The Effect of Variations in Chain Connectivity on Strain-Induced Crystallization in Unvulcanized EPDM Elastomers

Abstract

Abstract The introduction of a small amount of chemical or physical crosslinks has an enhancing effect on SIC in EPDM. This situation results from a decrease in stress relaxation or creep, so that the individual chains are deformed more affinely. At the same time, there is no deleterious effect of crosslinks on the crystallization of nonstretched EPDM because of its copolymeric nature. Therefore, the higher connected chains have a smaller melting entropy upon stretching, which promotes the crystallization process. In both EPDM green rubbers and black compounds, the amount of crystallinity and the end-melting temperature increase considerably with strain, in contrast to literature reports. These are desirable qualities for rubber tack. In spite of the positive effect of a higher chain connectivity on SIC, chain connectivity has a detrimental influence on tack. This effect is the result of a drastic reduction in bond formation caused by diminished material flowability and chain interdiffusion. Hence, we conclude that a tack-grade EPDM should, preferably, contain no branched or lightly crosslinked structures.