Thermal history dependent rheology, crystallization, and fusion joint strength of polyphenylene sulfide/carbon fiber thermoplastic composites

The impact of thermal history on ultimate thermoplastic composite properties is often overlooked, provided the final parts are compliant. This work investigated the influence of temperature and time of common manufacturing processes of poly phenylene sulfide (PPS) thermoplastic composites (TPC), which resulted in long chain branching and crosslinking of the PPS matrix. This was determined through measurable changes such as rheological properties, crystallization behavior, and fusion joint strength of PPS/CF TPCs. TPC melt processing steps included unidirectional (UD) tape manufacturing, large panel layup and consolidation, individual panel thermal treatment, and fusion joining. During thermal treatment, nine sets of two 10″ × 4″ panels were exposed to either 300°C, 320°C, or 340°C, for 5 min, 30 min, or 60 min. Using thermal profiles identical to TPC manufacturing steps, rheological and calorimetric techniques mimicked the thermal history of the PPS/CF composites to observe changes in PPS matrix properties at discrete process steps. Complex viscosity of PPS increased across all test frequencies, while percent crystallinity decreased, indicative of thermal degradation in the melt state. The increase in viscosity inversely related to single lap shear strength (SLSS) of fusion joined panels. Through combining matrix-level measurements (SAOS rheology and DSC) to composite mechanical testing, we gain significant insight regarding the influence of process-induced polymer matrix properties on fusion joint strength.