Nano-Silica/Urea-Formaldehyde Resin-Modified Fast-Growing Lumber Performance Study

Abstract

To promote the performance of fast-growing poplar wood for furniture applications, this study proposes and investigates the feasibility of modifying fast-growing poplar wood with a urea-formaldehyde resin impregnating agent by adding nano-SiO₂ as a way to improve its physical and mechanical properties. By observing the solubility of nano-SiO₂ addition in urea-formaldehyde resin, determine the optimal ratio of nano-SiO₂ addition to the solid content of the urea-formaldehyde resin solution. After the fast-growing poplar specimens were treated with nano-SiO₂/UF resin, the water absorption, wet swelling, dry shrinkage, nail grip, flexural strength, and modulus of flexural elasticity of the fast-growing poplar specimens were compared and analyzed to determine the effect of impregnation modification and the optimal impregnation ratio. The results showed that the physical and mechanical properties of fast-growing poplar wood were significantly improved by impregnating the fast-growing poplar wood with urea-formaldehyde resin with SiO₂, and the impregnation modification was beneficial to reducing the wet swelling and dry shrinkage of poplar wood, increasing its dimensional stability, improve the grip nail strength, and increase the flexural strength and flexural modulus of elasticity with the increase in nano-SiO₂ concentration.

1. Introduction

The furniture manufacturing industry is one of the important industries in China. Furniture production in China can reach a leading level in the world; its production, consumption, and export scale are constantly expanding. This also means the annual consumption of huge amounts of resources, especially timber resources. However, China is also one of the countries with the least forest (wood) resources per capita [1]. In the face of the increasing lack of wood resources, there is an urgent need to seek new furniture materials [2]. Using fast-growing wood as furniture material has become a trend of wood raw material selection in China’s furniture manufacturing industry [3].

Poplar is the main tree species in China’s plantations, accounting for the largest proportion of fast-growing and high-yield forests. The fast-growing poplar’s growth speed, wide distribution, numerous varieties, strong adaptability, wide growth cycle, cultivation, and processing technology are constantly being improved. Full development and utilization of poplar can reduce deforestation resources and protect the ecological environment. However, poplar also has the disadvantages of low density, loose material, poor dimensional stability, soft material, and so on. Also, it is usually used to manufacture pulp or processed into low-value-added products such as plywood [4]. It can not yet be used to make solid wooden furniture. In order to improve the added value and utilization rate of poplar and improve its physical and mechanical properties, it is necessary to modify the research of poplar, so that its utilization range can be extended to the field of solid wood and furniture [5].

Common modification methods include impregnation treatment, high-temperature heat treatment, densification treatment, and nanomaterial reinforcement [6,7,8]. Among them, the impregnation modification can often achieve the effect of one reagent with multiple effects [9,10]. The commonly used wood impregnation modification reagents are small molecule phenolic resin, urea-formaldehyde resin, melamine formaldehyde resin, plastic monomer, and so on [11,12,13,14]. Among them, urea-formaldehyde resin as an impregnating material is more common, as its price is low and it is easy to prepare. Moreover, it can improve the physical and mechanical properties of impregnated wood to a certain extent. Cao et al. [15] used modified urea-formaldehyde resin as an impregnating agent to compare and analyze the surface properties of poplar wood by heat treatment, impregnation, and impregnation combined with heat treatment. After the modification treatment, the color of the wood becomes darker, and the surface roughness and hydrophobicity increase. HOU et al. [16] studied and tested the two main factors affecting the impregnation time and impregnation pressure in the impregnation process of fast-growing poplar wood with low molecular weight and low viscosity urea-formaldehyde resin, and obtained an optimized impregnation process. SHI et al. [17] proposed preparing UF-SiO₂-wood composite materials by impregnating poplar samples in the modified agents of urea-formaldehyde resin and nano-silica and heating them. The results showed that the modified agents of urea-formaldehyde resin and nano-silica improved the mechanical property of poplar samples. In recent years, with the continuous development of nanotechnology and its excellent size effect, surface effect, and performance, it has been widely used in the field of wood protection and modification, and has become one of the hot spots of wood science at home and abroad [5,18]. For example, Shen et al. [19] used phenolic resin (PF) and nano-SiO₂ powder as the main modifiers and coupling agents, and the results showed that the density, hardness, and mechanical strength of wood were improved. Nano-SiO₂ can effectively improve the flame retardation, wear resistance, strength, and other properties of materials [20,21,22,23].

Inspired by this, the species selected in this study is fast-growing poplar, which is a popular economic tree species for large-scale planting. The rapid-growing poplar wood was impregnated with urea-formaldehyde resin mixed with nano-SiO₂ to investigate the optimal ratio between urea-formaldehyde resin and nano-SiO₂. The aim is to increase the added value of fast-growing poplar trees and provide the possibility of its application in furniture manufacturing materials. The effects on the physical and mechanical properties of the fast-growing poplar wood were measured and analyzed.

2. Materials and Methods

2.1. Experimental Materials

Fast-growing poplar wood (Populus spp., 107 poplar) from Heze, Shandong Province, China: specimen size of 300 mm × 75 mm × 75 mm and moisture content of 8%~12%.

The information on low molecular weight water-soluble urea-formaldehyde resin solution is shown in

Table 1. Information on urea-formaldehyde resin solutions.

Urea-Formaldehyde Resin Solution Information	Solid Content	Relative Molecular Weight	PH Value	Viscosity	Free Formaldehyde Content
numerical	49.1%	300–500	7.0–9.5	≥20 mPa·s	0.19%

Hydrophilic nano-silica: molecular weight 60.08, 99.8% purity, particle size 7–40 nm, and specific surface area 200 m²/g.

2.2. Experimental Design

A single-factor experimental method was used to control the vacuum level, vacuum time, pressurization pressure, and pressurization time to determine the solid content of urea-formaldehyde resin and the mass ratio of nano-silica to the solid content of urea-formaldehyde resin (w = 0, 1, and 2%) was used as the only variable factor.

2.3. Experimental Procedure

As shown in Figure 1, the important steps of silica-modified urea-formaldehyde resin impregnation of fast-growing poplar wood specimens include fast-growing poplar wood treatment, equipping with modified urea-formaldehyde resin impregnation agent, and vacuum-pressure impregnation treatment.

2.3.1. Preparation of Nano-SiO₂-Modified Urea-formaldehyde Resin

The solid content of water-soluble urea-formaldehyde resin was diluted to 20%–25% with distilled water, and the solid content was about 21.5% in the actual test. After drying the nano-SiO₂ particles, the nano-SiO₂ mass was weighed according to the mass ratio of the nano-SiO₂ to the solid content of the urea-formaldehyde resin solution with the following equation:

$$W=\frac{M_1}{M_0\times S}\times 100\%$$

where W is the mass ratio (%) of nano-SiO₂ to the solid content of the urea-formaldehyde resin solution, M₁ is the mass (g) of nano-SiO₂, M₀ is the mass (g) of the diluted urea-formaldehyde resin solution, and S is the solid content (%) of the diluted urea-formaldehyde resin solution.

The nano-SiO₂ is slowly poured into the urea-formaldehyde resin solution for ultrasonic dispersion in small amounts, and the dispersion is determined from 30 min to 90 min depending on the amount, with sufficient stirring to make the nano-SiO₂ fully dispersed in the urea-formaldehyde resin suspension without agglomeration or precipitation formation.

The variable factor of impregnation treatment is the ratio of nano-SiO₂ addition to the solid content of urea-formaldehyde resin solution (W). In this paper, W values of 0.5%, 1.5%, 2.5%, and 3.5% were selected for impregnation in the pre-experiments. The results of the pre-experiments showed that when W values were 2.5% and 3.5%, it took a long time to disperse the nano-SiO₂ in the impregnating agent, and the dispersed suspension very easily formed agglomerates and precipitation; after impregnation, it was found that some of the nano-SiO₂ agglomerated on the surface of the wood or precipitated at the bottom of the impregnation tank. Therefore, in this study, three levels of W were selected, 0%, 1%, and 2%, and three groups of poplar specimens were treated with these three different concentrations of modified impregnants. “W = 0%”, “W = 1%”, and “W = 2%” represent the poplar wood specimens impregnated in 0%, 1%, and 2% solutions with the mass ratio of silica nanoparticles to urea-formaldehyde resin solutions.

2.3.2. Preparation of Nano-SiO₂/UF-Modified Fast-Growing Poplar Wood

The fast-growing sawed poplar timber was further sawed into 300 mm × 20 mm × 20 mm, 20 mm × 20 mm × 20 mm, and 150 mm × 50 mm × 50 mm specimens of three sizes. The sawed specimens were sorted to remove the blocks with knots, worm eyes, cracks, and other defects, and the remaining blocks were sanded to remove burrs and the surface wood dust to make the surface smooth and clean.

The specimen was dried at 103 °C until the mass was constant. Then, vacuum impregnation treatment was performed. The specimen was vacuumed to −0.095 MPa for 120 min and kept at normal pressure for 30 min. It was pressured again to 0.7 MPa and held for 240 min.

2.4. Measurement and Characterization

2.4.1. Statistical Analysis

Comparison analyses of water absorption, wet swelling, dry shrinkage, nail grip, flexural strength, and flexural modulus coefficient of elasticity of fast-growing poplar treated with nano-SiO₂ urea-formaldehyde resin impregnated with 0%, 1%, and 2% of W was carried out.

2.4.2. Hygroscopicity

Hygroscopicity is the ratio of the quality of water absorbed by wood to saturation or within a specified period of time and the quality of all dry wood. After the wood absorbs water, there will be both suction water and free water inside the wood, which will not only affect its mechanical strength, but also cause the wood to decay more easily [24,25]. In this study, according to GB/T 1934.1-2009, hygroscopicity was measured to evaluate the physical properties of the specimen [26]. The hygroscopicity is calculated as follows:

$$A=\frac{M-M_0}{M_0}\times 100\%$$

where A is hygroscopicity; M is specimen’s quality after water absorption; M₀ is the total dry mass of the specimen.

2.4.3. Wet-Swelling Rate

In this study, the wet-swelling rate of the specimen was measured according to GB/T 1934.1-2009 [26]. The wet-swelling rate of wood is divided into from fully dry to air dry and from fully dry to saturated, which, respectively, refers to the ratio of the size or volume change in wood from fully dry moisture absorption to atmospheric relative humidity equilibrium or water absorption to saturation and the size or volume of the fully dry specimen.

2.4.4. Dry Shrinkage

The dry shrinkage in this experiment was measured according to GB/T 1934.1-2009 [26]. The dry shrinkage discussed in this study is mainly line dry shrinkage. Line dry shrinkage mainly regards full dry shrinkage and air drying shrinkage.

2.4.5. Nail-Holding Power

In this study, the gripping force of poplar wood was measured according to GB/T 14018-2009 [27]. Nail holding force is an important performance index of furniture wood [28]. Round steel nails are made of general low-carbon steel with a nail length of 45 mm and a straight diameter of 2.5 mm, specified by YB/T 5002-2017 [29]. Nail depth is 30 mm. The nail-holding force is measured immediately after the nail is driven in. A universal mechanical testing machine is loaded at a uniform speed, the nail is pulled out within 1–2 min, and the maximum load in the experiment is recorded.

2.4.6. Flexural Strength and Flexural Elastic Modulus

This experiment adopts universal mechanical testing machine to carry out three-point bending test. The specimens used for both the bending strength determination and the bending modulus of elasticity determination were 300 mm × 20 mm × 20 mm, and the length is along the grain direction. The modulus of elasticity of bending is used to measure the resistance of wood to deformation [30,31,32].

3. Results and Discussion

3.1. Effect of Impregnation Modification on the Physical Properties of Fast-Growing Poplar Wood

3.1.1. Hygroscopicity

The water absorption curves of the poplar wood material and the urea-formaldehyde resin-impregnated material with different amounts of SiO₂ nanoparticles were plotted by the measured water absorption rates, as shown in Figure 2. “Material” represents untreated poplar wood. It can be seen that the water absorption curves of both the material and the urea-formaldehyde resin-impregnated material with different amounts of SiO₂ nanoparticles showed a sharp and then slow rise. In the initial stage of water absorption, the wood absorbs water faster because the initial wood is completely dry and contains almost no water inside; thus, the internal space is large, and the external water can penetrate the wood more easily in the early stage of water absorption to remove the air inside the wood and fill it; at the later stage of water absorption, the remaining space inside the wood gradually decreases due to the removal of air and filling of water inside the wood before, the air remaining gradually decreases, and the required water content also decreases gradually.

Through mathematical statistics and mathematical simulation analysis of the curve, it can be found that there is a correlation between the number of days of water absorption and the water absorption rate; the number of days of water absorption and the water absorption rate show a logarithmic function relationship. The logarithmic function model y = a + blnx, and the correlation index R² is calculated, where the correlation index R² takes the value range of 0 ≤ R² ≤ 1. When R² is closer to 1, it means that the closer the measured value $y_i$ is to the regression value $\widehat{y}$, the better the fit of the regression equation is, and vice versa; the further the measured value is, the worse the fit is. As shown in

Table 2. Poplar wood material and impregnated wood–water absorption rate.

	Water Absorption (%)							y = a + blnx	R2
	1/4 d	1 d	2 d	4 d	8 d	13 d	15 d
Material	52	97	113	124	141	161	165	y = 0.26413ln(x) + 0.91779	0.98891
W = 0%	35	65	77	83	87	93	95	y = 0.13603ln(x) + 0.60703	0.94223
W = 1%	34	63	76	81	86	91	93	y = 0.13459ln(x) + 0.59401	0.94563
W = 2%	32	63	75	81	86	92	95	y = 0.14115ln(x) + 0.59637	0.94233

, the logarithmic function of water absorption days x, water absorption rate y, and the data of R² are established according to the data.

Table 2 shows the water absorption statistics of poplar wood material and poplar-impregnated wood. Comparing the material and impregnated wood, the water absorption of poplar material is much larger than that of the poplar-impregnated wood; within 6 h, the water absorption of poplar wood reaches 52%, while that of the impregnated wood is between 32% and 35%; after 1 day of water absorption, the water absorption of poplar material is close to 100%, while the water absorption of impregnated wood is less than 65%; after 15 days of water absorption, the water absorption of poplar material is more than 160%, while the impregnated wood is still below 100%. In summary, it can be shown that the urea-formaldehyde resin impregnation treatment can effectively reduce the water absorption of wood, and the water absorbed per unit time is lower than that of untreated poplar wood.

Due to the small gap in water absorption of each modified wood, 100% water absorption was selected as the reference node in this paper, and the regression equation derived was used to compare the length of time required to reach the node according to the water absorption of the specimen, i.e., according to y = a + blnx, x = e^(y−a)/b can be deduced. The reason for choosing 100% water absorption is that according to x = e^(y−a)/b, the larger the water absorption, the larger the time required. The larger the difference between the number of days, the more it reflects the difference between the various treatments. When the water absorption reached 100%, the days required for the materials with W = 0%, W = 1%, and W = 2% are, respectively, 1.3651 d, 17.9726 d, 20.4196 d, and 17.4542 d. The difference in hygroscopicity between unimpregnated specimens and impregnated specimens is very obvious. In practice, it is possible to judge how the wood behaves in long-term moisture absorption.

The addition of different amounts of SiO₂ nanoparticles to the urea-formaldehyde resin solution also had a certain effect on the water absorption of poplar wood. The data in the table show that the number of days required to reach 100% water absorption for the impregnated wood with a W of 1% is slightly greater than that for the other two impregnated wood; according to the comparison of the data for each period within 15 days, the difference between the water absorption of the three types of impregnated wood is within 2%, which indicates that the nanoparticles have some influence, but the effect is small. In summary, comparing the logarithmic functions of the three materials, we can see that when the W = 1%, the water absorption rate is lower than the other two, and the nano-SiO₂ has an effect on it, but the effect is small.

3.1.2. Wet-Swelling Rate

The wet-swelling rate of poplar wood material and the measured wet-swelling rate of poplar wood specimens modified by urea-formaldehyde resin and SiO₂ impregnation are shown in

Table 3. Impregnated material wet-swelling rate.

	From Full Dryness to Water Absorption Saturation Wet Swelling (%)			From Full Dry to Air Dry Wet-Swelling Rate (%)
	Radial	Tangential	Volume	Radial	Tangential	Volume
Material	5.9	7.2	14.3	1.5	1.8	3.6
W = 0%	2.7	5.1	8.2	1.1	1.6	3.1
W = 1%	3.1	4	7.1	1.1	1.3	2.7
W = 2%	3.4	4.2	7.5	1.2	1.2	2.7

. For the poplar material from fully dry to water absorption saturation, wet-swelling rate radial > 5%, tangential > 7%, and volume > 10%; for the poplar material from fully dry to air dry, wet-swelling rate radial and tangential are greater than 1.5%, volume > 3.5%. Whether the impregnated material is at 0%, 1%, or 2% of W, the moisture swelling rate from full dryness to water saturation in the radial and tangential is much lower than that of the material; radial < 3.5%, tangential < 5.5%, and within 10% of the volume. The wet-swelling rate from fully dry to air dry is also lower than that of the material, but the difference with the wet-swelling rate of the material is not large. In summary, the effect of impregnation modification on the wet-swelling rate is obvious, and the impregnation modification is beneficial for reducing the wet-swelling rate of poplar wood and increasing its dimensional stability.

From the effect of nano-SiO₂ addition, with the increase in SiO₂ addition, the radial wet-swelling rate showed a trend of becoming larger, and the change in the wet-swelling rate from full dryness to water absorption saturation was 0.3 percentage points, which was linear; while the change to an air-dry state was less than 0.1 percentage points, and the linearity was not obvious. With the increase in SiO₂, the tangential showed a decreasing trend, in which the greatest change was from 0 to 1%, indicating that the addition of SiO₂ can change the tangential wet-swelling rate, but when the W increased from 1% to 2%, the change in the tangential wet-swelling rate decreased, the wet-swelling rate was similar, and even the tangential wet-swelling rate showed a small increase. The change in the volumetric wet-swelling rate is similar to that of the tangential, and the main factor of the above change lies in the amount of SiO₂ added.

When SiO₂ was added to the urea-formaldehyde resin solution, the radial swelling rate increased slightly, the tangential and volumetric swelling rates showed a rapid decrease and then a slow increase, and the tangential and volumetric swelling rates of the impregnated modified poplar specimens were the smallest when the W was 1%.

3.1.3. Dry Shrinkage

The drying shrinkage of poplar-impregnated wood is shown in

Table 4. Impregnated material dry shrinkage rate.

	Full Dry Shrinkage Rate (%)			Air-Drying Shrinkage Rate (%)			Air-Dried Moisture Content
	Radial	Tangential	Volume	Radial	Tangential	Volume
Material	5.4	6.4	11.8	2.9	3.7	6.4	13.27%
W = 0%	5.4	8.5	13.7	2.8	5.2	8	12.81%
W = 1%	5.8	7.1	12.7	3.3	4.1	7	12.86%
W = 2%	5.8	6.9	12.6	3.3	4	7	13.27%

. The radial full dry shrinkage of the material is 5.4%, and the air dry shrinkage is 2.9%; the tangential full dry shrinkage is 6.4%, and the air dry shrinkage is 3.7%. The volume full dry shrinkage is 11.8%, and the air dry shrinkage is 6.4%. When only urea-formaldehyde resin was added, the radial dry shrinkage did not change much, and the tangential and volume both increased substantially; when nano-SiO₂ material was added, the radial dry shrinkage increased more significantly to 5.8%, which was 0.4 percentage points higher. However, when the nano-SiO₂ continued to increase to a W of 2%, the radial dry shrinkage was the same as when the W was 1%, and there was no significant decrease or increase. The tangential dry shrinkage and volumetric dry shrinkage decreased somewhat compared to the pure urea-formaldehyde resin-impregnated material and tended to the dry shrinkage of the material. Among them, the tangential and volumetric dry shrinkage rates varied more between 0% and 1% for the W, while the decrease tended to decrease when the W increased from 1% to 2%. As a whole, when urea-formaldehyde resin and nano-SiO₂ were added, both radial, tangential, and volumetric dry shrinkage rates were larger than those of the material, and the tangential dry shrinkage coefficients were larger than those of the radial dry shrinkage coefficients.

3.2. Effect of Impregnation Modification on the Mechanical Properties of Fast-Growing Poplar Wood

3.2.1. Nail-Holding Power

Table 5. Analysis of nail grip of impregnated material.

	The Moisture Content in the Test	Nail-Holding Power (N/mm)
		Radial	Tangential	Transection
Material	9.4	16.8	18.8	8.3
W = 0%	11.3	22.5	25.9	15.1
W = 1%	11.9	28.5	31.9	18.4
W = 2%	11.3	26.6	30.5	18.5

shows the measurement data of the nail strength of poplar-impregnated treated wood. From the table, we can see that the grip force of poplar fast-growing material is about 16.8 N/mm on the diameter side, 18.8 N/mm on the chord side, and 8.3 N/mm on the end side, and the grip force of urea-formaldehyde resin impregnated material and urea-formaldehyde resin impregnated material with nano-SiO₂ is greater than that of poplar material, which means that urea-formaldehyde resin impregnation can improve the grip force of poplar.

Figure 3 shows the effect of considering the amount of nano-SiO₂ addition on the nail grip force of poplar wood. When the W rises from 0% to 1%, the nail grip force also rises; when the amount of nano-SiO₂ continues to increase, the nail grip force no longer increases but will show a slight decrease. This may be caused by the excessive amount of nano-SiO₂. However, when the W is 2%, the nail strength is still greater than that of the impregnated wood without the addition of nanomaterials, indicating that the appropriate amount of nano-SiO₂ can effectively improve the nail strength of the wood.

In conclusion, the addition of nano-SiO₂ can improve the nail strength of impregnated wood, and the modification effect is best when the mass ratio of nano-SiO₂ to the solid content of urea-formaldehyde resin solution is 1%.

From Table 5 and Figure 3, it can also be seen that the material and impregnated material chord surface grip force is greater than the diameter surface grip force, but the two are closer, and the end surface grip force is the smallest. Figure 3 reflects the grain of the diameter, tangential, and end surfaces of the nail strength specimens and the location of the round steel nail holes. Due to the characteristics of wood, wood is a porous structural material, and the fiber conduit mouth is facing the longitudinal growth direction of the wood, as shown in Figure 3. When the steel nail is inserted, the steel nail squeezes the fiber catheter, resulting in the deformation of the fiber catheter in the contact part. However, due to the hollow inside of the catheter, the catheter is easily affected by plastic deformation and does not easily rebound. The extrusion between the catheter and the steel nail is relatively small, and the maximum static friction force is also small. Compared to the end face, as the tangential and longitudinal faces are transverse through the fiber conduit, the fiber wall can be closely attached to the steel nail, increasing the friction, so the grip force will have a certain increase compared to the end face. In the actual test, the difference in gripping force between the radial and tangential surfaces was small, and the tangential surface was slightly larger than the radial surface, because the fiber catheter arrangement on the tangential surface was more scattered than that on the radial surface, which increased the adhesion area to a certain extent.

Figure 4 shows the texture and hole position of the nail gripping force sample. The nail force of poplar wood materials mainly comes from the friction between the round steel nails and wood fiber conduits. Due to the small density of the poplar itself, the internal cavity and steel nail in the wood after the duct prevent plastic deformation, to a certain extent, so that the nail force of the poplar is not large [33,34,35]. After adding urea-formaldehyde resin, the resin filled the inner cavity of poplar wood, and the void between fibers and the catheter was also infiltrated into the urea-formaldehyde resin. To be nailed into the wood, the round steel nail needs to squeeze the wood fiber and the catheter. Due to the urea-formaldehyde resin in the interspaces between the fibers and in the conduit, it is difficult to squeeze the round steel nail easily, requiring greater force to nail it into the wood. When the wood is pulled out, the full wood fiber catheter increases the maximum friction force due to the extrusion of the round steel nail and the certain viscosity of the urea-formaldehyde resin. The round steel nail needs more force to exceed the maximum static friction force to be pulled out. The addition of nano-SiO₂, which is in powder form and has high hardness and good abrasion resistance, makes it easier to penetrate the wood fiber gaps and conduits and fill them, making the wood fiber and conduits more solid than the material and urea-formaldehyde resin-impregnated material, and mixing with urea-formaldehyde resin also increases the hardness of the wood fiber and conduits to a certain extent, increasing the force of nail extrusion and maximum static friction. It can be expected that, due to the properties of urea-formaldehyde resin and nano-SiO₂, the effect is more significant than the material in the scenario of applying wood screws. When there is too much nano-SiO₂, it may form agglomerates and cause blockage of the conduit opening, etc., so that the amount of urea-formaldehyde resin and nano-SiO₂ penetrating the wood conduit is reduced, thus affecting the nail-holding force.

3.2.2. Flexural Strength and Flexural Elastic Modulus

The specimens modified with urea-formaldehyde resin impregnates were treated with equilibrium moisture content to ensure that the moisture content was between 9% and 15%, and their moisture content was measured. The flexural strength and flexural modulus of elasticity at this moisture content were measured and converted to the values of strength and modulus of elasticity at 12% moisture content, and the measured data are shown in

Table 6. Impregnated material flexural strength and flexural elastic modulus data.

	ρ (g/cm3)	w (%)	MOR (MPa)		MOE (MPa)
			w	12%	w	12%
Material	0.446	10.5	62.6	58.9	7142	6986
W = 0%	0.52	12.9	87.6	90.8	9134	9262
W = 1%	0.534	12.9	89.3	92.6	9544	9677
W = 2%	0.594	11.1	96.4	93.1	10,152	10,019

.

The table shows that the flexural strength of the fast-growing poplar material is 58.9 MPa, and after impregnation modification, the flexural strength is greatly enhanced by 54%–58%. The flexural modulus of elasticity also performs similarly to the flexural strength, with an increase of 32%–43%. It can be seen that the modification by impregnation of urea-formaldehyde resin can enhance flexural strength very effectively. At the same time, the amount of nano-SiO₂ has different effects on flexural properties, and the MOR and MOE show an increasing trend with the increase in nano-SiO₂.

By impregnating the wood with urea-formaldehyde resin, the wood fibers are tightly bonded together, and secondly, the internal voids of the wood are filled so that they are not easily compressed, thus increasing the cost of being bent. The addition of nano-SiO₂ can increase the flexural strength and flexural modulus of elasticity. This is probably because nano-SiO₂ more easily enters into the internal voids of the wood and further fills the spaces not filled by the urea-formaldehyde resin, to a certain extent, to strengthen the wood fibers and ducts. However, due to the limited amount of addition, the modified wood with the addition of nano-SiO₂ shows little increase in performance compared to the urea-formaldehyde resin-impregnated wood.

4. Conclusions

To improve the physical and mechanical properties of fast-growing lumber and increase its product-added value in furniture applications, urea-formaldehyde resin impregnated with nano-SiO₂ was used to modify the fast-growing lumber. The main findings of this study are as follows:

• According to the values of water absorption per unit time of poplar-impregnated treated wood and its simulation curve, the nano-SiO₂ content has an effect on water absorption but does not play a decisive role; however, the impregnation treatment with urea-formaldehyde resin solution without the addition of other substances is beneficial in reducing the water absorption of wood, and the impregnation effect is good;
• Nano-SiO₂/UF resin impregnation modification treatment can effectively reduce the wet-swelling rate of poplar fast-growing wood and enhance its dimensional stability. Compared with no nano-silica, that is, W = 0%, when the W was 1%, the tangential and volumetric wet-swelling rates of impregnated modified poplar specimens were minimized;
• According to the comparison between poplar-impregnated treated wood and material, the impregnation treatment increased the dry shrinkage of poplar to some extent, which caused the relative weakening of dimensional stability;
• The nano-SiO₂/UF resin impregnation treatment modification can effectively improve the nail grip strength of poplar wood on all sides. The best modification effect of poplar wood was achieved when the urea-formaldehyde resin impregnating solution with W = 1% of nano-SiO₂ was added when the grip strength of poplar wood was strongest on the diameter, chord, and end surfaces;
• The flexural strength and flexural modulus of poplar fast-growing wood were improved by impregnation with nano-SiO₂/UF resin. The best modification effect of poplar wood was achieved by adding the urea-formaldehyde resin impregnation solution with W = 2% of nano-SiO₂, and the bending strength and bending modulus of elasticity of impregnated wood were optimized;
• The addition and content of nano-SiO₂ in urea-formaldehyde resin have different effects on different properties of specimens. In general, the urea-formaldehyde resin solution added with nano-silica has an effect on the physical properties of poplar specimens. However, compared with the urea-formaldehyde resin solution without nano-SiO₂, that is, W = 0%, the addition of nano-SiO₂ can effectively improve the mechanical properties of poplar wood specimens and increase its dimensional stability. The flexural strength and flexural elastic modulus improved with the increase in nano-SiO₂ concentration.

Improving the added value of poplar products and promoting the development of the agricultural and industrial economy by modifying fast-growing poplar wood with the combination of nano-silica and urea-formaldehyde resin is of great significance. At the same time, it also provides new possibilities for the use of fast-growing wood as furniture materials. It provides a new idea for the impregnation and modification of wood. However, the use of wood also needs to take into account the actual production costs. This study did not consider the energy consumption required in the modification process of fast-growing poplar wood, and did not compare it with natural forest wood for a more comprehensive consideration of wood utilization.