The Influence of Impregnation Methods and Curing Conditions on the Physical and Multiscale Mechanical Properties of Furfurylated Bamboo

Abstract

Furfurylation is an effective and green method for wood or bamboo modification that can significantly improve its physical and mechanical properties and the resistance against biological deterioration and the attack of subterranean termites. To elucidate the effect of furfurylation on the physical and multiscale mechanical properties of bamboo, the conditions of the furfurylation process were modified to cause an independent variation of the physical and multiscale mechanical properties in differently-treated bamboo samples. This was achieved by impregnating bamboo samples with solutions containing 15%, 30%, 50%, or 70% furfuryl alcohol (FA) by either of the two impregnation processes, vacuum pressure (V-P) and soaking (S) impregnation, while applying different curing conditions (wet- or dry-curing). The physical properties we measured included the absorption rate, weight percent gain (WPG), swelling efficiency (SE), and anti-swelling efficiency (ASE); the macro-mechanical properties involved the modulus of rupture (MOR), the modulus of elasticity (MOE), parallel-to-grain compressive strength (CS), and tensile strength (TS); the micro-mechanical properties included the tensile strength of bamboo’s vascular bundle and hardness and the indentation modulus of bamboo’s fiber cell walls. Finally, the correlation between the different physical and mechanical properties of the modified bamboo samples was analyzed. The results indicate that V-P impregnation made bamboo more permissible for the penetration of FA, while wet-curing was more conducive to ensuring a high curing rate. The dimensional stability of the bamboo samples treated with a high FA concentration through V-P impregnation and of those furfurylated by the S-Wet process using either medium or high FA concentrations was significantly increased. However, the dimensional stability of the bamboo samples modified with either low or medium FA concentrations decreased in both dry and wet curing. In terms of mechanical strength, furfurylation had little effect on the macro- and micro-mechanical properties of bamboo and was slightly improved in comparison to untreated samples. The results also showed a positive correlation between the macro- and micro-mechanical strength of the modified bamboo samples and a significant negative correlation between the mechanical strength and ASE. In soaking impregnation, the WPG and ASE were positively correlated, while the WPG and CS were negatively correlated. Interestingly, the correlation between the mechanical properties and ASE was not significant. Finally, both V-P-Wet and S-Wet approaches can be recommended for bamboo furfurylation, the former being time-saving and having a high curing rate in FA resin while significantly improving the moisture absorption and mechanical strength of bamboo. The advantage of the latter process is simplicity, a high utilization rate of FA, and a significant improvement in the dimensional stability of bamboo.

1. Introduction

Bamboo is an important forest vegetation worldwide, with integrated economic, ecological, and social benefits, and has utility in construction, furniture production, and other fields. However, bamboo exhibits several undesirable characteristics, such as easy cracking deformation and susceptibility to degradation by fungi and insects. These features are mainly caused by high levels of extractable polysaccharides present in bamboo. The extractable polysaccharides, such as starch and hemicellulose, contain plenty of hydroxyl groups, causing bamboo to become highly hygroscopic [1,2]. Numerous studies have been carried out on bamboo to improve its dimensional stability and durability. For instance, copper salt preservatives for wood, chitosan–copper complexes, and metal-free preservatives have been used to protect bamboo from decay by fungi [3,4,5]. However, the loss of preservatives is the main problem limiting the application of these preservatives in bamboo treatment. In addition, the dimensional stability of bamboo was found insufficiently improved by the preservative treatment. In recent years, heat treatment technology and chemical modification have also been used to protect and increase the utilization value of bamboo. Some studies suggest that heat treatment can effectively improve hygroscopicity, dimensional stability, anti-mildew properties, and decay resistance of bamboo [6,7] and the mechanical properties decrease obviously, when the heating temperature or treatment time exceeds a certain value [8]. Melamine formaldehyde, urea formaldehyde resin, and phenol formaldehyde resin were also used to improve the strength properties and the dimensional stability of bamboo [9,10]. However, the penetration effect of resin on bamboo is poor and it is difficult to reach a satisfactory load due to the anatomical structure of bamboo [11].

Furfurylation is an effective and green wood modification method, which is performed with a modification solution including FA monomers (or oligomers), water, catalysts, and stabilizers, which impregnate into the cell wall and are followed by polymerization in situ to form resin at elevated temperatures [12,13,14]. In the early 1950s, the furfurylation of wood was introduced by Dr. Alfred Stamm. The primary research focused on the selection of catalysts and the furfurylation effect on the physical–mechanical properties of modified wood. Various types of catalysts, such as zinc chloride (ZnCl₂) [15,16], maleic anhydride [17,18,19], citric acid [13,20,21], composite acidic catalysts [22], tartaric acid [23,24], and acrylic acid [25], have been investigated. Among them, maleic anhydride and citric acid have been the most widely used in the wood furfurylation industry. In recent years, researchers have used co-modification technology by simultaneously adding reagents and pretreating them with FA to increase the use value of the furfurylated wood and give it new functions. For instance, Dong et al. [26] modified fast-growing poplar wood with FA and nano-SiO₂. They suggested that the addition of nano-SiO₂ compensated for the reduction of the modulus of rupture of the FA-treated wood and gave it better flame retardant and thermal stability. In a different study [27], wood was modified by the coprecipitation of ferric and ferrous ions and then furfurylated to improve magnetic stability, acid resistance, and dimensional stability. A co-modification method that combinates aluminum sol-gel and furfurylation was also used to modify poplar wood, and the results showed that the addition of aluminum-based sol-gel further improved the water absorption and hydrophobicity of the modified sample [28]. Liu et al. [29] suggested that long and flexible aliphatic chains of epoxidized soybean oil (ESO) could increase the flexibility of FA resins through the ring-opening polymerization reacting with the FA and validated the application of ESO to increase the toughness of furfurylated wood. Yang et al. [30] adopted a two-step treatment that included a partial hemicellulose removal and furfurylation, reduced access to the hydroxyl group of fast-growing poplar wood (Populus euramericana), and further improved the dimensional stability of furfurylated wood.

Overall, furfurylation can significantly improve the physical and mechanical properties, as well as wood resistance against biological deterioration and the attack of subterranean termites. In addition, FA is a low-weight organic compound with a strong polarity that can easily penetrate the wood cell walls [12,31] and may have good applicability in bamboo that is characterized by low permeability. In our earlier studies, the authors attempted to evaluate if furfurylation could be applied in bamboo and found that furfurylated bamboo demonstrated excellent resistance to fungi and termite decay, with the WPG of FA as low as approximately 10%. In addition, we found that FA resin was not only distributed in cell cavities but also infiltrated the bamboo’s fiber cell walls, which was supported by indentation analysis and FA-IR imaging [32,33]. However, the effects of vacuum pressure impregnation and soaking impregnation on the dimensional stability of furfurylated bamboo were different. The former impregnation method significantly reduced the dimensional stability of bamboo furfurylated with 15%–50% FA, while the dimensional stability of bamboo furfurylated through soaking impregnation was improved [34,35]. It is well known that the structure of bamboo differs significantly from that of wood. At the anatomical level, bamboo is a natural composite material composed of parenchyma and a vascular bundle while lacking transverse transmission tissues, such as wood rays. Therefore, it can be assumed that the effect of the modification process on the macroscopic distribution of chemical compounds and the intrinsic properties of bamboo is more pronounced than that of wood.

Being a thermosetting resin, FA increases the thermal curing process when used to modify wood or bamboo, as compared to the traditional preservative treatment. Therefore, in addition to the FA concentration and impregnation method, curing conditions are the main factors that affect the diffusion and distribution of FA resin in bamboo during furfurylation.

To systematically investigate the effects of different modification processes, we aimed at assessing how FA concentration, impregnation process, and curing conditions affect the physical and multiscale mechanical properties of furfurylated bamboo. Physical properties included the absorption rate, WPG, swelling efficiency (SE), and anti-swelling efficiency (ASE). Macro-mechanical properties involved the modulus of rupture (MOR), the modulus of elasticity (MOE), parallel-to-grain compressive strength (CS), and tensile strength (TS), while micro-mechanical properties included the tensile strength of bamboo’s vascular bundle and hardness and the indentation modulus of bamboo’s fiber cell walls. Finally, the relationship between the different physical and mechanical properties of modified bamboo was analyzed by correlation analysis.

2. Materials and Methods

2.1. Materials

Furfuryl alcohol, sodium borate, citric acid, and oxalic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Analytical-grade chemicals and deionized water were used. Moso bamboo (Phyllostachys edulis (Carrière) J. Houz) strips were purchased from a bamboo flooring enterprise located in Zhejiang Province, China. Bamboo samples were processed to the dimensions provided in Figure 1a, subsequently oven-dried at 103 °C and weighed, and finally conditioned in a humidity control box (23 °C/65% RH) at least 30 days before furfurylation.

2.2. FA mixture Preparation

Furfuryl alcohol mixtures of various concentrations were prepared at room temperature, and they contained (w/v) 15%, 30%, 50%, or 70% FA, a 2% buffering agent (sodium borate), a 1.75% mixed organic acid catalyst (composed of 2 parts citric acid and 1-part oxalic acid), and deionized water. The listed chemicals were added in the respective order and thoroughly mixed using a magnetic stirring device.

2.3. Furfurylation of Bamboo

The bamboo samples were divided into five groups. The specific impregnation and curing processes of each group of samples are shown in

Table 1. The treatment process of bamboo furfurylation.

Group	FA	The Impregnation Process	The Curing Process
V-P-Wet	15	15 min vacuum impregnation followed by 1.5 MPa pressure impregnation for 1 h and 40 h of immersion in the mixture of FA and deionized water at atmospheric pressure.	Bamboo specimens were cured by wrapping in aluminum foil at 105 °C for 3 h.
	30
	50
	70
	W
S-Wet	15	Soaking in the mixture of FA and deionized water for 42 h at room temperature.
	30
	50
	70
	W
V-P-Dry	15	15 min vacuum impregnation followed by 1.5 MPa pressure impregnation for 1 h and 40 h of immersion in the mixture of FA and deionized water at atmospheric pressure.	Bamboo specimens were heated, and water was allowed to freely evaporate at 105 °C for 3 h.
	30
	50
	70
	W
S-Dry	15	Soaking in the mixture of FA and deionized water for 42 h at room temperature.
	30
	50
	70
	W
Control	——	——	——

. After curing, the aluminum foil that wrapped the wet-curing bamboo samples was removed. All the modified samples were dried until oven-drying. The absorption rate and WPG were calculated according to Equations (1) and (2), respectively.

$$\mathrm{Absorption} \mathrm{rate} =\frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}\times 100\%$$

(1)

$$\mathrm{WPG} =\frac{{\mathrm{m}}_2-{\mathrm{m}}_1}{{\mathrm{m}}_1}\times 100\%$$

(2)

where m₀ is the oven-dried mass (g) of a bamboo sample, m₁ is the mass (g) of a bamboo sample after impregnation, and m₂ is the oven-dried mass (g) of a furfurylated bamboo sample.

2.4. Preparation of a Bamboo’s Vascular Bundle

As shown in Figure 1b, untreated and modified bamboo samples with the dimensions of 20 × 50 × 5 mm³ were softened by soaking in deionized water. Then, under a microscope, a complete bamboo vascular bundle was selected, and the parenchyma outside it was removed using a thin blade to extract a complete bamboo vascular bundle. Before the tensile test, stiffeners made of poplar veneer were installed at both ends of the bamboo’s vascular bundle to stabilize the sample.

2.5. Testing the Physical and Mechanical Properties

2.5.1. Swelling Efficiency and Anti-Swelling Efficiency

The tangential, radial, and volume sizes of both dry- and wet-treated bamboo were measured. SE and ASE were calculated according to Equations (3) and (8), respectively.

$${\mathrm{SE}}_{\mathrm{T}}=\frac{{\mathrm{T}}_{\mathrm{wet}}-{\mathrm{T}}_{\mathrm{dry}}}{{\mathrm{T}}_{\mathrm{dry}}}\times 100\%$$

(3)

$${\mathrm{SE}}_{\mathrm{R}}=\frac{{\mathrm{R}}_{\mathrm{wet}}-{\mathrm{R}}_{\mathrm{dry}}}{{\mathrm{R}}_{\mathrm{dry}}}\times 100\%$$

(4)

$${\mathrm{SE}}_{\mathrm{V}}=\frac{{\mathrm{V}}_{\mathrm{wet}}-{\mathrm{V}}_{\mathrm{dry}}}{{\mathrm{V}}_{\mathrm{dry}}}\times 100\%$$

(5)

$${\mathrm{ASE}}_{\mathrm{T}}=\frac{{\mathrm{SE}}_{\mathrm{T}0}-{\mathrm{SE}}_{\mathrm{T}1}}{{\mathrm{SE}}_{\mathrm{T}0}}\times 100\%$$

(6)

$${\mathrm{ASE}}_{\mathrm{R}}=\frac{{\mathrm{SE}}_{\mathrm{R}0}-{\mathrm{SE}}_{\mathrm{R}1}}{{\mathrm{SE}}_{\mathrm{R}0}}\times 100\%$$

(7)

$${\mathrm{ASE}}_{\mathrm{V}}=\frac{{\mathrm{SE}}_{\mathrm{V}0}-{\mathrm{SE}}_{\mathrm{V}1}}{{\mathrm{SE}}_{\mathrm{V}0}}\times 100\%$$

(8)

where T_wet, R_wet, and V_wet are the tangential, radial, and volume size (mm) of the furfurylated bamboo samples in the wet states, respectively, and T_dry, R_dry, and V_dry are the tangential, radial, and volume size (mm) of the furfurylated bamboo samples in the dry states, respectively. SE_T0, SE_R0, and SE_V0 represent the tangential, radial, and volume coefficient of wet expansion of the control bamboo samples, respectively, and SE_T1, SE_R1, and SE_V1 represent the tangential, radial, and volume coefficient of wet expansion of the furfurylated bamboo samples, respectively.

2.5.2. Macro-Mechanical Properties

The MOR, MOE, parallel-to-grain CS, and TS of bamboo were tested according to the Chinese National Standards, GB/T 15780-1995, using a universal mechanical testing machine (AGS-X plus-50 kN, Shimadzu, Tokyo, Japan). The MOR and MOE of bamboo samples were tested in a three-point bending model with a testing speed of 2 mm/min. The MOR and MOE were determined using Equations (9) and (10), respectively.

$$\mathrm{MOR}=\frac{3{\mathrm{P}}_{\max }\mathrm{L}}{2{\mathrm{bh}}^2}$$

(9)

$$\mathrm{MOE}=\frac{{\mathrm{P}}_{\mathsf{\rho }}{\mathrm{L}}^3}{4\delta {\mathrm{bh}}^3}$$

(10)

where P_max is the maximum failure load of the sample, (N), L is the testing span between two supports (mm), b and h are the width and thickness of the samples (mm), respectively, P_ρ is the load increase (N), determined from of the straight-line section of the load-deflection curve, and δ is the mid-span deflection (mm) of the sample under P_ρ.

The CS was tested by loading bamboo samples at a constant loading rate and recording the maximum compression load. To test the TS, the two ends of a bamboo sample were clamped in the tester machine’s jaws, and the samples were vertically installed on the test machine and tested within 90 s with a uniform speed of loading to destroy the sample. The maximum tensile load was recorded.

2.5.3. Tensile Strength of a Bamboo’s Vascular Bundle

Figure 1b shows the schematic diagram of the preparation of a bamboo vascular bundle. The TS of the bamboo vascular bundle was tested using a miniature mechanical testing device (Instron 5848, US) with a 50 N sensor range. The test speed was 1.5 mm/min. The maximum tensile load was recorded.

2.5.4. Hardness and the Indentation Modulus of the Fiber Cell Wall

Nanoindentation was used to test the hardness and the indentation modulus of the bamboo’s fiber cell wall. Sample preparation and the testing methods of nanoindentation were adopted from our previous publications [32,33]. a TriboIndenter device (Hysitron, Minneapolis, MN, USA) with a Berkovich diamond tip, having a maximum radius of 100 nm, was used to test the indentation modulus and hardness of the bamboo cell wall. The target peak load was 250 μN, and the loading–unloading rate was 50 μN/s. The indenter was held at an approximately constant load for 6 s after attaining a peak load.

3. Results and Discussion

3.1. Physical Properties

The bamboo samples were furfurylated with mixtures containing different FA concentrations through either vacuum pressure impregnation or soaking impregnation. Owing to various settings, the FA mixture absorption rate varied between 87% and 55%. It can be clearly seen from Figure 2a that this parameter was obviously higher in V-P impregnation than in soaking impregnation. Regarding the effect of the FA concentration, the FA mixture absorption rate first increased and then decreased with an increase in the FA concentration from 15% to 70%. The highest absorption rate of bamboo impregnated either through the V-P method or by soaking was when 50% and 30% FA were applied, respectively. This phenomenon can be explained by the higher the FA concentration, the greater the resistance to its penetration into bamboo. V-P impregnation could provide more pores inside bamboo by applying the front vacuum, and the pressure could overcome the increased resistance caused by a higher concentration of FA during the impregnation process. In order to better understand the permeability of bamboo to the FA mixture, deionized water was applied as a control under the same impregnation conditions. It is worth noting that in the V-P impregnation process, the water absorption rate of bamboo was lower than that of the FA mixture, while it was obviously higher than the FA mixture absorption rate of bamboo treated by the soaking impregnation process.

It can clearly be seen from Figure 2b that the FA concentration, impregnation process, and curing conditions have considerable influence on the WPG of furfurylated bamboo. Regarding the FA concentration, the WPG of furfurylated bamboo increased with an increase in the FA concentration from 15% to 70%, except for the bamboo samples furfurylated through the S-Dry process, whose WPG did not increase further with the increase of the FA concentration above 30%. The positive correlation recorded between the WPG and the FA concentration is consistent with previous reports [14]. When the FA concentration increased from 30% to 70%, the WPG of the bamboo furfurylated through the S-Dry process did not increase further, which might be because the soaking treatment resulted in a limited immersion depth of the modifier, and some amount of the modifier was carried away by the evaporated water during the dry-curing stage. Considering the effect of the impregnation method on bamboo’s WPG, it can be seen that the WPG of V-P impregnation-modified bamboo is significantly higher than that of soaking-modified bamboo. This was mainly due to the fact that in the former impregnation method, bamboo possessed a higher absorption rate. In terms of the effect of the curing conditions, the WPG of wet-cured bamboo was higher than that of dry-cured bamboo, indicating that the wet-curing process can retain FA resin better than the dry-curing process. This was also confirmed by calculating the fixation rate obtained by the comparison between the WPG and the absorption rate. The results showed that the fixation rates of bamboo furfurylated through the V-P-Wet, S-Wet, V-P-Dry, and S-Dry processes were 31%, 27%, 19%, and 16%, respectively. Among the furfurylated bamboo samples, the highest WPG (37%) was determined in those furfurylated with the 70% FA mixture through the V-P-Wet process. This result is lower than that of wood materials treated under similar conditions [22], but it is obviously higher than that of other modifiers, such as phenolic resin-modified bamboo [36].

The swelling behavior of the furfurylated bamboo that was previously oven-dried to be saturated with water is presented in Figure 3. The hydrophobic FA resin filled the cell cavities and cell wall pores, thus reducing the swelling of the bamboo cell wall. However, a higher swelling efficacy and a negative ASE were observed for bamboo furfurylated with 15%–50% FA through V-P impregnation, which showed that the furfurylation did not improve the dimensional stability of bamboo but made the dimension change of the modified bamboo significant during the process of moisture absorption. As can be seen in Figure 3(b1,b2), this negative effect was canceled with the increase in the FA concentration, and when reaching 70%, the dimensional stability of bamboo was significantly improved with an ASE value as high as around 50%. Similar results were found in our previous study [34].

There are two possible explanations for this phenomenon. First, both curing and drying processes induced the collapse of parenchyma cells. The parenchyma cell is a kind of thin-walled structure in bamboo with a volume ratio of about 50% [37]. Due to the large cavity and thin wall, the integrity or stiffness of the cell walls is further weakened by the removal of part of the hemicellulose by the acid catalyst during the furfurylation process, which makes them prone to collapse during the rapid drying process. However, the deformation caused by solidification and drying can be recovered to a certain extent by soaking them in water for enough time. The effect of the curing method on the dimensional stability of bamboo also supports this explanation since the effect of the dry-curing treatment on the dimensional stability of furfurylated bamboo was lower in comparison to the wet-curing treatment, which can be seen in Figure 3(a1,b1,a2,b2), respectively. Second, high amounts of extractable polysaccharides present in bamboo were dissolved during the impregnation and curing treatments, which also represents the main reason for the deterioration of the dimensional stability of furfurylated bamboo. Previous studies have reported numerous starch grains in bamboo parenchyma cells being dissolved or even removed using an acidic FA mixture during V-P impregnation [32]. This can also explain why the dimensional stability of bamboo modified with 70% FA was improved, as a sufficient amount of FA resin filled the cell cavities vacated after the extraction.

In terms of bamboo furfurylation through soaking impregnation, this treatment, in comparison to V-P impregnation, showed a lower effect on the FA penetration depth and removal degree of hemicellulose and a weaker negative effect on the dimensional stability of bamboo. As can be seen in Figure 3(a3,b3), the dimensional stability of soaked bamboo was even improved. With an increase in the FA concentration from 30% to 70%, the V-ASE of bamboo furfurylated through wet-curing increased from nearly 20% to about 50%, except for bamboo furfurylated with 15% FA, which showed a decrease. The improvement in the dimensional stability of bamboo soaked after dry-curing was not ideal, as shown in Figure 3(a4,b4).

3.2. Mechanical Properties

On a macro level, the effects of the FA concentration, impregnation, and curing methods on the MOR, MOE, and CS of furfurylated bamboo are presented in Figure 4. It was found that these parameters have a similar variation trend with the change in the FA concentration under the same combination of impregnation and curing treatments. In the V-P-Wet treatment, it was observed that the maximum MOR, MOE, and CS values of furfurylated bamboo were determined in the samples treated with 15% FA, having about a 32%, 24%, and 31% increase, respectively, as compared with the values of untreated bamboo. When the FA concentration increased to 30%, the same parameters increased by 28%, 17%, and 24%, respectively, in comparison to untreated bamboo. As the FA concentration further increased to 50%, the MOR, MOE, and CS values of bamboo increased by 12%, 10%, and 13%, respectively, compared to the control. In short, with the increase of the FA concentration, the mechanical strength tends to increase less. When the FA concentration reached 70%, a decrease of 15%, 12%, and 15% was observed for the MOR, MOE, and CS, as compared to the control. The influence trend of the V-P-Dry treatment on the MOR, MOE, and CS of furfurylated bamboo was similar to that of the V-P-Wet treatment, which also applies when comparing the influence trends of the S-Wet and S-Dry treatments. In the latter case, it was observed that the MOR, MOE, and CS of furfurylated bamboo decreased slightly in comparison to untreated bamboo. As shown in Figure 4, when the FA concentration was 50%, the MOR, MOE, and CS of furfurylated bamboo decreased most obviously with the reduction by 16%, 14%, and 10%, respectively, in comparison to untreated bamboo. Additionally, it is worth noting that the MOR, MOE, and CS of bamboo treated with deionized water were slightly increased under the V-P-Wet, V-P-Dry, and S-Wet conditions. This phenomenon can be explained by the fact that some hydrophilic groups of bamboo cell wall compounds were reduced after the treatment with deionized water, resulting in a lower equilibrium water content of the treated bamboo than that of the untreated bamboo, which led to the improvement of the mechanical properties.

The effect of furfurylation on the bamboo mechanical properties at the organizational level was evaluated by examining its vascular bundle TS. Figure 5 shows the effects of the FA concentration, impregnation, and curing methods on the TS of furfurylated bamboo, as well as on the TS and MOE of a bamboo vascular bundle. It can be clearly seen that furfurylation had little effect on the TS of bamboo, except for the S-Dry process, which resulted in a decrease in the TS of the modified bamboo compared with the untreated bamboo. The TS of furfurylated bamboo under the V-P-Wet, V-P-Dry, and S-Wet conditions increased slightly.

Nanoindentation is a powerful tool for measuring hardness and the modulus of elasticity of the wood cell walls [38]. In order to understand the relationship between micro- and macro-mechanics, nanoindentation was applied to test the mechanics at the cellular level, including the hardness and indentation modulus of the control and furfurylated bamboo fiber cells. The effects of the FA concentration, impregnation, and curing methods on the hardness and the indentation modulus of furfurylated bamboo are presented in Figure 6. Although the degree of improvement was different due to the various treatment processes, both parameters were found to be improved in almost all the furfurylated bamboo samples, as compared with those of the control samples. The result showed that FA could be immersed in bamboo’s fiber cell wall and polymerized. Comparatively, both the hardness and indentation modulus of the fiber cell walls of furfurylated bamboo impregnated through the V-P process were higher than in bamboo furfurylated with soaking impregnation under both wet- and dry-curing conditions. Both parameters increased at first and then decreased with the increase in the FA concentration from 15% to 70%, and the maximum values were recorded in bamboo furfurylated with 30% FA through V-P impregnation and wet-curing, reaching double and triple values of those recorded in the control samples, respectively. This result can be explained by that the pore volume of the cell wall is limited, and the proper filling of FA resin can significantly increase the hardness and indentation modulus of the cell wall, while excessive swelling or a high concentration of the modified liquid will lead to the degradation of the cell wall components, and the resin enhancement will be offset. Regarding the furfurylation of bamboo through the soaking treatment, the hardness and indentation modulus of bamboo furfurylated with different FA concentrations were also increased, as compared with those recorded in the control, but the difference between the values scored through different FA concentrations was not obviously different.

3.3. Correlational Analyses

In order to further clarify the relationships between the physical and mechanical properties of furfurylated bamboo, a correlation analysis of these parameters across the samples, either being furfurylated through V-P impregnation or soaking impregnation, was carried out. Figure 7a shows the correlation analysis of the physical and mechanical properties of all the samples; no significant correlation between the WPG of furfurylated bamboo and its physical and mechanical properties can be observed. However, either a significant or an extremely significant positive correlation between the macro- and micro-mechanical properties was found, except for the MOE of the VB of furfurylated bamboo, but either a significant or an extremely significant negative correlation can be observed between the mechanical properties and ASE. Extremely significant positive correlations between the ASE-T, ASE-R, and ASE-T were also recorded. Figure 7b shows the correlation analysis of the physical and mechanical properties of the samples furfurylated through V-P impregnation. The WPG was negatively correlated with the TS of the VB and negatively correlated with other mechanical properties but not significantly. The MOE and CS were positively correlated with each other, while other mechanical strength parameters were positively correlated but not significantly. In addition, the MOE and CS were negatively correlated with ASE. Regarding the correlation analysis of the physical and mechanical properties of the samples furfurylated by soaking impregnation (Figure 7c), a positive relationship was found between the WPG and ASE, and a negative relationship was observed between the WPG and CS. Interestingly, the correlation between the mechanical properties and ASE was not significant.

4. Conclusions

To clarify the effect of furfurylation on the physical and multiscale mechanical properties of bamboo, furfurylation was performed using different FA concentrations (15%, 30%, 50%, or 70%) through either of the two impregnation processes, vacuum pressure or soaking impregnation, while applying different curing conditions (wet- or dry-curing). Based on the results, a conclusion can be drawn that V-P impregnation allowed for better penetration of the agent into bamboo, and wet-curing provided the full curing of the agent to ensure a higher curing rate. Moreover, in the V-P impregnation process, the dimensional stability of bamboo modified with either low or medium FA concentrations decreased in both dry- and wet-curing, while it increased in bamboo modified with high FA concentrations. The S-Wet modification was able to significantly improve the dimensional stability of bamboo at both medium and high FA concentrations. In terms of mechanical strength, furfurylation had little effect on the macro- and micro-mechanical properties of bamboo, and the V-P impregnation treatment using either low or medium FA concentrations significantly improved these properties to some extent, regardless of the curing method used. In comparison to untreated materials, the mechanical strength of bamboo modified by soaking did not change significantly. The results of a correlation analysis across the samples showed either a significant or an extremely significant positive correlation between the macro- and micro-mechanical parameters of the strength of the modified bamboo and a significant negative correlation between the mechanical strength parameters and ASE. The results of a correlation analysis of the samples furfurylated by soaking impregnation showed a positive relationship between the WPG and ASE and a negative relationship between the WPG and CS. Interestingly, the correlation between the mechanical properties and ASE was not significant.

In summary, both V-P-Wet and S-Wet are highly recommended processes for bamboo furfurylation. The advantages of the V-P-Wet approach are that it is time-saving, has a high utilization rate of FA resin during curing, and significantly improves the mechanical strength of bamboo. S-Wet has the advantages of simplicity, a high utilization rate of FA, and a significant improvement in the dimensional stability of bamboo. Additionally, the long-term durability and potential environmental impact of furfurylated bamboo should be further clarified.