Effect of Two Types of Chitosan Thermochromic Microcapsules Prepared with Syringaldehyde and Sodium Tripolyphosphate Crosslinking Agents on the Surface Coating Performance of Basswood Board

Abstract

In order to investigate the effect of thermochromic microcapsules on the surface coating performance of basswood board, two types of microcapsules prepared with syringaldehyde and sodium tripolyphosphate crosslinking agents were added to a UV primer and coated on the surface of basswood board. The color-change effect of the surface coating on basswood board with microcapsules added with syringaldehyde as the crosslinking agent was better than that with microcapsules added with sodium tripolyphosphate as the crosslinking agent, and the color difference varied more significantly with temperature. The effect of the two types of microcapsules on the glossiness of the surface coating on basswood board was relatively weak. The glossiness of the surface coating on basswood board with microcapsules containing syringaldehyde as the crosslinking agent showed an overall increasing trend with the increase in microcapsules, and the change trend was relatively gentle. The glossiness of the surface coating on basswood board with microcapsules containing sodium tripolyphosphate as the crosslinking agent increased first and then decreased as the amount of microcapsules added increased. The addition of microcapsules with syringaldehyde as the crosslinking agent had no significant effect on the reflectance in the visible light band of the surface coating on basswood board. Among the two groups of samples, the hardness increase in the surface coating on basswood board with syringaldehyde as the crosslinking agent was more significant. The adhesion level of the coating on the surface of the basswood board with the two microcapsules did not change. Neither of the microcapsules had a significant effect on the impact resistance of the surface on basswood board. In the comprehensive analysis, the surface coating on basswood board with microcapsules added with syringaldehyde as the crosslinking agent at a content of 4.0% had better comprehensive performance, better surface morphology, better color-change effect, and moderate mechanical properties. The color difference was found to be 21.0 at 25 °C, the reflectivity was found to be 57.06%, the hardness was found to be 3H, the adhesion was found to be five, and the impact resistance was found to be three.

1. Introduction

Wood is the only renewable natural resource among the four major materials. Due to its unique texture and superior processing performance, it has always maintained a core position in the field of building decoration materials [1,2,3,4,5,6,7]. The improvement in people’s own taste has led to higher requirements for wood in terms of a visual sense. Thermochromic wood can not only provide people with wonderful visual effects but can also meet users’ personalized needs for decorative materials, so it has broad development potential in the field of furniture decoration [8,9,10,11,12,13]. Microencapsulation of color-changing materials can effectively expand the application range of thermochromic wood. In addition, it is necessary to treat the wood surface before use, and its color-changing characteristics mainly depend on the wood surface. Therefore, adding color-changing microcapsules into the coating of wood products is an efficient way of utilization [14,15]. Microcapsule technology can effectively prevent the core material from being affected by the outside world, improve the color-changing effect of the core material, and has become the main research object of color-changing coatings [16].

In microcapsule technology, choosing a suitable wall material is crucial, among which chitosan has attracted much attention due to its excellent performance. Chitosan is a light-yellow powder that is easily soluble in inorganic acids such as acetic acid and hydrochloric acid but not in water. It is derived from chitin, the second most abundant natural polymer in nature, making it a widely available and renewable biomass resource. By removing some acetyl groups from chitin, chitosan is produced, which can be considered an inexhaustible natural polymer material [17]. As a natural non-toxic material, chitosan is often employed in microcapsule technology, where it serves as a wall material to cover various materials and achieve specific functional outcomes. Furthermore, due to its excellent degradability and biocompatibility, chitosan is widely used in industries such as food, textiles, and medicine [18,19,20]. He et al. [21] synthesized photochromic microcapsules using spiropyran compounds as a core material and chitosan as a wall material to prepare photochromic hydrophobic fabrics. After 20 cycles of UV irradiation, it still had a photochromic effect, indicating that microencapsulation greatly improves the fatigue resistance of the photochromic dyes. Teng et al. [22] prepared microcapsules containing garlic essential oil (GEO) using chitosan grafted with gallic acid (GA), which enhanced the inhibition of nitrite-producing bacteria. Yin et al. [23] prepared chitosan microcapsules and seaweed salt solutions containing cinnamon essential oil, which can effectively inhibit the decrease in vitamin C content in mango and delay the appearance of mango respiratory peak. Thermochromic materials have the characteristic of having a temperature memory function and have great application potential in aerospace, military, anti-counterfeiting technology, construction, and other fields. In recent years, a variety of thermochromic materials have been prepared using different methods, with various discoloration mechanisms [24]. Zhu et al. [25] prepared thermochromic microcapsules using urea-formaldehyde resin as the wall material and thermochromic compounds as the core material by in situ polymerization and studied the thermochromic properties of the microcapsules in wood and wood coatings. The results showed that the microcapsules, when compounded with wood and coatings, also had good thermochromic properties and had broad application prospects in the preparation of intelligent materials. Hu et al. [26] developed an intelligent multifunctional wood material. During the coating process of medium-density fiberboard, thermochromic microcapsules were added to the coating. Microcapsules have a sensitive color-change phenomenon. The color of medium-density fiberboard can change between blue and brown at 20–29 °C.

Spray drying is a widely used and practical method for preparing microcapsules [27,28]. The drying process lasts for a short time, and the microcapsule emulsion can be directly dried into powder. The drying output is high, and the efficiency is high [29]. Previous studies have identified the optimal preparation conditions for the color-changing compound. Specifically, the optimal conditions were found to be a mass ratio of crystal violet lactone, bisphenol A, and decanol of 1:3:50, a reaction temperature of 50 °C, a reaction time of 1.5 h, and a stirring speed of 400 rpm. Under these conditions, the color-changing temperature of the compound closely matched real-life scenarios, the discoloration temperature range was moderate, and the solution appeared clearest when colorless. Based on the above results, the color-changing compound with this ratio was selected as the core material of the microcapsule. As one of the core components of the color-changing compound, bisphenol A plays a key role. However, since bisphenol A is widely considered to be a potentially toxic and carcinogenic compound, its safety in use deserves attention. Although the application of bisphenol A in coatings improves the color-changing performance, its potential health effects still need to be considered. In this experiment, bisphenol A was encapsulated in the microcapsule core material, and there was no free bisphenol A, thus ensuring the application safety of the microcapsules.

In the preparation process of microcapsules, a crosslinking agent, as a key chemical substance, can effectively connect polymers through their reaction, thereby forming a more stable material structure. In this experiment, chitosan was used as the wall material, and the color-changing compound was used as the core material. The chitosan-coated color-changing microcapsules were prepared by spray drying. The type of crosslinking agent was selected as the research variable to explore the effects of two crosslinking agents, syringaldehyde and sodium tripolyphosphate (STPP), on the performance of microcapsules. Thermochromic microcapsules with different mass fractions were added to UV coatings and coated on the surface of basswood boards. The color difference, optical and mechanical properties of the coatings were studied. The purpose was to make the coating containing microcapsules maintain the optical and mechanical properties of the original coating under the condition of obtaining a better color-changing effect. This provides a technical basis for the application of color-changing microcapsules and the preparation of color-changing coatings.

2. Materials and Methods

2.1. Test Materials

The materials required in this test are shown in

Table 1. List of test raw materials.

Test Material	Molecular Formula	Manufacturer
Crystal violet lactone	C26H29N3O2	Wuhan Huaxiang Biotechnology Co., Ltd., Wuhan, China
Bisphenol A	C15H26O2	Shanghai APB Chemical Reagent Co., Ltd., Shanghai, China
1-Decanol	C10H22O	Shanghai MacLean Biochemical Technology Co., Ltd., Shanghai, China
Chitosan	(C6H11NO4)n	Shanghai Sinopharm Reagent Co., Ltd., Shanghai, China
Acetic acid	C2H4O2	Shanghai Sinopharm Reagent Co., Ltd., Shanghai, China
(Z)-Sorbitan mono-9-octadecenoate (Span-80)	C24H44O6	Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China
NaOH	NaOH	Fuzhou Feijing Biotechnology Co., Ltd., Fuzhou, China
Syringaldehyde	C9H10O4	Shanghai Een Chemical Technology Co., Ltd., Shanghai, China
STPP	Na5P3O10	Shanghai MacLean Biochemical Technology Co., Ltd., Shanghai, China

. The size of the basswood board used in the test was 100 mm × 100 mm × 5 mm, and the UV primer was provided by Jiangsu Haitian Technology Co., Ltd., Jurong, China. The UV primer included epoxy acrylic resin, polyester acrylic resin, trihydroxy methacrylate, trimethyl methacrylate, leveling agent, photoinitiator 1173 (2-hydroxy-2-methylpropiophenone), defoamer, etc. The equipment used in this test is shown in

Table 2. Experimental equipment.

Test Machine	Machine Model	Manufacturer
Electronic balance	JCS-W	Yongkang Huanyu Weighing Equipment Co., Ltd., Yongkang, China
Heat-collecting constant-temperature heating magnetic stirrer	DF-101S	Shenzhen Dingxinyi Experimental Equipment Co., Ltd., Shenzhen, China
Ultrasonic emulsifier disperser	TL-650CT	Jiangsu Tianling Instrument Co., Ltd., Yancheng, China
Small spray dryer	JA-PWGZ100	Shenyang Jingao Instrument Technology Co., Ltd., Shenyang, China
UV curing machine	CF-11	Bai De hong En Co., Ltd., Huzhou, China
Temperature measuring gun	TN400	Nomi Electronic Technology Co., Ltd., Changzhou, China
Portable color-difference meter	SC-10	Shanghai Hechen Energy Technology Co., Ltd., Shanghai, China
Gloss meter	X-rite ci60	Shenzhen Lai Te Instrument Equipment Co., Ltd., Shenzhen, China
Scanning electron microscopy	Quanta-200	Thermo Fisher Tech., Inc., Shanghai, China
Fourier transform infrared spectrometer	VERTEX 80V	Bruke Co., Ltd., Karlsruhe, Germany
UV spectrophotometer	U-3900	Hitachi Scientific Instruments (Beijing) Co., Ltd., Beijing, China
Universal mechanical testing machine	AGS-X	Shimazu Factory, Kyoto, Japan
Fine roughness tester	J8-4C	Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China
Pencil hardness tester	HT-6510P	Quzhou Aipu Measuring Instrument Co., Ltd., Quzhou, China
Paint film impactor	QCJ-40	Quzhou Aipu Measuring Instrument Co., Ltd., Quzhou, China
Paint film adhesion tester	QFH-A	Quzhou Aipu Measuring Instrument Co., Ltd., Quzhou, China

.

2.2. Microcapsule Preparation Method

According to the parameters shown in

Table 3. Preparation parameters of the test.

Sample (#)	Crosslinking Agent	m(Crystal Violet Lactone)/m(Bisphenol A): m(1-Decanol)	m(Core Material)/m(Wall Material)	Temperature (°C)	Stirring Speed (rpm)
1	C9H10O4	1:3:50	3:1	60	600
2	Na5P3O10	1:3:50	3:1	60	600

, two crosslinking agents, syringaldehyde and STPP, were used to prepare chitosan thermochromic microcapsules, which were named 1# microcapsules and 2# microcapsules, respectively. “#” was the sample number unit. The amount of raw materials used to prepare chitosan thermochromic microcapsules is shown in

Table 4. Detailed list of test raw materials.

Sample (#)	Chitosan (g)	1% Acetic Acid (mL)	Color-Changing Compound (g)	Span-80 (g)	Distilled Water (g)	Syringaldehyde (g)	Anhydrous Ethanol (mL)	STPP (g)	Distilled Water (g)
1	1.8	180	5.4	0.3	5.7	3.6	10	-	-
2	1.8	180	5.4	0.3	5.7	-	-	4.5	85.5

.

(1) Preparation of wall material: 1.8 g of chitosan was weighed and dissolved in 180 mL of 1% acetic acid solution to obtain the chitosan solution. A magnetic stirrer was added to the beaker, and the beaker was placed in the water bath at a set speed of 600 rpm and a temperature of 60 °C until the chitosan powder was completely dissolved to obtain the chitosan wall material solution.

(2) Preparation of the core material: First, the temperature of the water bath was raised to 30 °C, and 67.5 g of decanol was accurately weighed into the beaker, and the beaker was heated in the water bath to heat the decanol to a molten state. Then 4.05 g of bisphenol A and 1.35 g of crystal violet lactone were added to the beaker, and the solution in the beaker was evenly stirred with a magnetic stirrer. After stirring evenly, the water bath was gradually heated to 50 °C and stirred at 400 rpm for 1.5 h to obtain the core emulsion. The core emulsion was cooled to room temperature, at which time the solution was colorless.

(3) Emulsification of the core material: 0.3 g of Span-80 and 5.7 g of distilled water were added in a beaker. After full stirring, an emulsifier solution with a mass fraction of 5% was obtained. The prepared emulsifier was dripped into 5.4 g of core material emulsion, the beaker was placed in the water bath, the temperature was adjusted to 60 °C, the rotating speed was 600 rpm, and the reaction time was carried out for 20 min. The stirred solution was placed in the ultrasonic emulsifier, and the ultrasonic treatment was carried out for 5 min to ensure that the emulsifier was evenly wrapped on the outer surface of the core material.

(4) Crosslinking reaction of microcapsules: The temperature of the water bath was set at 35 °C and the rotating speed was set at 600 rpm. The wall material solution was absorbed by a dropper and added to the core material emulsion. The core material and wall material solution were thoroughly mixed for 1 h. Then 0.5 mol/mL of NaOH solution was added to adjust the pH value of the solution to about 5, and the solution was placed in the ultrasonic emulsifier disperser. After 5 min of ultrasonic treatment, the crosslinking agent solution was added into the water bath for crosslinking reaction for 3 h. The temperature of the water bath was 60 °C, and the rotating speed was 600 rpm. Then, the obtained solution was spray-dried at the inlet temperature of 110 °C, the outlet temperature of 64 °C, and the feed rate of 100 mL/h. The dried powder was chitosan thermochromic microcapsule powder.

2.3. Preparation Method of the Surface Coating on Basswood Board

The coating process of the basswood board is the manual painting method. Before use, the basswood board was placed at room temperature with a relative humidity of 50.0% ± 5.0% for 7 d to make the moisture content reached about 14.5%. To sand the surface of the basswood until it was smooth and flat, 800 grit sandpaper was used, then a brush was used to remove loose powder. Microcapsules 1# and 2# were added to the UV primer at an addition amount of 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, and 7.0%, respectively, among which the UV primer without microcapsules was the blank control group. The total mass of each group of coatings was controlled to be 3.00 g. The corresponding mass of microcapsules and UV primer were weighed according to

Table 5. List of materials for coatings.

Microcapsule Content (%)	Microcapsule Quantity (g)	UV Primer Quantity (g)
0	0.00	3.00
1	0.03	2.97
2	0.06	2.94
3	0.09	2.91
4	0.12	2.88
5	0.15	2.85
7	0.21	2.79

. After mixing the microcapsules and UV primer evenly, the mixture was applied evenly on the surface of the basswood with a soft brush. Then the basswood board was placed on the conveyor belt of the UV curing machine, the conveying speed was adjusted to 0.05 m/s, and the curing time to 30 s. After the curing was completed, it stood for 1 min to allow the coating surface to cool to room temperature, and then the second sanding, coating, and curing were performed.

2.4. Testing and Characterization

2.4.1. Micromorphology Characterization

The microstructure of the paint film was characterized by optical microscopy and scanning electron microscopy [30].

2.4.2. Chemical Composition Test

The chemical composition of microcapsules and paint films was tested and characterized by infrared spectrometers. In the infrared test, the microcapsule powder was made into thin sheets by the powder press.

2.4.3. Optical Performance Test

(1) Color difference: According to the test method of GB/T11186.3-1989 [31], the portable color-difference meter was used for testing. After the colorimeter was calibrated, the test hole was placed on the coating to be tested, the test button was pressed, and the L, a, and b values were recorded. The L, a, and b values represent the values of the test sample after the temperature changes. L₀, a₀, and b₀ represent the values of the sample’s starting test temperature (−5 °C). Each set of data was measured three times, and the average value was taken [32,33,34]. Formula (1) for the color difference (ΔE) between two points is as follows:

$$\Delta E={[{(L-L_0)}^2+{(a-a_0)}^2+{\left(\right(b-b_0)}^2]}^{{\displaystyle \frac{1}{2}}}$$

(1)

(2) Glossiness: The test was conducted in accordance with GB/T4893.6-2013 “Tests for physical and chemical properties of furniture surface paint films Part 6: Gloss determination method” [35]. A portable gloss meter was used to conduct the test. After calibrating the machine, the test hole was aligned with the coating to be tested, and the gloss value of the coating on the screen at incidence angles of 20, 60, and 85 was recorded [36,37,38,39].

(3) Reflectivity: An ultraviolet spectrophotometer was used to test the reflectivity of the paint film within the visible light wavelength range. Reflectivity refers to the ratio of the intensity of reflected light to the intensity of incident light when a beam of light shines on an object and is usually expressed as a percentage [40,41,42].

2.4.4. Mechanical Properties Test

(1) Hardness: The test was conducted in accordance with GB/T6739-2006 “Paints and varnishes-Determination of film hardness by pencil method” [43]. A pencil hardness tester was used to conduct the test. Several 6H-6B pencils were prepared, each with a flat lead core of about 4 mm exposed. The pencil was inserted into the hardness tester, and the tip of the pencil was pressed against the paint layer. The tip of the pencil was parallel to the paint film and pushed at a speed of approximately 0.5 mm/s over a distance of 1 cm.

(2) Adhesion: The test was conducted according to the standard GB/T4893.4-2013 “Tests for physical and chemical properties of furniture surface paint films Part 4: Adhesion cross-cutting method” [44]. First, a grid tool was used to draw a right-angle grid horizontally and vertically. Then a tape was stuck on the grid, pressed, and then torn off, and the shedding of the coating was observed on the tape. The adhesion grade of the coating is divided into 0, 1, 2, 3, 4, and 5, and the corresponding paint film shedding area is 5%, 15%, 35%, 55%, and more than 60%, respectively.

(3) Impact resistance: According to GB/T4893.9-1992 “Furniture surface paint film impact resistance test” [45], a paint film impactor was used to conduct the test. A sample was placed in the horizontal base, the 1.0 kg impact block was dropped freely from a certain height to impact the sample, and the coating was observed for cracks and spalling. Before the experiment, the coating without microcapsules was tested, and the impact height was 10 mm, 25 mm, 50 mm, 100 mm, 200 mm, 400 mm. Each height was impacted five times from low to high until the impact pattern caused grade 5 cracks to stop. After testing, it was found that in the coating without microcapsules grade 5 cracks appeared when the height was 50 mm, so the height was set to 50 mm to test the impact resistance of each coating at this height. Each coating was impacted five times. By comparing the data, as in

Table 6. Grade of impact site.

Impact Resistance Level	Variation in Paint Film Surface
1	No visible changes.
2	There are no cracks on the paint film surface, but impact marks are visible.
3	There are slight cracks on the paint film surface, usually 1–2 ring cracks or arc cracks.
4	There are moderate to severe cracks on the paint film surface, usually 3–4 ring cracks or arc cracks.
5	The paint film surface is severely damaged, usually with more than 5 ring cracks, arc cracks, or paint film falling off.

, the average of the five impact levels was rounded to an integer. The average was the impact resistance level.

3. Results and Discussion

3.1. Morphological Analysis

3.1.1. Macroscopic Morphology Analysis

The color change of 1# microcapsule was shown in Figure 1. The color of the microcapsule after freezing (−5 °C) was dark green, changing the blue color of the original discoloration complex, which was preliminarily judged to be related to the crosslinking agent added. Syringaldehyde is a brownish-yellow substance, which exhibited a dark-green color when mixed with the discoloration compound. When the microcapsules were cooled to room temperature, the color change was still not obvious, so the heating measures were taken. When placed in an oven and heated to 35 °C, it gradually turned yellow-green. When heated to 45 °C, the yellow-green became lighter. After 55 °C, the color of the microcapsules no longer changed. When syringaldehyde was used as a crosslinking agent to react with the color-changing compound, the brown color of syringaldehyde may have masked the original color change of the compound or formed a stable color state during the discoloration process, rendering the color change insignificant.

The color change of 2# microcapsule is shown in Figure 2. After freezing (−5 °C), the microcapsules turned light yellow, changing the blue color of the original color-changing compound. Preliminary judgment was that this was related to the added crosslinking agent. STPP is a white substance, which appeared light yellow when mixed with a color-changing compound. When the microcapsules were cooled to room temperature, the color did not change. The temperature was raised by placing the microcapsules in an oven and heating them to 55 °C. During the entire heating process, the color of the microcapsules did not change significantly, which may be because the crosslinking agent STPP used was less sensitive to temperature changes. In addition, STPP may have formed a relatively stable chemical structure with chitosan, further inhibiting the effect of temperature changes on the color of the microcapsules.

3.1.2. Microtopography Analysis

Figure 3 shows SEM images of 1# and 2# microcapsules. Spherical microcapsules with relatively uniform particle size and high yield were successfully produced by spray drying. Upon comparing the two SEM images, it can be clearly seen that there are large pieces of adhered and irregularly shaped materials in the 1# microcapsule, and the particle size distribution varies greatly. Although the microcapsules also appear spherical, there is obvious adhesion and agglomeration. The particle size distribution of 2# microcapsules is relatively uniform. The microcapsules are spherical, but there is an obvious agglomeration phenomenon, and the surface of the microcapsules is loose and porous. Both microcapsules showed a severe aggregation, which may be related to the wall material and the crosslinking between chitosan and aldehyde groups in aqueous medium.

The microscopic morphology of the paint film with added 1# microcapsules is shown in Figure 4. When the content of the microcapsules was 0%, the surface of the paint film was relatively flat. When the content of microcapsules was 1.0%, the surface of the paint film had a slightly raised granular sense. When the content was 4.0%, the particle sense of the paint film surface increased. When the content reached 7.0%, due to the aggregation of a small amount of microcapsules in the paint, the paint film presented a small area of raised wrinkles. This showed that when the content of the 1# microcapsules was low, there was little effect on the smoothness of the coating. The micromorphology of the paint film with the added 2# microcapsule is shown in Figure 5. When the content of 2# microcapsule was 0%, the surface of the paint film was relatively flat. When the content of 2# microcapsules was 1.0%, the convexity on the paint film surface was small. When the content of 2# microcapsule was 4.0% and 7.0%, the wrinkles and granularity on the surface of the paint film were stronger. This was because the 2# microcapsule was unevenly dispersed in the paint, forming agglomerates, resulting in larger particles appearing on the surface of the paint film. When the content of 2# microcapsule in the paint film was 4.0% or above, it would have a negative impact on the surface morphology of the paint film.

3.2. Chemical Composition Analysis

Figure 6 shows the infrared spectra of the blank paint film and the paint film with two kinds of microcapsules added. The absorption peak at 1721 cm⁻¹ was the absorption peak of C=O in the paint [46]. Due to the strengthening of intermolecular and intramolecular hydrogen bonds, the vibration frequency moved to a lower wave number, the peak area became larger, and the broad peak at 3449 cm⁻¹ was attributed to the overlapping peaks of –OH and N–H, indicating the formation of hydrogen bonds between chitosan-syringaldehyde microcapsules. The characteristic absorption peak of –RC=N– appeared at 1721 cm⁻¹, indicating that the amino group of chitosan and the aldehyde group of syringaldehyde form a Schiff base [47,48]. The characteristic peaks of the thermochromic complex also appeared in the infrared spectrum of the microcapsule. The 2967 cm⁻¹ was attributed to the stretching vibration peak of C–CH₃ in bisphenol A. The ester carbonyl C=O absorption peak of the non-lactone ring structure appeared at 1610 cm⁻¹, and 1183 cm⁻¹ corresponded to the symmetrical stretching absorption peak of the carboxylate, proving that the lactone ring in the molecule is open to form a conjugated chromogenic structure [49]. This proves that after adding the prepared 1# microcapsule to the UV primer, the wall material and core material components belonging to the 1# microcapsule still exist, and there is no chemical reaction between the 1# microcapsule and the UV primer. Although the chitosan content in the two formulations was the same, there was no obvious signal in the FTIR spectrum of the chitosan coated with 2# microcapsules. This phenomenon may have occurred due to the thick coating layer of the 2# microcapsules, which obscured the FTIR characteristic peak of chitosan. Additionally, STPP, used as a crosslinking agent, may have interfered with the FTIR spectrum, affecting the infrared absorption characteristics of chitosan.

3.3. Analysis of the Influence of Microcapsule Content on the Optical Properties of Basswood Surface Coating

3.3.1. Effect of Microcapsule Content on Color Difference of Surface Coating on Basswood Board

The effects of different contents of microcapsules on the color difference of the surface coating on basswood board are shown in

Table 7. The color difference of the surface coating on basswood board with 1# microcapsule at different temperatures.

Microcapsule Content (%)	Colorimetric Parameters	−5 °C	0 °C	5 °C	10 °C	15 °C	20 °C	25 °C	30 °C	35 °C	40 °C	45 °C	50 °C	55 °C
0.0	L	81.8	81.9	82.2	82.4	81.9	81.8	81.9	82.0	82.8	82.4	82.6	82.8	81.8
	a	6.8	6.9	6.9	7.0	7.1	7.0	7.0	6.8	7.0	6.9	6.9	7.0	6.9
	b	29.7	29.8	29.9	30.1	29.9	29.8	29.8	30.1	30.1	29.9	29.9	30.0	30.5
	ΔE	-	0.2	0.5	0.8	0.4	0.2	0.2	0.5	1.1	0.6	0.8	1.1	0.8
1.0	L	82.8	82.6	81.3	81.0	81.7	81.2	79.3	80.3	83.2	81.1	81.2	81.4	82.4
	a	6.2	6.0	5.7	6.4	6.1	5.6	7.2	7.1	5.6	5.8	6.3	7.0	6.0
	b	41.6	41.6	41.4	40.4	42.5	41.7	38.9	38.7	39.5	39.9	41.0	42.1	40.5
	ΔE	-	0.3	1.6	2.2	1.6	1.7	4.5	3.9	2.1	2.5	1.7	1.7	1.2
2.0	L	78.2	80.4	71.5	72.8	71.1	78.7	72.2	71.7	71.2	71.3	71.9	71.7	73.0
	a	8.8	7.2	10.2	9.2	9.6	7.9	10.1	8.2	9.2	9.5	10.6	8.7	9.2
	b	42.1	43.9	38.4	37.0	40.8	45.3	37.2	36.3	36.4	35.2	37.9	38.0	37.9
	ΔE	-	3.3	6.9	7.4	7.3	3.4	7.3	8.7	9.0	9.8	7.8	7.7	6.7
3.0	L	77.9	78.5	77.7	72.7	71.8	71.4	69.4	68.8	69.9	71.9	68.4	68.4	69.3
	a	5.4	5.6	4.9	6.9	6.8	7.3	9.4	10.4	11.8	6.4	6.7	7.3	8.3
	b	48.8	48.7	48.9	42.4	44.9	46.9	41.8	42.0	40.8	40.2	40.7	40.3	42.5
	ΔE	-	0.6	0.6	8.4	7.4	7.0	12.5	8.5	13.0	10.5	12.6	12.9	11.1
4.0	L	82.0	82.1	74.8	74.4	80.6	73.0	67.9	68.2	68.5	69.0	67.9	72.5	72.8
	a	5.9	5.3	5.7	5.7	5.4	7.7	7.0	6.9	9.3	11.9	7.2	7.1	5.7
	b	54.9	52.7	54.3	53.9	54.1	47.2	39.6	38.2	39.7	40.2	39.6	42.3	42.1
	ΔE	-	2.3	7.2	7.7	1.7	12.0	21.0	21.7	17.0	19.8	20.9	15.8	15.8
5.0	L	77.8	76.2	78.7	73.9	74.5	81.7	75.4	75.6	75.2	76.6	75.4	78.0	76.1
	a	4.8	4.9	4.4	5.6	6.7	4.2	7.3	7.0	7.3	7.2	7.8	8.3	12.8
	b	56.6	56.6	56.7	55.5	55.2	59.6	56.3	56.3	55.2	58.7	57.2	59.6	58.9
	ΔE	-	1.6	1.5	4.2	4.1	5.0	6.5	3.1	3.9	3.4	3.9	4.8	8.5
7.0	L	77.1	76.7	74.8	71.4	71.5	70.3	69.1	67.6	67.4	68.4	67.5	67.3	68.6
	a	7.2	6.9	7.5	8.2	8.2	9.3	7.3	8.7	8.3	8.0	9.5	7.9	9.1
	b	61.1	61.0	60.9	55.1	54.8	58.4	54.1	52.7	52.9	54.4	52.4	52.8	54.8
	ΔE	-	0.5	2.3	8.3	8.5	7.6	10.6	12.8	12.8	11.0	13.1	12.6	10.8

and

Table 8. The color difference of the surface coating on basswood board with 2# microcapsule at different temperatures.

Microcapsule Content (%)	Colorimetric Parameters	−5 °C	0 °C	5 °C	10 °C	15 °C	20 °C	25 °C	30 °C	35 °C	40 °C	45 °C	50 °C	55 °C
0.0	L	81.8	81.9	82.2	82.4	81.9	81.8	81.9	82.0	82.8	82.4	82.6	82.8	81.8
	a	6.8	6.9	6.9	7.0	7.1	7.0	7.0	6.8	7.0	6.9	6.9	7.0	6.9
	b	29.7	29.8	29.9	30.1	29.9	29.8	29.8	30.1	30.1	29.9	29.9	30.0	30.5
	ΔE	-	0.2	0.5	0.8	0.4	0.2	0.2	0.5	1.1	0.6	0.8	1.1	0.8
1.0	L	73.0	74.6	69.0	73.0	73.8	69.5	69.8	68.3	68.8	68.7	69.4	71.0	72.1
	a	8.1	8.0	9.0	8.3	8.7	8.4	8.0	8.1	9.2	9.4	6.8	8.4	9.4
	b	21.3	21.5	21.2	22.2	23.1	19.3	20.9	19.7	18.6	20.3	20.8	21.6	21.0
	ΔE	-	1.6	4.1	0.9	1.0	4.0	3.5	5.0	5.1	4.6	3.9	2.0	1.7
2.0	L	74.9	75.3	74.7	74.8	73.8	82.7	78.4	79.3	78.1	78.5	78.7	84.6	78.6
	a	8.6	8.7	8.3	8.1	9.8	6.3	7.1	6.5	5.7	6.4	5.9	5.9	7.2
	b	28.8	28.8	28.0	33.1	28.0	32.7	29.6	30.1	28.8	26.9	29.1	32.4	28.9
	ΔE	-	0.4	1.0	4.3	1.7	9.0	3.9	5.1	4.3	4.6	2.8	10.7	3.8
3.0	L	72.1	71.8	70.7	73.9	74.5	74.2	70.9	71.7	71.1	71.5	71.9	70.7	71.1
	a	10.9	13.8	10.1	9.2	9.2	9.5	10.3	9.8	10.7	14.8	12.1	9.0	10.0
	b	22.0	23.1	20.9	20.4	23.1	24.0	21.4	21.1	21.8	22.7	22.8	19.6	21.1
	ΔE	-	3.1	2.0	3.0	3.1	3.2	1.5	1.5	0.5	4.0	1.5	3.4	1.6
4.0	L	76.1	80.8	82.0	74.1	80.2	81.3	78.1	70.7	71.8	74.1	73.7	73.0	73.7
	a	6.3	6.2	5.3	8.7	6.9	5.4	8.1	7.6	7.7	8.1	8.9	8.4	9.4
	b	31.1	30.9	32.3	25.7	31.6	32.3	28.0	25.6	28.8	24.0	23.3	23.2	24.4
	ΔE	-	1.8	6.1	6.2	4.1	5.4	4.1	7.8	5.1	7.6	5.6	8.8	7.8
5.0	L	76.6	84.2	76.2	76.4	77.1	83.8	72.6	69.8	71.5	75.3	70.0	71.4	75.3
	a	6.9	5.5	7.7	7.8	7.9	8.8	8.2	8.9	10.7	8.7	11.0	7.1	9.0
	b	22.6	21.5	20.7	23.6	25.7	21.6	23.7	21.4	25.6	25.5	23.0	22.7	25.6
	ΔE	-	7.7	2.2	1.4	3.3	7.5	4.4	7.2	7.3	3.7	7.8	5.2	3.9
7.0	L	74.6	73.0	74.8	78.3	71.8	71.8	73.9	72.7	74.2	77.2	76.9	73.1	78.9
	a	9.1	10.2	8.4	8.6	7.6	10.2	15.4	9.0	13.0	7.9	6.2	6.4	4.6
	b	34.1	32.6	29.4	35.4	34.6	31.2	33.1	31.1	33.3	33.8	40.0	31.8	36.1
	ΔE	-	2.5	4.8	4.0	3.2	4.2	6.4	3.7	4.0	3.4	7.0	3.9	6.5

. The L value represents the lightness or darkness of the measured coating color, and the larger the L value, the brighter the color. The a value represents the red-green value. A positive a value means the color is reddish, and a negative a value means the color is greenish. The b value represents the yellow-blue value. A positive b value represents a yellowish color, and a negative b value represents a bluish color. Since the 1# microcapsules prepared by the addition of syringaldehyde crosslinking agent were yellow-green, the b value representing yellowness tends to increase with the increase of microcapsule content. The L value representing lightness and darkness, and the a value representing red and green values, changed less. The surface coating on basswood board with 1# microcapsules mostly changed color significantly at around 25 °C, which meant it started to change color. When the mass fraction of 1# microcapsule added was 4%, the color difference changed most significantly with the change of temperature, and the color difference was 21.0 at a temperature of 25 °C. As the temperature rose, the color-difference value of adding 2# microcapsules at the same mass fraction fluctuated within a certain range without obvious changes. Because the color of 2# microcapsules prepared by adding the STPP crosslinking agent was light yellow and lighter in color, it had little effect on the color of the surface coating on basswood board. When the mass fraction of 2# microcapsules was 2%, the color difference changed most significantly with the change of temperature, and the color difference was 10.7 at a temperature of 50 °C. The individual color-difference values in Table 8 were relatively large, probably because the paint film was transparent at the experimental temperature (−5 °C–55 °C), and the original wood color of the basswood board affected the experimental data. Moreover, the color of different parts of the same wooden board was uneven, which caused the measured individual color-difference values to be too large compared to other color-difference values. In previous studies, the application of different microcapsule materials in thermochromic coatings showed various color-difference effects. For example, Li et al. [50] studied color-changing microcapsules using urea-formaldehyde resin as wall material and found that the color-difference change was most significant at 80 °C, with the maximum color difference reaching 15.80. However, in this study, the color difference of the 1# microcapsules using chitosan as the wall material reached 21.0 at 25 °C, which shows that chitosan, as a green microcapsule wall material, has higher discoloration sensitivity at relatively low temperatures and is especially suitable for application scenarios near room temperature.

Significance analysis was performed on the data in Table 7 and Table 8. Based on the coating-color-difference data obtained at a temperature of 25 °C, the non-repeated two-way ANOVA method was used for significance analysis. Three values of the F, the p-value_, and the F_crit were obtained, respectively. It should be noted that the F represents the test statistic, a statistical measure used in hypothesis testing calculations. The p-value represents the significance level, which evaluates the range and interval of the overall parameter and evaluates the probability that the experiment may occur. The F_crit is the critical value of F at the corresponding significance level. Among them, F > F_crit indicates that there is a difference between the two sets of data. F < F_crit means there is no difference between the two sets of data. The standard for determining significant differences is 0.01 < p-value < 0.05, indicating a significant difference. A p-value ≤ 0.01 means the difference is extremely significant. p-value > 0.05 means not significant. As shown in

Table 9. Significance analysis of color difference.

Difference Source	SS	df	MS	F	p-Value	Fcrit
content	176.0271429	6	29.33785714	1.541605675	0.306188138	4.283865714
crosslinking agent	106.4257143	1	106.4257143	5.592313178	0.055915742	5.987377607
error	114.1842857	6	19.03071429
total	396.6371429	13

, the results obtained according to the above method are F < F_crit and p-value > 0.05, indicating that the content of microcapsules in the paint film has no significant impact on the color difference of the coating. This may be because the change in microcapsule content has not reached the critical point of producing significant color difference. Although the change in content has a certain effect, it is not enough to show a statistically significant difference. Although the crosslinking agent species did not reach the threshold of p-value < 0.05 in the significance analysis, the p-value close to 0.05 indicates that it may have an impact on the coating performance.

3.3.2. Effect of Microcapsule Content on the Glossiness of Surface Coating on Basswood Board

The changes in the glossiness of the surface coating on basswood board with different addition amounts of 1# microcapsules and 2# microcapsules added to the UV primer are shown in

Table 10. Gloss of the coating for different microcapsule content.

Sample	Microcapsule Content (%)	Gloss at 20° (GU)	Gloss at 60°(GU)	Gloss at 85° (GU)
Basswood surface coating with 1# microcapsules added	0.0	9.3	37.7	34.8
	1.0	8.6	36.0	38.3
	2.0	9.6	38.7	45.9
	3.0	14.8	46.5	43.5
	4.0	12.7	45	44.1
	5.0	10.4	40.2	43.8
	7.0	17.1	49.2	55.5
Basswood surface coating with 2# microcapsules added	0.0	9.3	37.7	34.8
	1.0	25.3	64.9	68.2
	2.0	36.0	76.9	81.2
	3.0	32.0	66.2	69.3
	4.0	22.9	61.4	60.0
	5.0	17.9	60.0	63.5
	7.0	16.3	53.2	67.4

. The glossiness of the surface coating on basswood board with 1# microcapsule added increased as the amount of microcapsule added increased, and the change trend was relatively gentle. The glossiness of the surface coating on basswood board with 2# microcapsules added showed an overall trend of first increasing and then decreasing as the amount of microcapsules added increased. When the microcapsule addition amount was 3%, the glossiness of the coating was the highest. This was due to the microcapsule powder was relatively coarse. When the microcapsule content was between 3% and 7%, as the microcapsule content increased, the particle size of the coating increased, the diffuse reflection of the coating surface increased, and the glossiness of the coating was reduced. Cellulose was unevenly distributed in the microcapsule wall material and was more prone to agglomeration. However, because it was added to the UV primer, its impact on the overall paint film was relatively small. In summary, the effects of 1# and 2# microcapsules on the gloss of the coating were relatively weak.

3.3.3. Effect of Microcapsule Content on the Reflectivity of the Surface Coating on Basswood Board

The reflectivity values and reflectivity curves in the visible light band on the surface of basswood with different contents of 1# microcapsules added are shown in

Table 11. The effect of adding 1# microcapsule on the surface of basswood board.

Sample	Microcapsule Content (%)	Visible Light Reflectance (%)
Basswood surface coating with 1# microcapsules added	0	64.57
	1.0	60.33
	2.0	58.66
	4.0	57.06
	5.0	59.45
	7.0	54.64

and Figure 7, respectively. The reflectivity of the visible light band on the surface on basswood board with different contents of 1# microcapsules added slightly decreased with the increase in microcapsule content. However, the spacing between each group of curves was very close, and the numerical differences were small. This showed that the influence of 1# microcapsules on the visible light band reflectivity of the surface coating on basswood board was not significant.

3.4. Analysis of the Effect of Microcapsule Content on the Mechanical Properties of Surface Coating on Basswood Board

The effect of microcapsule content on the mechanical properties of surface coating on basswood board is shown in

Table 12. Effect of microcapsule content on mechanical properties of coating surface.

Sample	Microcapsule Content (%)	Hardness	Adhesion Level (Level)	Impact Resistance Level (Level)
Basswood surface coating with 1# microcapsules added	0.0	2H	5	3
	1.0	2H	5	4
	2.0	2H	5	4
	3.0	4H	5	3
	4.0	3H	5	3
	5.0	H	5	4
	7.0	H	5	4
Basswood surface coating with 2# microcapsules added	0.0	2H	5	3
	1.0	4H	5	3
	2.0	3B	5	4
	3.0	H	5	3
	4.0	4H	5	3
	5.0	2H	5	4
	7.0	H	5	4

. Among the two groups of samples, the hardness of the surface coating on basswood board with 1# microcapsules added increased most significantly. When the addition amount was 3.0%, the coating hardness reached 4H. When the microcapsule content was 7.0%, the hardness of both coatings decreased slightly, and the hardness was H. According to the data in the Table 12, the adhesion level of the surface coating on basswood board with different contents of the two microcapsules added was 5, which was the same as the adhesion level of the surface coating on basswood board without adding microcapsules, indicating that the adhesion of the coating had not changed. This showed that when the content of the two microcapsules was 7.0% or less, it would not affect the adhesion of the coating on the surface of basswood boards and could be used in practice. The impact resistance level of the surface of basswood without microcapsules was level 3, which means that there were one to two circles of slight cracks on the coating surface. The impact resistance level of the surface coating on basswood board with the addition of the two types of microcapsules remained at levels 3 and 4, which showed that neither of the two microcapsules would have a significant impact on the impact resistance of the basswood board surface.

4. Conclusions

Two kinds of chitosan thermochromic microcapsules prepared with syringaldehyde and STPP crosslinking agent were added into UV primer and applied on the surface of basswood board. The morphology, chemical composition, optical properties, and mechanical properties of the surface coating on basswood boards were tested. The results showed that the 1# microcapsule coating prepared with syringaldehyde had a better color-changing effect than the 2# microcapsule coating prepared with STPP, and the color difference changed more obviously with temperature. When the content of microcapsules with syringaldehyde as the crosslinking agent was 4.0% and the temperature was 25 °C, the color difference of the surface coating on basswood board was the largest, reaching 21.0. With the increase in microcapsule content, the surface gloss of the basswood surface coating with microcapsules using syringaldehyde as the crosslinking agent showed an overall upward trend, and the change trend was relatively gentle. The reflectance of the visible light band of the coating decreased slightly. The hardness of the coating was significantly improved, with little change in adhesion level and impact resistance. With the increase in microcapsule content, the surface glossiness of the surface coating on basswood board with microcapsules using STPP as the crosslinking agent showed an overall trend of first increasing and then decreasing. The adhesion level and impact resistance of the coating changed little. This study demonstrated the potential of chitosan as a green and environmentally friendly material in thermochromic microcapsules, providing a new perspective for the application of eco-friendly materials in the field of coatings. Future research should be the further optimization of the preparation process conditions of microcapsules to enhance their color-changing effect and overall performance, thereby expanding their potential applications in coatings and other related fields.