Explain the Properties of Poly Tetra Fluoro Ethylene?
PTFE is a linear chain polymer of great molecular strength, known for its chemical inertness and low coefficient of friction. PTFE is not oxidised easily, it is resistant to all common solvents and remains stable at extremes of atmospheric temperatures. It was earlier thought that lowest coefficient of friction could be obtained with PTFE sliding against PTFE. But on the basis of tests conducted on several frictional interfaces, it has been conclusively proved that the frictional coefficient of PTFE sliding against grounded stainless steel surface is lower than PTFE on PTFE. However PTFE has poor creep properties i.e. it exhibits permanent compression under loads.
It also has poor bonding properties, and therefore, always used in thin sheets (upto 3 mm) recessed in a steel plate with half the thickness of PTFE embedded. Addition of certain fillers such as glass fibre, graphite or bronze to PTFE increases the wear resistance and creep properties but it also increases the frictional resistance. Fillers of molybdenum sulphide or silica though improves wear and creep properties without appreciable increase in friction. In bridge bearings, pure PTFE is therefore rarely used. Although, friction between steel and PTFE is the minimum, yet it is highly susceptible to intrusion of dust. Elaborate arrangements are, therefore, must to prevent entry of dust particles on the sliding surface. Silicon grease is generally used as a lubricant for PTFE surfaces. Dust seals are also recommended around PTFE bearings to prevent the ingress of dust.