The frictional properties of polyurethane elastomers are generally consistent with the established friction properties of rubber and vary from a coefficient of friction approximately 0.2 for the harder to around 2 to 3 for softest grades. The soft grades exhibit high values for coefficient of friction owing to the large true area of contact developed and the friction coefficients drop as the hardness increases. As with abrasion tests, however, values of friction obtained in the laboratory can be misleading and should only be taken as a rough guide as to actual service behavior. Such factors as surface cleanliness, lubrication by airborne dust or abrasion debris, and small traces of fluids can greatly influence the frictional charecteristics in practical applications.
The slip velocity has a small effect on the coefficient of friction, higher slip velocities giving slightly higher coefficient of friction. The coefficient of friction is higher on smooth moulded plastic surfaces than on the relatively rough surface of rough ground steel. The application of higher load increases the frictional force although there is a tendency for the coefficient of friction to fall with time. This effect is most probably due to lubrication of the interface by abrasion debris.
The coefficient of friction can be considerably reduced by lubrication, and when using polyurethanes for bearing applications it is generally beneficial to use oil or grease for this purpose. Where the provision of external lubrication is undesirable it has been claimed that additive can be incorporated into the polyurethane which makes them essentially self-lubricating. The additives generally employed are molybdenum disulphide, graphite and silicone oils. Caution must be observed when making use of these additives since in certain case it is known that their presence can have an adverse effect upon the ageing charecteristics of the polyurethane.