Introduction

Silicone rubber, a versatile polymeric material, has become indispensable in high voltage outdoor insulation over the past two decades. Its ability to resist environmental degradation while maintaining electrical insulation properties makes it an ideal material for outdoor insulators. Central to its effectiveness is hydrophobicity, a property that ensures longevity and reliability in challenging conditions.  

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Hydrophobicity refers to the property of a material that resists the spread of water on its surface. When a material is hydrophobic, water does not form a continuous film but instead beads up into droplets. This is crucial for outdoor insulators because continuous water films can create conductive paths on the insulator’s surface, leading to leakage currents, electrical discharges, and potentially catastrophic flashovers.

The Contact Angle Measurement:

The degree of hydrophobicity is often quantified using the contact angle, which is the angle at the interface where a water droplet meets a solid surface. A contact angle greater than 90° signifies strong hydrophobicity, where the surface resists wetting and water droplets remain spherical. Conversely, a contact angle less than 90° indicates that the surface is more hydrophilic, allowing water to spread out and potentially form a conductive layer.

The contact angle provides insight into the surface energy, roughness, and heterogeneity of the material. For silicone rubber insulators, maintaining a high contact angle is essential for ensuring that the insulator remains effective in preventing electrical conduction across its surface, especially in polluted or humid environments.

Electrical Insulation Performance: Hydrophobicity is crucial for maintaining the insulating properties of silicone rubber. In polluted environments, contaminants can accumulate on the insulator’s surface. If the surface becomes hydrophilic, water can create a continuous film that increases surface conductivity, leading to leakage currents and potentially severe electrical discharges.

Material Longevity and Aging: Silicone rubber insulators are exposed to various environmental stressors, such as UV radiation, humidity, and extreme temperatures. These factors can degrade the material over time. Hydrophobicity acts as a protective barrier, preventing water from penetrating the surface and slowing down the aging process.

Hydrophobicity in silicone rubber insulators is primarily attributed to the presence of low molecular weight (LMW) siloxanes on the surface. These LMW components migrate from the bulk material to the surface, where they form a hydrophobic layer. The dynamic nature of this process allows the material to recover its hydrophobic properties even after temporary loss due to environmental exposure.

Surface Energy and Wetting Behavior

Surface energy plays a pivotal role in determining the wetting behavior of a material. Silicone rubber has a low surface energy, which is why it exhibits strong hydrophobicity. The lower the surface energy, the more likely it is that water will form droplets rather than spreading out across the surface. This property is particularly important in environments where the insulator is exposed to rain, fog, or other forms of moisture.

The Role of LMW Siloxanes

Low molecular weight siloxanes are crucial for maintaining hydrophobicity. These small molecules have high mobility within the silicone rubber matrix, allowing them to migrate to the surface and form a protective hydrophobic layer. The presence of these LMW components is not only essential for initial hydrophobicity but also for the material’s ability to recover hydrophobicity after exposure to environmental stressors.

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Accurate measurement and monitoring of hydrophobicity are essential for ensuring the long-term performance of silicone rubber insulators. Various techniques have been developed to assess the hydrophobicity of these materials:

STRI Hydrophobicity Classification: This manual method classifies the surface hydrophobicity of insulators into seven classes, from HC1 (highly hydrophobic) to HC7 (completely hydrophilic). While effective, this method is subject to human judgment and variability.

One of the manual methods to detect the hydrophobicity class in outdoor environment can be descibed as follows. First, the surface to be studied (50-100 cm2) is sprayed with water. The obtained drop pattern is observed and attributed to one of the seven hydrophobicity classes. As a help, the examiner has a set of reference images of typical wetting patterns representing each HC class. The disadvantage of this method is that the measure is dependent on human judgment.

” Note that samples during the spraying for hydrophobicity test should be kept at inclinations between 10 and 35° from the horizontal plane. These inclinations should be chosen since they represent well typical inclinations of insulator surfaces in actual service. In order to make measurements comparable, all directions and distances between camera, illumination levels, and sample position should
be also fixed”

Sessile Drop Technique: The most commonly used method involves placing a water droplet on the material’s surface and measuring the contact angle. This technique provides a precise measurement of the material’s hydrophobicity and is widely used in both laboratory and field settings.

Dynamic Hydrophobicity Measurement: This approach monitors changes in hydrophobicity over time and under varying environmental conditions, such as UV exposure, temperature fluctuations, and pollution. It helps in understanding how these factors influence the material’s water-repelling properties.

Scanning Electron Microscopy (SEM): SEM provides detailed images of the material’s surface, revealing micro-cracks, surface roughness, and the extent of contamination. This technique is particularly useful for analysing the physical factors that influence hydrophobicity.

Leakage Current Monitoring: This method measures the electrical current that leaks across the surface of the insulator, which is directly related to the level of hydrophobicity. A loss of hydrophobicity leads to increased leakage current, making this a valuable tool for continuous monitoring.

Electric Fields: The presence of strong electric fields can accelerate the degradation of hydrophobicity, particularly in areas with high contamination levels. These fields can cause localized electrical discharges, known as corona discharges, which can lead to a significant loss of surface hydrophobicity.

Temperature: Elevated temperatures can enhance the diffusion of LMW siloxanes to the surface, aiding in the recovery of hydrophobicity. However, prolonged exposure to high temperatures can also accelerate the aging of the material, potentially leading to a permanent loss of hydrophobicity.

Humidity: High humidity levels can promote the formation of water films on the surface, leading to a loss of hydrophobicity. However, humidity can also facilitate the migration of LMW siloxanes to the surface, helping to restore hydrophobicity. The overall impact of humidity on hydrophobicity depends on the balance between these factors.

UV Radiation: UV radiation has varying effects on different polymeric materials. For silicone rubber, UV exposure can enhance the diffusion of LMW siloxanes, thereby maintaining or even improving hydrophobicity. In contrast, other materials like EPDM may become more hydrophilic with prolonged UV exposure.

Pollution and Contaminants: The accumulation of pollutants on the insulator surface can reduce hydrophobicity by altering the surface energy. Hydrophilic contaminants can create a conductive path for electrical currents, increasing the risk of flashovers. The ability of silicone rubber to transfer hydrophobicity to the pollution layer is a key factor in maintaining performance in polluted environments.

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The recovery of hydrophobicity can occur through several mechanisms:

Diffusion of LMW Siloxanes: After exposure to environmental stressors, LMW siloxanes migrate to the surface to restore hydrophobicity. This process is influenced by factors such as temperature, humidity, and the presence of an electric field.

Reorientation of Polymer Chains: In some cases, the reorientation of polymer chains at the surface can contribute to the recovery of hydrophobicity. This is particularly important when the surface has been altered by factors such as oxidation or contamination.

Surface Energy Considerations: The inherent surface energy of the material plays a role in hydrophobicity recovery. Materials with low surface energy are more likely to repel water and maintain their hydrophobic properties.

While silicone rubber insulators generally exhibit strong hydrophobicity recovery, certain conditions can hinder this process:

Severe Pollution: In environments with heavy pollution, the accumulation of hydrophilic contaminants can overwhelm the recovery process, leading to a persistent loss of hydrophobicity.

Prolonged UV Exposure: Although UV radiation can aid in the diffusion of LMW siloxanes, prolonged exposure can also lead to the breakdown of the polymer chains, reducing the material’s ability to recover hydrophobicity.

Mechanical Damage: Physical damage to the insulator surface, such as abrasion or cracking, can impair the diffusion of LMW siloxanes and hinder hydrophobicity recovery.

Hydrophobicity is a critical property that underpins the performance and reliability of silicone rubber insulators in outdoor electrical applications. By understanding the mechanisms that govern hydrophobicity, the factors that influence it, and the ways to measure and enhance it, engineers and material scientists can continue to improve the effectiveness of these essential components.