Diffusion of the Curing Agent through two Epdm Rubber Sheets with Different% Peroxide during their Simultaneous Cure

Abstract

When two sheets of EPDM with different percent peroxide as curing agent, 2% for the one and 10% for the other, are firmly pressed and cured together at a convenient temperature, a transfer of the curing agent has been proved to take place through the interface between these two sheets. The following two facts are observed. On the one hand, when this cured bi-layer system is immersed in toluene, a large swelling is observed but the two layers are not separated, in spite of the different rate and extent of swelling for each EPDM taken separately. On the other hand, the hardness does not vary discontinuously at the interface between the two sheets, but on the contrary it varies continuously through the thickness of the two layer system. This variation of the hardness through the thickness gives an idea of the gradient of peroxide concentration which has been developed during the stage of heating before the onset of cure. The kinetics of absorption of toluene is controlled by diffusion, and the two parameters, with the diffusivity and the capacity of absorption at equilibrium, are evaluated for the two-layer system by using the values measured on the two EPDM components with different percentages of peroxide taken separately. Modelling of the process of absorption and evaporation has been possible from these values for the bi-layer system, explaining the torsion of the bi-layer sheet during the stage of absorption and its stability. An application of this matter transfer during the cure process opens a way for re-treading a truck tyre with a new rubber band.