Recycled polypropylene blends as novel 3D printing materials

Consumer-grade plastics can be considered a low-cost and sustainable feedstock for fused filament fabrication (FFF) additive manufacturing processes. Such materials are excellent candidates for distributed manufacturing, in which parts are printed from local materials at the point of need. Most plastic waste streams contain a mixture of polymers, such as water bottles and caps comprised of polyethylene terephthalate (PET) and polypropylene (PP), and complete separation is rarely implemented. In this work, blends of waste PET, PP and polystyrene (PS) were processed into filaments for 3D printing. The effect of blend composition and styrene ethylene butylene styrene (SEBS) compatibilizer on the resulting mechanical and thermal properties were probed. Recycled PET had the highest tensile strength at 35 +/- 8 MPa. Blends of PP/PET compatibilized with SEBS and maleic anhydride functionalized SEBS had tensile strengths of 23 +/- 1 MPa and 24 +/- 1 MPa, respectively. The noncompatibilized PP/PS blend had a tensile strength of 22 +/- 1 MPa. PP/PS blends exhibited reduced tensile strength to ca. 19 +/- 1-3 MPa with the addition of SEBS. Elongation to failure was generally improved for the blended materials compared to neat recycled PET and PS. The glass transition was shifted to higher temperatures for all of the blends except the 50-50 wt. % PP/PET blend. Crystallinity was decreased for the blends, but was highest in the 75-25 wt. % PP/PS and the 50-50 wt. % PP/PET blend with SEBS-maleic anhydride. Solvent extraction of the dispersed phase revealed polypropylene was the matrix phase in both the 50-50 wt. % PP/PET and PP/PS blends.