Research on the Impregnation Process and Mechanism of Silica Sol/Phenolic Resin Modified Poplar Wood

Abstract

Phenolic resin-modified materials partially reduce the toughness of the wood. In this study, organic–inorganic composite modifiers were used to modify the wood. Silica sol/phenolic resin was prepared through in-situ polymerization, and poplar wood was modified using a vacuum pressure impregnation process, enhancing its toughness. Orthogonal experiments were conducted, and the impact toughness of the modified poplar wood was used as the evaluation index. Through orthogonal experiments, using the impact toughness of modified poplar as the evaluation indicator, it was found that when the average particle size of the silica sol is 8–15 nm, the pressure is 1.2 MPa, and the pressurization time is 3 h, the impregnation-modified poplar’s impact toughness reaches its optimum, improving by 84.1% and 135.4% compared to the raw material and phenolic resin impregnated wood, respectively. The Fourier Transform Infrared Spectroscopy (FT-IR) results indicated that the characteristic absorption peak of Si-O-Si appears in the poplar wood after impregnation, confirming the formation of new silicon-oxygen (Si-O) chemical bonds. X-ray Photoelectron Spectroscopy (XPS) analysis revealed that a chemical reaction occurs between the impregnation liquid and the wood, generating Si-O-C. Subsequently, through Dynamic Mechanical Analysis (DMA) and Thermogravimetric (TGA) analysis, it was understood that this chemical reaction significantly enhances the thermal stability and toughness of the impregnated material, making it superior to the original poplar material. The TGA further unveiled that, compared to untreated poplar, the thermal stability of the impregnated material has been notably improved. Lastly, Scanning Electron Microscopy (SEM) analysis demonstrated that the composite impregnation liquid successfully permeates and fills the interior of the poplar cells.

1. Introduction

Fast-growing poplar wood has attracted attention due to its potential as a rich resource of natural biomaterials; however, its deficiencies in dimensional stability and mechanical properties have limited its application value in various fields [1,2,3]. It is well-known that phenolic resin impregnation modification is a feasible and widely accepted approach to enhance its mechanical properties and add value, given the low cost and availability of phenolic resin [4,5,6]. Typically, low molecular weight thermosetting phenolic resins are used for wood impregnation modification, significantly strengthening the physical and mechanical properties of wood [5,7,8,9,10]. Despite this, this method can also introduce issues such as wood swelling, surface deformation, and reduced impact toughness [4,11,12]. In addition, some studies have inferred that the resin primarily resides on the surface of the wood based solely on the density increase of cedarwood boards from 393 kg/m³ to 454 kg/m³ and resin content of 412 kg/m³. Such conclusions drawn merely from changes in density and mass lack precision [4]. Moreover, existing research has not fully elucidated the specific distribution of resin in wood, or its impact on modified material properties and impregnation process parameters, and the principles of impregnation enhancement of wood’s mechanical properties and the structure–effect relationship between the modifier and material properties remain unrevealed.

In recent years, research on toughening thermosetting resins with inorganic nanoparticles has begun to attract academic attention [13,14,15,16]. The essence of this method is to introduce inorganic nanoparticles into thermosetting resins, utilizing the nanoparticles’ ultra-small size and high surface area to interact with thermosetting resin molecules at the microscopic scale, thereby improving the toughness and heat resistance of wood [14,15,17,18,19].

Nano silica sol, a common type of inorganic nanoparticle, is recognized for its excellent thermal stability and mechanical properties. Therefore, we used silica sol, a homogeneous solution, as a substitute for SiO₂ particles to react, successfully enhancing the reaction rate through in-situ polymerization and preparing a uniform and stable silica sol/phenolic resin composite impregnating solution. The introduction of this method demonstrated the advantages of organic–inorganic co-polymer modification in the interaction between the phenolic resin and wood, thus achieving excellent impact performance not possessed by singular modification [20,21,22,23]. Therefore, this preparation method holds immense potential for widespread applications and is expected to play a significant role in the production of various high-performance composite materials [24,25,26].

In this study, three different particle sizes of silica sol, all with a mass fraction of 40% (accounting for 40% of the phenol mass), were blended with phenolic resin using the in-situ polymerization method. The modified poplar wood was subjected to vacuum pressure impregnation, and the effects of the type of impregnating solution, pressure, and impregnation time on the weight gain rate, water absorption rate, and mechanical properties of the modified poplar wood were thoroughly investigated, thereby optimizing the process conditions for vacuum pressure impregnation modification of poplar wood. Additionally, with the aid of Fourier-Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), and Environmental Scanning Electron Microscopy (SEM), this study comprehensively analyzed the changes in the microstructure and thermal stability of the modified poplar wood, further revealing the mechanism of silica sol/phenolic resin impregnation enhancement for modified poplar wood. This provides a solid theoretical foundation and technical support for subsequent research on composite modifiers and modified wood materials.

2. Experiment

2.1. Experimental Materials

The specimen processing was conducted in accordance with the GB/T 1929–2009 standards [27] for sawing and sampling of physical and mechanical test pieces of wood. This study used Populus davidiana, Chinese Aspen, originating from Jiaohe Forestry Field in Jilin Province, with a density of approximately 0.42 g/cm³. All used samples were flawless materials, with no cracks or knots. The original specifications of the samples were 500 mm (L) × 120 mm (R) × 20 mm (T). Each one was cut into three types of specimens: 300 mm × 20 mm × 20 mm, 20 mm × 20 mm × 20 mm, and 70 mm × 50 mm × 50 mm. All samples were dried to a moisture content of 7%–12% before testing. Each test sample is subjected to ten valid repeated tests, and the statistical average value is taken.

Table 1. Experimental Materials.

Chemical Name	Raw Material Specification	Manufacturer
Phenol	Analytical Grade	Liaoning Quanrui Reagents Co., Ltd. Jinzhou, China
Sodium Hydroxide	Analytical Grade	Tianjin Damo Chemical Reagent Factory. Tianjin, China
Formaldehyde	Analytical Grade	Tianjin Fuchen Chemical Reagent Factory. Tianjin, China
Silica Sol	Average Particle Size 8–15 nm, 15–30 nm, 80–120 nm	Jinan Yinfeng Silicon Products Co., Ltd. Jinan, China
Silane Coupling Agent KH-560	97%	Shandong Yousu Chemical Technology Co., Ltd. Linyi, China
Anhydrous Ethanol	Analytical Grade	Tianjin Damo Chemical Reagent Factory. Tianjin, China
Distilled Water	-	Laboratory-made. Jinlin, China

presents the experimental materials.

Table 2. Information for low molecular weight water-soluble phenolic resin solutions.

Information on Phenolic Resin Solutions	Solid Content	Relative Molecular Weight	pH Value	Viscosity	Free Formaldehyde
numerical	61.28%	350–382	7–9	54.5 mPa·s	0.34%

lists the information for low molecular weight water-soluble phenolic resin solutions.

2.2. Experimental Procedure

As illustrated in Figure 1, the process of treating fast-growing poplar wood samples with silica sol-modified phenolic resin primarily involves several key steps. The first step is the preparation of fast-growing poplar wood specimens. Next, the poplar is soaked in a liquid through a vacuum pressure impregnation process, ensuring the solution fully enters the wood cells. Finally, the modified wood is dried in an appropriate environment, followed by systematic testing and evaluation of its properties. Table 1 lists the information for low molecular weight water-soluble phenolic resin solutions.

Figure 1. Experimental flow [28].

Figure 1. Experimental flow [28].

2.3. Preparation of Modified Silica Sol

The modification procedure involves utilizing a silica sol as the primary component. Start by taking 200 g of this sol and integrating it with an equimolar mixture of ethanol and water, each weighing 100 g. To ensure homogeneity, the concoction undergoes ultrasonic agitation for half an hour. Once this step is completed, the mixture is transferred to a specialized three-necked flask. A constant-temperature water bath, set at a precise 60 °C, aids in maintaining the ideal conditions for the next phase of the procedure. At this juncture, 9 g of KH-560 is gradually introduced into the mixture. Ensuring thorough mixing, this reaction is stirred continuously for 6 h. Upon its completion, one can observe a characteristic pale blue hue indicative of the modified silica sol solution. This solution is then carefully decanted and set aside, awaiting its subsequent applications.

2.4. Preparation of Silica Sol/Phenol–Formaldehyde Resin Composite Impregnation Solution

Phenol and formaldehyde were weighed according to a molar ratio of 1:2.1 and added to a beaker. Subsequently, a modified silica sol with a mass fraction of 40% (accounting for 40% of the weight of phenol) was added. The mixture of phenol, formaldehyde, and modified silica sol was subjected to ultrasonic oscillation for 30 minutes to ensure the even dispersion of the modified silica sol in the solution. This solution was then poured into a three-necked flask. Based on 5% of the total weight of phenol and formaldehyde, a 30% NaOH solution (where the weight of sodium hydroxide accounts for 30% of the total solution mass) was added to adjust the pH to 9–10. The reaction was set at a temperature of 90 °C and was allowed to proceed for 2 h. After the reaction was terminated, a silica sol/phenolic resin composite impregnating solution was obtained.

2.5. Preparation of Silica Sol/Phenol–Formaldehyde Resin Impregnated Modified Material

Begin by drying poplar samples of varying dimensions: 300 mm × 20 mm × 20 mm, 20 mm × 20 mm × 20 mm, and 70 mm × 50 mm × 50 mm. Ensure their moisture content is maintained between 7% to 12%. Once dried, place these wood materials into an impregnation tank. Proceed to evacuate the tank, aiming for a pressure range of −0.095 to −0.2 MPa. Maintain this vacuum state for 30 minutes. The pressure differential, both internally and externally of the tank, will naturally draw the impregnation solution inside. Following this, apply a specific pressure within the tank and sustain it for a determined duration. Upon completion, depressurize the tank and extract the now-impregnated and modified poplar wood. Initiate the drying phase by exposing the wood to ambient air for a period of 24 to 48 h. Subsequently, continue the drying process for the modified material until its moisture content once again falls within the range of 7% to 12%. This ensures the wood is optimally conditioned for subsequent water absorption and mechanical property assessments.

2.6. Testing and Characterization

(1) Weight Gain Test: The weight gain of the wood is calculated according to Formula (1) to determine the weight gain rate.

$$WPG=\frac{M_1-M_0}{M_0}\times 100\%$$

(1)

In Formula (1): WPG represents the weight percentage gain (%), M0 represents the oven-dry mass before impregnation (g), and M1 represents the oven-dry mass after impregnation (g). Each test sample is subjected to ten valid repeated tests, and the statistical average value is taken.

(2) Utilizing the DWD-100E Universal Mechanical Testing Machine (from Jinan Testing Group Limited, Times Group, Jinan, China) and the JB-300B Impact Testing Machine (from Jinan Testing Metals Group Limited, Jinan, China), tests were conducted respectively in accordance with the relevant standards GB/T1927.9–2021 [29] “Method for Determining Bending Strength of Flawless Small Specimens of Timber”, GB/T1927.10–2021 [30] “Method for Determining Bending Elastic Modulus of Flawless Small Specimens of Timber”, and GB/T1927.19–2021 [31] “Method for Determining Hardness of Flawless Small Specimens of Timber”, on the bending strength, elastic modulus, and hardness of the impregnated material and the raw material. Employing the JB-300B Impact Testing Machine (from Jinan Testing Metals Group Limited) and following the standard GB/T1927.17–2021 [32] “Method for Determining Impact Toughness of Flawless Small Specimens of Timber”, tests were conducted on the impact toughness of the impregnated material, raw material, and phenolic resin modified material. Using the FA2004 Precision Balance (from Tianjin Tianma Hongji Instrument Co., Ltd., Tianjin, China) and adhering to the standard GB/T1927.7–2021 [33] “Method for Determining Water Absorption of Flawless Small Specimens of Timber,” tests were performed on the water absorption rate of the impregnated material and the raw material. Each test sample is subjected to ten valid repeated tests, and the statistical average value is taken.

(3) Fourier Transform Infrared (FT-IR) spectroscopy: Infrared spectroscopy tests were conducted on both the poplar wood raw material and the impregnated material to study the impact of the silica sol/phenolic resin composite impregnating solution on the chemical structure of poplar wood. A WQF-510A infrared spectrometer (from Beijing Rayleigh Analytical Instrument Corporation, Beijing, China) was utilized. The test samples were ground using a pulverizer and passed through a 200-mesh sieve. The samples were prepared using the KBr pellet method and were then tested. The wavenumber range was 500–4000 cm⁻¹, and the infrared spectra of both the poplar wood raw material and the impregnated material were recorded. Each sample was tested four times.

(4) X-ray Photoelectron Spectroscopy (XPS) Analysis: In order to further clarify the reactions occurring within the wood due to the silica sol/phenolic resin composite impregnation liquid, XPS analysis was conducted on both the poplar raw material and the impregnated material. The ESCLAB250Xi type X-ray photoelectron spectroscopy analyzer (manufactured by the American company Thermo Ltd. Waltham, MA, USA) was used. Test specimens were crushed and passed through a 200-mesh sieve for XPS analysis. An Al–Kɑ source was employed with an X-ray beam energy of 100 W, a grating diameter of 200 mm, a sensitivity of 350 kcps, and the vacuum level in the analysis chamber was maintained at 10⁻⁸ bar. Each sample was tested four times.

(5) Dynamic Mechanical Analysis (DMA) Testing: Using the DMA242E dynamic thermal mechanical analyzer (manufactured by the German company Netzsch Instruments, Selb, Germany), the dynamic viscoelastic properties of the raw poplar material and the impregnated material were tested. The three-point bending method was adopted. The sample size was 40 mm × 20 mm × 3 mm, with an amplitude of 60 μm. The testing frequency was set at 2 Hz, the temperature range was from 30 °C to 350 °C, and the heating rate was 3 °C/min. Each sample was tested four times, and the average value was taken.

(6) Thermogravimetric Analysis (TGA): To investigate the impact of the silica sol/phenolic resin composite impregnation liquid on the thermal properties of poplar, tests were conducted on the thermal properties of both the raw material and the impregnated material. Thermogravimetric Analysis (TG–DTG): Using the TG 209 F3 thermogravimetric analyzer (manufactured by the German company Netzsch), the samples were tested for thermal weight loss. The testing temperature ranged from 30 °C to 800 °C with a heating rate of 10°C/min under an N₂ atmosphere, and the sample size was approximately 5 mg. Each sample was tested four times.

(7) Scanning Electron Microscopy (SEM): To ascertain the distribution condition of the composite impregnation liquid within the poplar, SEM was utilized to observe the microstructure of both the poplar raw material and the impregnated material. The samples were examined using a Quanta 200 Environmental Scanning Electron Microscope (ESEM) from Philips Electron Optics (PEI), Netherlands. Cross-sectional and radial sectional specimens of both raw and impregnated materials were taken, each measuring 10 mm × 10 mm × 10 mm. The wood was softened by boiling, and thin wood slices of 8 mm × 8 mm × 1.5 mm were cut with a sharp blade 2–5 mm away from the wood, then dried to constant weight for reserve. The specimens were subjected to gold sputtering before ESEM scanning, and the scanning areas on the cross, radial, and tangential sections were analyzed. Each sample was tested four times.

3. Results and Discussion

3.1. Research on the Process of Silica Sol/Phenolic Resin Impregnation and Modification Treatment of Poplar Wood

Table 3. Orthogonal experimental factor table.

Level	Factor A	Factor B	Factor C
	Composite Impregnation Solution	Applied Pressure/MPa	Pressurization Duration/h
1	S15/PF	0.8	1
2	S30/PF	1.0	2
3	S80/PF	1.2	3

presents the 3-factor, 3-level design of the L₉ (3³) orthogonal experiment, with weight gain, absorption rate, and mechanical properties as the main evaluation indicators for impregnation effectiveness. The optimal process conditions for the composite impregnation and modification of poplar wood were selected.

Table 4. Orthogonal experimental results of weight gain rate and water absorption rate.

Experiment	Weight Gain Rate/%	Water Absorption Rate/%
A1B1C1	59.55	95.23
A1B2C2	63.04	78.27
A1B3C3	92.28	34.36
A2B1C3	90.54	30.83
A2B2C1	56.46	78.65
A2B3C2	66.25	40.37
A3B1C2	56.41	90.49
A3B2C3	95.04	45.20
A3B3C1	56.25	101.69
Control material	-	150.20

shows the results of the orthogonal experiment for weight gain and water absorption rate, while

Table 5. Orthogonal experimental results of mechanical properties.

	MOR/Mpa	MOE/GPa	Impact Toughness /kJ/m2	Hardness /kN
A1B1C1	70.10	42.41	67.50	1.374
A1B2C2	96.90	48.13	56.25	1.537
A1B3C3	92.00	33.85	101.25	2.801
A2B1C3	101.40	40.98	90.00	2.763
A2B2C1	79.20	33.06	57.50	1.275
A2B3C2	71.20	26.96	74.06	1.628
A3B1C2	67.12	34.93	65.94	1.230
A3B2C3	86.00	36.37	80.00	1.841
A3B3C1	78.70	34.12	66.56	1.327
Control material	65.26	6.36	55.00	1.210
Phenolic resin-modified wood	-	-	43.00	-

displays the results of the orthogonal experiment for mechanical properties.

Table 6. Orthogonal Test Results and Analysis of Weight Gain Rate, Water Absorption Rate, andMOR.

Level Value	Weight Gain Rate/%			Water Absorption Rate/%			MOR/MPa
	A	B	C	A	B	C	A	B	C
k1	71.62	68.83	57.42	69.28	72.18	91.86	86.33	79.54	76.00
k2	71.08	71.51	61.90	49.95	67.37	69.71	83.93	87.37	78.41
k3	69.23	71.59	92.62	79.13	58.81	36.80	77.27	80.63	93.13
R	2.39	2.76	35.2	29.18	13.37	55.06	9.06	7.83	17.13
Primary and secondary factors	C > B > A			C > A > B			C > A > B
Optimal scheme	A1B3C3			A2B3C3			A1B2C3

presents the results and analysis of the orthogonal experiment for weight gain, water absorption rate, and flexural strength, while

Table 7. Orthogonal Test Results and Analysis of MOE, Impact Toughness, and Hardness.

Level Value	MOE/Gpa			Impact Toughness/kJ/m2			Hardness/kN
	A	B	C	A	B	C	A	B	C
k1	41.46	39.44	36.53	75.00	74.48	63.85	1.904	1.993	1.325
k2	33.67	39.19	36.67	73.85	64.58	65.42	1.889	1.551	1.465
k3	35.14	31.64	37.07	70.83	80.62	90.42	1.466	1.919	2.468
R	7.79	7.80	0.54	4.17	16.04	26.57	0.438	0.442	1.143
Primary and secondary factors	B > A > C			C > B > A			C > B > A
Optimal scheme	A1B1C3			A1B3C3			A1B1C3

provides the results and analysis of the orthogonal experiment for elastic modulus, impact toughness, and hardness.

The optimal process conditions for impregnation and modification of poplar wood were determined using the vacuum pressure impregnation method. From Table 4, Table 5, Table 6 and Table 7, it can be observed that the poplar wood treated with the silicon sol/phenolic resin composite impregnation solution exhibited varying degrees of improvement in physical and mechanical properties. However, the influence of each factor on the performance of the impregnated material differed in importance.

The factors affecting weight gain, impact toughness, and hardness were ranked as follows: pressing time (C) > pressure (B) > composite impregnation solution (A). The optimal combination for weight gain and impact toughness was C3B3A1, while for hardness, it was C3B1A1.The factors influencing water absorption rate and flexural strength were ranked as follows: pressing time (C) > composite impregnation solution (A) > pressure (B). The optimal combinations were C3B3A2 for water absorption rate and C3B2A1 for flexural strength. The factor affecting elastic modulus followed the order: pressure (B) > composite impregnation solution (A) > pressing time (C). The optimal combination was C3B1A1. In conclusion, it can be seen that pressure and pressing time have a more significant influence on the performance of the impregnated material.

3.1.1. Polymer Weight Gain Analysis

Weight gain is an important indicator for evaluating the impregnation effect of wood, as it can determine whether the impregnating solution effectively penetrates the internal structure of the wood. A higher weight gain indicates a greater amount of impregnating solution filling the wood’s pores. From the variance results in

Table 8. Significance of Various Factors on the Weight Gain Rate.

Factor	SS	df	MS	F-Ratio	p-Value
A	9.426	2	4.713	0.218	Not significant
B	14.806	2	7.403	0.342	Not significant
C	2202.829	2	1101.414	50.926	Significant
Error	43.255	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.00.

, it can be observed that F_0.01 > F_C > F_0.05 > F_0.1 > F_B > F_A, indicating that the pressure time (C) has the greatest influence on the weight gain of Poplar Wood, followed by the pressure (B), and the composite impregnating solution (A) has the smallest effect. The impregnating solution reacts with the wood polymers. After curing, it deposits on the cell walls and within the cell cavities, blocking the channels through which water molecules enter the wood. Consequently, water molecules cannot bind with the internal -OH groups of the wood. Based on this experiment, it can be concluded that a higher pressure allows the composite-impregnating solution to penetrate the wood more effectively. Therefore, for weight gain, it is suitable to select a pressure of 1.2 MPa and a pressure time of 3 h.

3.1.2. The Water Absorption Weight Gain Rate Analysis

The wood cell wall consists mainly of hydrophilic groups, such as a large number of hydroxyl groups [34]. From the variance results in

Table 9. Significance of Various Factors on the Water Absorption Rate.

Factor	SS	df	MS	F-Ratio	p-Value
A	1322.010	2	661.005	8.106	Not significant
B	275.459	2	137.730	1.689	Not significant
C	4605.366	2	2302.683	28.238	Significant
Error	163.093	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.00.

, it can be observed that F_0.01 > F_C > F_0.05 > F_0.1 > F_A > F_B. Among these three factors, the pressure time (C) has the greatest impact on the water absorption weight gain rate, followed by the composite impregnation solution (A), while the pressure pressure (B) has the smallest impact. This indicates that the longer the pressure time, the more components of the impregnation solution can diffuse into the wood interior under pressure. After solidification, the impregnation solution deposits in the cell walls and cell lumens, blocking the channels through which water molecules can enter and bind with the internal -OH groups of the wood. Additionally, smaller silica sol particle size allows for easier penetration into the wood interior after being combined with phenolic resin, leading to a decreased water absorption capacity of the wood and improved dimensional stability. On the other hand, larger particle size silica sol, when combined with phenolic resin, can cause blockage of micropores on the wood’s ray cell walls or ray cell membranes. Therefore, a longer pressure time can be chosen as a parameter. Based on the range and variance analysis, within the scope of this experiment, a pressure time of 3 h and a silica sol particle size of 15 nm can be selected.

3.1.3. The Analysis of Flexural Strength

The bending strength of wood represents its maximum capacity to withstand bending. It is a key parameter in designing wooden structures and furniture and a primary indicator for evaluating its mechanical properties. From

Table 10. Significance of Various Factors on the Modulus of Rupture.

Factor	SS	df	MS	F-Ratio	p-Value
A	132.199	2	66.100	0.291	Not significant
B	107.790	2	53.895	0.237	Not significant
C	516.218	2	258.109	1.137	Not significant
Error	454.186	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.00.

, it can be observed that F_0.01 > F_0.05 > F_0.1 > F_C > F_A > F_B, indicating that the influence of pressing time (C) is the greatest, followed by the composite impregnation solution (A), and the impact of pressure (B) is the least. Table 4 shows that the flexural strength of poplar wood significantly improves after treatment with silica sol/phenolic resin impregnation. The composite impregnation solution fills the internal voids of the wood, sharing the load with the poplar wood matrix and preventing cracking during load-bearing. Additionally, the impregnation solution can chemically bond or form hydrogen bonds with poplar wood. With a longer pressing time, more impregnation solution is filled inside the wood. However, when the particle size of the silica sol becomes larger after the composite with phenolic resin, it becomes difficult for it to penetrate the wood, thereby affecting the mechanical properties of the impregnated material. Therefore, for flexural strength, a pressing time of 3 h and a particle size of 15 nm for the silica sol are more suitable.

3.1.4. The Analysis of Flexural Modulus of Elasticity

From the variance analysis of the factors in

Table 11. Significance of Various Factors on the Modulus of Elasticity.

Factor	SS	df	MS	F-Ratio	p-Value
A	102.915	2	51.458	1.189	Not significant
B	117.737	2	58.868	1.360	Not significant
C	0.465	2	0.233	0.005	Not significant
Error	86.548	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.000.

, it can be observed that F_0.01 > F_0.05 > F_0.1 > F_B > F_A > F_C, indicating that the effect of pressure (B) on the flexural modulus of wood is the greatest, followed by the effect of composite impregnation solution (A), while the effect of pressing time (C) is the smallest. As shown in Table 4, the flexural modulus of impregnated poplar wood is significantly improved compared to the untreated material. This is mainly attributed to the filling of the cell lumen and cell walls with the silicon sol/phenolic resin impregnation solution, the cross-linking reaction between the composite impregnation solution and the hydroxyl groups, and the formation of a network structure between cellulose fibers, thereby enhancing the flexural modulus of the wood. Therefore, considering the flexural modulus along with other physical and mechanical properties, the optimal parameters would be a pressure of 1.2 MPa and a pressing time of 3 h.

3.1.5. Impact Toughness Analysis

Impact toughness analysis can reflect the micro-defects and impact resistance of wood, serving as an important indicator to assess its toughness or brittleness. The variance analysis results in

Table 12. Significance of Various Factors on Impact Toughness.

Factor	SS	df	MS	F-Ratio	p-Value
A	27.796	2	13.898	1.100	Not significant
B	392.966	2	196.483	15.546	Not significant
C	1333.055	2	666.527	52.738	Significant
Error	25.277	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.00.

indicate that F_0.01 > F_C > F_0.05 > F_B > F_0.1 > F_A, with a significant effect of pressing time (C), followed by pressure (B), and the least effect from the composite impregnation solution (A). As shown in Table 4, poplar wood treated with silica sol/phenol–formaldehyde resin composite impregnation solution exhibits a noticeable improvement in impact toughness compared to the raw material. This enhancement is attributed to the three-dimensional network structure and high strength and toughness properties of the nanoscale SiO₂ [35,36]. The reinforcement and toughening effect of the silica sol/phenol–formaldehyde resin composite led to an increase in the impact toughness of the modified poplar wood. Therefore, for impact toughness, the appropriate values for pressing time and pressure are 3 h and 1.2 MPa, respectively.

3.1.6. Hardness Analysis

Hardness represents the ability of wood to resist indentation by hard objects and is an important factor to consider in components such as flooring [37].

Table 13. Significance of influence of various factors on hardness.

Factor	SS	df	MS	F-Ratio	p-Value
A	0.371	2	0.185	3.580	Not significant
B	0.209	2	0.104	2.015	Not significant
C	2.333	2	1.166	22.524	Significant
Error	0.104	2

Note: F_0.01(2, 2) = 99.00. F_0.05(2, 2) = 19.00. F_0.1(2, 2) = 9.00.

presents the significant analysis of radial hardness in wood, where the impact of pressing time (C) on wood hardness is significant, followed by the influence of composite impregnation solution (A), while the impact of pressure (B) is minimal. This is attributed to the infiltration of the composite impregnation solution into the wood, which solidifies and forms a hard resin while creating a cross-linked structure within the wood, resulting in an increase in hardness. Considering the overall effect, a silica sol particle size of 15 nm and a pressing time of 3 h are recommended. In summary, the optimal impregnation treatment process is as follows: a composite of silica sol with an average particle size of 8–15 nm and phenolic resin, a pressure of 1.2 MPa, and a pressing time of 3 h. To further explore the strengthening mechanism of silica sol/phenolic resin composite impregnation solution on poplar wood, the selected impregnation modification process is applied to treat the poplar wood and characterization analysis is conducted comparing the untreated material (control wood) with the silica sol/phenolic resin impregnated wood (impregnated wood).

3.2. Study on the Impregnation Mechanism of Silica Sol/Phenolic Resin Modified Poplar

3.2.1. Fourier Transform Infrared Spectroscopy Analysis

We performed infrared spectroscopy testing on poplar wood samples and impregnated materials to investigate the influence of silica sol/phenol–formaldehyde resin composite impregnation solution on the chemical structure of poplar wood. From Figure 2, it can be observed that the material exhibits stretching vibration peaks and bending vibration absorption peaks of -OH at 3419 cm⁻¹ and 1631 cm⁻¹, respectively, while the absorption peak around 2917 cm⁻¹ corresponds to the stretching vibration of methyl and methylene groups. In comparison to the material, impregnated poplar wood shows characteristic absorption peaks of Si-O-Si around 817 cm⁻¹ and 460 cm⁻¹ and a stretching vibration absorption peak of Si-O-Si near 1062 cm⁻¹, which may also be attributed to the stretching vibration of Si-O-C [38]. The vibrational peaks at these positions are significantly enhanced in the impregnated material compared to the material alone, and the absorption peak around 460 cm⁻¹ corresponds to the bending vibration of Si-O-Si.

The stretching vibration characteristic peak around 2917 cm⁻¹ shows no significant change before and after the modification of poplar wood. However, in the impregnated material, the vibrational peaks at 3419 cm⁻¹ and 1631 cm⁻¹ are noticeably enhanced. This is primarily attributed to the cross-linking reaction between the composite impregnation solution and the wood, resulting in a reduction in the number of free hydroxyl groups in the wood. The composite impregnation solution forms more hydrogen bonds with poplar wood. In summary, after modification with silica sol/phenol–formaldehyde resin composite impregnation solution, the poplar wood retains its original characteristic absorption peaks and also exhibits absorption peaks corresponding to Si-O-Si. This indicates that the modified wood with composite impregnation solution involves not only physical filling but also chemical bonding.

3.2.2. X-ray Photoelectron Spectroscopy Analysis

In order to further elucidate the reactions occurring within the wood upon treatment with silica sol/phenol–formaldehyde resin composite impregnation solution, XPS analysis was conducted on both the poplar wood material and impregnated material, and the results are shown in Figure 3a,b. From Figure 3a,b, it can be observed that both the material and impregnated material exhibit strong peaks around 285 eV and 532 eV, corresponding to the absorption peaks of carbon (C) and oxygen (O) atoms, respectively. After treatment with silica sol/phenol–formaldehyde resin impregnation, the surface chemical composition of the wood undergoes changes. The presence of silicon (Si) in the impregnated material indicates the effective penetration of silica sol into the interior of the wood. Figure 4 presents a narrow scan of the Si element on the surface of the impregnated material, revealing the presence of Si-O-C chemical structures within the wood. This confirms the formation of chemical bonds between silica sol, phenol–formaldehyde resin, or wood, which is consistent with the infrared spectroscopy results.

The surface chemical structure of wood can be determined by analyzing the intensity and chemical shifts of the C1s peak in the wood. Carbon (C) in wood can exist in four different forms, with the three main forms commonly found in natural wood. Figure 5a,b show the high-resolution XPS spectra of C1s for the raw and modified materials after peak deconvolution, with four peaks being fitted for both, as illustrated in

Table 14. Fitting data of C1s splitting peaks on the surface of control and impregnated wood.

Material	C1/%	C2/%	C3/%	C4/%
Control wood	57.81	33.35	5.79	3.05
Impregnated material	39.60	46.52	10.09	3.78

. The C1 content in the impregnated material decreases, possibly due to the oxidation of the unstable terminal groups of the three major components in the wood into small molecule acidic substances. On the other hand, the increase in C2 and C3 content is due to the chemical cross-linking reaction between the impregnation solution and the functional groups within the wood, leading to the generation of oxygen-containing groups such as -C-O, -C=O, and -O-C=O [39]. The chemical reaction between the wood and the silica sol/phenol-formaldehyde resin composite impregnation solution, resulting in the formation of Si-O-C, contributes to the increase in the relative content of C2.

3.2.3. Dynamic Mechanical Analysis

DMA was employed to analyze the dynamic viscoelasticity of poplar wood materials and impregnated materials. When the storage modulus of a material decreases rapidly, it indicates better toughness [40]. Figure 6 shows the variation curve of the storage modulus with temperature for both the materials and impregnated materials, exhibiting a decreasing trend with increasing temperature. At low temperatures, the energy of molecular motion in wood is low, and under external forces, only various functional groups on branches, main chains, or side chains, as well as individual chain segments, can move. Consequently, the deformation caused by external forces is minimal, and when the force is removed, the deformation of wood immediately recovers. Therefore, the storage modulus of wood is relatively high [41]. With the continuous increase in temperature, the thermal energy of wood molecules gradually increases, leading to a decrease in the storage modulus. The impregnated materials show an improved storage modulus compared to the materials within a certain temperature range, primarily due to the reinforcing effect of the composite impregnating solution [42]. As seen in Figure 7, the loss modulus of the impregnated materials is significantly higher than that of the materials, indicating that under increasing temperature conditions, the impregnated materials exhibit superior material properties to poplar wood materials. The peak loss modulus values for poplar wood and impregnated materials are 318 MPa and 893 MPa, respectively. Therefore, the impregnated materials possess higher toughness than the materials, and the glass transition loss peak temperature of the impregnated materials is higher than that of the materials, mainly due to possible crystallization in the amorphous region of the impregnated materials [43].

Loss tangent represents the ease of molecular viscous flow within the wood. A higher value indicates more difficult molecular flow and greater energy loss. As observed from Figure 8, with increasing temperature, the peak loss tangent of the impregnated materials shifts towards higher temperatures, and the curve’s peak value for the impregnated materials is higher than that of the materials. The corresponding temperatures for the peak loss tangents of poplar wood and impregnated materials are 61 °C and 157 °C, respectively, indicating the glass transition temperatures of both materials. The higher glass transition temperature of the impregnated materials is attributed to the reinforcing effect of the silica sol/phenolic resin composite impregnating solution within the wood [44], indicating that the thermal stability of impregnated poplar wood is superior to the materials.

3.2.4. Thermogravimetric–Differential Thermal Gravimetric Analysis

To investigate the influence of silica sol/phenolic resin composite impregnating solution on the thermal properties of poplar wood, thermal performance tests were conducted on the materials before and after modification. Figure 9 and Figure 10 represent the TG–DTG curves of poplar wood before and after modification. From Figure 9, it can be observed that the thermal decomposition of poplar wood can be mainly divided into four stages. In the temperature range of 25–120 °C, it is primarily attributed to the evaporation of moisture inside the wood. In the temperature range of 120–220 °C, the main components of the wood decompose into substances such as CO and CO₂. At temperatures ranging from 220 to 400 °C, the wood undergoes intense thermal decomposition, with a mass loss ranging from approximately 45.0% to 79.5%. At temperatures between 400 and 800 °C, the wood starts to combust, and the residual mass fraction of the modified material at 800 °C is 21.81%, significantly higher than that of the control material (9.83%). This indicates that the impregnated material exhibits improved heat resistance compared to the original material.

From the DTG curve, it can be observed that the maximum decomposition rate temperature of the original material is 358.7 °C, which is approximately 59 °C higher than that of the impregnated material (300.2 °C). This indicates that the poplar wood treated with silica sol/phenolic resin impregnation undergoes earlier decomposition and char formation, attributed to the composite impregnating solution disrupting the crystalline structure. Therefore, it can be concluded that the thermal stability of poplar wood impregnated with silica sol/phenolic resin is significantly improved compared to the original material. Additionally, the cross-linking structure formed between the silica sol/phenolic resin composite impregnating solution and the wood facilitates the charring process.

3.2.5. Scanning Electron Microscopy Analysis

To determine the distribution of the composite impregnating solution within poplar wood, SEM was utilized to observe the microstructure of both the original material and the impregnated material. Figure 11a,b shows the cross-sectional SEM images of the materials. A comparison reveals that the untreated material contains hollow and permeable vessels, while the impregnated material, due to the penetration of the silica sol/phenolic resin into the wood and subsequent polymerization and solidification upon high-temperature drying, exhibits vessels filled with the impregnating solution. Figure 11c,d displays the radial section SEM image, where it can be observed that most of the pits and ray cells along the rays of the impregnated material are filled with resin, indicating successful permeation of the impregnating solution into the wood vessels and ray cells, achieving a desirable deposition effect. Figure 11e,f presents the tangential section SEM image, demonstrating the uniform distribution of the silica sol/phenolic resin composite impregnating solution within the vessels and pits of the impregnated material. Infrared spectroscopy analysis further confirms that the composite impregnating solution undergoes cross-linking reactions with the active groups of the wood, enabling its stable presence within the wood structure [45], thereby enhancing the dimensional stability and mechanical properties of the wood.

4. Conclusions

In this endeavor, silica sol of varied particle sizes and phenolic resin were harnessed to concoct a composite impregnation liquid via in-situ polymerization, followed by the application of a vacuum pressure impregnation technique on poplar wood, aiming for a significant enhancement in its mechanical prowess. A systematic exploration was conducted concerning the pivotal factors such as the type of composite impregnation liquid, applied pressure, and impregnation duration on the physical and mechanical attributes of poplar wood, facilitated by rigorous experimental design and analysis to ascertain the optimal process conditions. With the aid of analytical apparatus including FT-IR, TGA, XPS, and SEM, this investigation delved into the chemical structure, thermal stability, and microstructural metamorphoses of the modified poplar wood, unraveling the precise distribution and interaction mechanics of the silica sol/phenolic resin composite impregnation liquid within the wood.

The deductions are delineated as follows:

• The variety of composite impregnation liquid, the exerted pressure, and impregnation duration bear a significant imprint on the physical and mechanical attributes of poplar wood. The apex process conditions include employing S15/PF composite impregnation liquid, a pressure magnitude of 1.2 MPa, and an impregnation span of 3 h.
• Unlike preceding studies that solely deduced based on density and mass alterations, the current investigation, steered by FT-IR, XPS, and SEM analysis outcomes, distinctly unveiled the formation of Si-O-C chemical bond liaisons between the silica sol/phenolic resin composite impregnation liquid and the wood, achieving not merely physical filling but also a notable elevation in the dimensional stability and mechanical characteristics of poplar wood. Especially through the silica sol/phenolic resin composite impregnation, the impact toughness of poplar wood was remarkably amplified, showing an augmentation of 84.1% and 135.4% compared to the untreated material and material solely treated with phenolic resin, respectively.
• DMA analysis exhibited that the ingress of composite impregnation liquid markedly amplified the thermo–mechanical performance of impregnated poplar wood, rendering its thermal and toughness supremacy over the original material.
• TGA curve exploration displayed that, juxtaposed with the original material, the impregnated material boasts superior thermal stability with diminished mass loss in the carbonization phase, thereby significantly boosting its thermal resistance.
• The innovation of this research lies in the efficacious modification of poplar wood through the silica sol/phenolic resin composite impregnation liquid, probing deeply into the distribution of the impregnation liquid in the wood and the influence of impregnation process parameters on the attributes of the modified material, elucidating the principles of impregnation-enhanced mechanical properties of wood, and the structure–activity rapport between the modifier and material properties. This novel methodology furnishes fresh avenues and technical buttress for the modification of poplar wood and other biomass materials, propelling the advancement in wood science and engineering technology.