What compressible rubber like gasket material is the least permeable to CO2? I am trying to can a carbonated solution at 120 PSI.  I am trying to figure out the best material to use for the gasket on the valve cap. I am carbonating my fluid with $\ce{CO2}$ and it is loosing pressure over time due to the permeability of $\ce{CO2}$ through my gasket. I have tried about 8 materials (aluminum, copper, graphite,  buna, FMK A, Viton, ECH, PVDF) I was not able to compress the metals enough with my equipment we also dismissed a number of other materials that are known to be highly permeable to $\ce{CO2}$ or are not feasible and have not found a suitable gasket yet. I also tried using $\ce{N2O}$ as my gas but it seems to have the same permeability as $\ce{CO2}$. my gas has to be soluble in water and non toxic. I know that Viton and FMK A are the same but one supplier said theirs was more pure than anyone else.
Viton is a trade name for a fluoroelastomer type. There are different grades which have different properties. I've had a Viton seal be deformed in a vessel when using tetrahydrofuran as a solvent, whereas Kalrez does not. I've seen different Viton grades again not have great compatibility with acetone. So do check out the compatibility of each supplier's product.
We have two seals that we use for $\ce{CO2}$ applications. EPDM and PTFE.  These both cope with 100 bar pressure.