Several factors can affect the degree to which a seal is gas permeable. Use of lubrication decreases permeability, as does applying greater squeeze. Use of harder compounds and smaller cross-sections can also help reduce permeability. Butyl rubber allows the least gas permeation. Tetrafluoroethylene (PTFE), chloroprene, epichlorohydrin, polyurethane, nitrile, and fluorocarbon are also good choices. Silicone and fluorosilicone allow the most gas permeation.

In applications involving pressures of 500 psi or higher, compressed gases enter through flaw sites on the seal’s surface and fill the O-ring’s micropores until equilibrium is reached. During an equilibrium shift (as during decompression), the gases expand, creating blisters in lower durometer O-ring compounds and fractures in harder compounds (see Figure 55). Instances of such “explosive decompression” can be reduced through careful choice of materials. Harder, high shear modulus compounds have the most resistance to explosive decompression because they have the strength to dissipate the fracture energy as it propagates through the O-ring. For contact with carbon dioxide (CO2), such as in air guns, polyurethane is definitely the best choice. For more on explosive decompression see Diagnosing O-Ring Failure: Explosive Decompression.

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