(603c) Effect of Polypropylene Melt Flow Index on the Material Extrusion Based Additive Manufacturing of Polypropylene/Hydrogenated Resin Blends

Material extrusion (MatEx) based additive manufacturing (AM) process can produce parts with intricate features that are difficult to fabricate using traditional subtractive manufacturing techniques. However, the MatEx process suffers from two major drawbacks: the mechanical properties of the parts produced using MatEx are often inferior to the bulk material properties and only a limited number of materials are currently compatible with this mode of AM. Isotactic polypropylene (PP) is a popular thermoplastic material which undergoes rapid crystallization that leads to significant trapped residual stress when processed using MatEx, resulting in poor geometric tolerance and mechanical performance.

In this work, PP having different melt flow indices (MFIs) were blended with low molecular weight hydrocarbon resins having different degrees of hydrogenation. The influence of the PP MFI on the crystallinity of the pure polymers and PP/hydrogenated resin blends as well as the melting and crystallization temperature was investigated. For example, the addition of the resins lowered the melting temperature by 7⁰C while the crystallization temperature of pure PP (MFI=12g/10 min) decreased from 133.3⁰C to 118.6⁰C in the blends; thereby increasing the time required for the material to crystallize from the melt. Next, the blends were extruded using a single screw extruder to obtain filaments that can be used in the MatEx process. An understanding of the modifications in the crystallization process significantly impact the z-axis adhesion and residual stress state, which directly affect mechanical properties and warpage in the printed parts. Tensile bars were printed in two different orientations to analyze the mechanical performance and study the effect on part isotropy. The maximum tensile strength and modulus of the printed parts was found to decrease by 21% and 15% respectively. However, moderate post-printing annealing achieved mechanical properties that approached injection molded PP parts by improving the interlayer bonding of the parts. We can leverage these results to modify processing conditions to optimize mechanical properties of the parts generated using MatEx.