Frictional / Surface Properties of Virgin Unfilled PTFE

Coefficient of Friction OF PTFE

Although the low coefficient of friction of PTFE is widely known, it is interesting to reflect that no reference was made to this characteristic until about ten years after the discovery of the polymer. Credit for this first publication goes to Shooter and Thomas who measured the coefficient of friction using a Bowden-Leben machine with loads of between 1 and 4 kg (2.2 and 8.8 Ib) and sliding velocities from 0.1 to 10mm / s (0.02 to 2 ft / min). They reported that the coefficient was 0.04. Other workers report that while Amonton’s law is fairly well obeyed at moderate loads the coefficient of friction rises steeply at very light loads, say below 100g (31 / 2 oz). Thompson et al. who studied the coefficient at high loads found the extremely low figure of 0.016 at a load of 1360kg (3000 Ib) . The coefficient of friction is dependent also on the sliding velocity, a high speed resulting in a high coefficient. By combining a low load and a high sliding velocity of 1.89m / s (370 ft / min), Flom and Porile found the high value of 0.36 for the coefficient. In their pioneer paper Shooter and Thomas claimed that, at the very low speeds they used, the coefficient was independent of temperature over the range 20 to 200°C (68 to 392°F). However, later work has shown that temperature has some effect. King and Tabor report that the coefficient remains steady at about 0.1 over the range 100 to -45°C (212 to -49°F). On further cooling the coefficient rises to about 0.2 but does not alter further even when the polymer is cooled to 80°C (-112°F). For the behaviour at elevated temperatures the best guide is the work of McLaren and Tabor who demonstrated that the coefficient of friction fell with increase in temperature. Makinson and Tabor have also examined the effect of sliding velocity and substantially agree with the variation in coefficient of friction with velocity given above. They have found that whereas at low velocities a thin continuous film of PTFE is laid down on the other slide surface (in this case glass), at higher velocities the PTFE is torn off in discrete fragments. Of less general importance than the dependence on load, velocity and temperature, but still of interest is the observation by Tabor and Williams that the coefficient is influenced by the orientation of the polymer, the coefficient being about 30% higher when sliding was across the chains than when it was along them.

ANGLE OF CONTACT

Zisman and his co-workers have studied the contact angles made with PTFE by a wide range of liquids. A few of their results with common liquids are summarised:

Fox and Zisman found that there was a critical surface tension (ca 17.5 to 20.5 dynes / cm) below which liquids would wet PTFE (i.e., would spread on a smooth polymer surface).