Graded expanded beads of polypropylene

Abstract

Polypropylene foams are very popular as high-performance and economical cellular materials. We explored the feasibility of producing non-uniform foam structures using commercial polypropylene to enhance its mechanical properties compared to the uniform structures representing the benchmark foams. The gas sorption stage has been designed with time-varying boundary conditions to induce a graded concentration profile that results, after foaming, in non-uniform density structures. We report the design and results of the foaming process in terms of pressure and temperature history, cell structure, density and mechanical properties of the produced foams. The experimental campaign conducted on a random co-polypropylene, blown with CO₂, confirms the possibility of achieving non-uniform polypropylene foams with higher density in the bead periphery and tunable average density (from 80 to 150 kg/m3). Finally, an unconventional foamed bead molding process has been explored to create sintered parts for mechanical characterization. Interestingly, the elastic modulus and the compression strength at 25% of deformation for the graded foams are ca. 10% higher than the uniform foams ceteris paribus. The explored technology is a simple and economical one-step method in autoclaves that can be scaled up industrially.