Effects of coupling agents on dispersion and structure in polypropylene/clay nanocomposites

This work focuses on improving the interactions of layered silicates with bulk polypropylene to optimize the nanolayer exfoliation. Silane coupling agents were selected based on their affinity with the bulk polypropylene: this was obtained by solubility parameter calculations. Maleated polypropylenes with different extents of bound maleic anhydride were also evaluated for their effectiveness in nanolayer exfoliation.;To study the interactions with silane coupling agents, the clay pretreated with a C18-surfactant was treated additionally with phenyl-trimethoxy silane. 5 wt% of this clay was melt blended with 10 wt% maleated polypropylene PP-g-MA and with 85 wt% of two different polypropylene grades having widely different molecular weights. The resulting nanocomposites were analyzed for microstructure and mechanical performance using X-ray diffraction (XRD), transmission electron microscopy (TEM), melt rheology and tensile modulus measurements. Nanocomposites with the most exfoliated structure were obtained when the chosen silane was located at the edges of the organoclay. This was confirmed with higher (almost twice) relative viscosity of the composites with silane modification at the organoclay edges as compared to composites with organoclay. The analysis of several TEM images also supports the rheology data. The improvements in tensile modulus were greatest for the composites with this clay, indicative of improved coupling. The effects of shear and extensional flows on nanolayer morphology were investigated by extrusion through uniform bore and converging dies. Nanolayer restacking was observed; the levels for which were reduced for the nanocomposites reinforced with edge silated clay particularly at higher residence times in shear flows, as quantified by image analysis of the nanocomposite extrudates. Our results also show that compatibilizers having greater amounts of covalently bound maleic anhydride yield the most exfoliated structure.