It is double PTFE encapsulated O rings of the type used in mechanical seal for chemical and thermal resistance. PTFE (Polytetrafluoroethylene) O-Rings outperform seals made from most other plastics. This is mainly due to their outstanding endurance of temperature ranges from (-184C to 260C) and known resistance to highly corrosive gases and chemicals, even with long-term exposure. PTFE has very little mechanical backbone and O-Rings will creep and take a compression set if not contained when placed under a load. When installing, care should be taken to avoid nicking or gouging by stretching over threads or sharp shoulders. Provide corner breaks or radii if possible, or use tools to lead rings over sharp areas.