Extrusion additive manufacturing of semi-crystalline PET/PP blends

Semi-crystalline polymers are widely used in molding manufacturing processes, but are difficult to process via filament material extrusion additive manufacturing (AM). The significant warping and shrinking that occurs during melt cooling and polymer crystallization often leads to build failure in FFF. Four material-centric methods enabling FFF processing with semi-crystalline polymers have previously been hypothesized in literature: i) adding micron-scale filler materials, ii) using copolymers to disrupt inherent crystallizability, iii) blending a secondary, immiscible polymer with the matrix polymer, and iv) using a semi-crystalline ionomer. All four methods direct morphology formation during solidification of the extruded melt In this work, the authors present an investigation of approach (iii) by printing with a series of poly(ethylene terephthalate) (PET)/polypropylene (PP)/maleic anhydride-graft-polypropylene (MA-g-PP) polymer blends. It is shown that these blends can be extruded into a filament and then processed via filament material extrusion to form a printed structure. Extrapolations from this demonstrated success indicate generalizable trends for target morphology in FFF. Choosing polymers with disparate peak melting temperatures, such as PET (T_m = 255 °C) and PP (T_m = 165 °C), allows for volumetric changes associated with crystallization to be spread out over a larger localized area and over a wider processing time. Maintaining a mobile PP phase while the PET phase crystallizes and holds the desired part shape enables manufacturing without reaching a level of critical distortion resulting in build failure. These conclusions suggest that blending polymers is one path to expanding the families of polymers processable via material extrusion AM, including looking toward capturing post-consumer waste streams for additive manufacturing feedstock.