Polypropylene/Organoclay Nanocomposites Prepared by Supercritical CO2 Assisted Extrusion Process

Abstract

Polypropylene (PP) nanocomposites were prepared by melt intercalation in an intermeshing co-rotating twin-screw extruder with the assistance of supercritical CO2 injection. The effect of molecular weight of PP-MA (maleic anhydride modified polypropylene) on clay dispersion and mechanical properties of nanocomposites were investigated. After injection molding, the tensile properties and impact strength were measured. The best overall mechanical properties were found for composites containing PP-MA with the highest molecular weight. The basal spacing of clay inthe composites was measured by X-ray diffraction(XRD). Nano-scale morphology of samples was observed by transmission electron microscopy (TEM). The crystallization kinetics was measured by differential scanning calorimetry (DSC) and by optical microscopy at a fixed crystallization temperature. For well dispersed two-component system, PP- MA330k/clay, the crystallization kinetics and the spherulite size remained almost unchanged and the impact strength decreased with increasing the clay content. On the other hand, the intercalated three component system, PP/PP-MA330k/clay, containing some dispersed clay as well as the clay tactoids, showed much smaller size of spherulites and a slight increase in impact strength with increasing the clay content. The influence of supercritical CO2 on mixing was evaluated together with the effect of initial melting temperature. Increasing initial melting temperature causes gradual decrease in bulk cristallization kinetics with exception of the 240-260°C temperature range for system without CO2. Optical microscopy revealed large number of small spherulites for system without CO2 after initial melting at 250°C. After 28 min of initial induction period of crystallization many small spherulites appeared in the vicinity of large spherulite for the system with CO2 indicating beginning of homogenous nucleation.