Influence of the recycling process parameters on CFRTP waste properties

Nowadays, the combination of continuous fibers and thermoplastic polymers as the matrix to continuous fiber-reinforced thermoplastics (CFRTP) is receiving increasing attention due to their potential advantages such as excellent weight-specific mechanical properties, short cycle times, storability, repeated meltability, good formability and the use of alternative joining processes enabling automated large volume manufacturing processes which allow various applications in different industries including transportation, construction among others. As the production rate of these materials increases, the amount of waste for disposal increases, for which recycling strategies need to be established to ensure the sustainability of CFRTP. Hence, these recycling strategies must be developed and evaluated economically and ecologically to close the loop and achieve a circular economy to process recycled fiber-reinforced pellets from CFRTP waste to valuable products e.g., by injection molding. This study presents a mechanical recycling approach from CFRTP waste to injection molded test specimens and evaluates the impact of the individual recycling steps along the recycling chain on the fiber length as the fiber length is detrimental to the resulting mechanical properties. First, the CFRTP waste processability is investigated and conditions for size reduction by cutting and shredding into feedstock for extrusion are defined. Second, fiber-reinforced pellets are produced by twin-screw extrusion. The fiber volume content and the process parameters screw speed and temperature during compounding are varied and the influence of these parameters on the fiber length is determined. Third, the extruded pellets are further processed by injection molding. Here, the influence of screw speed, back pressure, and processing temperature as well as the initial fiber length in the extruded granules on the resulting properties is investigated. Quantitative correlations between material properties and processing parameters are presented and suggestions for gentle processing during recycling are given.