The hydrophobicity transfer test, also known as hydrophobicity migration testing, evaluates the ability of silicone rubber composite insulators to restore water repellency on polluted surfaces, one of the most important properties governing pollution flashover performance. This technical article explains the underlying mechanisms, sample requirements, test items, measurement methods, and acceptance criteria, with reference to standards such as IEC TS 62073, IEC 61109, IEC 62217, and GB/T 19519.
Technical Overview
Hydrophobicity is the water-repellent condition of a solid surface: when a surface is hydrophobic, water forms discrete, mobile droplets instead of a continuous film. For outdoor insulation, this behavior is decisive. On a hydrophobic insulator surface, polluted moisture cannot form conductive films, leakage currents remain low, and the risk of dry-band arcing, tracking, and pollution flashover is greatly reduced.
Silicone rubber, the dominant housing and shed material for composite insulators, possesses a further unique capability known as hydrophobicity transfer, or hydrophobicity migration. Natural contaminants such as salt, dust, and industrial deposits are intrinsically hydrophilic. However, mobile low-molecular-weight (LMW) siloxane species diffuse from the rubber bulk into the porous pollution layer and encapsulate the contaminant particles, rendering the pollution layer itself water repellent after a period ranging from several hours to a few days.
This migration capability distinguishes silicone rubber from glass, ceramics, and most other polymeric housing materials such as EPDM, which remain hydrophilic once polluted. It is also distinct from hydrophobicity recovery, which describes the regeneration of water repellency on the bare polymer surface after temporary loss caused by corona discharge, arcing, or weathering. Both phenomena are driven by the same mobile LMW siloxane fraction, which is why the migration test is widely used to assess formulation quality.
The hydrophobicity transfer test quantifies how quickly and how completely a material transfers water repellency to a defined artificial pollution layer. Measurement is generally performed either by the spray method, which assigns a wettability class from HC1 (strongly hydrophobic) to HC7 (hydrophilic), or by the contact angle method, both of which are described in IEC TS 62073. The test forms part of material qualification and type testing for composite insulators under standards such as IEC 61109, IEC 62217, GB/T 19519, and GB/T 22079, and it is specified in many utility purchasing specifications for polluted service environments.
Testing Samples
Silicone Rubber Plate Specimens
Cured slabs of housing-grade HTV (high-temperature vulcanizing) silicone rubber, commonly in sizes of 50 mm by 100 mm or 100 mm by 100 mm with a thickness of 2 mm to 6 mm, are the standard sample type for material qualification. Specimens are molded from the production compound under representative curing conditions. Several specimens, typically three to five, are tested for each material or batch to account for statistical scatter.
Sections Taken from Finished Insulators
To verify the actual product rather than the raw compound, samples are cut from the sheds or housing of finished composite insulators. These sections confirm that the molding and vulcanization process has not depleted the mobile siloxane fraction needed for migration. Utility acceptance tests and failure investigations frequently rely on such product-derived samples.
RTV Coating Specimens
Room-temperature vulcanizing (RTV) silicone coatings applied to glass or porcelain substrates are tested for hydrophobicity transfer to qualify anti-pollution flashover coatings used in substations and on ceramic insulator strings. Coating thickness, typically in the range of 0.3 mm to 0.5 mm, is recorded because it influences the available LMW reservoir.
Reference and Comparison Materials
Test campaigns often include reference silicone samples with known migration behavior and, where required, non-silicone polymeric materials such as EPDM as negative controls. These help validate the pollution application and classification procedure for each individual test series.
Testing Items
The scope of a hydrophobicity migration test typically covers the following items:
- Initial hydrophobicity of the clean, unpolluted surface, recorded as a baseline wettability class and contact angle
- Hydrophobicity class of the artificial pollution layer after defined migration times, commonly 24 h, 48 h, and 96 h after pollution deposition
- Static contact angle of the polluted surface at the same time intervals
- Advancing and receding contact angles and contact angle hysteresis, where a more complete wettability characterization is required
- Verification of the applied pollution severity, expressed as salt deposit density and non-soluble deposit density, on control specimens
- Optional assessment of migration capability after preconditioning such as thermal aging, UV exposure, or corona treatment, to evaluate long-term service performance
Testing Methods
Specimen Preparation and Conditioning
Specimens are gently cleaned, typically with deionized water, and dried for at least 24 hours under ambient laboratory conditions. The initial hydrophobicity is then measured and recorded; qualified housing materials normally exhibit HC1 to HC2, or a static contact angle well above 100 degrees, before pollution is applied.
Artificial Pollution Application
A defined artificial pollution layer is applied to simulate field contamination. A slurry of sodium chloride and an inert powder such as kieselguhr or diatomite in deionized water is sprayed or brushed uniformly onto the specimen surface to achieve target severities, for example a salt deposit density of approximately 0.1 mg/cm² and a non-soluble deposit density of approximately 0.5 mg/cm², unless the product standard or client specification dictates other values. The specimens are then allowed to dry in a dust-free environment.
Migration Conditioning
Polluted specimens are stored in still air under controlled conditions, typically around 23 °C with moderate relative humidity and away from direct light, so that the migration process proceeds without external disturbance. Hydrophobicity measurements are repeated at scheduled intervals, most commonly after 24 h, 48 h, and 96 h; some test programs add earlier points such as 12 h or extended points up to 168 h to record the full migration curve.
Spray Method (Wettability Class)
Following the spray technique described in IEC TS 62073, a fine water mist is applied to the polluted surface from a distance of approximately 25 cm for 20 to 30 seconds. The resulting wetting pattern is photographed immediately and compared with reference images to assign a hydrophobicity class from HC1 to HC7. HC1 and HC2 represent strongly hydrophobic behavior with discrete droplets, while HC6 and HC7 indicate continuous water films and essentially hydrophilic behavior.
Contact Angle Method
In the contact angle technique, sessile water droplets, usually on the order of 5 microliters, are placed on the polluted surface and measured with a goniometer or by image analysis. Static contact angles are averaged over at least five droplets distributed across at least three positions on each specimen. Advancing and receding angles may additionally be recorded to quantify contact angle hysteresis, which reflects the uniformity of the transferred hydrophobicity.
Evaluation and Acceptance Criteria
Results are reported as hydrophobicity class and contact angle as a function of migration time. Typical acceptance criteria for housing-grade silicone rubber require the pollution layer to reach HC1 to HC3, or a static contact angle of at least 90 degrees, within 96 hours; premium HTV compounds commonly achieve HC1 to HC2 within 24 hours. The exact limits are defined by the applicable product standard, such as IEC 61109, IEC 62217, GB/T 19519, or GB/T 22079, as well as by client technical specifications.
Test Report
A complete report documents specimen identification and origin, formulation and curing information where relevant, the pollution method and verified deposit densities, environmental conditions, the measurement schedule, the measurement method used, all hydrophobicity classes and contact angles with time, photographic records of spray patterns, and any deviations from the referenced procedure.
Applications
- Material qualification and type testing of composite insulators for AC and DC overhead lines under IEC 61109, IEC 62217, GB/T 19519, and GB/T 22079
- Factory quality control and incoming inspection of silicone rubber compounds used for sheds and housings
- Qualification of RTV anti-pollution flashover coatings for substations and for retrofitting ceramic insulator strings
- Assessment of field-aged insulators to determine whether migration capability has been retained after years of service
- Failure analysis and root-cause diagnosis of pollution flashover incidents
- Research and development of silicone formulations, including the effect of LMW content, filler loading, and crosslink density on migration rate
- Input data for insulator selection and pollution performance evaluation under IEC 60815-3 in heavily polluted service environments
FAQ
What is the difference between hydrophobicity transfer and hydrophobicity recovery?
Hydrophobicity transfer (migration) describes the movement of water-repellent properties from the silicone rubber into a pollution layer deposited on top of it. Hydrophobicity recovery describes the reappearance of water repellency on the polymer surface itself after it has been temporarily lost, for example through corona discharge or surface arcing. Both rely on mobile LMW siloxane species, but they are evaluated with different test procedures.
How is the hydrophobicity class defined?
The spray-based classification assigns the surface a class from HC1 to HC7. HC1 and HC2 correspond to discrete droplets on a dry-looking surface (strongly hydrophobic), intermediate classes show progressively connected water patterns, and HC6 and HC7 correspond to continuous wet films (hydrophilic). The classification references are published in IEC TS 62073.
Which standards govern hydrophobicity migration testing?
Measurement techniques are defined in IEC TS 62073. Product and material requirements that call for hydrophobicity transfer evaluation include IEC 61109, IEC 62217, IEC 60815-3, GB/T 19519, GB/T 22079, and related power industry standards such as DL/T 810 for DC composite insulators.
How long does a migration test take?
Counting specimen conditioning, pollution application, and the standard 96-hour observation window, a complete test normally takes about one to two weeks. Extended programs involving additional aging preconditioning or longer migration intervals may take longer.
What results indicate good material quality?
High-quality housing-grade silicone rubber typically migrates the pollution layer to HC1 to HC2, or to static contact angles above 90 degrees, within 24 to 48 hours, and holds this level at 96 hours. Slow migration, incomplete transfer, or large scatter between specimens indicates an inadequate LMW siloxane fraction or processing deficiencies.
Can the test be performed by external laboratories?
Yes. Accredited third-party laboratories routinely perform hydrophobicity migration testing as part of type test programs, batch release testing, and independent failure investigations, issuing reports that are accepted by utilities and manufacturers worldwide.