Unique elastomer gasket is co-molded with two materials: Perfluoroelastomer with an outer Silicone centering ring. The chemically resistant perfluoroelestomer section is the only portion exposed to the wetted area of the reaction. The soft pliable sealing surface provides improved sealing under full vacuum, as opposed to encapsulated o-ring materials and envelope gaskets. Gasket is for use with 300mm Duran flat ground flange on Chemglass 30L, 50L, 75L and 100L reactor system vessels.