Effect of the Cooling Process on Crystallization and Consolidation of Thermoplastic GFRP Bars With a Melt Pultrusion Technology

Thermoplastic composites offer many advantages over conventional thermoset counterparts, including excellent toughness, reprocessing capabilities, reduced carbon footprint, and so on. A thermoplastic composite profile was proposed for manufacturing using melt pultrusion technology with thermoplastic prepreg tapes. However, how to reduce the internal defects and surface topography deficiencies of the pultruded profiles remains a tough issue. The present study investigated the influences of the cooling process on the crystallization behavior and consolidation of polypropylene-based glass fiber reinforced polymer (GFRP) bars using melt pultrusion technology. The degree of crystallinity of PP was increased from 33.0% to 43.9% when prepreg tapes experienced consistent radial cooling with a uniform cooling rate below 30°C/min within the crystalline temperature range of 100°C–128°C. Thermal deconsolidation or an enlarged transverse temperature gradient led to an asynchronous contraction of polymers and incomplete crystallization, thus forming pronounced internal defects and surface texture deficiencies. Full consolidation for bars of 10 mm diameter was achieved when the cooling die temperature was maintained around the peak crystalline temperature (110°C). The optimized interfacial adhesion between the fiber phase and the matrix phase yields the flexural strength of 566 MPa and interlaminar shear strength of 23.8 MPa.