To some degree all elastomers are permeable to gases. The rate of gas permeation through an o-ring varies by material compounding; material hardness; degree of squeeze; presence or absence of lubrication; size of o-ring cross section; and the pressure, temperature and type of gas being sealed.
Typically, harder compounds containing more carbon black feature lower diffusion rates.
In the case of Nitriles, increasing acrylonitrile content results in decreased permeability.
Laboratory tests indicate that lubricated o-rings are significantly less permeable than unlubricated rings. These same tests further demonstrate that increased seal squeeze results in decreased permeability in unlubricated situations.
In general, Butyl is best in airtight situations. FKM is best for vacuum applications.
Note: Fluorosilicone and Silicone provide the least resistance to gas permeation.