They consist of practically inert material, atoxic, with low coefficient of friction and non-reagents to most chemical substances, except in very special situations (concentration and temperature). Other feature is its impermeability that keeps its properties in humid environments.
They may be also considered as:

  Resistant to practically all known organic and inorganic compounds;
  Inert to room temperature;
  Resistant to temperatures superior to 390 ºF (200 ºC).