(33a) Mitigation of Hysteresis in High Solids Content Polymers Using Photorheometry

Highly filled (also called high solids content or highly loaded) polymer systems are a vital component in the field of chemical engineering, specifically to areas such as ceramics, composites, and energetics. These high solids content materials are comprised of a polymer matrix packed with a large fraction of filler – often over 40% by volume. At these loading levels, interparticle interactions dominate a material’s rheological behavior, which often define its applicability to direct ink write (DIW) additive manufacturing (AM) processes. For example, a yield stress forms at high loadings, which enables precision starting/stopping of material deposition, while shear thinning allows the ink to remain fluid-like at high shear rates through the nozzle. Unfortunately, high shear rates associated with DIW also break down the interparticle network of highly loaded systems, which causes a lag in network reformation post-extrusion, known as hysteresis. As such, the system remains fluidic temporarily after extrusion, and inks can potentially sag or even flow across the print bed. This results in a poor resolution material with inadequate interlayer adhesion. A potential solution has been the use of in-situ or ex-situ UV light during printing to quickly recover and maintain the fidelity of printed structures.

This work highlights the use of oscillatory photorheometry to analyze the role that an in-situ UV cure can play in quickly restoring a material’s interparticle network after subjection to high levels of stress. The research focuses primarily on how loading level, distribution, and transparency of fillers can contribute to thixotropy and hysteresis, specifically when an in-situ UV cure is applied to encourage network reformation. This is accomplished through a three-interval thixotropy test, where 1) a low stress time sweep is conducted to gather low-strain data, 2) high stress time sweep data is gathered until a new equilibrium is reached, and finally 3) UV is optionally turned on while baseline, low stress data is gathered again. Comparison of the third step with and without UV exposure allows conclusions to be drawn about how hysteresis of a highly filled system post-extrusion can be mitigated using varying intensities of UV light.