A PTFE envelope gasket is a composite seal consisting of a protective, chemical-resistant Polytetrafluoroethylene (PTFE) outer shell (envelope) enclosing a resilient filler material (e.g., graphite, asbestos-free fibre, or rubber). It combines the chemical inertness of PTFE with the compressibility of the filler, making it ideal for aggressive, high-temperature, or pharmaceutical applications where contamination is not allowed.