stanier, The first place you might want to check is, "Non-Metallic Flat Gaskets for Pipe Flanges, American National Standard, ASME B16.21" American Society of Mechanical Engineers, New York, 1992.
Playing the game of gaskets, gaskets, gaskets! What's available, and will it work with the process medium at the given P/T?
Hope this helps, goodluck!
saxon