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.

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Foaming properties of the three NC‐based (nitrocellulose‐based) propellants, namely, single‐base propellant, NG (nitroglycerine) propellant and TEGDN (triethylene glycol dinitrate) propellant were investigated in the batch foaming process by using supercritical CO2 as the physical foaming agent. Burning characteristics of the foamed NC‐based propellants were also investigated in this work. For this study, the CO2 desorption of the three NC‐based propellants were plotted by the gravimetric method. The morphology and burning characteristics of these foamed NC‐based propellants were characterized by scanning electron microscope (SEM) and closed vessel experiment. The test data revealed that the energetic plasticizer has a considerable effect on the pore formation in the NC matrix although it has little effect on the CO2 solubility in the NC‐based propellants. Moreover, the SEM images showed the foaming temperature also plays an important role in the pore parameters of foamed propellants. Furthermore, the data of closed vessel experiment indicated that the burning characteristics of foamed NC‐based propellants largely depend on the pore parameters, and the porous structure of foamed propellants would considerably increase the mass conversion rate.

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