Reversible Thixotropic Rheological Properties of Graphene-Incorporated Epoxy Inks for Self-Standing 3D Printing

As three-dimensional (3D) printing has emerged as a new manufacturing technology, the demand for high-performance 3D printable materials has increased to ensure broad applicability in various load-bearing structures. In particular, the thixotropic properties of materials, which allow them to flow under applied external forces but resist flowing otherwise, have been reported to enable rapid and high-resolution printing owing to their self-standing and easily processable characteristics. In this context, graphene nanosheets exhibit unique π-π stacking interactions between neighboring sheets, likely imparting self-standing capability to low-viscosity inks. Herein, we develop a thermally curable graphene-incorporated epoxy ink system that exhibits shear-thinning characteristics and upright standing capability owing to its high static yield stress (∼1,680 Pa). The reversible liquid-to-solid phase transition of the composite ink, absent in the pristine epoxy ink, is clearly identified by its viscoelastic properties and dynamic yield stress. This thixotropic composite ink enables the continuous filament printing of 10 stacked layers without the spreading of injected filaments. Significantly, the 3D-printed composite structure, post-thermal curing, exhibits robust structural integrity and is free from weld lines or voids at the stacked interfaces. Combined with the clearly elucidated processing-structure-property relationships of the ink system, our results highlight its potential for a wide spectrum of applications.