Upcycling of Polypropylene Glass Fibre Composite Regenerates Through a New Technological Approach

The transition toward lightweight and sustainable mobility demands not only advanced materials but also fully recyclable products and circular manufacturing technologies. Fibre-reinforced thermoplastic composites, particularly glass fibre¿reinforced polypropylene (GF-PP), offer substantial weight reduction and CO¿ emission savings. However, their recyclability and reuse within closed material cycles remain challenging. Conventional production of polypropylene (PP) organosheets and unidirectional (UD) tapes generates up to 40% manufacturing waste, most of which is currently downcycled or incinerated, undermining the environmental benefits of lightweight construction. To overcome these limitations, this PhD research developed and optimized a single-step Injection Moulding Compounder (IMC) process as an innovative and energy-efficient route for recycling and upcycling glass fibre¿reinforced thermoplastics. This work contributes to the broader LightCycle initiative, which aims to establish a closed-loop process for fibre-reinforced composite production. In the first stage, the homogeneity and reinforcing efficiency of post-industrial GF-PP composites blended with recycled polypropylene (rPP) were thoroughly investigated through detailed thermal, rheological, and mechanical analyses. The addition of GF-PP composites did not significantly alter rPP processability, which is advantageous for industrial implementation. However, different behaviors were observed depending on the composite type: organosheet-derived GF-PP increased viscosity and enhanced tensile strength by up to 30%, whereas UD tape¿based GF-PP primarily acted as a filler. Compounding and granulation reduced fibre length, highlighting the importance of controlling fibre¿fibre interactions and shear conditions during processing. Subsequent optimization using Design of Experiments (DOE) and Response Surface Methodology (RSM) identified the optimal formulation, 50 wt.% GF-PP flakes, 5 wt.% additive, and a screw speed of 150 rpm, yielding an elastic modulus of 4.1 GPa and a tensile strength of 48.6 MPa with satisfactory flowability. Rheological modelling confirmed that GF-PP and additive content predominantly influenced viscosity and shear response, indicating improved fibre¿matrix interaction. Furthermore, machine learning (ML) models trained on the experimental dataset achieved a high predictive accuracy (R² > 0.85) for stiffness and strength, confirming the capability of data-driven approaches to predict and optimize recycled composite performance. In the final stage, the IMC process was experimentally validated as a sustainable, single-step manufacturing technology for GF-rPP composites. Numerical simulations using Ansys POLYFLOW® provided insights into the effect of melt flow design on pressure build-up and residence time, which were successfully validated through experimental trials. Compared with the conventional two-step compounding and injection moulding route, the IMC process achieved 10¿12% higher tensile strength, fibre retention, and enhanced stress transfer efficiency. The integration of a FIFO melt reservoir further improved the process stability, reduced the residence time, and minimized the thermal degradation and energy consumption. Although the IMC system demonstrated a high potential, further optimization is required to integrate its multiple control units into a unified automated platform, enabling real-time process monitoring and adaptive control. Future research should also evaluate the recyclability of IMC-produced parts through multiple processing cycles and explore the influence of initial fibre length and processing temperature on long-term performance. Moreover, the characteristics of IMC like low-shear, controlled-temperature environment presents strong potential for processing biopolymers and natural fibre composites, extending its applicability