The Internal Heating of Ebonite during Vulcanization

Abstract It is known that the reaction between rubber and sulfur to form ebonite is strongly exothermic, and under certain conditions may cause a marked internal rise of temperature during vulcanization. This heating is sufficient in many cases to accelerate the rate of vulcanization of the center of the mass to an important extent, so that the degree of vulcanization of the center differs from that of the surface, the product thus being non-uniform in physical properties. In more severe cases the temperature may rise sufficiently to cause decomposition, evolution of gas and porosity, or even violent explosion. This internal heating is favored by a high temperature of vulcanization, by great thickness of the mass to be vulcanized, by the addition of accelerators of vulcanization, and by the omission from the mixing of inert compounding ingredients, which act as diluents. It is, perhaps, not so commonly known that the heating is greater in an open steam cure, owing to the lower rate of dissipation of heat from the surface, though this factor may have been one of those leading to the common adoption of the water immersion method of cure. In technical practice the inconvenience of this phenomenon is avoided as far as possible by a control of the factors mentioned, including such devices as the inclusion of semi-vulcanized ebonite dust or crumb, building-up from semi-vulcanized sheets, “stepped” or rising cures, or, where possible, by designing the object so as to facilitate conduction of heat from the center by means of recesses or metal inserts.