Anisotropic Swelling of EPDM Rubber Discs

From the experimental values, there are two kinetics for the swelling, the one expressed in terms of the change in the thickness, and the other in terms of the change in the radius. 

The kinetics of these changes in dimensions can be calculated by using the same parameters of diffusion shown in Table 7.4, except for the increments of thickness and radius Ax and Ar shown in Table 7.5 to take into account the anisotropy of the rubber disc to swelling. Rather good agreement is thus obtained between the calculated and experimental kinetics, by considering that accuracy is not so favorably attained for these measurements of dimensions that those made on the weight. 

FIGURE 7.8 Kinetics of the increase in dimensions of EPDM rnbber discs with 2% peroxide. Experiments (dots) calcnlated (fnll line). (With permission of Elsevier, Eignre 4 in Polymer, 2002, 43, p. 4261, K. Azaar, I. D. Rosea, and J. M. Vergnand.) 

The anisotropy of these rubber discs, the swelling being larger in the thickness than in the radius, whatever the percent peroxide used for the cure, may be explained by an orientation of the macromolecules along the flat parallel faces of the discs, which is obtained during the cure when the rubber is pressed into the slabs of the mold. 

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