Crack growth rubber abrasion

The question now arises which fracture processes, if any, are strongly affected by the local chemical structure Two examples are considered below tearing and crack growth, and abrasive wear. Under certain conditions these failure processes are found to depend upon particular features of the elastomer molecule and they are therefore distinctly different, even for closely-related chemical structures. Natural rubber can usefully be compared with cis 1, 4-polybutadiene in this respect, because, although their chemical structures are superficially similar, large differences are observed in their resistance to tearing and in the mechanism of wear. 

Champ, Southern and Thomas recently applied the tearing criterion to the study of rubber abrasion. Their results were expressed as the crack growth per cycle r as a function of maximum T, tearing energy value, attained during each cycle (approximately 10 < T < 10 erg/cm )... 

A novel theory has been developed which directly relates the abrasion of rubber by a knife edge to its crack-growth characteristics. The approach is based on fracture mechanics which has been successfully used to interpret other failure processes in rubbers. The theory treats the removal of the rubber when a steady state has been reached in the development of the abrasion pattern in the rubber surface. It is suggested that crack growth occurs into the rubber from stress concentrations in the abrasion pattern. An important point is that once a steady state is established the detailed way in which the particles of rubber are finally detached is unimportant. The crack growth behaviour of the rubber is measured independently using established techniques. 

Although a considerable amount of work has been done on the mechanism of rubber abrasion , it is still not clear what the fundamental process is. It has been proposed that in some cases the rubber is removed by a tensile rupture process, and in other cases by a fatigue mechanism. However, no quantitative theory has been developed to relate the rate of abrasion to independently measured fundamental strength properties. In this paper we propose a theory which relates the abrasion produced in a particularly simple abrasion process, the passing of a blade over the rubber surface, to the crack growth behaviour of the material. The approach used is that which has been referred to as fracture mechanics. It has been successfully applied to the tear > °, crack growth > and fatigue behaviour of rubbers. 

Fig. 5b. Comparison of abrasion and crack growth data. Isomerized natural rubber o abrasion crack growth...

The addition of a rigid polymer to a soft matrix results in an increase in the modulus of elasticity of the total polyblend and is known as rubber reinforcement. It also raises both tensile strength and tear strength, and reduces abrasion, cut growth, and flex cracking. 

Blends of halobutyl or brominated poly(isobutylene-co-p-methylstyrene) with high diene rubbers are used in tire sidewalls and tread compoimds (137-139). In sidewalls, ozone resistance, crack cut growth, and appearance are critical to their performance. Properly formulated blends with high diene rubbers that exhibit phase cocontinuity yield excellent sidewalls (140). The property balance for tire tread compounds can be enhanced by the incorporation of a more damping halobutyl or brominated poly(isobutylene-co-p-methylstyrene) rubber phase (141). Improvements in wet-, snow-, and ice-skid resistances and in dry traction without compromises in abrasion resistance and rolling resistance for high performance tires can be accomplished by using bromobutyl or brominated poly(isobutylene-co-p-methylstyrene) up to 30 pbr in tread compoimds (142). 

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