Rheological behavior of (short) carbon fiber/thermoplastic composites. Part II: The influence of matrix type

This study is the second of a series of two papers on the rheological prope rties of VGCF/thermoplastic composites, its aim being to compare the perfor mance of different thermoplastic matrices when reinforced with sub-micron V apor Grown Carbon Fibers (VGCF). For this purpose, two polymers (polypropyl ene, PP, and polycarbonate, PC) were used to produce composites with differ ent levels of fiber incorporation. The highest content VGCF composites were produced, in pellet form, using a co-rotating twin-screw extruder. These w ere subsequently diluted during the injection molding stage to obtain compo sites with different levels of fiber content. The theological characterizat ion was performed, using the latter materials, by means of both capillary a nd rotational rheometries, results being presented and discussed in terms o f reinforcing capability and its susceptibility to temperature, for the rel evant functions in both shear (steady and oscillatory) and extensional flow s. The results show that VGCF have a higher reinforcing capability, both th eological and mechanical, when incorporated in the PP matric. This better p erformance can be due either to better adhesion between the fibers aid PP i n comparison to PC, or to a higher degree of melt penetration of non-wetted fiber rich areas and lower fiber length degradation, owing to the lower vi scosity of PP in the extrusion shear rate range.