Time dependent characteristics of sulfonated EPDM containing zinc stearate I. thermal behavior
The crystallization and melting of zinc stearate (ZnSt) in ZnSt-plasticized compositions of ethylene–propylene–diene terpolymer (EPDM) and sulfonated EPDM (S-EPDM) have been studied by differential scanning calorimetry. While immiscible in both EPDM and S-EPDM, ZnSt is compatible with the latter. This compatibility presumably arises from either electrostatic or dipole–dipole interactions between the carboxylate functionality of ZnSt and the sulfonate groups of the polymer. The crystallization behavior of ZnSt is significantly perturbed when the material is added to S-EPDM, and the crystalline ZnSt phase that results is strongly time dependent. ZnSt first crystallizes as small imperfect crystals when the compositions are cooled from the melt, but it eventually anneals at room temperature into larger, more perfect crystals. Time dependent changes in the mechanical properties of these compositions are also demonstrated and it is suggested that this may be due in part to changes in the ZnSt morphology.