Elegant Way of Strengthening Polymer-Polymer Interface Using Nanoclay

A unique mechanism responsible for enhancing the autohesive tack strength of ethylene propylene diene rubber (EPDM) was elucidated by studying the interfacial strength of an unvulcanized EPDM elastomer joint in the presence of nanoclay. The tack strength significantly increased with nanoclay concentration up to 4 parts per 100 grams of rubber (phr), beyond which it dropped. For example, the tack strength of 4 phr of the nanoclay-loaded sample was nearly 137% higher than that of neat EPDM rubber. The influence of nanoclay in the bond formation and the bond separation steps of the tack test was understood by analyzing various tack governing factors such as green strength, creep compliance, entanglement molecular weight, relaxation time, the self-diffusion coefficient, and the monomer friction coefficient (zeta(0)). Furthermore, the ability of EPDM rubber to undergo strain-induced crystallization (SIC) during straining (at the time of bond separation in the peel test experiment) in the presence of nanoclay was also investigated. When the clay concentration was 4 phr, there was a slight reduction in the extent of molecular diffusion at the tack junction due to the nanoclay reinforcement; however, the diffusion was sufficient enough to establish entanglements across the interface, Furthermore, the less diffused chains of the nanocomposite samples showed greater bond breaking resistance than the unfilled sample due to the higher zeta(0) value owing to the nanoclay reinforcement. It was also observed that the presence of nanoclay reduced the amount of crystallinity in the unstrained state and hence favored diffusion of elastomer chains across the interface. In addition, the presence of nanoclay significantly increased the ability of the EPDM elastomer to crystallize due to the alignment of nanoclay during straining, thus providing greater bond breaking resistance to the diffused elastomer chains. At higher clay loading (>4 phr), the elastomer chains could not establish entanglements across the interface due to extremely slow diffusion and aggregated platelets on the rubber surface, and therefore the tack strength decreased.