Heat-Triggered Cell-Wall Furfurylation Enables Ultrastable Wood Densification

Wood densification offers a route to high-performance structural materials, yet permanent fixation of highly compressed wood remains challenging because hygrothermal stimuli can activate stress relaxation and dimensional recovery of the cell wall. Here, we report a heat-triggered cell-wall furfurylation strategy for stabilizing highly densified wood. A ternary acidic catalyst package composed of NH4Cl, citric acid, and itaconic anhydride remains relatively mild at room temperature but exhibits progressively enhanced acidity during heating, thereby coordinating furfuryl alcohol penetration and early stage cell-wall plasticization with subsequent curing during hot pressing. Process-resolved compression measurements support a staged transition from plasticization-dominated deformation to progressive polymer-assisted fixation. The resulting densified wood exhibits set recovery below 0.5% after immersion in water at 95 °C, together with markedly improved resistance to fungal decay and termite attack. The pronounced softening effect enables a thickness reduction of 69% without visible macroscopic fracture, while the impact toughness remains 34.9% higher than that of untreated wood. The parallel-to-grain compressive strength and modulus of elasticity increased to 3.95 and 5.62 times those of untreated wood, respectively. Chemical imaging at the cell-wall scale and scattering analyses support the association of furfuryl-derived polymer with cell-wall regions, particularly lignin-rich domains, together with enhanced cellulose microfibril packing and orientation. This work establishes progressive heat-triggered regulation of cell-wall-confined furfurylation as a strategy for expanding the processing-property window of nondelignified densified wood, reconciling high-ratio deformation, near-complete hygrothermal fixation, and toughness retention.