Cellular Morphology Optimization of Polypropylene/CaCO ₃ Composite Films Designed for Piezoelectric Applications

In this work, the cellular morphology of polypropylene (PP)/calcium carbonate (CaCO ₃ ) composite films was optimized with respect to piezoelectric cellular films criteria. To do this, a series of PP films filled with CaCO ₃ 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 CaCO ₃ 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.