Additive Manufacturing Filled Polymer Composites for Environmental Contaminants: Material Extrusion Processing, Structure and Performance

Research interest in Additive Manufacturing (AM) as an enabling technology for customizable parts is rapidly expanding. While much AM research focus is on high performance feedstocks and process optimization to obtain parts with improved mechanical properties, interest in the environmental applications of AM has recently increased. The lower cost and greater accessibility AM is leading to novel environmental research solutions in wastewater treatment and toxicity reduction by capitalizing on the increased affordability and accessibility of 3D printing (3DP) technologies for customizable, high surface area structures. The novelty and focus of this dissertation is exploration of Material Extrusion (MatEx) based Fused Filament Fabrication (FFF) of filled polymer composites as a disruptive technology enabler for deployable and retrievable structures in environmental media for adsorption, destruction and toxicity reduction of harmful chemicals. This dissertation addresses research questions that generally answer, "why AM for environmental applications?". The inherent layer-by-layer design provides larger surface area structures for interaction with contaminated media. Polylactic acid (PLA) was selected due to its green sources and biocompatibility relative to synthetic polymers and its wide processing window allowing shear thinning and "printability" despite the elevated viscosity and modulus of highly filled composites. The filler selected for contaminant adsorption was microporous zeolite, which has affinity for ammonia, radionuclides and Per- and Polyfluorinated Substances (PFAS). The filler selected for contaminant destruction was photocatalytic TiO2 nanoparticles which can degrade organic chemicals, harmful algal bloom toxins and PFAS. A preliminary research hurdle was overcome by demonstrating that immobilization of zeolite and TiO2 in a PLA binder matrix did not prevent adsorption or free radical release, respectively. The first major research objective involved investigation of high surface area printed PLA-zeolite geometries with different zeolite loadings and found that while ammonia was reduced, there were diminishing returns with increased loading in terms of mass standardized adsorptive performance due to insufficiently exposed zeolite. The research solution leveraged AM print process parameters to increase the macroporosity of the printed composite structure to create voids and channels allowing water infiltration and chemical adsorption to zeolite. Faster printing of larger roadways generated macrostructural voids that were maintained by extrusion at lower temperature for rapid solidification. The second research objective involved compounding different loadings and dispersion states of TiO2 in PLA to demonstrate immobilization of TiO2 closer to UV-light penetration water improves photocatalysis. Higher 32% w/w TiO2 loadings were heavily agglomerated and more difficult to print process due to high viscosity, rapid liquid-solid transition (G'>G") and particle network recovery during printer retractions, leading to nozzle clogging. Lower 20% w/w loading was more conducive to larger production printing due to lower viscosity, longer viscosity recovery times for retractions and thus generally a wider processing window. While altering twin screw processing parameters reduced TiO2 agglomerates in filaments, leading to increases in crystallinity (due to seeding effects and chain scission) and lower viscosity recovery, photocatalytic performance was not significantly improved. Evidence presented showed that larger particle agglomerates were more toward the inside of printed surfaces and thus less available to UV-light irradiation. This location of larger particles is supported by previous theoretical and empirical investigations showing larger particles migrate at a faster velocity away from the outer walls of confined extrudates within non-Newtonian flow fields due to normal forces, leaving more smaller particles toward outer surfaces. This research provided novel contributions to the environmental and AM research communities and pioneered a convergence of these fields into an interdisciplinary community of practice focused on better characterization and processing in environmental applications to improve structure-environmental property relationships. Future research should build on these findings to enhance performance through multi-functional materials that adsorb and destroy contaminants. The reactive surface area should be further increased through by high surface area designs and print parameter optimized porous structures providing a continuum of meso- to microporosity as confirmed by chemical flux and mass transfer studies for additional AM technologies (e.g., Direct Ink Write).