Preparation of short carbon fiber/polypropylene fine-celled foams in supercritical CO2

This article reports an attempt to improve polypropylene fine-celled foaming with the incorporation of short carbon fibers. The carbon fibers with uniform dispersion in polypropylene matrix were obtained in reference to the way of papermaking. The composites were prepared via melt compounding in a twin-screw extruder followed by injection molding. Three concentrations of carbon fibers, 5, 15 and 25 wt.%, were used. The foaming was carried out by a batch foaming process with supercritical CO2 as the physical blowing agent. The results showed the foamed composites had well-defined closed cell structure, but large amount of unfoamed regions and non-uniform cell size distribution were observed at the lowest carbon fibers content. Further incorporating carbon fibers decreased the crystallinity and increased the melt viscoelasticity. As a result, the composites had higher CO2 solubility with the increment of carbon fibers content. It is found that the foamed composite with 25 wt.% carbon fibers had the most uniform size distribution and better cell morphology compared to the other two composites. The increment in the cell density with increasing carbon fibers content indicated that the bubble nucleation was dominated by the heterogeneous mechanism with the addition of carbon fibers. The poorly bonded interface between carbon fibers and polypropylene could serve well as nucleation sites.