Liquid Silicone Rubber (LSR) is often utilized for seals, valves and diaphragms, due to its high elongation (between 400 and 700% at room temperature) and tensile strength over a wide temperature range. The tensile properties of thermoset rubbers and thermoplastic elastomers need to be measured/tested in order to verify that the quality control standards of Silicone Rubber Liquid are met, as well as to determine whether or not the material is fit for its purpose. The test should be performed according to ASTM D412, which describes two test methods: A and B.
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.
While today’s popular non-stick bakeware products manufactured out of Liquid Silicone Rubber (LSR) are flexible to the touch, they maintain a certain degree of resistance to deformation. While ear buds or in-ear headphones, on the other hand, require a close and comfortable fit with a flexibility that enables it to match the shape of the inner ear for a more natural feel.
The elemental structure of Liquid Silicone Rubber (LSR) enables the material to have a very low compression set; normally in the range of 20 – 40% (22 h @ 350˚C). A low compression set means LSR retains its elastic properties, even during prolonged application of compressive stresses (resist creep), such as in sealing or vibration dampening part applications. This resistance to creep allows LSR to maintain its high sealing force during a product’s lifetime. This method allows for producing molded silicone rubber products using liquid silicone rubber with outstanding results.