Earlier studies suggested that the adhesion of rubber to a substrate is increased by slight surface roughening. Rolling resistance experiments confirm this effect for very soft vulcanisates and unvulcanised rubber. Elastic analyses are shown not to predict the increased adhesion. The important factors suggested as giving rise to the increase for vulcanisates are stress relaxation during the contact period and loss at final detachment of the elastic energy built up in 'islands' of contact during separation. For the unvulcanised rubbers flow is shown to be the major factor. It leads to a high degree of intimate contact with rough surfaces, and also to energy dissipation around regions of locally higher adhesion where yielding occurs during peeling.
The influence of the stressistrain curve shape on the cracking of a brittle elastic material is analysed in terms of the energy balance theory of fracture mechanics. Both r-shaped and J-shaped elastic curves are investigated. For some cracking tests, for example the trouser tear and lubricated cutting tests, the shape of the stressistrain curve is found to have little effect on the crack extension force; this is determined mainly by the fracture surface energy of the material. However, in other tests such as tension, the shape of the stressistrain curve does influence the fracture stress, but not by a large amount when compared with the effect of reinforcement. Whether an r-shaped or J-shaped curve leads to a higher fracture stress depends on such factors as the fracture energy and the size of the preexisting crack. It thus appears that no general statement can be made about the effect of the shape of the stressistrain curve upon the crack resistance of materials, contrary to views expressed previously in the literature.
In cold climates the correct performance of rubber components such as seismic isolators depends on them maintaining their elastic properties when exposed to prolonged periods at low temperatures. The high damping compounds developed for seismic isolation are normally especially prone to crystallization when exposed to subzero temperatures for periods of a few weeks. The effect of low-temperature crystallization on the mechanical stiffening of natural rubber is evaluated. The relationship between the shear modulus and amount of crystallization is measured using a technique in which the dimensional change and stiffness are monitored simultaneously. The relationship is found to be approximately independent of the crosslink density and the temperature of crystallization. It appears not to be realistically modeled by considering the crystals to behave as rigid filler particles but good qualitative agreement with experiment was obtained by modeling the crystals as a network of threads. Partially crystalline rubbers are found to yield under the application of a large stress like other partially crystalline polymers. Mechanisms for suppressing crystallization in rubber are discussed and the low-temperature stiffening of specially formulated rubber compounds for seismic isolation is presented. These results show that carefully formulated high damping natural rubber compounds can give adequate performance at low temperatures.
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