The study of physico-mechanical properties of SiO2-impregnated wood under dry and saturated conditions

This research has demonstrated that SiO2 impregnation under high vacuum pressure of − 90 kPa can significantly reduce porosity by almost 10%, and improve the mechanical and viscoelastic properties of spruce wood under dry and saturated states. Characterization methods, such as impact test, DMA, SEM, EDS, porosity and SAXS tests were conducted on non-treated and − 90 kPa treated spruce wood samples under dry, saturated and submerged states to analyze the synergistic effect of high vacuum SiO2 impregnation pressure on wood properties. The results showed that high vacuum impregnation pressure had a significant positive reinforcing effect on wood’s properties. It increased the impact resistance of wood under dry and saturated conditions. Additionally, the high vacuum impregnation technique was able to overcome the water-induced softening effect and caused a significant increase in the storage modulus through uniform dispersion of the SiO2 particles in the wood’s vascular structure. Consequently, the impregnation reinforced the wood and ameliorated its capacity to absorb energy. High vacuum impregnation was also able to counteract the plasticizing effect of the water molecules and significantly increased the loss modulus by increasing the internal friction and cohesion of the wood components with the addition of the nanoparticles to the vascular system, which increased the wood’s capacity to transform and dissipate energy. Quantitatively and qualitatively, impregnation under a vacuum pressure of − 90 kPa exhibited an effective obstruction of the vascular structure of spruce wood. In all conditions, high vacuum-impregnated samples showed significant enhancements over non-treated samples. This research demonstrated that high vacuum SiO2 impregnation is an effective wood processing technique. Multiple materials and applications could benefit from this research wherein high strain-rate deformations are expected to occur or when simultaneous elastic behaviour of wood and its damping energy is needed. This study could also pave the way for research on the synergistic effect of SiO2 impregnation and water absorption on the viscoelastic behaviour of wood.


Introduction
Wood exhibits good mechanical properties and a high strength-to-weight ratio.It also offers shorter construction times (Brunner et al. 2000;Burrows 2013).Additionally, wood is renewable and a good sustainable material when harvested and exploited appropriately.For many reasons, wood is an attractive material to the construction industry (Upland 2017).However, wood's physico-mechanical properties are dependent on its water content (Greer and Pemberton 2020;Olsson and Salmén 1997).
Most mechanical properties of wood are influenced and altered when its moisture content changes below the fibre saturation point (FSP) (Green et al. 1999;Wagner et al. 2015).This phenomenon is related to the plasticizing and softening effects of water molecules in wood (Wagner et al. 2015).Water molecules introduce free space between long cellulose molecular chains and facilitate their movement, which causes a significant change in physicomechanical properties.Moreover, the high water uptake capacity of wood makes it susceptible to degradation and lowers its durability (Illston et al. 2001).
Researchers and industries have developed many treatments to decrease the water absorption capacity of wood (Rowell et al. 1985;Williams and Feist 1999).However, one of the most commonly known and implemented solutions to mitigate wood's water uptake and enhance durability consist of externally applied coatings which are not highly effective as they do not prevent water uptake capacity in wood (Rowell et al. 1985).Over time, coated wood, just like non-coated wood, will swell due to the incompatibility of the products with the wood.Additionally, over time, the coatings will decay and wash away.Therefore, products need to be reapplied for optimal performance, which requires more maintenance (Rowell et al. 1985;Beaulieu and Biermeier 2020).A prospective method to decrease the water absorption capacity of wood to stabilize and enhance its mechanical properties consists of obstructing the wood's vascular system with dense materials like ceramics (Grosse et al. 2018;Xu et al. 2020;Zhang et al. 2019;Przystupa et al. 2020;Lin and Feng 2012).Ceramic nanoparticles, in an aqueous colloid state, can be used to fill this porous vascular structure (Boulos et al. 2017).Through this process, the wood imbibes a colloidal solution through the vascular system.As the liquid phase evaporates, the nanoparticles agglomerate, fill the lumens and obstruct the vascular structure.
Unlike the properties of wood that are negatively affected by moisture content (e.g., stiffness, hardness, and durability), the impact strength is enhanced under high relative humidity and saturated conditions (Bučar et al. 2015).However, this opposite behaviour limits the use of wood in applications in which energy absorption is required (e.g., guard rails post and off-set block, fence post, decking boards, etc.).In other words, although an increase in water content improves impact strength, it causes wood to lose its stiffness and become more susceptible to degradation.
The impact strength of a solid material depends on its ability to absorb and dissipate energy under instantaneous load and deformation (Bučar et al. 2015).

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Wood Science and Technology (2023) 57:1039-1059 However, the intricate nature of wood and the effect of moisture content make the impact behaviour of wood multifaceted (Mindess and Madsen 1986;Jansson 1992).The impact resistance of wood is dependent on fibre orientation.For coniferous trees, such as spruce, the impact resistance is higher in the radial rather than in the tangential direction (Bučar et al. 2015).The impact strength of wood is also related to its density.A higher density leads to higher impact strength (Bučar et al. 2015).A few studies have investigated the effect of moisture content on the impact strength of wood.However, the accuracy of the data is debatable.For example, Kollmann et al. (1984) showed in their study that impact strength increased with an increase in moisture content when the moisture content was above 20%, but the relationship was incongruous when the moisture content was below 20%.
The effect of water content on the impact strength of wood can be better revealed if the viscoelastic behaviour of wood is characterized in parallel.Viscoelastic properties (Storage Modulus, Loss Modulus, and Tan δ) are a measure and indication of the capacity of a material to absorb and dissipate energy (Tschoegl et al. 1989).Ultimately, all together, they identify the viscous and elastic responses of materials when they undergo oscillating loads or deformations (Tschoegl et al. 1989).When wood undergoes such loads, some energy is dissipated as heat (Green et al. 1999).Energy dissipation occurs through internal friction and is reflected in the viscoelastic properties, and the dissipation mechanism depends on the temperature and moisture content of the wood (Green et al. 1999).There is an optimal moisture content that varies with temperature below the Fiber Saturation Point (FSP) at which internal friction is minimal (Green et al. 1999).When moisture content drops below the optimal point, static friction increases, which improves energy absorption (i.e., storage modulus).On the other hand, when moisture content increases toward the FSP, kinetic friction increases, which improves energy dissipation (i.e., loss modulus).An increase in moisture content beyond the FSP reduces internal friction and thus both storage and loss modulus due to the wood softening and plasticizing effects of water molecules (Green et al. 1999).
In addition, there is an optimal temperature at which the internal friction is minimal; this temperature depends on moisture content.The temperature at which minimal internal friction is achieved is negatively correlated with moisture content.A decrease in moisture content increases the temperature threshold.For wood having a temperature above 0 °C and moisture content above 10%, the internal kinetic friction strongly increases with temperature rise.However, for dry wood, internal kinetic friction usually decreases as the temperature rises (Green et al. 1999).
Our previous research studied the effects of vacuum pressures on SiO 2 impregnation of wood in a dry state (Lemaire-Paul et al. 2023).The study revealed that SiO 2 impregnation performed at a high vacuum pressure of − 90 kPa was the optimal impregnation condition that showed the best results (Lemaire-Paul et al. 2023).The results revealed that − 90 kPa SiO 2 impregnation could significantly increase wood's density, reduce water absorption, and also simultaneously increase the storage modulus and loss modulus of spruce wood in an oven-dried condition (Lemaire-Paul et al. 2023).However, the synergistic effect of moisture content and SiO 2 impregnation on spruce wood's impact and viscoelastic properties are still unknown.

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Accordingly, this article will pursue the previous study by focusing on the − 90 kPa SiO 2 impregnation condition, taking that it is the optimal impregnation condition, and further characterize the aforementioned properties of spruce wood under dry and saturated conditions.Therefore, impact strength and the alteration of viscoelastic properties of samples before and after vacuum impregnation with respective to dry and saturated conditions will be analyzed.It is worth mentioning that there are currently few studies on the effect of SiO 2 impregnation on the mechanical properties of wood, as well as little information on their impact and viscoelastic behaviour.Therefore, this article is taking a step forward to fill the gap of knowledge accordingly.Additionally, this article will make use of scanning electron microscopy and small angles of X-ray scattering to further analyze and validate the results and hypotheses.

Materials
The samples used in this study were prepared from spruce wood of the Picea glauca species, commonly known as Canadian spruce or White spruce.Spruce is the most frequently used type of wood in construction in eastern Canada.LUDOX HS-40 colloidal silica, a water-based colloidal suspension of nano-silica, was used as an impregnation solution.This suspension contained a 40% solution of 12 nm SiO 2 particles, with a pH of 9.5, and has an aqueous density of 1.3 g/cm 3 at 25 °C.The solution was purchased from Sigma-Aldrich.

Sample preparation
The wood specimens were cut and sanded to the required dimensions for the tests and characterization techniques.The wood specimens contained both earlywood and latewood.The sample's dimensions (length × width × thickness) were: (25 mm × 10 mm × 2 mm) for dynamic mechanical analysis, (127 mm × 12.7 mm × 7 mm) for impact test, and (33 mm × 14.5 mm × 14.5 mm) for porosity analysis.The samples were progressively sanded using sandpaper with grits of 80, 100, 120, 220, and 600 to remove any residues on the specimens and reach a consistent surface roughness.The length of the samples was cut along the longitudinal wood fibres' axis, and the base cross-section (width and thickness) was in the tangential-radial plane.The samples were placed in an oven at 103 °C for 24 h prior to the tests to dry them in order to minimize their humidity content.To test the samples under saturated conditions, when required, samples were kept in distilled water at room temperature and weighed periodically until the difference between the two consequent measurements became less than 1%.To test the samples in the submerged condition when required, the samples were first saturated with the same procedure explained previously, and then further kept and tested under distilled water.

Impregnation process
The wood specimens were impregnated with the SiO 2 colloid using a dip-coating technique.Although the samples can naturally absorb the solution, air entrapped within the macropores will impede the effective impregnation of the sample.To mitigate this issue and improve the effectiveness of the impregnation, the process was conducted under vacuum pressure using the Buehler Cast-N-Vac 1000 vacuum chamber.The most effective vacuum impregnation pressure was previously determined in our previous research to be − 90 kPa (Lemaire-Paul et al. 2023).Accordingly, for this article, the samples were impregnated under negative pressures of − 90 kPa for one hour.Following the impregnation process, the samples were placed in an oven at 50 °C for one hour followed by 100 °C for five minutes to accelerate the agglomeration of the SiO 2 nanoparticles inside the vascular system.

Sample designations and descriptions
Table 1 presents the samples' designations with their respective descriptions.These designations will be used throughout the tables and figures presented in this paper.

Impact test analysis
The mechanical behaviour of treated and non-treated samples was assessed through impact testing under dry and saturated states.Twenty samples of each condition were tested.The tests were performed using a Zwick Roell HIT5.5PPendulum Impact Tester with a charpy setup with a 5.4 J hammer and by following the standard ASTM D6110.

Dynamic mechanical analysis under dry and submerged condition
The treated and non-treated wood samples were tested by Dynamic Mechanical Analysis (DMA) equipped with a submerged 3-point bending clamp setup.Ten samples of each condition were tested.The synergistic effect of the vacuum pressure impregnation process on the viscoelastic properties of wood (i.e., storage modulus, loss modulus, and Tan δ) under submerged conditions were studied.Accordingly, the viscoelastic properties were measured before and after each impregnation under dry and submerged conditions.Multi-strain tests were carried out using a TA Instruments DMA Q800 testing machine set to a frequency of 1 Hz and a temperature range of 5-40 °C with a heating rate of 5 °C/min.The temperature range was selected to analyze the effect of impregnation at ambient, below ambient, and above ambient temperature.The temperature range for testing in submerged conditions is restrained above 0 °C to prevent freezing, and below 80 °C to prevent high water evaporation.In conjunction, considering there was no significant change in properties over 40 °C, the upper limit of 40 °C was selected to save time.

Scanning electron microscopy
The treated and non-treated wood samples were analyzed with a Scanning Electron Microscope (SEM) to observe morphology and fracture mechanics at failure following impact tests.The fractures occur in the middle of the samples, which is the maximum distance from each extremity.Accordingly, after impact, the samples were cut in half at the middle location to reveal the inner section of the vascular system of the wood.Prior to SEM analyses, all samples were polished and coated with a vapourdeposited 20 μm layer of Pd-Au.Microscopy was carried out using a JSM-7500F FESEM SEM at 2-3 kV.

Porosity analysis
The porosity of samples before and after impregnation was determined by identifying the true and apparent densities of the samples.First, the true density of the samples was obtained using a micromeritics gas pycnometer (AccuPyc II 1340).Helium was used for the measurements.The density of each sample was measured three times to determine the mean value both before and after impregnation.Second, the apparent density of the samples was measured with respect to ASTM D2395 Test Method A. These two measurements were performed on each sample before and after impregnation.Accordingly, the porosity of the samples before and after the impregnation was calculated using Eq. ( 1), where Apparent Density is the density of the samples obtained using ASTM D2395 Test Method A, and True Density is the density of the samples obtained using the pycnometer.

Small angle X-ray scattering
Treated and non-treated wood samples were tested with Small Angle X-ray Scattering (SAXS) to complement the results obtained through microscopy.The results help to indicate the impregnation efficacy and confirm the presence of SiO 2 within the samples.These tests were conducted with a Bruker AXS Nanostar system. (1) Wood Science and Technology (2023) 57:1039-1059

Statistical analysis
A two-sample t-test was applied to compare the mean values of each key property (Livingston 2004).A confidence interval of 95% was considered as long as the P-value was below 0.05, indicating significant differences among the values.Consequently, the solid volume fraction of the wood sample (i.e., ApparentDensity TrueDensity ) increased thereby decreasing its porosity.Table 2 presents the statistical analysis conducted to compare the porosity of the samples before and after impregnation.The P-values of the statistical analyses revealed that the change in porosity before and after impregnation was significant.

Small angle X-ray scattering
Figure 2 displays the results of SAXS tests carried out on both non-treated and − 90 kPa SiO 2 -treated samples.The spectrogram of the − 90 kPa sample reveals a broad peak between 2 thetas of 0.5 and 1 degrees, indicating the presence of nanoparticles with a particle size of 11.8 nm (the conversion calculations were not included) in the wood structure that is absent in the non-treated spectrogram.Additionally, Fig. 3 shows that the count number for the − 90 kPa sample (73,586 counts) is twice that of the non-treated wood (36,191 counts).This finding suggests that the amorphous material exists in the form of particles, as evidenced by the significant increase in scattering count observed in the − 90 kPa sample.These results confirm a drop in porosity due to the agglomeration of SiO 2 nanoparticles in the void space within the vascular structure of the wood after impregnation.

Impact test analysis
Impact tests are destructive; accordingly, it was not possible to directly assess the effect of the SiO 2 impregnation by testing the same samples before and after impregnation.Instead, the different conditions were tested separately, and averages were compared.To minimize the effect of the substrate variability, a minimum of 12 specimens were tested under dry and saturated conditions.The impact test results of both the − 90 kPa and NT oven-dried wood samples are depicted in Fig. 4. As can be seen from the results, the − 90 kPa samples displayed substantially improved impact resistance (372.52 J/m) compared to the NT samples Fig. 2 SAXS test results for non-treated and − 90 kPa treated samples (270.63J/m).As can be seen in Table 3 which shows the statistical analysis of the impact tests, the results are significant.This suggests that the vacuum impregnation treatment was effective in enhancing the impact resistance of wood under dry conditions.Vacuum impregnation can alter the impact resistance of wood in two ways.Initially, the wood substrate's ductility is enhanced by the impregnation process.The vacuum assists in the infiltration of the alkaline liquid phase of collide through the wood's vascular structure, leading to the degradation-solubilisation of wood components (Lemaire-Paul et al. 2023).Consequently, the cellulose micro-fibrils become more mobile, resulting in greater wood ductility.This is achieved by removing lignin and hemicellulose and shortening the cellulose macromolecules.Moreover, it incorporates SiO 2 nanoparticles, resulting in improved impact resistance.This enhancement occurs through various mechanisms including: • Reinforcement: Nanoparticles such as silica, alumina, and carbon nanotubes have high fracture toughness.When these nanoparticles are diffused into the wood, they can reinforce the wood matrix, providing additional toughness to the wood.This reinforcement can enhance the wood's ability to absorb higher impact energies (Xu et al. 2020).• Interfacial strengthening: SiO 2 nanoparticles can form strong bonds with the hydroxyl groups of wood components due to their high surface area and reactivity (i.e., the ability to form hydrogen bonds).The interfacial bonds have the potential to enhance adhesion between the wood matrix and nanoparticles, ultimately increasing the energy required for crack propagation through the interface between the matrix and nanoparticles (Zhu et al. 2022, Zhang et al. 2017).• Damping: SiO 2 nanoparticles can also improve the damping properties of wood, which is the ability to absorb vibrations due to the increase in internal friction between wood and the nanoparticles (Lemaire-Paul et al. 2023).The damping effect can alleviate stress concentrations, preventing crack initiation and propagation and ultimately increasing the amount of absorbed energy under impact loads (Sujon et al. 2021, Novel et al. 2020).
Figure 5 shows the results of the impact tests of the − 90 kPa and NT samples under saturated conditions.The − 90 kPa samples demonstrated an average impact energy of 570.80 J/m in contrast to the NT samples with 281.61 J/m.As can be seen in Table 4 which shows the statistical analysis of the impact tests, the results are significant.
According to the results, − 90 kPa was 2 times more resistant to impact under saturated conditions compared to NT.It is worth noting that the − 90 kPa under saturated conditions exhibited a significant increase in impact resistance compared to the − 90 kPa under dry conditions.The enhanced impact resistance observed under saturated conditions may be attributed to the following mechanisms associated with the presence of water: • Water molecules can infiltrate the cell walls and increase the mobility of cellulose microfibrils.This heightened mobility results in increased internal kinetic friction and cohesion between the microfibrils (Brett et al. 2019), which in turn requires a greater amount of energy to initiate cracks.Additionally, the presence of water within the wood vascular structure introduces a viscous barrier against crack propagation, increasing the energy required to propagate cracks (Liyu et al. 2003).However, these two mechanisms should not be the main reasons for the substantial increase in the impact resistance of spruce wood, as the absorbed energy of dry and saturated NT showed only an insignificant increase.• Water molecules play a crucial role in strengthening and reinforcing mechanisms, by facilitating the formation of hydrogen bonds.In particular, the presence of water molecules mediates the agglomeration of nanoparticles and enhances the bonding between SiO 2 particles and hydroxyl groups in wood components.As a result, loosely bound or unbound SiO 2 particles agglomerate and reinforce the vascular system of spruce wood while increasing the interfacial adhesion between the agglomerated particles and cell walls.The authors suggest that these  factors synergistically contribute to the impact strength of saturated spruce wood.
The results of the study also confirm the synergistic effect of SiO 2 impregnation and water content at saturation on the impact resistance of spruce wood.

Scanning electron microscopy
Figures 6 and 7 present SEM micrographs of the fractured surfaces of the NT and − 90 kPa impact test samples in dry and saturated states, respectively.The SEM analysis was performed in conjunction with Energy Dispersive Spectroscopy (EDS) to survey the presence and dispersion of silica nanoparticles in the wood's structure.
The EDS micrographs with the element spectrum that highlight the sample's comare presented in Appendix A (see online resource 1).The low-magnification micrograph of dried NT (longitudinal-tangential plane) showed minimal deformation, as demonstrated in Fig. 6a, without any signs of defibrillation or fibre pullouts.Moreover, upon closer examination using higher magnifications, a brittle fracture surface with sharp edges was revealed (Fig. 6c).Micrographs of the NT specimens under saturated conditions showed distortions in fibre orientation (Fig. 6b).Furthermore, the defibrillation of cellulosic microfibrils was observed in Fig. 6d, resulting from the softening effect of water molecules that caused a slight increase (approximately 4%) in absorbed energy during impact testing.In other words, the presence of water molecules had a mild softening effect on the fracture mechanism, causing it to deviate slightly from a brittle mode of failure.However, this effect was not significant enough to transform it into a ductile fracture.The SEM analysis of the − 90 kPa samples under dry conditions showed that the samples underwent defibrillation along the longitudinal axis as demonstrated in Fig. 7a.This observation supports the proposed hypothesis of degradation and solubilization of wood components, including lignin and hemicellulose.Consequently, the fibres displayed increased mobility and defibrillation, implying that the sample could undergo more plastic deformation during the high strain rate impact test.Micrographs taken at high magnification (Fig. 7c) revealed the presence of sporadic SiO 2 particles, ranging from 0.5-5 µm, adhered to the inner walls of the lumens.The SEM micrographs, coupled with the EDS analysis in Appendix A (see online resource 1), confirmed the existence of silicon and oxygen in these particles, providing evidence of the presence of SiO 2 .
The morphology of the fractured surface underwent significant changes in crack propagation and impact resistance in the saturated state.The low-magnification micrograph revealed the presence of fibre pullouts, as shown in Fig. 7b, due to alterations in crack propagation pathways.As discussed in the previous section, the presence of water improved interfacial adhesion between particles and lumen cell walls and facilitated uniform precipitation of SiO 2 particles on the inner walls of the lumens.This strengthened the walls and altered the crack propagation path, forcing it to cross the lumens (i.e., in the direction of the tangential axis) at some weak points.As a result of these changes, fibre pullouts occurred, causing the fibres to detach from the wood structure by overcoming the friction force.This dissipated energy and increased the impact-absorbed energy.Moreover, upon high-magnification observation, it was revealed that water molecules played a mediating role in the agglomeration of SiO 2 particles within the size range of 2-10 µm.

Dynamic mechanical analysis in dry and submerged conditions
Figure 8 shows the dynamic mechanical analysis results obtained from − 90 kPa and NT in dry and submerged conditions.The dry − 90 kPa samples exhibited the highest storage modulus of 5923 MPa, followed by the dry NT samples, the submerged − 90 kPa, and the submerged NT samples with the storage moduli of 3725 MPa, 1720 MPa, and 1000 MPa, respectively.The results showed that in the dry condition, the − 90 kPa sample had 1.6 times more rigidity than non-treated wood.The tests under submerged conditions showed a similar increase in magnitude in the storage moduli (1.72 times) of the − 90 kPa and NT samples.It should be noted that both NT and − 90 kPa experienced a significant loss in their storage moduli of at least 70% after being submerged in water.However, the drop in storage modulus was less pronounced for − 90 kPa than NT.It is worth mentioning that the impregnation process successfully reinforced spruce wood and it was expected that the storage modulus of the samples would increase according to the rule of mixture (Callister et al. 2018).However, after submersion, water molecules acted as plasticizers, causing a substantial decrease in E′ as the cellulose fibrils became more mobile due to the abundance of water molecules in the cell walls.It is important to note that the samples were already saturated.Despite the plasticizing effect of water molecules, the presence of SiO 2 particles and the strong interfacial adhesion between the particles and the cell wall (i.e., cellulose microfibrils) prevented the plasticizing effect from exceeding 76% at 25 °C.In contrast, the plasticizing effect of water molecules on NT resulted in a greater reduction of up to 84%.The P-values revealed that the change in storage modulus was significant for these comparisons (Table 5).
The results of the dynamic mechanical analyses conducted on the − 90 kPa and NT samples under dry and submerged conditions are presented in Fig. 8c and d, respectively.The data demonstrate an increase in the loss modulus (E′′) of both NT and − 90 kPa as the temperature was raised from 5 to 40 °C.While − 90 kPa exhibited a higher E′′ compared to NT, both samples showed an approximately 11% increase in E′′.It is known that loss modulus is a viscoelastic property that characterizes the amount of energy dissipation through dynamic internal friction.The impregnation process resulted in a degradation-dissolution phenomenon, which increased the mobility of cellulosic microfibrils, leading to an increase in internal kinetic friction between the fibrils and the wood matrix.This increased friction is the reason for the higher loss modulus observed in the − 90 kPa sample.However, the increase in temperature had an indifferent impact on the E′′ of both NT and − 90 kPa under dry conditions.This is because temperature affects the internal energy of materials, which is independent of microstructure and depends on their vibrational mode (i.e., atomic and crystalline structures, as well as temperature).
As discussed previously, the addition of water to wood changes its mechanical behaviour by acting as a plasticizer.Typically, plasticizers, like water molecules, increase the free volume in the wood matrix, enabling cellulose chains to move more easily and reducing the dissipation of energy.However, this plasticizing effect was less pronounced for − 90 kPa due to the interfacial adhesion of cellulose microfibrils with SiO 2 particles, which hindered their movement.In contrast, under submerged conditions, E′′ increased between 5 °C and 6.1 °C for NT and between 5 °C and 8.2 °C for − 90 kPa.This observation highlights the complex nature of the plasticizing effect of water.The increase in E′′ could be due to the higher viscosity of water at low temperatures.Specifically, within the range of 5-6.1 °C and 5-8.2 °C for NT and − 90 kPa, respectively, the water molecules acted as a viscous body linked to the interior surface of lumens via hydrogen bonds.Another interesting finding is that the difference between the loss modulus of NT under dry and submerged conditions between 5 and 6.1 °C was statistically insignificant.However, this difference was statistically significant for − 90 kPa.One possible explanation for this behaviour is the formation of a higher number of hydrogen bonds between water molecules and SiO 2 particles deposited on the interior surface of lumens.This led to an increase in internal kinetic friction, ultimately enhancing the loss modulus.
Figure 8 displays the Tan δ results of the − 90 kPa and NT samples under both dry and submerged conditions.Tan δ is the ratio of the loss modulus to the storage modulus, which measures the damping energy of a material.Generally, materials with high Tan δ exhibit predominantly viscous behaviour, while those with low Tan δ are characterized by elastic properties.Among the samples, the submerged NT samples exhibited the highest Tan δ value of 0.23, followed by the submerged − 90 kPa samples with 0.17.This finding confirms the greater plasticizing impact of water on NT, which resulted in a higher rate of decrease in storage modulus relative to the loss modulus, leading to a softer wood matrix.It is important to note that while Tan δ is a measure of damping in materials, this property is related to low-strain rate deformation, and we cannot expect the same behaviour in high-strain rate deformation, such as impact tests.As mentioned in the impact test section, the softening effect of water in wood slightly ameliorated its impact resistance, but it is not the primary mechanism of enhancement for impregnated samples.Table 5 summarizes the statistical analyses conducted to compare the viscoelastic properties of the − 90 kPa and NT samples under dry and submerged conditions.The results revealed that the change in Tan δ before and after impregnation under dry and submerged conditions was significant for all the samples and conditions except for − 90 kPa dry compared to non-treated dry over 25 °C.A decrease in Tan δ means a reduction of viscous behaviour and an increase in elastic response.These results confirmed that impregnation had a positive effect on the stabilization and enhancement of spruce wood properties under dry and submerged conditions.

Conclusion
The research findings reveal that applying SiO 2 impregnation under a high vacuum pressure of − 90 kPa leads to a nearly 10% reduction in porosity and enhanced mechanical and viscoelastic properties in spruce wood under both dry and saturated conditions.To investigate the combined impact of these conditions and vacuuminduced SiO 2 impregnation on wood attributes, several characterization techniques were employed.These methods included impact testing, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), porosity testing, and small-angle X-ray scattering (SAXS).These analyses were carried out on untreated and − 90 kPa treated spruce wood samples in various states: dry, saturated, and submerged.
SAXS results indicated effective SiO 2 impregnation under higher vacuum pressure, with noticeable SiO 2 presence in the wood's vascular structure.SEM and EDS analyses demonstrated agglomerated SiO 2 particles within the vascular system of spruce wood.Across all conditions and states, samples impregnated under high vacuum pressure exhibited significant impact strength improvement.Impact resistance increased by 1.38 times in dry conditions and 2 times in saturated conditions.Post-fractography analysis confirmed the enhancement of impact resistance through reinforcement, interfacial strengthening, and damping mechanisms, particularly pronounced when water was present.
The vacuum-induced impregnation substantially increased storage modulus by 1.6-fold (dry state) and 1.72-fold (submerged state).Additionally, loss modulus experienced a 1.44-fold (dry) and 1.95-fold (submerged) increase for − 90 kPa compared to non-treated samples.Notably, non-treated and − 90 kPa samples exhibited reduced loss modulus following submersion.Vacuum impregnation countered the water-induced softening and plasticizing effects by maintaining dynamic internal friction between cell walls and SiO 2 particles.This research's implications extend to various materials and applications, particularly those involving rapid loading and deformation or simultaneous elastic behaviour and damping energy in wood.Furthermore, this study opens avenues for exploring the synergistic effects of SiO 2 impregnation and water absorption on wood's viscoelastic behaviour.To advance the research, durability tests under

Figure 1
Figure1shows the results of the porosity analysis conducted on − 90 kPa before and after impregnation.The results showed that − 90 kPa impregnation caused a reduction in porosity by almost 10%.Accordingly, SiO 2 impregnation conducted under − 90 kPa of vacuum pressure was confirmed effective and capable of significantly reducing porosity.Based on our previous study, it was found that the impregnation process effectively removes the trapped air within the wood's vascular system and improves the permeability of the colloid into the open pores of the wood (Lemaire-Paul et al. 2023).After the drying process, the nanoparticles tend to aggregate, forming micrometric particles, resulting in the occupation of the void space with solid material.In this respect, our previous study showed that − 90 kPa impregnation significantly increased the density by 118 kg/m 3 (Lemaire-Paul et al. 2023).Consequently, the solid volume fraction of the wood sample (i.e., ApparentDensity TrueDensity ) increased thereby decreasing its porosity.Table2presents the statistical analysis conducted to compare the porosity of the samples before and after impregnation.

Fig. 1
Fig. 1 Porosity change before and after impregnation

Fig. 3 Fig. 4
Fig. 3 2D-SAXS patterns obtained from Non-Treated and the − 90 kPa treated samples with the X-ray beam directed to the radial axis of the samples

Fig. 5
Fig. 5 Impact strength of the − 90 kPa treated and non-treated saturated wood samples

Fig. 6
Fig. 6 Fractograpghs of the NT samples under dry (a-c) and saturated conditions (b-d)

Fig. 7
Fig. 7 of the − 90 kPa samples in dry (a-c) and saturated conditions (b-d)

Fig. 8
Fig. 8 Dynamic mechanical analysis results of the treated and non-treated samples in the dry and submerged conditions

Table 1
Sample designations and descriptions *Suffix: sub = under submerged state, sat = under saturated state **No suffix = dry state

Table 3
Statistical analysis of the impact strength of the − 90 kPa treated and non-treated oven dry wood samples

Table 4
Statistical analysis of the impact strength of the − 90 kPa treated and non-treated saturated wood samples

Table 5
DMA test results of the − 90 kPa dry and submerged storage modulus, loss modulus, and Tan δ before and after the SiO 2 impregnations