The densification of solid wood is a well-studied technique that aims to increase the strength and hardness of the material by permanently compressing the wood tissue. To optimise the densification process in this study, a pre-treatment with sodium sulphite was used (delignification). With delignification prior to densification, one achieves higher compression ratios and better mechanical properties compared to densification without pre-treatment. The reactivity of syringyl (dominant in hardwoods) and guaiacyl (dominant in softwoods) lignin towards delignification is different. The influences of this difference on the delignification and densification of softwoods and hardwoods need to be investigated. This study aimed to densify wood after delignification and investigate how variations in chemical composition between coniferous and deciduous species affect the densification process. Scots pine and Eurasian aspen specimens with a similar initial density were investigated to study the influence of the different lignin chemistry in softwoods and hardwoods on the densification process. Both timbers were delignified with sodium sulphite and sodium hydroxide and subsequently densified. While the delignification was twice as efficient in aspen than in pine, the compression ratios were almost identical in both species. The Brinell hardness and compressive strength showed a more significant increase in aspen than in Scots pine; however, one exception was the compressive strength in a radial direction, which increased more effectively in Scots pine. Scanning electron microscopy (SEM) revealed the microstructure of densified aspen and Scots pine, showing the crushing and collapse of the cells.

Wood modification is an excellent way to improve material properties, prolong service life, and pave the way for new applications for timber in the built environment. The aim of this study is to establish the influence of wood species in the modification process with phenol urea formaldehyde resin. Seven hardwoods and four softwoods were compared in an identical treatment. Since the wood species is the main variable, the study provides a clear insight as to its influence on modification—something not previously investigated. Small wood blocks of all eleven species were impregnated with low-molecular weight phenol urea formaldehyde resin and were subsequently heat-cured at 150 °C. Mass and dimensions of specimens were carefully monitored throughout three cycles of oven-drying and water-soaking to determine anti-swelling efficiency, bulking coefficient, and swelling coefficient. The results showed a clear relationship between density, bulking-, and swelling coefficients. Beyond this well-known relationship it was observed that species with similar density (e.g., lime and sycamore) had significantly different bulking coefficients and anti-swelling efficiencies. Reasons for these differences, which are independent of density, are discussed. The soaking solution was analysed to determine the nature of leached substances showing a correlation between the amount of leachate and the UV-absorbance (272 nm) of the solution.

Wood modification using low molecular weight thermosetting resins improves the biological durability and dimensional stability of wood while avoiding increasingly regulated biocides. During the modification process, resin monomers diffuse from the cell lumen to the cell wall, occupying micropore spaces before in situ curing at 150 degrees C. This study investigated the mechanism of cell wall diffusion at multiple scales, comparing two test groups where diffusion was either facilitated or restricted. Antiswelling efficiency tests demonstrated improved dimensional stability when diffusion was facilitated. Differential scanning calorimetry showed that bound water was excluded more effectively from the cell wall if cell wall diffusion was enabled. Solid-state NMR spectroscopy (1H MAS and 13C MAS) with relaxation time analysis indicated that resin migrated to distinct locations within the cell wall, influenced by diffusion and drying conditions. These findings highlight how optimizing cell wall diffusion can significantly improve the performance of wood modification processes using thermosetting resins.

Biological pretreatment with Phanerochaete chrysosporium was evaluated as a route to facilitate thermo-mechanical densification of poplar. Twenty-four specimens were incubated for 16 weeks either with vermiculite (enhanced decay) or without, then hot-pressed radially from 25 mm to 8 mm, while maintaining a temperature of 120 °C. Vermiculite-assisted samples showed a much higher mean mass loss (29.0% vs. 5.9%) but a lower mean densification degree (74.4% vs. 115.8%) compared to non-vermiculite samples. Set recovery after 72 h at high relative humidity remained approximately 40% for both variants. Mechanical testing revealed that vermiculite-assisted specimens suffered significant longitudinal strength loss (L 10.1 ± 5.6 N·mm⁻²). In contrast, the non-vermiculite samples showed significant gains in radial strength (R 31.4 ± 10.7 N·mm⁻²) compared to undensified poplar controls. The applied fungal pretreatment did not result in selective delignification sufficient to improve dimensional stability after densification.