the process.

Factors Monitored During Testing

 • Recovery Time: Duration required for the material to regain its hydrophobic state.

 • Recovery Efficiency: Degree to which hydrophobicity is restored compared to the original state.

 • Environmental Conditions: Recovery behavior under different temperatures, humidity levels, or pollutant types.

Testing hydrophobic recovery provides critical insights into the durability and reliability of silicone rubber insulators, especially in polluted and high-stress environments.