Optimization of Preparation Process for Chitosan-Coated Pomelo Peel Flavonoid Microcapsules and Its Effect on Waterborne Paint Film Properties

Abstract

In order to prepare chitosan-coated pomelo peel flavonoid microcapsules with antibacterial properties, chitosan was used as the wall material for the purpose of coating the core material, pomelo peel flavonoids. The pH of the microcapsule crosslinking reaction was 7.5, the mass ratio of the microcapsule core material to the wall material was 1:1, and the concentration of the emulsifier was 1%. The microcapsules obtained under these preparation conditions exhibited superior performance, morphology, and dispersion. Additionally, the yield and coating rates were recorded at 22% and 50%, respectively. To prepare the paint film, the microcapsules were added into the coatings at varying concentrations of 0%, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%. The antibacterial efficacy of the paint film for both bacteria was progressively enhanced with the incorporation of microcapsules. The antibacterial efficacy against Staphylococcus aureus was observed to be higher than that against Escherichia coli. As the content of microcapsules increased, the color difference in the paint film increased, the gloss loss rate increased, and the light transmission rate reduced. The tensile property and elongation at break reduced, and the roughness increased. At a microcapsule content of 6.0%, the paint film exhibited superior overall performance, with an antibacterial efficacy against Escherichia coli and Staphylococcus aureus of 46.3% and 56.7%, respectively. The color difference was 38.58. The gloss loss rate was 41.0%, the light transmission rate was 90.4%, and the paint film exhibited a large elastic region, with an elongation at break of 21.5% and a roughness of 1.46 μm.

1. Introduction

With the innovation of technology, users expect more from furniture products [1,2,3,4,5]. As one of the commonly used materials in homes, wood materials have the characteristics of texture, easy to work with, light weight, and high strength [6,7,8]. However, wood has the property of dry shrinkage and wet expansion [9,10,11] and is vulnerable to damage such as decay and mold during use [12,13,14]. Wood absorbs moisture easily from the air, providing a favorable environment for bacteria to thrive [15]. Paint films are widely used in people’s daily life in direct contact with the surface of household products. To protect the substrate of wooden furniture, the surface is usually coated [16,17,18,19,20]. The paint films on furniture surfaces are important carriers of bacterial transmission, so improving the antibacterial property of paint films is one of the effective means to stop the direct transmission of bacteria [21,22,23,24]. At present, plant-based antibacterial agents are the most extensively studied and conveniently extracted natural antibacterial agents. They use certain parts of plants containing bactericidal active substances or extract effective ingredients to achieve antibacterial effects. They have the advantages of high efficiency, low toxicity or nontoxicity, no pollution, and high selectivity and are not easily resistant to drugs [25,26,27]. Natural antibacterial agents have problems such as poor processing performance and unstable chemical properties [28]. Therefore, they cannot be directly applied in waterborne paint and need further research and promotion. Microcapsule technology is used to cover natural antibacterial or antibacterial substances to make antibacterial microcapsules [29,30], expanding the application scope of natural antibacterial agents [31,32].

Flavonoids are a class of compounds widely present in nature, found in certain plants and berries, with a total of about 4000 species. They often exist in the form of free or glycosidic compounds in plants and are currently a highly regarded natural active product with excellent physiological functions [33,34,35]. Lopes et al. [36] investigated the efficacy of glycosides and flavonoids in inhibiting Staphylococcus aureus RN4220 and Staphylococcus aureus SA1199B. The results showed that the tested flavonoids inhibited biofilm formation in Staphylococcus aureus strains. Liu et al. [37] studied the chemical composition of flavonoids extracted from pomelo peel. And its antibacterial activity against the selected pathogen Vibrio anguillarum and biosensor strain Chromobacterium violaceum CV026 was determined. These studies indicate that pomelo peel extract can be used for the production of antibacterial agents. Chitosan is a natural biopolymer which is widely used in various industries because of its excellent film forming property, antibacterial activity, and biodegradability [38,39]. Chen et al. [40] used chitosan as the wall material and water-soluble vitamin C and oil-soluble lemon essential oil as the core materials and prepared antibacterial microcapsules by the single condensation method. The results showed that both chitosan and oil-soluble lemon essential oil had antibacterial properties, so the paints with microcapsules had antibacterial properties. Utami et al. [41] encapsulated Holothuria atra microcapsules through the ionic gelation process of chitosan and sodium tripolyphosphate (STPP). The results showed that the prepared microcapsules had high antibacterial activity against Escherichia coli and Staphylococcus aureus. Escherichia coli and Staphylococcus aureus are commonly used for antibacterial performance testing. Escherichia coli is a Gram-negative bacterium, while Staphylococcus aureus is a Gram-positive bacterium, and the two bacteria can represent different bacterial species. Escherichia coli and Staphylococcus aureus are considered to be the two most common bacteria [42,43], which are widely found in nature and are closely related to human health.

Therefore, chitosan-coated pomelo peel flavonoid microcapsules were prepared with pomelo peel flavonoid as the core material, chitosan as the wall material, and STPP as the crosslinking agent. The microcapsules were prepared by the ion condensation method [44]. Different contents of microcapsules were added to the paint films to explore their application in the paint film. The performances of the paint film with added microcapsules were analyzed. The content of microcapsules in the paint film was controlled to make sure that the antibacterial properties of the paint film are improved while the film still has good properties. This provides a certain technical basis for the application of antibacterial microcapsules and the preparation of antibacterial paint film.

2. Materials and Methods

2.1. Materials

The deacetylation degree range of chitosan is 80.0% to 95.0%. A mold made of silicone material with a diameter of 50 mm × 50 mm × 10 mm was used to prepare the paint films. The waterborne acrylic topcoat used in the test was from Jiangsu Haidian Technology Co., Ltd., Jurong, China. The pomelos were Shatian Pomelo from Rong County, Yulin, China. Two microorganisms, Staphylococcus aureus and Escherichia coli, were used to test the antibacterial performance of the paint films. In addition, the materials used in this experiment were all chemicals. The experimental materials are shown in

Table 1. List of materials.

Material	Molecular Formula	CAS No.
Chitosan	(C6H11NO4)n	9012-76-4
Acetic acid	CH3COOH	64-19-7
Tween-80	C24H44O6	9005-65-6
Sodium tripolyphosphate	Na5P3O10	7758-29-4
Anhydrous ethanol	C2H6O	64-17-5
Staphylococcus aureus	-	-
Escherichia coli	-	-
Nutrient agar medium	-	-
Nutritious broth	-	-

. The equipment used in the experiment is shown in

Table 2. Experimental equipment.

Equipment	Model	Manufacturing
Optical microscope (OM)	AX10	Carl Zeiss AG. Oberkochen, Germany
Scanning electron microscope (SEM)	Quanta-200	Thermo Fisher Scientific, Waltham, MA, USA
Magnetic stirrer	DF-101Z	Nanbei Scientific Instrument Technology Co., Ltd., Beijing, China
Fourier Transform Infrared (FTIR) spectrometer	VERTEX 80V	Bruker Corporation, Karlsruhe, Germany
Humidity chamber	HWS-50	Shanghai Shangyi Instrument Equipment Co., Ltd., Shanghai, China
Freeze-dryer	YTLG-10A	Shanghai Yetuo Technology Co., Ltd., Shanghai, China
Colony counter	XK97-A	Hangzhou Qiwei Instrument Co., Ltd., Hangzhou, China
Portable colorimeter	SC-10	Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China
Gloss meter	HG268	Shenzhen ThreeNH Technology Co., Ltd., Shenzhen, China
Ultraviolet spectrophotometer	U-3900	Hitachi Scientific Instruments (Suzhou) Co., Ltd., Suzhou, China
Universal mechanical testing machine	AGS-X	Shimadzu Manufacturing Co., Ltd., Kyoto, Japan
Fine roughness tester	JB-4C	Suliang Instrument Technology Co., Ltd., Suzhou, China

.

2.2. Method of Preparing Microcapsules

As shown in

Table 3. The orthogonal test schedule.

Level	Factor A pH Value	Factor B m(Core Material):m(Wall Material)	Factor C Concentration of Emulsifier (%)
1	6	1.0:1	1
2	8	1.2:1	3

, the optimal preparation process for the preparation of chitosan-coated pomelo peel flavonoid microcapsules was investigated by designing a three-factor two-level orthogonal test and a one-factor test. The orthogonal tests were designed, and the influencing factors were the pH for the crosslinking reaction of the microcapsules, the mass ratio of the core and wall materials of the microcapsules, and the concentration of the emulsifier during the emulsification of the microcapsules. Samples of microcapsules with different parameters were obtained. The experimental arrangement is shown in

Table 4. The orthogonal test microcapsule samples.

Sample	pH	m(Core Material):m(Wall Material)	Concentration of Emulsifier (%)
1	6	1.0:1	1
2	6	1.2:1	3
3	8	1.0:1	3
4	8	1.2:1	1

. According to the various test results, the maximum influence of the preparation of microcapsules was explored. The maximum influence factor was used as the variable of the one-factor test, and the preparation process of microcapsules was optimized through the test so as to obtain a better preparation process for the microcapsules.

The preparation of microcapsules was divided into four steps. The first step was to prepare the chitosan wall material solution, the second step was to prepare the core material emulsion of pomelo peel flavonoids, the third step was to mix the wall material solution with the core material emulsion, and the fourth step was to add STPP into the mixed solution to make it crosslinked with the wall material. The specific preparation steps are shown in [45].

Table 5. The schedule of test material dosage.

Test	Sample	Chitosan (g)	1% Acetic Acid Solution (g)	Pomelo Peel Flavonoids (g)	Anhydrous Ethanol (g)	Emulsifier (g)	Deionized Water (mL)	STPP (g)
Orthogonal test	1	0.80	39.20	0.80	7.20	0.72	71.28	0.80
	2	0.80	39.20	0.96	8.64	2.11	68.29	0.80
	3	0.80	39.20	0.80	7.20	2.10	69.90	0.80
	4	0.80	39.20	0.96	8.64	0.70	69.70	0.80
One-factor test	5	0.80	39.20	0.80	7.20	0.72	71.28	0.80
	6	0.80	39.20	0.80	7.20	0.72	71.28	0.80
	7	0.80	39.20	0.80	7.20	0.72	71.28	0.80
	8	0.80	39.20	0.80	7.20	0.72	71.28	0.80
	9	0.80	39.20	0.80	7.20	0.72	71.28	0.80

shows the dosage of materials.

2.3. Preparation of Waterborne Paint Films with Different Contents of Microcapsules

Waterborne paint films with different contents of microcapsules were coated in the silicone mold. The ordinary content of the surface paint of the substrate was 60 g/m²–80 g/m², and a total of 4–6 coats were applied. To simulate the regular use of the paint on the surface of the substrate and to take into account the loss during actual use, the total waterborne paint mass of the paint film was set at 400 g/m², and the thickness was about 80 μm. The amount of paint film prepared is detailed in

Table 6. The preparation of the paint film.

Content of Microcapsules (%)	Mass of Microcapsules (g)	Mass of Waterborne Paint (g)
0	0	1.00
3.0	0.03	0.97
6.0	0.06	0.94
9.0	0.09	0.91
12.0	0.12	0.88
15.0	0.15	0.85

.

The optimal microcapsule sample 7 prepared in the one-factor test was added to the waterborne paint at different concentrations of 0%, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%. The microcapsules and the waterborne paint were thoroughly stirred by a glass rod and coated in silicone molds. The silicone molds were left at room temperature for 1 h and then cured in an oven at 50 °C for 30 min. Finally, the paint film was slowly removed from the silicone mold to obtain a complete paint film sample for testing.

2.4. Performance Test

2.4.1. The Microcapsule Yield and Coverage Rate

After the preparation of microcapsules, the product quality was measured, and the percentage of the product quality to the raw material quality was calculated, that is, the yield of the microcapsules.

The weighed microcapsule powder, denoted as M₁, fully ground, was added to a beaker filled with anhydrous ethanol and soaked for 2 days. In order to fully dissolve the cores, the beakers were placed in a water bath at 60 °C for 4 h. After soaking, complete filtration, and rinsing, the resulting product was oven-dried at 50 °C until the weight did not change. The resulting product was recorded as M₂. This was the weight of the wall material of the microcapsule. The formula for calculating the coverage rate of microcapsules is shown in Formula (1).

$$P=\left(M_1-M_2\right)/M_1\times 100\%$$

(1)

2.4.2. The Morphology and Chemical Composition

An OM was used to observe and analyze the microstructure of the microcapsules. For observation, the light source was first turned on, and the slide was placed on the carrier table and fixed with a pressure clamp. The focus was adjusted until the field of view was clear, and the picture was taken.

An SEM was used to observe and analyze the microstructure of the microcapsules and the paint film. The SEM was used to characterize the microstructure of the paint films. In order to characterize the microcapsules by SEM, a processed sample was placed on the sample stage, a door of the sample compartment was closed, and the vacuum pump was turned on to pump the air. The relevant parameters were set according to the sample characteristics and observation need, and the desired image was captured and saved. When the particle size of the microcapsules was analyzed, the SEM image of the microcapsules obtained was analyzed by software to generate a histogram of the particle size of the microcapsules.

An FTIR spectrometer was used to determine the chemical composition of microcapsules and the paint films with added microcapsules. When the paint film samples were tested by the FTIR spectrometer, the parameters were adjusted so that the background spectra were first acquired, and subsequently, the paint film samples were placed in the sample chamber for testing. The KBr pressing method was used for the testing of microcapsule samples.

2.4.3. The Antibacterial Performance

Escherichia coli (ATCC25922) and Staphylococcus aureus (ACTT6538) were used to test the antibacterial properties of the coating. According to GB/T 21866-2008 [46], the testing was divided into bacterial strain preservation, bacterial strain activation, bacterial suspension preparation, sample testing, and calculation results. The specific experimental steps can be found in [46,47]. The antibacterial rate R of the paint film was calculated according to Formula (2), where B represents the average number of recovered colonies after 48 h of blank coating and C represents the average number of recovered colonies after 48 h of antibacterial paint film, in CFU/piece.

$$R=\left(B-C\right)/B\times 100\%$$

(2)

2.4.4. The Optical Performance

Based on the GB/T 11186.3-1989 [48], a portable colorimeter was used to measure the color value of the paint film and calculate the color difference in the paint film. After the portable colorimeter was calibrated, a point was selected on the paint film to test, and the color values of L, a, and b were noted. The values of the blank paint film were denoted as L₁, a₁, and b₁, and those of the film containing microcapsules were denoted as L₂, a₂, and b₂. The color difference ΔE of the paint film was calculated by Formula (3), where ΔL = L₂ − L₁, Δa = a₂ − a₁, and Δb = b₂ − b₁.

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

(3)

Based on GB/T 4893.6-2013 [49], a gloss meter was used to test the glossiness of the paint film and record the glossiness of the paint film under three sets of incident angles (20°, 60°, and 85°). G₀ was the glossiness of the paint film without microcapsules added, G₁ was the glossiness of the paint film containing microcapsules, and the gloss loss rate G_L at 60° incidence angle of the paint film was calculated. The calculation formula is shown as Formula (4).

$$G_L=\left(G_0-G_1\right)/G_0\times 100\%$$

(4)

An ultraviolet spectrophotometer was used to measure the light transmittance of the paint film in the visible band with a wavelength range of 380 nm–780 nm.

2.4.5. The Mechanical Performance

The paint film was tested by a universal mechanical testing machine. During the test, the two ends of the paint film were fixed by the fixture, and the paint film was stretched until it broke. According to Formula (5), the elongation at break of the film was denoted as e, and the original length of the film was denoted as L₀. When the film broke, the distance beyond the original length was denoted as L_n. The elongation at break of the paint film was the ratio of the distance over the original length to the original length.

$$e=L_n/L_0\times 100\%$$

(5)

A fine roughness tester was used to test the roughness of the paint film.

3. Results and Discussion

3.1. The Microcapsules’ Yield and Coverage Rate

Four kinds of microcapsule samples were obtained by a three-factor and two-level orthogonal test. The yield of the orthogonal test for the preparation of microcapsules is shown in

Table 7. Analysis of range and variance results of microcapsule yield rate in orthogonal tests.

Category	Sample	Factor A pH Value	Factor B m(Core Material):m(Wall Material)	Factor C Concentration of Emulsifier (%)	Yield (%)
Range	1	6	1.0:1	1	20
	2	6	1.2:1	3	20
	3	8	1.0:1	3	22
	4	8	1.2:1	1	21
	Mean value 1	20.000	21.000	20.500
	Mean value 2	21.500	20.500	21.000
	R	1.500	0.500	0.500
	Factor primary and secondary levels	A > B = C
	Optimal level	A2	B1	C2
	Optimal solution	A2 B1 C2
Variance	Sum of squared deviations	2.250	0.250	0.250
	Degree of freedom	1	1	1
	Fratio	2.455	0.273	0.273
	Fcritical value	10.100	10.100	10.100
	Significance

. Sample 3 has the highest yield, and the result is 22%. According to Table 7, it can be found that the yield results of microcapsule samples have little difference. According to the yield range analysis, the primary and secondary effects of the three factors are A > B = C. Therefore, the biggest factor that influences the microcapsule yield is A, which is the pH value of the microcapsule crosslinking reaction. The mass ratio of the microcapsule core material to wall material and the emulsifier concentration had the same effect on the yield. The optimal level of microcapsule yield was A2 B1 C2, namely, a crosslinking reaction pH of 8, a core–wall ratio of 1:1, and an emulsifier concentration of 3%. As shown by the result data in Table 7, the size of factors affecting the yield of microcapsules is basically consistent with the range result. Through the sum of squared deviations, it can be seen that factor A has a much greater impact on the yield than the other two factors. Therefore, combining the yield results of microcapsules, it can be concluded that the pH value of the microcapsule crosslinking reaction has the greatest influence on the yield of the microcapsules. The optimum preparation process of the microcapsules was when the pH value of the microcapsule crosslinking reaction was 8, the mass ratio of the microcapsule core material to wall material was 1:1, and the emulsifier concentration was 3%.

Table 8. Analysis of range and variance results of microcapsule coverage rate in orthogonal tests.

Category	Sample	Factor A pH Value	Factor B m(Core Material):m(Wall Material)	Factor C Concentration of Emulsifier (%)	Coverage Rate (%)
Range	1	6	1.0:1	1	45
	2	6	1.2:1	3	31
	3	8	1.0:1	3	53
	4	8	1.2:1	1	51
	Mean value 1	38.000	49.000	48.000
	Mean value 2	52.000	41.000	42.000
	R	14.000	8.000	6.000
	Factor primary and secondary levels	A > B > C
	Optimal level	A2	B1	C1
	Optimal solution	A2 B1 C1
Variance	Sum of squared deviations	196.000	64.000	36.000
	Degree of freedom	1	1	1
	Fratio	1.986	0.649	0.365
	Fcritical value	10.100	10.100	10.100
	Significance

shows the coverage rate results of the chitosan-coated pomelo peel flavonoid microcapsules in the orthogonal test. Sample 3 had the highest coverage rate of 53%. According to the analysis of range results, the order of the three influencing factors is A > B > C. Therefore, the factor that has the largest influence on microcapsule coverage rate is the pH value of the crosslinking reaction, followed by the mass ratio of the microcapsule core–wall material and finally the emulsifier concentration in the preparation. A2 B1 C1 is the optimal coverage rate level of the microcapsule preparation. The variance results of the impact of various factors on the coverage rate are basically consistent with the range results. From the sum of squares data, it can be seen that the influence of the pH value of the microcapsule crosslinking reaction on the result of the coverage rate is much greater than that of others. Thus, it can be concluded that factor A, that is, the pH value in the microcapsule crosslinking reaction, has the biggest influence on the coverage rate of microcapsules. The optimal process for the coverage rate of microcapsules is the microcapsule crosslinking reaction with a pH value of 8, a core–wall ratio of 1:1, and a concentration of the emulsifier of 1%.

According to the results of the orthogonal test, the pH value of the microcapsule crosslinking reaction is the most significant factor influencing the yield and coverage rate of chitosan-coated pomelo peel flavonoid microcapsules. Therefore, based on the orthogonal test, a one-factor test was designed with the pH value of the microcapsule crosslinking reaction as the variable. According to the orthogonal test results, the yield of microcapsule samples did not vary much, so the preparation parameters of the one-factor test focused on the results of the coverage rate in the orthogonal test. When the mass ratio of the core–wall material is 1:1, the coverage rate and yield are better. When the emulsifier concentration is 1%, the coverage rate is better, and the high coverage rate is conducive to improving the antibacterial property of the waterborne paint, so the emulsifier concentration is determined to be 1% in the one-factor test. The results of the one-factor test are shown in

Table 9. The yield and coverage rate of microcapsules in the one-factor test.

Sample	pH Value	Yield Rate (%)	Coverage Rate (%)
5	5.5	24	38
6	6.5	20	41
7	7.5	22	50
8	8.5	22	47
9	9.5	23	44

. In the one-factor test, sample 5 with a pH value of 5.5 had a higher yield but a lower coverage rate. This may be because the pH value was too low, resulting in a large amount of residual chitosan wall material being coated. It can be proved that the coating of microcapsules should be carried out under alkaline conditions. When the pH exceeds 5.5, the yield of microcapsules slightly increases with the increase in pH value. When the pH value reaches 7.5, the coverage rate of microcapsules is up to 50%, and with the increase in pH value, the coverage rate of the microcapsules gradually decreases, which may be because the high pH value leads to the formation of microcapsules too fast, and the core material is reduced. It is proved that when pH value is 7.5, it is more suitable for pomelo peel flavonoids coated with chitosan. The optimal preparation conditions are a core-to-wall ratio of 1:1, an emulsifier concentration of 1%, and a pH value of 7.5 during the crosslinking reaction. Based on the results of the microcapsules’ yield, coverage rate, and morphology in the one-factor test, sample 7 with a pH value of 7.5 at the time of the crosslinking reaction was selected as the optimal sample and added to the waterborne paint film to further explore the application of microcapsules in the waterborne paint.

3.2. The Morphological and Chemical Composition of the Microcapsules

The macroscopic morphology of the pomelo peel flavonoids, chitosan, and chitosan-coated pomelo peel flavonoid microcapsule sample 7 is shown in Figure 1. The pomelo peel flavonoids are irregular large granular yellow crystals, and the chitosan is a beige powder. The microcapsules prepared with these two are in a yellow powder shape. This is because during the preparation of the microcapsules, the core emulsion mixed with the wall solution, resulting in staining. At the same time, the chitosan wall material itself is thinner and has a beige color, and there are also more encapsulated core materials. The color of the core materials is revealed through more micropores, making the color of the microcapsules darker. This can preliminarily prove that the successfully prepared microcapsules contain two substances, pomelo peel flavonoids and chitosan.

The micromorphology of the chitosan-coated pomelo peel flavonoid microcapsules prepared by orthogonal test is shown in Figure 2. Microscope images show that there are two kinds of materials in the microcapsule: the wall material, chitosan, and the core material, pomelo peel flavonoids. The bright white spot inside the microcapsule is the pomelo peel flavonoids, which can preliminarily prove the success of the microcapsule coating. It can be observed that microcapsule sample 1 and sample 2 prepared at pH 6 in the orthogonal test have better dispersibility and more microcapsule particles. The agglomeration of microcapsule sample 3 and sample 4 was serious when the pH value was 8. This may be due to the high pH value during the reaction of microcapsules, which leads to a fast crosslinking reaction of the microcapsules. When the core material is not fully coated, too much crosslinking is formed in the wall material; the microcapsules were adhered together by a large number of walls that were not coated with the core material and could not be presented as separate spheres.

Figure 3 shows the microscopic morphology of the chitosan-coated pomelo peel flavonoid microcapsules in the one-factor test. The particles of chitosan in the microscope image are too small, and the observation analysis is limited. Therefore, the microscopic morphology of microcapsules in the one-factor test is mainly observed by the SEM. It can be observed that microcapsule sample 5 in the one-factor test is basically in an amorphous state, which indicates that the crosslinking reaction of microcapsules is not suitable when the pH is 5.5, and microcapsules cannot be formed when the pH is too low. The agglomeration phenomenon of microcapsule sample 8 was more serious, and the agglomeration phenomenon of microcapsule sample 9 was the most serious, but it was obvious that the microcapsules had formed into a spherical state, indicating that the microcapsules were successfully prepared. However, a high pH value will lead to a sticky agglomeration of microcapsules, which is not conducive to microcapsule dispersion and is not suitable for the microcapsule crosslinking reaction. The crosslinking reaction between the chitosan and STPP occurred under alkaline conditions. When the solution is at a lower pH value, STPP is more likely to protonate, forming positively charged STPP molecules. The positively charged STPP interacts electrostatically with the negatively charged chitosan molecules, promoting the crosslinking reaction. When the pH value is higher, the protonation degree of STPP decreases, and the electrostatic interaction between STPP and chitosan is weakened, so the crosslinking reaction is weakened. In Figure 3, the microcapsules prepared at pH 7.5 for sample 7 were clearly observed to be more complete spherical microcapsules and well dispersed, with fewer impurities compared to the other samples. Therefore, considering the yield, coverage rate, and microscopic morphology of the chitosan-coated pomelo peel flavonoid microcapsules, sample 7 with a 22% yield and a 50% coverage rate was selected. Figure 4 shows the particle size distribution of microcapsules prepared by the one-factor test. As shown in Figure 4, the particle size of the microcapsules prepared at different pH values is relatively dispersed, mainly between 2 and 7 μm, indicating that the chitosan-coated pomelo peel flavonoid microcapsules have different particle sizes and poor morphology. The average particle size of the four samples is not significantly different. The average particle size of sample 6 is 5.19 μm, that of sample 7 is 5.19 μm, that of sample 8 is 5.28 μm, and that of sample 9 is 5.38 μm.

Figure 5 shows the infrared spectra of the wall material, the chitosan-coated pomelo peel flavonoid microcapsules, and the core material. In the infrared spectra of the chitosan wall material, 3433 cm⁻¹ is a vibration absorption peak of the chitosan structure -OH, 1647 cm⁻¹ is the vibration absorption peak of an amide group Ι, 1088 cm⁻¹ is a C-O-C absorption peak, and 891 cm⁻¹ is the stretching vibration absorption peak of a pyranoside ring. In an infrared spectrum of the pomelo peel flavonoids, the vibration peaks at 1372, 1242, and 1058 cm⁻¹ are the characteristic peaks of pomelo peel flavonoids, the absorption peak at about 3390 cm⁻¹ is a hydroxyl association, and the absorption peak at 2932 cm⁻¹ is CH₂-. In addition, 1740 cm⁻¹ and 1646 cm⁻¹ are the C-O-induced stretching vibration peaks, and 877 cm⁻¹ is the absorption peak caused by substituent positions on the benzene ring. The absorption curves also showed the absorption peaks of the chitosan and pomelo peel flavonoids [50]. The chemical components of the chitosan and pomelo peel flavonoids were present in the microcapsules, which proved the successful preparation of microcapsules.

3.3. The Morphology of the Paint Film

The macroscopic morphology shown in Figure 6 is the paint film with different contents of sample 7. The paint film was yellow and transparent after microcapsules were added. With the increase in microcapsule content in the paint film, the transparency gradually decreased, and the color of the paint film gradually deepened. This is because the microcapsules are yellow; the addition of the waterborne paint changes the color of the film. The microcapsules coated with chitosan have good dispersibility and no obvious granular sensation in the waterborne paint. When the proportion of microcapsule mass in the total paint film mass is less than 15.0%, the morphology of the paint film is not affected except for the color change, and no cracking phenomenon appears, which preliminarily proves that the chitosan-coated pomelo peel flavonoid microcapsules are suitable for addition to the waterborne paint. When the proportion of microcapsule mass in the total paint film mass reaches 15.0%, the edges of the paint film have already been damaged. This is because the chitosan increases the viscosity of the paint film. The excessive content of microcapsules in the waterborne paint leads to increased viscosity and poor flat property, making it difficult to form the paint film. This indicates that the paint film with 15.0% microcapsules is no longer suitable for application.

The microstructure of the paint film with added microcapsule sample 7 is shown in Figure 7. With the addition of 3.0% microcapsules, the paint film has a slightly raised granular feeling. When the content is 6.0% and 9.0%, the granular feeling on the paint film surface is more obvious. When the content reaches 12.0%, the agglomeration of a lot of microcapsules in the paint film, the paint film presents a large area of raised folds. It can be seen that when the content of microcapsules is above 9.0%, it will seriously affect the flatness.

3.4. The Chemical Composition of the Paint Film

Figure 8 shows the FTIR image of the paint film without microcapsules and added microcapsules with a content of 9.0%. In Figure 8, around 1726 cm⁻¹ is the characteristic peak of the C=O in the waterborne acrylic resin in the waterborne acrylic paint, and around 1144 cm⁻¹ is the vibration peak of the C-O in both the waterborne paint and microcapsules. About 3390 cm⁻¹ is the characteristic peak of the hydroxy-association of pomelo peel flavonoids in microcapsules, and 2932 cm⁻¹ is the absorption peak of CH₂-. The C-O-C absorption peak of the chitosan structure in the microcapsules is about 1088 cm⁻¹, and the stretching vibration absorption peak of the pyranoside ring is at 891 cm⁻¹. It can be proved that the microcapsules are used in the waterborne paint, and the chemical composition of the paint film is not destroyed.

3.5. The Antibacterial Properties of the Paint Film

The average number of recovered bacteria and the antibacterial rate are shown in

Table 10. The antibacterial properties.

Content of Microcapsules (%)	Average Number of Recovered Escherichia coli (CFU·Piece−1)	Antibacterial Rate against Escherichia coli (%)	Average Number of Recovered Staphylococcus aureus (CFU·Piece−1)	Antibacterial Rate against Staphylococcus aureus (%)
0	190	-	432	-
3.0	134	29.5 ± 0.6	289	33.1 ± 0.8
6.0	102	46.3 ± 1.6	187	56.7 ± 1.5
9.0	73	61.6 ± 0.9	126	70.8 ± 1.2
12.0	51	73.2 ± 1.4	74	82.9 ± 2.7
15.0	32	83.2 ± 1.1	36	91.7 ± 1.7

. As shown in Table 10, the antibacterial rate against both bacteria gradually increased with the increase in the microcapsule content, indicating that the paint film with added microcapsules has an antibacterial effect on both bacteria. When the content of microcapsules in the paint film reaches 15%, the antibacterial rate of the paint film against the two kinds of bacteria could reach 83.2% and 91.7%, respectively. Both the wall material chitosan and the core material pomelo peel flavonoids have antibacterial activity, which further improves the antibacterial rate of the microcapsules.

Figure 9 shows the antibacterial rate of paint films with different microcapsule contents. The antibacterial activity of the paint film against Staphylococcus aureus is slightly higher than that against Escherichia coli. The results show that the chitosan-coated pomelo peel flavonoid microcapsules successfully had an antibacterial effect in the waterborne paint film and effectively improved the difficult mixing of the pomelo peel flavonoids in waterborne paint. The pomelo peel flavonoids that have antibacterial properties work together with the chitosan that also has antibacterial properties through the micropores of the microcapsules to inhibit bacteria.

The chitosan-coated pomelo peel flavonoid microcapsules had a higher antibacterial rate compared to melamine-resin-coated pomelo peel flavonoid microcapsules [51]. When the content of melamine-resin-coated pomelo peel flavonoid microcapsules in the paint film was 6%, the antibacterial rate against Escherichia coli was 50.5%, and the antimicrobial rate against Staphylococcus aureus was 40.5%. When the content of the chitosan-coated pomelo peel flavonoid microcapsules in the paint film was 6%, the antibacterial rates against Escherichia coli and Staphylococcus aureus were 46.3% and 56.7%, respectively. This is due to the fact that only the core material is antibacterial in the melamine-resin-coated pomelo peel flavonoid microcapsules, and the wall material is not antibacterial, whereas chitosan, which is used as the wall material of the microcapsules in the chitosan-coated pomelo peel flavonoid microcapsules, is also antibacterial, so the antibacterial rate of the microcapsules is increased.

A significance analysis was conducted on the data in Table 10. The significance analysis was conducted using a non-repeated two-factor variance method. The three values of F, p-value, and F crit separately were calculated (

Table 11. Significance analysis of antibacterial properties.

Item	SS	df	MS	F	p-Value	F Crit
Content of microcapsules	10,537.29	5	2107.457	243.8058	0.00	5.050329
Antibacterial species	142.83	1	142.83	16.5236	0.009682	6.607891
Error	43.22	5	8.644
Total	10,723.34	11

). Among them, F is the test statistic, a statistical measure used for hypothesis testing calculations. The p-value represents a significance level, evaluates the range and interval of the overall parameters, and calculates the probability of the experiment occurring. F crit is the critical value of F on the corresponding significance level. Among them, F > F crit indicates that there is a difference between the two sets of data. F < F crit means there is no difference between the two sets of data. The criteria for determining significant differences are 0.01 < p-value < 0.05, indicating significant differences. A p-value ≤ 0.01 means the difference is extremely significant. A p-value > 0.05 indicates insignificance. According to the above methods, the results obtained are F > F crit and p-value < 0.01, indicating that the content of microcapsules in the paint film has a significant effect on the antibacterial properties of the paint film.

3.6. The Optical Properties of the Paint Film

(1)

The chromaticity value and color difference

The chromaticity value of and color difference in paint films with different microcapsule contents is shown in

Table 12. The chromaticity value of and color difference in paint films with different microcapsule contents.

Content of Microcapsules (%)	L	a	b	ΔE
0	81.91	1.73	−2.27	-
3.0	44.93	0.97	0.50	37.09
6.0	43.50	0.97	1.30	38.58
9.0	43.00	0.90	1.57	39.11
12.0	42.20	0.87	1.60	39.91
15.0	41.40	0.63	1.87	40.74

. When the paint film contains 15% microcapsules, the color difference can reach up to 40.74. Because of the addition of yellow granular microcapsules to the paint, the color of the paint film deepens, and the L value in the paint film gradually decreases as the microcapsules increase. The a values representing the red–green values are positive in all samples, indicating that the color was reddish. Because the color of the paint film itself tends to be blue, the b value of the sample without microcapsules is negative. After adding microcapsules, the b value changed from negative to positive, indicating that the color became yellowish and gradually increased as the microcapsules increased, indicating that the yellow color of the microcapsules themselves had an obvious influence on the paint film.

(2)

Glossiness and gloss loss rate

Table 13. The glossiness and gloss loss rate of paint films with different microcapsule contents.

Content of Microcapsules (%)	20° (%)	60° (%)	85° (%)	Gloss Loss Rate (%)
0	6.10	17.45	31.17	-
3.0	4.47	14.13	14.93	19.1
6.0	2.40	10.30	7.27	41.0
9.0	2.13	10.17	4.10	41.7
12.0	2.10	10.17	3.40	41.7
15.0	1.77	8.10	3.27	53.6

shows the glossiness and gloss loss rate of paint films with different microcapsule contents. At different incidence angles, the glossiness decreases as the microcapsules increase. Because the chitosan-coated pomelo peel flavonoid microcapsules have color, the addition of microcapsules enhances the diffuse reflection of light on the surface of the paint film, which affects the glossiness. When the paint film contained 9.0% microcapsules, the gloss loss rate of the paint film was 41.7%; when the microcapsule content was higher than 9.0%, the gloss loss rate of the paint film began to decrease as the microcapsules increased. This is because of the good dispersibility of the chitosan-coated pomelo peel flavonoid microcapsules in the waterborne paint, which has little effect on the flatness of the dried paint film.

(3)

Visible light transmittance

The visible light transmittance of paint films with different microcapsule contents is shown in

Table 14. The visible light transmittance of paint films with different microcapsule contents.

Content of Microcapsules (%)	Visible Light Transmittance (%)
0	89.89
3.0	93.25
6.0	90.40
9.0	90.47
12.0	87.94
15.0	85.54

. The visible light transmittance is higher than 85% on the whole. The addition of microcapsules has little effect on the visible light transmittance. When the paint film contained 3.0% microcapsules, the visible light transmittance value was 93.25%. Figure 10 shows the light transmittance curve of microcapsule paint film with different additive content. As the microcapsule content increases, the light transmittance of the paint film decreases slightly. All paint films tend to be relatively transparent, exhibiting a stable visible light wavelength range. This indicates that adding microcapsules has little effect on the transparency of the paint film and is suitable for practical applications.

3.7. The Mechanical Properties of the Paint Film

(1)

Roughness

The effect of microcapsule content on the roughness of the paint film is shown in

Table 15. The effect of different microcapsule content on the roughness.

Content of Microcapsules (%)	Roughness (%)
0	0.27
3.0	0.46
6.0	1.46
9.0	2.60
12.0	2.65
15.0	2.70

. The roughness increased with the increase in microcapsules. The addition of microcapsules results in uneven particles on the paint film surface. When the microcapsule content increased, the concave and convex feeling of the paint film was enhanced, which affects the flatness of the paint film and makes the roughness rise continuously. However, because the microcapsules prepared with chitosan have better dispersibility in the waterborne paints, the overall roughness of the paint film is low.

(2)

Elongation at break

As an important parameter characterizing the uniform or stable deformation of a material, elongation at break is an index used to judge the ductility of a material, indicating the ability of the paint film to deform without rupturing when subjected to an external force. Higher elongation at break indicates better ductility of the paint. The tensile properties of the paint film are shown in Figure 11. When the microcapsule content in the paint film was 3.0% and 6.0%, the elastic region of the paint film was the largest, and the ductility of the paint film was higher. This means that the paint film has better flexibility and ductility and can better adapt to the deformation of the substrate, thereby improving the overall performance of the paint. But when the microcapsule content exceeded 9.0%, the ductility of the paint film began to decrease gradually. This shows that the appropriate amount of microcapsules can enhance the ductility of the film, but this may be because the chitosan-coated pomelo peel flavonoid microcapsules dispersed in waterborne paints have a certain viscosity, which thickens the paint and regulates the viscosity of waterborne paints to a certain extent. However, the microcapsule content is too high, which will reduce the ductility of the paint film. Too many particles of microcapsules added to the paint film can easily produce agglomeration; microcapsules cannot be evenly distributed in the paint film, resulting in stress concentration, and these stresses will be released when the paint film is subjected to external forces, thus reducing the elongation at break.

Table 16. The elongation at break.

Content of Microcapsules (%)	Elongation at Break (%)
0	18.9
3.0	22.3
6.0	21.5
9.0	16.1
12.0	10.5
15.0	7.9

shows the effect of the microcapsule content on the elongation at break. The elongation at break was 18.9% when the paint film did not contain microcapsules. When the content of microcapsules added to the paint film was 3%, the elongation at break was 22.3%. The chitosan-coated pomelo peel flavonoid microcapsules have a certain viscosity, and an appropriate amount of dispersion in the waterborne paints can thicken the paint and improve the viscoelasticity. The elongation at break of the membrane decreased when the content of microcapsules in the paint film was greater than 3.0%. When the content was 15.0%, the elongation at break was 7.9%. When the microcapsule content is too high, the viscoelasticity decreases, the paint film becomes brittle, and the relative tensile property of the paint film gradually decreases, which leads to a decrease in the elongation at break.

4. Conclusions

The preparation procedure for the optimal chitosan-coated pomelo peel flavonoid microcapsules is as follows: the pH value of the microcapsule crosslinking reaction is 7.5, the core–wall ratio is 1:1, and the concentration of the emulsifier is 1%. The morphology of microcapsules is better, and the dispersion is relatively uniform. With the increase in microcapsule content, the antibacterial rate of the paint film with added microcapsules against the two kinds of bacteria increased gradually, and the antibacterial rate against Staphylococcus aureus was higher than that against Escherichia coli. The color difference in the paint film gradually increased, the gloss gradually decreased, the loss of light increased, the roughness gradually increased, and the light transmittance decreased. The ductility of the paint film could be enhanced with the appropriate amount of microcapsules. The best tensile properties and the highest elongation at break of 22.3% were obtained when the paint film contained 3.0% microcapsules. When the content of microcapsules exceeds 6.0%, the tensile performance and elongation at break of the paint film significantly decrease. When the content of microcapsules was 6.0%, the comprehensive performance of the paint film was superior, the antibacterial rate of the paint film against Escherichia coli was 46.3%, the antibacterial rate of the paint film against Staphylococcus aureus was 56.7%, the color difference was 38.58, the gloss loss rate was 41.0%, the transmittance rate was 90.4%, and the paint film had a large elastic area, with an elongation at break of 21.5% and a roughness of 1.46 μm. These results provide a technical reference for the application of pomelo peel flavonoid microcapsules in the preparation of waterborne antibacterial paints.