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The hydrophobic methyl side groups of Liquid Silicone Rubber (LSR) account for its low surface energy and water repellence. An average value for silicones is 24 dynes/cm or 0.024 N/m. The surface hydrophobicity of a solid surface is determined by its free surface energy. ASTMD2578 (or ISO 8296) is the most employed technique to study loss and recovery of hydrophobicity of silicones, which is calculated by measuring the contact angle. The method is very time efficient, and inexpensive instruments can be utilized. It is often defined on the basis of the static contact angle between the surface and a water droplet, in which a surface can be considered hydrophilic if the contact angle is 90˚.

The most commonly utilized method for contact angle measurements of surface hydrophobicity is the Sessile Drop Technique where a droplet of a purified liquid is placed on a surface using a syringe. The resulting angle between the droplet and surface is measured, generally using a goniometer or a camera/projector. The angle formed by adding liquid to the droplet, causing it to advance over the surface, is called the advancing contact angle. The angle formed by removing liquid from the droplet, causing it to recede over the surface, is the receding contact. The difference between the advancing and the receding contact angle (referred to as the hysteresis) is dependent on the surface roughness and the surface heterogeneity. Both the contact angle, as well as the hysteresis, are also dependent on the contact time of the droplet and the surface. The contact angle recorded for silicones shows a decrease with increasing contact time of the water droplet. Researchers suggest that segmental motions of the polymer and water diffusion into the rubber may cause these time dependent effects.

Liquid Silicone Rubber - Hydrophobic vs Hydrophilic

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[1] M.J. Owen, “Siloxane Surface Activity” in Silicon-Based Polymer Science, A Comprehensive Resource, Eds. J, Ziegler and F.W. Gordon Fearon, Advances in Chemistry Series 224, American Chemical Society, Washington DC, 1990

[2] ASTM D 2578: Standard Test Method for Wetting Tension of Polyethylene and Propylene Films; ASTM International, 2009

[3] R. Bärch and H.-J. Winter, “On the Evaluation of Influences on the Hydrophobicity of Silicone-Rubber Surfaces”, 10th Inter, Symp. on HV Eng., Montreal, Canada, pp. 13-16, 1997

[4] H. Hillborg, U.W. Gedde, “Hydrophobicity Changes in Silicone Rubbers”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 6, No. 5, p. 703-718, 1999

[5] Osswald, T.A.; Baur, E.; Brinkmann, S.; Oberbach, K.; Schmachtenberg, E.: International Plastics Handbook; 4th edition, Hanser Gardner Publications, Cincinnati, 2006

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and LSR/Thermoplastic (Two-Shot) components.

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