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38 TECHNOLOGY JUL/AUG 2025 FDM ASIA | www.fdmasia.com

Marko Milivojevic photography Mechanical Performance
The longitudinal mechanical properties of self-densified wood
not only far exceed those of natural wood, but also significantly
outperform those of many widely studied compressed densified

woods.
Fig. 3a compares the tensile stress-strain curves of natural
wood and self-densified wood. Both curves show a linear
deformation behaviour before tensile failure. The tensile strength
Note that the cellulose fibrils were swelling, not dissolution. of the self-densified wood is approximately 496.1 MPa, which
No fine, disordered, or poorly aligned regenerated cellulose is 9.0 times higher than that of the original wood (55.3 MPa).
fibres were observed in the SEM images. The cell, nanofiber Fig. 3b compares the tensile strength of self-densified
bundles, and microfibrils within the wood maintained their wood, natural wood, and compressed densified wood, a
longitudinal alignment without significant changes. widely recognised method for enhancing strength. The results

During the swelling process, the zoomed-in SEM images show that self-densified wood performs similarly to or even
show that the density of cellulose microfibrils in the wood cell better than compressed densified wood in tensile strength.
wall significantly decreased from 288 to 102 fibres per μm2, Self-densified wood exhibits exceptional specific strength,
demonstrating the successful swelling process. reaching 396.8 MPa/(g·cm–3), making it a promising new
This process is essential to the following self-densification ultra-lightweight engineering material. It not only significantly
as it effectively exposes more hydrophilic hydroxyl groups of enhances tensile strength but also maintains exceptional
cellulose fibrils, which would greatly facilitate the re-formation toughness, with its fracture work exceeding ten times that of

of H-bonds between adjacent nanofibers. natural wood and its impact toughness of 75.2 kJ/m² is six
The crystalline structure of the self-densified wood was times greater than natural wood, similar to compressed wood.
found to be similar to that of natural wood (cellulose I), This effectively resolves the conflict between strength and
indicating that the crystalline structure of cellulose remained toughness in traditional materials, highlighting its superior
unchanged during the LiCl/DMAc swelling process. mechanical properties, making it highly suitable for high-
Finally, the swelling wood was washed with distilled water strength, high-toughness applications.
and air-dried at room temperature. The fibrils in wet swelling The Shore D hardness also significantly increases from
wood were covered with water molecules that bond weakly 42° to 87°. The compressive strength of the self-densified
with the –OH groups of cellulose. wood is about two times higher than that of natural wood.

This weak hydrogen bonding led to a looser interfacial The flexural strength of self-dense wood along the growth
connection between cellulose and water molecules, thereby direction is approximately five times that of natural wood.
weakened the synergistic interactions between fibres. The results of studies have shown that, similar to natural
During water evaporation, the capillary pressure in the wood, self-dense wood exhibits consistent flexural strength
xylem conduit caused spontaneous shrinkage of the bulk when the bending load is applied parallel to the wood's
wood. As the adjacent cellulose fibrils came into contact, transverse plane.
and stronger hydrogen bonds formed between the hydroxyl In contrast, compression-dense wood demonstrates

groups of cellulose molecules (CNC-CNC), ultimately resulting significant directional dependence. When the bending load
in a highly densified and compact structure. axis is parallel to the compression direction, the longitudinal