Direct Ink Writing of Fully Bio-Based Liquid Crystalline Lignin/Hydroxypropyl Cellulose Aqueous Inks: Optimization of Formulations and Printing Parameters

Following the recent demonstration of the potential to direct ink write lyotropic blends of organosolv lignin (OSL) and hydroxypropyl cellulose (HPC), this study aims to optimize the formulations and direct ink writing parameters for fully bio-based lignin/HPC inks. A prescreening identifies the theoretical window of printability for different compositions for formulations based on OSL solutions of 45, 47.5, and 50% solid contents and OSL/HPC wt %/wt % ratios of 30/70, 40/60, and 50/50. Measurements of shear–viscosity and recovery behavior evidence the shear-thinning contribution of HPC and the viscosity recovery contribution of lignin. Shape fidelity, morphology, and mechanical properties of printed monolayers from the prescreened formulations reveal band texture morphology in printed parts and the best performance for the inks with a solid content of 49% and an OSL/HPC ratio of 50/50 (Young's modulus of 995 MPa, ultimate tensile strength of 18.5 MPa, and toughness of 2.8 MJ m–3). The ink composition has a more significant impact on final properties than the printing parameters, albeit a decreased printing speed of 5 mm s–1 and increased pressure of 3.5 bar contribute favorably to properties. Finally, multiple layer objects were successfully printed for compositions having solid contents of 49 and 50.5% and OSL/HPC ratios of 40/60 and 50/50.