Effect of Multiple Extrusions on the Impact Properties of Polypropylene/Clay Nanocomposites

Polypropylene (PP)-based polymer nanocomposites
containing organically modified montmorillonite
(OMMT) with and without maleic anhydride grafted PP,
were compounded by twin-screw extrusion. The extrusion
process was repeated various numbers of times to increase
the extruder residence time (TR) and, through that, the particle
dispersion. Rheological measurements fitted to a modified
Carreau–Yasuda model defining a melt yield stress
were used to indicate changes in the particle dispersion with
regard to TR. This analysis showed a monotonically
increased dispersion of clay particles in the PP matrix with
increasing extruder TR. The small-strain tensile properties
were tested at both ambient (20C) and elevated (90C) temperatures,
and no significant changes were observed in the
tensile strength or modulus as a function of TR. Instrumented
Izod impact tests showed that the nanocomposite
impact strength (ri) increased monotonically with increased
TR by 70% from least dispersed to best dispersed, which was
still 20% below the level for neat PP. Both the fracture initiation
energy and propagation energy increased with TR, but
the primary effect on ri came from the fracture propagation
energy, which delivered 80% of the improvement.