(187k) Material Extrusion Based Additive Manufacturing with Blends of Polypropylene and Hydrocarbon Resins

Filament material extrusion (MatEx) is one of the most commonly used techniques in additive manufacturing due to its operational flexibility, easy maintenance, low energy demands, low material wastage and low cost. However, in spite of notable development in the MatEx process, there are currently only a handful of materials that can be printed consistently using this technique. Isotactic polypropylene (PP), a popular thermoplastic material, undergoes rapid crystallization that leads to significant trapped residual stress when processed using MatEx, resulting in poor adhesion to the printing platform, poor geometric tolerance and mechanical performance. In this work, the effects of low molecular weight hydrocarbon resins on the thermal behavior, crystallization, and morphology of their blends with PP are studied. The addition of the resins to the pure PP matrix lowered the crystallization temperature of PP from 121.8ºC to 116.0ºC, which enables additional diffusion during the solidification process. Polarized optical microscopy demonstrates the differences in crystalline morphology, which impacted the structure at the interlayer boundaries between deposited roads in particular. The combination of modifications in crystallization rate, time, and morphology significantly affect the z-axis adhesion and residual stress state, which directly controls the mechanical properties and part warpage. Tensile bars were printed in two different orientations to analyze the mechanical performance and study part anisotropy. The maximum tensile stress for one of the printed 80/20 blends is 32.5 MPa which is higher than that of the printed PP part (26.8 MPa). The improvement in the tensile strength is due to a combination of changes in crystallinity, morphology, and improved interlayer adhesion during printing. The parts were annealed post-printing to improve polymer chain diffusion across the layers thereby improving interlayer adhesion and resulting in tensile modulus and strength values in excess of 90% of injection molded (IM) PP parts.