pu90 doesn't absorb co2 so when it is exposed to atmosphere it won't swell like all the other seals do
silicone only absorbs it a little but is very soft and will be damaged easily
epdm if smothered in silicone oil is ok but will swell then go down
viton will swell and i find will them break easily
nbr will swell
the interesting thing with compatibility charts is they don't take into account exposing the o ring to atmosphere after it has been in contact with co2
these pick show before and after being exposed to liquid co2 for 24hrs