Study on the Effect of Acrylic Acid Emulsion on the Properties of Poplar Wood Modified by Sodium Silicate Impregnation

Abstract

Inorganic silicate impregnation-modified fast-growing wood shows improved mechanical properties and thermal stability, but inorganic silicate agent loss and moisture absorption affect its processability. This study proposes a method to improve the impregnating agent loss and modified wood moisture absorption of poplar wood modified by using an acrylic acid emulsion/sodium silicate composite. The acrylic acid emulsion coated the sodium silicate and cell wall surfaces with a cured film that blocked water molecules from entering the modified wood. The acrylic acid emulsion adhered to the wood and sodium silicate, thus reducing impregnating agent loss. The addition of the acrylic acid emulsion maintained the excellent mechanical properties of sodium silicate-modified poplar wood and greatly improved its bending strength. The water absorption and moisture swelling rate were significantly decreased, and the dimensional stability of modified poplar wood was more than 50% higher than that of unmodified poplar wood. The thermogravimetric analysis (TG)results showed that the addition of organic components reduced the heat resistance of modified wood, but the thermal stability was still higher than unmodified wood. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that acrylic esters in acrylic acid emulsion reacted with hydroxyl groups on sodium silicate and wood to form covalent bonds that improved the impregnating agent’s resistance to loss and reduced the moisture absorption of the wood. The modified poplar wood showed better dimensional stability and water resistance.

1. Introduction

Due to its fast growth, low weight, and moderate hardness, poplar (Populus L.) is widely used in construction, furniture, and papermaking applications [1,2]. Impregnated modified poplar is an environmentally friendly and durable wood product that has gradually received attention for research and applications [3,4,5,6]. The modification of poplar by inorganic substances can improve its structure and performance, thus achieving its high-value utilization. Silicification modification is widely used to prepare inorganic composite wood with improved strength, weather resistance, and fire resistance. Alkali metal silicate solution, also known as water glass, generates silicic acid gel and silica solids after hardening. Silicic acid gel strengthens surfaces during high-temperature drying, and the mesh skeleton of the silica solid makes it heat-resistant [7]. Therefore, sodium silicate modification can enhance the compressive strength and hardness of poplar wood, while also imparting fire resistance and smoke suppression properties [8,9]. Sodium silicate-impregnated modified poplar wood has a high moisture absorption rate during use, making it susceptible to dampness and mold. Additionally, there is easy loss of impregnating agent and size instability, which affect its service life and quality.

Dong et al. [10] and Yona et al. [11] showed that the combination of inorganic and organic agents can partially compensate for their respective deficiencies. Chen et al. [12] used a mixture of hydroxymethyl urea and sodium silicate to impregnate poplar wood. Sol-gel modifiers were fixed in the micropores of wood by heating and drying, followed by in situ gel polymerization to form a Si-O-Si framework. The modified wood obtained higher mechanical strength and lower moisture absorption. Bi et al. [13] used sodium silicate and dimethyloldihydroxyethyleneurea as a composite modifier to perform vacuum-pressure impregnation modification on fast-growing poplar trees, thereby forming a bridging structure. This treatment immobilized sodium silicate and significantly improved the size stability of poplar wood. To improve the performance of poplar wood, Liu et al. [14] prepared a glucose-urea-melamine resin/sodium silicate composite modifier and then performed impregnation modification on plantation poplar wood. The synergistic effect of organic-inorganic compounds greatly improved the physical and mechanical properties of wood, reduced its moisture absorption, and improved its size stability. However, all of the above organic-inorganic composite systems contained aldehyde agents, which raises concerns about their safety and makes them difficult to apply in indoor environments [15].

Acrylic acid emulsions are milky white or nearly transparent viscous liquids composed of pure acrylic ester monomers copolymerized into an emulsion. They are versatile, high-performance emulsions with a small particle size, outstanding water resistance, weatherability, and good adhesion to various surfaces such as brick, wood, and steel.

Its excellent film-forming properties enable good coverage of the target, and acrylic emulsions can be added to silicate cement to restrict the penetration of corrosive impregnating agents, thus protecting concrete and extending its service life. Compared with untreated concrete, the corrosion rate after exposure is reduced by 96% [16]. Coating the surface of medium-density fiberboard with an acrylic emulsion improves the wetting of the board surface and enhances adhesion between the coating and the wood due to the rapid movement of particles and film formation of plasma-treated samples [17]. To improve the performance of artificial poplar wood, water-based acrylic resins have been used for surface impregnation. Single-sided densified wood was developed, which showed high dimensional stability, surface hardness, and bending performance [18]. In addition to being applied to the surface, the excellent flowability of acrylic emulsions also makes them suitable for impregnating the pores of wood. Gao et al. [19] designed an eco-friendly acrylic resin system to fill the cell cavities of poplar wood to produce a new type of poplar plywood with improved mechanical strength and dimensional stability, addressing the low strength and dimensional stability of fast-growing poplar wood.

Based on the excellent water resistance and weather resistance properties of acrylic emulsion and its adhesion to wood substrates, a waterborne acrylic resin and sodium silicate were used in this study to modify poplar wood by co-mixing and immersion. The good film-forming property and water resistance of the acrylic emulsion improved the hygroscopicity of silicates and enhanced the impregnating agent’s adhesion. This study explores the modification of poplar wood via immersion in inorganic sodium silicate, improving its properties.

2. Materials and Methods

2.1. Materials

Fast-growing poplar (Populus L.) from Chenzhou, Hunan province was used as the experimental wood and processed according to national standards based on the desired performance tests. There were four types of mechanical test pieces: bending strength test piece 20 mm × 20 mm × 300 mm (R × T × L), hardness test piece 50 mm × 50 mm × 70 mm (R × T × L), compressive strength test piece 20 mm × 20 mm × 30 mm (R × T × L), and density test piece 20 mm × 20 mm × 20 mm (R × T × L), with 48 samples of each. Smooth and knot-free samples were selected from all six sides, with burrs removed before use. They were then placed in a drying oven for gradient drying and backup. A sodium silicate solution with a modulus of 3.4 M and a solids content of 45% was purchased from Hunan Hetang Chemical Co., Ltd., Xiangtan, China. An acrylic emulsion with a solids content of 40.0% and viscosity of 152 mPa·s was purchased from Shenzhen Jitian Chemical Co., Ltd., Shenzhen, China. Ultra-pure water was made in the laboratory.

2.2. Preparation of Modified Poplar Wood

Prepare 5L of acrylic emulsion with 7% mass fraction, 30% sodium silicate solution with mass fraction and acrylic/sodium silicate compound solution respectively. Sawn poplar wood was divided into four groups: untreated poplar wood (NP) as the control group, and the other three groups were treated with impregnation modification. The preparation procedure is shown in Figure 1. The reserve poplar wood was placed in the impregnation tank and treated with three different agents using the vacuum-positive pressure cycling method [20]. The key procedure parameters were a vacuum degree of −0.095 MPa and a retention time of 5 min. The pressure was 0.5 MPa, and the retention time was 30 min. The above method was repeated for four cycles to prepare one group of acrylic acid-modified poplar wood (AA), one group of sodium silicate-modified poplar wood (SS), and one group of acrylic acid/sodium silicate-modified poplar wood (AA/SS), respectively. Then, the modified poplar wood surface was cleaned and air dried for 24 h.

The specimens were placed in an oven for gradient drying at 40/60/80 °C and then dried to a constant mass at 103 ± 2 °C. After being removed, they were placed in a drying dish to cool to room temperature for standby.

2.3. Characterization Test Methods

2.3.1. Hardness Test

The hardness was tested according to the GB/T1941-2009. The number of tested specimens per group was 12 (The number of subsequent tests is consistent with this). Each specimen was tested twice on three cutting surfaces: longitudinal, radial, and tangential. The speed of compression was 3–5 mm/min, and the depth of compression was 5.64 mm. The load reading was accurate to 10 N.

2.3.2. Bending Strength and Compressive Strength Test

The bending strength of wood was performed according to the standard GBT1936-2009. The dimensions of the sample were 300 mm × 20 mm × 20 mm, with a radius of curvature of 30 mm for both the pressure head and the support and a distance of 240 mm between the supports. The force was applied on the tangential surface of the sample.

The compressive strength of wood along the grain was measured according to the standard GB/T1935-2009. The dimensions of the sample were 30 mm × 20 mm × 20 mm, with the length along the grain direction. The sample was loaded at a constant speed, and the failure time should be between 1.5 and 2.0 min.

2.3.3. Water Absorption Test and Swelling Test

According to the test method for wood water absorption specified in GB/T 1934.1-2009, the specimen size should be 20 mm × 20 mm × 20 mm. During the test, the sample was pressed into the water surface at least 50 mm, and the surface moisture should be absorbed with absorbent paper each time the weight was measured until the difference between the moisture content of the last two measurements was less than 5%. After soaking for 7 d, the size was tested with an accuracy value of 0.001 mm according to the wet swelling test method specified in GB/T 1927.8-2021.

2.3.4. Fourier-Transform Infrared (FTIR) Spectroscopy

A small poplar wood sample was placed into a wood grinder and crushed into a powder. The FT-IR test equipment is the Thermo Fisher iS50. The sample powder with a particle size smaller than 0.074 mm (200 mesh) was collected and pressed into transparent KBr slices for analysis. The wavenumber range of the FTIR spectra was 400–4000 cm⁻¹.

2.3.5. X-ray Diffraction (XRD)

The thin slices of poplar wood were dried in a vacuum drying oven at 60 °C for 48 h to remove moisture. XRD was used to test the crystallinity index (CrI) of the sample under a voltage of 36 kV and a current of 20 A. The test angle range of the sample was 5–40°, and the scanning speed was 4° min⁻¹. The calculation formula of the CrI was as follows:

$$\mathit{CrI}=\frac{I_{002} {- I}_{\mathrm{am}}}{I_{002}}$$

(1)

where I₀₀₂ is the maximum diffraction peak intensity of the main crystalline peak 002 (a.u.), and I_am is the diffraction peak intensity at 18° (a.u.).

2.3.6. Scanning Electron Microscopy (SEM)

After gold sputtering the poplar wood standard sample, it was fixed on the loading platform with a conductive adhesive. The morphology of the end and radial sections of the poplar wood was observed by SEM (FEI Quanta200, Hillsboro, OR, USA) under a 20 kV accelerating voltage.

2.3.7. Thermogravimetric Analysis (TGA)

A suitable amount of wood powder was added to a micro-crucible and tested using a thermogravimetric analyzer (German Netzsch TGA209F1, Selb, Germany) under an air atmosphere and heated from room temperature to 750 °C at a heating rate of 10 °C/min.

2.3.8. Statistical Analysis

The type of ANOVA test was Least Significant Difference (LSD). According to the minimum significant difference standard of 95% confidence level, the results of ANOVA-test showed significant difference (p < 0.05).

3. Results

3.1. Effect of Acrylic Emulsion on the Mechanical Properties and Thermal Stability of Sodium Silicate-Modified Poplar Wood

Sodium silicate-modified poplar wood has significantly improved mechanical properties and thermal stability. However, the addition of an acrylic emulsion dilutes sodium silicate, which is likely to affect the properties of sodium silicate-modified poplar wood. Figure 2a showed the SEM images of the modified poplar wood. AA modified poplar wood due to the low concentration of acrylic emulsion mainly adhered to the cell walls. Although SS modified poplar has better filling effect, it is difficult to bind closely with the cell wall. AA/SS modified poplar wood combines the advantages of both modifiers perfectly. The internal filler of the wood has a smooth surface and is tightly bound to the cell walls. To investigate the influence of the acrylic emulsion on the mechanical properties of poplar wood, compressive strength, bending strength, and hardness tests were conducted, as shown in Figure 2b,c.

The acrylic emulsion slightly improved the compressive and bending strength of poplar wood. The acrylic emulsion penetrated the poplar cell cavities, and after the water evaporated, the components only needed to infiltrate and adhere to the cell walls, where they played a binding role in the original structure during lateral force loading, helping it withstand greater loads. Therefore, its improvement relative to the natural poplar wood was worse when subjected to longitudinal stresses. After sodium silicate cures, it becomes brittle, so the compressive strength of poplar modified by sodium silicate was significantly improved, but the bending strength was reduced. The addition of acrylic emulsion improved its toughness [21]. The compressive strength of the AA/SS modified poplar was still significantly improved, and the bending strength was better than that of the AA modified poplar. After the composite modified solution entered the wood, the acrylic emulsion improved the strength by soaking into the cell walls, where it also mixed with most of the sodium silicate to improve the toughness after curing [22].

Figure 2c showed the hardness test results, which showed the same trend as the compressive strength. Hardness mainly reflects the surface strength of wood. Acrylic emulsion does not improve the hardness, while filling and crystal hardness after sodium silicate curing improved the surface hardness of poplar. The hardness of the AA/SS modified poplar was similar to that of the SS modified poplar, indicating that in the AA/SS compound system, the sodium silicate curing agent was mainly responsible for the mechanical properties of the modified wood.

An organic acrylic emulsion was used to explore its effect on the heat resistance of the original inorganic salt system. Therefore, modified materials and acrylic curing materials were subjected to thermogravimetric analysis, as shown in Figure 2d,e. According to the TG results, the thermal stability of AA/SS modified poplar was much lower than that of SS modified poplar due to the addition of acrylic emulsion. The addition of organic filler reduced the thermal stability after modification, but it was still greater than that of the natural poplar wood [23]. The acrylic acid cured material almost completely decomposed at 700 °C, with only 1.48% residue, which could not provide it with heat-resistant substances to improve the thermal stability of the modified poplar wood.

According to the DTG results, the addition of sodium silicate reduced the thermal decomposition rate of the poplar because, after solidification, the inorganic minerals reduced the heat conduction and flammability, thus achieving flame retardancy by reducing the combustibles content. Due to its alkalinity, it changed the chemical composition and crystal structure of poplar after impregnation, and the thermal decomposition temperature of cellulose decreased from about 330 °C to about 270 °C after heating. The earlier pyrolysis temperature promoted the carbonization of wood, avoided the generation of flammable gases at higher temperatures, and avoided or delayed violent combustion (explosions). The addition of acrylic emulsion increased the pyrolysis rate of poplar at 430–450 °C [24]. Analysis of the AA solid material showed that acrylic acid thermally decomposed at this temperature, but due to its combination with sodium silicate, only minor changes during pyrolysis were found in AA/SS modified poplar.

The addition of an acrylic emulsion to the sodium silicate modified poplar system improved the bending strength of the sodium silicate modified poplar and maintained other strength properties such as compressive strength and hardness. The heat resistance of the AA/SS modified poplar was slightly lower, but it still showed a greater improvement in the overall heat stability of the modified poplar.

3.2. Effect of Acrylic Emulsion on Water Resistance

The modification of poplar wood with sodium silicate significantly improved its strength and thermal stability, but there were some drawbacks in high-humidity environments. After sodium silicate curing in poplar wood, sodium silicate is exposed to the environment, resulting in high hygroscopicity and a tendency to absorb moisture and mold. In addition, complete curing of sodium silicate in the multi-level voids of poplar wood is difficult to achieve, leading to the loss of the agent, instability in wood size due to water entering the cell walls, and shortened service life and quality. To explore the water resistance performance enhancement effect of acrylic acid emulsion on sodium silicate-modified poplar wood, the water absorption rate, swelling rate, and ASE of natural poplar, AA modified poplar, SS modified poplar, and AA/SS modified poplar wood were tested, and the results are shown in Figure 3.

The water absorption rates of the samples of various modified poplar woods after soaking in pure water for 60 h are shown in Figure 3a. Due to the different degrees of internal void filling, the water absorption rates of modified materials significantly decreased. Natural poplar wood has more space to accommodate water due to its voids, resulting in the highest water absorption rate of 145%. The acrylic acid emulsion adhered to the cell wall in AA modified poplar wood, which blocked some small pores, making it difficult for water to penetrate deeply into the wood. However, since the cell cavities were unfilled, there was still some space available for water storage. The decreased water absorption rate in SS modified poplar wood was because sodium silicate filled the cell cavity after curing, preventing them from accommodating more water, so it only penetrated through loosely combined gaps and soaked along the cell wall. The water absorption rate of AA/SS modified poplar wood was the lowest, as both the cell wall and cell cavity were blocked by the two agents, resulting in a significant enhancement. However, it still had a water content of 40% since impregnation modification did not adequately fill and block all voids. A large amount of water was also attached to the surface of the samples during tests.

The swelling rate of AA/SS was also significantly improved, as measured by the tangential swelling rate (Figure 3b). The addition of sodium silicate did not significantly improve the swelling rate. Although the cell cavities were filled, moisture could still migrate within the cell walls, leading to volume expansion. The improvement was more significant after adding acrylic acid emulsion. The acrylic acid emulsion better formed a tightly attached film to the cell walls. Although moisture could still penetrate the gap and cell cavity, it had difficulty directly infiltrating the cell walls, thus reducing cell wall expansion. Based on this, the filling of the cell walls further enhanced the effect, giving the AA/SS composite the best size stability. Anti-swelling effectiveness (ASE) is a direct manifestation of size stability, which was enhanced in a step-wise manner after adding acrylic acid emulsion. Figure 3d showed the drug loading of poplar wood after modification with sodium silicate and leaching of agent after water immersion for 7 days. The addition of the acrylic acid emulsion in sodium silicate slightly increased the loading amount, and the bonding performance of acrylic acid emulsion made sodium silicate easier to retain. The agent leaching rate decreased from 35.3% to 17.7%. After observing the surface morphology of the modified poplar wood, it was initially determined that the good film-forming ability of acrylic acid prevented sodium silicate from directly contacting water and being dissolved and lost [25], as shown in Figure 3e. In addition, the linear polymerization of the acrylic acid emulsion allowed it to be interwoven into the sodium silicate. Its good molecular activity made the structure more compact and cured, making it difficult to dissolve. By adding acrylic acid emulsion, the moisture absorption and stability of sodium silicate-impregnated modified poplar wood were improved, increasing its water resistance and ultimately improving its service life and quality.

3.3. Influence of Acrylic Acid Emulsion on the Chemical Structure and Moisture Barrier Mechanism

To investigate the impact of curing agent additives on the crystal structure of cell walls, several modified poplars were analyzed using XRD (Figure 4a). The relative crystallinity of modified poplar wood was calculated by the Segal method [26], and the relative crystallinity of natural poplar, SS, AA, and AA/SS modified poplar wood was 56.4%, 54.6%, 44.0% and 48.3%, respectively. The results showed that impregnation can reduce the relative crystallinity of poplar. The infiltration of the solution may affect the crystallinity of the cell walls, especially the acrylic acid emulsion, which can penetrate and adhere to cell walls, causing the fiber bundles to swell. While enhancing the binding force, it will also induce disorder in the crystalline region and reduce the relative crystallinity.

The FTIR spectra of modified poplars are shown in Figure 4b. The peak at 1735 cm⁻¹ was due to C=O vibrations and is characteristic peak of hemicelluloses in poplar. Hemicellulose is the main component of moisture absorption in wood, and degradation of hemicellulose reduced the moisture absorption of modified poplar wood. This peak disappeared in SS and AA/SS modified poplar, indicating that the alkalinity of sodium silicate changed the chemical composition of the cell wall, which promoted the penetration and fixation of the curing agent in the cell walls. The mixing of acrylic emulsion and sodium silicate did not change the chemical properties of the sodium silicate solution. The peaks at 1600 cm⁻¹ and 900 cm⁻¹ are the vibration peaks of C=C, and the increased peak intensity in AA and AA/SS modified poplar demonstrates that AA impregnated poplar and was retained. The peak at 453 cm⁻¹ is the characteristic peak of Si-O-Si. Sodium silicate hydrolyzes into a silica tetrahedral structure in water, so this peak intensity increased.

Figure 4c showed the FTIR spectrum of AA emulsion and AA/SS composite solids. XPS was carried out on the natural poplar and modified poplar to further demonstrate the mechanism responsible for the performance improvement after compositing AA and sodium silicate. The XPS survey spectra are shown in Figure 4d. The addition of SS resulted in the appearance of peaks of Si and Na elements in the spectrum of modified poplar, indicating the distribution of sodium silicate in modified poplar.

To further verify the enhancement mechanism, the C 1s, O 1s, and Si 2p XPS spectra were obtained (Figure 5). According to data in the reference [27,28], there are four types of carbon atom binding forms and two types of oxygen atom binding forms, as shown in

Table 1. Assignment of the carbon and oxygen peak components, C1s, and O1s for modified poplar.

Group	Symbol	Chemical Bond Form	Binding Energy
Carbon	C1	C-C/C-H	284–286 eV
	C2	C-O	286–288 eV
	C3	C=O/O-C-O	288–289 eV
	C4	O-C=O	>289 eV
Oxygen	O1	O-C=O	530–532 eV
	O2	C-O	532–534 eV

. C1 corresponds to carbon atoms that only bond with other carbon atoms or hydrogen atoms, and this component mainly comes from lignin and wood extracts. C2 is due to carbon bonding with a single non-carboxylic oxygen atom, mainly derived from cellulose in wood. The C3 represents carbon atoms bonded to one carbonyl oxygen or two non-carbonyl oxygen atoms. C4 represents a carbon atom bonded to one carbonyl oxygen and one non-carbonyl oxygen.

As shown in Figure 5a, the C1 and C2 peaks are important structural components in modified poplar, and their changes after modification were small. The addition of AA in AA modified poplar increased the content of saturated C atoms in poplar, and the area of the C1 peak increased. Due to the presence of a large amount of C=O bonds in the ester group, the area of the C3 peak also increased. In SS modified poplar, the C2 peak was slightly weakened, mainly due to hemicelluloses destruction and a decrease in the structure of C atoms connected to non-carbonyl O atoms. The hydrolysis of sodium silicate with wood to form Si-O-C bonds also increased the C3 peak. In AA/SS modified poplar, changes in C atoms were mainly reflected in the C1 and C3 peaks. Besides the large number of saturated C atoms introduced by AA, some C1 peaks also transformed into C3 peaks. In addition to the above reasons, the acrylic acid emulsion polymerized and interlaced with sodium silicate, and the combination of silicate ions and ester groups of acrylic acid generated many Si-O-C bonds.

Oxygen atoms in XPS spectra can be divided into two components (Figure 5b). In the peak spectrum of Oxygen atom, the O1 peak comes from an oxygen atom connected to a carbon atom via a double bond. The O2 peak with higher binding energy represents an oxygen atom connected to a carbon atom via a single bond. The O1 peak increase is more significant in SS and AA/SS modified poplar wood. The former is due to the alkaline action that causes partial hemicellulose exposure. The latter is caused by the external introduction of ester groups, as well as the combined action of the carboxyl groups in acrylic acid and the hydroxyl groups on the cell wall. As shown in Figure 5c, the Si-O-C structure was formed inside both SS and AA/SS modified poplar, which proved that sodium silicate was able to form a chemical bond with wood and acrylic emulsion in the presence of sodium silicate [29].

Combining the XPS results, the strengthening mechanism of water-based acrylic acid emulsion on sodium silicate-modified poplar is shown in Figure 5d. The polymerization of water-based acrylic acid emulsion formed a linear high polymer, which was interlaced in cured sodium silicate, providing an enveloping effect. The active groups on the branch chain easily captured the sodium silicate groups to form a Si-O-C structure, thus greatly improving its resistance to loss in water. The small-molecule active substances in acrylic acid emulsion penetrated more into the cell wall, combined with the hydroxyl groups on the cellulose to form ester groups, thus preventing water from penetrating and enhancing the size stability of modified poplar.

4. Conclusions

In order to enhance the application range of fast-growing poplar wood, this study addressed the defects of the poor water resistance and easy loss of the agent in sodium silicate-modified poplar wood. By modification with a composite system (acrylic emulsion and sodium silicate), the water resistance and agent fixation were further improved while maintaining the advantages of inorganic sodium silicate-modified poplar wood. The mechanical property tests showed that the addition of acrylic emulsion did not reduce the mechanical properties. On the contrary, the bending strength was improved, and the shortcomings of sodium silicate’s brittleness and poor toughness were compensated. The addition of organic components slightly reduced the heat resistance, but the overall effect was insignificant, and the AA/SS modified poplar wood still had outstanding heat resistance. The acrylic emulsion attached to the surface of the cell walls and formed a thin film that reduced the number of pathways for water to penetrate the cell walls. This improved the size stability of poplar wood and strengthened the fixation of silicates in the pores of poplar wood after compositing, nearly doubling its water-leaching resistance. By analyzing the internal binding mechanism, carboxyl groups in the acrylic emulsion formed ester bonds with hydroxyl groups on cellulose, and the film adhesion, size stability, and water resistance performance were enhanced. The linear acrylic polymer molecules were interlaced with sodium silicate, and the silicate ions were captured and fixed, thus greatly improving the water-leaching resistance. The results of the study are expected to be able to advance the application of fast-growing poplar wood in outdoor humid environments.