Investigation and modelling of the water transport properties in unfilled EPDM elastomers

The water sorption properties (i.e. diffusivity and solubility) of three unfilled vulcanized EPDMs were investigated in wet atmosphere (typically between 0 and 95% RH) and in pure distilled water (denoted as 100% RH) at 70 °C with an IGAsorp dynamic sorption analyzer. In the 0–0.5 activity range, all EPDMs obey to the usual Fick and Henry's laws. In contrast, at activities higher than 0.5, the water sorption behavior of all EPDMs deviates from these two laws because of the formation of water clusters. Park's equation was used for simulating accurately the sorption isotherms and thus, accessing Henry's constant and the average size of water clusters. In wet atmospheres, clusters would be composed of about 8 ± 4 molecules corresponding approximately to the size of free volume holes. In contrast, in pure distilled water, clusters consist in a much larger number of water molecules (between 55 and 71) presumably because of an exacerbated swelling of EPDM samples. Thus, immersion in water solution is more drastic than exposure to fully saturated wet atmosphere. The impact of the EPDM formulation (nature and content of crosslinking agent) on these experimental results is clearly of the second order of magnitude