Abrasion-Reinforcement: Methods of Evaluation

Abstract

Abstract There are basically three methods by which abrasion-reinforcement may be studied (1) conventional laboratory physical property measurement and empirical prediction of reinforcement (2) laboratory abrasion tests for an actual measurement of reinforcement under accelerated conditions and (3) a study of the mechanism of abrasion with a theoretical deduction of the important physical properties relevant to the phenomenon of abrasion. None of the above are without serious difficulties in their execution at the present state of rubber technology. Known reinforcing materials such as fine particle size carbon blacks increase modulus, tensile strength, tear strength and hardness while decreasing breaking elongation. Measurements of these properties can be very helpful in the selection of reinforcing materials but such measurements used alone are not entirely reliable. They must be supplemented however with actual laboratory abrasion measurements. The choice of a laboratory abrasion test that is strongly correlated with service performance is mandatory. Such a laboratory test must be capable of reliably evaluating all rubbers and all types of reinforcing materials without serious ambiguity. Sources of such ambiguity are abrasive contamination, unrealistic severity effects and an altered dependency of abrasion vs conventional physical properties when compared to tire test results. A laboratory abrasion tester, the Pico abrasion machine, is shown to do a much better job at reliably predicting service preformance for a variety of rubber compositions than the other presently used abrasion machines. The earliest serious work in attempting to deduce the mechanism of wear was by Wiegand who decided that discrete particles were removed. Subsequent work by Sohallamach and others using radiotracers agree in principle with the basic concept of Wiegand. Schallamach has demonstrated that during severe wear a wear pattern or set of ridges at right angles to the direction of abrasion develops. From a study of the motion of this system of ridges he has deduced that rubber is removed by a tearing action or some other rupture process from the under side of the ridges. This is accomplished through a bending back of the forward pointing ridges during contact with the abrading surface. Stiehler on the other hand holds that wear occurs on a “molecular” scale due to localized degradation of the surface layer of the rubber with subsequent volatilization of the degraded fragments. The arguments for discrete particle removal seem to be supported by sufficient evidence to discount the “molecular” removal theory. It must be acknowledged however that under mild abrasion conditions the particles removed are microscopic in size. Zapp has shown in a qualitative treatment that, everything else being equal, the lower the dynamic modulus of a rubber formulation the greater the resistance to abrasion. Schallamach has advanced farther than anyone in explaining what factors or mechanical properties govern the resistance to abrasion in his study of the wear of slipping wheels. The wheels which he refers to are solid rubber wheels of small diameter used to carry a load in much the same manner as pneumatic tires. From the intensive study of solid wheels he has shown the following. At constant angle of slip the abrasion of a slipping wheel is proportional to the resilience of the rubber and is also proportional to the stiffness of the wheel. The term stiffness may be thought of as synonymous with dynamic hardness. When, however, abrasion is carried out at constant side force then the abrasion is again directly proportional to the resilience but is inversely proportional to the stiffness. if the composite qualitative picture as outlined above is correct the tearing strength or tearing energy should play an important role. A brief review of the only really quantitative and theoretical work of the tearing of rubber is given. This work has been carried out by Greensmith, Thomas and Mullins at the NRPRA. It can be concluded from their work that no clear cut evidence is on hand that shows that the tear strength of known abrasion resistant formulations is materially greater than for very poor wearing compounds. While there is some evidence indicating that formulations reinforced with HAF do possess higher tear strengths than corresponding gum vulcanizates under some conditions these conditions are not the ones that are present during abrasion. Unfortunately the tear strengths of vulcanizates at high rates of strain have not been determined due to the extreme experimental difficulties involved. Such measurements are needed to determine what role tear strength really plays in the overall abrasion process.