Direct Ink Writing 3D Printing of High Graphite Loading Polybenzoxazine Composite

ABSTRACT

Polybenzoxazine is a class of high‐performance thermoset that can be reinforced with graphite to gain great thermal stability, chemical resilience, and robust mechanical strength. However, polybenzoxazine composites are challenging to process via extrusion‐based additive manufacturing. Most polybenzoxazines polymerize via thermal‐assisted ring opening to crosslink, but the temperature increase melts benzoxazine monomers and destroys a printed part's morphology before crosslinking. Yet, extrusion‐based additive manufacturing requires the extrudate to liquefy during processing. Despite the conundrum, additive manufacturing of polybenzoxazine composites can reduce waste materials, enable rapid prototyping, and generate parts otherwise challenging to cast with a mold. Adding composite fillers, such as graphite, to benzoxazine monomers would complicate the viscoelastic behaviors of the composite precursor mixture, and the addition also generates a semisolid consistency. A semisolid mixture is stationary like a solid until agitated by an external force, at which point the semisolid will flow like a liquid with applied force. This viscoelastic behavior allows graphite and benzoxazine monomer mixtures to be 3D printed via direct ink writing, and the printed object can hold its morphology during subsequent thermal crosslinking. The object can be further stabilized during thermal crosslinking by burying the printed object in fine inert sand that acts as conformal temporary mold. As a proof of concept, bisphenol‐A and aniline type polybenzoxazine reinforced with graphite was 3D printed and thermally crosslinked while maintaining the printed shape.