Through-plane electrical conductivity of injection-molded polypropylene/carbon-fiber composite foams

Low-pressure injection foam molding of polypropylene/carbon-fiber composites with the aid of physical blowing agent (N2) was conducted and their foaming behavior and through-plane electrical conductivity were characterized. The proper processing window coupled with an effective degree of foaming (up to 20% void fraction) and gas content (0.3 wt.% N2) resulted in an increase of the conductivity up to four orders of magnitude and significant improvement in its uniformity along the injection-molded sample. Foaming enhanced the through-plane conductivity through: (a) an enhanced inter-connectivity of fibers through biaxial orientation around the growing bubbles, (b) a less degree of in-plane fiber orientation (especially within the skin layer) by the plasticizing effect of gas, and (c) a decreased skin layer thickness by the plasticizing gas. The results also show that the injection flow rate and melt temperature played an important role on the foams’ conductivity through their effects on the foam morphology in the core and the skin layer thickness.