The Effects of Urea–Formaldehyde Resin-Coated Toddalia asiatica (L.) Lam Extract Microcapsules on the Properties of Surface Coatings for Poplar Wood

Abstract

Urea–formaldehyde resin was used as a wall material and Toddalia asiatica (L.) Lam extract was used as a core material to prepare urea–formaldehyde resin-coated Toddalia asiatica (L.) Lam extract microcapsules (UFRCTEMs). The effects of UFRCTEM content and the mass ratio of core-to-wall material (M_core:M_wall) on the performance of waterborne coatings on poplar surfaces were investigated by adding microcapsules to the waterborne topcoat. Under different M_core:M_wall of microcapsules, as the content of microcapsules increased, the glossiness and adhesion of the coatings gradually decreased, and the color difference value of the coatings gradually increased. The cold liquid resistance, hardness, and impact resistance of the coatings were all improved, and the roughness of the coatings increased. The antibacterial rates of the coatings against Escherichia coli and Staphylococcus aureus were both on the rise, and the antibacterial rate against Staphylococcus aureus was slightly higher than that against Escherichia coli. When the microcapsule content was 7.0% and the M_core:M_wall was 0.8:1, the surface coating performance on poplar wood was excellent. The glossiness was 3.43 GU, light loss was 75.55%, color difference ΔE was 3.23, hardness was 2H, impact resistance level was 3, adhesion level was 1, and roughness was 3.759 µm. The cold liquid resistance was excellent, and resistance grades to citric acid, ethanol, and cleaning agents were all 1. The antibacterial rates against Escherichia coli and Staphylococcus aureus were 68.59% and 75.27%, respectively.

1. Introduction

Wood, as a renewable biomass material, plays an extremely important role in ecological civilization, national economic construction, and people’s daily lives [1,2,3]. Wood is mainly composed of three components: lignin, cellulose, and hemicellulose, which make it susceptible to pests and diseases, as well as bacterial and fungal erosion [4,5]. Hard broad-leaved trees have a long growth cycle and are scarce in resources [6,7,8]. Poplar has the advantages of fast production speed, moderate material, tough wood, corrosion resistance, and strong stability [9,10]. Poplar is a fast-growing wood that can alleviate the current shortage of wood resources and has a wide range of application prospects in wooden furniture [11]. However, poplar has problems such as loose fiber structure, low density, easy moisture absorption and deformation, and difficulty in drying, which greatly limits its application range [12,13]. These issues have resulted in higher requirements for the antibacterial effect on the surface of poplar wood. Therefore, the antibacterial treatment of wood is an important means to extend its service life, improve its utilization level, and save wood resources [14]. As an environmentally friendly coating, waterborne coatings can have special properties that affect wood, such as antibacterial, anti-corrosion, heat-resistant, waterproof, and fireproof characteristics [15,16,17]. It has been found that excessive use of chemical antibacterial agents can easily cause environmental pollution, carcinogenic effects on humans and other organisms, and corrosion of metal objects [18,19,20]. Plant-derived antibacterial agents obtained from natural plants are extracted through physical or chemical separation and have advantages such as safety, efficiency, wide range of sources, wide variety, and minimal toxic side effects [21,22,23]. Therefore, a large number of plant extracts have been developed and applied in fields such as cosmetics, natural fungicides, and feed [24]. However, when plant extracts are directly used as wood antibacterial agents, there are various shortcomings such as a narrow range of insect-resistant bacteria, easy degradation, easy loss, short efficacy period, and sensitivity to external environmental factors (such as temperature, air humidity, light, rain, etc.), which result in overall poor antibacterial effect and limit its application range. Therefore, further research and expansion are needed [25].

The extract from the roots, stems, and leaves of Toddalia asiatica (L.) Lam contains a certain degree of bioactive substances such as antibacterial, antioxidant, and insecticidal properties [26]. The chemical components of Toddalia asiatica (L.) Lam anhydrous ethanol extract contain alkaloids, coumarins, triterpenes, and flavonoids with antibacterial activity [27]. Coumarin compounds can reduce the pathogenicity and drug resistance of bacteria by inhibiting the bacterial quorum sensing system, reducing the expression of related virulence factors, and forming biofilms [28]. Flavonoids act on the cell membrane of microorganisms, altering the permeability, and achieving the goal of inhibiting or killing bacteria [29]. Alkaloids can induce the release of membrane-bound cell wall autolytic enzymes, ultimately leading to lysis [30]. The antibacterial effect of terpenes is mainly attributed to their ability to interact with microbial membranes and destroy them, as well as the increased antibacterial and antifungal activity of terpenes by hydroxyl, ketone, and aldehyde groups [31]. However, it was not easily dispersed when directly applied to coatings, and antibacterial properties were not guaranteed. It is even more difficult to apply them in surface coatings for wooden products, and further research is needed to expand their application functions. The use of microcapsule technology can encapsulate some natural antibacterial agents or antibacterial substances to make antibacterial microcapsules, improve the processing performance of antibacterial agents, enhance the stability of natural antibacterial agents, and improve their usability, expanding their application scope. Therefore, Toddalia asiatica (L.) Lam extract was prepared into microcapsules and applied in wood antibacterial coatings. Raj et al. used different materials to sequentially extract the active ingredients from the leaves of Toddalia asiatica (L.) Lam. The results showed that the extract exhibited antibacterial activity against selected bacteria (Staphylococcus epidermidis, Enterobacter aerogenes, Shigella flexneri, Klebsiella pneumoniae, and Escherichia coli) and fungi (Aspergillus flavus, Candida krusei, and Botrytis cinereal) [32]. Roshan et al. prepared chitosan-based nanocapsules of Toddalia asiatica (L.) Lam essential oil (neTAEO) and the results confirmed that neTAEO exhibited stronger antifungal and aflatoxin B1 inhibitory activity than Toddalia asiatica (L.) Lam essential oil, and had greater development prospects [33].

Three types of urea–formaldehyde resin-coated Toddalia asiatica (L.) Lam extract microcapsules (UFRCTEMs) with the mass ratio of core-to-wall material (M_core:M_wall) of 0.6:1, M_core:M_wall of 0.8:1, and M_core:M_wall of 1.2:1 were prepared. Three types of UFRCTEMs were added to waterborne coatings at concentrations of 1.0%, 3.0%, 5.0%, 7.0%, and 9.0%, respectively. Then, the waterborne coatings were applied on the surface of poplar wood. By testing and analyzing the microstructure, chemical composition, optical properties, mechanical properties, cold liquid resistance, and antibacterial properties of the surface coating of poplar wood, the influence of different UFRCTEMs and addition contents on the comprehensive performance of the surface coating on poplar wood was explored, providing a reference for the coating process of antibacterial coatings.

2. Materials and Methods

2.1. Materials and Instruments

The leaves of Toddalia asiatica (L.) Lam were obtained from Lingshan County, Qinzhou, China. The leaves were placed in a 40 °C oven to dry to a constant weight. The leaves were pulverized into powder by a crusher. The size of the poplar wood was 50 mm × 50 mm × 8 mm, which was smoothed using #800 and #1000 sandpaper. The coatings used were a waterborne acrylic topcoat and a primer, both from Jiangsu Haitian Technology Co., Ltd., Nanjing, China.

2.2. Preparation Method of Microcapsules

2.2.1. Preparation Method of Toddalia asiatica (L.) Lam Extract

The leaf powder was mixed with anhydrous ethanol, heated in a water bath, and centrifuged. A vacuum pump combined with a Buchner funnel was used to filter the solution. The specific preparation method used in this study is the same as in Reference [34].

2.2.2. Preparation Method of Microcapsules

According to Reference [35], three types of UFRCTEMs with 0.6:1, 0.8:1, and 1.2:1 M_core:M_wall were prepared.

Table 1. The proportion of the microcapsule raw materials.

Sample (#)	Mcore:Mwall	Urea (g)	37% Formaldehyde (g)	Wall Material (g)	Toddalia asiatica (L.) Lam Extract (g)	Ethanol (g)	Core Material (g)	Emulsifier (g)	Deionized Water (g)
1	0.6:1	10.00	16.22	16.00	0.19	9.41	9.60	1.04	50.96
2	0.8:1	10.00	16.22	16.00	0.26	12.54	12.80	1.43	70.07
3	1.2:1	10.00	16.22	16.00	0.38	18.82	19.20	2.09	102.41

shows the proportion of the UFRCTEM raw materials.

2.3. Painting Method for Poplar Board

According to the technical specifications for the application of waterborne wood coatings on furniture surfaces, a uniform layer of coating was manually applied to the poplar board, with a one-time application amount of 60 g/m² to 80 g/m². The coating was applied with a brush-painting technique. A coating method of two layers of primer and two layers of topcoat was adopted. The selected coating amount for each layer was 80 g/m², and the total coating amount for the primer and topcoat was 320 g/m². The thickness of each coating on the surface of the wood was about 80 μm, and the total thickness was about 320 μm. Taking into account the loss error during the coating process, the actual consumption of the coating was 1.8 times the theoretical application amount. The total mass of the coating applied to the surface of the poplar wood was 1.44 g.

Table 2. List of materials used for the waterborne coatings.

Microcapsule Content (%)	Waterborne Primer (g)	Microcapsules (g)	Waterborne Topcoat (g)
0	0.720	0	0.720
1.0	0.720	0.007	0.713
3.0	0.720	0.022	0.698
5.0	0.720	0.036	0.684
7.0	0.720	0.051	0.669
9.0	0.720	0.065	0.655

shows the materials used for the waterborne coatings.

The specific coating process for the surface coating of poplar wood was as follows: A #360 sandpaper was used to treat the rough edges of the poplar board, and a #800 sandpaper was used to remove the surface of the poplar board and polish it smooth. Then, the first layer of primer was applied with a brush. After the first application of primer, the coating was leveled at room temperature for 20 min before being transferred to an oven to dry. After the coating was completely cured, it was taken out and polished with #800 sandpaper. The above-described procedures of brushing, leveling, and curing were repeated. The total mass of the topcoat was kept constant, and the three UFRCTEM samples, #1, #2, and #3, prepared in the early stage were added to the topcoat in a mass ratio of 1.0%, 3.0%, 5.0%, 7.0%, and 9.0%, and they were mixed evenly. The first layer of the topcoat was applied using a brush. The above-described procedures of brushing, leveling, and curing the topcoat were repeated. Then, #1000 sandpaper was used to polish the coating before the second layer of topcoat was applied. In addition, a pure primer and a pure topcoat were applied to the poplar board as control group specimens for future use.

2.4. Testing and Characterization

2.4.1. Performance Characterization of Microcapsules

(1)

Coverage rate (C): UFRCTEMs with a mass of M₁ were weighed. M₂ was the weight of the weighing filter paper. The UFRCTEMs were soaked in ethanol, filtered, and dried after 24 h. The total mass of the dried filter paper and wall material was M₃. The calculation of the coverage rate is shown in Formula (1).

$$C={\displaystyle \frac{\left(M_1{+M}_2\right)-M_3}{M_1}}$$

(1)

(2)

Yield rate (Y): The total mass of materials used for preparing the UFRCTEM samples was denoted as M₁. The mass of the UFRCTEMs after drying was recorded as M₂. The calculation of the yield rate is shown in Formula (2).

$$Y={\displaystyle \frac{M_2}{M_1}}$$

(2)

(3)

Analysis of microstructure and chemical composition: a Zeiss optical microscope (OM, Carl Zeiss AG, Oberkochen, Germany) was used to observe the morphology of the UFRCTEMs. Scanning electron microscopy (SEM, Tescan, Brno, the Czech Republic) was used to analyze the microstructure of the UFRCTEMs and coatings. Fourier-transform infrared spectroscopy (FTIR, Brucker AG, Karlsruhe, Germany) was used to analyze the chemical composition of the UFRCTEMs and coatings.

2.4.2. Color Difference Testing of Coatings

In the light of GB/T 11186.3-1989 [36], the chromaticity value of the coatings was measured and recorded using a SEGT-J colorimeter (Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China) and a color difference was calculated. The color difference ΔE between the coating with UFRCTEMs added and the pure coating was calculated using the color difference shown in Formula (3), where ∆L = L₁ − L₂, ∆a = a₁ − a₂, and ∆b = b₁ − b₂; ΔL represent the difference in brightness of the coating; Δa represents the red–green difference in the coating; and Δb represents the yellow–blue difference in the coating.

$$\mathsf{\Delta }E={[{\left(\mathsf{\Delta }L\right)}^2+{\left(\mathsf{\Delta }a\right)}^2+{\left(\mathsf{\Delta }b\right)}^2]}^{{\displaystyle \frac{1}{2}}}$$

(3)

2.4.3. Test for Glossiness and Reflectivity of Coatings

The sample was treated according to the requirements of GB/T 4893.6-2013 [37]. The glossiness values of the coating at three incidence angles of 20°, 60°, and 85° were tested and recorded using a glossmeter (Shenzhen Linshang Technology Co., Ltd., Shenzhen, China), with the unit being GU.

The reflection curve of the coating in the visible light wavelength range (380–780 nm) was tested and recorded using a Hitachi UV spectrophotometer (Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China). The reflectance R value was calculated using Formula (4).

$$R={\displaystyle \frac{{\int }_{380}^{780}r\left(\lambda \right)i\left(\lambda \right)d\left(\lambda \right)}{{\int }_{380}^{780}i\left(\lambda \right)d\left(\lambda \right)}}$$

(4)

where i(λ) is the standard radiation intensity for sunlight, and the unit is W∙m⁻²∙nm⁻¹. r(λ) is the reflectance value obtained through testing.

2.4.4. Roughness Testing of Coatings

The roughness value was tested and recorded using a JB-4C roughness tester (Cangzhou Oupu Testing Instrument Co., Ltd., Cangzhou, China). The macro knob was rotated to fine tune the probe position until the red display point was at the zero-scale line. The test button was activated and data were recorded. The unit of roughness value is µm.

2.4.5. Cold Liquid Resistance Test of Coatings

According to GB/T 4893.1-2021 [38], 10% citric acid solution, undescended ethanol with a volume fraction of 96%, and a cleaning agent (Guangdong Baiyun Cleaning Group Co., Ltd., Guangzhou, China) were chosen as the experimental liquids. The damage to the surface was inspected under specified lighting conditions. The test results were evaluated using numerical levels.

2.4.6. Antibacterial Performance Testing of Coatings

According to GB/T 21866-2008 [39], test operations were carried out. Firstly, Escherichia coli (ATCC25922, Shanghai Shifeng Biotechnology Co., Ltd., Shanghai, China) and Staphylococcus aureus (ACTT6538, Shanghai Shifeng Biotechnology Co., Ltd., Shanghai, China) were subjected to live bacterial manipulation. An amount of 24 g of agar medium (Huankai Microbial Technology Co., Ltd., Guangdong, China) and 1000 mL of distilled water were weighed to prepare an agar plate medium, and a sterilization treatment was performed. Slanted preserved bacterial strains were inoculated onto agar plates and placed in a constant temperature and humidity incubator (Shanghai Zhetu Scientific Instrument Co., Ltd., Shanghai, China) with a relative temperature of 38 °C for cultivation for 18–20 h. Next, the required bacterial suspension was prepared. Finally, sample testing was conducted according to References [35,40]. The formula for calculating the antibacterial rate is shown in Formula (5), where R represents the antibacterial rate and the unit is %. B represents the average number of recovered colonies of pure coating samples after 48 h, in CFU/piece. C represents the average number of recovered bacteria in the antibacterial coating sample after 48 h, in CFU/piece.

$$R={\displaystyle \frac{B-C}{B}}\times 100\%$$

(5)

2.4.7. Hardness, Impact Resistance, and Adhesion Testing of Coatings

(1) Hardness: according to GB/T 6739-2022 [41], a pencil with a hardness of 9B-9H was used, which was determined by a QHQ-A portable pencil hardness tester (Shenzhen Weichuangjie Testing Instrument Co., Ltd., Shenzhen, China). The pencil was inserted diagonally at a 45° angle into the pencil hardness tester with a load of 750 g for hardness testing.

(2) Impact resistance: according to the content of GB/T 4893.9-2013 “Physical and chemical properties testing of furniture surface coating—Part 9: determination of impact resistance” [42], the impact resistance of the wood surface coatings was tested with a coating impactor (Dongguan Jiaxin Measuring Instrument Co., Ltd., Dongguan, China). A magnifying glass was used to observe the number of cycles of surface rupture of the coating to evaluate the impact resistance level. Each sample was subjected to 5 impacts. The nearest integer to the arithmetic mean of the evaluation level was taken as the result of the level evaluation. The impact resistance level increased sequentially from level 5 to level 1, and the evaluation table for the coating impact site level is shown in

Table 3. Evaluation table of coating impact position grade.

Level	Changes in Coating on Wood Surface
1	No visible changes (no damage).
2	No cracks on the surface of the coating, but visible impact marks.
3	There are mild cracks on the surface of the coating, usually 1–2 circular or arc cracks.
4	There are moderate to severe cracks on the surface of the coating, usually 3–4 circular or arc cracks.
5	The surface of the coating is severely damaged, with usually more than 5 cycles of ring cracks, arc cracks, or coating detachment.

.

(3) Adhesion: According to GB/T 4893.4-2013 [43], the adhesion of the coating was tested using a coating adhesion tester (Quzhou Aipu Measuring Instrument Co., Ltd., Quzhou, China). The coating was cross-cut with the blade at a vertical angle of 90 degrees. A 3M adhesive tape was applied on the grid surface and peeled off at an angle close to 60°, quickly and smoothly. The adhesion level decreased from level 0 to level 5.

3. Results and Discussion

3.1. Morphology and Chemical Composition Analysis of Microcapsules

3.1.1. Microscopic Morphology Analysis of Microcapsules

SEM images of the UFRCTEMs are shown in Figure 1. Figure 1A–C show the morphology of UFRCTEM samples #1–#3 under low magnification, while Figure 1D–F show the morphology of the UFRCTEMs under high magnification. The UFRCTEMs with M_core:M_wall of 0.6:1 had more spherical and rounded shapes, with a small difference in particle size. The UFRCTEMs with M_core:M_wall of 0.8:1 were spherical, plump, and had a relatively uniform particle size distribution, but they aggregated more severely. The UFRCTEMs with M_core:M_wall of 1.2:1 had large adherent and irregularly shaped substances, and the difference in UFRCTEM particle size was relatively small. As the M_core:M_wall increased, the agglomeration phenomenon of the microcapsules continued to strengthen.

3.1.2. Chemical Composition Analysis of Microcapsules

As shown in Figure 2, the absorption peak at 3350 cm⁻¹ was the stretching vibration peak of C-O in the core material and -NH and -OH in the wall material [44]. The characteristic peaks of C=O and C=N in the urea–formaldehyde resin appeared at 1639 cm⁻¹ and 1550 cm⁻¹, respectively. The absorption peak at 1247 cm⁻¹ was caused by the stretching vibration of C-N and the deformation vibration of N-H in the urea–formaldehyde resin, indicating the presence of the chemical composition of urea–formaldehyde resin in the UFRCTEMs [45]. The characteristic peaks of C=N and C-O in coumarin compounds in the core material were located at 1600 cm⁻¹ and 1110 cm⁻¹, which existed on the absorption curve of the microcapsules, proving the presence of Toddalia asiatica (L.) Lam extract in the UFRCTEMs [46].

The above results indicate that the UFRCTEMs contain characteristic peaks of the core and wall materials.

3.2. Optical Performance Analysis of Coatings

The results of the coating glossiness and light loss rate for the UFRCTEMs with different M_core:M_wall and different contents are shown in

Table 4. Glossiness and light loss rate of poplar surface coatings with different M_core:M_wall and different content.

Sample (#)	Mcore:Mwall	Microcapsule Content (%)	Glossiness (GU)			Light Loss Rate (%)
			20°	60°	85°
-	-	0	2.20	14.03	33.23	-
Surface coating on poplar wood with #1 microcapsules added	0.6:1	1.0	1.87	7.17	10.57	48.90
		3.0	1.33	5.20	2.33	62.94
		5.0	1.10	4.40	1.53	68.64
		7.0	1.17	4.30	1.10	69.35
		9.0	0.90	2.53	0.87	81.97
Surface coating on poplar wood with #2 microcapsules added	0.8:1	1.0	1.60	7.00	9.93	50.11
		3.0	1.40	6.23	2.77	55.60
		5.0	1.23	4.60	1.13	67.21
		7.0	1.13	3.43	0.93	75.55
		9.0	0.97	2.83	0.77	79.83
Surface coating on poplar wood with #3 microcapsules added	1.2:1	1.0	1.63	8.50	14.30	39.42
		3.0	1.40	6.17	3.43	56.02
		5.0	1.27	4.30	1.57	69.35
		7.0	1.13	3.80	1.00	72.92
		9.0	0.87	2.80	0.93	80.04

. The data at a 60° incidence angle are plotted in Figure 3. Compared with the surface coating of poplar wood without UFRCTEMs added, the glossiness of the three coatings prepared with UFRCTEMs with different M_core:M_wall decreased with the increase in UFRCTEM content, and the difference in glossiness data between them was relatively small. When the M_core:M_wall of the added UFRCTEMs was 0.8:1, the overall glossiness of the coating was slightly higher, and the optical performance was relatively excellent. When the content of the UFRCTEMs in the coating was between 1.0% and 5.0%, the glossiness on the surface of poplar wood decreased rapidly. When the content of the UFRCTEMs was greater than 5.0%, the trend of glossiness changes tended to be gentle. When the M_core:M_wall of the UFRCTEMs was 0.8:1, the optical performance of the coating was better, and the maximum glossiness of the coating was 7.00 GU.

The lower the light loss of the wood surface coating, the better the optical performance of the coating [47,48]. Table 4 shows that when the UFRCTEMs with the same M_core:M_wall were added to the coating, the light loss was positively correlated with the content of UFRCTEMs in the coating.

Table 5. Chromaticity and color difference of poplar surface coatings with different M_core:M_wall and different content.

Sample (#)	Mcore:Mwall	Microcapsule Content (%)	Chromaticity Parameter						ΔE
			L1	a1	b1	L2	a2	b2
-	-	0	82.35	5.30	29.95	82.25	5.55	29.85	-
Surface coating on poplar wood with #1 UFRCTEM added	0.6:1	1	81.90	6.25	28.85	81.50	6.10	28.70	0.98
		3	78.35	8.75	27.00	78.20	8.20	26.60	2.41
		5	76.25	10.20	27.15	76.05	9.80	27.10	3.39
		7	77.50	8.20	30.45	77.20	8.05	29.80	2.01
		9	69.25	12.35	29.50	69.00	11.95	29.45	6.53
Surface coating on poplar wood with #2 UFRCTEM added	0.8:1	1	81.55	4.25	26.65	81.50	4.20	26.70	1.83
		3	79.65	7.15	28.05	79.25	6.85	27.75	1.35
		5	75.95	10.40	27.35	75.65	10.15	26.80	3.60
		7	76.35	8.15	25.75	76.15	7.90	25.45	3.23
		9	75.60	11.95	28.55	75.10	11.70	28.05	4.13
Surface coating on poplar wood with #3 UFRCTEM added	1.2:1	1	78.90	6.85	27.70	78.70	6.45	27.60	1.49
		3	78.40	6.30	31.00	78.05	6.20	30.70	1.22
		5	75.55	7.25	30.30	75.50	6.85	30.40	2.54
		7	76.10	7.65	30.00	76.05	7.35	29.60	2.36
		9	76.65	9.15	27.65	76.10	8.95	27.25	2.94

and Figure 4, respectively, show the results and trends of coating chromaticity values for UFRCTEMs with different M_core:M_wall and contents. Compared with the coating without UFRCTEMs in Figure 4A, the brightness value decreased with the continuous increase in UFRCTEM content. The addition of UFRCTEMs causes protrusions on the coating, weakening the coating’s ability to reflect light and reducing the brightness value. Figure 4B,C show a positive correlation between the red and green values and the content of UFRCTEMs. The yellow and blue values generally show a trend of first increasing, then decreasing, and then increasing with the increase in UFRCTEM content. In Figure 4D, the color difference value shows an overall upward trend with the increase in UFRCTEM content. For the wood surface coating containing the UFRCTEMs with M_core:M_wall of 0.6:1, the minimum color difference of the coating was 0.98 when the microcapsule content was 1.0%. The minimum color difference value with 0.8:1 M_core:M_wall of UFRCTEMs added to the coating was 1.35 when the UFRCTEM content was 3.0%. When M_core:M_wall of 1.2:1 UFRCTEMs was added to the coating, the minimum color difference of the coating was 1.22 when the UFRCTEM content was 3.0%.

Comparing the color difference values corresponding to the coatings prepared by three types of UFRCTEMs, the overall color difference with M_core:M_wall of 1.2:1 added was relatively small. Because the UFRCTEMs with M_core:M_wall of 1.2:1 have a higher content of core material, the prepared UFRCTEMs have a darker color compared to the other two samples, which can balance the color difference caused by the wood grain on the surface of the poplar.

Figure 5 shows the effects of UFRCTEMs with different M_core:M_wall on the surface coating reflectivity of poplar wood. The reflectivity curves of wood surface coatings prepared using three types of UFRCTEMs were highly similar. As the content of UFRCTEMs increased, the overall reflectivity showed an upward trend.

Table 6. The effects of UFRCTEMs with different M_core:M_wall on the poplar surface coating reflectance R value.

Microcapsule Content (%)	Reflectance R Value of Surface Coating on Poplar Wood
	0.6:1	0.8:1	1.2:1
0	0.6193	0.6193	0.6193
1.0	0.6472	0.5652	0.6298
3.0	0.6014	0.6399	0.6176
5.0	0.5890	0.6144	0.5715
7.0	0.6250	0.6699	0.6310
9.0	0.5770	0.5899	0.5471

shows the effect of UFRCTEM content on the coating reflectivity R value of UFRCTEMs with different M_core:M_wall. The content of UFRCTEMs had a relatively small impact on the reflectivity R value of the coating, and it was consistent with the change in the reflectivity curve. When adding UFRCTEMs with M_core:M _wall of 0.6:1, the coating reflectivity showed a trend of first increasing, decreasing, and then increasing again. When the UFRCTEM content was 1.0%, the largest reflectivity value was 0.6472. When the M_core:M_wall of UFRCTEMs was 0.8:1, the reflectivity fluctuated and generally showed an upward trend. When the content was 7.0%, the largest reflectivity value was 0.6699. When the M_core:M_wall of UFRCTEMs was 1.2:1, the reflectivity showed a trend of first increasing and then decreasing. When the UFRCTEM content was 7.0%, the largest reflectivity value was 0.6310.

The higher the reflectivity of the coating, the weaker the light absorption ability, and thus the corresponding heat absorption ability was also weaker. The absorption capacity of coatings for solar infrared and ultraviolet rays decreases, weakening the accumulated heat on the coating surface, thereby extending the lifespan of wooden substrates and their surface coatings [49].

3.3. Cold Liquid Resistance of Coatings

Table 7. Cold liquid resistance grades of the poplar surface coatings with different M_core:M_wall and different content.

Sample (#)	Mcore:Mwall	Microcapsule Content (%)	Cold Liquid Resistance Level of Surface Coating on Poplar Wood (Level)
			Citric Acid	Ethanol	Cleaning Agents
-	-	0	2	3	3
Surface coating on poplar wood with #1 UFRCTEMs added	0.6:1	1	1	2	3
		3	1	2	2
		5	1	2	2
		7	1	1	2
		9	1	1	1
Surface coating on poplar wood with #2 UFRCTEMs added	0.8:1	1	1	2	2
		3	1	2	2
		5	1	2	2
		7	1	1	1
		9	1	1	1
Surface coating on poplar wood with #3 UFRCTEMs added	1.2:1	1	1	2	2
		3	1	2	2
		5	1	1	1
		7	1	1	1
		9	1	1	1

shows the cold liquid resistance level of surface coatings on poplar wood with different M_core:M_wall and different amounts. The citric acid resistance level of the surface coatings on poplar prepared by using three types of UFRCTEMs increased to level 1. The ethanol and cleaning agent resistance levels upgraded from level 3 to level 1 or 2. The addition of UFRCTEMs had a positive protective effect on the liquid resistance of the surface coating on poplar wood. As the M_core:M_wall of UFRCTEMs increased, the liquid resistance of the coating became increasingly excellent. When the M_core:M_wall of UFRCTEMs was 0.6:1 and the UFRCTEM content was 9.0%, the liquid resistance performance of the coating was level 1. The liquid resistance of the coating was level 1 when the M_core:M_wall of UFRCTEMs was 0.8:1 and the UFRCTEM content was greater than or equal to 7.0%. When the M_core:M_wall of UFRCTEMs was 1.2:1 and the UFRCTEM content was greater than or equal to 5.0%, the liquid resistance performance of the coating was level 1.

The addition of UFRCTEMs to waterborne coatings can protect the coating from erosion by the test liquid. Poplar is affected by various stains, watermarks, and other factors during use. Improving the cold liquid resistance of the coating can provide protection for the substrate and surface coating on poplar wood, thereby extending the service life of the coating and the poplar wood [50].

3.4. Mechanical Properties of Surface Coatings

3.4.1. Hardness of Surface Coating on Poplar Wood

The hardness of wood surface coating refers to the resistance of the coating to a series of mechanical forces such as scratches, impacts, and squeezing, and is an important indicator of the mechanical strength of the coating. As shown in

Table 8. The hardness of the poplar surface coatings with different M_core:M_wall of UFRCTEMs and different content.

Microcapsule Content (%)	Hardness
	0.6:1	0.8:1	1.2:1
0	B	B	B
1.0	B	B	B
3.0	HB	HB	HB
5.0	HB	HB	2H
7.0	HB	2H	2H
9.0	2H	2H	3H

, the hardness of the surface coating on poplar wood changed. The hardness of the surface coating on wood without the addition of UFRCTEMs was B. For UFRCTEMs with M_core:M_wall of 0.6:1 and 0.8:1, the hardness of the coating increased from B to 2H with the increase in microcapsule content. For UFRCTEMs with M_core:M_wall of 0.8:1, the coating hardness increased from B to 3H with the increase in microcapsule content.

The larger the M_core:M_wall of microcapsules, the higher the hardness of the surface coating of the poplar wood. This is because UFRCTEMs are small particles with a certain volume; adding UFRCTEMs to waterborne coatings can fill the pores in the coating matrix. The addition of microcapsules increases the solid content of waterborne coatings, thereby increasing the density of the coating and gradually increasing the hardness [51].

3.4.2. Impact Resistance of Surface Coating on Poplar Wood

The impact resistance of a layer, also known as impact strength, is the ability of a surface coating to withstand heavy impact without cracking.

Table 9. Impact resistance grades of poplar surface coatings with different M_core:M_wall of UFRCTEMs and different content.

Microcapsule Content (%)	Impact Resistance Level (Level)
	0.6:1	0.8:1	1.2:1
0	5	5	5
1.0	4	4	4
3.0	4	4	3
5.0	4	3	3
7.0	3	3	3
9.0	3	3	3

shows the changes in the impact resistance level of surface coatings on poplar wood. Figure 6, Figure 7 and Figure 8 show the surface condition of the poplar coatings after impact resistance testing. The impact resistance level of the wood surface coating without UFRCTEMs was level 5, and the performance was poor and did not meet the qualified standards. When the M_core:M_wall of UFRCTEMs in the coating was 0.6:1, the impact resistance level of the coating was increased from level 4 to level 3. When the content of UFRCTEMs exceeded 7.0%, the surface coating of the poplar wood met the qualified standard for surface coating of wooden furniture (GB/T 3324-2017) [52]. When the M_core:M_wall was 0.8:1, the impact resistance level of the coating was increased from level 4 to level 3. When the content of UFRCTEMs exceeded 5.0%, the surface coating of the poplar wood met the qualified standard. When the M_core:M_wall was 1.2:1 and the microcapsule content was greater than 3.0%, the impact resistance level of the coating reached the qualified standard.

The M_core:M_wall of UFRCTEMs had a significant impact on the impact resistance of the coating. As the M_core:M_wall of UFRCTEMs increased, the impact resistance gradually improved. This is because the structure of UFRCTEMs is dense, and when they are added as solid powder fillers to waterborne coatings, UFRCTEMs can enhance the mechanical strength and suppress the cracking of the coating due to stress [53].

3.4.3. Adhesion of Surface Coating on Poplar Wood

Table 10. Adhesion grades of the poplar surface coatings with different M_core:M_wall and different content.

Microcapsule Content (%)	Adhesion Level (Level)
	0.6:1	0.8:1	1.2:1
0	0	0	0
1.0	1	1	1
3.0	1	1	1
5.0	1	1	1
7.0	1	1	2
9.0	2	2	2

reflects the changes in the adhesion level of the surface coating of poplar wood. For UFRCTEMs with M_core:M_wall of 0.6:1 and 0.8:1, when the microcapsule content was between 1.0% and 7.0%, the adhesion level of the coating remained stable at level 1. When the content of microcapsules exceeded 7.0%, the adhesion of the coating decreased to level 2. For UFRCTEMs with M_core:M_wall of 1.2:1, when the microcapsule content was between 1.0% and 5.0%, the coating adhesion was level 1. When the content of microcapsules exceeded 5.0%, the adhesion of the coating decreased to level 2. Overall, the adhesion performance of UFRCTEMs in the coating was superior when the M_core:M_wall was 0.6:1 and 0.8:1, while the surface adhesion level of UFRCTEM coatings with M_core:M_wall of 1.2:1 was poor.

The addition of UFRCTEMs disrupts the uniformity of waterborne coatings, causing an agglomeration and stress concentration in the coating, and thus reducing the coating adhesion [54].

3.4.4. Roughness of Surface Coating on Poplar Wood

The roughness values of coatings on poplar wood are shown in

Table 11. Surface coating roughness of the poplar with different M_core:M_wall and different content.

Microcapsule Content (%)	Roughness (µm)
	0.6:1	0.8:1	1.2:1
0	0.260	0.260	0.260
1.0	1.030	0.951	1.387
3.0	2.806	2.457	2.194
5.0	2.161	3.407	2.508
7.0	3.522	3.759	3.947
9.0	4.571	4.961	4.226

. The roughness value of the surface coating on poplar without microcapsules was 0.260 µm. When adding UFRCTEMs with M_core:M_wall of 0.6:1, the coating roughness increased from 1.030 µm to 4.571 µm as the microcapsule content gradually increased. When the M_core:M_wall of UFRCTEMs was 0.8:1, the coating roughness increased from 0.951 µm to 4.961 µm with the increase in microcapsule content. When the M_core:M_wall of the added UFRCTEMs was 1.2:1, the coating roughness increased from 1.387 µm to 4.226 µm as the microcapsule content gradually increased.

This is because the UFRCTEMs contain solid powder and cannot be uniformly dispersed when mixed with waterborne coatings, resulting in an uneven coating surface. In addition, the manual brushing process used also affects the uniformity and smoothness of the coating, thus increasing the roughness of the coating to a certain extent. Overall, the M_core:M_wall of three UFRCTEM samples had a certain degree of influence on the coating roughness on the wood surface. The coating with 0.8:1 of M_core:M_wall had a higher roughness, followed by the coating with 0.6:1 of M_core:M_wall, and the coating with 1.2:1 of M_core:M_wall had a lower roughness. This is because the morphology of UFRCTEMs can affect the surface smoothness of the coating. The UFRCTEMs with M_core:M_wall of 0.8:1 tend to agglomerate more severely, resulting in an uneven dispersion of UFRCTEMs in waterborne topcoats and an increase in coating roughness values.

3.5. Antibacterial Properties of Surface Coatings on Poplar Wood

Table 12. Average number of recovered colonies and antibacterial rate of the poplar surface coatings.

Mcore:M wall	Microcapsule Content (%)	Average Number of Recovered Escherichia coli (CFU·Piece−1)	Antibacterial Rate of Escherichia coli (%)	Average Number of Recovered Staphylococcus aureus (CFU·Piece−1)	Antibacterial Rate of Staphylococcus aureus (%)
-	0	347	-	368	-
0.6:1	1	190	45.25	195	47.01
	3	135	61.10	143	61.14
	5	124	64.27	104	71.74
	7	96	72.33	98	73.37
	9	95	72.62	77	79.08
0.8:1	1	197	43.22	181	50.82
	3	153	55.91	139	62.23
	5	125	63.98	101	72.55
	7	109	68.59	91	75.27
	9	98	71.76	73	80.16
1.2:1	1	180	48.13	147	60.05
	3	143	58.79	135	63.32
	5	122	64.84	103	72.01
	7	104	70.03	89	75.82
	9	96	72.33	85	76.90

shows the antibacterial rates of surface coatings on poplar against Escherichia coli and Staphylococcus aureus, and the average number of recovered colonies. The overall antibacterial rate of the three types of microcapsule coatings against Staphylococcus aureus was slightly higher than that of Escherichia coli, and the coating with M_core:M_wall of 0.8:1 had the best comprehensive antibacterial rate. As the content of UFRCTEMs in the coating increased, the antibacterial rate of the coating also increased. Figure 9 shows the trend of antibacterial rate changes. Figure 10 and Figure 11 show the bacterial colonies of Escherichia coli in the culture dish after antibacterial testing with microcapsules #2 and #3. For Escherichia coli, when the content of UFRCTEMs in the coating was between 1.0% and 7.0%, the antibacterial rate increased significantly. When the content of UFRCTEMs was greater than 7.0%, the increase in antibacterial rate was gradual. For Staphylococcus aureus, when the content of UFRCTEMs in the coating was less than 5.0%, the antibacterial rate curve increased significantly. When the content of UFRCTEMs was greater than 7.0%, the antibacterial rate curve gradually stabilized. When adding UFRCTEMs with M_core:M_wall of 0.6:1, the maximum antibacterial rate against Escherichia coli was 72.62%, and the maximum antibacterial rate against Staphylococcus aureus was 79.08%. When the M_core:M_wall of UFRCTEMs was 0.8:1, the maximum antibacterial rate against Escherichia coli was 71.16%, and the maximum antibacterial rate against Staphylococcus aureus was 80.16%. When the M_core:M_wall of UFRCTEMs was 1.2:1, the maximum antibacterial rate against Escherichia coli was 72.33%, and the maximum antibacterial rate against Staphylococcus aureus was 76.90%.

The addition of antibacterial microcapsules enhanced the antibacterial performance of waterborne coatings on the surface of poplar wood, proving that UFRCTEMs do indeed exert antibacterial effects.

3.6. Microscopic Morphology and Chemical Composition of Surface Coating on Poplar Wood

The coating with a 7.0% addition of UFRCTEMs had a better overall performance. Therefore, SEM analysis was performed on the coatings prepared using microcapsules with different M_core:M_wall at 7.0% addition (Figure 12). The surface of the coating without UFRCTEMs was relatively flat, while the coating with M_core:M_wall of 0.8:1 and 1.2:1 was relatively rough. Among them, the coating with M_core:M_wall of 1.2:1 had significant protrusions on the surface. After adding microcapsules with M_core:M_wall of 0.6:1, the coating showed less agglomeration and a smoother surface. At 7.0% additive content, the coating roughness values for M_core:M_wall of 0.6:1, 0.8:1, and 1.2:1 were 3.522 µm, 3.759 µm, and 3.947 µm, respectively. The observed microstructure of the surface coating on the poplar wood was consistent with the roughness value analysis results.

Figure 13 shows the infrared spectra of the waterborne coating on the surface of poplar with UFRCTEMs added and without UFRCTEMs added. The characteristic peak of around 1727 cm⁻¹ belonged to the stretching vibration peak of C=O in waterborne coatings. The stretching vibration peaks of -CH₃ and C=O in the urea–formaldehyde resin wall material of UFRCTEMs were located at 2924 cm⁻¹ and 1639 cm⁻¹, respectively [55]. The characteristic peak at 1114 cm⁻¹ was the absorption peak of C-O in the UFRCTEM core material.

The above findings indicate that the chemical composition of the waterborne coating did not change after mixing with UFRCTEMs, and these results are consistent with the infrared analysis results of the UFRCTEMs.

3.7. Sectional Views Coatings with Different M_core:M_wall on Wood Surface

Figure 14 shows views of poplar wood coated with a UFRCTEM coating and a waterborne coating. Figure 14A shows the poplar layer, waterborne primer layer, and waterborne topcoat layer with UFRCTEMs added. The yellow circle indicates a small amount of waterborne primer penetrating and filling the conduit holes of poplar wood, while the red circle represents the UFRCTEMs distributed in the waterborne topcoat. In Figure 14A, the waterborne topcoat layer containing UFRCTEMs had poor flatness, while in Figure 14B, the section of the waterborne coating was relatively smooth and even. The results in Figure 9 are consistent with the optical and mechanical analysis results of the coating.

4. Conclusions

UFRCTEMs were added to waterborne paint and applied to poplar wood to explore the effects of different contents and M_core:M_wall on the performance of waterborne coatings on the surface of poplar wood. Under different M_core:M_wall of UFRCTEMs, as the content of UFRCTEMs increased, the glossiness and the visible light reflectance gradually decreased, and the light loss and the color difference gradually increased. Adding microcapsules to the coating improved the cold liquid resistance, enhanced the hardness, increased the surface roughness, and enhanced the impact resistance. However, to some extent, it weakened the adhesion. As the content of UFRCTEMs increased, the antibacterial rate against both bacteria showed an upward trend, and the overall antibacterial rate against Staphylococcus aureus was slightly higher than that against Escherichia coli. The UFRCTEMs in the coating did not react chemically with waterborne coatings, ensuring excellent performance of the UFRCTEMs. The coating on a poplar surface with M_core:M_wall of 0.8:1 and UFRCTEM content of 7.0% had excellent comprehensive performance, ensuring good optical, cold liquid resistance, mechanical, and antibacterial properties of the coating. The glossiness was 3.43 GU, light loss was 75.55%, reflectance R value was 0.6699, color difference ΔE was 3.23, hardness was 2H, impact resistance level was level 3 at an impact height of 50 mm, adhesion level was level 1, and roughness value was 3.759 µm. The cold liquid resistance was excellent, with resistance to citric acid, ethanol, and cleaning agents of grade 1. The antibacterial rates against Escherichia coli and Staphylococcus aureus were 68.59% and 75.27%, respectively. These findings provide technical references for the application of UFRCTEMs in antibacterial coatings on poplar surfaces and expand their potential application prospects in industries such as coatings. The obtained microcapsule powder was relatively rough and adhesive, with a low encapsulation rate. Therefore, it was necessary to separate and purify the main antibacterial components of Toddalia asiatica (L.) Lam extract to prepare antibacterial microcapsules with better performance. The antibacterial performance of waterborne coatings on poplar wood surfaces was investigated after 48 h, without testing and analyzing the durability of microcapsules and the maximum antibacterial time of waterborne coatings containing microcapsules. Only microcapsules were added to waterborne topcoats, without exploring the effects of different methods of adding microcapsules and different types of wood on the surface coating. In the future, more in-depth exploration will be conducted on the above limitations.