Preparation of Polypropylene Foams with Bimodal Cell Structure Using a Microporous Molecular Sieve as a Nucleating Agent

A novel approach for fabricating polypropylene (PP) foam with bimodal cellular structure utilizing CO2 as a physical foaming agent was established by adding porous molecular sieve (PMS) as a nucleating agent in a batch foaming process. The mechanism for the formation of a bimodal cellular structure was studied. It was found that the PMS and the PP crystal nucleus acted as heterogeneous nucleation sites, among which the nucleation from PMS with the adsorption property of the foaming agent could induce the formation of large cells in the particle-rich regions, and the nucleation from the PP crystal was developed into small cells in the PP matrix The effects of foaming temperature and foaming pressure on the cellular structure of PP foams were investigated. By controlling the foaming parameters, we obtained the bimodal foams with the average diameters of 16-33 and 3.1-7.0 mu m for large cells and small cells, respectively, and foam materials with various densities (0.054-0.22 g/cm(3)) were obtained. The compression properties of foam materials with bimodal and unimodal cellular structures were compared, and PP foam with excellent compressive properties was prepared.