Effect of Thermochromic and Photochromic Microcapsules on the Surface Coating Properties for Metal Substrates

Abstract

A coating with thermochromic and photochromic microcapsules can enhance a product’s attractiveness. Different coating processes may affect the performance of coatings. Therefore, the micromorphology, chemical composition, chromatic difference, gloss, hardness, adhesion, impact resistance, roughness, cold liquid resistance, and ultraviolet photooxidation resistance of the surface coating on the metal substrate were assessed by choosing three coating processes. The thermochromic color difference of the coating with photochromic microcapsules in the primer and thermochromic microcapsules in the topcoat changes greatly. When the temperature reached 80 °C, the maximum color difference of the coating was found to be 23.0. The color difference of the coating with the thermochromic microcapsules in the primer and photochromic microcapsules in the topcoat was the most obvious, with a color difference of 71.7. The gloss of the coating mixed with thermochromic microcapsules and photochromic microcapsules was the highest, which was found to be 81.7 GU. The coating gloss of thermochromic microcapsules in the primer and photochromic microcapsules in the topcoat was found to be 15.6. The mechanical property of the coating mixed with thermochromic microcapsules and photochromic microcapsules was the best—the hardness was found to be 2H, the adhesion was found to be level 1, and the impact resistance was found to be 12.5 kg·cm. The mechanical property of the coating prepared by the other two coating sequences was poor. The coating prepared by the three finishing processes on the metal substrate had sufficient cold liquid resistance, and the gloss of the coating before and after the cold liquid resistance changed slightly. By studying the coating process of thermochromic coating and photochromic coating, a technical reference is provided for creating dual-function intelligent coatings.

1. Introduction

In industry, transportation, construction, marine, and aerospace fields, and for daily necessities [1,2,3], metal is an essential engineering material. Cast iron, steel, and other common metals are available. Metal materials are characterized by durability, hardness, plasticity, and fire resistance [4]. It is often necessary to apply several layers of coatings on the surface of metal substrates in order to better protect the metal substrate. Coatings have progressively evolved into intelligent, multipurpose materials as a result of scientific and technological advancements [5,6,7,8]. An intelligent coating has additional features such as discoloration, corrosion resistance, superhydrophobic ability, and self-healing properties in addition to its inherent qualities of protecting the substrate and enhancing aesthetics [9,10,11,12]. This means that it can respond to changes in the external environment in a controlled way and produce adaptability. Multi-functional coatings, which are affordable and attractive, not only boost the added value of products but also satisfy the unique and varied needs of consumers.

The color performance of the product is greatly influenced by the coating’s color. In addition to shielding the substrate from the damaging effects of the outside environment (water, air, and light), the pigment employed in the coating must also have adequate endurance and not fade over time [13]. Even though some pigments have vibrant color and sheen, they are poorly resistant to light, acid, alkali, and solvents, and they will change color or fade when exposed to the outside environment over an extended period of time [14]. When the pigment is encapsulated inside microcapsules, the wall material of the microcapsules can shield the pigment from the environment and increase the pigment’s available life [15]. Additionally, people’s needs are becoming more varied and customized as high technology becomes more widely accepted, and consumers also enjoy the appealing appearance of color-changing coatings. Photochromic materials and thermochromic materials based on electron transfer are gaining more and more attention in the field of color-changing materials because of the ability to modify their electronic behavior [16,17].

Thermochromic compounds are generally composed of chromogenic agents, leuco agents, and solvents [18]. The color is decided by the chromogenic agents. The leuco agent determines the type of color. The temperature of the color shift depends on the solvent. Zhu et al. [19] combined 2-anilino-6-(dibutylamino)-3-methylfluoran, bisphenol A, and 1-tetradecanol in a 1:2:60 ratio to create a thermochromic compound, which was then encapsulated in a urea–formaldehyde (UF) polymer by in situ polymerization. These microcapsules demonstrated a suitable intelligent thermochromic effect when they were added to wood and wood coatings. Pedaballi et al. [20] investigated and contrasted the effects of oleic acid, tris-(2-butoxyethyl) phosphate, and oleyl phosphate on the dispersion of commercial thermochromic microcapsules and established the significance of these microcapsules in thermochromic coatings. They proposed that the depletion stabilization effect is the most plausible mechanism of oleyl phosphate’s favorable dispersion effect on microcapsules. The commercially available encapsulated reversible thermochromic pigments, which have optical properties that change reversibly with temperature and have been employed in architectural coatings, were thoroughly described and their compatibility with aggressive cement matrix was examined by Perez et al. [21]. Three component organic pigments were used to encapsulate melamine formaldehyde (MF) microcapsules, and this process was investigated. Yang et al. [22] created cholesteric liquid crystal microcapsules by in situ polymerization utilizing cellulose nanocrystal-stabilized Pickering emulsion as a template, which included cellulose nanocrystals and MF resin hybrid shells. Cholesteric liquid crystal microcapsules and a curable adhesive were successfully combined to create a coating slurry, and the slurry was then coated on a substrate and dried to create a thermochromic coating. Crystal violet lactone [23] has a fast thermal response, high chromaticity, and suitable oil solubility. It is a leuco agent that is frequently utilized in color-changing systems. However, because the crystal violet lactone’s conjugated system is broken, it is colorless. As a result, the color expression requires bisphenol A, a chromogenic agent. The tetradecanol as the solvent has a low melting point [24]. The UF resin [25] is durable, strong, transparent, and chemically resistant. It also has sufficient hardness and wear resistance.

Photochromic microcapsules are tiny containers made of a substance that changes color when exposed to ultraviolet (UV) light [26]. Metal coatings that incorporate photochromic microcapsules can boost the value of metal products. Jia et al. [27] used in situ polymerization to microencapsulate photochromic components in urea melamine formaldehyde resin. Using a straightforward drop coating technique, microencapsulated photochromic material/polydimethylsiloxane composite was applied to wood to create photochromic wood. To create intelligent photochromic wood, Hu et al. [28] coated plywood with coatings containing photochromic microcapsules. It was discovered that when the microcapsule concentration was increased from 2.5 to 10 wt.%, the chromatic difference of photochromic wood grew from 7.45 to 21.58. A popular green coating for environmental preservation on the market is waterborne acrylic resin coating [29]. Water is used as a solvent in the waterborne acrylic resin coating to save resources and lessen VOC volatilization. The weather resistance, adhesion, and gloss of the waterborne coating are all adequate [30]. Alkyd resin continues to be a high-performance and widely used resin. The outdoor endurance, high gloss, flexible coating, adhesion, high weather resistance, aging resistance, strong light and color retention, and some heat, water, and cold liquid resistance are all strengths of the alkyd resin coating [31].

In this study, tetradecanol, bisphenol A, and crystal violet lactone were chosen as the primary components of the thermochromic microcapsules. UF resin as the wall material can effectively address the color-changing compound’s current flaws and increase its range and service life. The photochromic microcapsules that turn rose red when exposed to natural light are chosen. Under natural light irradiation, the microcapsules can totally change color in about 20 s due to their quick light responsiveness and high stability. The goal of creating a coating with both thermochromism and photochromism functions in this paper was achieved by applying thermochromic and photochromic microcapsules to the coating. This exploration can satisfy customers’ individualized needs and lay the groundwork for the creation of intelligent films.

2. Materials and Methods

2.1. Experimental Materials

The crystal violet lactone (C₂₆H₂₉N₃O₂, M_W: 415.527, CAS: 1552-42-7) was purchased from Nantong Runfeng Petrochemical Co., Ltd., Nantong, China. The bisphenol A (C₁₅H₁₆O₂, M_W: 228.286, CAS: 80-05-7) was bought from Shanghai HaiHai Chemical Co., Ltd., Shanghai, China. The tetradecanol (C₁₄H₃₀O, M_W: 214.387, CAS: 112-72-1) was obtained from Guangzhou Jiangshun Chemical Technology Co., Ltd., Guangzhou, China. Urea (CH₄N₂O, M_W: 60.06, CAS: 57-13-6) was purchased from Guangzhou Suixin Chemical Co., Ltd., Guangzhou, China. The 37% formaldehyde water solution (CH₂O, M_W: 30.03, CAS: 50-00-0) was purchased from Shandong Xinjiucheng Chemical Technology Co., Ltd., Jinan, China. Triethanolamine (C₆H₁₅NO₃, M_W: 149.19, CAS: 102-71-6) was acquired from Xingtai Xinlanxing Technology Co., Ltd., Xingtai, China. Acetic acid (C₂H₄O₂, M_W: 60.052, CAS: 64-19-7), citric acid monohydrate (C₆H₁₀O₈, M_W: 210.139, CAS: 5949-29-1), and hydrochloric acid (HCl, M_W: 36.461, CAS: 7647-01-0) were purchased from Jinan Xiaoshi Chemical Co., Ltd., Jinan, China. Absolute ethanol (C₂H₆O, M_W: 46.068, CAS: 64-17-5) was purchased from Guangzhou Chengyi Nuoyi Instrument Co., Ltd., Guangzhou, China. Arabic gum powder (CAS: 9000-01-5) was purchased from Nanjing Jinyou Biotechnology Co., Ltd., Nanjing, China. Acetone (C₃H₆O, M_W: 58.08, CAS: 67-64-1) was purchased from Guangzhou Jiangshun Chemical Technology Co., Ltd., Guangzhou, China. Ethyl acetate (C₄H₈O₂, M_W: 88.105, CAS: 141-78-6) was purchased from Jinan Zhengkang Chemical Co., Ltd., Jinan, China. Xylene (C₈H₁₀, M_W: 106.165, CAS: 1330-20-7) was purchased from Jinan Zhengkang Chemical Co., Ltd., Jinan, China. The main components of commercial photochromic microcapsules include polyformaldehyde melamine (CAS: 9003-08-1, 32–36 wt.%), styrene-maleic anhydride maleic acid monomethyl ester polymer (CAS: 31959-78-1, 6.5–8.0 wt.%), 4-(1-phenylethyl)-o-xylene (CAS: 6196-95-8, 50–60 wt.%), and 1,3,3-trimethylindoline-6’-(1-piperidinyl) spironoxazine (photochromic purple dye, CAS: 114747-45-4, 2.6–4.0 wt.%), and they were purchased from Shenzhen Oriental Rainbow Company, Shenzhen, China. Waterborne acrylic resin coating (mainly composed of waterborne acrylic emulsion, polyurethane emulsion, additives, and water) was purchased from Jiangsu Anyi Chemical Co., Ltd., Nantong, China. Alkyd resin coating was purchased from Nanjing Panfeng Chemical Co., Ltd., Nanjing, China. Aluminum alloy metal plates (50 mm × 50 mm × 0.5 mm) were purchased from Dongguan Guangouli Metal Materials Co., Ltd., Dongguan, China.

2.2. Synthesis of Color-Changing Compound and Thermochromic Microcapsules

(1)

Synthesis of the color-changing compound

Tetradecanol (30.00 g) was first weighed and placed in a beaker. The beaker was placed in the DF-101 digital display thermostatic water bath (Shenzhen Dingxinyi Experimental Equipment Co., Ltd., Shenzhen, China), where the tetradecanol was heated to a molten state at 50 °C. Then, 1.50 g of bisphenol A and 0.50 g of crystal violet lactone were then added. The rotor was added to stir. The temperature was steadily raised to 90 °C after the mixture was well stirred. After stirring at 400 rpm for 1.5 h, the mixture became clear and transparent. The color-changing compound was a dark blue solid after cooling to room temperature.

(2)

Preparation of the wall material prepolymer

First, 16.84 g of 37% formaldehyde water solution and 8.00 g of urea were weighed and added to a beaker. When the urea was fully dissolved, the beaker was placed in the water bath and stirred at room temperature. To adjust the pH of the solution to 8.5, a few drops of triethanolamine were added. The temperature was raised to 70 °C. The mixture was stirred at 300 rpm for 1 h.

(3)

Preparation of core material

First, 5.48 g Arabic gum powder, 104.25 g distilled water, and 8.32 g color-changing compound were weighed and put into a beaker. The beaker was placed into a water bath at 50 °C after a rotor was added, and the mixture was then steadily stirred until the compound was entirely dissolved. The water bath was then heated to 65 °C and stirred for 20 min at 1600 rpm. The mixture was put into the TL-650CT ultrasonic emulsification disperser (Jiangsu Tianling Instrument Co., Ltd., Yancheng, China) for 5 min so that the emulsifier was equally wrapped on the outside surface of the color-changing compound.

(4)

Encapsulation of microcapsules

The beaker containing the ultrasonic core material emulsion was put into a 35 °C water bath for gradual stirring. Drop by drop, the wall material prepolymer was mixed into the prepared core material solution. Following that, the water bath’s rotational speed was changed to 500 rpm, 1.16 g of silica and 1.16 g of sodium chloride were added, and then 8 wt.% citric acid monohydrate was gradually added. The reaction was continued for 1 h after the pH of the solution was adjusted to about 2.5. To obtain the microcapsule emulsion, the temperature was raised to 68 °C, and the stirring speed was adjusted to 250 rpm to continue the reaction for 30 min. After the microcapsule emulsion was cooled to room temperature, it was filtered by an SHZ-DIII desktop circulating water vacuum pump (Shanghai Yuhua Instrument Equipment Co., Ltd., Shanghai, China) and dried in a 35 °C 202-0AB electric constant temperature blast drying oven. The blue powder obtained was the thermochromic microcapsules.

2.3. Finishing Process

A primer layer was obtained by first weighing the microcapsules and coatings of the appropriate quality according to

Table 1. Experimental materials list.

Finishing Process	Thermochromic Microcapsules Concentration (%)	Photochromic Microcapsules Concentration (%)	Thermochromic Microcapsules Mass (g)	Photochromic Microcapsules Mass (g)	Alkyd Acrylic Resin Coating Mass (g)	Waterborne Acrylic Resin Coating Mass (g)
photochromic primer + thermochromic topcoat	10	10	0.2	0.2	1.8	1.8
thermochromic primer + photochromic topcoat	10	10	0.2	0.2	1.8	1.8
photochromic and thermochromic coating	10	10	0.2	0.2	3.6	0

, stirring them evenly, and then painting them onto the surface of metal boards with a BEVS1803 coating preparation machine (Guangzhou Keyu New Material Technology Co., Ltd., Guangzhou, China). After they had been dried at room temperature for 1 h, they were finely sanded with 800 mesh sandpaper [32]. In Figure 1, the experimental flow is depicted. The coating was applied in a coating amount of 15–20 g/m², and it was then dried for 1 day in the oven at 30 °C. After drying, the thickness of the primer layer was about 60 μm [33]. The same procedure was used to apply the topcoat. Finally, a double-layer coating on the metal substrate was successfully obtained.

In different finishing sequences, the photochromic and thermochromic microcapsules were added to the coating and coated on the metal substrate, respectively. Table 1 displays the multi-functional coating’s finishing sequences. Alkyd resin was the foundation of the coating containing thermochromic microcapsules, while the waterborne acrylic resin was the base of the coating with photochromic microcapsules. The previous experiments showed that when our prepared thermochromic microcapsules were added to the waterborne acrylic resin coating, the discoloration performance of the microcapsules was reduced, the microcapsules in the paint film would be severely reunited, and the surface of the paint film would be uneven [34]. Therefore, the alkyd resin coating served as the base for the combined usage of thermochromic and photochromic microcapsules.

2.4. Testing and Characterization

(1)

Micromorphology characterization and chemical composition testing

The software “Nano measurer”, with a measurement capacity of 100, was used to gauge the particle size distribution of microcapsules [35]. To describe the micromorphology of the prepared coating, a VERITAS scanning electron microscope (SEM, Shanghai Junzhun Instrument Equipment Manufacturer, Shanghai, China) was chosen. The ATR tablet pressing method was used to manufacture the coating, and a BOEN-85697F Fourier transform infrared spectrometer (FTIR, Fribourne Industrial Development Co., Ltd., Shanghai, China) was chosen to study the coating’s chemical composition.

(2)

Chromatic difference testing

After a SEGT-J chromatic difference meter (Beijing Shidai Shanfeng Technology Co., Ltd., Beijing, China) was calibrated, the test hole with the sample was aligned to test and record the values of L, a, and b. On one test item, a total of 4 tests were performed. The L value denotes lightness. The color becomes brighter with the higher L value. The red–green color is represented by the value a. The color is red if the a is positive. The color is green if the a is negative. Yellow–blue color is represented by the b value. The color is yellow when the b is a positive value. The color is blue when the b is negative. L₁, a₁, and b₁ represent the data of the coating before the treatment, and L₂, a₂, and b₂ represent the data of the coating after the treatment. The chromatic difference in the coating before and after treatment (ΔE) is calculated [36] according to Formula (1):

$$\Delta E={\left[{\left(L_1-L_2\right)}^2+{\left(a_1-a_2\right)}^2+{\left(b_1-b_2\right)}^2\right]}^{1/2}$$

(1)

(3)

Gloss testing

The gloss tests were carried out with a DT60 gloss meter (Changzhou Dude Precision Instrument, Changzhou, China) according to GB/T4893.6-2013 [37]. After the calibration by pressing the test key, the lens cover of the gloss meter was taken off. The test sample was aligned with the test hole to record the coating gloss at 20°, 60°, and 85°. With the gloss data at a 60° incidence angle as a reference, the light loss rate of the film before and after adding microcapsules was calculated according to Formula (2). G_L stands for the light loss rate, G₀ for the gloss of the film without microcapsules, and G₁ for the gloss of the film with microcapsules.

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

(2)

(4)

Mechanical properties and roughness testing

According to GB/T 6739-2006, a QHQ-A portable paint film hardness tester (6H–6B pencils, Dongguan Huaguo Precision Instrument Co., Ltd., Dongguan, China) was used to gauge the hardness of the film on the metal substrate [38]. The values 6B–6H were from softest to hardest. The maximum hardness of the pencil was recorded as the coating’s hardness when there is no indentation on the coated surface. The adhesion grade of the coating was examined using the QFH-HG600 film scribing device from Shanghai Le’ao Test Instrument Co., Ltd., Shanghai, China. There were 6 degrees, with grade 0 denoting the coating’s best adhesion and grade 5 denoting the coating’s poorest adhesion. The impact strength of the coating was evaluated using a BEVS1601 paint film impactor tester (Guangzhou Xinyi Laboratory Equipment Co., Ltd., Guangzhou, China) in accordance with GB/T 1731-1993 [39]. The larger the number, the better the impact resistance. An SJ-210 precise roughness tester, which is offered by Shenzhen Fengteng Precision Instrument Co., Ltd., Shenzhen, China, was used to gauge the coating’s roughness.

(5)

Cold liquid resistance testing

Acetic acid, ethanol, coffee, and 15 wt.% NaCl solution were chosen as the cold liquid resistance testing agents of the coating to evaluate the coating’s cold liquid resistance in accordance with GB/T4893.1-2005. The coating’s center was chosen as the test area for its resistance to cold liquids. With tweezers, the filter paper from different testing agents was removed after being soaked for 5 s. It was placed on the coating surface, and then a glass cover was placed on the testing sample surface for 24 h. After removing the glass cover and the filter paper, the remaining liquid was wiped off. After the coating was completely dry, the coating surface was observed, and the chromatic difference and gloss of the testing area were tested to determine the coating’s level of cold liquid resistance. Grade 1 indicates that the testing area is not distinguished from other areas on the sample [40].

(6)

Ultraviolet (UV) photooxidation aging resistance testing

According to GB/T 1865-2009 [41], the artificially accelerated aging test (UV photooxidation) was performed in a UV weather resistance test chamber (Nanjing Environmental Testing Equipment Co., Ltd., Nanjing, China). The irradiance of the xenon light is 50 W/m². The film based on the metal substrate was placed in the UV test chamber. Every 24 h until the coating had no discoloration performance, the chromaticity value of the coated surface was checked.

3. Results and Discussion

3.1. Coating Morphology Characterization

The SEM morphology of the coatings on the metal substrates under three distinct finishing sequences is shown in Figure 2. The film surfaces of samples with “photochromic primer + thermochromic topcoat”, “thermochromic primer + photochromic topcoat”, and “photochromic and thermochromic coating” were all smooth. This is because the particle size of photochromic microcapsules is smaller and can be more evenly dispersed in the coating, according to Figure 3. The photochromic microcapsules and the thermochromic microcapsules can be uniformly distributed on the substrate surface with a concentration of 10 wt.%, so the coating surface is relatively smooth [42].

3.2. Infrared Spectrum Analysis

The infrared spectrum diagram of the coatings applied to a metal substrate in accordance with different finishing procedures is shown in Figure 4. The peak at 1131 cm⁻¹ belongs to the characteristic absorption peak of CH₃O. The peak at 3353 cm⁻¹ is the stretching vibration peak of the N–H bond and O–H bond [43]. The stretching vibration peak of the carbonyl group of second-order acyl appears at 1637 cm⁻¹. The peak at 2957 cm⁻¹ indicates the asymmetric stretching vibration of –CH₂–. These peaks are characteristic of UF resin [44]. The peak at 1730 cm⁻¹ is the characteristic peak of C=O in the ester bond. The peaks at 2849 and 2929 cm⁻¹ are the characteristic peaks of C–H in methyl and methylene groups. All the above are the characteristic peaks of alkyd resin coating [45]. The flexural vibration absorption peak in the C–H plane of the benzene ring appears at 697 cm⁻¹, which is the characteristic peak of styrene–maleic anhydride copolymer [46]. The typical peaks of the triazine ring of polyformaldehyde melamine’s stretching vibration appeared at 816 cm⁻¹ and 1343 cm⁻¹ [47].

3.3. Influence of Temperature on the Chromatic Difference of the Film

The chromaticity values of the coatings applied to the metal substrate in three different finishing sequences during the heating process are displayed in

Table 2. Chromaticity values of the film under different finishing sequences during the heating process.

Finishing Process	Chromaticity Values	Room Temperature	55 °C	60 °C	65 °C	70 °C	75 °C	80 °C	85 °C
photochromic primer + thermochromic topcoat	L	58.3	56.4	59.3	61.2	59.3	57.5	59.4	62.7
	a	−2.5	−1.2	−0.2	0.9	1.4	1.4	3.7	3.2
	b	10.8	17.7	22.6	24.3	27.6	29.4	33.0	32.6
	ΔE	-	7.3	12.1	14.2	17.3	19.1	23.0	22.8
thermochromic primer + photochromic topcoat	L	72.8	71.2	73.1	72.8	73.5	73.0	73.0	71.5
	a	1.7	0.9	1.0	1.3	2.3	2.8	3.6	2.9
	b	6.2	5.9	7.3	8.4	9.2	9.9	10.4	9.7
	ΔE	-	1.8	1.4	2.2	3.1	3.9	4.6	4.0
photochromic and thermochromic coating	L	57.7	60.6	61.3	61.1	60.7	62.5	62.1	61.5
	a	−1.9	−2.8	−2.3	−2.0	−1.0	−0.3	0.5	−1.0
	b	10.2	16.0	18.8	20.6	23.5	25.2	27.6	28.0
	ΔE	-	6.6	9.3	11.0	13.7	15.3	18.1	18.2

. The coatings’ chromatic difference changing trend is shown in Figure 5. Figure 6 illustrates the color change in the alkyd resin film with photochromic and thermochromic microcapsules during the heating process. On the metal substrate, the sample with photochromic primer and thermochromic topcoat underwent a significant color variation. At 60 °C, there were also noticeable color changes. The film totally changed its color, and the chromatic difference was at its greatest when the temperature approached 80 °C. The film’s chromatic difference was also noticeable when the two microcapsules were combined in the alkyd resin coating. The film with the smallest chromatic difference was one with the thermochromic primer and photochromic topcoat.

3.4. Influence of Visible Light on Chromatic Difference of the Film

Table 3. Photochromic chromaticity value of the film under different finishing sequences.

Finishing Process	Before Discoloration			After Discoloration			ΔE
	L1	a1	b1	L2	a2	b2
photochromic primer + thermochromic topcoat	58.3	−2.5	10.8	49.9	49.2	47.4	63.9
thermochromic primer + photochromic topcoat	72.8	1.7	6.2	8.6	32.2	15.6	71.7
photochromic and thermochromic coating	57.7	−1.9	10.2	2.0	−12.0	−2.1	62.5

displays the chromatic difference between the coatings applied to the metal substrate in various finishing sequences both before and after photochromism. Figure 7 shows a photochromic diagram of the alkyd resin film with photochromic microcapsules and thermochromic microcapsules after different exposure times of visible light. The chromatic difference before and after color change reaches 71.7 for the sample with thermochromic primer and photochromic topcoat. The photochromic performance of the alkyd resin coating mixed with thermochromic microcapsules and photochromic microcapsules was relatively the worst, with a chromatic difference of 62.5. Because the topcoat was a layer of coating with thermochromic microcapsules instead of a layer of coating with photochromic microcapsules, which affected the film’s light response chromaticity to a certain extent, the chromatic difference of the film sample with the photochromic primer is lower than the film sample with the photochromic microcapsules in the topcoat [48].

3.5. Influence of Finishing Process Parameters on Gloss of Coatings

Table 4. The gloss of the film under different finishing sequences.

Finishing Process	Gloss (GU)
	20°	60°	85°
photochromic primer + thermochromic topcoat	36.6	65.9	56.4
thermochromic primer + photochromic topcoat	4.0	15.6	33.5
photochromic and thermochromic coating	73.7	81.7	80.2

displays the gloss of the films applied to metal substrates using various finishing procedures. The alkyd resin coating containing photochromic microcapsules and thermochromic microcapsules had the highest gloss on the metal substrate, which was 81.7 GU at 60°. The film with thermochromic primer + photochromic topcoat had the lowest coating gloss (15.6 GU) at 60°. This is due to the fact that 10 wt.% of photochromic microcapsules was added to the topcoat, resulting in a large number of particles and an intensified diffuse reflection of the coating surface, which lowered the gloss of the coating [49].

3.6. Influence of Finishing Process Parameters on Mechanical Properties and Roughness of Coatings

Table 5. Mechanical properties and roughness of the film under different finishing sequences.

Finishing Process	Mechanical Properties			Roughness (μm)
	Hardness	Adhesion (Grade)	Impact Resistance (kg∙cm)
photochromic primer + thermochromic topcoat	2H	2	6.0	2.7
thermochromic primer + photochromic topcoat	2H	1	9.0	1.2
photochromic and thermochromic coating	2H	1	12.5	0.7

displays the mechanical characteristics and roughness of the coatings applied to the metal substrate in various finishing sequences. This table shows that the samples coated with three methods had high hardness of 2H. The alkyd resin coating with the mixed photochromic and thermochromic microcapsules and the coating with thermochromic microcapsules in the primer and photochromic microcapsules in the topcoat had an adherence of grade 1, which was better. For the sample with photochromic microcapsules in the primer and thermochromic microcapsules in the topcoat, grade 2 refers to poor adhesion. With an impact resistance of 12.5 kg∙cm, the alkyd resin coating combined with thermochromic microcapsules and photochromic microcapsules was the best. The impact resistance of the films created in the other two finishing sequences was worse. With a roughness of 0.7 μm, the alkyd resin coating surface combined with thermochromic microcapsules and photochromic microcapsules is the smoothest. The film surface of “photochromic primer + thermochromic topcoat” has a roughness of 2.7 μm, which was the highest. Previous experiments [34,48] indicated that the best concentration of the thermochromic microcapsules and the concentration of the photochromic microcapsules are both 10.0%. As a result, there are some similarities between the coating hardness of the thermochromic microcapsules in the topcoat and the photochromic microcapsules in the topcoat. Because the photochromic microcapsules have lower particle sizes than the thermochromic microcapsules, their coatings have stronger impact resistance than those of thermochromic microcapsules [50]. At the same concentration, the photochromic microcapsules have more particles, and the wall of the microcapsules can transfer the impact force more, thus improving the impact resistance.

3.7. Influence of Finishing Process Parameters on Cold Liquid Resistance of the Film

Table 6. The chromatic difference of the film before and after cold liquid resistance tests under different finishing sequences.

Finishing Process	Chromatic Difference for Cold Liquid Resistance
	Acetic Acid	Coffee	15 wt.% NaCl Solution	Ethanol
photochromic primer + thermochromic topcoat	2.5	3.9	2.4	0.8
thermochromic primer + photochromic topcoat	2.2	0.8	3.8	2.6
photochromic and thermochromic coating	2.2	2.8	2.9	2.7

,

Table 7. Gloss of the film before and after cold liquid resistance tests under different finishing sequences.

Finishing Process	Gloss before Cold Liquid Resistance Tests (GU)	Gloss after Cold Liquid Resistance Tests
		Acetic Acid (GU)	Coffee (GU)	15 wt.% NaCl Solution (GU)	Ethanol (GU)
photochromic primer + thermochromic topcoat	65.9	67.6	65.2	68.8	66.4
thermochromic primer + photochromic topcoat	15.6	14.0	16.8	15.2	15.7
photochromic and thermochromic coating	81.7	80.0	79.3	76.9	79.1

and

Table 8. Cold liquid resistance grade of the film under different finishing sequences.

Finishing Process	Cold Liquid Resistance (Grade)
	Acetic Acid	Coffee	15 wt.% NaCl Solution	Ethanol
photochromic primer + thermochromic topcoat	1	1	1	1
thermochromic primer + photochromic topcoat	1	1	1	1
photochromic and thermochromic coating	1	1	1	1

display the coatings’ chromatic differences, gloss levels, and degrees of cold liquid resistance after being applied in various finishing sequences to metal substrates. After the acetic acid, coffee, 15 wt.% NaCl solution, and ethanol cold liquid resistance tests, there was no noticeable color change in the coatings applied in the three finishing sequences, and their cold liquid resistance performance was all grade 1. The three finishing sequences did not appreciably alter the gloss of the coatings. In the coffee resistance tests, the coating color changed significantly, and the chromatic difference value became large. The greatest color change after coffee liquid resistance occurred when the metal substrate was applied with photochromic microcapsules in the primer and thermochromic microcapsules in the topcoat. This is due to the fact that the coating becomes rougher due to the increased particle size and concentration of the thermochromic microcapsules. The coating’s chromatic difference increased as a result of coffee’s ease of penetration into the coating [51]. The three methods did not appreciably alter the gloss of the coatings.

3.8. Influence of Finishing Process Parameters on UV Photooxidation Aging Resistance of the Film

The chromaticity values and chromatic difference of the coatings applied to the metal substrate using the best (photochromic and thermochromic coating) and control (photochromic primer + thermochromic topcoat) finishing sequences before and after UV photooxidation are shown in

Table 9. The chromatic difference of the film before and after UV photooxidation aging under different finishing sequences.

Finishing Process	Situation	L	a	b	ΔL	Δa	Δb	ΔE
photochromic primer + thermochromic topcoat	before aging	58.30	−2.48	10.75	−14.20	−4.78	−1.25	15.03
	after aging	72.50	2.30	12.00
photochromic and thermochromic coating	before aging	57.68	−1.93	10.15	1.18	1.58	−14.65	14.78
	after aging	56.50	−3.50	24.80

. The chromatic difference values of the sample with thermochromic microcapsules in the topcoat and the sample with mixed microcapsules in the coating were 15.03 and 14.78, respectively. It is clear that the coating with the mixed photochromic and thermochromic microcapsules had improved aging resistance.

4. Conclusions

Thermochromic and photochromic coatings were prepared by three methods: photochromic primer + thermochromic topcoat, thermochromic primer + photochromic topcoat, and photochromic and thermochromic coating. The coating’s comprehensive performance was the best on metal substrates when thermochromic and photochromic microcapsules were combined in the alkyd resin coating. At this time, the thermochromic temperature was 80 °C, the chromatic difference was 18.1, the photochromic chromatic difference was 57.9, the hardness was 2H, the grade of adhesion was 1, the impact resistance was 12.5 kg cm, the roughness was 0.7 μm, and the liquid resistance of grade was 1. These results can meet the unique needs of clients and establish the foundation to produce intelligent coatings. It was not discussed whether the compatibility between the two different coatings would affect the overall performance of the film, which is the goal of future research.