Production of plastic powder feedstocks for additive manufacturing by precipitation

Funded by Deutsche Forschungsgemeinschaft (DFG) – Project number 409808524

The overall aim of this project within priority program 2122 “Materials for Additive Manufacturing” is the development of novel semi-crystalline polymer powders for selective laser sintering (SLS) via a liquid-liquid phase separation (LLPS) and subsequent crystallization for polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyoxymethylene (POM) and polyvinylidene fluoride (PVDF). Selection criteria for appropriate solvents for LLPS will be deduced by an iterative screening method based on Hildebrand and Hansen solubility parameters. In-situ model LLPS experiments employing dynamic light scattering, optical microscopy and impedance spectroscopy –supported by ex-situ electron microscopy- will allow for deeper insights into the phase separation and particle formation mechanisms. A detailed mechanistic understanding of the underlying mechanism of LLPS, i.e. nucleation, droplet coalescence and growth and elucidation of their dependencies on system composition, time-temperature history will be achieved. LLPS will be implemented on the mini plant scale, where the effect of process parameters (e.g. influence of stirring, cooling regime) on material and bulk solid properties of the obtained polymer particles will be systematically studied. Moreover, in-situ functionalization of the particles with nanoparticulate flowing aids and additive enhancement with thermal stabilizers (antioxidants) will be studied, scale-up criteria will be deduced and powder amounts applicable for studies on SLS processing behavior (several 100 grams to the kilogram scale) will be produced. A detailed structural characterization of the obtained SLS particles with respect to crystallinity, polymorphism, thermal characteristics and morphology will be performed employing amongst others dynamic scanning calorimetry, X-ray diffraction, vibrational spectroscopy (IR, Raman) and electron microscopy. The effect of process parameters on molar mass distributions of the polymer and melt viscosity will be assessed by gel permeation chromatography and melt rheology. Bulk solid characteristics such as the product particle size distribution and the powder flowability which are seen to be most important for processing will be characterized by laser diffraction particle sizing, respectively shear testers and powder application model experiments. The gained understanding of the structure-property relationships and the information on SLS processability provided by the cooperation partners will be utilized for further LLPS process optimization to tailor the desired properties of the novel SLS powders. Novel SLS powders with improved material behavior, i.e. chemical resistance (PET, PBT, PVDF), impact resistance (PBT), high stiffness, excellent dimensional stability (PET, PBT, POM) shall be developed widening the field of application of SLS-manufactured parts.

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