Polytetrafluoroethylene (PTFE), (- CF₂ - CF₂ -) n, is produced under pressure through polymerization of tetrafluoroethylene (CF₂ = CF₂) obtained from difluoromonochloromethane (CHC/F₂) catalytically at 1000 °C. PTFE displays excellent heat resistance but its coefficient of thermal expansion is greatly dependent on the temperature. When subjected to force, it deforms plastically even at room temperature. Fillers such as fibre glass or graphite can reduce this cold flow, the coefficient of thermal expansion and the wear rate. PTFE is resistant to virtually all aggressive liquid and gaseous media. Exceptions are: molten or dissolved alkali metals, a number of chlorofluorocarbons, fluorine and chlorofluoride at higher pressures and temperatures, amino polyesters at t > 160 °C and styrene at t > 80 °C. For material characteristics see materials for secondary seals.