Rheological behavior of well-dispersed polypropylene/halloysite nanotube composites prepared by water-assisted mixing extrusion

Abstract

Composites of polypropylene (PP) with 5 wt% halloysite nanotubes (HNTs) were prepared by conventional extrusion and water-assisted mixing extrusion, respectively. Transmission electron microscopy showed reduced aggregation of the HNTs in the composites prepared by injection of water during extrusion, with more efficient dispersion of the HNTs. The shear rheological behavior was systematically investigated to understand the effect of the HNT dispersion on the structure of nanocomposites prepared with water injection. The application of small amplitude oscillation shear at different temperatures led to an obvious increase in the storage modulus and complex viscosity of the composites prepared with water injection; and the viscoelastic properties were not affected by temperature. In using the modified Krieger–Dougherty model, the enhanced shear viscosity was correlated with the absorption of more polymer layers on the surface of the HNTs. Furthermore, the results of start-up flow tests showed that the PP/HNT nanocomposites prepared with water injection exhibited larger overshoots at various applied shear rates. Finally, the reversal flow responses of the nanocomposites were interpreted in terms of evolution of the microstructure during the rest period through a dynamic model. In this study, two competitive kinetic constants, namely, the build-up and breakdown coefficients in the dynamic model, were considered to be related to the interaction of PP-HNTs. Thus, it could be deduced from the model that due to the increased build-up coefficient of the composites with water injection, the structural parameter was more rapidly recovered to its initial state.