Phenolic resin impregnated modified materials have partially reduced the toughness of the wood. In this study, organic-inorganic composite modifiers were used to modify the wood. Silica sol/phenolic resin composite impregnating solution was prepared by an in-situ polymerization method and used to impregnate poplar wood under vacuum pressure. Orthogonal experiments were conducted, and the impact toughness of the modified poplar wood was used as the evaluation index. The results showed that the best impact toughness was achieved when the average particle size of silica sol was 8-15 nm, the pressure was 1.2 MPa, and the pressure time was 3 hours. The impact toughness of the impregnated modified poplar wood was improved by 84.1% compared to the untreated material. FT-IR results revealed the characteristic absorption peak of Si-O-Si in the impregnated poplar wood, confirming the formation of new chemical bonds. XPS analysis indicated that a chemical reaction occurred between the impregnating solution and the wood, resulting in the formation of Si-O-C. Furthermore, DMA analysis revealed that this chemical reaction significantly improved the thermal stability and toughness of the impregnated material, surpassing the original poplar wood. In addition, TG analysis revealed a significant improvement in thermal stability of the impregnated material compared to untreated poplar wood. Finally, SEM analysis demonstrated the successful infiltration and filling of the poplar wood by the composite impregnating solution.
Research Article
Research on the Impregnation Process and Mechanism of Silica Sol/Phenolic Resin Modified Poplar Wood
https://doi.org/10.21203/rs.3.rs-3114749/v1
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toughness
modified
poplar wood
silica sol/phenolic resin.
Fast-growing poplar wood, as a natural biomass material, is abundant in resources. However, its poor dimensional stability and low mechanical properties have diminished its value in usage. (Bi et al. 2022). In order to enhance its mechanical properties and added value, functional modifications need to be carried out to address its defects. (Xie et al.2012). Phenolic resin impregnation modification is one of the most common methods of modification. (Xu et al. 2022;Guan et al.2022). Phenolic resin is characterized by its low cost and easy availability of raw materials. (Zhang et al.2018). When used as an impregnation modifier, low molecular weight thermosetting phenolic resin is commonly used for wood impregnation modification. Its advantage lies in enhancing the physical and mechanical properties of the wood. (Guan et al.2018; Liu and Wang 2006;Yue et al.2010; Furuno et al.1993). However, its disadvantages include causing wood swelling, surface deformation, and reduced impact toughness, which can lead to wastage during subsequent processing. (Wang et al. 2014). The use of inorganic nanoparticles to toughen phenolic resin is a current research focus. ( Sun et al.2021;Zhang et al.2021). This method involves introducing inorganic nanoparticles into phenolic resin. The inorganic nanoparticles, due to their extremely small particle size and high surface area, can interact with the phenolic resin molecules at the microscopic level, thereby enhancing the toughness and heat resistance of the wood. (Zhang et al.2021).
Nano silica sol, as a commonly used inorganic nanoparticle, is utilized to enhance the toughness and heat resistance of phenolic resin due to its excellent thermal stability and mechanical properties. (Chen et al.2015;Zhang et al.2021;Zhang et al. 2021). In the process of preparing composite materials, nano silica sol is mixed with phenolic resin to form a homogeneous mixture. After curing and heat treatment, the nano silica sol is uniformly dispersed in the matrix of the phenolic resin, resulting in a high-performance nanocomposite material. This composite material not only inherits the good properties of phenolic resin but also benefits from the addition of nano silica sol, resulting in a significant improvement in toughness and heat resistance. (Wang et al.2006;Tan et al.2009;Zhang et al.2015). It is worth noting that the addition of nano silica sol does not alter the fundamental properties of phenolic resin. On the contrary, it allows for fine-tuning of the composite material's performance by adjusting its content. Therefore, this preparation method has tremendous potential for widespread applications and is expected to play a significant role in the manufacturing of various high-performance composite materials. (Pappas et al.2005;Liu et al. 2010;Liu and Hu 2004;Shams et al.2004;Qin et al.2023).
To address the issues of low toughness in phenolic resin-modified fast-growing poplar wood, this experiment employs three different particle sizes of silica sol and phenolic resin to form a composite through in-situ polymerization. The modified poplar wood is then treated using a vacuum pressure impregnation process. The study investigates the effects of the type of composite impregnation solution, pressure, and duration on the weight gain, water absorption, and mechanical properties of the modified wood. The goal is to optimize the process conditions for vacuum pressure impregnation modification of poplar wood. Furthermore, analytical techniques such as Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) are employed to analyze the microstructure, thermal stability, and other changes in the modified poplar wood. The aim is to explore the mechanism of silica sol/phenolic resin impregnation-enhanced modification of poplar wood. This research aims to provide necessary technical support and scientific theory guidance for the development of composite modifiers and subsequent impregnation of modified wood.
2.1 Experimental materials
Test specimen: Chinese White Poplar (Populus davidiana), with a density of approximately 0.42 g/cm3, was selected as the experimental material. The wood was prepared in three different sizes: 300 mm × 20 mm × 20 mm, 20 mm × 20 mm × 20 mm, and 70 mm × 50 mm × 50 mm. These wood specimens were dried to a moisture content ranging from 7% to 12% and kept as reserves. A total of 10 specimens were prepared for each test. Phenol, analytical grade, supplied by Liaoning Quanrui Reagent Co., Ltd., China.Sodium hydroxide, analytical grade, supplied by Tianjin Damao Chemical Reagent Factory, China.Formaldehyde, analytical grade, supplied by Tianjin Damao Chemical Reagent Factory, China.Silica sol, with average particle sizes of 8-15 nm, 15-30 nm, and 80-120 nm, provided by Jinan Yinfeng Silica Products Co., Ltd., China.Silane coupling agent KH-560, purity of 97%, supplied by Shandong Yousu Chemical Technology Co., Ltd., China.Anhydrous ethanol, analytical grade, supplied by Tianjin Damao Chemical Reagent Factory, China.Distilled water, prepared in the laboratory.
2.2 Experimental
InstrumentsDHG-9075-Ⅲ Electric Blast Drying Oven, Shanghai Yiheng Technology Co., Ltd., China;DWD-100E Universal Mechanical Testing Machine, Jinan Experiment Group Co., Ltd., Shidai Group, China;SY8200T Ultrasonic Cleaner, Shanghai Shengyuan Ultrasonic Instrument Equipment Co., Ltd., China;JB-300B Impact Testing Machine, Jinan Shijin Group Co., Ltd., China;VPI250 Vacuum Pressure Impregnation Tank, Shenyang Weike Vacuum Technology Co., Ltd., China;FA2004 Precision Balance, Tianjin Tianma Hengji Instrument Co., Ltd., China;01120028 Vernier Caliper, Shanghai Taihai Gongliangju Co., Ltd., China;WQF-510A Infrared Spectrometer, Beijing Rui Li Analytical Instrument Co., Ltd., China;ESCALAB250Xi X-ray Photoelectron Spectroscopy Analyzer, Thermo Fisher Scientific Inc., USA;DMA242E Dynamic Mechanical Analyzer, NETZSCH Instrument Manufacturing GmbH, Germany;TG 209 F3 Thermogravimetric Analyzer, NETZSCH GmbH, Germany;Quanta200 Environmental Scanning Electron Microscope, Philips (PEI) Netherlands.
2.3 Preparation of modified silica sol
Weigh 200 g of silica sol and add it to a mixture of ethanol and water, with both ethanol and water weighing 100 g each. After ultrasonic agitation for 30 minutes, transfer the mixture to a three-necked flask. Set the temperature of a constant-temperature water bath to 60℃. During the reaction, add 9 g of KH-560 dropwise and stir the reaction for 6 hours. After the reaction is complete, a pale blue modified silica sol solution is obtained. Pour out the modified silica sol and let it stand for further use.
2.4 Preparation of Silica Sol/Phenol-Formaldehyde Resin Composite Impregnation Solution
Phenol and formaldehyde were taken in a molar ratio of 1:2.1, and added to a beaker. Different mass fractions of modified silica sol (calculated based on the mass percentage of phenol) were added. The mixture of phenol, formaldehyde, and modified silica sol was subjected to ultrasonic oscillation for 30 minutes to ensure uniform dispersion of the modified silica sol in the solution. The mixture was then transferred to a three-neck flask. Sodium hydroxide solution with a mass fraction of 30% (based on 5% of the total amount of phenol and formaldehyde) was added. The reaction temperature was set at 90°C, and the reaction was terminated after 2 hours, resulting in the formation of the silica sol/phenol-formaldehyde resin composite impregnation solution.
2.5 Preparation of Silica Sol/Phenol-Formaldehyde Resin Impregnated Modified Material
Drying the poplar wood materials of three different specifications until the moisture content reaches 7% to 12%. Placing the poplar wood materials into the impregnation tank and evacuating the tank to -0.095 to -0.2 MPa, maintaining the vacuum for 30 minutes. Utilizing the pressure difference between the inside and outside of the tank to draw the impregnation solution into the tank. Then applying a certain pressure to the tank and maintaining it for a certain period. Finally, releasing the pressure and removing the impregnated and modified poplar wood. The wood is first air-dried for 24 to 48 hours, followed by drying the modified material until the moisture content reaches 7% to 12% for water absorption and mechanical property testing.
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.
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).
（2）The physical and mechanical properties of the impregnated material are evaluated through various tests. The water absorption, bending strength, modulus of elasticity, hardness, and impact toughness are determined following the standards specified as GB/T1927.7-2021 for water absorption, GB/T1927.9-2021 for bending strength, GB/T1927.10-2021 for modulus of elasticity, GB/T1927.19-2021 for hardness, and GB/T1927.17-2021 for impact toughness of defect-free small wood specimens. These standards provide the necessary guidelines and procedures for conducting the respective tests and assessing the performance of the impregnated wood.
（3）Fourier Transform Infrared (FT-IR) spectroscopy: To investigate the structural changes of yangmu (Populus davidiana) before and after modification, the samples were crushed using a grinder and sieved through a 200-mesh screen. Sample preparation was done using the KBr pellet method, and the samples were then subjected to Fourier Transform Infrared (FT-IR) spectroscopy. The infrared spectra of both the yangmu raw material and the impregnated material were measured in the wavenumber range of 500-4000 cm-1
（4）XPS (X-ray Photoelectron Spectroscopy) Analysis: The samples were crushed and sieved through a 200-mesh screen for X-ray photoelectron spectroscopy analysis. The analysis was conducted using an Al Kα source with an X-ray beam energy of 100 W. The analyzer had a grating diameter of 200 mm and a sensitivity of 350 kcps. The analysis chamber was maintained at a vacuum level of 10-8 bar.
（5）DMA (Dynamic Mechanical Analysis) Testing: To determine the dynamic viscoelastic properties of both the untreated wood material and the impregnated material, a three-point bending method was employed. The samples were prepared with dimensions of 40 mm × 20 mm × 3 mm. The DMA testing was conducted with a specified amplitude of 60 μm, a testing frequency of 2 Hz, and a temperature range of 30-350 ℃. The heating rate was set at 3 ℃/min. Each sample was tested four times, and the average values were calculated to ensure accuracy and repeatability of the results.
（6）Thermogravimetric Analysis (TGA): The samples were subjected to thermogravimetric analysis to determine their thermal decomposition behavior and weight loss characteristics. The TGA measurements were conducted under a controlled nitrogen (N2) atmosphere. The samples, with an approximate weight of 5 mg, were heated from 30 to 800 ℃ at a heating rate of 10 ℃/min. During the analysis, the weight of the samples was continuously monitored as a function of temperature, allowing for the determination of the thermal stability and decomposition patterns of the samples.
（7）Scanning Electron Microscopy (SEM): Cross-sectional and radial-sectional samples were obtained from both the untreated material and the impregnated material, with dimensions of 10 mm × 10 mm × 10 mm. The wood specimens were softened by boiling them in hot water. Sharp blades were used to cut thin wood slices measuring 8 mm × 8 mm × 1.5 mm from the wood at a distance of 2-5 mm from the surface. These thin wood slices were dried until a constant weight was achieved and kept for further use.The specimens were subjected to gold sputter coating prior to electron microscopy scanning. The electron microscopy scanning focused on the labeled regions of interest, including the cross-sectional, radial-sectional, and tangential-sectional surfaces.This analysis provided detailed observations of the microstructure and surface characteristics of the samples, allowing for a closer examination of their internal composition and arrangement.
3.1 Research on the Process of Silica Sol/Phenolic Resin Impregnation and Modification Treatment of Poplar Wood
Table 1 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 2 shows the results of the orthogonal experiment for weight gain and water absorption rate, while Table 3 displays the results of the orthogonal experiment for mechanical properties. Table 4 presents the results and analysis of the orthogonal experiment for weight gain, water absorption rate, and flexural strength, while Table 5 provides the results and analysis of the orthogonal experiment for elastic modulus, impact toughness, and hardness.
Table 1 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 |
Table 2 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 |
Table 3 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 |
Table 4 Orthogonal Test Results and Analysis of Weight Gain Rate, Water Absorption Rate, and MOR
|
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 |
Table 5 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 |
The optimal process conditions for impregnation and modification of poplar wood were determined using the vacuum pressure impregnation method. From Tables 2, 3, 4, and 5, 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) > pressing 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) > pressing pressure (B). The optimal combinations were C3B3A2 for water absorption rate and C3B2A1 for flexural strength.The factor affecting elastic modulus followed the order: pressing pressure (B) > composite impregnation solution (A) > pressing time (C). The optimal combination was C3B1A1.In conclusion, it can be seen that pressing 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 6, it can be observed that F0.01 > FC > F0.05 > F0.1 > FB > FA, indicating that the pressure time (C) has the greatest influence on the weight gain of yang wood, followed by the pressure (B), and the composite impregnating solution (A) has the smallest effect. The composite impregnating solution penetrates into the wood, solidifies, fills the wood's pores, and reacts with the components of the wood's cell walls, depositing inside 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 hours.
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 (Yao and Pu 2009) . From the variance results in Table 7, it can be observed that F0.01 > FC > F0.05 > F0.1 > FA > FB. 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 hours and a silica sol particle size of 15 nm can be selected.
3.1.3 The analysis of flexural strength
The flexural strength of wood refers to its ability to resist bending and fracturing, and it is an important parameter to consider in the design of furniture, wooden structures, and other components (Sjostrom 1993). From Table 8, it can be observed that F0.01 > F0.05 > F0.1 > FC > FA > FB, indicating that the influence of pressing time (C) is the greatest, followed by the composite impregnation solution (A), and the impact of pressing pressure (B) is the least. Table 3 shows that the flexural strength of Yangmu 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 Yangmu matrix and preventing cracking during load-bearing. Additionally, the impregnation solution can chemically bond or form hydrogen bonds with Yangmu. With 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 hours 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 9, it can be observed that F0.01 > F0.05 > F0.1 > FB > FA > FC, indicating that the effect of pressing time (B) on the flexural modulus of wood is the greatest, followed by the effect of composite impregnation solution (A), while the effect of pressing pressure (C) is the smallest. As shown in Table 3, the flexural modulus of impregnated Yangmu 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 crosslinking 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 pressing pressure of 1.2 MPa and a pressing time of 3 hours.
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(Bodig and Jayne 1982). The variance analysis results in Table 10 indicate that F0.01 > FC > F0.05 > FB > F0.1 > FA, with a significant effect of pressing time (C), followed by pressing pressure (B), and the least effect from the composite impregnation solution (A). As shown in Table 3, Yang 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. The reinforcement and toughening effect of the silica sol/phenol-formaldehyde resin composite lead to an increase in the impact toughness of the modified Yang wood. Therefore, for impact toughness, the appropriate values for pressing time and pressing pressure are 3 hours and 1.2 MPa, respectively
Table 6 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：F0.01(2，2)=99.00 F0.05(2，2)=19.00 F0.1(2，2)=9.00
Table 7 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：F0.01(2，2)=99.00 F0.05(2，2)=19.00 F0.1(2，2)=9.00
Table 8 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：F0.01(2，2)=99.00 F0.05(2，2)=19.00 F0.1(2，2)=9.00
Table 9 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：F0.01 (2，2)=99.00 F0.05 (2，2)=19.00 F0.1 (2，2)=9.00
Table 10 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：F0.01 (2，2)=99.00 F0.05 (2，2)=19.00 F0.1 (2，2)=9.00
Table 11 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：F0.01 (2，2)=99.00 F0.05 (2，2)=19.00 F0.1 (2，2)=9.00
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 (Hoadley 2000). Table 11 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 pressing 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 hours 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 pressing pressure of 1.2 MPa, and a pressing time of 3 hours. 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 1a, 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 (Deng et al.2021). 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 crosslinking 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.
Table 12 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 |
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, X-ray photoelectron spectroscopy (XPS) analysis was conducted on both the poplar wood material and impregnated material, and the results are shown in Figure 1b,1c . From Figure 1b,1c, 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 1d 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. From Figure 1e,1f, it can be observed that the C1 content decreases in the impregnated material, which may be attributed to the oxidation of unstable end groups of the three major components in the wood into small molecular acid substances. On the other hand, the increase in C2 and C3 content is due to the chemical crosslinking 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 (Matuana et al.2011). 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
Dynamic mechanical analysis (DMA) was employed to analyze the dynamic viscoelasticity of Yang wood materials and impregnated materials. When the storage modulus of a material decreases rapidly, it indicates better toughness (Miyagawa et al.2010). Figure 2a 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(Li et al.2015). 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 (Li et al. 2012).. As seen in Figure 2b , 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 Yang wood materials. The peak loss modulus values for Yang 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(Huang et al. 2019).
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 2c, 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 Yang 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 (Lam et al.2016), indicating that the thermal stability of impregnated Yang 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 Yang wood, thermal performance tests were conducted on the materials before and after modification. Figures 2d and 2e represent the TG-DTG curves of Yang wood before and after modification. From Figure 2d, it can be observed that the thermal decomposition of Yang 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 Yang 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 Yang 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 Yang wood, SEM (Scanning Electron Microscopy) was utilized to observe the microstructure of both the original material and impregnated material. Figure 3a,3b shows the cross-sectional SEM images of the materials. A comparison reveals that the original 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 3c,3d 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 3e,3f 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(Lang et al.2013), thereby enhancing the dimensional stability and mechanical properties of the wood.
The study employed silica sol of different particle sizes and phenolic resin to prepare composite impregnating solutions through in-situ polymerization. Yang wood was subjected to impregnation and modification under vacuum pressure. The results demonstrated that the type of composite impregnating solution, applied pressure, and duration of pressure significantly influenced the physical and mechanical properties of Yang wood. The optimal processing conditions were found to be the use of S15/PF composite impregnating solution, a pressure of 1.2 MPa, and a pressure duration of 3 hours. Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) results revealed the formation of Si-O-C chemical bonds between the composite impregnating solution and the wood, enhancing the dimensional stability and mechanical properties of Yang wood. Dynamic mechanical analysis (DMA) indicated that the impregnation with the composite solution improved the thermal-dynamic properties of the impregnated material, resulting in better thermal performance and toughness compared to the original material. Thermogravimetric analysis (TGA) curves demonstrated that the impregnated material exhibited improved thermal stability, with reduced mass loss during the charring stage, indicating enhanced heat resistance.
Author Contributions: All authors are responsible for the full content of this submission and approve the submission.
Research funding: This research was supported by funding from multiple sources. Firstly, it was funded by the National Natural Science Foundation of China (No. 31972947 and 32171712). Secondly, the research was also supported by Jilin Innovative and Entrepreneurial Talents, project number 2023QN44, sponsored by Jilin Human Resources and Social Security Department. In addition, important financial support for this research was provided by the Jilin Provincial Key Science and Technology Tackling Project (No. 202202095NC) and the Jilin Provincial Science and Technology Development Innovation Platform (Base) and Talent Special Project (No. 20230202092NC), both sponsored by the Jilin Provincial Department of Science and Technology.
Conflict of interest statement: The authors declare that they have no conflict of interest with this article.
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No competing interests reported.