Tuning Mesostructure to Enhance Flame Retardancy of Direct-Ink-Written Wood-Based Materials

Abstract Direct ink writing with wood flour and cellulose-based biopolymeric inks offers a sustainable and scalable approach for manufacturing recyclable building and packaging materials with tailored architectures. However, these materials remain flammable, and their cellulose-rich composition makes them susceptible to smoldering. Smoldering is a concealed, flameless combustion process that can persist undetected and generate localized hot spots that trigger the transition to flaming, posing fire hazards. Here, we show that fire resistance can be enhanced without additives by controlling pore morphology through the postprinting drying method. Freeze-dried specimens retain a highly porous network and form porous char during smoldering, promoting oxygen transport, extensive char oxidation to ash, and sustained propagation. In contrast, ambient-dried specimens form a dense, glassy, nonporous char through levoglucosan coalescence and solidification. This char restricts oxygen transport, weakens heat feedback, and rapidly quenches smoldering with minimal ash formation. SEM and Raman spectroscopy show that this contrast arises primarily from char mesostructure and pore topology rather than local carbon–carbon bonding. A coupled reaction–diffusion–heat-transfer model reproduces the transition from sustained propagation to quenching as effective oxygen diffusivity decreases. These findings establish that postprinting drying-rate control enables significant tunability of mesostructure for improving the fire safety of 3D-printed wood-based materials.