Improvement of dimensional stability and mechanical properties of polypropylene using recycled nanosilica for material extrusion-based additive manufacturing

Material extrusion-based additive manufacturing (ME-AM) is an innovative method of producing parts, but polypropylene (PP) is not commonly used in this method due to high warpage tendency and low mechanical properties after printing. In this study, two types of recycled nanoparticles were obtained from battery separators: low purity (91% purity) and high purity (98% purity). The effect of these nanoparticles on the crystallization behavior, mechanical properties and warpage of PP parts prepared with ME-AM, were investigated. Incorporating the low purity nanosilica resulted in a remarkable 345% increase in impact strength compared to the neat sample. This was attributed to the formation of a substantial amount of beta crystals within the PP crystalline structure, as confirmed by various analyses. The addition of the low purity nanosilica increased thermal conductivity by 52%, whereas the high purity nanosilica only achieved a 42% increase. This enhancement resulted in a noticeable warpage reduction of the printed samples. The incorporation of both types of recycled nanosilica enhanced crystallization temperature (Tc) and rate, thereby improving the dimensional uniformity of the filaments. Also, these rises in Tc and crystallization rate reduced fluidity by 25%, which enhanced surface quality and minimized oozing. The addition of these nanoparticles led to a 13% increase in elastic modulus without negatively affecting recyclability. Due to the easier recycling process and superior enhancement of the impact strength, the addition of the low purity nanosilica became a more affordable option for manufacturing PP-based composite filaments.