Friction of rubber

FIGURE 22.13 Simple analytical model of hysteresis friction of rubber moving over a rough road profile left). Tire on a wet road track, where hysteresis energy losses dominate the traction behavior right). (From Kluppef M. and Heinrich, G., Kautschuk, Gummi, Kunststojfe, 58, 217, 2005. With permission.)... 

Friction of rubber is a very complicated phenomenon but is generally thought of as being composed of two parts, adhesive friction and hysteresis... 

When rubber is brought into contact with another surface it deforms elastically and the real area of contact will increase with increasing normal load and, hence, the coefficient of friction will decrease with increasing normal load. It is also apparent that the real contact area is dependent on the surface geometry of the test piece. It is, hence, desirable to measure the friction of rubber over the range of normal forces of interest and to test with the surface geometry to be used in service, which may mean using the product or part of it as the test piece. 

The distinction is sometimes made between static and dynamic friction, implying that there is one level of the coefficient of friction just at the point when movement between the surfaces starts and another level when the surfaces are steadily separating. There can of course be no measure of friction without movement so that static friction is actually friction at an extremely low velocity and thereafter the coefficient of friction of rubbers may vary markedly with velocity. Hence, it is necessary to measure friction over the range of velocities of interest. Friction is also dependent on temperature, which can lead to inaccuracies at high velocities because of heat build-up at the contacting surfaces. 

Product areas where friction of rubber is particularly important are roads and floor surfaces where it is convenient if measurements can be made in-situ. Consequently, a considerable number of portable devices have also been developed. 

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

Another approach to static friction is to measure friction at a number of very low-velocities and then to extrapolate to zero velocity. This gives a static friction" value for zero contact time and is likely to be much lower than the former method, although it may be argued that extrapolation in these circumstances is wholly inappropirate and that no real measurement of static friction has been made at all. However, there is no doubt that this approach to measurement has given a much greater insight into the nature of the friction of rubber-like polymers. 

Adhesion in the Friction of Rubber-like Polymers Theories of Rubber Friction Adhesion in the Friction of Polymers The Effect of Pressures Effect of Speed and Temperature The Interfacial Layer Boundary Lubrication... 

Figure 7. Data from Grosch showing that the friction of rubber at various speeds and temperatures can be treated by the WLF transform to lie on a single "master" curve.

These theories of rubber friction have at least two major defects in common. First they ignore the nature of the counterface the friction is apparently determined solely by the properties of the rubber. But is the friction of rubber on glass the same as rubber on P.T.F.E. Secondly they do not consider whether sliding takes place truly at the interface or within the rubber itself. 

It is observed that the level of the friction-speed curve on a wet surface is lower than that on a dry surface. The decrease in friction caused by the presence of water is not uniform on the various countersurfaces. In particular,the reduction in friction of rubber on hard polymers was small. On many other solids, there was a significant reduction in friction. A typical result is shown in Figure lA. 

The use of a damper to suppress stick-slip, although convenient, would seem to be open to the objection that it will produce an average value of the periodic force observed in an undamped system. Our observations reported in our paper on the friction of rubber on ice indicate that the perk value of the frictional force is more appropriate. Without repeating our reasons for believing this, there is the experimental observation that increasing the transducers strong stiffness increases the minimum value of the periodic force without affecting the maximum. It would affect, therefore, that an infinitely stiff mechanical system would produce a non-periodic force equal to the maximum value obtained in a stick-slip measurement. 

This paper describes the effect of velocity and temperature on the friction coefficient of both filled and unfilled rubber vulcanizates sliding on smooth ice. It has been shown that the mechanism of the friction of rubber on ice is the same as that on other smooth surfaces under similar low sliding speed conditions and that the maximum friction coefficients are similar. The Williams Landel and Ferry equation is used to superpose curves of the velocity dependence of the friction coefficient at different temperatures to produce a master curve and therefore to demonstrate the viscoelastic nature of the frictional mechanism. The frictional behaviour depends on the condition of the ice track and a tentative explanation for this observation is suggested. The frictional properties of vulcanizates containing various amounts of a reinforcing carbon black filler have been studied. 

It is a common experience that the friction of rubbers and of most other materials on ice is extremely low. Under some circumstances the reasons for the low coefficient of friction may be ascribed to frictional melting or to pressure melting on the ice. The friction of rubber on smooth surfaces has been carefully studied j but until recently > little scientific work has been done to investigate the mechanism of the friction of rubber on ice. This paper describes all the recent work which we have carried out on the subject and shows that under some circumstances the friction of rubber on ice may be very high indeed with friction coefficients well in excess of unity. 

Measurements of the friction coefficient over a wide range of speeds and temperatures have been made. A typical set of results is shown in Figs. 2 and 3 where it can be seen that Increasing the sliding velocity may lead to an increase or a decrease in the friction coefficient depending on the temperature at which the experiment was done. Behaviour of this sort has been reported for the friction of rubber on other smooth surfaces such as glass or polished steelThe individual curves at each temperature may be... 

At low sliding speeds the friction of rubber on ice arises from the viscoelastic properties of the rubber. The behaviour is similar to that obtained on other smooth surfaces and results can be shifted to form mastercurves according to the WLF transform. 

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