Foam extrusion of polypropylene–rice husk composites using CO2 as the blowing agent

The biodegradability and low cost of natural fibers make them attractive for composite applications. However, the density of natural fibers is higher than that of polymers, which increases the density of natural fiber composites. This drawback could be reduced by foaming. In this study, a polypropylene/rice husk natural fiber composite was extrusion foamed by a single-screw extruder using CO2 as the blowing agent. The effects of coupling agent, screw rotation speed, CO2 content, and die temperature on the cell structure were studied. Because moisture and volatile organic compounds from rice husks could be released during the extrusion process and deteriorate the cell structure, the thermal degradation behavior of rice husks was simulated using thermogravimetric analysis. The experiment results showed that the addition of a coupling agent improved the cell morphology and reduced the cell size of the composite. Before the coupling agent was added, there was a clear relation between the foam density and both the screw speed and the CO2 loading level. However, the relation disappeared after the coupling agent was added. The foam density decreased much more significantly with the decreasing die temperature. The lowest foam density achieved in this experiment was 0.6 g/cm3. Melt fracture phenomena were observed when the die temperature was below 160°C. In addition, a bouquet-like anisotropic cell structure was observed at a low die temperature of 147°C