Friction and molecular structure: the behaviour of some thermoplastics

Abstract

This paper describes an experimental study of the friction and transfer of some thermoplastic polymers sliding over clean surfaces. With PTFE and high density polyethylene sliding at low speeds on smooth surfaces of glass or polished metals there is a marked difference between static and kinetic friction. The static friction is of the order of $\mu =0.2$ and is accompanied by transfer of lumps of polymer several hundred angstroms thick. This frictional behaviour is 'normal' and may be explained in terms of the bulk properties of the polymers. However, once sliding has commenced and the slider acquires a preferred orientation the friction falls to a much lower value $(\mu <0.1)$ and the transfer is either in the form of an extremely thin film (PTFE) or discrete streaks less than 10 nm thick (PE). This behaviour does not depend on the degree of crystallinity or on the crystalline texture of the polymer. If the molecular structure is changed by introducing an appreciable number of bulky side groups (e.g. TFE-HFP copolymers, PCTFE or low density PE) the kinetic friction remains the same as the static value. The friction and transfer are those of 'normal' polymers and resemble polypropylene and amorphous polymers such as PMMA and PVC in their behaviour. The results suggest that the low friction and light transfer of PTFE and high density PE during sliding are essentially due to their smooth molecular profiles.