Rheological behavior of (Short) carbon fiber/thermoplastic composites. Part 1: The influence of fiber type, processing conditions and level of incorporation

The aim of this work is to understand the influence of type of (short) carbon fibers, processing conditions and fiber incorporation level on the rheological behavior of carbon fiber/polypropylene (PP) composites. For this purpose, two types of fibers (sub-micron Vapor Grown Carbon Fibers, VGCF, and ex-PAN, PAN), three different extruder screw geometries and three different fiber incorporation contents were studied. The rheological characterization was performed by means of capillary and rotational rheometry, results being presented and discussed in terms of reinforcing capability in both shear (steady and oscillatory) and extensional flows. The results show that VGCF have a generally higher influence on the rheological behavior of the composites than the PAN fibers. However, because of their higher intrinsic rigidity, PAN fibers give rise to composites with better mechanical properties than the VGCF ones. It is also shown that the influence of the screw geometry on fiber damage and, consequently, on the behavior of the composites, is weak, fiber degradation occurring mainly in the compounding stage. The incorporation level has the expected influence, i.e., it produces gradual changes in all the properties considered in this study.