Cellular Morphology Optimization of Polypropylene/CaCO3 Composite Films Designed for Piezoelectric Applications

In this work, the cellular morphology of polypropylene (PP)/calcium carbonate (CaCO3) composite films was optimized with respect to piezoelectric cellular films criteria. To do this, a series of PP films filled with CaCO3 micro-particles of three different particle sizes (3, 6 and 12 microns) were developed at various weight concentrations (3 to 44 wt. %). Before going through a gas diffusion expansion (GDE) step to inflate the initiated cells, all composite films were first produced via extrusion/calendaring then biaxially stretched to initiate the cellular structure by interfacial delamination between the CaCO3 particles and PP matrix. After biaxial stretching and GDE, it was observed that only films with filler contents above 23 wt.% and 12 μm particle size presented a well-developed cellular structure. By optimizing the extrusion, biaxial stretching and GDE steps, we were able to generate a required cellular morphology where the average cell height ranged between 5 and 8 μm, which is considered good for further Corona charging. Also, the average cell aspect ratio (cell length divided by cell height) ranged between 4 and 10 with an average cell wall thickness between 7 and 12 μm, which is also considered as optimum for good piezoelectric properties.