Gelation Phenomenon Observed in EPDM Compound with Carbon Black

The phenomena of gelation observed in EPDM compounds with carbon black are known as being troublesome during the processing of rubber. In addition, it is mysterious that the gelation disappears after processing and suddenly appears again. We proposed a new index related to the phenomenon of gelation observed in EPDM compounds. That is, apparent free energy of activation (Ea) derived from reological tests is introduced as the index. Ea is calculated from the Arrhenius equation of shift factor, aT, which is obtained from the temperature-dependence of complex viscosities. Two types of EPDM compounds were prepared; one whose gelation increased with increased mixing time, and another, which did not show the gelation owing to the addition of bis-3-(triethoxysilylpropyl)tetrasulfane (TESPT). Ean and Ea0, the free energy of activation for both compounds, respectively, were obtained. The degree of gelation is defined as 100 (Ea0-Ean)/Ea0. Taking into account the degree of gelation defined above, rubber extrusion and rubber properties were studied. The dieswell decreased with the increase in mixing time, and the elongation of vulcanized EPDM also had the same tendency. Changes in these phenomena were proportional to the degree of the gelation index. Interestingly, a middle-T2 value measured by pulsed NMR decreased with the decrease in Ean. As a conclusion, the Ea value may be an effective index to determine the phenomena of gelation observed in EPDM compounds with carbon black.