Polyurethane properties : Coefficient of friction

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.

Polyurethane Properties : Effect of low temperature

Low temperature environments affect the properties of polyurethane elastomers, but no degradation occurs and effect is completely revesible. The major effect is an increase in Young’ s modulus below 0 *C with a corresponding increase in hardness, tensile strength, tear strength and torsional stiffness and a decrease in resilience.

The two properties that can have an important bearing on the use of polyurethane at low temperature are the decrease in resilience which has already been mentioned and the increase in stiffness. In the use of polyesterurethane at low temperatures another phenomenon can sometimes be observed. This is that of crystallization of the polyurethane when held at moderately low temperature foe some period of time. When crystallized the polymers are considerably harder and their stiffness charecteristics are similar to those normally observed at much lower temperatures. The effect is reversible and can be removed by the application of heat or by the generation of internal heat by flexing. The use of mixed polyesters reduces this tendency to crystallization.

Polyurethane in Shoes and Shoe Machinery

The use of solid polyurethane rubber for ladies’s shoe heel top pieces has been known for several year. Cast grades of medium to high hardness have generally been employed for this purpose, but more recently the thermoplastic polyurethane have been graining ground. The material cost of polyurethane are high compared with more conventional heeling materials and to be competitive the processing cost must be as low as possible. The high cost has so far prevented the use of solid polyurethanes for the soles of shoes but the development of the cellular varieties of the solid materials has reawakened interest in this outlet. By using cellular grades with a specific gravity of 0.4, material cost are reduce to a third of those of the solid. Provided the process can be refined sufficiently to keep the costs low, soles for hard-wearing applications are a distinct possibility. In this case closed cell matreial is necessary to prevent excessive uptake of water.

The process for shoe manufacture is becoming highly automated. The high hardness, toughness and flexibility of the solid polyurethanes make them excellent materials for many items in this application. Replacement of a natural rubber diaphragm by polyurethane increased the diaphragm life from two months to a yeas.