The friction and lubrication of polymers

Abstract

This paper describes a study of the friction of a typical polar polymer, nylon, and a typical non-polar polymer, polyethylene. The friction of unlubricated surfaces varies with load and geometry in a manner that can be explained in terms of the multi-asperity model of Howell & Lodge (1954), modified to allow for a deformation mode that is intermediate between elastic and plastic deformation. Nylon is markedly plasticized by water and its effect as it diffuses into the bulk of the polymer has been studied using hardness measurements. Initially the water produces a softening restricted to the surface layers and this leads to a modest reduction in friction. After a protracted period the bulk softening of the polymer causes an increase in the area of contact (cf. Rubenstein 1961) and the friction again rises. If one of the surfaces is a hydrophilic substance such as glass a more marked reduction in friction occurs when water is introduced to the surface. There is strong evidence that this due to the formation of a weakly held water film, a few molecules thick, attached to the hydrophilic surface. The film survives because the local pressures are not too severe. For this reason water provides effective lubrication for glass sliding on polyethylene or rubber but not for glass on glass where the contact pressures are much higher. Long chain acids produce negligible plasticization of the polymers but lubricate by forming adsorbed surface films. They are not as effective on polymers as on metals and this work supports the view of Fox & Zisman (1952) that this is because the films are not fully condensed. Polyethylene surfaces are lubricated by surface films of oleamide or stearamide. These materials may also be incorporated, in small quantities, within the bulk of the polymer and can diffuse to the surface to provide an effective lubricating layer. Non-polar liquids and some silicones produce a small reduction in the friction of polymers; the effect is more marked if one of the surfaces is of glass (or metal). It is suggested that these liquids form weakly adsorbed films which survive the relatively gentle stresses at the contact regions and so reduce the amount of polymer-polymer or polymer-glass interaction.