Quantifying the Impact of Multiple Twin-Screw Extrusion Cycles on Discontinuous Carbon Fiber Composites

To enable re-use, recycled fiber-filled thermoplastics may require intermediate processing, such as twin-screw compounding. These intermediate processes can introduce additional processing-induced degradation and performance variability. This study evaluated the recyclability of carbon-fiber-reinforced acrylonitrile butadiene styrene (CF-ABS) through ten consecutive pilot-scale extrusion cycles to determine the primary degradation mechanism as a function of lifetime and to quantify the impact on mechanical performance. Chemical analysis revealed that the ABS matrix remained chemically robust throughout repeated extrusion cycles, whereas optical microscopy showed that fiber breakage was significant. Fiber length exponentially decayed with repeated cycles, with the decay rate stabilizing between the 3rd and 5th cycle, resulting in a 62% reduction in weight-averaged fiber length from C0 to C10. The reduction in fiber length was found to govern both the rheological and mechanical behavior of the composite material. Viscosity and storage modulus decreased with decreasing fiber length while tan(δ) and elongation at break increased, shifting toward values characteristic of neat ABS and indicating a transition from fiber-dominant to matrix-dominant behavior. The tensile strength decreased 35% after the 10th cycle, and its decay behavior correlated with the decay rate of the measured fiber length. This correlation enabled the development of a simple analytical model to predict tensile strength, which predicted tensile strength more accurately than the Kelly–Tyson model (RMSE of 38 reduced to 3.5) and required fewer experimentally derived inputs. Overall, this study demonstrated that fiber breakage, rather than chemical degradation of the ABS matrix, dictates performance of recycled CF-ABS.