Solid-State ¹³C NMR Study of Accelerated-Sulfur-Vulcanized ¹³C-Labeled ENB−EPDM

Abstract

Accelerated-sulfur-vulcanized 13 C-labeled EPDM with and without carbon black and extender oil was analyzed using 13 C solid-state NMR to determine the chemical structure of the network. High-resolution solid-state 13 C NMR reveals that sulfur cross-linking takes place at the allylic positions of the ENB independent of the presence of carbon black and oil. From the integrated intensities of the 13 C signals, the conversion of ENB into a cross-link can be quantitatively determined during the vulcanization process. The ENB conversion for gumstock EPDM is ∼10% after 10 min of vulcanization at 150 °C, which is a typical optimum vulcanization time in commercial applications. In the presence of carbon black the ENB conversion is marginally faster and reaches ∼12% in 10 min, while a maximum conversion of ∼20% was obtained. The efficiency of the ENB conversion was ∼20% less at 150 °C and ∼30% less at 180 °C in the compound with carbon black and oil compared to the compound without carbon black and oil. The substitution at the 9-position of ENB is always preferred over each of the two 3-positions. In turn, the substitution on the 3-exo position is always preferred over the 3-endo position, which is different from earlier model studies. When the material is heated for extended periods (10−20 min), oxidation and reversion of the cross-links starts to occur. Reversion is enhanced upon a temperature increase to 180 °C and yields a 4,5,6,7-tetrahydro-4,7-methanobenzo[ b ]thiophene compound. The length of the sulfur bridge in compound A and B is rather short, i.e., 1 or 2.