The effect of strain-hardening on the morphology and mechanical and dielectric properties of multi-layered PP foam/PP film

Polypropylene (PP) foams are important in various industrial applications due to their unique mechanical and electrical properties. However, fabrication of microcellular PP foams is still limited because of polymer's rheological properties. In this work, the influence of rheological properties and the layer confinement effect on the microcellular morphology of foam/film structure and the mechanical and dielectric properties were investigated. Two types of long chain branched polypropylenes (BPPs) were used and named as BPP1 and BPP2, respectively. Talc micro-particles were used as a nucleating agent for crystallization. In uniaxial extensional flow, both BPPs exhibited strong strain-hardening behavior with the Trouton ratios well above 3 at all extensional rates, regardless of the talc content; however, BPP2 exhibited more strain-hardening than BPP1. 16 and 32 layered foam/film structures were then fabricated with BPP2 and 32 layered foam/film was fabricated with BPP1. The lower strain-hardening of BPP1 caused cell collapse and showed very little evidence of film layers separating the foam layers, while the stronger strain-hardening of BPP2 resulted in stable cell layers near the surface of the sheet and a non-coalesced cell structure. Mechanical properties in compression showed large differences in compressive strain due to the difference in cell structures. Dielectric property measurement on biaxially oriented 32-layer BPP2 foam/film, the one that showed the best cell structure and mechanical properties, showed much higher apparent dielectric constant (6-7) and higher release and stored energy densities than those of neat biaxially oriented polypropylene films. (C) 2015 Elsevier Ltd. All rights reserved.