Diffusion of curing agent through the thickness of a bi-layer EPDM system during the cure

Two EPDM rubber layers with different level of curing agent (2 and 10%) pressed and cured together exhibit interesting properties when immersed in toluene. In spite of the large swelling taking place, different for each of the two layers, leading to a strong flexural force, the two components of the bi-layer remain bonded. Simultaneously, a hardness gradient has been measured through the thickness of the final material. This hardness gradient is assumed to result from the diffusion of the curing agent through the thickness of the bi-layer system during the cure process. Deeper insight into the phenomenon is obtained in making calculations by taking into account these facts, either on the process of cure of the bi-layer system or on the process of absorption of liquid leading to large assymmetric swelling. Application of this phenomenon could be made in re-treading old tyres with a roller band made of a new rubber.