Material Extrusion-Based Additive Manufacturing with Blends of Polypropylene and Hydrocarbon Resins

Filament-based material extrusion additive manufacturing (MEAM) is one of the most commonly used techniques in additive manufacturing (AM). In spite of recent notable development in the MEAM process, there is still a need to develop more materials that can be printed consistently using this technique. Isotactic polypropylene (PP), a popular thermoplastic material, undergoes rapid crystallization and subsequent volume contraction. This can lead to residual stress buildup in PP parts when processed using MEAM, resulting in poor adhesion to the printing platform, poor geometric tolerance, and mechanical performance. In this work, the effects of varying composition of low molecular weight hydrocarbon resins incorporated to PP are investigated. Specifically, the thermal behavior, crystallization, morphology, and printability of the blends are studied. The rapid crystallization of PP has been delayed by the addition of hydrocarbon resins that provided a larger time window for the residual stresses to relax. The addition of the resins to the pure PP matrix lowers the crystallization temperature of PP from 121.8 to 116.0 °C, which further enables additional diffusion during the solidification process. Polarized optical microscopy demonstrates the differences in crystalline morphology, which is expected to impact the structure at the interlayer boundaries between deposited layers. The combination of modifications in crystallization rate, time, and morphology significantly affects the interlayer adhesion and residual stress state, which directly controls the mechanical properties and part warpage of printed parts. Tensile bars of the different blends were printed in two different orientations to analyze the mechanical performance and study part anisotropy. The maximum tensile stress of pure PP (26.8 ± 2.1 MPa) printed at a ±45° raster angle increased with addition of 20 wt % hydrocarbon resin (32.4 ± 3.0 MPa) when printed under the same conditions. 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 postprinting 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 PP parts.