Heat Build-Up and Blowout of Rubber Blocks

The phenomenon of explosive “blowout” of thick rubber blocks, under repeatedly applied, severe compressive loads, is due entirely to the development of high internal temperatures. If the compound is electrically conductive, the phenomenon can be duplicated in a microwave oven without imposing any mechanical loads. Blowout appears to consist of the expansion to burst of pressurized cavities within the rubber. Pressure appears to be generated internally by a volatile constituent or decomposition product of the rubber compound. Expansion is restrained by elastic stresses set up in the rubber as the cavity expands. Bursting is made easier in some compounds because they soften markedly at high temperatures and thus lose resistance to cavity expansion. Different elastomers have strikingly different blowout temperatures. Butyl rubber compounds blow out at relatively low temperatures, about 180°C, whereas NR and SBR compounds blow out at temperatures of about 200°C or higher. Also, different vulcanizate structures have different blowout temperatures. For example, materials with C-C or monosulfidic crosslinks show higher blowout temperatures than those with polysulfidic crosslinks. Polybutadiene compounds did not blow out at all. Instead, they developed internal cavities that grew in size and number but never burst open to the exterior, probably because the internal pressure never reached sufficiently high levels. It is concluded that the principal cause of blowout is the generation at high temperatures of sufficient quantities of a volatile decomposition product, but the reaction is specific to the particular elastomer and crosslinking system employed. It does not appear to consist of simple thermal decomposition of the hydrocarbon elastomer, which would require higher temperatures than those observed at blowout.