Date of Award

Spring 2-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Polymer Science and Engineering

Committee Chair

Jeffrey S. Wiggins

Committee Chair School

Polymer Science and Engineering

Abstract

Relationships between polyurethane (PU) design and chemorheological properties were correlated with ambient reactive extrusion (ARE) additive manufacturing (AM) processing conditions. PUs synthesized from varying monomeric and polymeric isocyanates, hard segment concentrations, crosslinkers, and additives were systematically designed to elicit differences in chemorheological properties and efficacy. Variations in PU composition and experimental procedures were evaluated to establish a baseline of PUs for ARE. PUs were synthesized and monitored throughout polymerization by rheological and real-time infrared spectroscopy techniques to observe moduli development and functional group conversion, respectively. Observed experimental properties were correlated with critical ARE parameters, such as dimensional stability at deposition and reaction progression between layer deposition.

Fumed silica (FS) was incorporated into polymeric methylenediphenyl diisocyanate (MDI)-based PUs to modify viscosity and elucidate the influence of nano-scale additives on network development and ARE print quality. Increased FS concentration imparted significant differences in rheological gelation profiles. While a dependence on print toolpath and reaction progression of initial layers was observed, FS-containing PUs demonstrated increased dimensional stability and print quality when compared to unfilled PUs synthesized from monomeric diisocyanates.

Highly-filled modified MDI-based PUs (~70 wt% glass microspheres) exhibited profound increases in dimensional stability and resolution, yielding the highest aspect ratio of height to wall thickness (130.1 mm : 1.62 mm) in ARE literature, to date. Evaluation of mechanical properties according to flat and upright build orientations, with transverse or longitudinal raster direction, indicated elastic modulus was isotropic regarding raster direction, while ultimate tensile strength was dependent upon raster direction with a 13.4% difference in flat builds and 28% difference in upright builds. The slight mechanical anisotropy was attributed to small voids along the layer-layer interface observed via scanning electron microscopy; nonetheless, the ­in situ interlayer polymerization yielded printed specimen with reduced anisotropy when compared to traditional thermoplastic AM methods.

All studies remained central to eliciting chemorheological variations in PUs and the effects of PU design on ARE printability. Evaluation of PUs with varying initial reaction rates and rheological responses indicated ARE print quality had a significant dependence on material chemorheological properties, while PU kinetics and interlayer material consolidation dictated mechanical properties.

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