Preparation of Tung Oil Microcapsules Coated with Chitosan Sodium Tripolyphosphate and Their Effects on Coating Film Properties

Abstract

To address the high drying temperature, low yield, and low coating rate that characterize traditional chitosan/gum arabic microcapsules, this study used chitosan/sodium tripolyphosphate (STPP) ionic crosslinking to construct a composite wall, combined with optimized emulsifier compounding (T-80/SDBS), to prepare tung oil self-healing microcapsules. Orthogonal testing determined the following optimal parameters: a core-to-wall ratio of 2.0:1.0, a T-80/SDBS ratio of 4.0:6.0 (HLB = 12.383), an STPP concentration of 4%, and a spray-drying temperature of 120 °C. With these parameters, a yield of 42.91% and coating rate of 68.50% were achieved. The microcapsules were spherical (1–6 μm), with chitosan–STPP electrostatic interactions forming a dense wall. Adding 5% microcapsules to the UV topcoat enabled self-healing after 60 s UV curing: the scratch-healing rate reached 25.25% (width decreased from 11.13 μm to 8.32 μm), the elongation at break increased by 110% to 9.31%, the light transmission remained >82.50%, and the color difference (ΔE = 2.16) showed no significant change versus unmodified coating.

1. Introduction

Coatings are widely used as surface protection and decorative materials in the fields of construction, automobiles, furniture, and industrial equipment [1,2,3,4,5,6]. Traditional coatings are susceptible to mechanical damage, chemical corrosion, or environmental aging during long-term use [7,8,9,10,11], leading to cracking, peeling, or functional failure of the coating and shortening the service life of the material [12,13,14]. In recent years, with the rising demand for environmental protection and high performance, ultraviolet (UV)-curable coatings have attracted attention due to their fast curing, low volatile organic compound (VOC) emissions, and high hardness [15,16,17,18]. However, traditional UV coatings still suffer from insufficient toughness and a lack of damage self-healing ability, limiting their application in precision or high-durability scenarios [19,20,21,22,23]. Self-healing technology can significantly extend the service life of materials and reduce maintenance costs by giving them the ability to heal themselves after damage through their internal design [24,25,26,27,28,29]. Currently, self-healing mechanisms mainly include microencapsulated healing agents, reversible chemical bond reorganization, and microvascular network delivery [30,31,32,33,34]. Among them, microencapsulated self-healing technology has become a research hotspot because of its easy process and high healing efficiency [35,36,37].

The technology encapsulates the healing agent in microcapsules. When the coating is damaged, the microcapsule ruptures to release the healing agent, which heals the damaged area through polymerization, crosslinking, and other reactions [38,39,40,41]. However, the preparation parameters of microcapsules (e.g., core-to-wall ratio, wall stability, etc.) directly affect the healing effect, and how to optimize the performance of microcapsules to adapt to different coating systems is still a technical difficulty [42,43].

Self-healing microcapsules consist of a wall material with good mechanical strength and encapsulation stability, and a core material that is the healing agent (e.g., monomer, oil, or catalyst) [44,45,46]. In existing studies, natural polymers (e.g., chitosan, gum arabic) are widely used as wall materials due to their biocompatibility, degradability, and low cost [47]. However, the traditional chitosan–gum arabic microcapsules have defects such as a high drying temperature and a low yield and coating rate [48]. To overcome these problems, this study innovatively used sodium tripolyphosphate (STPP) and crosslinked it with chitosan to form a composite wall material, utilizing the phosphoric acid root in STPP to form electrostatic interaction and hydrogen bonding with the amino and hydroxyl groups of chitosan to enhance the stability of the wall material. The core is made of tung oil, which is rich in conjugated double bonds and reactive groups that can polymerize under oxidation to heal cracks. The core-to-wall ratio, emulsifier ratio, and other parameters were optimized through orthogonal and one-way tests to significantly improve the yield and coating rate of microcapsules, which provided technical support for the development of high-efficiency self-healing UV coatings. Externally reinforced self-healing materials utilize physical mixing to place the healing agent in the material. When the material is damaged, the wall ruptures and the healing agent flows out and cures to achieve the effect of filling the crack [49,50]. Self-healing materials based on microcapsule technology contain a variety of typical systems, such as a dicyclopentadiene/Grubbs catalyst system, an epoxy resin-based system, an isocyanate system, a dry oil system, a polar solvent system, a silicone system, and a silicone oil-based system, which are wrapped around the healing agent through microcapsules, releasing the core healing agent to realize the self-healing function when the material is damaged [51,52,53].

Li et al. prepared tung oil-loaded microcapsules protected with polyurea formaldehyde (PUF) shells using in situ polymerization [54]. The microcapsules maintained a spherical shape with a core content greater than 80.0 wt%. The microcapsules had good thermal stability. A bifunctional coating was prepared by incorporating tung oil microcapsules in epoxy resin. Under scarring or abrasion conditions, tung oil was released from the microcapsules, realizing the self-repairing and self-lubricating functions of the coating. The tung oil had excellent film-forming properties when in contact with oxygen. The self-healing anti-corrosion and self-lubricating anti-wear properties were evaluated by salt immersion corrosion test and wear test. Ji et al. prepared microcapsules using sodium alginate, chitosan, and Bifidobacterium longum as a wall material, an emulsification method and an endogel encapsulation method, respectively [55]. Both the alginate microcapsules and the chitosan-coated alginate microcapsules possessed sphere-forming properties. The alginate microcapsules retained their spheronization properties after freeze-drying.

Microcapsules made from chitosan and gum arabic as wall materials have the problems of high temperature during drying and low yield and coating rate of the microcapsules produced [56]. To solve this problem, sodium tripolyphosphate was used to crosslink with chitosan to form the wall material in order to produce microcapsules with better yield and encapsulation. Self-healing microcapsules were prepared by the ionic crosslinking method, using chitosan and sodium tripolyphosphate as wall materials and tung oil as the core material. Polyoxyethylene sorbitan monooleate (T-80) and sodium dodecylbenzene sulfonate (SDBS) were used as emulsifiers to be mixed with the core to form the emulsion. Sodium tripolyphosphate (STPP) is crosslinked with chitosan to form the wall. By changing the solution pH, chitosan crosslinks with STPP, and the phosphate and sodium ions in STPP can form electrostatic interactions, as well as hydrogen bonding with the amino and hydroxyl groups in chitosan, so that the microcapsule obtains a more stable shell. In this study, a four-factor, three-level orthogonal test was designed, and a one-factor test was further used to explore the preparation parameters that could produce microcapsules with a better coating rate and yield. The produced chitosan sodium tripolyphosphate-coated tung oil (CST-T) microcapsules were added to the UV topcoat to investigate the effect of the addition of different CST-T microcapsules on the performance of the coating film.

2. Test Materials and Methods

2.1. Materials

The materials used for the microcapsule preparation are detailed in

Table 1. List of materials used in test.

Name	Molecular Formula	CAS No.	Manufacturer
Chitosan	(C6H11NO4)n	9012-76-4	Sinopharm Chemical Reagent Co., Ltd., Beijing, China
Gum Arabic Powder	C12H36	9000-01-5	Tianjin Zhonglian Chemical Reagent Co., Ltd., Tianjin, China
Acetic Acid (CH3COOH)	C2H4O2	64-19-7	Sinopharm Chemical Reagent Co., Ltd. Beijing, China
Tung Oil	C65H100O14	-	Shanghai Shenmeng Home Furnishing Co., Ltd., Shanghai, China
Tannic Acid	C76H52O46	1401-55-4	Tianjin Zhonglian Chemical Reagent Co., Ltd., Tianjin, China
Polyoxyethylene Sorbitan Monooleate	C24H44O6	9005-65-6	Tianjin Beichen Fangzheng Reagent Co., Ltd., Tianjin, China
Sodium Dodecylbenzene Sulfonate	CH3(CH2)11C6H4SO3Na	25155-30-0	Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China
Sodium Tripolyphosphate (SDPP)	Na5P3O10	7758-29-4	Sinopharm Chemical Reagent Co., Ltd., Beijing, China

. A UV topcoat (supplier: Jiangsu Haidian Technology Co., Ltd., Jiangsu, China), comprising polyurethane acrylic resin, propylene glycol diacrylate, hexylene glycol diacrylate, photoinitiator, functional fillers, matting agents, wax additives, defoamer, dispersant, and anti-settling agents, was applied to 75 mm × 25 mm glass substrates using silicone molds (50 mm × 25 mm × 25 mm). The experimental equipment used is cataloged in

Table 2. List of equipment used in experiment.

Name	Model Number	Manufacturer
Collector-type constant-temperature heating magnetic stirrer	DF-101Z	Shanghai Yixin Scientific Instrument Co., Ltd., Shanghai, China
Ultrasonic emulsifying disperser	BILONG-500	Shanghai Biron Instrument Co., Ltd., Shanghai, China
Small spray-dryer	JA-PWGZ100	Shenyang Jing’ao Instrument Technology Co., Ltd., Shenyang, China
Circulating-water vacuum pumps	SHZ-D	Shanghai Sematic Instrument Co., Ltd., Shanghai, China
Air-drying oven	DHG-9240A	Shanghai Aojin Instrument Manufacturing Co., Ltd., Shanghai, China
UV-curing machine	620#	Huzhou Tongxu Machinery Equipment Co., Ltd., Huzhou, China
Optical microscope	AX-10	Carl Zeiss Co., Ltd., Baden-Württemberg, Germany
Fourier infrared spectrometer	VERTEX 80V	Bruker Technology Co., Ltd., Hamburg, Germany
Scanning electron microscope	QUANTA-200	Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA
Roughness tester	J8-4C	Shanghai Tai Ming Optical Instrument Co., Ltd., Shanghai, China
Gloss meter	HG268	Shenzhen SUNSHI Technology Co., Ltd., Shenzhen, China
Spectrocolorimeter	CR7	Shenzhen SUNSHI Technology Co., Ltd., Shenzhen, China
Universal mechanical testing machine	AGS-X	Shimadzu Manufacturing, Kyoto, Japan
Ultraviolet spectrophotometer	U-3900	Hitachi Instrument Co., Ltd., Suzhou, China

.

2.2. Preparation of CST-T Microcapsules

As shown in

Table 3. Orthogonal test design.

Level	Factor A Core-to-Wall Ratio	Factor B Emulsifier Compounding Ratio (T-80:SDBS)	Factor C Concentration of Crosslinker STPP (%)	Factor D Spray-Drying Temperature (°C)
1	1.0:1.0	0.5:9.5	2	125
2	2.0:1.0	2.0:8.0	3	130
3	3.0:1.0	3.5:6.5	4	135

, a four-factor, three-level orthogonal test was designed to investigate the more favorable conditions for the preparation of CST-T microcapsules. We prepared different microcapsule samples by varying the preparation conditions. Our aim was to investigate how these conditions would affect the microcapsule yield and coating rate. This study was conducted to identify the optimal conditions for achieving an improved yield and coating rate. We investigated four factors in microcapsule preparation: the core-to-wall ratio, emulsifier ratio, the STPP concentration (as crosslinker), and the spray-drying temperature. Each factor was tested at three distinct levels. The selection of factor levels for the four-factor, three-level orthogonal test is shown in Table 3. The specific experimental arrangement is shown in

Table 4. Orthogonal microcapsule samples.

Samples	Core-to-Wall Ratio	Emulsifier Compounding Ratio (T-80:SDBS)	Concentration of Crosslinking Agent STPP (%)	Spray-Drying Temperature (°C)
1	1.0:1.0	0.5:9.5	2	125
2	1.0:1.0	2.0:8.0	3	130
3	1.0:1.0	3.5:6.5	4	135
4	2.0:1.0	0.5:9.5	3	135
5	2.0:1.0	2.0:8.0	4	125
6	2.0:1.0	3.5:6.5	2	130
7	3.0:1.0	0.5:9.5	4	130
8	3.0:1.0	2.0:8.0	2	135
9	3.0:1.0	3.5:6.5	3	125

.

For microcapsule sample 1,

Table 5. Schedule of test material dosage.

Type of Test	Samples	Chitosan (g)	1% Acetic Acid (g)	Tung Oil (g)	T-80 (g)	SDBS (g)	Pure Water for Emulsification (g)	STPP (g)	Pure Water for STPP (g)
Orthogonal test	1	0.800	79.200	0.800	0.119	2.261	76.820	0.200	9.800
	2	0.800	79.200	0.800	0.476	1.904	76.820	0.300	9.700
	3	0.800	79.200	0.800	0.833	1.547	76.820	0.400	9.600
	4	0.800	79.200	1.600	0.117	2.233	76.050	0.300	9.700
	5	0.800	79.200	1.600	0.470	1.880	76.050	0.400	9.600
	6	0.800	79.200	1.600	0.823	1.528	76.050	0.200	9.800
	7	0.800	79.200	2.400	0.116	2.214	75.270	0.400	9.600
	8	0.800	79.200	2.400	0.466	1.864	75.270	0.200	9.800
	9	0.800	79.200	2.400	0.816	1.516	75.270	0.300	9.700
One-way test	10	0.800	79.200	1.600	0.470	1.880	76.050	0.400	9.600
	11	0.800	79.200	1.600	0.587	1.763	76.050	0.400	9.600
	12	0.800	79.200	1.600	0.705	1.645	76.050	0.400	9.600
	13	0.800	79.200	1.600	0.823	1.528	76.050	0.400	9.600
	14	0.800	79.200	1.600	0.940	1.410	76.050	0.400	9.600
	15	0.800	79.200	1.600	1.058	1.292	76.050	0.400	9.600
	16	0.800	79.200	1.600	1.175	1.175	76.050	0.400	9.600

shows the material dosages in the orthogonal experimental design with four factors at three levels each.

A 3% emulsifier solution was prepared by dissolving 0.119 g of T-80 and 2.261 g of SDBS in deionized water. Subsequently, 0.800 g of tung oil (1:1 core-to-wall ratio) was added to this emulsifier solution. The mixture was stirred at 50 °C and 1800 rpm for 1 h using a magnetic stirrer. Subsequently, it was ultrasonicated (1 s on/2 s off pulses) for 5 min to form a homogeneous milky-white core emulsion.

A 1% chitosan solution was prepared by dissolving 0.800 g of chitosan in 79.200 g of 1% acetic acid, followed by stirring (50 °C, 1000 rpm, 1 h). This solution was added dropwise to the core emulsion at pH 4.5, then mixed under identical stirring conditions for 1 h. Separately, 0.200 g of STPP was dissolved in 9.800 g of deionized water to form a 2% solution of STPP. This solution was added dropwise to the emulsion–wall mixture and crosslinked (50 °C, 1000 rpm, 2 h). After 24 h aging at room temperature, the product was spray-dried (inlet: 125 °C; feed rate: 150 mL/h) to obtain microcapsule powder.

2.3. Preparation of UV Coating Film with Addition of CST-T Microcapsules

CST-T microcapsules (5 wt%) were incorporated into the UV topcoat films according to

Table 6. Details of coating materials.

Samples	Microencapsulated Mass (g)	UV Topcoat Mass (g)
10	0.05	0.95
11	0.05	0.95
12	0.05	0.95
13	0.05	0.95
14	0.05	0.95
15	0.05	0.95
16	0.05	0.95

. Specifically, 0.95 g of UV topcoat and 0.05 g of microcapsules were magnetically stirred (800 rpm, 15 min). The mixture was then applied to glass substrates, leveled for 10 min, and UV-cured (60 s).

2.4. Testing and Characterization

2.4.1. Sample Characterization of Microcapsules

Fourier transform infrared (FTIR) spectra of microcapsules were measured using a Bruker instrument model Equinox 55 spectrometer (Bruker Technology Co., Ltd., Hamburg, Germany), and FTIR spectra of polymers were measured using the attenuated total reflection (ATR) mode in the spectral range of 500–4000 cm⁻¹.

Yield test: After the sample had been dried, the mass of the collected sample was weighed and recorded as M_a, and the mass of the raw material used in the sample-making was recorded as M_b; the ratio of the mass of the collected sample to the mass of the input material was the yield of the sample. The formula for the yield Y is shown in Equation (1).

$$Y={\displaystyle \frac{M_a }{M_b}} \times 100\%$$

(1)

Encapsulation rate test: A certain mass of the sample M₀ was weighed and poured into a mortar, then grinded sufficiently to rupture the microcapsule walls. The ground sample was poured into a beaker and a certain amount of ethanol was added to the beaker so that the sample was well soaked. After 48 h, the samples were filtered using a vacuum filtration machine. The samples were continuously rinsed during the filtration process to separate the residual core material from the wall material. After filtration, the resulting product was dried in a blast oven at 50 °C until a constant weight was reached, at which time the weight was recorded as M₁. C was calculated as shown in Equation (2).

$$C={\displaystyle \frac{M_0-M_1}{M_0}} \times 100\%$$

(2)

Characterization of morphology: The scanning electron microscope QUANTA-200 was used to characterize the microscopic morphology of microcapsules and coating films. To characterize the sample morphology using SEM, the sample was fixed on the sample stage with conductive adhesive. The sample was mounted on the sample holder according to the experimental requirements. The scanning electron microscope sample chamber was opened and the sample tray was mounted on the sample holder. The sample chamber was closed and a vacuum was applied to the sample chamber. After the vacuum was drawn, the sample was observed and images were taken through the software interface.

Characterization of chemical composition: The chemical composition of microcapsules and varnish films were tested and characterized using the FTIR spectrometer VERTEX 80V (Bruker Technology Co., Ltd., Hamburg, Germany). To test the lacquer film samples, the surface of the ATR crystals was cleaned first. The samples to be tested were prepared to the appropriate size. The parameters were adjusted according to the experimental requirements of the sample. The sample to be measured was placed on the surface of the ATR crystal, ensuring the sample was in good contact with the surface, and the sample was pressed by rotating the sampling holder. Spectra were collected and data were saved on the software interface. The KBr pressing method was used for testing the microcapsules and other powdered samples. A 1 mg sample of microcapsules was placed in a mortar with 100 mg of KBr powder and a pestle was used for thorough grinding. Subsequently, the powder was poured into a mold for tableting. Finally, the insert plate containing the mold cover and the sample ingot was inserted directly into the sample holder of the instrument for measurement.

2.4.2. Coating Film Performance Tests

Self-healing performance test: The self-healing performance of the coating film was tested using the scratch test. The surface of the coating was scratched with a razor blade, and the width of the scratch was observed and recorded with an optical microscope, which was recorded as W₁. Then, 48 h later, the scratch was observed again and the width of the scratch was recorded as W₂; the self-healing rate of the coating film was calculated from W₂ and W₁. The formula for calculating the self-healing rate H is shown in Equation (3).

$$H={\displaystyle \frac{W_1-W_2}{W_1}} \times 100\%$$

(3)

Glossiness test of coating film: According to GB/T 4893.6-2013 [57], the coating film was tested for glossiness. The glossiness tester HG268 can be used to measure the glossiness presented by the coating when it is irradiated by light sources of different angles. To test the surface gloss of a coating using the glossiness tester, the coating was covered with the measuring window at the bottom of the instrument and the data were subsequently recorded by clicking on the measuring button. The surface gloss of the coating with the addition of microcapsules was recorded as G₁, and the surface gloss of the coating film without microcapsules was recorded as G₀. The formula for calculating the rate of loss of gloss G is shown in Equation (4).

$$G={\displaystyle \frac{G_0-G_1}{G_0}} \times 100\%$$

(4)

Color difference test of coating film: The Colorimeter CR7 was used to measure the color difference of the coating film surface. During the test, the color value of the UV topcoat without microcapsules was measured first, and then the color value of the coating film after microcapsules were added was measured, and the instrument automatically calculated the color difference value ΔE. The L value stood for the degree of lightness and darkness: the larger the L value was, the brighter the color of the sample was, and vice versa. The a value was used to measure the tendency of the color to be reddish–greenish: if the a value was negative, it meant that the color was greenish, and if the a value was positive, it meant that the color was reddish. The b value was used to indicate the tendency of the color to be yellowish-blue: if the b value was negative, it meant the color was bluish, and if the b value was positive, it meant the color was yellowish.

Visible light transmittance test of coating film: Visible light transmittance is the ability of light to pass through a sample, and the percentage of the luminous flux passing through the sample to its incident luminous flux is the light transmittance of the prepared coating film. The UV spectrophotometer U3900 was used to test the visible transmittance of coatings with and without microcapsules, and the visible transmittance τ of each sample was calculated according to Equation (5). τ__t(λ) represents the measured transmittance of the sample at wavelength λ as measured by the UV spectrophotometer, v(λ) is the CIE standard bright-vision visual function, and d(λ) is the relative spectral power distribution of the light source.

$$\tau ={\displaystyle \frac{\sum _{\lambda }d\left(\lambda \right)v\left(\lambda \right){\tau }_t\left(\lambda \right)}{\sum _{\lambda }d\left(\lambda \right)v\left(\lambda \right)}}$$

(5)

Roughness test of coating film: The fine roughness tester J8-4C was used to test the roughness. The sample was placed on the sample stage and the probe position was adjusted using the knob. When the probe was in contact with the sample surface, the test was started and the data were recorded.

Elongation at break test: The universal mechanical testing machine AGS-X was used to test the elongation at break of the coating film. Each sample formulation was tested in quintuplicate (n = 5), and results are reported as mean values. The UV topcoat, after mixing it with the microcapsules, was poured into a mold, cured, and demolded. The ends of the coating film were clamped in the fixture of the universal mechanical testing machine. The speed during stretching was set to 0.5 mm/min and the samples were stretched to break. The elongation at break of the coating film was calculated as shown in Equation (6), where e denotes the elongation at break of the coating film at the break point, L₀ is the initial distance between the upper and lower fixtures when the coating film was stretched, and L₁ is the distance between the upper and lower fixture arms when the coating film was broken.

$$e={\displaystyle \frac{L_1-L_0}{L_0}} \times 100\%$$

(6)

3. Results and Discussion

3.1. Yield and Encapsulation Rate of CST-T Microcapsules

A four-factor, three-level orthogonal experimental design was implemented to optimize the encapsulation efficiency of the chitosan–STPP/tung oil (CST-T) microcapsules, yielding nine distinct formulations. As quantified in

Table 7. Analysis of microcapsule coverage rate in orthogonal tests.

Samples	Factor A Core-to-Wall Ratio	Factor B Emulsifier Compounding Ratio (T-80:SDBS)	Factor C STPP Concentration (%)	Factor D Spray-Drying Temperature (°C)	Coating Rate (%)
1	1.0:1.0	0.5:9.5	2	125	42.00
2	1.0:1.0	2.0:8.0	3	130	50.00
3	1.0:1.0	3.5:6.5	4	135	52.00
4	2.0:1.0	0.5:9.5	3	135	50.00
5	2.0:1.0	2.0:8.0	4	125	68.00
6	2.0:1.0	3.5:6.5	2	130	62.00
7	3.0:1.0	0.5:9.5	4	130	48.00
8	3.0:1.0	2.0:8.0	2	135	58.00
9	3.0:1.0	3.5:6.5	3	125	68.00
Mean value 1	48.00	46.67	54.00	59.33
Average value 2	60.00	58.67	56.00	53.33
Average value 3	58.00	60.67	56.00	53.33
range	12.00	14.00	2.00	6.00
Factor priority level	B > A > D > C
Optimal preparation program	A2 B3 C3 D1

, the encapsulation rates ranged from 42.00% (sample 1) to 68.00% (samples 5 and 9). Statistical analysis revealed the significance hierarchy of factors affecting encapsulation efficiency as B > A > D > C, indicating that the emulsifier ratio (ratio of T-80 to SDBS) exerted the strongest influence, followed by the core-to-wall ratio and the spray-drying temperature, while the STPP concentration demonstrated minimal impact. The optimal condition combination was identified as A2B3C3D1, with a core-to-wall ratio of 2.0:1.0, a T-80-to-SDBS ratio of 3.5:6.5, an STPP concentration of 4%, and spray-drying temperature of 125 °C. These parameters produced microcapsules with maximized encapsulation efficiency.

Analysis of variance for the microcapsule encapsulation rates is presented in

Table 8. Analysis of variance table for encapsulation rates.

Considerations	Square Sum (e.g., Equation of Squares)	df	Mean Square	F	p
Factor A Core-to-wall ratio	496.000	2	248.000	11.160	0.004 **
Factor B Emulsifier compounding ratio	688.000	2	344.000	15.480	0.001 **
Factor C Crosslinker concentration	16.000	2	8.000	0.360	0.707
Factor D Drying Temperature	144.000	2	72.000	3.240	0.087
Inaccuracies	200.000	9	22.222

Note: * p < 0.05, ** p < 0.01.

. The factor significance hierarchy for the chitosan–sodium tripolyphosphate-coated tung oil microcapsule yield was as follows: emulsifier ratio (T-80:SDBS) > core-to-wall ratio > spray-drying temperature > STPP concentration. Factors A (core-to-wall ratio) and B (emulsifier ratio) showed highly significant effects (p < 0.01) on encapsulation rates, whereas Factors C (STPP concentration) and D (temperature) demonstrated no statistical significance. These findings align with the range analysis of encapsulation rates.

The results of the yield of microcapsules in the four-factor, three-level orthogonal test were analyzed as shown in

Table 9. Analysis of microcapsule yield rate in orthogonal tests.

Samples	Factor A Core-to-Wall Ratio	Factor B Emulsifier Compounding Ratio (T-80:SDBS)	Factor C Concentration of Crosslinker STPP (%)	Factor D Spray-Drying Temperature (°C)	Yield (%)
1	1.0:1.0	0.5:9.5	2	125	37.32
2	1.0:1.0	2.0:8.0	3	130	32.48
3	1.0:1.0	3.5:6.5	4	135	41.94
4	2.0:1.0	0.5:9.5	3	135	32.87
5	2.0:1.0	2.0:8.0	4	125	50.68
6	2.0:1.0	3.5:6.5	2	130	37.98
7	3.0:1.0	0.5:9.5	4	130	27.49
8	3.0:1.0	2.0:8.0	2	135	35.08
9	3.0:1.0	3.5:6.5	3	125	25.09
k1	37.25	32.56	36.79	37.70
k2	40.51	39.41	30.15	32.65
k3	29.22	35.00	40.04	36.63
range	11.29	6.85	9.89	5.05
Factor priority level	A > C > B > D
Optimal preparation program	A2 B2 C3 D1

. The highest yield of all the samples produced in the orthogonal test was obtained for the sample 5 microcapsules, with 50.68%, followed by the sample 3 microcapsules with 41.94%, while the sample 9 microcapsules had the lowest yield of 25.09%. The effect of the four factors on microcapsule yield was A > C > B > D, i.e., the core-to-wall ratio had the greatest effect on microcapsule yield, followed by STPP concentration and emulsifier ratio. The levels of each factor more favorable for making high yield microcapsules were A2 B2 C3 D1, respectively. This indicates that the microcapsule samples produced in this orthogonal test when the microcapsules were prepared under the conditions of core-to-wall ratio of 2.0:1.0, the ratio of emulsifier T-80 to SDBS of 2.0:8.0, the concentration of STPP of 4%, and the spray-drying temperature of 125 °C achieved a high yield.

The analysis of variance of the results of the encapsulation rate of microcapsules in the orthogonal test is shown in

Table 10. Analysis of variance table for yield rate.

Considerations	Square Sum (e.g., Equation of Squares)	df	Mean Square	F	p
Factor A: Core-to-wall ratio	405.082	2	202.541	18.323	0.001 **
Factor B: Emulsifier compounding ratio	144.772	2	72.386	6.548	0.018 *
Factor C: Crosslinker STPP concentration	305.019	2	152.509	13.797	0.002 **
Factor D: Drying temperature	84.894	2	42.447	3.840	0.062
inaccuracies	99.486	9	11.054

Note: * p < 0.05, ** p < 0.01.

. The order of influence on microcapsule yield was as follows: the core-to-wall ratio, the concentration of the crosslinker STPP, the ratio of T-80 to SDBS in the emulsifier, and finally, the drying temperature. Factor A: the core-to-wall ratio, Factor B: the emulsifier compounding ratio, and Factor C: the crosslinker STPP concentration all had a significant effect on the results of the microencapsulation yield. Factor D: the spray-drying temperature had no significant effect on the results of the microcapsule encapsulation rate.

In order to obtain a better coating rate and yield, a one-way test was designed based on the results of the orthogonal test. In the orthogonal test, the factor that had the greatest effect on the encapsulation rate of the microcapsules was Factor B: the emulsifier compounding ratio. Factor A: the core-to-wall ratio had the greatest effect on the yield of microcapsules, and there was a higher yield of microcapsules when A2 was used. Since microcapsules with a high encapsulation rate provide more restorative agents when applied, a one-factor test was designed focusing on the results of the encapsulation rate test. According to the results of orthogonal tests, it was found that the preparation conditions for the samples with the best yield and the best coating rate were somewhat similar. The samples with the optimal coating rate were prepared under the conditions of A2 B3 C3 D1, and the samples with the optimal yield were prepared under the conditions of A2 B3 C3 D1. Therefore, a one-way test was designed to prepare microcapsules using the ratio of the two emulsifiers as a variable. Considering the effects of the levels of other factors on the experiments, levels A2 C3 D1 were used for the other three factors in the one-way test, i.e., a core-to-wall ratio of 2.0:1.0, a crosslinking agent STPP concentration of 4%, and a spray-drying temperature of 120 °C, respectively.

The yield and encapsulation rate of microcapsules obtained in the one-way test are shown in

Table 11. Yield and coverage rate of single-factor tests.

Samples	T-80-to-SDBS Ratio	HLB Value of Emulsifiers	Yield (%)	Coating Rate (%)
10	2.0:8.0	11.510	40.39	49.50
11	2.5:7.5	11.728	40.97	60.00
12	3.0:7.0	11.947	44.08	65.00
13	3.5:6.5	12.170	40.77	55.00
14	4.0:6.0	12.383	42.91	68.50
15	4.5:5.5	12.601	35.73	69.25
16	5.0:5.0	12.819	34.95	63.50

. The highest yield was 44.08% for microcapsule Sample 12 prepared at an emulsifier ratio of 3.0:7.0; this was followed by microcapsule sample 14 prepared at an emulsifier ratio of 4.0:6.0 with a yield of 42.91%. The lowest yield was 34.95% for microcapsule sample 16 prepared at an emulsifier ratio of 5.0:5.0. The HLB value of the compounded emulsifier was calculated as shown in Equation (7). The HLB value of the compounded emulsifier was calculated from the HLB values of the two emulsifiers used, and the mass was added. The HLB value of T-80 was 15.000, and the HLB value of SDBS was 10.638. W₁ represents the mass of T-80 added to the complex emulsifier, and W₂ represents the mass of SDBS added to the complex emulsifier.

$$\mathrm{HLB}={\displaystyle \frac{W_1\times 15.000+{ W}_2\times 10.638 }{W_1+W_2}}$$

(7)

The yield of microcapsules produced in the one-way test showed a general trend of increasing and then decreasing. The HLB value of the emulsifier gradually decreased as the content of SDBS in the compound emulsifier increased, so the selection of emulsifiers with lower HLB values was not conducive to the enhancement of the yield of CST-T microcapsules. The highest encapsulation rate of 69.25% in the univariate test was achieved for microcapsule sample 15, followed by sample 14 with 68.50%, while the lowest encapsulation rate was only 49.50% for microcapsule sample 10. The encapsulation rate of the microcapsules produced in the test basically showed a trend of increasing and then decreasing, reaching a maximum under the preparation conditions of two emulsifiers in a ratio of 4.5:5.5, and the emulsifier used in the preparation of sample 15 had a higher HLB value compared to the other samples in the one-way test. The HLB value of the emulsifier was positively correlated with the encapsulation rate of sodium chitosan tripolyphosphate-coated tung oil microcapsules.

The STPP concentration critically influenced the capsule wall density through ionic crosslinking. At lower concentrations, insufficient crosslinking between chitosan’s amine groups and STPP’s phosphate anions resulted in porous walls, reducing encapsulation efficiency. Conversely, excess STPP caused over-aggregation of chitosan–STPP complexes, leading to irregular capsule morphology and lowered yield.

The optimal HLB value (12.383 for T-80/SDBS = 4:6) stabilized the oil-in-water emulsion by balancing its hydrophilic–lipophilic properties. A higher HLB value increased the emulsion viscosity, hindering droplet breakup during ultrasonication and yielding larger, less uniform capsules. A lower HLB value reduced the interfacial film rigidity, promoting the coalescence of tung oil droplets and lowering the encapsulation rate.

3.2. Morphology and Chemical Composition of CST-T Microcapsules

3.2.1. Morphology of Microcapsules

According to the microscopic morphology of the microcapsules produced in the one-way test, the microcapsule samples were all spherical, as shown in Figure 1. Except for sample 10, where the adhesion between the microcapsules was stronger and separate spheres could not be presented, the microcapsules made under the other conditions could be observed to present complete spheres for each microcapsule when observed by SEM. This was due to the 2.0:8.0 ratio of the two emulsifiers used in the preparation of sample 10. The HLB value of the emulsifier at this case was 11.510, which is a lower HLB value of the emulsifier used in the preparation compared to other microcapsule samples. These results indicate that lower HLB values of emulsifiers have a negative effect on microcapsule morphology.

The particle size distribution of the microcapsules produced in the one-way test is shown in Figure 2. In the samples of CST-T microcapsules produced by the one-way test, the particle sizes of microcapsule sample 10 were mainly distributed in the range of 1–4 μm, those of microcapsule sample 11 in the range of 1–3 μm, those of microcapsule sample 12 in the range of 1–4 μm, those of microcapsule sample 13 in the range of 2–4 μm, those of microcapsule sample 14 in the range of 1–5 μm, those of microcapsule sample 15 in the range of 1–6 μm, and those of microcapsule sample 16 in the range of 1–4 μm. The differences in the particle sizes of the microcapsules were small, due to the fact that the particle sizes of the microcapsules were mainly affected by the stirring speed. The higher the magnetic stirring speed, the smaller the particle sizes of the sample. The samples had a similar particle size distribution because the magnetic stirring speed used in preparing the microcapsules was the same. This also indicates that the particle size was minimally affected by the emulsifier.

3.2.2. Chemical Composition Analysis of CST-T Microcapsules

Figure 3 shows the infrared spectra of the wall materials, chitosan and sodium tripolyphosphate; the core material, tung oil; and microcapsule sample 15. During testing, measurements for chitosan, sodium tripolyphosphate, and the microcapsules were taken using the KBr compression method, and tung oil was tested by dropping it directly on the surface of ATR crystals.

The C-H telescopic vibration peak at 2854 cm⁻¹, the C=O telescopic vibration peak at 1746 cm⁻¹, and the conjugated double bond bending vibration at 991 cm⁻¹ are characteristic peaks of tung oil, which indicates the presence of tung oil in the sample. The vibration absorption peak of -OH at 3434 cm⁻¹, the -CH telescopic vibration peak at 2873 cm⁻¹, and the -NH₂ vibration absorption peak at 1647 cm⁻¹ are characteristic peaks of chitosan, indicating that chitosan is present in the sample. Sodium tripolyphosphate in the wall material exhibits a -OH vibrational absorption peak at 3434 cm⁻¹, a characteristic peak of the P=O bond at 1212 cm⁻¹, a peak at 1168 cm⁻¹ from the telescopic vibration of the P-O bond, and a peak at 899 cm⁻¹ due to the P-O bond’s bending vibration. In the IR spectrum of microcapsule sample 15, the vibrational absorption peak at 1647 cm⁻¹ from -NH₂ in chitosan, the telescopic vibration peak of P-O at 1168 cm⁻¹ for sodium tripolyphosphate, and the bending vibration peak of P-O at 899 cm⁻¹, which are shifted, as well as the new peak at 1575 cm⁻¹, which arises from the interaction of the amino group of chitosan with the phosphate group of sodium tripolyphosphate, suggest that the wall of the microcapsules consists of both chitosan and sodium tripolyphosphate [58,59].

The characteristic peak of tung oil can be observed in the spectrum of microcapsule sample 15, and it can be judged that the core material, tung oil, was been successfully encapsulated in the wall material composed of chitosan and sodium tripolyphosphate, and the CST-T microcapsule was successfully prepared.

3.3. Morphology and Chemical Composition of UV Coating Films with Added CST-T Microcapsules

3.3.1. Morphology of UV Coating Film

Sample 12 microcapsules, with the highest yield in the one-way test, and sample 15 microcapsules, with the highest encapsulation rate in the one-way test, were selected to be added to the UV topcoat at a level of 5%, respectively, and poured into a silicone mold to make a coating film. As shown in Figure 4, the surface morphology of the UV topcoat, the UV topcoat with the addition of 5% sample 12 microcapsules, and the UV topcoat with the addition of 5% sample 15 microcapsules, respectively, were photographed using SEM. The surface of the blank UV topcoat is relatively flat, while the surface of the coating film with 5% microcapsules added shows uneven particles, but the surface of the coating film with the addition of these two microcapsule samples has a similar flatness due to the similar particle size of the microcapsules in sample 12 and sample 15.

3.3.2. Chemical Composition Analysis of UV Coating Film

The FTIR spectra of the UV topcoat film with the addition of microcapsule sample 15, and the UV topcoat film without the addition of microcapsules, used as a blank control sample, are shown in Figure 5. The main components of the UV topcoat are urethane acrylic resin, propylene glycol diacrylate, and hexylene glycol diacrylate, and the common characteristic peaks of these components can be observed in the C=C telescopic vibration peak at 1608 cm⁻¹, the C=O vibration peak at 1725 cm⁻¹, and the C-H telescopic vibration peak at 2920 cm⁻¹. These vibrational peaks of the blank UV topcoat can also be observed in the spectrum of the UV topcoat with the addition of sample 15 microcapsules, in addition to the vibrational absorption peak of -OH at 3434 cm⁻¹ and the -CH stretching vibrational peak at 2920 cm⁻¹, brought about by the microcapsules in the UV topcoat with the addition of sample 15 microcapsules [60,61]. The characteristic peaks of the UV topcoat and sample 15 microcapsules appear at the same time in the infrared spectra of the UV topcoat with the addition of microcapsules, which indicates that the microcapsules added to the UV topcoat were able to be maintained in a stable state.

3.4. Self-Healing Properties of UV Coating Films with the Addition of CST-T Microcapsules

The microcapsules produced in the one-way test were added to the UV topcoat at a level of 5%, respectively, and the mixed coatings were coated on glass panels for the scratch test to test the self-repairing properties of the coating film. The scratch results are shown in Figure 6 and Figure 7, and

Table 12. Self-healing rate of each sample in scratch test.

Samples	T-80-to-SDBS Ratio	Scratch Width Before Repair (μm)	Scratch Width After Repair (μm)	Healing Rate (%)
UV topcoats without added microcapsules	-	15.44	15.44	-
MC-UV10	2.0:8.0	14.11	13.32	5.60
MC-UV11	2.5:7.5	12.96	11.55	10.88
MC-UV12	3.0:7.0	13.76	12.72	7.56
MC-UV13	3.5:6.5	9.16	8.43	7.97
MC-UV14	4.0:6.0	11.13	8.32	25.25
MC-UV15	4.5:5.5	13.47	10.44	22.49
MC-UV16	5.0:5.0	17.91	15.6	12.90

; there was 48 h between measurements. The coating film with the addition of 5% sample 14 microcapsules had the highest repair rate of 25.25%, with a scratch width of 11.13 μm before repair and 8.32 μm after repair. The UV topcoat film with the addition of sample 15 microcapsules also obtained a high repair rate of 22.49%, with the scratch width reduced from 13.47 μm before repair to 10.44 μm after repair. The lowest repair rate was for the UV topcoat film with the addition of sample 10 microcapsules, with a repair rate of only 5.60%, and the width of scratches reduced from 14.11 μm before repair to 13.32 μm after repair. The repair rates of UV topcoats with the addition of sample 10, 11, 12, and 13 microcapsules were all low, ranging from 5% to 11%. The UV topcoats with the addition of sample 14 and 15 microcapsules presented higher repair rates, which decreased again when sample 16 microcapsules were added. With an increase in the SDBS content of the emulsifier in the microcapsules, the repair rate of the UV coating films with the addition of microcapsules showed an overall tendency to increase first and then decrease. The UV topcoats prepared with sample 14 and 15 microcapsules when the two emulsifiers were in ratios of 4.0:6.0 and 4.5:5.5 had good repair rates.

Figure 8 shows the relationship between the ratio of the two emulsifiers in the preparation of the microcapsules and the self-repair rate of the UV topcoat film with the addition of microcapsules. All the microcapsule samples produced in the one-way test resulted in some self-repairing properties of the coating film when added to the UV topcoat. The UV topcoat with the addition of sample 14 microcapsules, prepared with an emulsifier ratio of 4.0:6.0, showed the highest self-healing rate. When 5% sample 14 microcapsules were added to the UV topcoat, a repair rate of 25.25% was achieved when scratches appeared on the surface of the coating film. This was followed by the UV topcoat with the addition of sample 15 microcapsules, prepared with an emulsifier ratio of 4.5:6.5, which also resulted in a self-healing rate of 22.49% of the coating film. The self-healing rate was as low as 5.60% for the coating film with the incorporation of sample 10 microcapsules prepared with an emulsifier ratio of 2.0:8.0. Comparing the results of the one-way test with those of the scratch test, it was found that the two emulsifiers were in the ratio of 4:6 for the preparation of the film with sample 14 microcapsules, and the HLB value of the emulsifier was 12.383; the two emulsifiers were in the ratio of 4.5:5.5 for the preparation of the film with sample 15 microcapsules, and the HLB value of the emulsifier was 12.601; the two emulsifiers were in the ratio of 2.0:8.0 for the preparation of the film with sample 10 microcapsules, and in this case, the HLB value of the emulsifier was 11.510. The test results showed that when the ratio of the two emulsifiers was 4.0:6.0, i.e., the HLB value of the emulsifiers was 12.383, the addition of microcapsules to the UV topcoat at a content of 5% improved the self-healing properties of the coating film.

While absolute healing efficiencies (5–25%) were modest, microcapsules prepared with emulsifier ratios of 4.0:6.0 (sample 14 microcapsules, HLB = 12.383) and 4.5:5.5 (sample 15 microcapsules, HLB = 12.601) exhibited significantly higher scratch closure (22–25%) compared to those with emulsifiers with a lower HLB value (5–8%, e.g., sample 10 microcapsules). This trend suggests that the HLB value of the emulsifiers critically influences healing agent release and crack-filling efficacy. Future work will focus on improving healing magnitude via capsule size/distribution optimization.

The 25.25% healing efficiency for sample 14 (HLB = 12.383) arose from two factors: (1) a dense chitosan–STPP wall ensured mechanical rupture under scratch stress, releasing tung oil; (2) tung oil’s conjugated double bonds underwent oxidative polymerization upon O₂ exposure, filling cracks via in situ film formation. Emulsifiers with lower HLB values (e.g., sample 10: HLB = 11.51) produced agglomerated capsules, hindering uniform dispersion and healing agent release.

3.5. Mechanical Properties of UV Coating Films with Added CST-T Microcapsules

The mechanical properties of roughness and elongation at break of the coating films were measured. As shown in

Table 13. Mechanical properties of coatings with different microcapsules.

Samples	Roughness (μm)	Elongation at Break (%)
UV topcoats without added microcapsules	0.31	4.43
MC-UV10	0.88	8.89
MC-UV11	0.76	9.31
MC-UV12	0.58	5.40
MC-UV13	0.54	5.32
MC-UV14	0.66	5.17
MC-UV15	0.54	6.92
MC-UV16	0.74	6.78

, because the UV topcoat had better leveling, the surface of the film without microcapsules was smooth, and the roughness was only 0.31 μm. The roughness of the surface of the UV topcoat film with 5% CST-T microcapsules increased significantly, and the overall trend of the film first decreased and then increased. The highest roughness reached 0.88 μm for the UV topcoat film with sample 10 microcapsules. The roughness of the UV topcoat film with sample 11 microcapsules was 0.76 μm, and the surfaces of the UV topcoat films with sample 15 microcapsules and sample 13 microcapsules were still relatively flat, with a roughness of 0.54 μm, which may be attributed to the fact that the sample 10 microcapsules were more agglomerated than the other microcapsule samples, which could not easily be uniformly dispersed when mixed into the UV topcoat and thus generated more agglomerates, producing flatness of the surface of the films with the addition of sample 15 microcapsules and sample 13 microcapsules.

Figure 9 shows the stress–strain curves of the coating films. As shown in Table 13, the elongation at break of the coating film without the addition of microcapsules is only 4.43%, which is due to the fact that UV coatings usually contain components with high hardness, such as resins and monomers, and thus UV coatings are usually less ductile and have lower elongation at break. The elongation at break of the coating film increased after the addition of the microcapsules in the one-factor test. The coating film with the addition of sample 11 microcapsules had the highest elongation at break of 9.31%, indicating that the toughness of the coating film was enhanced with the addition of microcapsules.

3.6. Optical Properties of UV Coating Films with Added CST-T Microcapsules

Figure 10 and

Table 14. Transmittance of coatings with different microcapsules.

Samples	Visible Light Transmittance (%)
With no microcapsules	87.90
With sample 10 microcapsules	79.56
With sample 11 microcapsules	82.74
With sample 12 microcapsules	86.31
With sample 13 microcapsules	82.87
With sample 14 microcapsules	83.81
With sample 15 microcapsules	82.50
With sample 16 microcapsules	82.88

show the visible light transmittance of the coating films with the addition of microcapsules. The UV topcoat without microcapsules has the highest visible light transmittance of 87.90%, and the visible light transmittance of the UV topcoat with microcapsules is decreased compared with that of the UV topcoat without microcapsules. The transmittance rate of all the films with microcapsules shows a general trend of increasing and then decreasing. The UV topcoat with sample 10 microcapsules has the lowest transmittance of 79.56%, which is 8.34% lower than that of the UV topcoat without microcapsules. This may be due to the poor morphology of the sample 10 microcapsules prepared under the conditions of a 2.0:8.0 ratio of the two emulsifiers, and the serious agglomeration of the microcapsules, which could not be distributed uniformly in the UV topcoat, resulting in a significant decrease in the transmittance rate of the coating film.

Notably, the coating with sample 12 microcapsules (HLB = 11.947) achieved 86.31% visible light transmittance, nearing the blank coating’s value of 87.90%. This anomaly stems from its optimal emulsifier ratio (T-80-to-SDBS = 3.0:7.0), which promoted particle dispersion (Figure 1C) and minimized interfacial light scattering. By contrast, a higher SDBS content (e.g., sample 10, HLB = 11.510) induced agglomeration, while a higher T-80 content (e.g., sample 16, HLB = 12.819) reduced matrix compatibility, with both lowering the visible light transmittance.

While coating thickness is a known factor that influences visible light transmittance, our controlled application process ensured uniform thickness (50 ± 5 μm) across all samples. Thus, the observed visible light transmittance differences primarily arose from light scattering at microcapsule–matrix interfaces. Agglomerated microcapsules (e.g., sample 10) exacerbated scattering, reducing the visible light transmittance by ~8.34%, whereas optimized microcapsules (samples 14–15) minimized scattering losses (<5.40% reduction).

Table 15. Surface glossiness of coatings with different microcapsules.

Samples	Glossiness (GU)			Loss of Light (%)
	Angle of Incidence of Light Source: 20°	Angle of Incidence of Light Source: 60°	Angle of Incidence of Light Source: 85°
No microcapsules	12.00	49.30	56.40	-
Sample 10 microcapsules	7.80	32.90	25.20	33.27
Sample 11 microcapsules	7.20	30.00	20.20	39.15
Sample 12 microcapsules	4.90	29.70	21.20	39.76
Sample 13 microcapsules	7.70	32.00	33.20	35.09
Sample 14 microcapsules	8.20	34.50	39.50	30.02
Sample 15 microcapsules	5.30	31.80	28.00	35.50
Sample 16 microcapsules	5.40	30.70	27.60	37.73

shows the gloss of UV topcoat films containing different microcapsules measured at 20°, 60°, and 85° incident angles, along with the gloss loss at 60°. Films without microcapsules had high gloss values of 12.00 GU at 20°, 49.30 GU at 60°, and 56.40 GU at 85°. Adding microcapsules prepared with different emulsifiers reduced the gloss. These films showed gloss values between 29.70 GU and 34.50 GU at 60°. Compared to the film without microcapsules, the gloss loss at 60° ranged from 30.02% to 39.76%. This similar gloss loss indicates that microcapsules made using different emulsifier combinations had a comparable effect on film gloss when added at the same amount. The lowest gloss loss was 30.02%. This occurred when adding 5% sample 14 microcapsules to the UV topcoat. This result shows that films containing sample 14 microcapsules had better surface gloss than films containing microcapsules made using only one emulsifier factor. The gloss values for films with sample 14 microcapsules were 8.20 GU at 20°, 34.50 GU at 60°, and 39.50 GU at 85°.

The minimal loss in transparency at 5% microcapsule loading stemmed from sub-wavelength capsule sizes, minimizing light scattering. The 110% increase in elongation at break was attributed to microcapsules acting as stress concentrators, inducing matrix plastic deformation and dissipating energy. However, an excessive amount of the emulsifier SDBS (sample 16: T-80/SDBS = 5:5) increased the surface roughness (0.74 μm) due to hydrophilic particle aggregation, reducing gloss.

Table 16. Chromaticity and color difference values of coatings.

Samples	Colorimetric Value			Color Difference Value ΔE
	Brightness Value L	Red and Green Value a	Yellow and Blue Value b
No microcapsules	76.93	−2.24	5.08	-
Sample 10 microcapsules	74.10	−1.62	6.25	3.12
Sample 11 microcapsules	73.32	−2.17	5.65	3.66
Sample 12 microcapsules	75.37	−2.00	5.54	1.64
Sample 13 microcapsules	74.76	−2.03	5.70	2.27
Sample 14 microcapsules	74.82	−2.21	5.38	2.14
Sample 15 microcapsules	74.90	−2.04	5.77	2.16
Sample 16 microcapsules	73.43	−2.17	5.80	3.58

shows the chromaticity and color difference values of UV topcoat films without microcapsules and with different CST-T microcapsules. The film without microcapsules had high brightness, with a brightness value of 76.93. The brightness value of the UV topcoat film with CST-T microcapsules was slightly darker than that of the blank UV topcoat, and the brightness of the film with sample 11 microcapsules was the lowest, with a brightness value of 73.32. Since the CST-T microcapsules were white powder, the addition of microcapsules had little change on the red–green and yellow–blue values of the coating film compared with those of the UV topcoat without the addition of microcapsules, indicating that the addition of microcapsules did not have much of an effect on the red–green and yellow–blue tendencies of the color of the coating film. The color difference between all the films with microcapsules and those without microcapsules showed a tendency of decreasing and then increasing. The color difference of the UV topcoat with sample 12 microcapsules was the smallest, at only 1.64.

4. Conclusions

Orthogonal optimization employing four factors at three levels revealed the emulsifier ratio between T-80 and SDBS to be the dominant variable governing CST-T microcapsule performance. The optimal parameters, under which a peak encapsulation efficiency of 68.50% and yield of 42.91% were achieved, were identified to be the following: a core-to-wall ratio at 2.0:1.0, a T-80-to-SDBS ratio of 4.0:6.0, an STPP concentration of 4%, and a spray-drying temperature of 120 °C. At a loading level of 5% into the UV topcoat, these microcapsules induced characteristic non-monotonic responses, with the surface roughness, elongation at break, and visible light transmittance exhibiting unimodal trends with increasing SDBS content, while the color difference ΔE followed a concave profile. The UV topcoar with sample 15 microcapsules demonstrated optimal comprehensive performance at this loading, delivering 22.49% self-healing efficiency alongside critical properties: 82.50% visible light transmittance, 35.50% light loss, an ΔE value of 2.16, a surface roughness of 0.54 micrometers, and an elongation at break of 6.92%.