Preservation of wood structure using stabilized alkaline bleaching agents: a novel and mild approach to enhance whitening for lignin-rich wood template

Raw balsa wood was purchased from Specialized Balsa Wood, LLC in the USA in two different thicknesses: 1 and 3 mm. The density of the balsa wood ranges from 0.15 to 0.17 g/cm³. Raw birch wood with a thickness of 1 mm was provided from VNT Trinciati S.p.a (Italy), with a density of 0.70 g/cm³. Birch wood with a thickness of 3 mm was sourced from Woodenave in China, with a density of 0.70 g/cm³. Both species were cut into two sets of 50 sample units, with known dimensions of 25 × 25 × 1 mm³ and 25 × 25 × 3 mm³ (tangential x longitudinal x radial). This was done to guarantee an accurate comparison between the two wood species.

For the bleaching process, a 30% v/v hydrogen peroxide (H₂O₂) solution (supplied by Sigma Aldrich), potassium hydroxide (KOH) granules (Sigma Aldrich, purity ≥ 85%), and sodium bicarbonate (NaHCO₃) powder (Solvay S.p.a, purity: 99%) were used.

An alkaline H₂O₂ solution was prepared by dissolving KOH granules in 30% v/v H₂O₂ to reach a 2 M concentration. Sheets of balsa wood, with thicknesses of 1 and 3 mm, were immersed in 20 mL of such solution at room temperature. The wood: solution ratio was 1:10 g/mL (1:16 v/v). The samples were left to soak at room temperature in a 60 mm diameter glass crystallizer under the action of a glass weight for 2 h, until completely bleached. Then, they were immersed in a 0.5 M NaHCO₃ solution to wash and stabilize the final color. Afterwards, the samples were rinsed thoroughly with deionized water to remove residual components and dried in an oven at 65 °C overnight.

The procedure used for balsa wood was applied to birch specimens, with the difference that samples were treated twice with the alkaline H₂O₂ solution for 4 h, until complete bleaching. The wood: solution ratio was 1:3 g/mL (1:16 v/v).

The morphological analysis was performed using Field-emission Scanning Electron Microscopy (FESEM, Hitachi S4000, Tokyo, Japan) to analyze the wood’s hierarchical structure and porosity features before and after bleaching.

The percentage of lignin removed was determined gravimetrically using an analytical balance ED224S Extend (Sartorius), with measurements repeated on five samples of each type. For each sample, the weight before and after treatment was measured after drying in an oven at 105 °C for 4 h. The percentage of lignin removed was calculated using Eq. 1:

The UV-visible analysis was conducted using an Avantes AvaSpec-ULS2048XLUSB2 model spectrophotometer (Apeldoorn, The Netherlands) alongside an AvaLight HAL-S-IND tungsten halogen light source. The detector and light source were linked via fibre optic cables to a probe with a 1.5 mm diameter FCR-7UV200-2-1.5 × 100. The incident and detecting angles were both set at 45° from the surface normal to eliminate specular reflectance. The spectral range of the detector was from 350 to 1100 nm, and the analysis was collected from 400 to 750 nm.

The ATR-FTIR structural analyses were performed on bleached and raw wood sheets using a Nicolet IS50 IR spectrometer (Thermo Fisher Scientific). The spectra of the analyzed samples were collected at an IR beam temperature of 35 °C, with 32 scans performed at a 4 cm^-1 resolution in a spectral range of 4000 to 550 cm^-1.

The relative lignin content was estimated from FTIR spectra by calculating the ratio between the intensity of the aromatic skeletal vibration of lignin at 1509 cm⁻¹ (I₁₅₀₉) and the intensity of the C–H deformation in cellulose at 893 cm⁻¹ (I₈₉₃​). The relative lignin content for each treated sample was then expressed as a percentage with respect to the corresponding untreated native wood, considered as 100%, according to the Eq. 2:

For each specimen, spectra were collected on three different points of the sample surface, and the reported values correspond to the average of these measurements.

The relative crystallinity percentage was determined using an Empyrean X-Ray Diffractometer (XRD) from Malvern Panalytical, equipped with a Cu source at 40 kV and 40 mA. The samples were tested in the 2θ range of 5–40° with a scan rate of 3°/min. The diffractometer was equipped with soller slits (0.04 rad), anti-scatter slit (P7.5), beam mask (10 mm), divergence (¼ °), and axial (½ °) slits. All wood slices were dried at 60 °C for 24 h before analysis. In particular, through Segal’s method, the crystallinity percentage was determined using Eq. 3:

Where: (i) C_rI is the percentage of relative crystallinity; (ii) I₀₀₂ represents the maximum intensity of the diffraction angle for the cellulose I₀₀₂ lattice; (Popescu2009) and (iii) I_am is the scattering intensity of the amorphous background diffraction at 2θ, which is approximately 18°.

The diffractograms were deconvoluted using a fitting procedure with the OPUS V. 5.5 software to calculate the entire crystalline contribution. Multiple runs were performed to achieve the minimal residual error, with a Root Mean Square Error (RMSE) of less than 2 × 10⁻⁴. Five components were included, utilising five Gaussian bands to deconvolve the crystalline phases (101, 10 − 1, 012, and 200) and the amorphous halo. The resulting fitting curve closely matches the experimental curve. Following the deconvolution, the crystalline index was successfully determined using Eq. 4:where the term I_c, which replaces I₀₀₂, indicates the total area of the crystalline component. This value is derived from the sum of the integral contributions attributed to the deconvoluted crystallographic planes 101, 10 − 1, 012, and 200.

The specific surface area (SSA) of the wood samples was analyzed utilizing N₂ physisorption measurements performed at 77 K. Before analysis, the samples were outgassed at 70 °C for 16 h, resulting in a residual pressure lower than 10^− 6 Torr. The N₂ measurements were performed in the relative pressure range from 5 × 10^− 5 to 1 P/P₀ using a Quantachrome Autosorb 1MP instrument. The apparent surface areas were determined using the Brunauer-Emmett-Teller (BET) equation within the relative pressure range from 0.05 to 0.25 P/P₀. Pore size distributions and cumulative pore volume were obtained by applying the QSDFT model of N₂ at 77 K on carbon (slit pore, equilibrium model).

The tensile strength of the samples was determined using a Shimadzu AGSX tensile/compressive machine. A total of twenty samples measuring 10 × 100 × 1 mm³ and twenty samples 10 × 100 × 3 mm³ (tangential x longitudinal x radial) were tested. The twenty samples, consisting of ten balsa wood and ten birch wood, were divided into two sets of five samples each. One of these sample sets for the two chosen species was treated with a whitening solution before testing. Sample preparation and tensile tests were performed in accordance with ISO 13061-6. Tests on treated and untreated balsa and birch samples are the average of five measurements conducted for each sample. The displacement rate was 1 mm/min, with the clamp distance ranging from 60 to 70 mm. A 1 kN force sensor was used for the thinner samples, while a 50 kN force sensor was employed for the thicker samples. Strain was measured as the ratio of cross-head displacement to the initial distance between the grips. Stress was calculated as a ratio of force to the cross-sectional area.

The pictures of the 3-mm samples obtained by the discoloring process are shown in Fig. 1, both in the fiber direction and in the inner cross-section. These images confirm a homogeneous bleaching, with a uniform white color characterizing both surface and bulk of the specimens, overcoming the thickness limitations reported in previous studies (< 2 mm) (Li et al. 2017; Xia et al. 2021a).

The color of the wood may vary depending on the treatment conditions. In an alkaline environment, lignin may undergo chromogenic reactions, leading to darkening (Liu et al. 2023). However, when combined with a mild oxidizing agent, such as H₂O₂, chromophore units, including benzene rings, quinoids, vinyl groups, phenolic hydroxyls, and carbonyls, are deconjugated (Ding et al. 2022), generating intermediates such as coniferaldehyde and quinones while preserving the overall lignin macrostructure.

This effect is achieved through the synergistic combination of H₂O₂ and the basic activator KOH, without the need for external energy sources.

KOH’s high solubility in concentrated H₂O₂ ensures uniform distribution of the reagents, while careful control of reaction conditions prevents instability, uncontrolled exothermic reactions, or hazardous byproduct formation, preserving the structural integrity of the wood. The optimized combination of reagents allows deep penetration into both low-density and high-density structures of the sample, enabling effective treatment of wood species with thicknesses from 1 to 3 mm, and up to 1 cm for balsa, far exceeding previous thickness limitations (Li et al. 2017; Xia et al. 2021a). Operating at room temperature with carefully selected concentrations balances penetration efficiency and safety, minimizing the risk of lignin or cellulose degradation and preventing premature H₂O₂ decomposition. Treatment times of 2–4 h ensure complete saturation of the wood, promoting uniform bleaching throughout the sample.

During the process, chromophore structures are selectively reacted or removed while non-participating lignin is preserved.

The bleaching protocol involves two steps (Fig. 2): (i) immersion in an alkaline H₂O₂ solution until complete bleaching (2–4 h); (ii) a subsequent washing phase with aqueous NaHCO₃, which stabilizes the pH and the final color. This step acts as a buffer: as H₂O₂ is gradually consumed, NaHCO₃ prevents KOH from promoting the reconjugation of previously deconjugated lignin chromophores. By maintaining a controlled pH, this washing phase ensures uniform whitening and preserves the integrity of the bleached wood.

To validate the predicted penetration and bleaching uniformity, this method was applied to wood types with different densities and hierarchical structures. The results confirm the effective diffusion of reagents across various wood architectures, achieving uniform lignin modification while maintaining structural integrity, and demonstrating the versatility and scalability of the proposed treatment.

The influence of the bleaching treatment on the microstructure was evaluated through FE-SEM analysis. Figure 3 shows balsa wood (a–d) and Fig. 4 birch wood (a–d) before and after treatment.

The balsa wood sample was chosen as the representative example because it is the most sensitive wood species to chemical treatments. Specifically, its lower density, reduced lignin content, high porosity, and thinner cell walls permit easier access to the lignin within its hierarchical structure (Alqrinawi et al. 2024).

As shown in Fig. 3 at the microscopic level, low-magnification images (a and c) reveal that both samples exhibit the typical structure of balsa wood fibers, characterized by porosity with irregular polygonal shapes resembling hexagons (Borrega et al. 2015). The orientation of wood fibers is parallel to the tree’s growth direction, creating an orderly structure.

The pore diameters of both samples, at the microporous hierarchical level, range from 10 to 50 μm. As a result, no significant structural changes are observed before and after treatment, nor is there any fiber collapse during the delignification process (Liu et al. 2019).

A combination of lignin and hemicellulose mainly causes strong adhesion between neighboring fibers. The images at high magnifications (see images b and d, Fig. 3) show that the lignin, which is predominantly located in the middle lamella between the cell walls, has been preserved.

In contrast, the birch samples (Fig. 4a–d) exhibit a denser and more compact structure, with smaller and more uniformly distributed lumina compared to balsa. At low magnification (Fig. 4a and c), the characteristic arrangement of hardwood vessels is evident, showing no evidence of collapse or major structural alterations after bleaching. High-magnification images (Fig. 4b and d) confirm that lignin remains preserved in the middle lamella, ensuring adhesion between fibers and maintaining the integrity of the wood structure.

This observation confirms that the bleaching treatment with alkaline H₂O₂ effectively maintains the wood’s structure without creating new microporosities that occur with traditional delignification methods (Zhu et al. 2016a; Li et al. 2016b; Wu et al. 2019; Montanari et al. 2020).

The effect of lignin removal can be observed by monitoring changes in the colour of the wood surface and measuring the percentage of mass loss.

A UV-visible analysis was conducted to measure the reflectivity of the white template after the bleaching treatment and is reported in Fig. 5. Every treated sample shows a high reflectance level at 550 nm (92–97%), indicating that the samples produced using this approach are suitable for creating transparent composites (Li et al. 2017). Additionally, the samples made with this method demonstrate superior reflectance compared to the previously used bleaching techniques. For example, Xia et al. (2021b) reported an average reflectance of 93% for balsa wood.

The difference in measured reflectance values was smaller for birch wood than for balsa wood. This outcome may be attributed to the lower diffusion of the solution in birch wood, which has a higher density and lower porosity than balsa wood. As a result, the deeper chromophore groups in the lignin may be less accessible, reducing bleaching efficiency.

A gravimetric analysis was conducted to evaluate the effectiveness of the bleaching process in performing a semi-quantitative assessment of the percentage of all the removed components compared to the untreated sample. This treatment allows for the retention of most of the lignin, unlike conventional delignification methods, which typically result in the removal of over 80% of lignin and a total mass loss of around 25–26% for both balsa and birch wood (Li et al. 2017; Zhang et al. 2024; Alves-Ferreira et al. 2021).

In addition to lignin removal, certain cellulose, hemicellulose, and extractives in the raw sample may also be removed during extraction. Thus, lignin preservation is evaluated with further structural analysis. Table 1 presents the weight loss from both types of wood studied for the two thicknesses (1 and 3 mm).

The data presented in Table 1 demonstrates a similar trend for both balsa and birch samples. Specifically, as the wood’s thickness increases, the percentage of mass loss decreases. This percentage decreases from 9.6 to 8.6% for the balsa samples, while for the birch samples, from 7.0 to 6.4%. This result is due to a reduced diffusion of reagents within the bulk of the material, which leads to a less complete deconjugation of lignin and a lower solubilization of hemicellulose, resulting from the more challenging access to components in the central zone from the basic solution.

The higher mass losses for low-density wood confirm that balsa is more susceptible to overall structural modifications under the treatment conditions, due to its lower density, lower lignin content, and higher cellulose ratio. In contrast, denser hardwoods like birch necessitate more aggressive treatment conditions to achieve comparable overall modification of wood components (Alqrinawi et al. 2024; Santos et al. 2011). Nevertheless, the total weight loss varies between 9.5 and 6.5% for both wood species under mild room temperature conditions. These results are lower than the previous bleaching treatment proposed by Li et al. (2017), ranging from 12 for balsa to 10.6% for birch. In addition, the lower weight loss with our treatment could lead to a further high volume fraction transparent wood composite because a higher initial wood content in the white wood template leads to a lower resin content after infiltration and polymerization. It is crucial to consider the final composite’s sustainability and mechanical properties. Many resins used are petroleum-based, (Zhu et al. 2016b; Li et al. 2016b) and incorporating a wood template enhances the mechanical properties of the infiltrating resin (Li et al. 2016b).

Fourier-transform infrared spectroscopy in attenuated total reflectance (FTIR-ATR) was performed to assess the impact of the bleaching process on the chemical and physical structure of the treated balsa and birch samples. This assessment was conducted by analyzing the changes in signals related to the functional groups present before and after the lignin deconjugation process.

The analyses, as shown in Fig. 6, allowed for identifying the peaks corresponding to the lignocellulosic components present in the materials.

For clarity, the band’s assignments are presented in Table 2.

Due to wood’s significant structural and compositional variability, it is highly complex to correlate the vibrational mode of the functional groups that compose only lignin with the corresponding peaks. However, some of the distinguishable peaks can be associated with aromatic rings and can be compared to understand whether there are structural changes before and after treatment. By comparing the spectra of balsa and birch woods, both in their natural and bleached states, for the two thicknesses (see Fig. 6A-D), we observe that the intensity and shape of the broadband between 4000 and 2750 cm-1, which is related to the stretching of -OH groups, remain consistent. The peaks at 2919 and 2852 cm-1, which correspond to the stretching of the -CH₂ bonds in holocellulose, are also preserved. This behaviour indicates that the structure of the cellulose and hemicellulose skeleton remains unchanged after treatment with alkaline H₂O₂ (Yang et al. 2018).

The band at 1737 cm-1, associated with the balsa sample (Fig. 6A and C) and the birch sample (Fig. 6B and D), is attributed to hemicellulose and/or lignin and is absent after bleaching. This result is due to the cleavage of the lignin and hemicellulose ester groups due to the alkaline hydrolysis induced by KOH in the bleaching solution (Agarwal and McSweeny 1997; Horikawa et al. 2013). The bands at 1594 and 1509 cm-1, related to the aromatic ring stretching of lignin, remain stable before and after treatment for both types of wood. This demonstrates that most of the lignin has been deconjugated through the action of H₂O₂, which selectively acts on the chromophore groups while preserving its aromatic structure (Li et al. 2017; Wójciak et al. 2010). In addition to the qualitative observations, a semi-quantitative evaluation of the residual lignin was conducted by calculating the ratio of absorbance at 1509 cm⁻¹ to the absorbance at 893 cm⁻¹, which is associated with the β-glycosidic vibrations of cellulose, as reported in the literature (Kostryukov et al. 2022; Sun et al. 1997; Tryjarski et al. 2022). The analysis revealed that the relative lignin content decreased by approximately 18% in the balsa samples (1 and 3 mm) and by 27% in the 1 mm birch samples, and by 26% in the 3 mm birch samples, respectively (see Table S1). The differences in relative lignin loss between balsa and birch can be attributed to the distinct structural properties of the two species: balsa’s lower density, higher porosity, and thinner cell walls facilitate easier penetration of the bleaching solution, whereas birch’s denser structure and thicker cell walls limit diffusion, leading to variations in the degree of lignin deconjugation as detected by the surface-sensitive FTIR measurements.

The band at 1240 cm-1, associated with the stretching of the C-O bond in hemicellulose, is absent after treatment of both the balsa and birch bleached samples. This confirms the occurrence of alkaline hydrolysis in the presence of the basic agent. Finally, the bands at 1024 and 897 cm-1 related to the C-O ester stretching vibrations in methoxyl and β-O-4 linkages in lignin and the bending in the plane of the aromatic -CH groups of the lignin macromolecule, respectively, appear unchanged for all bleached materials, confirming the occurrence of deconjugation and not the complete removal of lignin, leaving the aromatic skeleton intact. The analyses shown indicate that after our process, a large part of the hemicellulose was removed through basic attack, while the lignin structure was only deconjugated. Furthermore, it can be assumed that the original molecular structure of cellulose was maintained even after the chemical treatment at room temperature. This behaviour may suggest that the treatment with H₂O₂ at basic pH is more selective for the deconjugation of lignin and the removal of hemicellulose, and does not degrade cellulose, in contrast to the delignification treatments. It is also important to note that the relative lignin values obtained from FTIR analysis are semi-quantitative and sensitive to surface changes. Therefore, they cannot be directly compared to the absolute Klason lignin contents reported in the literature. Instead, these values provide insights into structural modifications and relative compositional changes.

An X-ray diffraction analysis was conducted to assess the impact of bleaching treatment on the crystalline structure of wood samples.

Figure 7 presents the X-ray diffraction patterns for two types of wood samples at two thicknesses: raw and bleached balsa (diffractograms A and C for raw samples at 1 mm and 3 mm, respectively, and diffractograms A’ and C’ for bleached samples at the same thicknesses). Diffractograms B and D relate to raw birch samples at 1 mm and 3 mm, respectively, and diffractograms B’ and D’ for the corresponding bleached samples.

To evaluate the relative crystallinity of the samples, both the Segal method (Segal et al. 1959) and the Rietveld method (Rietveld 1967, 1969) were applied, as reported in the literature (Xu et al. 2013). The Segal method, which provides a general estimation of the crystallinity index, is reported in Table 3 together with Fig. S1 (Supporting Information). The Rietveld method, instead, involves deconvolution with Gaussian curves designed to refine and model the peaks, thus considering contributions from the entire crystalline component. This analysis is presented in Fig. 7; Table 3.

Table 3 reports the crystalline content of the wood samples calculated using both selected methods.

As demonstrated in Fig. 7, an analysis of the data reveals that the position of diffraction peak 200 in the area related to the presence of crystalline cellulose I remains unchanged for the bleaching of both balsa (Fig.s A’ and C’) and birch wood (Fig.s B’ and D’). This observation indicates that the bleaching process does not affect the crystallinity of cellulose type I (Lionetto et al. 2012). This type of cellulose is associated with the most intense peak in the diffractogram, as it is the most abundant, ordered and stable crystalline organisation of cellulose in wood and plants. (Newman 1994; Kulasinski et al. 2014) However, an increase in both relative and total crystallinity was observed and calculated using the Segal and Rietveld methods for all the samples except for balsa wood with 1 mm thickness. This finding suggests that the treatment influenced the amorphous regions by partially removing most of the hemicellulose and a small part of the lignin (9.6–6.4%, see Table 1) and rearranging the remaining hemicellulose, cellulose and lignin components (Xu et al. 2013). The results indicate that lignin and hemicellulose in the cell walls were partially degraded during bleaching due to basic attack at room temperature. As previously mentioned, KOH facilitates the hydrolysis of amorphous cellulose by breaking the ether bonds in the molecular chains, allowing the exposed hydroxyl groups to recombine through hydrogen bonding (Liu et al. 2023; Xu et al. 2013). As demonstrated in Table 3, which utilizes the Segal method to estimate the degree of crystallinity of cellulose type I, divergent responses may be observed between the two types of wood following treatment. These findings are consistent with results reported in the literature for balsa wood, which shows a crystallinity degree of 80–90% (Borrega et al. 2015), and birch wood, which ranges from 40 to 60% (Wikberg and Maunu 2004). The percentage of crystallinity in balsa wood remains stable before (82.9 and 81.2%) and after treatment (83.8 and 84.4%) for the two analyzed thicknesses. In contrast, birch exhibits a significant increase in crystallinity after bleaching, rising from 66.0 to 70.2% to 93.9 and 80.9% for 1 and 3 mm, respectively. Utilising the Rietveld method, which incorporates the contributions of all crystalline components, a discernible discrepancy in the behaviour of the two wood types upon treatment is evident (see Table 3). For the thicker balsa, an increase in crystalline content is shown, while a decline in crystalline content is evident in thinner balsa wood, decreasing from 64.4 to 54.7%. This reduction can be attributed to the bleaching solution’s strongly oxidizing action in the thinner and lower-density balsa wood, which leads to the unselective deconjugation/removal of part of the amorphous component and causes partial damage to the crystalline component (Alqrinawi et al. 2024; Quintana et al. 2015). On the contrary, for the two birch wood samples, an increase in the degree of crystallinity was observed even when utilizing the Rietveld method. The percentage increases from 50.2 to 84.0% at 1 mm and from 73.5 to 84.0% at 3 mm. This fact is attributed to the more orderly arrangement of cellulose microfibrils in the amorphous region following basic treatment (Tyufekchiev et al. 2019). In this instance, the amorphous lignin component exhibits enhanced resistance, making it more difficult to modify or remove while simultaneously protecting the crystalline part from degradation (Higuchi et al. 1977; Lourenço and Pereira 2018). Consequently, the oxidizing alkaline solution is an effective preservation agent for the cellulose component, achieving this by selectively targeting the amorphous fractions and as a consequence, the crystalline content associated with cellulose is enhanced (Lionetto et al. 2012). The assessment of the material’s crystallinity post-treatment was monitored, as higher crystallinity in the bleaching treatments indicates superior fibre orientation and less amorphous loss, which could lead to enhanced mechanical properties (e.g., fracture toughness) compared to the weak delignified woody materials (Frey et al. 2019).

Since the Rietveld method considers the entire contribution of all crystalline phases, unlike the Segal method that primarily targets the 002 diffraction plane, the results from the Rietveld refinement were deemed more reliable. Consequently, these results were used for further correlation with the mechanical properties discussed in Sect. "Mechanical properties".

The textural properties of the materials before and after treatment were analyzed to assess the impact of the process on pore size and distribution. Nitrogen (N₂) physisorption analysis was conducted at 77 K on wood samples, each 3 mm thick, including natural and bleached balsa and birch. The resulting isotherms are presented in Fig. S2 (Supporting Information). The N₂ physisorption isotherms for these wood materials are classified as type V. All hysteresis loops extend to low relative pressures (below 0.4 P/P₀) and remain visibly open throughout desorption.

In woody materials, Wang et al. (2023) suggest that the open hysteresis loops of N₂ at cryogenic conditions result from the weak interaction between the gas and the particular hierarchical structure of wood as indicated by the low value of the constant C (Table 4) calculated from BET model applied on the isotherms and which limits the accessibility of nitrogen in porosity (Lowell et al. 2012; Fukuzumi et al. 2013). Indeed, using the constant C derived from the BET model, it is possible to describe the intensity of the interaction between the adsorbed gas and the material’s surface. When the values of C are < 10, as in this case, the gas adsorption on the surface is unselective, resembling a liquid-like condensation (Lowell et al. 2012). Nevertheless, the C values can still be deemed reliable for applying the BET model (Lowell et al. 2012).

The analysis of the pore size distribution curves in Fig. 8 reveals the presence of micro- and mesopores in all the materials examined. These pores exhibit a dimensionality that varies uniformly between 15 and 100 Å, with a maximum distribution centred at approximately 20 Å in all cases. However, when the total porous volumes are considered, it becomes evident that the values related to the pore distributions are negligible (Fig. 8A; Table 4).

In particular, the overall quantity of adsorbed N₂ is lower for natural birch and balsa wood, at approximately 4.8 and 8.7 m²/g, in accordance with the literature data (Garemark et al. 2023; Montanari et al. 2020). The specific surface area calculated by the BET method for bleached balsa and birch wood is assessed at 44.3 and 41.8 m²/g, respectively (Table 4), with a total pore volume of 0.054 and 0.066 cc/g, respectively (Fig. 8B; Table 4). In addition, the overall pore volume of the two materials was observed to be lower than that of the two treated materials analyzed, with values of 0.018 and 0.024 cc/g, respectively (see Fig. 8B; Table 4). The slight variation in surface area aligns with the treatment applied to the materials, which aims to modify lignin in situ rather than remove it. This approach helps prevent microporosity formation in the cell wall, which could lead to partial detachment of the cellulose fibres and structural weakening (Liu et al. 2019). Deep delignification treatments can significantly increase the surface area and microporous volume of the resulting wood (Montanari et al. 2020, 2023). Consequently, the excessive increase in microporosity of white wood templates may create greater challenges for later infiltration with transparent resins, leading to longer processing times to achieve optimal performance in terms of high transmittance and low haze.

Tensile strength tests were conducted to evaluate the mechanical behaviour before and after treatment. The stress-strain curve of the treated and untreated wood is shown in Fig. S3 (Supporting Information). This analysis is important because the properties, interactions, and arrangement of components in wood materials directly affect their mechanical properties. Treatments designed to modify or selectively remove specific components, such as lignin and hemicelluloses, can significantly alter the mechanical characteristics of natural wood (Zhang et al. 2013). Bleaching treatments can notably increase the crystallinity of cellulose microfibrils, which strongly adhere to each other due to rigid lignin, leading to enhanced mechanical properties (Ottesen et al. 2019; Wang et al. 2014).

Figure 9 illustrates the ultimate tensile strength of the two types of wood examined in this study, comparing results before and after treatment at two different thicknesses: 1 mm and 3 mm.

Table 5 shows a clear correlation between cellulose crystallinity (CrI) and the mechanical behaviour of the wood species studied (see Table 3). The crystallinity values discussed in this section, and reported in Table 3, are derived from the Rietveld method, which was selected as the most reliable approach since it considers the entire crystalline contribution rather than only the intensity of the 002 reflection. For 1 mm birch, CrI increased significantly from 50% to 84% after treatment, resulting in a higher tensile strength of ~196 MPa compared to ~162 MPa in the native wood. This improvement is attributed to better alignment of cellulose fibrils and partial removal of amorphous lignin and hemicellulose (~7% of the total). In contrast, 1 mm balsa showed a slight decrease in CrI from 60% to 55%, while both birch and balsa exhibited a small increase in relative crystallinity via the Rietveld method, accompanied by a corresponding decrease in ultimate strength.

The effects of the bleaching treatment on mechanical properties correlate with changes in cellulose crystallinity (C_rI). Three trends were observed: (i) decreased properties when C_rI decreases; (ii) preserved properties when C_rI slightly increases; and (iii) improved properties when C_rI significantly increases.

Both 1 mm birch and 1 mm balsa samples were selected for this study. The thin 1 mm thickness allows the chemical solution to access the entire structure, making any modifications to the cellulose–lignin network more pronounced.

For 1 mm birch, selective deconjugation of lignin at room temperature preserves cellulose, promotes fibril alignment, and maintains lignin as a natural binder, enhancing rigidity and strength while supporting stress transfer along the fibers. Its rigid structure complements the flexibility of cellulose, allowing stress to be effectively transferred along the wood cell walls, which enhances compressive strength and dimensional stability (Kurei et al. 2024; Salmén et al. 2016). During processing, structural changes in lignin can further improve its bonding capabilities, illustrating its versatility as a binder (Wang et al. 2016). Unlike traditional delignification, which removes most lignin and compromises mechanical support (Li et al. 2017; Wu et al. 2019; Xia et al. 2021b; Bolognesi et al. 2024), this approach retains structural integrity.

In contrast, 1 mm balsa exhibited a moderate reduction in CrI (from 60% to 55%) and a corresponding decrease in tensile strength (from 22.7 to 10.4 MPa). Despite the preservation of about 80% of lignin and the absence of visible structural collapse in SEM images, the lower density and higher porosity of balsa make it more sensitive to slight changes in the cellulose–lignin network. The partial removal and redistribution of amorphous domains may induce local heterogeneities and a weaker inter-fibrillar interface, reducing stress transfer efficiency (Alqrinawi et al. 2024; Li et al. 2017). In low-density woods such as balsa, even mild chemical modification can lead to disproportionate mechanical losses, as mechanical strength relies more on cell wall cohesion than on overall lignin content (Alqrinawi et al. 2024).

In thicker or denser samples (3 mm birch), limited solution diffusion reduced fibril alignment, slightly decreasing mechanical performance, while balsa, being more porous, showed smaller differences. These results indicate that both CrI and lignin retention are crucial for maintaining mechanical integrity, and that optimizing solution penetration could further improve the properties of thicker samples.

Moreover, our approach to delignification and bleaching presents several advantages over traditional methods: (i) it is eco-sustainable, utilizing minimal hydrogen peroxide, avoiding using on external energy sources (like light or UV), and resulting in alkaline water as a by-product that can be readily purified for future applications of the recovered base (KOH) until it is completely exhausted. Additionally, it generates no toxic gases during H₂O₂ decomposition, unlike delignification or other bleaching techniques that emit volatile chlorine, sulfur compounds or ozone; (ii) the oxidation of chromophore groups is enhanced by the base’s more penetrating action, which releases K⁺ ions, occurring faster than in previously mentioned delignification and bleaching methods. Additionally, this method preserves the wood volume fraction with a total weight loss of less than 10%. To showcase the scalability of our process and the simplicity of solution diffusion at room temperature, we prepared a sample measuring 100 × 100 × 10 mm³ (longitudinal ×  tangential ×  radial) in just 3 hours of treatment performed on a dry sample, as shown in Fig. 10. Balsa was selected for this demonstration because its lower density facilitates homogeneous bleaching at larger thicknesses, allowing us to highlight the feasibility of extending the process to centimeter-scale specimens.

Conventionally, preparation times for samples of this thickness range from 6 to 14 h and are generally conducted on wet samples. Additionally, retaining a significant portion of lignin as a binder facilitates the creation of mechanically durable wood samples compared to traditional techniques, resulting in improved optical properties (92–97% reflectance) for both low- and high-density wood types after bleaching.