Rubbers tearing

The international standard method given in ISO 8147 uses a disc test piece 3 0.1 mm thick and between 35 and 40 mm in diameter, bonded to metal plates which are approximately 0.1 mm less in diameter than the rubber. The slightly smaller size of the metals is intended to prevent the rubber tearing from the edges of the metals during test. 

The observed M dependence of arrest glassy fracture energies is curious because this is the same dependence predicted by Lake and Thomas for the threshold tearing of elastomers. It was previously shown that this dependence is exhibited by the rubber tear of the DGEBA/DDS epoxies over a wide range of temperatures, even though they were far removed from the threshold region. 

Figure 8.23. Silicone rubber tear strength vs. surface area of silica. [Data from Okel T A, Waddell W H, Rubb. Chem. Teck ol.,6 , iioA, 1995, 59-76.]...

This basic joint configuration can cope very adequately with a wide variety of adherend types and adhesives. While the major limitation is poor peel and cleavage resistance, it performs well with appropriate materials - for example, flexible, compliant rubber bonded to a second, stiffer adherend. The stress induced at the joint s edge by distortion will be dissipated over a greater area than usual by the bending of the rubber and the more closely matching cohesive strengths of the adhesive and the rubbery material - compared with those of an adhesive and metal - could well mean that the joint will ultimately fail only because the rubber tears at extreme load. 

M = adhesion tear (adhesion failure) R = rubber tear (cohesion failure) RM = combines R M ... 

Particularly prone areas of bond stress in the early part of the components existence are concentrated by protrusions of metal components, such as bolt heads, into the rubber mass. An extensive load created in the area of such protrusions concentrates in their vicinity and localised bond failure can occur. This may not be evident to the observer but cavitation may have been created which can expand under stressing of the product in service, with resultant complete failure through rubber tear. 

Most of the experimental points for the upper regions of the curves represented excellent adhesion with rubber tearing bonds. 

To make the flaw grow, say by 1 mm, we have to tear the rubber to create 1 mm of new crack surface, and this consumes energy the tear energy of the rubber per unit area X the area of surface torn. If the work done by the gas pressure inside the balloon, plus the release of elastic energy from the membrane itself, is less than this energy the tearing simply cannot take place - it would infringe the laws of thermodynamics. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

This lower has a number of ramifications on the properties of polybutadiene. For example, at room temperature polybutadiene compounds generally have a higher resilience than similar natural rubber compounds. In turn this means that the polybutadiene rubbers have a lower heat build-up and this is important in tyre applications. On the other hand, these rubbers have poor tear resistance, poor tack and poor tensile strength. For this reason, the polybutadiene rubbers are seldom used on their own but more commonly in conjunction with other materials. For example, they are blended with natural rubber in the manufacture of truck tyres and, widely, with SBR in the manufacture of passenger car tyres. The rubbers are also widely used in the manufacture of high-impact polystyrene. 

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