EPDM-carbon black interactions and the reinforcement mechanisms, as studied by low-resolution H-1 NMR

Proton, low-resolution T-2 NMR relaxation experiments are used to study the adsorption of EPDM to the surface of carbon blacks and the structure of th e 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 surfac e after mixing. The estimated thickness of the interfacial EPDM is in the r ange of one to two diameters of the monomer unit (approximate to 1 nm). At temperatures of up to 160 degrees C, the mobility of EPDM chain units in th e interface is strongly hindered and is comparable to that in an unfilled E PDM at temperatures 10-15 deg above T-g. It is suggested that the sites of the interaction between the carbon black and the EPDM cause physical adsorp tion network junctions in the rubber matrix. The average molar mass of EPDM chains between the adjacent adsorption junctions in bound EPDM rubber is a bout 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 comparabl e to the average distance between the adjacent carbon black aggregates. Thi s suggests that the carbon black aggregates are interconnected by EPDM brid ging chains, and a continuous EPDM/carbon black physical network is formed in the bound rubber fraction of the compound. The results obtained for uncu red filled rubbers provide strong evidence of a "bimodal" structure of the physical network. The two types of EPDM chains and/or chain fragments, whic h have strongly different densities of EPDM-carbon black adsorption junctio ns, are present in the elastomer matrix outside of the EPDM-carbon black in terface. There is an EPDM fraction that is loosely bound to the carbon blac k by adsorption interactions. This loosely bound rubber has numerous adsorp tion network junctions, similar to those of the bound rubber. The other fra ction of EPDM, the extractable (unbound) rubber, has a relatively low numbe r of adsorption network junctions and can apparently be extracted from the compound. The fraction of loosely bound EPDM chains, as measured by NMR, in creases as the maximum possible EPDM-carbon black contact area per unit vol ume of the elastomer increases, regardless of the type of carbon black, and is relatively close to the content of the bound rubber. Results of mechani cal property tests on the carbon black filled vulcanisates reveal that seve ral 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 int eractions provide a significant contribution to the modulus in the low-stra in region, while they are broken at high strains.