Closed loop recycling approach for aged polyamide 12 feedstocks for powder bed fusion by precipitation from a moderate solvent combined with solvolysis

Abstract

A major challenge in the powder bed fusion (PBF) process with polyamides, which are the most commonly used powders, is the handling of already used powder (aged powder). During PBF, polyamide powders undergo chemical ageing, resulting in altered thermal and rheological properties of the feedstock material. Components made from aged powder are of inferior quality. Therefore, aged powder is usually refreshed with new (virgin) powder. Although, a substantial amount of aged powder accumulates over the course of processing. These excess quantities are either employed in other plastic manufacturing processes (downcycling) or, more frequently, disposed. A combined process of solvolysis with subsequent thermally induced liquid-liquid phase separation and polymer crystallisation (TIPS+S) is used to restore the thermal properties and rheological properties of aged polyamide 12 (PA12) and thus to make the powders suitable for PBF again. The degree of solvolysis of the aged PA12 is tuned by reaction temperature or reaction time. A kinetic model allows to determine suitable process parameters for TIPS+S depending on the ageing condition of the feed to match the thermal properties of the recycled PA12 powders to those of PA12 virgin powder. The optimised recycled PA12 powder shows comparable zero shear viscosity, particle size distribution, particle shape and flowability to the virgin material. Specimen built from equally mixed recycled and aged PA12 powder achieved the same mechanical properties like those obtain by PBF of equally mixed virgin and aged powder. Therefore, a closed loop recycling approach is feasible, in which aged PA12 powders from the PBF process are recycled by the proposed TIPS+S process.