Effect of particle shape on rheology and printability of highly filled reactive inks for direct ink writing

Highly filled inks including a reactive titanium-boron composite powder (with Ti center dot 2B composition), a polymeric binder, and a solvent mixture combining the main solvent with a plasticizer and surfactant, are prepared for material extrusion-based printing. To determine the effect of particle shape and loading on rheology and printability of the inks, both spherical and irregularly shaped powders with the same composition and close particle sizes are manufactured by high-energy milling and used to formulate 80%, 90%, and 95% (wt.) inks. All ink formulations show shear thinning and shear recovery behavior. The degree of shear thinning decreases with increased particle loading, and inks with spherical particles show a higher degree of shear thinning. Shear recovery is instantaneous for inks with spherical particles, whereas it takes over 10 min for inks with irregular particles to fully recover. All inks behave elastic, solid-like at low strain and become viscous, liquid-like at a critical strain, or yield stress (sigma(y)). For inks with irregular particles, critical yield stress is significantly higher (sigma(y) approximate to 1250 Pa for 80%, 1600 Pa for 90%, and 2800 Pa for 95%) as compared to spherical particles (sigma(y) approximate to 400 Pa for 80%, similar to 600 Pa for 90%, and 1700 Pa for 95%). Printability studies show that inks with spherical particles produce finer struts with a smaller width, and lower porosity. For 80%, 90%, and 95% inks with irregular particles, the smallest printable strut widths were approximately 250, 400, and 650 p.m, whereas for spherical particles they were 170, 300, and 360 p.m, respectively. We demonstrated printing of various 2D and 3D structures including heterogeneous structures printed using multiple ink formulations.