High Impact Toughness Polypropylene/CaCO3 Nanocomposites and the Toughening Mechanism

Considerable improvement in the impact toughness of an annealed polypropylene (PP) nanocomposite containing CaCO3 nanoparticles was achieved and its toughening mechanism was investigated. The nanocomposite was prepared by melt-blending PP and CaCO3 nanoparticles in a batch mixer. Injection-modeled impact and tensile bars were annealed at 150 °C for 2 h prior to the impact and tensile tests. The Young modulus and yield stress of the nanocomposite were found to increase after annealing. What is more, the annealed nanocomposite had much higher impact toughness than its unannealed counterpart. The average Izod impact strength of the 150 °C-annealed nanocomposite, containing 20 wt % (7.8 vol %) CaCO3 nanoparticles coated with 6 wt % stearic acid, was 168 J/m, which was 3.5 times higher than that of neat PP. Experiments, including X-ray diffraction, differential scanning calorimetry, scanning electron microscopy (SEM) and the instrumented falling-weight impact test, were performed to study the toughening mechanism of the annealed nanocomposite. SEM micrographs of the impact fracture surface and bulk morphology underneath the fracture surface of the broken Izod samples and the results of the instrumented falling-weight impact test suggested that the plastic deformation zone that formed in the crack-initiation stage was responsible for the high impact toughness of the annealed nanocomposite. The micromorphology ahead of the arrested crack tip of the annealed nanocomposite showed that the propagation of the crack occurred through shear-to-rupture of the ligaments at the crack opening. The size of the plastic deformation zone and the fracture energy were likely related to the increased matrix ligament strength after annealing. In summary, the annealing-reinforced ligaments were proposed to be responsible for the high impact toughness of the annealed nanocomposite.