Enhancement of Wood Coating Properties by Adding Silica Sol to UV-Curable Waterborne Acrylics

: In recent years, with the development of the coating industry and the increasing awareness of environmental protection, the modiﬁcation of waterborne wood coatings has become the focus of research. Generally, the system composed of silica sol modiﬁcation and UV curing can make up for the defects of poor mechanical properties, low hardness, and slow curing speeds of waterborne wood coatings. Herein, we used silica sol-reinforced UV-curable waterborne acrylic wood coatings and tested the related physical properties of the coatings. FT-IR analysis showed that the Si-O-Si bond appeared, indicating that the silica sol was successfully grafted onto the waterborne acrylic molecular chain. The results showed that the mechanical properties of the UV-curable waterborne acrylic wood coating ﬁlm reached their optimum when the content of silica sol was 1 wt%, the number of UV lamps was 3, and the drying time was 20 min. The corresponding values for wear resistance, hardness, adhesion, and impact strength were 0.106 g (high level), grade 3, and 90 kg · cm, respectively. However, when the content of silica sol is greater than 1 wt%, the related physical properties of the coatings will decrease. The results showed that the gloss of the coating decreased with increasing silica sol content. When the silica sol content was 2 wt%–6 wt%, the coating showed a matte gloss. This present work shows that the modiﬁcation process is simple, controlled, inexpensive, and meets the demand for UV-curable waterborne acrylic wood coatings in daily life.


Introduction
The coatings used in wood products are collectively referred to as wood coatings, which generally have the effect of protecting the wood and increasing its beauty [1][2][3]. Wood coatings, as an important coating species, account for up to 10% of the total coatings in each country, thanks to the expansion of the wood industry. Traditional wood coatings contain volatile organic compounds (VOCs) that restrict the utilization and development of traditional wood coatings [4,5]. As a result, the development of low-pollution wood coatings, such as represented by UV-curable water-based wood coatings, has become the focus of scientific research in various countries.
UV-curable water-based wood coatings are a kind of environmental coating that use water as a solvent, can reduce the content of VOCs, and have no irritating odor [6][7][8][9][10][11]. Moreover, it has many advantages, such as high production efficiency, rapid curing, low energy consumption, low pollution, green environmental protection, and energy savings [12][13][14][15]. Due to these advantages, UV-curing waterborne wood coatings are widely used in electronic components, printing, glass, wood coatings, construction, and other industries [16,17]. Therefore, the development of low-pollution wood coatings represented

Preparation of the Reinforced Coating
To test the coating film properties of the reinforced coating, it was applied to the surface of the eucalyptus multilayer board with a spray gun (G15/G40, Graco, Philadelphia, PA, USA). Typically, the modified WA is poured into the can of the spray gun. Then, spraying the eucalyptus multilayer board by the "crossed" method, the paint output per gun was controlled at 50 g/cm 2 . At last, the sprayed samples were placed in the drying oven (DZF-6012, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China) at 40 • C for 3 h. They will be taken out to test the related physical properties. The preparation process for reinforced coating is shown in Figure 1. mixed solution. After 2 h of magnetic stirring, the silica sol was obtained.

Preparation of UV-Curable WA Coating Reinforced by Silica Sol
A UV-curable WA coating was reinforced with different contents of silica sol by mechanical blending method. Briefly, we weighed 3 g of UV-curable WA resin into beaker and stirred for ten minutes. Then, the silica sol was added to the resin solution 1 wt%, 2 wt%, 3 wt%, 4 wt%, and 5 wt% of the UV-curable WA resin mass (Table 1). Su sequently, to obtain a uniform dispersion, it was stirred with a magnetic stirrer at 4 r/min for 2 h at 25 °C. Finally, the mixed solution was sonicated with an ultrasonic crush (HN-150Y, Jinan Tongxin Biotechnology Co., Ltd., Jinan, China) for 10 min to make sili sol uniformly dispersed in the UV-curable WA resin emulsion.

Preparation of the Reinforced Coating
To test the coating film properties of the reinforced coating, it was applied to t surface of the eucalyptus multilayer board with a spray gun (G15/G40, Graco, Philad phia, PA, USA). Typically, the modified WA is poured into the can of the spray gun. The spraying the eucalyptus multilayer board by the "crossed" method, the paint output p gun was controlled at 50 g/cm 2 . At last, the sprayed samples were placed in the dryi oven (DZF-6012, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China) at °C for 3 h. They will be taken out to test the related physical properties. The preparati process for reinforced coating is shown in Figure 1.

Characterizations
The particle size distribution of silica sol was carried out using a laser particle size analyzer (SALD-2300, Shimadzu Co., Ltd., Kyoto, Japan). A scanning electron microscope (SEM, JEOL-7800F) was carried out to observe the morphology of silica sol and coatings. Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Electron Corp., Madison, WI, USA) was operated to determine the characteristic absorption peaks of UV-curable WA coatings before and after silica sol enhancement. Furthermore, the physical properties of UV-curable WA coating were examined in accordance with the relevant Chinese Standards. The abrasion resistance was carried out using JM-V (Shanghai Tianchen Modern Environment Co., Ltd., Shanghai, China). We calculated the abrasion resistance of the eucalyptus multilayer board after 100 times of grinding, and the unit of abrasion resistance was mg/100 r according to GB/T 18103-2013. The pencil hardness method was adopted to analyze the hardness of the coating (GB/T 23999-2006). The adhesion of the coating was tested by a paint film scriber at 2 mm intervals according to GB/T 7657-2013. Meanwhile, the adhesion level was evaluated according to the peeling of the paint film. The impact resistance test is completed under the guidance of GB/T 7657-2013. The gloss of the UV-curable WA coating was tested by the glossmeter (BGD512-60, KSJ Photoelectrical Instruments Co., Ltd., Quanzhou, China) according to GB/T 9754-2007. Moreover, all the data were analyzed by analysis of variance.

Particle Size and SEM Analysis
According to the particle size test results in Table 2, the particle size distribution diagram of the silica sol is shown in Figure 2a. According to the test results in Table 2, when the silica sol concentration was 0.63 mol/L, the average particle size was 34.638 µm. Furthermore, the particle size of silica sol samples less than 62.938 µm accounted for 90% of the total, and 50% were smaller than 29.153 µm. Figure 2a shows the distribution range of the particle size of the silica sol. The particle size distribution of the sample was between 10 and 100 µm. Therefore, the analysis in Figure 2a is consistent with that in Table 2. Where Dav represents the average particle size of the silica sol, D is the volume average particle size, S/V is the specific surface area, and D10 represents 10% of the total volume for particles smaller than 11.117 µm, as are D50 and D90. This test adopted the intensity method of the laser particle size analysis to calculate the particle size.
The silica sol and the coatings before and after enhancement were observed under SEM, and the results are shown in Figure 2b-e. As can be seen from Figure 2b,c, the particle size of the as-prepared silica sol was relatively uniform and presented a spherical shape. The SEM images of the silica sol were consistent with the above particle size analysis. It can be seen from Figure 2d that the unmodified UV-curable WA resin coating had cracks and poor thermal stability, which were caused by the contraction of resin molecules when cold. And there was no granular substance in its coating film. However, as shown in Figure 2e, the surface layer of the reinforced film was evenly distributed with some white granular substances. Moreover, these particles were somewhat blurred in the SEM image, and their clear shape cannot be seen. It can be seen that the silica sol particles were not on the surface of the coating film but were coated in the inner layer. Therefore, the addition of silica sol enhanced the stability, hardness, water resistance, and alcohol resistance of the UV-curable WA film. The silica sol and the coatings before and after enhancement were observed SEM, and the results are shown in Figure 2b-e. As can be seen from Figure 2b,c, th ticle size of the as-prepared silica sol was relatively uniform and presented a sph shape. The SEM images of the silica sol were consistent with the above particle siz ysis. It can be seen from Figure 2d that the unmodified UV-curable WA resin coatin cracks and poor thermal stability, which were caused by the contraction of resin mol when cold. And there was no granular substance in its coating film. However, as in Figure 2e, the surface layer of the reinforced film was evenly distributed with white granular substances. Moreover, these particles were somewhat blurred in th image, and their clear shape cannot be seen. It can be seen that the silica sol particle not on the surface of the coating film but were coated in the inner layer. Therefo addition of silica sol enhanced the stability, hardness, water resistance, and alcoh sistance of the UV-curable WA film.

FT-IR of Reinforced Coating
To reveal the characteristic functional group of the reinforced coating, FT-IR spectra were introduced ( Figure 3). As shown in Figure 3, these characteristic peaks of the reinforced coating were consistent with the raw materials. Moreover, the solvent of the UV-curable WA resin was water, and the FT-IR analyses were performed by the attenuated total reflection method. The characteristic stretching vibration absorption peak at 3400 cm −1 is attributed to -OH. Meanwhile, the characteristic absorption peaks at 2960 cm −1 , 2870 cm −1 , and 1730 cm −1 are methyl, methylene, and carbonyl, respectively. The hydroxyl peaks at 3428 cm −1 narrowed with the addition of silica sol, which was due to the reduction of hydroxyl groups as a result of the dehydration condensation reaction between some hydroxyl functional groups on the UV WA resin molecules and the hydroxyl groups on the surface of the silica sol [36]. The FT-IR spectra of the added silica sol showed a significant enhancement of the stretching vibration peak at approximately 1000-1200 cm −1 , which was attributed to the condensation of the hydroxyl group on the silica sol with the silicon hydroxyl group on the main chain to form Si-O-Si [37]. However, the disappearance of the absorption peak of the epoxy group at 910 cm −1 is caused by the ring-opening reaction of the epoxy group present in the silica sol with the hydroxyl group on the main chain. The addition of silica sol makes the C=C vibration absorption peak at 829 cm −1 weak and narrow, indicating that the double bond between silica sol and the double bond on the main chain of UV WA resin has a cross-linking reaction under UV light. Therefore, the silica sol was successfully grafted onto the UV-WA resin chain by bridging action.
hydroxyl functional groups on the UV WA resin molecules and the hy the surface of the silica sol [36]. The FT-IR spectra of the added silica so icant enhancement of the stretching vibration peak at approximately which was attributed to the condensation of the hydroxyl group on the silicon hydroxyl group on the main chain to form Si-O-Si [37]. Howev ance of the absorption peak of the epoxy group at 910 cm −1 is caused by reaction of the epoxy group present in the silica sol with the hydroxyl g chain. The addition of silica sol makes the C=C vibration absorption peak and narrow, indicating that the double bond between silica sol and th the main chain of UV WA resin has a cross-linking reaction under UV lig silica sol was successfully grafted onto the UV-WA resin chain by bridg Figure 3. The FT-IR spectra of UV-curable WA before and after being reinforced 2 wt% concentration.

Mechanism Analysis
The reaction mechanism of silica sol-reinforced UV-curable WA is lowing equations (Equations (1)- (3)). WA and its ester monomers are p tion polymerization, which belongs to free radical polymerization. The sol will open the π bond of the WA monomer and undergo an addition monomer free radicals (Equations (1) and (2)). Monomer free radicals molecular chains one at a time. The addition of silica sol increases the n free radicals, which increases the chance of chemical bonding between WA molecule. Therefore, silica sol can be more fully grafted onto the addition, silica sol can react with active functional groups in WA molec energy chemical bonds (Equation (3)). However, when the silica sol was number of active centers will be generated in the system. This will increa generation and chain transfer. The stability of lotion becomes worse, and amount increases due to the increase in branch chains generated on the

Mechanism Analysis
The reaction mechanism of silica sol-reinforced UV-curable WA is shown in the following equations (Equations (1)- (3)). WA and its ester monomers are prepared by solution polymerization, which belongs to free radical polymerization. The addition of silica sol will open the π bond of the WA monomer and undergo an addition reaction to form monomer free radicals (Equations (1) and (2)). Monomer free radicals combine to form molecular chains one at a time. The addition of silica sol increases the number of primary free radicals, which increases the chance of chemical bonding between silica sol and the WA molecule. Therefore, silica sol can be more fully grafted onto the WA molecule. In addition, silica sol can react with active functional groups in WA molecules to form high-energy chemical bonds (Equation (3)). However, when the silica sol was excessive, a large number of active centers will be generated in the system. This will increase the rate of heat generation and chain transfer. The stability of lotion becomes worse, and the precipitation amount increases due to the increase in branch chains generated on the molecule.  Figure 4 shows the influence of the proportion of silica sol in the UV-curable WA coating on the abrasion resistance. And take the average of the three values as the final value of the weight loss. It can be seen from Figure 4 that when the proportion of silica sol in the coating reached 1-3 wt%, the abrasion resistance of the coating was significantly improved. However, when the proportion of silica sol was higher than 3 wt%, the wear resistance of the composite coating decreased. When the content of silica sol is 3 wt%, the lowest wear value is only 0.092 g. When the silica sol content was increased to 4 wt% and 5 wt%, the weight loss was significantly increased to 0.123 g and 0.128 g, respectively. At this time, the weight loss was higher than that of the unmodified coating. Meanwhile, according to the variance analysis, when the silica sol content was 1 wt%, 2 wt%, or 3 wt%, the abrasion resistance of the coating was significantly affected (p < 0.01). The reason for this phenomenon is that a small amount of SiO2 particles in silica sol can significantly improve the coating's wear resistance while having no effect on its transparency. With the increased use of silica sol as an additive, the phenomenon of self-agglomeration occurs due to the uneven distribution of SiO2 in the coating solution caused by its large specific surface area [38]. As a result, the wear value of the coating increases, which also means that the abrasion resistance of the reinforced coating decreases.  Figure 4 shows the influence of the proportion of silica sol in the UV-curable WA coating on the abrasion resistance. And take the average of the three values as the final value of the weight loss. It can be seen from Figure 4 that when the proportion of silica sol in the coating reached 1-3 wt%, the abrasion resistance of the coating was significantly improved. However, when the proportion of silica sol was higher than 3 wt%, the wear resistance of the composite coating decreased. When the content of silica sol is 3 wt%, the lowest wear value is only 0.092 g. When the silica sol content was increased to 4 wt% and 5 wt%, the weight loss was significantly increased to 0.123 g and 0.128 g, respectively. At this time, the weight loss was higher than that of the unmodified coating. Meanwhile, according to the variance analysis, when the silica sol content was 1 wt%, 2 wt%, or 3 wt%, the abrasion resistance of the coating was significantly affected (p < 0.01). The reason for this phenomenon is that a small amount of SiO 2 particles in silica sol can significantly improve the coating's wear resistance while having no effect on its transparency. With the increased use of silica sol as an additive, the phenomenon of self-agglomeration occurs due to the uneven distribution of SiO 2 in the coating solution caused by its large specific surface area [38]. As a result, the wear value of the coating increases, which also means that the abrasion resistance of the reinforced coating decreases.  Figure 4 shows the influence of the proportion of silica sol in the UV-curable WA coating on the abrasion resistance. And take the average of the three values as the fina value of the weight loss. It can be seen from Figure 4 that when the proportion of silica so in the coating reached 1-3 wt%, the abrasion resistance of the coating was significantl improved. However, when the proportion of silica sol was higher than 3 wt%, the wea resistance of the composite coating decreased. When the content of silica sol is 3 wt%, th lowest wear value is only 0.092 g. When the silica sol content was increased to 4 wt% and 5 wt%, the weight loss was significantly increased to 0.123 g and 0.128 g, respectively. A this time, the weight loss was higher than that of the unmodified coating. Meanwhile according to the variance analysis, when the silica sol content was 1 wt%, 2 wt%, or 3 wt% the abrasion resistance of the coating was significantly affected (p < 0.01). The reason fo this phenomenon is that a small amount of SiO2 particles in silica sol can significantly improve the coating's wear resistance while having no effect on its transparency. With th increased use of silica sol as an additive, the phenomenon of self-agglomeration occur due to the uneven distribution of SiO2 in the coating solution caused by its large specifi surface area [38]. As a result, the wear value of the coating increases, which also mean that the abrasion resistance of the reinforced coating decreases.

Hardness Analysis
The effect of silica sol content on the hardness of UV-curable WA coatings is shown in Figure 5. It can be seen from the figure that when the silica sol content reached 1 wt%, the hardness of the composite coating reached a high level, and the hardness was significantly improved. Although the amount of silica sol was small, it can fully react with UV-curable WA coatings to generate chemical bonds, which can significantly increase the hardness of the paint film [39]. However, the hardness of the coating film decreased when the silica sol content gradually increased. This is due to the uneven dispersion of silica in the silica sol, which makes the coating unable to form a continuous network structure. This is consistent with the results of the abrasion resistance of the composite coating discussed above. However, when the content of silica sol is 5 wt%, excessive SiO 2 will be deposited on the surface of the coating. This will instantly improve the hardness of the coating. Moreover, analysis of variance showed that the coating hardness was not significantly affected by silica sol content (p > 0.05). Therefore, when the percentage of silica sol in the coating was 1 wt%, the UV-curable WA coating reinforced by silica sol had excellent abrasion resistance and pencil hardness.

Hardness Analysis
The effect of silica sol content on the hardness of UV-curable WA coatin in Figure 5. It can be seen from the figure that when the silica sol content rea the hardness of the composite coating reached a high level, and the hardness cantly improved. Although the amount of silica sol was small, it can fully rea curable WA coatings to generate chemical bonds, which can significantly hardness of the paint film [39]. However, the hardness of the coating film decr the silica sol content gradually increased. This is due to the uneven dispersio the silica sol, which makes the coating unable to form a continuous netwo This is consistent with the results of the abrasion resistance of the composite cussed above. However, when the content of silica sol is 5 wt%, excessive deposited on the surface of the coating. This will instantly improve the har coating. Moreover, analysis of variance showed that the coating hardness wa icantly affected by silica sol content (p > 0.05). Therefore, when the percentage in the coating was 1 wt%, the UV-curable WA coating reinforced by silica sol h abrasion resistance and pencil hardness.  Figure 6 shows the effect of silica sol content on the adhesion of the UVcoating. Generally, the evaluation level of adhesion is from 1 to 7, and leve that the adhesion of the coating reached the highest level. It can be seen from F the amount of silica sol additive had a significant effect on the adhesion of th coating. Meanwhile, the smaller the amount of added silica sol, the higher t level was. Furthermore, the adhesion level of the coating becomes smaller an sion decreases as the addition of silica sol increases. When the content of sili wt%, the adhesion of the coating film reached grade 3. When the content of s 2 wt% to 4 wt%, the adhesion of the coating film remained at level 4. When a sol to 5 wt%, the adhesion continues to decrease to level 5. The analysis showed that the silica sol content had a significant impact on the adhesion o  Figure 6 shows the effect of silica sol content on the adhesion of the UV-curable WA coating. Generally, the evaluation level of adhesion is from 1 to 7, and level 1 indicates that the adhesion of the coating reached the highest level. It can be seen from Figure 5 that the amount of silica sol additive had a significant effect on the adhesion of the composite coating. Meanwhile, the smaller the amount of added silica sol, the higher the adhesion level was. Furthermore, the adhesion level of the coating becomes smaller and the adhesion decreases as the addition of silica sol increases. When the content of silica sol was 1 wt%, the adhesion of the coating film reached grade 3. When the content of silica sol was 2 wt% to 4 wt%, the adhesion of the coating film remained at level 4. When adding silica sol to 5 wt%, the adhesion continues to decrease to level 5. The analysis of variance showed that the silica sol content had a significant impact on the adhesion of the coating (p < 0.01). The adhesion of the coating refers to the firmness of the bond between the coating film and the substrate, which directly affected the ability of the coating to protect the substrate. When the silica sol was less, it can fully react with UV-curable WA coatings to make the internal and external stress of the coating reached a balance, so that the adhesion of the coating was enhanced. However, when there was too much silica sol, the reaction will be incomplete and a stable structure cannot be formed [40]. The firmness of the combination of the coating film and the substrate was reduced, thereby reducing the adhesion of the coating.

Adhesion Analysis
Forests 2023, 14, x FOR PEER REVIEW make the internal and external stress of the coating reached a balance, so that t of the coating was enhanced. However, when there was too much silica sol, will be incomplete and a stable structure cannot be formed [40]. The firmness bination of the coating film and the substrate was reduced, thereby reducing t of the coating.

Impact Resistance Analysis
The effect of silica sol content on the impact resistance of UV-curable film is shown in Figure 7. The impact resistance test of the coating surface detect the dynamic loading capacity of the coating surface. Figure 7 shows th content of silica sol was 1 wt%, the impact resistance of the coating reache which was significantly better than that of the coating without silica sol. Wh sol continued to be added, the impact resistance of the coating decreased b higher than the 50 kg·cm of the coating without silica sol. An analysis of varian showed that the impact resistance of the coating was significantly affected wh sol content was between 1 wt% and 2 wt%. However, the factor analysis s when the silica sol content was 3 wt%, 4 wt%, or 5 wt%, the p-value was great indicating that the impact strength had no significant impact. The reason for th enon was that a smaller amount of silica sol can react fully with the UV-curab ing substrate to form a chemical bonding interaction, resulting in a significan the impact resistance of the coating layer. As the silica sol content increased resistance of the coating decreased due to the agglomeration and self-cross-l tions of silica particles in the silica sol. Compared with the related literature shows excellent impact resistance performance [2,5].

Impact Resistance Analysis
The effect of silica sol content on the impact resistance of UV-curable WA coating film is shown in Figure 7. The impact resistance test of the coating surface was used to detect the dynamic loading capacity of the coating surface. Figure 7 shows that when the content of silica sol was 1 wt%, the impact resistance of the coating reached 90 kg·cm, which was significantly better than that of the coating without silica sol. When the silica sol continued to be added, the impact resistance of the coating decreased but was still higher than the 50 kg·cm of the coating without silica sol. An analysis of variance (p < 0.01) showed that the impact resistance of the coating was significantly affected when the silica sol content was between 1 wt% and 2 wt%. However, the factor analysis showed that when the silica sol content was 3 wt%, 4 wt%, or 5 wt%, the p-value was greater than 0.05, indicating that the impact strength had no significant impact. The reason for this phenomenon was that a smaller amount of silica sol can react fully with the UV-curable WA coating substrate to form a chemical bonding interaction, resulting in a significant increase in the impact resistance of the coating layer. As the silica sol content increased, the impact resistance of the coating decreased due to the agglomeration and self-cross-linking reactions of silica particles in the silica sol. Compared with the related literature, this result shows excellent impact resistance performance [2,5].

Glossiness Analysis
The glossiness of the UV-curable WA coating modified by silica sol is shown in Figure 8. Moreover, each sample was measured three times, and the average was taken as the final value. As shown in Figure 8, the addition of silica sol had a large impact on the glossiness of the coating. Furthermore, the one-way analysis of variance shows that the content of silica sol has a significant effect on the gloss of the coating (p < 0.01). It can be seen from the analysis of two adjacent factors that the p value between them is less than 0.05. Combined with Figure 8, it can be seen that the coating has excellent gloss when the silica sol content is greater than 1 wt%. The gloss of the coating shows a decreasing trend with the increase in silica sol. This is because silica sol is a colloidal solution with fine particles. The high nanoactivity of SiO2 during gel separation will bind the powder particles in the coating. In addition, SiO2 forms a new silicate inorganic polymer compound with inorganic salts and metal oxides and further hardens to form a film. Meanwhile, the fine particles are deeply penetrating the substrate and can penetrate its interior via capillary action [41]. Therefore, silica sols reacted chemically with UV-cured WA coatings to produce inorganic opaque substances, which greatly reduced the gloss and transparency of the coating. Accompanied by excellent gloss performance, it also has a certain degree of competitiveness.

Glossiness Analysis
The glossiness of the UV-curable WA coating modified by silica sol is shown in Figure 8. Moreover, each sample was measured three times, and the average was taken as the final value. As shown in Figure 8, the addition of silica sol had a large impact on the glossiness of the coating. Furthermore, the one-way analysis of variance shows that the content of silica sol has a significant effect on the gloss of the coating (p < 0.01). It can be seen from the analysis of two adjacent factors that the p value between them is less than 0.05. Combined with Figure 8, it can be seen that the coating has excellent gloss when the silica sol content is greater than 1 wt%. The gloss of the coating shows a decreasing trend with the increase in silica sol. This is because silica sol is a colloidal solution with fine particles. The high nanoactivity of SiO 2 during gel separation will bind the powder particles in the coating. In addition, SiO 2 forms a new silicate inorganic polymer compound with inorganic salts and metal oxides and further hardens to form a film. Meanwhile, the fine particles are deeply penetrating the substrate and can penetrate its interior via capillary action [41]. Therefore, silica sols reacted chemically with UV-cured WA coatings to produce inorganic opaque substances, which greatly reduced the gloss and transparency of the coating. Accompanied by excellent gloss performance, it also has a certain degree of competitiveness.

Glossiness Analysis
The glossiness of the UV-curable WA coating modified by silica sol is shown in Figure 8. Moreover, each sample was measured three times, and the average was taken as the final value. As shown in Figure 8, the addition of silica sol had a large impact on the glossiness of the coating. Furthermore, the one-way analysis of variance shows that the content of silica sol has a significant effect on the gloss of the coating (p < 0.01). It can be seen from the analysis of two adjacent factors that the p value between them is less than 0.05. Combined with Figure 8, it can be seen that the coating has excellent gloss when the silica sol content is greater than 1 wt%. The gloss of the coating shows a decreasing trend with the increase in silica sol. This is because silica sol is a colloidal solution with fine particles. The high nanoactivity of SiO2 during gel separation will bind the powder particles in the coating. In addition, SiO2 forms a new silicate inorganic polymer compound with inorganic salts and metal oxides and further hardens to form a film. Meanwhile, the fine particles are deeply penetrating the substrate and can penetrate its interior via capillary action [41]. Therefore, silica sols reacted chemically with UV-cured WA coatings to produce inorganic opaque substances, which greatly reduced the gloss and transparency of the coating. Accompanied by excellent gloss performance, it also has a certain degree of competitiveness.

Conclusions
In this study, we investigated the effect of silica sol on the chemical and physical properties of UV-cured WA wood coating films. UV-curable WA wood coatings with superior overall performance were prepared, which improved the mechanical properties of the coatings such as abrasion resistance, hardness, and impact resistance. The results show that when the content of silica sol in the UV-curable WA coating was 1 wt%, the mechanical properties (except adhesion) of the coating could meet the application requirements. At this time, the corresponding weight loss value of the coating was 0.106 g; the hardness had reached grade H, the adhesion had reached grade 3, and the impact resistance had reached 90 kg·cm. The reason for this phenomenon is that silica sol has excellent properties such as high rigidity, high strength, ultraviolet absorption, high adsorption, and high dispersion. Furthermore, the amount of silica sol additive has a great influence on the optical properties of UV-cured WA wood coatings. With the increase in silica sol addition, the optical properties of the coating tend to be matte, which meets the demand of the development of wood coating in the direction of matte. However, the physical/chemical interactions of silica microparticles with WA emulsion and statistical analysis need to be further investigated. This work demonstrates the potential application of silica sol in UV-curable WA wood coatings. Meanwhile, it also provides a new perspective for the design of new UV-cured wood coatings.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author, S.L., upon reasonable request.