Self-densified wood beats aluminum: a game-changer for green construction

Wood’s reputation as a low-carbon building block has been held back by one stubborn fact: even the best construction lumber is ten times weaker than steel. Mechanical densification—cooking planks in chemicals and squeezing them at 150 °C—has pushed strength above 500 MPa, but only along the grain; perpendicular properties fall, and the boards swell back when wet. A new study from Nanjing University now shows that wood can be coaxed to densify itself, no press required, yielding isotropic, metal-beating performance while remaining completely renewable.

The trick, reported in the Journal of Bioresources and Bioproducts, is to free cellulose nanofibrils from the rigid lignin matrix and let them migrate. Basswood blocks are first boiled in an alkaline sulfite bath that dissolves roughly half of the lignin. The partly delignified scaffolds are then soaked in lithium-chloride dimethylacetamide (LiCl/DMAc) at 100 °C. The solvent swells the cellulose, loosening hydrogen bonds and causing fibrils to balloon into the empty cell lumens. When the samples are simply air-dried, capillary tension pulls the network inward from every side, shrinking the cross-section by 79 % while the longitudinal dimension stays almost unchanged. The collapsed cell walls fuse into a compact, brick-like microstructure laced with newly formed cellulose-to-cellulose bonds.

Mechanical tests on the 1.1-mm-thick sticks reveal tensile strength of 496 MPa—nine times that of natural basswood and on par with aerospace-grade 6061-T6 aluminum. Because the shrinkage is symmetrical, the gain is not limited to the grain direction: transverse tensile strength jumps from 9 MPa to 63 MPa, eliminating the notorious weak plane that causes conventional densified boards to split. Charpy impact energy reaches 75 kJ m⁻², six-fold higher than untreated wood, while Shore D hardness climbs from 42 to 87, comparable to hard plastics. Even a wooden nail machined from the material drives through three 1-cm basswood layers without pre-drilling and withdraws at 310 N, outperforming a commercial steel wire nail of the same diameter.

Scanning electron microscopy shows that the self-densified fibres remain perfectly aligned, eliminating kink bands and micro-cracks that act as stress concentrators in hot-pressed wood. X-ray diffraction confirms that the cellulose-I crystal lattice is preserved, accounting for the extraordinary specific strength of 397 MPa cm³ g⁻¹—among the highest ever recorded for a bulk bio-based material. Importantly, the process works on low-density balsa as well, raising its tensile strength fourteen-fold and hinting that any fast-growing plantation species could be upgraded.

The team calculates that producing one cubic metre of the super-wood generates less than 150 kg of CO₂, roughly one-twentieth of the footprint for the same volume of aluminum. With no synthetic resin or energy-intensive pressing, the route is easily scaled to existing pulp mills, the authors say. They are now talking to automotive and prefabricated-building suppliers about prototype floor panels and electric-vehicle battery casings that could shave hundreds of kilograms off finished products.

The National Natural Science Foundation of China and the National Key R&D Program funded the work. Data and supplementary images are available through the journal’s website.