Silicone rubber is universally regarded as the best-in-class elastomer for extreme environments. In addition, silicone rubber is one of the most permeable elastomers. This property is a key advantage for silicone rubber in many design situations. However, some applications which require silicone performance in extreme environments also require low gas permeability. Applications that require such material characteristics cover a range of aerospace components such as inflatables, ducting, and diaphragms. This paper presents a technical review of the gas permeability characteristics of silicone rubber, including analysis of the gas penetration mechanism through the elastomer. The chemical structure of silicone rubber affects permeability, as do other governing factors such as temperature and gas type. In particular, phenyl vinyl methyl silicone (PVMQ) has the lowest glass transition temperature (-120!aC) of all elastomers,but its gas permeability is very high. Arlon has developed a proprietary technology to reduce the gas permeability of PVMQ. Arlon!ˉs technologcal approach produces large gains in gas permeability reduction without sacrificing excellent low temperature elastomeric flexibility.
The high flexibility of the silicon-oxygen chain in silicone provides !°openings!± which are fre volume and permit gas diffusion. The typical silicones such as vinyl methyl siloxane, (VMQ) and PVMQ, are two of the most permeable elastomers . High permeability of silicone can be applied in membranes for blood oxygenation, gas separation, drug delivery, and fundamental. The penetration studies to gain insight of the molecular structure of silicone elastomer mechanism of gas in polymer can also be applied to silicone rubber to help understand gas
permeability.
Free volume or !°holes!± exists in the rubber maix. !°Holes!± thermally rm and disappear with the movement of polymer chains. Gases are soluble in rubberlike substance. When rubber is exposed to a gas, solution occurs at the surface and the dissolved gas molecules diffuse into the interior. The diffusion of gas molecules in the rubber membrane is a process in which the gas molecules migrate from !°holes!± (free volumeto !°holes!± (free volume). The permeation of gas through a membrane involves in solution on one side, diffusion through the membrane to the other side, and finally evaporation out of membrane. The rate of permeation is a specific function of a given gas and rubber. The rate of permeation depends on both solubility and the diffusion rate.
