Sustainable recycled coloured polypropylene using fused granulate fabrication: influences of pigments, copolymer composition and morphology on material properties

The growing demand for sustainable polymers drives the utilisation of coloured recycled polymers in additive manufacturing. This study systematically investigates the influence of pigments, copolymer, and morphology on thermal, rheological, and mechanical behaviours of recycled polypropylene (rPP) processed using Fused Granular Fabrication (FGF). FTIR and XRD analyses identified polyethylene segments and pigments induced crystallinity modification in opaque samples (red, purple, black and blue), while clear and green samples retained well-defined isotactic structures. DSC and TGA indicated dual melting peaks and minor reductions in thermal stability in copolymer-rich samples. Rheological measurements showed that melt flow viscosity and shear-thinning behaviour depend on pigment-polymer interactions and copolymer composition. Mechanical testing revealed significant effects on tensile (13.3–23.8 MPa), flexural (19.9–30.3 MPa), and impact (3.5–7.5 kJ/m²) properties depending on printing temperature and formulation. Fractography revealed phase separation, cavitation, and pigment dispersion influencing energy dissipation. Successful single and multi-colour rPP showcases validated structural integrity for sustainable additive manufacturing applications.