Understanding the reinforcing behavior of expanded clay particles in natural rubber compounds

We report the unusual mechanical percolation behavior of expanded clay nanoparticles in a natural rubber (NR) matrix. This phenomenon is discussed in terms of fractal dimensions of the nanoparticle cluster. Highly exfoliated structures of nanoparticles in NR are obtained by a process we call the 'propping-open approach'. The impact of filler dispersion and rubber-filler interactions on the viscoelastic behavior of NR-clay nanocomposites is systematically investigated. We observe non-linear viscoelastic behavior (Payne effect) at very low nanoparticle concentrations which we attribute to the formation of a network-like structure of the exfoliated clay particles. We rely on the Kraus and Maier-Göritz models to interpret such nonlinear viscoelastic behavior. We find that the chain mobility of the NR is greatly reduced based on the viscoelastic master curves. The value of the mechanical percolation threshold (φ_p) and the fractal nature of nanoparticle clusters are determined through an analysis of the experimental data based on a theory put forward by Huber and Vilgis. The nature of rubber-filler interactions is further understood from swelling experiments utilizing the Kraus and Cunneen-Russell equations.