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˚.
Like all silicones, Liquid Silicone Rubber (LSR) is characterized by a flexible, fully saturated backbone of alternating silicon and oxygen atoms. The siloxane bonds have a partial ionic nature and give the material its high strength. The backbone is shielded by closely packed hydrophobic methyl side groups, which have low intermolecular forces between them. This allows the surface energy or surface tension to be low, while also being hydrophobic. This characteristic enables liquid silicone rubber to be water repellent, while allowing it to adhere only to materials with higher surface energy such as polyamide 66 or polycarbonates.