A recent article published in the Journal of Bioresources and Bioproducts proposed a self-densification strategy to fabricate super-strong wood by reassembling highly aligned wood fibers as functional units.
Notably, the process did not require hot pressing, and the resulting self-densified wood exhibited better mechanical properties than compressed densified wood and traditional metals like aluminum alloys.
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Background
Natural wood is a traditional building material widely employed in construction. However, the strength of natural wood is considerably lower than steel; it is often insufficient for advanced engineering applications.
Super-strong wood is generally synthesized via mechanical pressing in a direction perpendicular to the wood growth. This unidirectional compression entirely collapses the wood lumina and the porous cell walls.
Alternatively, compression densification of wood considerably improves its longitudinal and transverse tensile strength in the direction perpendicular to the compression.
However, the transverse tensile strength of compressed wood is significantly inferior to that of natural wood when the load axis is parallel to the compression direction, mainly due to stress concentrations, micro defects, viscoelastic nature, and shape memory effects.
Therefore, a new isotropic densification approach is necessary to enhance wood’s mechanical properties and structural stability in all directions for structural applications.
Methods
Natural wood blocks were submerged in a boiling lignin-eliminating solution comprising Na2SO3 and NaOH for 10 hours, followed by immersion in boiling deionized water to eliminate residual chemicals.
Subsequently, the partially delignified wood blocks were submerged in N,N-dimethylacetamide at 160 °C for one hour to remove residual water and activate wood cellulose.
Subsequently, LiCl was added to the solution, and the swelling reaction was performed at 100 °C for one hour, followed by placing the specimen at ambient temperature for 10 hours. Finally, the swelling wood sample was washed with deionized water, air-dried at ambient temperature, and spontaneously shrank to produce the self-densified wood.
The morphologies of the swelling wood, partially delignified wood, and self-densified wood were observed via scanning electron microscopy. Meanwhile, X-ray diffraction (XRD) patterns of self-densified and natural wood were recorded.
The chemical modifications in wood during delignification and swelling were analyzed by Fourier transform infrared spectroscopy in the attenuated total reflection mode.
The National Renewable Energy Laboratory (NREL) approach was adopted to quantify the lignin proportion in natural and self-densified wood. Finally, the wood samples’ mechanical properties, including tensile, three-point bending, and compressive properties, were tested using a universal test machine.
Results and Discussion
Super-strong, self-densified wood was synthesized via the proposed microstructural regulation approach.
During this process, natural wood shrunk uniformly in the transverse region while preserving its longitudinal dimension, yielding a 79% volume reduction and an almost threefold increase in density; the density became comparable to that of compressed wood.
The mechanical properties of self-densified wood in the longitudinal direction far exceeded those of natural wood and various compressed wood types. The tensile strain-stress curves of self-densified and natural wood exhibited a linear deformation trend before failure.
However, the self-densified wood’s tensile strength was approximately 496.1 MPa, 9 times more than that of natural wood. Additionally, the tensile strength of self-densified wood was similar to or even better than compressed wood.
Self-densified wood exhibited an excellent specific strength of 396.8 MPa·cm3/g, making it a promising, extremely lightweight engineering material. Additionally, it maintained excellent toughness, with its fracture work surpassing 10 times that of natural wood.
Additionally, its impact toughness (75.2 kJ/m²) was six times greater than that of natural wood. Thus, the conflict between toughness and strength was effectively resolved in self-densified wood, making it appropriate for high-strength and -toughness applications.
The considerable improvement in mechanical properties was ascribed to the distinct self-densification at different scales. Specifically, the self-densified wood maintained a highly dense and ordered structure.
During densification and tensile fracture surface, extremely aligned nanocellulose fibers were compacted and organized longitudinally, enhancing the interfacial area among the fibers.
At the molecular scale, during swelling and dehydration, the excess hydroxyl groups in cellulose molecular chains facilitated hydrogen bond development, destruction, and redevelopment.
Thus, the densification process promoted the development of several hydrogen bonds between the cellulose chains.
Conclusion
Overall, the researchers successfully demonstrated a self-densified, super-strong wood via microstructural regulation. The proposed method involved releasing cellulose fibrils from the lignin confinement through partial delignification.
These cellulose fibrils moved inward and filled the cell lumen via swelling and concluded with air-drying.
This distinct self-densification process endowed the resulting wood with a uniform microstructure and enhanced mechanical properties in all directions compared to natural wood.
The improvement in mechanical properties compensated for the performance heterogeneity due to compression in conventional densified wood and overcame its limitation of being applicable only in longitudinal load-bearing scenarios.
The developed self-densified super-strong wood is highly attractive for many structural and engineering applications, including complex mechanical scenarios. This versatility was demonstrated in this study through the fabrication of a wood nail.
Journal Reference
Huang, D. et al. (2025). Self-densified super-strong wood. Journal of Bioresources and Bioproducts. doi:10.1016/j.jobab.2025.03.001.https://www.sciencedirect.com/science/article/pii/S2369969825000167
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