Wood flour and Kraft lignin enable air-drying of the nanocellulose-based 3D-printed structures

The predominant technique for producing 3D-printed structures of nanocellulose involves freeze-drying despite its drawbacks in terms of energy consumption and carbon footprint. This study explores the less-energy-intensive drying approach by leveraging the valorization of forest residual streams. We utilized wood flour and Kraft lignin as fillers to facilitate room-temperature drying of the nanocellulose-based 3D printed structures. Various ink formulations, integrating cellulose nanofibers, wood flour, and lignin, were tested for direct ink writing (DIW). The formulations exhibited shear-thinning behavior and distinct yield stress with rising stress levels, ensuring the effective flow of the ink during DIW. Consequently, multilayered objects were printed with high shape fidelity and precise dimensions. Lignin and wood flour prevented structural collapse upon room-temperature drying. A reduced shrinkage was observed with the addition of lignin in freeze and room temperature drying. Moreover, the room-temperature dried samples were denser and demonstrated significantly higher resistance to applied compressive force, surpassing those reported for cellulose-based 3D composites in the existing literature. Remarkably, the trade-off effects of lignin are highlighted in terms of efficient stress-distributing and micro-scale sliding, enabling better strength. Along with wood flour, it further increases thermal stability. However, lignin hinders the hierarchical porous structure, the main ion transportation channels, reducing the double-layer capacitance of the carbonized structures. Overall, the results underscore the potential of all-biobased formulations for DIW for practical applications, highlighting their enhanced mechanical properties and structural integrity via the more sustainable drying method.