The iPP Printing Paradox: Interlayer Adhesion Through Crystallization

Reducing material consumption in polymer manufacturing is a key sustainability challenge because it directly lowers the demand for fossil‐based materials. Fused Filament Fabrication (FFF) offers a promising route by minimizing production waste. However, the mechanical performance of printed parts remains deficient as interlayer failure is difficult to prevent in FFF of thermoplastics. Polymer‐chain interdiffusion and re‐entangling across an interphase are typically too slow at printing time scales. Up to now, amorphous polymers predominate in FFF, whereas the use of semi‐crystalline polymers remains scarce due to printability issues such as warpage. Even though interlayer adhesion can be improved through the formation of crystals across the interphase rather than solely via interdiffusion and re‐entanglement. This study focuses on characterizing the crystal phases and fracture behavior at the interphase between two FFF‐printed strands of isotactic polypropylene (iPP). The influence of molecular weight and processing conditions on the crystal phase composition of industrial iPP grades is investigated. Although lower molecular weight enhances interdiffusion, results indicate that a higher molecular weight improves interlayer adhesion in iPP. This is because there is a stronger shear flow at similar print speed, which, as a result, forms more of the ductile β‐iPP polymorph at the interphase. Notably, an eightfold increase in fracture energy is observed for the highest molecular weight, with up to 50% β‐phase crystals, compared to predominantly α‐phase at low molecular weight and/or mild printing conditions. These findings establish a clear structure–property relationship in which molecular weight and print speed synergistically strengthen interlayer adhesion in the case of two strands in FFF‐printed polypropylene.