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Solid Lubricant Applications
Solid lubricants
PTFE (polytetrafluoroethylene)

PTFE or 'polytetrafluoroethylene' has been invented in 1938 by DuPont de Nemours. It consists only of carbon and fluorine atoms and has unique properties within the polymers:

  • extremely high melting temperature (327°C), it can be used up to 260°C in continuous use.
  • non-combustible, its resists pure oxygen up to elevated temperature and most chemicals, except strong bases above 300°C.
  • low surface tension and coefficient of friction.

PTFE resin is manufactured by free-radical polymerisation of tetrafluoroethylene in water, and under pressure. The water medium helps in removing the heat generated by the polymerisation and control the reaction rate. The polymer usually has extremely an high molecular weight, well in excess of 1 million. Two manufacturing technologies are used:

  • Suspension polymerisation: little or not surfactant is used so that the polymers grows a the surface of the solid particles. As the polymerisation proceeds, the size of the particles increase steadily up to 50 or 100 µ.
  • Dispersion polymerisation: some fluorinated surfactant is used during the polymerisation, so that the polymer grows proceed within a micelle, and the particle size of the polymer obtained is well controlled at about 0.2 to 0.3 µ.

PTFE metal powders processing techniquePTFE resin does not flow above its melting point, but remains an extremely viscous rubbery transparent mass that can NOT be processed by conventional methods such as extrusion or injection. Specific methods have been developped to manufacture parts from PTFE, derived from metal powders processing techniques. The basic principle is to shape PTFE powder into a pre-form followed by sintering at 380-400°C. Modified PTFE, manufactured by co-polymerisation, and known as FEP and PFA do overcome those problems of PTFE processing, but at the expense of other properties.

PTFE resin has the apperance of a white powder, usually with poor flow properties. It forms a "putty" when compressed or sheared due to fibrillation. This phenomenon can be attributed to the bonding between polymer particles due to the interlocking of polymer chains (see picture on the left).

PTFE wax (also called PTFE micropowder) is similar to PTFE resin, but the molecular weight is much lower (< 50,000 g/mol). It has also the appearance of a white powder but with no tendency to agglomerate. As it retains the hydrophobic and lubricating properties of PTFE resin but can be easily be incorporated in liquids and solids, PTFE wax is widely used as an additive in the chemical industry. It is however, not suitable to manufacture mechanical components as it turns to a brittle solid upon melting and cooling.