Preparation of Urea-Formaldehyde-Coated Cationic Red-Ternary System Microcapsules and Properties Optimization

Abstract

Thermochromic microcapsules were synthesized and optimized using crystal violet lactone, bisphenol A, and decanol as the core materials, a dispersible cationic red dye as the color-modifying additive, and urea-formaldehyde resin as the wall material, based on orthogonal and single-factor experiments. The effects of the proportion of cationic red dye in the core material, the mass ratio of formaldehyde to urea, the emulsifier HLB value, and the core–wall mass ratio on yield, encapsulation rate, thermochromic ΔE, and formaldehyde release of microcapsules were systematically investigated. The results showed that the core–wall ratio was the key factor affecting the comprehensive performance of the microcapsules. Through the comparison of orthogonal and single-factor tests, 11# microcapsule was identified as having the best overall performance in terms of ΔE, and encapsulation rate. The ΔE value was increased by about 165% compared with the lowest-performing sample, significantly enhancing the thermochromic response. The encapsulation rate was improved by nearly 40%, effectively enhancing the encapsulation quality and core stability, with overall performance standing out. The best preparation process was to add 0.5% of the core material mass of dispersible cationic red dye, the mass ratio of formaldehyde and urea was 1.2:1, the HLB value of emulsifier was 10, and the core–wall ratio was 1:1.1. The yield of 11# microcapsules prepared under this condition was 31.95%, the encapsulation rate was 68%, the thermochromic ΔE was 9.292, and the formaldehyde release concentration was 1.381 mg/m³. Furthermore, 11# microcapsules with different addition levels were introduced into the UV primer to evaluate their effects on the mechanical and optical properties of the coating. The results showed that the addition of microcapsules weakened the gloss and light transmittance of the coating, increased the surface roughness, and decreased the elongation at break. When the addition amount was 5%, the coating exhibited the best overall performance: UV-visible light transmittance reached 91.92%, 60° gloss was 42.2 GU, elongation at break was 9.3%, and surface roughness was 0.308 μm. This study developed a purple thermochromic microcapsule system by regulating the dispersible dye content and interfacial conditions. In coating applications, the system exhibited a strong ΔE response and excellent overall performance, offering great advantages over existing similar systems in terms of color-change efficiency, ΔE enhancement, and coating adaptability.

1. Introduction

The development of intelligent material science has driven innovation in environmentally responsive functional coatings [1,2,3,4,5,6]. At present, color-changing systems based on external stimuli such as light, heat, and electricity have already found applications in multiple fields [7,8,9,10,11]. Thermochromic materials with controllable reactions and simple conditions have attracted extensive attention. Taking the ternary system composed of crystal violet lactone, bisphenol A, and fatty alcohol as an example, its reversible thermochromic properties can be effectively protected through microencapsulation technology, providing technical support for the development of durable intelligent coatings. Microcapsules feature a core–shell structure that protects functional core materials from environmental degradation [12]. UV coatings, as an environmentally friendly coating system, are characterized by low energy consumption and high efficiency [13,14]. Modified UV coatings have already demonstrated properties such as self-healing, antibacterial, and flame retardant [15,16]. However, the limited functionality of traditional UV coatings restricts its broader application in the field of smart home [17]. By integrating thermochromic microcapsules into UV resin matrix, a novel coating system with both rapid curing and intelligent color-changing capabilities can be constructed [18]. Microcapsule technology has the advantages of low cost and easy preparation [19,20]. A number of fossil-based and bio-based microcapsules have also been widely used in various aspects of daily life for a long time [21]. Microencapsulation technology is widely used in functional coatings, powder pharmaceuticals and food preservation [22,23]. Combined with microcapsule technology, UV coatings can achieve intelligent functions [24,25,26].

Spray drying, as an efficient and controllable microcapsule preparation technology, has been widely used in food, medicine, cosmetics, pesticides and functional materials in recent years [27,28,29]. The method is to mix the substance to be coated with the wall material into an emulsion or suspension and then atomize it. Under the action of hot air, the solvent is rapidly evaporated to form microcapsules, which has the advantages of fast preparation speed, simple operation, low cost and suitable for active substances with high thermal stability [30]. Especially in the loading of fragrances, vitamin, probiotics, natural pigment and heat-sensitive functional materials, the spray drying method shows a good encapsulation effect and release control ability [31]. da Silva Acácio et al. [32] microencapsulated the essential oil of S. terebinthifolius by spray drying maltodextrin and gum arabic as embedding agents and SiO₂ as colloid adjuvant. The morphology of the microcapsules was analyzed by scanning electron microscope (SEM) and proved that the particles were regular spherical particles with a size ranging from 5 to 10 μm. Thermal stability of the microcapsules was studied by thermogravimetric analysis–differential scanning calorimetry (TGA-DSC), and the microcapsules remained stable at temperatures up to 200 °C. The microcapsules produced by spray drying were able to protect essential oils from external influences, such as thermal degradation. Takatani et al. [33] successfully prepared new microcapsules using ethyl laurate as the core material (phase change material; PCM) and gelatin as the shell. It was found that physical crosslinking of tannic acid was most effective in strengthening the shell under acidic to neutral pH conditions, and spray drying temperature of 120 °C was most suitable for producing well-dried microcapsules with minimal PCM loss. Currently, this technique has achieved certain progress in microcapsule particle size control, encapsulation efficiency improvement, and development of multifunctional wall materials. It has gradually become one of the mainstream processes for rapid drying and large-scale encapsulation of heat-sensitive substances, providing a strong support for the development and application of functional microcapsules [34,35,36].

However, the existing thermochromic microcapsule systems still face certain limitations in practical applications. For instance, insufficient encapsulation stability often leads to a shortened color-change lifetime, poor compatibility with coating compromises optical performance, and the high cost of some thermochromic dyes restricts their use in smart coatings. Conventional thermochromic systems are typically based on ternary components (chromophore, developer, and solvent) and lack regulation of auxiliary colorants, resulting in a narrow color-change range. In this study, dispersible cationic red dye was introduced as an auxiliary colorant. Its excellent lipophilicity and thermal responsiveness are expected to synergize with the ternary system to optimize the tunable color range and enhance the response toward the purple region. Moreover, as a cationic dye, it exhibits good dispersibility and outstanding color stability, enabling uniform distribution within the core material, which facilitates stronger core–wall interfacial bonding, improves the structural stability of microcapsules, and enhances compatibility with coating systems.

Thermochromic microcapsules were prepared using crystal violet lactone, bisphenol A, and decanol as the ternary core system, with urea–formaldehyde resin serving as the wall material, via in situ polymerization. The results confirmed that the obtained microcapsules exhibited certain thermochromic properties; however, the color-changing performance still required an improvement [37]. Therefore, the present study aimed to further optimize the preparation process of thermochromic microcapsules, with a particular focus on enhancing the color difference and thermochromic sensitivity. To achieve this, a cationic red dye with good dispersibility was introduced into the core system, and spray drying was employed for rapid drying treatment of the resulting microcapsule emulsion, leading to the construction of urea–formaldehyde-coated cationic red-ternary system thermochromic microcapsules. Orthogonal and single-factor experiments were conducted to systematically investigate the effects of the proportion of dispersible cationic red in the core material, the mass ratio of formaldehyde to urea, the HLB value of the emulsifier, and the core–wall ratio on the yield, encapsulation rate, thermochromic ΔE value, and formaldehyde emission of microcapsules. The results showed that the core–wall ratio was the key factor affecting the properties of the microcapsules. Furthermore, functional coatings with thermochromic properties were prepared by adding the optimized microcapsules into UV primer. The effects of different microcapsule contents on the optical and mechanical properties of the coatings were investigated. The research objective was to obtain microcapsules with superior thermochromic performance and to provide experimental evidence and theoretical support for an application in cross-color tone thermochromic systems and functional coatings.

2. Materials and Methods

2.1. Test Materials

The materials and equipment used in the test are shown in

Table 1. Test materials.

Test Materials	Molecular Formula	Molecular Mass	Purity	Manufacturer
Crystal violet lactone	C26H29N3O2	415.527	AR	Shanghai Haiyu Chemical Co., Ltd., Shanghai, China
Dispersible cationic red dye	C3H4N4O2	128.09	-	Jiangsu Yancheng Delong Chemical Co., Ltd., Yancheng, China
Bisphenol A	C15H16O2	228.286	AR	Shanghai Haiyu Chemical Co., Ltd., Shanghai, China
Decanol	C10H22O	158.28	AR	Guangdong Wengjiang Chemical Reagent Co., Ltd., Shaoguan, China
Urea	CH4N2O	60.06	AR	Guangzhou Suixin Chemical Co., Ltd., Guangzhou, China
Formaldehyde solution	CH2O	30.03	37%	Guangzhou Suixin Chemical Co., Ltd., Guangzhou, China
Triethanolamine	C6H15NO3	149.19	AR	Shandong Hengshuo Chemical Co., Ltd., Jinan, China
Citric acid monohydrate	C6H10O8	210.139	AR	Guangdong Fangxin Biotechnology Co., Ltd., Shaoguan, China
Gum arabic powder	N/A	-	AR	Langfang Qianyao Technology Co., Ltd., Langfang, China
Triton X-100	C16H26O2	250.376	AR	Jinan Xiaoshi Chemical Co., Ltd., Jinan, China
Span 80	C24H44O6	428.6	AR	Jinan Xiaoshi Chemical Co., Ltd., Jinan, China
Sodium chloride	NaCl	58.44	AR	Shanghai Sinopharm Reagent Co., Ltd., Shanghai, China
Silicon dioxide	SiO2	60.08	AR	Shanghai Sinopharm Reagent Co., Ltd., Shanghai, China

Note: N/A means not applicable.

and

Table 2. Test equipment.

Test Equipments	Model	Manufacturer
Constant temperature water bath pot	DF-101S	Muyang Hongguanriyi E-commerce Co., Ltd., Muyang, China
Scanning electron microscope (SEM)	Quanta-200	Thermo Fisher Technology Co., Ltd., Waltham, MA, USA
Zeiss optical microscope (OM)	AX10	Carl Zeiss AG., Baden, Germany
Infrared Spectrometer (FTIR)	VERTEX 80V	Brook Co., Karlsruhe, Germany
Glossmeter	HG268	Shenzhen Sanenshi Technology Co., Ltd., Shenzhen, China
Colorimete	SEGT-J	Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China
UV spectrophotometer	U-3900/3900H	Hitachi Instrument Co., Ltd., Suzhou, China
Roughness meter	J8-4C	Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China
Universal mechanical testing machine	AG-IC10OKN	Shimadzu Production House, Kyoto, Japan
UV single lamp curing machine	620#	Huzhou Tongxu Machinery Equipment Co., Ltd., Huzhou, China
Small spray dryer	JA-PWGZ100	Shenyang Jingao Instrument Technology Co., Ltd., Shenyang, China
Air quality detector	DM105D	Shenzhen Langdeli Technology Co., Ltd., Shenzhen, China
Handheld thermometer	2003Y	Putian Hanjiang Huafeng Plastic Co., Ltd., Putian, China

, respectively. The dispersible cationic red dye was a dark red powder dye produced with a composite dispersant, mainly composed of 3-amino-5-carboxy-1,2,4-triazole, and did not possess thermochromic properties. Bisphenol A was used as the color developer in the thermochromic system, functioning primarily to trigger color changes in response to temperature variations. During the synthesis of the microcapsules, the bisphenol A was completely encapsulated within the wall material, thereby avoiding direct exposure to the external environment and consequently reducing potential risks of human and environmental contact during application. The coating used in the experiments was a UV matte primer (HB-4301), provided by Jiangsu Haitian Technology Co., Ltd., Zhenjiang, China.

2.2. Preparation Method of Microcapsules

The orthogonal experiment was designed based on four factors during the microcapsule preparation process: the mass fraction of cationic red in the core materials, the mass ratio of formaldehyde solution to urea (W_F:W_U), the HLB value of the emulsifier, and the core–wall ratio. A single-factor experiment was designed to verify the key influencing factor. The selection of factors and levels for the orthogonal experiment is shown in

Table 3. Table of orthogonal test factor and level.

Level	Factor A	Factor B	Factor C	Factor D
	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value of the Emulsifier	Core–Wall Ratio
1	2.5	1.2:1	6.0	1:1
2	1.5	1.0:1	8.0	1:1.5
3	0.5	0.8:1	10.0	1:2

. The detailed layout of the orthogonal experiment is shown in

Table 4. Table of orthogonal test detailed layout.

Sample (#)	Factor A	Factor B	Factor C	Factor D
	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value of the Emulsifier	Core–Wall Ratio
1	2.5	1.2:1	6.0	1:1
2	2.5	1.0:1	8.0	1:1.5
3	2.5	0.8:1	10.0	1:2
4	1.5	1.2:1	8.0	1:2
5	1.5	1.0:1	10.0	1:1
6	1.5	0.8:1	6.0	1:1.5
7	0.5	1.2:1	10.0	1:1.5
8	0.5	1.0:1	6.0	1:2
9	0.5	0.8:1	8.0	1:1

.

Table 5. Materials of orthogonal test.

Sample (#)	Dye (g)	Urea (g)	Formaldehyd (g)	Distilled Water (Wall Material) (mL)	Gum Arabic (g)	Span 80 (g)	Triton X-100 (g)	Distilled Water (Core Material) (mL)	Core Material (g)	NaCl (g)	SiO2 (g)
1	0.32	8.00	12.62	256.00	3.92	4.61	-	162.13	12.80	1.79	1.79
2	0.20	8.00	10.52	240.00	5.33	-	-	101.33	8.00	1.12	1.12
3	0.08	8.00	8.42	125.00	1.31	-	0.77	39.39	3.11	0.44	0.44
4	0.10	8.00	12.62	256.00	4.27	-	-	81.07	6.40	0.90	0.90
5	0.18	8.00	10.52	240.00	5.04	-	2.96	152.00	12.00	1.68	1.68
6	0.06	8.00	8.42	125.00	1.27	1.49	-	52.52	4.15	0.58	0.58
7	0.04	8.00	12.62	256.00	3.58	-	2.11	108.09	8.53	1.19	1.19
8	0.03	8.00	10.52	240.00	1.82	2.16	-	76.00	6.00	0.84	0.84
9	0.03	8.00	8.42	125.00	4.15	-	-	78.79	6.22	0.87	0.87

and

Table 6. Materials of single-factor test.

Sample (#)	Dye (g)	Urea (g)	Formaldehyd (g)	Distilled Water (Wall Material) (mL)	Gum Arabic (g)	Triton X-100 (g)	Distilled Water (Core Material) (mL)	Core Material (g)	NaCl (g)	SiO2 (g)
10	0.06	8.00	12.62	256.00	5.37	3.16	162.13	12.80	1.79	1.79
11	0.05	8.00	12.62	256.00	4.88	2.87	147.39	11.64	1.63	1.63
12	0.05	8.00	12.62	256.00	4.48	2.63	135.11	10.67	1.49	1.49
13	0.05	8.00	12.62	256.00	4.13	2.43	124.72	9.85	1.38	1.38
14	0.04	8.00	12.62	256.00	3.84	2.26	115.81	9.14	1.28	1.28

are the material tables of orthogonal test and single-factor test, respectively. The mass ratio of emulsifier to core material was set at 1:1.5. To enhance the compactness and structural stability of the microcapsule, SiO₂ and NaCl were added as auxiliary dispersants. The ionic strength of the solution was adjusted by NaCl, whereby the polymerization of the wall material was promoted and an adsorption layer was formed on the microcapsule surface, suppressing flocculation and contributing to improved encapsulation efficiency and wall integrity. Meanwhile, SiO₂ was employed to enhance the dispersion stability of the core emulsion during high-speed stirring, effectively preventing stratification and agglomeration. Based on previous studies, the amounts of both additives were set at 14% of the core material mass [37].

The HLB value of the emulsification system in the experiments was calculated as follows: when the emulsifier was a combination of Triton X-100 and gum arabic powder, the HLB value was determined according to Equation (1); when the emulsifier was a combination of Span 80 and gum arabic powder, the HLB value was calculated using Equation (2). Here, A₁ and B₁ represent the mass ratios of the nonionic emulsifier in their respective systems, while A₂ and B₂ represent the ratios of gum arabic powder.

H = (A₁ × 13.4 + A₂ × 8.0)/(A₁ + A₂)

(1)

H = (B₁ × 4.3 + B₂ × 8.0)/(B₁ + B₂)

(2)

Take sample 10# as an example:

• Preparation and dispersion of core materials: The water bath was adjusted to 50 °C, and 80 g of decanol was added to a beaker. Then 4.8 g bisphenol A and 1.6 g crystal violet lactone were added with the mass ratio of crystal violet lactone: bisphenol A: decanol being 1:3:50. The mixture was stirred at 400 rpm for 1.5 h, followed by cooling to room temperature. The obtained product was the thermochromic core material, hereinafter also referred to as the thermochromic compound. The 5.37 g gum arabic and 3.16 g Triton X-100 were weighed as emulsifiers, and 0.06 g dispersible cationic red dye was added. Then 162.13 mL distilled water and 12.80 g thermochromic compounds were added. The system was adjusted to 65 °C and stirred at a relatively high speed for 30 min. Then ultrasonic emulsification was carried out for 5 min.
• Preparation of wall materials: According to the material list, 8.00 g urea and 12.62 g formaldehyde were weighed. After adding 256.00 mL distilled water, triethanolamine was added dropwise to adjust the system pH to 8.5. The beaker was sealed, the temperature was adjusted to 70 °C, and the mixture was maintained at 300 rpm for 1 h to pre-polymerize the wall material.
• Preparation of microcapsules: The urea-formaldehyde prepolymer was added dropwise into the core material emulsion at a stirring speed of 500 rpm and a temperature of 35 °C, followed by the addition of 1.79 g SiO₂ and 1.79 g NaCl. The 8% citric acid monohydrate solution was added dropwise to adjust the pH of the system to 2.5, and the mixture was stirred for 1 h. Then the temperature was then raised to 68 °C, the stirring speed was reduced to 250 rpm, and after stirring for 30 min, the microcapsule emulsion was left to stand at room temperature. The microcapsule emulsion was sealed and left to stand for 24 h, with a portion used for formaldehyde emission testing and the remainder subjected to spray drying. The inlet temperature of the spray dryer was set to 130 °C, and the peristaltic pump speed was set to 100 mL/h. After the machine was preheated and the outlet temperature stabilized around 60 °C, the drying process began. The spray-dried light purple powder constituted the final microcapsule product.

2.3. Preparation Method of the Paint Film:

The dimensions of the paint film mold were 50 mm × 50 mm × 20 mm, and the list of materials used for the UV primer mixture is shown in

Table 7. Materials table of blend UV primer.

Microcapsule Addition Amount (%)	Microcapsule Quality (g)	UV Primer Quality (g)
0	0.00	1.00
5	0.05	0.95
10	0.10	0.90
15	0.15	0.85
20	0.20	0.80
25	0.25	0.75
30	0.30	0.70

.

Orthogonal and single-factor experiments: The 0.30 g thermochromic microcapsules was added to the UV primer, maintaining the total mass of microcapsules and UV primer at 1.00 g. After thorough mixing, the mixture was poured into the paint film mold. After the mixed primer was naturally leveled, the mold was placed on the conveyor belt of the single-lamp curing machine, with the conveyor speed set to 0.01 m/s and the curing time approximately 1 min. After curing, the paint film was demolded for color-change performance testing.

Paint film test: The optimal microcapsules obtained from the single-factor test were added to the UV primer at the addition amounts of 0%, 5%, 10%, 15%, 20% and 25%. The total mass of thermochromic microcapsules and UV primer was kept at 1.00 g, and the mixture was thoroughly mixed and poured into the paint film mold, or the total mass was kept at 0.50 g and coated on a glass plate. After the primer mixture had leveled naturally, the mold was placed on the conveyor belt of the single-lamp curing machine, and the conveying speed was adjusted to 0.01 m/s, and the curing time was about 1 min. After curing, the paint film was demolded for optical and mechanical performance testing, with the paint film without microcapsules serving as the blank control group.

2.4. Testing and Characterization

2.4.1. Yield and Encapsulation Rate Test

The yield was denoted as P. The mass of the dried microcapsules obtained was denoted as m_p. The total mass of the main components of the core and wall materials used to prepare the sample was denoted as m_r. The yield was calculated according to Equation (3).

P = (m_p/m_r) × 100%

(3)

The encapsulation rate was denoted as P_c. Microcapsules with mass m₁ was ground thoroughly and placed into a beaker. The sample was soaked in ethanol and heated in 60 °C oven for 2 h. The mixture was stirred thoroughly every 15 min using a glass rod. After the reaction, the mixture was washed and vacuum-filtered with anhydrous ethanol. The filtered residue was then dried in a 60 °C oven. The mass of the dried residue was recorded as m₂. The encapsulation rate was calculated according to Equation (4).

P_c = [(m₁ − m₂)/m₁] × 100%

(4)

2.4.2. Color-Changing Performance and Formaldehyde Emission Test

Color-changing performance: The prepared paint films were placed in a refrigerator and cooled for a period of time to ensure the microcapsules were sufficiently chilled. A portable colorimeter was used to record the L, a, and b values of the coating at −20 °C [38]. Each sample was tested four times under low-temperature conditions. Then, after being heated in a 60 °C oven for a period of time, the same four tests were performed to obtain the L, a, and b values of the coating at high temperature. The color difference was calculated based on the average of the four test values at the two temperatures. Since the UV primer itself does not have thermochromic properties, the experimental results can be attributed to the color-changing performance of the microcapsules. The L value represents the brightness-darkness value of the sample, the a value represents the red-green value, and the b value represents the yellow-blue value [39,40]. The test data of the paint film at high temperature were denoted as L₁, a₁, and b₁, and those at low temperature were denoted as L₂, a₂, and b₂. The color difference ΔE was calculated according to Equation (5), where ΔL = L₂ − L₁, Δa = a₂ − a₁, Δb = b₂ − b₁.

ΔE = [(ΔL)² + (Δa)² + (Δb)²]^1/2

(5)

Formaldehyde emission: The 0.50 g of the microcapsule emulsion was placed into a sealed 250 mL glass beaker, quickly capped, and allowed to stand at room temperature (25 ± 2 °C) for 24 h. An air quality detector was used to measure the formaldehyde concentration inside the beaker during the experiment. The detectable concentration range for HCHO formaldehyde testing was 0–1.999 mg/m³. Each sample was tested independently four times, and the HCHO concentration values were recorded. The results reflect the differences in free formaldehyde levels under various preparation conditions.

2.4.3. Normalization Analysis

To comprehensively evaluate the overall performance of the prepared thermochromic microcapsules, four parameters—yield rate, encapsulation rate, ΔE, and formaldehyde emission—were subjected to normalization analysis. During the weighted normalization process, all experimental data were first converted into positive indicators and standardized using the maximum–minimum method. The normalized values were calculated according to Equation (6), where n represents the normalized indicator value, n_x denotes the original experimental value of sample x under the given indicator, and n_min and n_max refer to the minimum and maximum values of that indicator among all samples, respectively. After this transformation, all indicators were linearly mapped onto the [0, 100] range, facilitating cross-comparison.

n = (n_x − n_min)/(n_max − n_min) × 100

(6)

On this basis, to more accurately reflect the importance of each indicator in the practical application of microcapsules, weighting coefficients (w_i) were introduced to process the different indicators. The final comprehensive score was calculated according to Equation (7), where S denotes the overall performance score of a sample, n_j is the normalized value of the j-th sample under a given indicator, w_i represents the weight of the i-th indicator, and N is the total number of indicators.

$$S={\sum }_{i=1}^N{w}_i·n_j{\sum }_{i=1}^N{w}_i=1$$

(7)

Considering the research objectives and application requirements of thermochromic microcapsules, the weighting scheme was determined as follows: the ΔE directly characterizes the thermochromic performance and serves as the core indicator for evaluating the functionality of microcapsules; therefore, it was assigned the highest weight of 0.35. The encapsulation rate reflects the structural integrity and stability of the microcapsules, which are critical to maintaining functionality and prolonging service life, and was thus assigned a weight of 0.30. Formaldehyde emission represents the environmental safety of the material, closely related to practical applications and ecological requirements, and was given a weight of 0.20. Yield rate reflects the feasibility and efficiency of the preparation process, playing a role in enabling large-scale production, but its relative importance is lower compared with functionality and environmental safety; therefore, it was assigned a weight of 0.15. This weighting method ensures the comparability of multiple indicators and comprehensively reflects the overall level of performance, environmental safety, and process feasibility, thereby providing a more scientific basis for selecting the optimal microcapsules.

2.4.4. Microscopic Morphology Test

Optical microscopy (OM): An appropriate amount of the sample was placed on a glass slide and covered with a coverslip. The slide was then positioned on the observation stage of an optical microscope and examined under appropriate magnification.

Scanning electron microscopy (SEM): An appropriate amount of the sample was fixed onto a sample plate using double-sided tape for gold spraying treatment. After gold spraying treatment, the sample plate was placed on the observation stage inside the SEM chamber for vacuuming. Once the pressure reached the required level, the sample was observed under suitable magnification.

2.4.5. Chemical Composition Test

The chemical composition of the prepared microcapsules and paint films was analyzed using a Fourier-transform infrared (FTIR) spectrometer.

2.4.6. Optical Performance Test

Gloss: The gloss of the paint film was measured using a glossmeter. The gloss values were recorded under three different incident angles (20°, 60°, and 85°) to compare the differences [41].

Transmittance: The light transmittance of the paint film in the visible wavelength range was measured using a UV spectrophotometer. Transmittance refers to the ratio between the actual intensity of light that passes through a medium and the original light intensity, typically expressed as a percentage.

2.4.7. Mechanical Performance Test

Elongation at break: The prepared paint film was cut into standard dimensions, and both ends were clamped using a universal testing machine [42,43,44]. The film was stretched at a rate of 0.5 mm/min until it fractured [45]. The elongation at break of the paint film was calculated according to Equation (8), where e represents the elongation at break, L₀ is the initial distance between the upper and lower clamps when the film is taut, and L^′ is the distance between the clamps at the point of fracture.

e = [(L^′ − L₀)/L₀] × 100%

(8)

Roughness: The surface roughness of the paint films was measured using a roughness tester, and the recorded values were used to analyze the variation trends in roughness among different coating samples [46,47,48,49].

3. Results and Discussion

3.1. Analysis of Microcapsule Preparation Results

3.1.1. Analysis of Microcapsule Yield and Encapsulation Rate

The yield analysis of microcapsules in the orthogonal experiment is shown in

Table 8. Analysis on the yield rate of microcapsules by orthogonal test.

Category	Sample (#)	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value	Core–Wall Ratio	Yield Rate (%)
Range	1	2.5	1.2:1	6.0	1:1	40.37 ± 0.96
	2	2.5	1.0:1	8.0	1:1.5	27.09 ± 1.70
	3	2.5	0.8:1	10.0	1:2	36.76 ± 1.32
	4	1.5	1.2:1	8.0	1:2	43.91 ± 1.18
	5	1.5	1.0:1	10.0	1:1	46.01 ± 1.41
	6	1.5	0.8:1	6.0	1:1.5	38.70 ± 1.63
	7	0.5	1.2:1	10.0	1:1.5	45.91 ± 1.15
	8	0.5	1.0:1	6.0	1:2	38.96 ± 1.74
	9	0.5	0.8:1	8.0	1:1	41.32 ± 1.05
	Mean value 1	34.74	43.40	39.34	42.56
	Mean value 2	42.87	37.35	37.44	37.23
	Mean value 3	42.06	38.93	42.89	39.88
	Range	8.14	6.04	5.45	5.33
	Factor primary and secondary level	A > B > C > D
	Optimal level	A2	B1	C3	D1
	Optimal scheme	A2B1C3D1

. Among the samples, Sample 5# exhibited the highest yield at 46.01%, followed by Sample 7# with a yield of 45.91%, and Sample 4# with a yield of 43.91%. The lowest yield was observed in Sample 2#, which was only 27.09%. The proportion of cationic red in the core materials had the greatest influence on the microcapsule yield. The order of influence of the four factors was A > B > C > D. The optimal synthesis condition for higher yield was A2B1C3D1. The analysis of variance for yield is presented in

Table 9. Analysis of variance table for yield rate.

Factor	SS	df	MS	F	p-Value
Factor A: Mass fraction of cationic red in the core materials (%)	240.979	2	120.490	20.829	0.000 **
Factor B: WF:WU	117.977	2	58.988	10.197	0.005 **
Factor C: HLB value of the emulsifier	91.763	2	45.882	7.931	0.010 **
Factor D: Core–Wall ratio	85.290	2	42.645	7.372	0.013 *
Error	52.063	9	5.785
Total	588.072	17

Note: * p ≤ 0.05, ** p ≤ 0.01.

. The ANOVA results indicated that all four factors had a significant effect on the microcapsule yield rate.

The encapsulation rate analysis of microcapsules in the orthogonal experiment is shown in

Table 10. Analysis of microcapsules encapsulation rate by orthogonal test.

Category	Sample (#)	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value	Core–Wall Ratio	Encapsulation Rate (%)
Range	1	2.5	1.2:1	6.0	1:1	68 ± 1.63
	2	2.5	1.0:1	8.0	1:1.5	64 ± 1.41
	3	2.5	0.8:1	10.0	1:2	51 ± 1.63
	4	1.5	1.2:1	8.0	1:2	68 ± 2.16
	5	1.5	1.0:1	10.0	1:1	63 ± 0.82
	6	1.5	0.8:1	6.0	1:1.5	49 ± 2.16
	7	0.5	1.2:1	10.0	1:1.5	59 ± 1.63
	8	0.5	1.0:1	6.0	1:2	63 ± 0.82
	9	0.5	0.8:1	8.0	1:1	59 ± 1.41
	Mean value 1	61.00	65.00	60.00	63.33
	Mean value 2	60.00	63.33	63.67	57.33
	Mean value 3	60.33	53.00	57.67	60.67
	Range	1.00	12.00	6.00	6.00
	Factor primary and secondary level	B > C = D > A
	Optimal level	A1	B1	C2	D1
	Optimal scheme	A1B1C2D1

. Samples 1# and 4# exhibited the highest encapsulation rate, reaching 68%. The mass ratio of formaldehyde to urea had the greatest influence on the encapsulation rate of the microcapsules. The order of influence of the four factors was B > C = D > A. The optimal synthesis condition for higher encapsulation rate was A1B1C2D1. As the W_F:W_U ratio decreased, it became increasingly difficult for the capsule wall to form. The analysis of variance for encapsulation efficiency is shown in

Table 11. Analysis of variance table for encapsulation rate.

Factor	SS	df	MS	F	p-Value
Factor A: Mass fraction of cationic red in the core materials	3.111	2	1.556	0.350	0.714
Factor B: WF:WU	507.111	2	253.556	57.050	0.000 **
Factor C: HLB value of the emulsifier	109.778	2	54.889	12.350	0.003 **
Factor D: Core–wall ratio	108.444	2	54.222	12.200	0.003 **
Error	40.000	9	4.444
Total	768.444	17

Note: ** p ≤ 0.01.

. The ANOVA results indicated that factor A had a p-value > 0.05 and thus showed no significant effect on the microcapsule encapsulation rate. Therefore, when designing the single-factor experiment, factor A was first excluded as a variable. Factors B, C, and D all exhibited significant effects on the encapsulation rate of the microcapsules.

3.1.2. Analysis of Color-Changing Performance and Formaldehyde Emission of Microcapsules

The results of color difference and formaldehyde emission from the microcapsules in the orthogonal experiment are shown in

Table 12. The results of microcapsules ΔE and formaldehyde emission by orthogonal test.

Sample (#)	Low Temperature Chromaticity Value (−20 °C)			High Temperature Chromaticity Value (60 °C)			ΔE	Formaldehyde Emission (mg/m3)
	L1	a1	b1	L2	a2	b2
1	37.700	47.750	−4.250	42.000	49.300	−3.775	4.595 ± 0.141	1.303 ± 0.018
2	55.575	49.425	−7.875	53.225	49.750	−5.300	3.501 ± 0.289	1.383 ± 0.007
3	31.725	36.500	−3.950	37.250	43.650	−2.075	9.228 ± 0.211	1.315 ± 0.011
4	48.650	52.100	−7.550	51.725	53.750	−5.125	4.250 ± 0.178	1.330 ± 0.015
5	47.450	52.850	−2.250	37.625	50.825	1.075	10.568 ± 0.279	1.316 ± 0.010
6	45.175	47.100	−8.225	47.975	49.725	−6.050	4.411 ± 0.199	1.383 ± 0.008
7	51.475	38.175	−13.150	52.200	45.000	−11.200	7.135 ± 0.316	1.303 ± 0.013
8	32.575	34.350	−9.375	36.475	40.100	−6.975	7.351 ± 0.205	1.348 ± 0.011
9	49.350	41.550	−13.275	52.950	43.725	−10.600	4.985 ± 0.120	1.291 ± 0.009

. As the temperature gradually increased from low temperature, the color of the microcapsules changed from dark purple to light purple gradually. With the rise in temperature, ΔE values generally increased, indicating that the microcapsules had thermochromic performance. Heating–cooling cycle experiments were further conducted to monitor the ΔE variation during the cooling process. The results, as shown in

Table 13. The results of microcapsules chromaticity values and ΔE with decreasing temperature.

Sample (#)	High Temperature Chromaticity Value (60 °C)			Low Temperature Chromaticity Value (−20 °C)			ΔE
	L1	a1	b1	L2	a2	b2
1	43.800	48.700	−3.100	39.300	49.100	−3.300	4.522 ± 0.159
2	53.200	49.500	−5.800	56.200	50.200	−8.100	3.844 ± 0.123
3	37.700	42.300	−1.600	31.900	35.800	−3.800	8.985 ± 0.224
4	50.900	53.200	−5.500	47.000	52.400	−7.600	4.501 ± 0.163
5	39.200	50.400	1.700	44.800	56.900	−2.800	9.688 ± 0.260
6	48.500	49.800	−5.800	45.800	46.900	−7.900	4.484 ± 0.132
7	51.300	45.100	−11.400	51.300	37.900	−12.000	7.225 ± 0.127
8	38.500	38.500	−7.100	34.100	32.800	−9.500	7.590 ± 0.094
9	52.400	46.600	−10.300	51.600	42.800	−13.400	4.969 ± 0.155

, indicated that the color change of the microcapsules exhibited a certain degree of reversibility and cycling stability, with the ΔE trend remaining stable after multiple cycles. Sample 5# showed the highest ΔE value of 10.568, demonstrating the best thermochromic performance. This was followed by sample 3# with a ΔE of 9.228, and Sample 8# with a ΔE of 7.351. Regarding formaldehyde emission, Sample 9# exhibited the lowest emission at 1.291 mg/m³, followed by Sample 1# and 7#, both at 1.303 mg/m³. However, the formaldehyde emission of all samples was significantly higher than international safety standards. Compared with the limit set by the World Health Organization (WHO) indoor air quality guidelines (0.1 mg/m³), the release level was markedly higher; it also exceeded the residential and public building air quality standards (maximum allowable concentration of 0.05 mg/m³ and daily average concentration of 0.01 mg/m³) [50,51,52]. This indicated that the current microcapsule system still required further optimization in terms of environmental safety. To reduce formaldehyde emissions, future studies could explore the incorporation of components such as chitosan, which has formaldehyde adsorption capability, or the use of green wall materials with low free formaldehyde content to partially replace urea-formaldehyde resin, thereby enhancing the environmental performance of the material.

Table 14. Analysis on the ΔE results of microcapsules by orthogonal test.

Category	Sample (#)	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value	Core–Wall Ratio	ΔE
Range	1	2.5	1.2:1	6.0	1:1	4.595
	2	2.5	1.0:1	8.0	1:1.5	3.501
	3	2.5	0.8:1	10.0	1:2	9.228
	4	1.5	1.2:1	8.0	1:2	4.250
	5	1.5	1.0:1	10.0	1:1	10.568
	6	1.5	0.8:1	6.0	1:1.5	4.411
	7	0.5	1.2:1	10.0	1:1.5	7.135
	8	0.5	1.0:1	6.0	1:2	7.351
	9	0.5	0.8:1	8.0	1:1	4.985
	Mean value 1	5.78	5.33	5.45	6.72
	Mean value 2	6.41	7.14	4.25	5.02
	Mean value 3	6.49	6.21	8.98	6.94
	Range	0.72	1.81	4.73	1.93
	Factor primary and secondary level	C > D > B > A
	Optimal level	A3	B2	C3	D3
	Optimal scheme	A3B2C3D3

presents the analysis of the color difference results of microcapsules from the orthogonal experiment. The emulsifier HLB value (C) had the most significant influence on the ΔE variation in the microcapsules. The order of influence among the four factors was C > D > B > A. The synthesis condition under the optimal ΔE was A3B2C3D3. The HLB value of the emulsifier had a significant impact on the microcapsule ΔE, primarily because it strongly influenced the emulsification process and the integrity of the microcapsule structure. At low HLB values, poor emulsification led to larger and uneven emulsion droplets, resulting in incomplete encapsulation and weakened thermochromic performance. An appropriate HLB value helped form a stable emulsion system, ensuring uniform dispersion and effective encapsulation of the thermochromic material, thereby enhancing the color-change response, reflected in a higher ΔE. Conversely, excessively high HLB values may lead the system overly hydrophilic, impairing wall material film formation and similarly reducing color-change performance. Therefore, at an HLB of 10, the microcapsule performance was balanced and ΔE reached its maximum. The analysis of variance for ΔE is shown in

Table 15. Analysis of variance table for ΔE results.

Factor	SS	df	MS	F	p-Value
Factor A: Mass fraction of cationic red in the core materials	1.841	2	0.921	3.326	0.083
Factor B: WF:WU	9.867	2	4.934	17.825	0.001 **
Factor C: HLB value of the emulsifier	72.548	2	36.274	131.053	0.000 **
Factor D: Core–wall ratio	13.310	2	6.655	24.044	0.000 **
Error	2.491	9	0.277
Total	100.057	17

Note: ** p ≤ 0.01.

. The ANOVA results indicated that factor A had a p-value > 0.05 and thus showed no significant effect on the ΔE of the microcapsules. Factors B, C, and D all exhibited significant effects on the microcapsules ΔE.

Since formaldehyde emission is a negative indicator, directly including it in the comprehensive evaluation would lead to biased results. Therefore, in this study, its values were converted into positive parameters by taking the difference from the maximum permissible limit of the detection method (1.999 mg/m³). This transformation not only ensures comparability among indicators of different parameters but also facilitates unified evaluation after normalization. Within this framework, the lower the formaldehyde release, the larger the difference obtained, and thus the higher the normalized score, indicating better environmental performance of the material. The analysis of formaldehyde emission from microcapsules in the orthogonal experiment is shown in

Table 16. Analysis on the formaldehyde emission by orthogonal test.

Category	Sample (#)	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value	Core–Wall Ratio	Formaldehyde Emission After Positive Pretreatment (mg/m3)
Range	1	2.5	1.2:1	6.0	1:1	0.696
	2	2.5	1.0:1	8.0	1:1.5	0.616
	3	2.5	0.8:1	10.0	1:2	0.684
	4	1.5	1.2:1	8.0	1:2	0.669
	5	1.5	1.0:1	10.0	1:1	0.683
	6	1.5	0.8:1	6.0	1:1.5	0.616
	7	0.5	1.2:1	10.0	1:1.5	0.696
	8	0.5	1.0:1	6.0	1:2	0.651
	9	0.5	0.8:1	8.0	1:1	0.708
	Mean value 1	0.67	0.69	0.65	0.70
	Mean value 2	0.66	0.65	0.66	0.64
	Mean value 3	0.69	0.67	0.69	0.67
	Range	0.03	0.04	0.03	0.05
	Factor primary and secondary level	D > B > C > A
	Optimal level	A3	B1	C3	D1
	Optimal scheme	A3B1C3D1

. The core–wall ratio had a significant impact on the formaldehyde emission during microcapsule preparation. The order of influence among the four factors was D > B > C > A. The optimal synthesis condition for achieving low formaldehyde emission was A3B1C3D1, indicating that formulations with lower formaldehyde content are more environmentally friendly. The analysis of variance for formaldehyde emission is shown in

Table 17. Analysis of variance table for formaldehyde emission.

Factor	SS	df	MS	F	p-Value
Factor A: Mass fraction of cationic red in the core materials	0.003	2	0.001	13.297	0.002 **
Factor B: WF:WU	0.004	2	0.002	20.780	0.000 **
Factor C: HLB value of the emulsifier	0.004	2	0.002	17.753	0.001 **
Factor D: Core–wall ratio	0.008	2	0.004	42.636	0.000 **
Error	0.001	9	0.000
Total	0.020	17

Note: ** p ≤ 0.01.

. The ANOVA results indicated that all four factors had significant effects on the formaldehyde emission of the microcapsules.

3.1.3. Analysis of Normalized Orthogonal Experiment Results and Single-Factor Comprehensive Performance of Microcapsules

The weighted normalization analysis was performed on the experimental data of microcapsules prepared through orthogonal experiments, converting each index such as yield, encapsulation rate, color difference (ΔE), and formaldehyde emission into a standardized percentage scale using the range method. This enabled comprehensive evaluation on a unified scale. After weighted normalization, the final comprehensive scoring results are shown in

Table 18. Normalized score of orthogonal test results.

Sample (#)	Normalized Scores				Average of Weighted Total Scores
	Yield Rate	Encapsulation Rate	ΔE	Formaldehyde Emission
1	70.19	100.00	15.48	86.96	63.34
2	0.00	78.95	0.00	0.00	23.69
3	51.10	10.53	81.04	73.91	53.97
4	88.94	100.00	10.59	57.61	58.57
5	100.00	73.68	100.00	72.83	86.67
6	61.38	0.00	12.88	0.00	13.72
7	99.47	52.63	51.42	86.96	66.10
8	62.76	73.68	54.47	38.04	58.19
9	75.22	52.63	20.99	100.00	54.42

. Among them, Sample 5# achieved the highest score of 86.67, demonstrating the best overall performance, particularly excelling in color difference between high and low temperatures. Sample 7# ranked second, with a total score of 66.10, indicating a relatively high comprehensive performance as well. Figure 1 shows the normalized scores radar chart of microcapsules by orthogonal test, which provides a clear visual comparison of the comprehensive advantages and shortcomings of each sample.

The analysis results of the average total score with equal weighting in the orthogonal experiment are shown in

Table 19. Analysis of factors affecting the score of orthogonal tests of microcapsule samples.

Sample (#)	Mass Fraction of Cationic Red in the Core Materials (%)	WF:WU	HLB Value	Core–wall Ratio	Average of Weighted Total Scores Based on Range Analysis
1	2.5	1.2:1	6.0	1:1	63.34
2	2.5	1.0:1	8.0	1:1.5	23.69
3	2.5	0.8:1	10.0	1:2	53.97
4	1.5	1.2:1	8.0	1:2	58.57
5	1.5	1.0:1	10.0	1:1	86.67
6	1.5	0.8:1	6.0	1:1.5	13.72
7	0.5	1.2:1	10.0	1:1.5	66.10
8	0.5	1.0:1	6.0	1:2	58.19
9	0.5	0.8:1	8.0	1:1	54.42
Mean value 1	47.00	62.67	45.08	68.14
Mean value 2	52.99	56.18	45.56	34.50
Mean value 3	59.57	40.70	68.91	56.91
Range	12.57	21.97	23.83	33.64
Factor primary and secondary level	D > C > B > A
Optimal level	A3	B1	C3	D1
Optimal scheme	A3B1C3D1

. Range analysis indicated that the core–wall mass ratio had the most significant effect on the comprehensive score. The order of influence among the four factors was D > C > B > A, and the optimal synthesis condition for the microcapsules was A3B1C3D1. That was, 0.5% cationic red dye was added, the ratio of formaldehyde to urea was 1.2:1, the ratio of core–wall was 1:1, and the HLB value of emulsifier was 10. At the same time, in the orthogonal experiment, the microcapsule score showed a trend of first decreasing and then increasing with the reduction in the core–wall ratio. Therefore, on this basis, the single-factor experiment was designed using the same process, with the core–wall ratio range set between 1:1 and 1:1.4 to further verify its influence on microcapsule performance. Preliminary experiments revealed that when the core–wall ratio was below 1:1, the microcapsules were difficult to form stably, and both encapsulation efficiency and structural integrity decreased; therefore, this range was not included in the present tests. Microcapsules numbered 10# to 14# correspond to core–wall ratios of 1:1, 1:1.1, 1:1.2, 1:1.3, and 1:1.4, respectively.

Table 20. The results of microcapsule comprehensive performance by single-factor test.

Sample (#)	Low Temperature Chromaticity Value (−20 °C)			High Temperature Chromaticity Value (60 °C)			Yield Rate (%)	Encapsulation Rate (%)	ΔE	Formaldehyde Emission (mg/m3)
	L1	a1	b1	L2	a2	b2
10	62.975	28.775	−8.725	61.000	22.525	−8.725	38.53 ± 1.28	63 ± 1.41	6.555 ± 0.181	1.414 ± 0.010
11	59.125	26.925	−5.975	65.400	20.075	−5.750	31.95 ± 1.78	68 ± 1.63	9.292 ± 0.209	1.381 ± 0.013
12	70.750	13.650	−5.150	67.450	12.975	−10.150	27.44 ± 1.15	65 ± 0.82	6.029 ± 0.126	1.340 ± 0.009
13	61.025	18.450	−12.200	54.975	16.475	−17.250	27.08 ± 1.27	56 ± 1.63	8.124 ± 0.201	1.331 ± 0.015
14	56.500	24.075	−14.750	54.400	19.950	−16.950	30.41 ± 0.98	59 ± 0.82	5.125 ± 0.162	1.451 ± 0.012

and

Table 21. Normalized score of single-factor test results.

Sample (#)	Normalized Scores				Average of Weighted Total Scores
	Yield Rate	Encapsulation Rate	ΔE	Formaldehyde Emission
10	100.00	58.33	34.30	30.83	50.67
11	42.58	100.00	100.00	58.33	83.05
12	3.21	75.00	21.69	92.50	49.07
13	0.00	0.00	71.97	100.00	45.19
14	29.11	25.00	0.00	0.00	11.87

present the comprehensive performance results of microcapsules from the single-factor experiments, along with their normalized scores. Figure 2 shows the normalized score radar chart of microcapsules by single-factor test. Sample 11# showed outstanding performance across all indicators, with the best ΔE of 9.292 and the highest encapsulation rate of 68%. Its weighted normalized comprehensive score was 83.05, higher than that of the other single-factor samples. Although sample 5# showed a slightly higher score (86.67), sample 11# was derived from the single-factor optimization experiment, and its preparation conditions were more representative of the actual improvement effects under factor influences. Moreover, it exhibited the best encapsulation rate, indicating a more complete microcapsule structure with better long-term stability and application reliability. Meanwhile, although its ΔE was lower than that of sample 5#, it was still at a relatively high level, ensuring a desirable thermochromic effect. Further heating–cooling cycle experiments were conducted, and the results shown in

Table 22. The results of microcapsules chromaticity values and ΔE with decreasing temperature.

Sample (#)	High Temperature Chromaticity Value (60 °C)			Low Temperature Chromaticity Value (−20 °C)			ΔE
	L1	a1	b1	L2	a2	b2
10	56.200	23.400	−8.100	60.800	27.700	−8.400	6.304 ± 0.139
11	67.000	26.100	−9.200	63.100	32.700	−4.100	9.208 ± 0.074
12	68.600	13.100	−10.500	65.000	14.100	−5.900	5.926 ± 0.102
13	51.400	17.800	−16.200	57.900	17.600	−11.400	8.083 ± 0.143
14	48.200	25.100	−21.400	48.400	29.100	−18.400	5.004 ± 0.091

demonstrated that the ΔE variation trend of 11# remained stable during the cycles. This finding was consistent with the cycling stability results obtained in the orthogonal test, thereby further confirming its superior reversible thermochromic performance. In summary, sample 11# was selected as the preferable microcapsule due to its advantages in stability, overall performance balance, and representativeness in the optimization experiment. With a suitable emulsification environment and a relatively high core–wall ratio, Sample 11# achieved a good balance between the structural integrity of the wall and the functionality of the core material. This enabled the thermochromic material to effectively exhibit color change while being well protected by the robust wall shell. The optimal preparation conditions were the addition of 0.5% dispersible cationic red dye relative to the core material mass, a formaldehyde-to-urea ratio of 1.2:1, an emulsifier HLB value of 10, and a core–wall mass ratio of 1:1.1.

3.1.4. Microscopic Morphology Analysis of Microcapsules

As shown in Figure 3, a moderate addition of the cationic red dye helped improve the dispersion of the core material and enhanced the uniformity of the microcapsules. However, when the addition level was too high, the excessive cationic groups interfered with the stability of the emulsion system, leading to an incomplete emulsification of the core material and, consequently, intensified aggregation of the microcapsules. Therefore, an appropriate amount of cationic red dye was crucial for ensuring desirable morphology and performance of the microcapsules. As the mass ratio of formaldehyde to urea decreased, the particle size of microcapsules increased, and the spherical morphology of the overall microcapsules was improved first and then weakened. This suggested that when the formaldehyde-to-urea ratio was between 1.2:1 and 1.0:1, a small amount of dimethylol urea was formed, which enhanced the crosslinking degree of the microcapsule wall material without causing excessive crosslinking or agglomeration. With the increase in the emulsifier HLB value, the formation of microcapsules improved, and the particle size of the microcapsules became more uniform at an HLB value of 10. When the HLB value was 6, the microcapsules tended to rupture or became unformed. This indicated that an emulsifier with an HLB value of 10 achieved a better hydrophilic–lipophilic balance with the oil phase (core material) used in this study under aqueous conditions, thereby enhancing emulsion stability and facilitating microcapsule formation. Meanwhile, as the core–wall ratio decreased, some microcapsule particles became larger, and the particle size of the microcapsules changed from uniform distribution at the beginning to varying sizes, with increased agglomeration. This showed that with the increase in wall material, microcapsules were more likely to form, but the excessive presence of wall material in the system could lead to rapid binding between the core and wall materials, resulting in particle size irregularities and excessive crosslinking.

The particle size distribution of microcapsules was analyzed in combination with Figure 4. Microcapsules 1–3# exhibited overall particle sizes within the small-to-medium range, with relatively uniform morphology, indicating stable microcapsule structures under these conditions. Microcapsules 4# showed the narrowest size distribution within the small particle range, with an average diameter of 3.42 ± 1.85 μm and regular morphology, corresponding to its highest encapsulation rate. This demonstrated the positive effect of small and uniform particle sizes on improving encapsulation performance. Microcapsules 5# had a larger mean size of 5.11 ± 2.72 μm, with pronounced agglomeration in OM images; however, it exhibited the highest yield rate and ΔE. This suggested that under moderate dye proportion and relatively high HLB values, dense structures could form, enabling strong thermochromic performance despite evident agglomeration. Microcapsules 6 and 7# had similar particle size distributions, but stronger agglomeration was observed in 6#, which was consistent with its significantly lower encapsulation rate compared to microcapsules 7#. Microcapsules 8#, with an average size of 3.68 ± 2.01 μm, showed moderate performance, without remarkable advantages in ΔE or encapsulation rate. Microcapsules 9#, although having a relatively large mean size of 5.62 ± 2.90 μm, exhibited a distinctly lower ΔE. This indicated that at lower contents of cationic red dye, the insufficient chromophore dosage led to weaker color-change effects, even when particle size was relatively large.

Figure 5 shows the OM images of microcapsule samples prepared from the single-factor experiments. As the core–wall ratio decreased, excessive crosslinking of the wall material occurred, causing microcapsules to stick together and further aggravated aggregation. More and more large-sized microcapsules were produced, and the uniformity of particle size also decreased, which was unfavorable for subsequent addition into UV coatings. Microcapsules with uneven particle sizes, when added to UV coatings, could not only worsen aggregation but also negatively impacted the mechanical and optical properties of the coating. Uneven particle sizes in the coating may cause localized high stress, affecting the mechanical properties of the coating such as flatness, gloss, and hardness. During long-term use, a local stress could also reduce the service life of the coating.

The analysis was based on the SEM microstructures of microcapsules in the single-factor experiment shown in Figure 6 and the particle size distribution shown in Figure 7. As the core–wall ratio decreased, the phenomenon of excessive crosslinking in the microcapsules became more severe. The particle size distribution of microcapsules 10#, 11#, and 12# fell within the smaller range of 1–7 μm, indicating smaller particle sizes, with mean particle sizes of 4.43 ± 2.21 μm, 4.11 ± 2.60 μm, and 5.22 ± 2.66 μm, respectively. Sample 10# and 11# exhibited lighter crosslinking and more uniform particle sizes. Sample 11# microcapsule showed the most concentrated distribution within the small particle size range. The particle size of Sample 13# was generally larger, with the mean particle size increasing to 6.06 ± 2.83 μm. While in Sample 14# microcapsule, almost no independent microspheres could be found, and the particle size range was too broad—small microcapsules coexisted with ones larger than 11 μm—resulting in the worst microstructure. This is because, as the core–wall ratio decreased, the excessive wall material caused even the formed microcapsules to cross-link more easily during the rapid drying of the spray-drying process under external force. The effect of the core–wall ratio on crosslinking behavior can be attributed to the relative content of the wall material. When the core–wall ratio is too high, the wall material is insufficient to fully encapsulate the core, resulting in incomplete crosslinking, a loose shell structure, and consequently reduced encapsulation efficiency and microcapsule stability. Conversely, when the core–wall ratio is too low, the crosslinking density of the wall material is enhanced, but aggregation and uneven deposition are more likely to occur, thereby reducing the encapsulation performance. The microcapsules 10# and 11# exhibited better microstructural performance. This morphological difference was consistent with the performance data discussed earlier. Sample 11# exhibited better results in both encapsulation rate and ΔE, indicating that a uniform particle size distribution and a moderate crosslinking structure contribute to improving the integrity of the capsule wall and the stability of the color change. In contrast, the strong agglomeration observed in Samples 13# and 14# reduced the encapsulation rate and was accompanied by a decrease in ΔE, suggesting that agglomeration and excessive crosslinking weakened the thermochromic performance. These findings demonstrated that the microscopic morphological characteristics were intrinsically related to the comprehensive performance indicators of the microcapsules, further confirming the key role of the core–wall ratio in regulating the structure and function of microcapsules.

3.1.5. Chemical Composition Analysis of Microcapsules

From

Table 23. Characteristic peak of infrared spectrum.

Peak Numbers (cm−1)	Characteristic Peak	Substance	Formation Reasons
3321	–OH	Bisphenol A	Stretching and vibration peak
2922	C–CH3	Bisphenol A	Stretching and vibration peak
1513 and 1463	Benzene ring C=C skeleton	Bisphenol A	Stretching and vibration peak
1251	C–OH	Bisphenol A	Absorption peak
1055	C–O	Bisphenol A	Stretching and vibration peak
1613	Lactone ring carbonyl C=O	Crystal violet lactone	Stretching and vibration peak
1177 and 1055	Ester group C–O–C	Crystal violet lactone	Symmetrical stretching and vibration peak
3435	N–H	Disperse cationic red	Absorption peak
1734	Ester carbonyl group with non-lactone ring structure	Core materials	Absorption peak
3321	-OH in the carboxyl group	Core materials	Absorption peak
3353	N–H, O–H	Wall materials	Stretching vibration peak
2963	–CH2–	Wall materials	Asymmetric stretching vibration
1566	The N–H of the amide	Wall materials	Flexural vibration
1390	C–N	Wall materials	Stretching and vibration peak
1136	CH3O	Wall materials	Absorption peak

and Figure 8, it can be seen that the absorption peak at 3353 cm⁻¹ originates from the overlapping stretching vibrations of N–H and O–H, the peak at 2963 cm⁻¹ corresponded to the asymmetric stretching vibration of –CH₂–, the peak at 1635 cm⁻¹ was assigned to the carbonyl stretching vibration of secondary amide (amide I) in the UF wall material, and the peak at 1136 cm⁻¹ corresponded to the characteristic absorption of CH₃O. These characteristic peaks were all observed in the infrared spectra of the microcapsules, indicating the presence of the wall material.

In the infrared spectra of microcapsules containing cationic red dye, characteristic peaks of the core material were also observed. A new ester carbonyl C=O absorption peak appeared at 1734 cm⁻¹ in the dye-containing microcapsules, which was absent in both the dye-free control microcapsules and the pure cationic red dye. This peak arose because the incorporation of the dye induced the opening of the lactone ring in the core material, forming a conjugated chromophore structure and generating a new ester carbonyl. Meanwhile, the symmetric stretching vibration of the carboxylate at 1390 cm⁻¹ further confirmed the occurrence of the ring-opening reaction.

The absorption peak at 3435 cm⁻¹ originated from the N–H stretching vibration of the amino groups in the dispersible cationic red dye, which was clearly present in the spectrum of the pure dye. In the 11# microcapsules containing the dye, this peak was slightly weakened and showed a minor shift, indicating possible hydrogen bonding or electrostatic interactions between the dye and the wall/core materials. The spectrum of 11# microcapsules simultaneously exhibited characteristic peaks of the wall material, core material, and the new features introduced by the dye, confirming that the dye had been successfully encapsulated while retaining its inherent color properties, thereby demonstrating the successful preparation of the microcapsules.

Based on the test results of other microcapsule properties, the 11# microcapsule exhibited the best overall performance when the core–wall ratio was 1:1.1. Its yield reached 31.95%, encapsulation rate was 68%, thermochromic ΔE was 9.292, and formaldehyde emission concentration was 1.381 mg/m³. The 11# microcapsules were then incorporated into the coating to evaluate various coating properties, aiming to determine the optimal addition level of microcapsules in the UV coating system.

As shown in Figure 9, due to the high saturation of the red background, it was difficult to visually distinguish changes in the blue hue with the naked eye. However, the images captured using the same camera settings revealed variations within the red-purple color range. At low temperatures, the microcapsules appeared dark purple, while at high temperatures, they turned light purple, indicating that a clear color change occurred between –20 °C and 60 °C. Quantitative analysis of ΔE showed that the instrumentally measured ΔE values could effectively reflect the thermochromic response of the microcapsules, compensating for the subjectivity and limitations of visual observation.

3.2. Paint Film Performance Analysis

3.2.1. Analysis of Paint Film Optical Performance

The effects of adding 11# microcapsules on the gloss and transmittance of the UV primer are shown in

Table 24. Effect of adding 11# microcapsule on the glossiness and transmittance of UV primer.

Amount of Microcapsules Added (%)	Gloss (GU)			60° Light Loss Rate (%)	Transmittance (%)
	20°	60°	85°
0	73.7	112.1	91.6	-	94.06
5	15.8	42.2	53.7	62	91.92
10	26.4	63.8	72.9	43	84.95
15	28.0	60.0	64.5	46	75.20
20	16.7	49.0	46.6	56	67.51
25	19.8	48.9	47.8	56	65.92
30	16.8	35.8	29.7	68	56.58

. At a 5% addition, the UV coating exhibited increased gloss loss at the 60° angle; however, at 10%, gloss loss decreased, followed by an increase again at 15%. This trend is attributed to the relatively poor microstructure of the 11# microcapsules. At low addition levels, the uneven dispersion of microcapsules caused localized surface irregularities in the high-solids UV coating, reducing glossiness. When the addition amount increased to 10%, the light loss rate decreased, indicating that the microcapsules were relatively uniformly dispersed in the coating, and the coating surface became smoother. The maximum capacity was achieved at 15% loading, beyond which excessive microcapsules reduced surface smoothness and consequently weakened gloss.

The optical performance of the coatings is shown in Figure 10. As the amount of microcapsules increased, the transmittance of the coating decreased. The transmittance of the coating without microcapsule addition was 94.06%, which dropped to 56.58% when 30% of the 11# microcapsules were added. This change indicated that microcapsules had a significant impact on the optical properties of the coating. The decrease in transmittance might be related both to the pigments within the microcapsules, which could absorb certain wavelengths of light, and to scattering effects caused by the microcapsule particles and their aggregation [53,54]. It also highlighted that excessive addition could reduce transparency, which was unfavorable for applications requiring high light transmittance. Within the 5–15% addition range, the coating transmittance remained relatively high, providing a reasonable balance between thermochromic functionality and basic optical performance. However, when the addition exceeded 15%, transparency dropped significantly, making it unsuitable for transparent or semi-transparent primer systems that required high transmittance.

3.2.2. Analysis of Paint Film Mechanical Performance

Table 25. Effect of adding 11# microcapsule on the elongation at break and roughness of UV primer.

Amount of Microcapsules Added (%)	Elongation at Break (%)	Roughness (μm)
0	10.6	0.089
5	9.3	0.308
10	6.8	0.425
15	6.6	0.640
20	2.4	0.536
25	2.1	0.706
30	0.9	0.927

shows the effects of different addition levels of 11# microcapsules on the elongation at break and surface roughness of the coating. Adding only 5% microcapsules reduced the elongation at break from 10.6% to 9.3%, indicating that the microcapsules, acting as rigid particles, introduced localized stress concentrations within the coating matrix. Meanwhile, the limited interfacial interaction between the microcapsules and the polymer matrix hindered local mechanical load transfer, affecting the continuity of the matrix. In addition, during UV curing, the microcapsules caused slight shading in certain regions, subtly interfering with the polymer crosslinking process. As the amount of addition increases, the roughness gradually rose, starting from a relatively high level. Considering the relatively poor microstructure of the 11# microcapsules, the amount of the UV primer coating should not be too high. The mechanical properties were better at 5% addition, and the elongation at break and roughness showed excellent results. From the perspective of performance weighting, considering the application requirements of UV primers in transparent or semi-transparent systems, this study prioritized the optical properties of the coating, particularly transmittance, while also taking mechanical properties into account as necessary for reliability. Although a 5% microcapsule addition slightly reduced glossiness, transmittance remained within an acceptable range, and overall optical performance was satisfactory. With microcapsule additions of 10% or more, uneven distribution within the coating led to a decline in optical properties and a noticeable deterioration in mechanical performance. Therefore, based on the combined performance in optical properties and color difference, the optimal addition level of 11# microcapsules in the UV primer was determined to be 5%, in order to achieve the best overall coating performance.

3.2.3. Analysis of Paint Film Morphology

The SEM images of UV primer coating with different mass fractions of 11# microcapsules are shown in Figure 11. Surface irregularities caused by microcapsule aggregation were observed. At a 5% addition level of 11# microcapsules, the UV primer exhibited poor surface morphology with relatively large protrusions. The aggregation of microcapsules led to poor leveling, which further resulted in an uneven UV coating surface. When the amount of additives was further increased, the surface of the coating with 10% 11# microcapsules showed more extensive but less pronounced protrusions. This may have contributed to a further increase in surface roughness, while the impact on gloss appeared to be less severe than at the 5% level. Although this study did not quantitatively analyze the size, coverage area, or defect density of the aggregates, the evolution of surface morphology indicated that the dispersion and aggregation behavior of the microcapsules in the coating had a significant impact on the observed trends in gloss and roughness. While considering pursuing high thermochromic ΔE, the optical mechanical properties of the coating itself cannot be too affected. Therefore, considering the comprehensive influence of microcapsules on coating performance, the optimal addition amount was determined to be 5% of 11# microcapsules, at which point the surface morphology could be accepted to a certain extent.

3.2.4. Chemical Composition Analysis of Paint Film

Figure 12 shows the infrared spectra of UV primer and microcapsules. The absorption peaks at 1176 cm⁻¹ and 1724 cm⁻¹ corresponded to the C–O and C=O stretching vibrations, respectively, which were characteristic peaks of the main components in the UV primer, namely polyester acrylate resin, trimethylolpropane triacrylate, and trimethylolpropane acrylate. The peak at 2922 cm⁻¹ was attributed to the C–CH₃ stretching vibration, originating from both bisphenol A in the core material and epoxy acrylate resin in the UV primer. This peak was enhanced in the spectrum of the UV primer with 10% 11# microcapsules. The peak at 1635 cm⁻¹ represented the ester carbonyl C=O stretch of a non-lactone ring structure, while the peak at 1390 cm⁻¹ corresponded to the symmetric stretching of carboxylate, indicating that the lactone ring had opened to form a conjugated chromogenic structure. Importantly, the coating with the addition of microcapsules did not produce a new peak that was different from the microcapsules and the coating, suggesting that the addition of microcapsules did not hinder the curing process of the UV primer nor induce chemical reactions between the two. Thus, the microcapsules were confirmed to be suitable for addition in UV primer.

When 11# microcapsules were added to the UV primer at a loading of 5%, the overall performance of the UV coating was optimized. The UV–visible light transmittance reached 91.92%, the 60° gloss was 42.2 GU, and the 60° gloss loss rate was 62%. In terms of mechanical properties, the elongation at break was 9.3%, and the surface roughness was 0.308 μm. The 11# microcapsules also exhibited excellent thermochromic performance, with a ΔE value of 9.292 between low and high temperatures.

4. Conclusions

Through orthogonal experiment and single factor experiment, the preparation process of urea-formaldehyde-coated cationic red-ternary system thermochromic microcapsule was optimized, and its thermochromic ability in coating system was verified. The effects of the proportion of dispersed cationic red in the core material, the mass ratio of formaldehyde to urea, the HLB value of the emulsifier, and the core–wall ratio on the overall microcapsule performance were systematically investigated. Among these factors, the core–wall ratio had the most significant impact. The optimal preparation conditions were: dispersed cationic red that accounts for 0.5% of the mass of the core material, a formaldehyde-to-urea ratio of 1.2:1, an emulsifier HLB value of 10, and a core–wall ratio of 1:1.1. As the core–wall ratio decreased, excessive crosslinking of the wall material led to microcapsule agglomeration and sticking, resulting in a decrease in yield, the encapsulation rate and ΔE value first increase and then decrease, and the formaldehyde emission increases first and then decreases. When the core wall ratio was 1:1.1, the microcapsules exhibited an optimal comprehensive performance, with a yield of 31.95%, encapsulation rate of 68%, thermochromic ΔE of 9.292, and formaldehyde emission of 1.381 mg/m³. When 5% amount of 11# microcapsules was added, the UV coating exhibited the best comprehensive properties: UV–visible transmittance of 91.92%, 60° gloss of 42.2 GU, gloss loss of 62%, elongation at break of 9.3%, and roughness of 0.308 μm. Future studies will further determine the ΔE–temperature response curves and color-change temperature parameters of UV primer films containing thermochromic microcapsules at different temperatures, and explore their application on substrates such as wood, in order to more systematically evaluate their thermochromic performance and practical potential. Effort will also be devoted to optimizing the wall material formulation by exploring bio-based alternatives with low free formaldehyde release or formaldehyde adsorption capability, in order to enhance the green characteristics and environmental friendliness of the microcapsule system.