Concentrated ceramic particle suspensions can be extruded to form 3D structures for biomedical, aviation, and electronic applications using the additive manufacturing technique of direct ink writing (DIW). Designing suspensions or “inks” for DIW requires a balance to be achieved between material strength and viscosity. Non-adsorbing polymers are often added to DIW inks as viscosity and yield stress modifiers. Understanding the mechanisms by which free polymers contribute to the concentrated suspension rheology, and notably how they affect the onset of shear thickening (an undesirable phenomenon for DIW), is crucial to the design of 3D printable polymer-ceramic suspensions. In this work, we examine the rheology of aqueous colloidal alumina suspensions containing non-adsorbing polyvinylpyrrolidone (PVP) of varying molecular weights and at concentrations within the dilute and semi-dilute non-entangled regimes. PVP addition causes large increases in both viscosity and dynamic yield stress. Discontinuous shear thickening occurs above a critical onset stress (τ_min) which is dependent on both PVP loading and molecular weight. We relate the overall suspension rheology to the polymer conformation in the suspending medium, demonstrating how non-adsorbing polymer addition can extend the processing window during extrusion by increasing τ_min. The fundamental insights gleaned herein regarding how non-adsorbing polymers contribute to suspension rheology can help to inform the design of 3D-printable polymer-ceramic suspensions and better understand their potential processing limitations.

(left) Direct ink writing of a polymer-ceramic suspension with test shape (middle top) and best printability (middle bottom) at 55 vol% alumina. (right) Steady-shear data showing discontinuous shear thickening for 55 vol% alumina suspensions containing varying loadings of non-adsorbing PVP.

Jeffrey Youngblood

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