Anisotropic swelling of thin EPDM rubber discs by absorption of toluene

Anisotropic swelling of thin EPDM discs is observed when they are in contact with toluene resulting from the liquid absorption. The process is controlled by transient diffusion, with a concentration-dependent diffusivity and a finite coefficient of convective transfer at the liquid–rubber interface. Moreover a rather high swelling takes place during the stage of absorption, leading to an increase in dimensions of the flat discs. The increase in the thickness of the discs is around 1.17–1.18 times larger than that in the radius, exhibiting an anisotropic behaviour. The flat discs have been cured by pressing them in the heated slabs of a press, leading to a kind of orientation of the molecules. The state of cure of around 96% has been determined either by calorimetry in scanning mode or by a moving die rheometer under isothermal conditions at three temperatures within the 170–190 °C range. Various percent peroxide as a vulcanising agent have been used, e.g. 2, 10, and 20%. While the diffusivity is about similar for the three samples, the extent of liquid absorbed decreases with the percent peroxide. A numerical model based on finite differences taking into account all these facts has been built in order to describe the process.