(392b) Process-Structure-Property Relationships in Additively Manufactured Polypropylene Blends

Material extrusion (MatEx) based additive manufacturing (AM) of polymers has gained great popularity due to its versatility, operational simplicity, and cost-effectiveness. However, AM or 3D printing of semicrystalline polymers are still largely relegated to prototyping applications. Major problems involving shrinkage and warpage of the printed parts must be addressed before such polymers can be used for printing functional parts that can be employed in load bearing applications. Moreover, there can be a great deal of variation in the morphology and crystallization behavior of the polymer depending upon the processing conditions employed and the inherent material properties. Thus, the goal of this work is to provide fundamental understanding of the complex changes that take place during MatEx-based printing of semicrystalline polymer blends and thereby assist in developing tailored compositions for consistent and reliable AM processing.

In the present work, we blend polypropylene (PP) with polyolefin-based copolymers and evaluate the compatibility of the blends with MatEx-based AM. Differences in crystallization halftimes and crystallographic structures have a profound impact on the interlayer adhesion and residual stress state, which directly controls the mechanical performance and warpage of the printed parts. We investigate the effect of high shear in the printer nozzle on the flow-induced crystallization of the blends and couple with the melt flow behavior through detailed rheological characterizations. Variations in the crystal structure, degree of orientation, and degree of crystallinity, resulting from different printing conditions, significantly influence the tensile properties of the printed parts. The findings from this study can be leveraged in toolpath planning, process parameter optimization, and new feedstock development for MatEx, highlighting the current limitations as well as providing valuable insights into necessary processing modifications in order to enable MatEx of next generation semicrystalline polymers.