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.
Silicone rubber has excellent high temperature properties and can be used in silicone rubber molding. Silicone rubber molding can operate normally from -50°C to +250°C. This makes silicone rubber components ideal for the automotive industry or high temperature sterilization environments as in medical applications.
The density or its reciprocal, the specific volume, is important for the shrinkage during processing and is greatly affected by temperature and pressure. The specific volume is often plotted as a function of pressure and temperature, in what is known as a pvT diagram (see image on the left below for an example of a pvT diagram). The measurement is defined in ISO 17744.