Flow response and microstructure of polymers reinforced with discontinuous fibers

The reinforcement of plastics with fibers has been practiced for many years. Only recently have long discontinuous fibers been compounded with thermoplastics by the method of pultrusion which preserves fiber length while promoting wetting of each fiber by the matrix polymer. Reinforcement of linear polymers with high strength fibers enhances physical properties and creates formable thermoplastic composites. Conventional compounding of fiber and polymer in a pelletizing extruder creates pellets. These are typically cylindrical, about 3×3 mm; containing fibers with a length distribution averaging 0.3 mm (but always <3 mm). In distinction to such `short fiber' (SF) materials, greater fiber length can be achieved by the process of pultrusion. In this technology, continuous tows of fiber are wetted by molten polymer; `pultruded' through a die; and chopped to any desired length. The resulting pellets are typically 3 mm diameter, 12 mm long, and contain 30 to 60 weight percent of fibers with l/d≥700. These long fiber (LF) pellets can be processed e.g. by injection molding, to give parts with net shape and good dimensional tolerances, high stiffness, tensile, and impact strength. In order to understand the distribution of discontinuous fibers in molded parts, and to comprehend the flow behavior of these non-Newtonian fiber-filled suspensions, we have examined the melt rheology of short and long fiber reinforced thermoplastics.