The Evolution of Crystal Structure in Polypropylene as It Goes From Pellet to Powder to Print in Powder Bed Fusion Additive Manufacturing

Abstract

ABSTRACT Polymer powder bed fusion (PBF/P) is an additive manufacturing technology that fabricates end‐use products for many applications. Thermal‐induced phase separation (TIPS) has been adapted to produce spherical isotactic polypropylene (PP) powders suitable for PBF/P. In this study, the evolution of the i‐ PP crystalline structure is tracked as the material evolves from the as‐received pellets to TIPS produced powders to PBF/P printed parts in i ‐PP molecular‐weight blends. TIPS‐produced powders show composition‐dependent lamellar structures: long periods in the powder MW blends are larger than those in the pellets when the 12k i‐ PP composition is < 10% or > 33%, but similar to those in the 12k i‐ PP pellet when the 12k composition is 10%–33%. This is attributed to entropically driven surface enrichment of 12k chain and their early crystallization in the powder particles of blends between 10% and 33% 12k i‐ PP. Outside of this composition range, the larger molecular weight i‐ PP dominates the crystallization. In contrast, printed parts exhibit similar long periods across all blend compositions, suggesting that an energy density of the laser fully melts the crystals, which erases the thermal history of the blends. These results demonstrate the utility of scattering methods to guide the rational design of novel PBF/P feedstocks.