Exfoliation of Nanolayer Assemblies for Improved Natural Rubber Properties: Methods and Theory

Natural rubber (NR) based nanocomposites were prepared by conventional processing techniques using various nanoclays like unmodified clays (hectorite and montmorillonite, NA) and modified montmorillonite. In the case of modified montmorillonite (15A) at 4phr loading, the 300% modulus, tensile and tear strength exhibited increments of 77%, 20% and 18% over the gum, respectively. This was due to the attainment of a state of improved dispersion, resulting from a balance of exfoliated-intercalated morphology. The unmodified clay failed to affect the above parameters significantly.  Property-morphology correlation of the hybrids was performed through dynamic mechanical and swelling studies in conjugation with X-ray diffraction (XRD). The obvious limitation of unmodified montmorillonite based NR nanocomposites prepared by the conventional method was alleviated by using latex blending technique. Pre-exfoliated aqueous suspension of NA was blended with NR latex under special conditions and the mixture was subsequently cured by pre-vulcanization as well as conventional curing. These as-prepared nanocomposites generated predominantly exfoliated morphology as revealed by the XRD and transmission electron microscopy studies, along with substantially improved physical and mechanical properties. For example, 4phr of NA loaded prevulcanized nanocomposites registered 25% and 138% increase in tensile strength and modulus, respectively, compared to the gum vulcanizate. These nanocomposites exhibited higher glass transition temperature and storage moduli over their unfilled counterparts. A simple theory based on effective surface area factor was proposed to account for the structure-property correlations.