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This paper describes a study of the friction of several types of rubber against hard surfaces over a wide range of temperatures and sliding velocities. The highest velocity did not exceed a few centimetres per second so that frictional heating was negligible. The results show that the friction increases with the sliding velocity to a maximum value and then falls. The application of the Williams, Landel & Ferry (1955) transform shows that the frictional behaviour of a rubber sliding at various velocities and temperatures on a given surface can entirely be described by a single ‘master curve’ and the glass transition temperature of the material. The master curve on a rough abrasive track shows, in general, two peaks—one of these occurs at a velocity related to the frequency with which the track asperities deform the rubber surface. This maximum is absent on a smooth track and thus reflects the deformation losses produced by the passage of the asperities over the rubber surface. The other peak occurs in general at much lower velocities; it coincides in position with the single maximum obtained on a smooth surface. Introduction of a fine powder (MgO) into the interface between the rubber and track eliminates this peak on both smooth and rough surfaces; it is therefore attributed to molecular adhesion. Comparison with the relaxation spectrum of the rubber gives a fundamental jump distance of the order of 60 Å. It appears therefore that friction arises from adhesion and deformation losses, and that both are directly related to the visco-elastic properties of the rubber.

This paper describes a study of the friction of several types of rubber against hard surfaces over a wide range of temperatures and sliding velocities. The highest velocity did not exceed a few centimeters per second so that frictional heating was negligible. The results show that the friction increases with the sliding velocity to a maximum value and then falls. The application of the Williams, Landel and Ferry transform shows that the frictional behavior of a rubber sliding at various velocities and temperatures on a given surface can entirely be described by a single master curve and the glass transition temperature of the material. The master curve on a rough abrasive track shows, in general, two peaks—one of these occurs at a velocity related to the frequency with which the track asperities deform the rubber surface. This maximum is absent on a smooth track and thus reflects the deformation losses produced by the passage of the asperities over the rubber surface. The other peak occurs in general at much lower velocities; it coincides in position with the single maximum obtained on a smooth surface. Introduction of a fine powder (MgO) into the interface between the rubber and track eliminates this peak on both smooth and rough surfaces; it is therefore attributed to molecular adhesion. Comparison with the relaxation spectrum of the rubber gives a fundamental jump distance of the order of 60 A. It appears, therefore, that friction arises from adhesion and deformation losses, and that both are directly related to the viscoelastic properties of the rubber.

The paper gives a brief survey of the state of friction and abrasion research with a view of the possibility to use laboratory methods for the development of new compounds with optimal traction and abrasion properties. It shows that viscoelasticity plays a decisive role in friction and in this way measurements of the dynamic properties give a good indication of the possibilities for good traction properties. However, friction is still a good deal more complex than the modulus or loss factor curves. It takes in different frequency ranges and temperatures in the contact area so that a direct laboratory measurement of these properties is still very desirable. If the speed and temperature correspond to the log aTv values experienced in practice and the laboratory track structure and texture is not too far removed from that of road surfaces, the correlation with road tests is high. To simulate the structure and texture of road surfaces with durable laboratory surfaces, a combination of two surfaces may be necessary. Abrasion is not only influenced by the strength properties of the rubber but also by oxidation and thermal degradation. To give these processes the correct weight in the laboratory, the testing conditions have to be mild and a combination of several conditions is necessary in order to demonstrate the complexity of interactions, which can lead to ranking reversals. Energy dissipation, speed, and abrasive surface structure and texture are identified as prime variables to achieve a high correlation with road wear. Since viscoelasticity, encompassing not only polymer but also filler, oil-extension, curing and other compound additives, plays a major role in both friction and wear, the rolling resistance of the compound is always effected and has to be taken into account. Modern polymerization methods and new filler concepts make it possible to change the viscoelastic properties in such detail that high friction and—to the degree to which strength contributes to wear—high wear resistance can be combined with low rolling resistance. This development has certainly not reached its climax yet. Exciting times lie ahead for tire compounders, polymer- and filler chemists alike.

The temperature dependence of abrasion of various gum and black-filled rubbers on silicon carbide paper is closely similar to the temperature dependence of their energy density at break determined at a high rate of extension. The conclusion is drawn that this type of abrasion is predominantly due to tensile failure, as had been envisaged in earlier work. The effective rate of extension during abrasion at a sliding velocity of 1 cm/sec is about 10,000 per cent/sec. The volume of abraded rubber is approximately proportional to the ratio between frictional energy dissipation and energy density at break. The proportionality constant is greater for black-filled than for gum rubbers. The effects of temperature and velocity on the abrasion of non-crystallizing gums are interrelated by means of the Ferry transform, indicating the viscoelastic nature of the abrasion process operating on these compounds.