IEC TS 62073-2003 pdf – Guidance on the measurement of wettability of insulator surfaces.
1 Scope and object The methods described in this technical specification can be used for the measurement of the wettability of the shed and housing material of composite insulators for overhead lines, substations and equipment or ceramic insulators covered or not covered by a coating. The obtained value represents the wettability at the time of the measurement. The object of this standard is to describe three methods that can be used to determine the wettability of insulators. The determination of the ability of water to wet the surface of insulators may be useful to evaluate the condition of the surface of insulators in service, or as part of the insulator testing in the laboratory. 2 Terms and definitions For the purposes of this document, the following definitions apply. 2.1 wettability ability of a surface to be wetted by a liquid (e.g. water) 2.2 hydrophobicity and hydrophilicity 2.2.1 hydrophobicity low level of wettability by water of a surface. A hydrophobic surface has a low surface tension and thus is water-repellent 2.2.2 hydrophilicity high level of wettability by water of a surface. A hydrophilic surface has a high surface tension and thus is wetted by water (in the form of a film)
4 Method A – Contact angle method 4.1 General The contact angle method is a measurement that involves the evaluation of the contact angle formed between the edge of a single droplet of water and the surface of a solid material. If done on a horizontal surface, the advancing and receding contact angles can be measured by adding water to or withdrawing water from the droplet. The contact angles depend strongly on the surface roughness and contact angles measured on polluted surfaces may differ significantly from contact angles measured on smooth, clean and planar surfaces. 4.2 Equipment Different commercial equipment for measuring the contact angle is available. Simple measurements are made using a magnifying device with a graduated reticle (goniometer) fixed on a frame with a syringe for application of the droplet on the test specimen. Another method involves magnifying the droplet using a light projector (behind the droplet) and projecting an image of the droplet onto a graduated background. Some equipment includes camera, display and computer for analysis of the measurements. 4.3 Measurement procedure 4.3.1 General recommendations General recommendations include: a) the receding contact angle ( r θ ) reflects the wetting properties of an insulator more than the advancing contact angle ( a θ ) and the static contact angle ( s θ ); b) it is often necessary to cut out a test specimen from the insulator under investigation. The test specimen selected should be as planar as possible and the size should allow for the application of at least three droplets on separate surface areas adjacent to each other. The surface to be measured should not be touched and the specimen should be carefully stored until the measurement has been performed. The measurement should be performed as soon as possible;
c) the water used should not contain impurities affecting the water surface tension (e.g. tensides, solvents, oil residues, etc.). De-ionized water is suitable; d) the volume of water in the droplet is not very critical. Volumes in the range 5 µl to 50 µl may be used. 50 µl is the recommended volume. For rough surfaces, a larger droplet volume may be needed. To limit a possible influence of the water droplet volume, the volume should be kept as constant as possible when comparing different specimens; e) the measurement of the contact angles should be performed as soon as possible (within a minute) after the application of the droplet on the surface. This is especially important when the ambient temperature is high and the relative humidity is low, which increases the rate of evaporation of the droplet. If the measurement is performed in a chamber with saturated water vapour, it eliminates the influence of evaporation. NOTE Small droplet volumes have the advantage that the contact angle is less influenced by gravity. On the other hand, for rough surfaces and other surfaces that could have high advancing contact angles and low receding angles, a too small droplet volume makes the measurement of the dynamic contact angles very difficult. A small droplet volume will also be more sensitive to evaporation, which could affect the measurement. The optimal droplet volume may thus be dependent on type of surface and ambient temperature and humidity