EPDM−Carbon Black Interactions and the Reinforcement Mechanisms, As Studied by Low-Resolution1H NMR

Abstract

Proton, low-resolution T2 NMR relaxation experiments are used to study the adsorption of EPDM to the surface of carbon blacks and the structure of the physical EPDM/carbon black network in cured and uncured compounds. It is shown that a layer of immobilized EPDM is formed at the carbon black surface after mixing. The estimated thickness of the interfacial EPDM is in the range of one to two diameters of the monomer unit (≈1 nm). At temperatures of up to 160 °C, the mobility of EPDM chain units in the interface is strongly hindered and is comparable to that in an unfilled EPDM at temperatures 10−15 deg above Tg. It is suggested that the sites of the interaction between the carbon black and the EPDM cause physical adsorption network junctions in the rubber matrix. The average molar mass of EPDM chains between the adjacent adsorption junctions in bound EPDM rubber is about 1800−2500 g/mol and depends on the content and the type of the filler. The mean end-to-end distance between the adsorption junctions is comparable to the average distance between the adjacent carbon black aggregates. This suggests that the carbon black aggregates are interconnected by EPDM bridging chains, and a continuous EPDM/carbon black physical network is formed in the bound rubber fraction of the compound. The results obtained for uncured filled rubbers provide strong evidence of a “bimodal” structure of the physical network. The two types of EPDM chains and/or chain fragments, which have strongly different densities of EPDM−carbon black adsorption junctions, are present in the elastomer matrix outside of the EPDM−carbon black interface. There is an EPDM fraction that is loosely bound to the carbon black by adsorption interactions. This loosely bound rubber has numerous adsorption network junctions, similar to those of the bound rubber. The other fraction of EPDM, the extractable (unbound) rubber, has a relatively low number of adsorption network junctions and can apparently be extracted from the compound. The fraction of loosely bound EPDM chains, as measured by NMR, increases as the maximum possible EPDM−carbon black contact area per unit volume of the elastomer increases, regardless of the type of carbon black, and is relatively close to the content of the bound rubber. Results of mechanical property tests on the carbon black filled vulcanisates reveal that several factors contribute to the reinforcing effect of the filler. Besides the hydrodynamic effects, the occluded EPDM, the chemical cross-links, and the formation of the physical EPDM−carbon black network, the filler−filler interactions provide a significant contribution to the modulus in the low-strain region, while they are broken at high strains.