High solids content materials are largely defined by their ability to form interparticle networks upon incorporation of sufficient filler, termed the percolation threshold. This threshold is believed to originate from particle-particle and particle-matrix interactions. Rheological complications arise from these interactions, including the presence of yield stresses, shear thinning, and thixotropy. Current efforts aim to tailor these properties in order to characterize the printability of highly filled systems, e.g. greater than 40 volume % filler, using rotational rheometry for the purpose of additive manufacturing (AM).<br/><br/>Compared to their solution cast counterparts, AM products can be made with more complex geometries and lower void fractions while possessing superior mechanical properties. However, several criteria must be met for an ink to be considered printable. The material must 1) flow when subjected to its static yield stress, 2) decrease in viscosity during the printing process, and 3) recover structure sufficiently fast post-extrusion to prevent sag and promote interlayer adhesion. This work analyzes the effects of particle geometry (spherical, cubic, or angular), size, and loading level on the rheological properties relevant to printability.<br/><br/>Preliminary results suggest that particle size can drastically influence viscosity profiles when fillers approach the nanometer scale. Filler geometry appears to have an influence on viscosity profiles, although the transition from micro- to nano-scale fillers appears to be far more significant. Expectedly, the Cox-Merz rule fails to match viscosity readings when continuous flow and oscillation data are compared. This is predominantly due to the presence of slip and edge fracture during continuous flow in a parallel plate setup. Future work focuses on three interval thixotropy testing. This test, which subjects the material to a sudden jump in stress followed by a period of recovery, tracks rheological properties indicative of structure and restructure, and has potential to simulate the extrusion process.

Characterization of high solids content materials for additive manufacturing via rotational rheometry

ACS Spring Meeting 2021

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