Influence of Two Types of Microcapsule Composites on the Performance of Thermochromic UV Coatings on Bleached Poplar Wood Surfaces

Abstract

To meet the growing demand for intelligent surfaces in furniture and interior design, this study developed thermochromic UV coatings for bleached poplar. While conventional UV coatings are valued for their ecofriendliness and rapid curing, their functionality remains limited; integrating thermochromic capability offers a highly promising solution. We examined how the combination of two microcapsule systems (UF@TS and UF@TS-R) influenced the performance of UV coatings on bleached poplar by applying a two-primer/two-topcoat protocol with varied microcapsule loadings to impart color-changing behavior. The effects were then analyzed from multiple perspectives—type, application layer, and concentration gradient—covering optical and mechanical properties as well as thermochromic response. Results indicated that the optimum performance was achieved when UF@TS was incorporated into the UV topcoat and UF@TS-R into the UV primer at specific mass concentrations. The resulting coating delivered temperature-responsive color variation, providing both theoretical and technical support for developing high-value-added UV finishes for wooden furniture and advancing the use of fast-growing timber in high-end applications.

1. Introduction

As an environmentally friendly coating system, UV coatings have technical characteristics such as low energy consumption and high efficiency [1,2,3]. UV coatings have already been modified to have properties such as self–healing [4], antibacterial effects [5,6,7], and flame retardance [8,9,10]. However, the single-function nature of traditional UV coatings limits their application expansion in the smart home field [11]. To address this gap, thermochromic systems based on external stimuli such as heat have emerged as promising solutions [12], while advanced coating technologies have also provided new approaches [13]. As society, science, and technology progress, because wood is a versatile and nontoxic material, it has been widely used in various fields of life [14,15]. Fast–growing poplar, with convenient processing and strong adaptability, is widely planted all over the world [16,17]. However, fast-growing poplar is generally characterized by inherent defects such as insufficient mechanical strength [18,19,20], though novel joint structures may compensate for this limitation [21,22]. Its traditional applications are predominantly in low-value-added fields such as packaging materials [23,24]. Enhancing the physicochemical properties of fast–growing poplar through surface functionalization modification techniques [25,26], including grinding pretreatment [27], has become the key approach to expanding its applications in home furnishings. The development of intelligent materials science has driven the innovation of environmentally responsive functional coatings [28,29,30]. To date, color–changing systems based on external stimuli such as light, heat, and electricity have been applied in various fields [31]. Thermochromic materials with controllable reactions and simple conditions have attracted much attention [32,33,34,35,36].

In order to improve the performance of UV coatings and endow them with intelligent characteristics, Li et al. prepared a modified silicone–acrylate prepolymer, which combined the advantages of UV curing and moisture curing, featuring a rapid curing rate and overcoming the issues of UV–curing shadow areas and thick coatings [37]. Sen et al. developed coating materials with antibacterial properties without the use of antibacterial agents. They synthesized two different quaternary imidazole compounds to modify silica nanoparticles and prepared UV–curable coatings based on the thiolene reaction mechanism; these coatings exhibited certain antibacterial properties [38]. Hong et al. synthesized a kind of UV–curable urethane acrylate oligomer (UV–UAO) containing ammonium salts. Thus, without the need for cumbersome and complicated synthesis and fabrication processes, UV–curable antifog coatings could be obtained [39]. While most studies have focused on improving curing rate or antibacterial properties, research on thermochromic UV coatings for wood remains scarce. Herein, a new stratified microcapsule strategy is proposed.

Microcapsules, with their core–shell structure, are capable of protecting the core materials with specific functions from environmental erosion. The combination of thermochromic microcapsules and UV resin matrix can build a new type of coating system with both rapid curing and intelligent imaging functions [40]. The influence of composites of urea–formaldehyde–coated ternary–system thermochromic microcapsules (UF@TS) and urea–formaldehyde–coated cationic red-ternary-system microcapsules (UF@TS-R) on the optical, mechanical, thermochromic, and aging properties of UV coatings was examined. Moreover, a comparison was made between these properties and those of UV coatings containing only UF@TS-R. The results provided theoretical and technical support for the development of high-value-added UV coatings for wooden furniture surfaces.

2. Materials and Methods

2.1. Materials and Equipment

A detailed list of materials and equipment for coating preparation is shown in

Table 1. List of materials.

Name	Molecular Formula	CAS No.	Manufacturer
Crystal violet lactone (CVL)	C26H29N3O2	1552-42-7	Wuhan Huaxiang Biotechnology Co., Ltd., Wuhan, China
Bisphenol A	C15H16O2	80-05-7	Shanghai Haiyu Chemical Co., Ltd., Shanghai, China
N-decyl alcohol	C10H22O	112-30-1	Shanghai MacLean Biochemical Technology Co., Ltd., Shanghai, China
Urea	CH4N2O	57-13-6	Guangzhou Suixin Chemical Co., Ltd., Guangzhou, China
37% formaldehyde solution	CH2O	50-00-0	Shandong Xinjiucheng Chemical Technology Co., Ltd., Jinan, China
Trolamine	C6H15NO3	102-71-6	Shandong Chengkai New Materials Co., Ltd., Linyi, China
Monohydrate citric acid	C6H10O8	5949-29-1	Jinan Xiaoshi Chemical Co., Ltd., Jinan, China
Gum acacia	N/A	9000-01-5	Nanjing Jinyou Biotechnology Co., Ltd., Nanjing, China
Triton X-100	C16H26O2	9002-93-1	Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China
Hydrogen peroxide	H2O2	7722-94-1	Hangzhou Kaseng Chemical Technology Co., Ltd., Hangzhou, China
Sodium hydroxide	NaOH	1310-74-2	Shandong Taixi Chemical Co., Ltd., Jinan, China
Sodium silicate	Na2SiO3	6834-92-0	Shandong Zhengyu Chemical Technology Co., Ltd., Jinan, China

. The coating used in the experiments was a UV primer, which was provided by Jiangsu Haitian Technology Co., Ltd., Zhenjiang, China. Its main constituents encompassed epoxy acrylate resin, polyester acrylate resin, trimethylolpropane triacrylate (TPGDA), trimethylolpropane trimethacrylate (TMPTA), photoinitiator, defoamer, and leveling agent. The solid content of the UV primer was in excess of 98.0%.

Commercially standardized poplar wood (Populus L.) was procured from Shanghai Lingyu Wood Industry Co., Ltd., Shanghai, China to ensure batch uniformity such that moisture content and color differences among all samples remained within a narrow range. The poplar wood samples had dimensions of 50 mm × 50 mm × 3 mm. Poplar heartwood was utilized in this experiment. Compared with sapwood, poplar heartwood exhibits a deeper, mottled color, which better revealed the effects of bleaching treatment on the samples. The primary constituents of the topcoat included polyurethane acrylate resin, propylene glycol diacrylate, neopentyl glycol diacrylate, photoinitiator, matting powder, defoamer, and leveling agent, among others. This topcoat was provided by Jiangsu Haitian Technology Co., Ltd., Zhenjiang, China, with a solid content exceeding 98.0%. A list of test equipment is shown in

Table 2. List of test equipment.

Equipment	Model	Manufacturer
Pencil hardness tester	HT-6510P	Shenzhen Junda Times Instrument Co., Ltd., Shenzhen, China
Coating impactor	QCJ-50	Shanghai Meiyu Instrument Technology Co., Ltd., Shanghai, China
Coating adhesion gripper	QFH-A	Shenzhen Junda Times Instrument Co., Ltd., Shenzhen, China
Constant temperature water bath pot	DF-101S	Shuyang Hongguan Riyi E-commerce Co., Ltd., Shuyang, China
Scanning electron microscope (SEM)	Quanta-200	Thermo Fisher Scientific, Waltham, MA, USA
Gloss meter	HG268	Shenzhen Three Enshi technology Co., Ltd., Shenzhen, China
Spectrocolorimeter	SEGT-J	Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China
Fourier infrared spectrometer (FTIR)	VERTEX 80V	Bruker GMBH., Billerica, MA, USA
Ultraviolet spectrophotometer	U-3900/3900H	Hitachi Scientific Instruments (Beijing) Co., Ltd., Beijing, China
Roughness meter	J8-4C	Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China
Universal mechanical testing machine	AG-IC10OKN	Shimadzu Production House, Kyoto, Japan

.

2.2. Microcapsule Preparation Method and Experimental Design

2.2.1. Method of UF@TS Preparation

Microcapsule structure terminology: The term “wall material” refers to the urea–formaldehyde (UF) resin shell that encapuslates the thermochromic core, forming a core–shell architecture. The term “core material” refers to the thermochromic ternary system, comprising crystal violet lactone (CVL), bisphenol A, and decanol, encapsulated within the UF resin shell. A graphical exemplification of the microcapsule structure is shown in Figure 1.

Preparation and dispersion of color-changing blends: The water bath was adjusted to 50 °C. Subsequently, 80 g of decanol was weighed and placed into a beaker. Following this, in accordance with a mass ratio of CVL to bisphenol A to decanol of 1:3:50, 4.8 g of bisphenol A and 1.6 g of CVL were added. After stirring at 400 rpm for 1.5 h, 1.74 g of gum acacia and 1.02 g of Triton X–100 were measured as emulsifiers and added to 52.52 mL of distilled water; then, 4.15 g of the color–changing blend was incorporated. The mixture was subsequently stirred at a higher speed for 30 min at 65 °C. After 5 min of ultrasonication, the emulsifiers completely emulsified the core material emulsion.

Preparation of the wall material: Amounts of 8.00 g of urea and 8.42 g of formaldehyde were weighed and added to 125.00 mL of distilled water. Triethanolamine was then added dropwise to adjust the pH of the system to 8.5. Subsequently, the mixture was sealed and stirred in a water bath adjusted to 70 °C, with the stirring speed maintained at 300 rpm for 1 h.

Preparation of UF@TS: The core material emulsion was placed in a water bath at 35 °C. The wall material was then added dropwise to the core material while maintaining a stirring speed of 500 rpm. Subsequently, 0.58 g of SiO₂ and 0.58 g of NaCl were added; this was followed by the dropwise addition of an 8% citric acid monohydrate solution. Once the pH was reduced to 2.5, the mixture was stirred magnetically for 1 h. The temperature of the water bath was then adjusted to 68 °C, and the stirring speed was reduced to 250 rpm. After reacting for 30 min under these conditions, the UF@TS emulsion was obtained. The prepared emulsion was sealed in a beaker and left undisturbed for three days. Subsequently, it was filtered using deionized water under a vacuum pump. After drying in an oven at 35 °C, a light blue UF@TS powder was obtained.

2.2.2. Method of UF@TS-R Preparation

Preparation and dispersion of color–changing blends: The water bath was adjusted to 50 °C. Subsequently, 80 g of decanol was weighed and placed into a beaker. Following this, in accordance with a mass ratio of CVL to bisphenol A to decanol of 1:3:50, 4.8 g of bisphenol A and 1.6 g of CVL were added. After stirring at 400 rpm for 1.5 h, 1.74 g of gum acacia and 1.02 g of Triton X–100 were measured as emulsifiers and added to 52.52 mL of distilled water; then, 4.15 g of the color–changing blend was incorporated. The mixture was subsequently stirred at a higher speed for 30 min at 65 °C. After 5 min of ultrasonication, the emulsifiers completely emulsified the core material emulsion.

Preparation of the wall material: Amounts of 8.00 g of urea and 12.62 g of formaldehyde were weighed and added to 256.00 mL of distilled water. Triethanolamine was then added dropwise to adjust the pH of the system to 8.5. Subsequently, the mixture was sealed and stirred in a water bath adjusted to 70 °C, with the stirring speed maintained at 300 rpm for 1 h.

Preparation of UF@TS-R: The urea–formaldehyde prepolymer was added dropwise to the core material emulsion. The stirring speed was set at 500 rpm, and the temperature was maintained at 35 °C. Subsequently, 1.63 g of SiO₂ and 1.49 g of NaCl were added; this was followed by the dropwise addition of an 8% citric acid monohydrate solution to adjust the pH of the system to 2.5. The mixture was then stirred for 1 h. The temperature was subsequently raised to 68 °C, and the stirring speed was reduced to 250 rpm. After stirring for 30 min, the UF@TS-R emulsion was sealed and left undisturbed at room temperature for 24 h. Subsequently, spray drying was conducted using a spray dryer with an inlet temperature of 130 °C and a peristaltic pump rate of 100 mL/h. Once the machine had completed preheating and the outlet temperature had stabilized at around 60 °C, the drying process commenced. The pale purple powder obtained after drying was identified as UF@TS-R.

2.3. Pretreatment and Coating Method for Bleached Poplar Wood Samples

Bleaching treatment of poplar wood samples: An aqueous solution of NaOH with a concentration of 4 g/L and a volume of 800 mL was prepared by adding 3.2 g of NaOH. This solution was employed to conduct pretreatment on ten pieces of fast–growing poplar wood, with the treatment duration controlled at 15 min. Subsequently, the wood samples were thoroughly washed. A bleaching agent was prepared by mixing 290 g of hydrogen peroxide with a mass fraction of 7.7% and 16 g of sodium silicate and then adding deionized water to make up to a total volume of 800 mL. The bleaching agent was placed in a magnetic–stirrer–equipped water bath with the temperature set at 65 °C, and the poplar wood samples were immersed in it for bleaching over a period of 6 h. After bleaching, the poplar wood samples were thoroughly washed and air dried at room temperature to minimize warping caused by rapid drying.

Coating method for bleached poplar wood samples: Bleached poplar samples were finished by hand using a two-primer/two–topcoat system. Prior to coating, the bleached poplar substrates were pretreated by sanding with 800–grit abrasive paper to remove wood fuzz and then wiped clean of surface residues. UF@TS was selected as the additive and mixed with UV primer at mass fractions of 0% (blank control), 5%, 10%, 15%, 20%, and 25%. The total mass of each coating system was strictly controlled at 1.000 g. In the coating process, the coating was applied by precisely dispensing 0.500 g of homogenized composite UV primer onto the substrate center, followed by manual uniform spreading with an applicator rod. After allowing it to level for 1 min, the primer was cured for 60 s in a single–lamp curing machine with a conveyance rate of 0.1 m/s. The primer application process was repeated, followed by two coats of topcoat application. The formulations of the coatings with different amounts of thermochromic microcapsules added are shown in

Table 3. Formulations of coatings with different amounts of thermochromic microcapsules added.

Sample (#)	Coating Method	Level of Microcapsule Addition (%)	Mass of Thermochromic Microcapsules (g)	Mass of Single–Layer UV Coating (g)	Spreading Rate (g/m2)
0	Without microcapsule addition	0	0.000	0.500	200
1-1	UF@TS-R added to both primer and topcoat	5	0.025	0.475	190
1-2		10	0.050	0.450	180
1-3		15	0.075	0.425	170
1-4		20	0.100	0.400	160
1-5		25	0.125	0.375	150
2-1	UF@TS-R added to primer, UF@TS added to topcoat	5	0.025	0.475	190
2-2		10	0.050	0.450	180
2-3		15	0.075	0.425	170
2-4		20	0.100	0.400	160
2-5		25	0.125	0.375	150

. Among them, samples 2-1# to 2-5# had UF@TS-R added to both the primer and the topcoat. Samples 2-6# to 2-10# had UF@TS-R added to the primer and UF@TS added to the topcoat. According to ISO 1514:2024 [41], the spreading rate (SR) was calculated using Equation (1), where m denotes the mass of coating used for a single layer and s represents the substrate area.

$$\mathrm{S}\mathrm{R}={\displaystyle \frac{\mathrm{m}}{\mathrm{s}}}$$

(1)

Unlike the existing microcapsule–coating preparation described by Wang et al. [31]—which requires 35 °C oven drying (20 min per layer × 4 layers) and a total drying time of at least 80 min—the present study employed UV–curing technology. Each layer cured in only 60 s, giving an overall curing time of no more than 4 min and a significant increase in production speed.

2.4. Tests and Characterization

2.4.1. Morphological Analysis

The whiteness value of the bleached poplar wood was determined by measuring the color coordinates (L, a, b) before and after the bleaching treatment. The central point of each sample was selected for chromaticity measurement four times before and after bleaching, and the mean values were taken as the initial and postbleaching chromaticity values, respectively. The initial whiteness (W₀) and the whiteness after treatment (W) were calculated, and the difference, ΔW, represented the increase in whiteness. The specific calculations are given by Equations (2) and (3).

$$\mathrm{W}=100-{\left[\right(100-\mathrm{L}{)}^2+{\mathrm{a}}^2+{\mathrm{b}}^2]}^{1/2}$$

(2)

$$\mathsf{\Delta }\mathrm{W}=\mathrm{W}-{\mathrm{W}}_0$$

(3)

Given the high magnification of SEM, a small piece was removed from the surface of the bleached poplar wood panel as required. The sample to be tested was fixed and sputtered with gold, then placed into a scanning electron microscope for vacuum operation. The sample was scanned and photographed after adjusting the brightness and focus.

2.4.2. Micromorphology

FTIR was used to analyze the chemical composition of the microcapsules and the coating, thereby confirming uniform microcapsule deposition on the substrate and the preservation of their structural integrity after curing. The samples were kept dry to avoid interference from water vapor in the spectra.

2.4.3. Optical Performance Test

Glossiness: According to GB/T 4893.6-2013 [42], the glossiness of the coating was tested. The glossiness was measured at the incidence angles of 20°, 60°, and 85°.

Transmittance: According to ISO 2813: 2014 [43], an ultraviolet spectrophotometer was used to test the transmittance of the coating. The prepared coatings were placed in the sample chamber, with blank samples used as controls, and scanned over a wavelength range of 380–780 nm.

2.4.4. Testing of Mechanical Properties and Roughness

Impact Resistance: In accordance with GB/T 4893.9–2013, the impact resistance of the coating was tested [44]. The samples to be tested were horizontally fixed on the testing platform, and the impact steel ball was adjusted to the preset height of 50 mm. Five different test points were selected for each sample to conduct the impact test. After the impact test, the impact area was visually inspected under natural light with the aid of a magnifying glass, and the morphological characteristics of the damage at each impact point were recorded. The impact resistance was divided into five grades, with Grade 1 representing the best impact resistance and Grade 5 representing the worst. The final impact grade of the sample was determined by calculating the average degree of damage at the five impact points.

Hardness: In accordance with GB/T 6739–2022, the hardness of the coating was tested using a pencil hardness tester [45]. The front end of the test pencil was sharpened and ground to the standard shape specified, and then, the pencil was fixed in the hardness tester. During the test, a fixed load was applied to the surface of the coating at a constant speed of 0.5 mm/s for a scratch test. Each set of samples was tested sequentially with pencils of different hardness levels ranging from 6H to 6B, with each scratch length being no less than 10 mm. After the test, the samples were observed. The highest pencil hardness grade that did not produce a visible scratch was taken as the pencil hardness value of the coating.

Adhesion: In accordance with the standard GB/T 4893.4–2013, the adhesion of the coating was tested using a multiblade cutting tool [46]. This multiblade cutting tool was used to perform two sets of orthogonal cuts on the surface of the coating to be tested at a constant rate, with the cutting depth ensuring penetration through the entire coating to the surface of the wood substrate, forming a grid pattern. Subsequently, an adhesive tape was closely applied to the cut area and then rapidly stripped off to observe the degree of damage. Adhesion was divided into six grades from 0 to 5, with Grade 0 indicating no coating detachment and optimal adhesion. Grade 5 indicated that more than 65% of the coating was removed, representing the worst adhesion.

Roughness: According to ISO 25178–601: 2025 [47], a roughness tester was used to obtain a roughness value. A coated glass plate was placed on the testing table, the position of a stylus was adjusted to contact the coating, and the coating roughness was tested and recorded.

2.4.5. Color–Changing Performance Test

To investigate the color change of the coating during heating, the chromaticity values were measured four times at every 10 °C interval. Specifically, the values were recorded at −20 °C, −10 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C. Each target temperature was maintained for 2 min, and the mean values of L, a, and b from the four replicates were taken as the final chromaticity data. Subsequently, the differences between successive measurements were calculated as ΔL, Δa, and Δb, and ΔE was derived according to Equation (4).

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

(4)

When the color difference showed an obvious step–like change, the inflection point temperature was regarded as the color–changing temperature of the microcapsules. The temperature range selected corresponds to the temperature range of furniture in daily use and can well reflect the color–changing effect of the coating in daily use.

2.4.6. Coating Aging Performance Test

In accordance with GB/T 1740–2007 and the Feller artificial accelerated aging test method, the aging performance of the coating was tested.

Table 4. Coating degradation rating criteria.

Degradation Level	Degradation Mode
	Rusting	Blistering	Discoloration	Cracking
1	0(S0)	0(S0)	Extremely slight	0
2	1(S1)	1(S1), 1(S2)	Slight	1(S1)
3	1(S2)	3(S1), 2(S2), 1(S3)	Noticeable	1(S2)
4	2(S2), 1(S3)	4(S1), 3(S2), 2(S3), 1(S4)	Significant	2(S2)
5	3(S2), 2(S3), 1(S4), 1(S5)	5(S1), 4(S2), 3(S3), 2(S4), 1(S5)	Severe	3(S3)

Note: When multiple degradation modes coexisted, the overall rating corresponded to the most severe observed failure mode.

presents the comprehensive degradation rating criteria for coatings specified in GB/T 1740–2007, which was employed in this experiment to assess the aging grade of the coatings. The oven was maintained at 120 °C, and according to the time–temperature equivalence principle, all samples underwent continuous isothermal aging at 120 °C for 12 h in a forced–air circulation oven [48]. The aging–related changes in the UV coating on the surface of the bleached poplar wood were characterized by testing the color–changing performance and the variation in gloss.

2.4.7. Statistical Significance of the Difference

The results of tests were assessed in terms of the statistical significance of the difference concerning the coating processes used on the bleached poplar wood samples and the changing mass fractions of thermochromic microcapsules in coatings. The nonrepeated two–way ANOVA method was used for significance analysis. Analysis of variance at a significance level of 0.05 was performed to detect significant differences in coating properties. SS is the sum of squares from the mean square, representing the sum of squares between groups and within groups; df is the degree of freedom; and MS is the mean square obtained by dividing the sum of squares by the degrees of freedom. F represents the test statistic, which is used to calculate the hypothesis test. p-value indicates the level of significance, which is used to evaluate the range and interval of overall parameters and calculate the probability of a possible experiment. F crit represents the F value at the corresponding significance level. When F is less than F crit, there is no difference between the two groups of data under analysis. The criterion for judging the significance of the difference were as follows. If 0.01 < p-value < 0.05, the difference was significant. If p-value ≤ 0.01, the difference was very significant. If p-value > 0.05, there was no difference. The results obtained by the above methods were F > F crit and p-value < 0.01.

3. Results and Discussion

3.1. Microscopic Morphology of Wood Surfaces Coated with Thermochromic UV Coatings

Figure 2 shows SEM images of the cross–section of a bleached poplar wood panel coated with UV coating at an oblique angle. After the addition of microcapsules, the UV coating changed from smooth to rough. Both before and after the addition of microcapsules, the coating had a good combination with the wood substrate. The coating penetrated into the gaps of the wood, showing good sealing properties. The addition of 10% microcapsules did not adversely affect the good combination of the coating and the bleached poplar wood. The coating penetrated into the pores near the surface of the coating, indicating good curing properties of the UV coating.

The SEM morphology of coatings on the surface of a bleached poplar wood panel is shown in Figure 3. The surface of coating 2-4# exhibited significant agglomeration, but the overall surface was relatively flat. Both coatings 2-3# and 1-3# showed the morphology of microcapsules, but coating 1-3# had obvious grooves and more irregular, uneven agglomerations at the edges, resulting in a poorer overall morphology. This was because UF@TS-R exhibited poor dispersion in the coating; when it was used in both the primer and the topcoat, the roughness worsened because of a double–overlay effect. Incorporating UF@TS in the topcoat effectively alleviated this phenomenon, yielding a smoother surface morphology. Moreover, the addition of microcapsules has a cumulative impact on coating–surface defects. Excessive microcapsules not only disrupt uniform contact among the curing components but may also combine with matting agents to cause further agglomeration and powdering.

3.2. Impact of Three Types of Blended Microcapsules on the Optical Properties of Coatings

As shown in Figure 4, the peak at 1062 cm⁻¹ was characteristic of the wood after bleaching, which was masked after coating. Because of agglomeration of the microcapsules, there were fewer microcapsules under the probe of the device, resulting in smaller peaks for the relevant components of the microcapsules. However, the O-H stretching vibration peak at 3691 cm⁻¹, which was characteristic of the wall material of the microcapsules, could still be observed. This indicates that during the coating process of UV coatings containing microcapsules on the surface of bleached poplar wood, no chemical reactions affecting the curing of the coating occurred. Both types of coatings were successfully prepared on the surface of bleached poplar wood.

3.3. Analysis of the Optical Properties of Coatings on the Surface of Bleached Poplar Wood

The changes in the gloss and reflectance of the thermochromic coatings on the surfaces of the two bleached poplar specimens prepared by different addition methods are listed in

Table 5. Changes in glossiness and reflectivity of surface coatings of bleached poplar with different coating processes.

Sample (#)	Glossiness (GU)			60° Light Loss Rate (%)	Reflectance (%)
	20°	60°	85°
0	2.2	5.0	16.0	-	79.77
1-1	1.2	2.4	4.1	52	63.47
1-2	1.0	1.7	2.7	66	58.54
1-3	0.8	1.6	4.8	68	52.69
1-4	0.7	1.3	2.7	74	55.67
1-5	0.7	1.6	2.5	68	45.77
2-1	1.6	3.2	3.2	36	68.75
2-2	1.3	2.0	1.7	60	66.02
2-3	0.9	2.8	10.7	44	57.18
2-4	1.1	3.1	3.5	38	57.45
2-5	0.8	2.8	6.0	44	54.85

. The results of the significance analyses for the tabulated data are presented in Tables S1–S5. As shown in Tables S1–S3, the coatings exhibited a statistically significant difference in gloss only at an incident angle of 20°. As shown in Tables S4 and S5, the coatings demonstrated a significant difference in reflectance, whereas the 60° gloss loss ratio did not differ significantly. The gloss first decreased and then increased with rising microcapsule loading. Among coatings 1-1# to 1-5#, only 1-5# exhibited a slight increase in gloss, while among coatings 2-1# to 2-5#, both 2-4# and 2-5# showed an increase in gloss. This indicated that the addition of UF@TS-R exerted a more pronounced negative effect on coating gloss, and only at an extremely high loading level could it avoid further compromising the inherently low gloss of the UV topcoat.

As shown in the UV–visible reflectance spectra in Figure 5, coatings 1-1# to 1-5# appeared purplish–red, while the primers of coatings 2-1# to 2-5# were red and the topcoats were blue, resulting in an overall purplish–red appearance. Therefore, there were significant differences in reflectance between the two types of coatings. The reflectance of coatings 2-1# to 2-5# was higher than that of coatings 1-1# to 1-5#. This was because the topcoat and primer, which were of different colors, had high reflectance for different wavelength ranges of light. However, this did not mean that the cover rate of coatings 2-1# to 2-5# was also higher. The reflectance of the coating decreased overall after the addition of microcapsules and did not change significantly with the addition of microcapsules. The reflectance decreased the most at 25% addition, where the reflectance of coating 1-5# decreased to 45.77% and the reflectance of coating 2-5# decreased to 54.85%.

3.4. Analysis of the Mechanical Properties and Roughness of Coatings on the Surface of Bleached Poplar Wood

Table 6. Mechanical properties of surface coatings of bleached poplar with different coating processes.

Sample (#)	Mechanical Properties
	Adhesion (Grade)	Hardness	Impact Resistance (Grade)	Roughness (μm)
0	1	6H	4	0.280
1-1	2	6H	5	2.650
1-2	3	6H	5	4.091
1-3	4	6H	4	4.812
1-4	4	5H	4	6.241
1-5	5	5H	5	10.555
2-1	2	6H	4	0.555
2-2	2	6H	4	1.236
2-3	3	6H	3	3.289
2-4	3	6H	4	4.048
2-5	4	5H	3	6.142

shows the differences in the mechanical properties of coatings on the surface of bleached poplar wood with different coating processes. The results of the significance analyses for the tabulated data are presented in Tables S6–S9. Table S6 indicates that the coatings differed significantly in extensibility. Tables S7 and S8 reveal no significant difference in hardness or impact resistance. Table S9 confirms a significant difference in surface roughness; as the amount of microcapsules added increased, the adhesion tended to increase. The rate at which adhesion decreased for coatings 1-1# to 1-5#, without UF@TS addition, was faster than that for coatings 2-1# to 2-5#. The presence of an excessive amount of UF@TS-R in the primer had a significant impact on the adhesion of the coating, and the addition of this microcapsule in the topcoat further exacerbated this adverse effect. This was attributed to poor microscopic morphology and severe agglomeration, which increased the internal stress of the coating and weakened its overall ability to resist tensile forces.

The hardness of coating 2-5# decreased to 5H; that of coatings 1-4# and 1-5# also dropped to 5H. This indicates that the coatings containing UF@TS-R were softer. This was because the extensive agglomeration of microcapsules led to the formation of visible lumps on the coating surface, causing a rapid decrease in coating hardness in these areas.

The impact resistance grade of the coatings of the two groups of samples basically showed a tendency to first decrease and then increase with the increasing amounts of microcapsule addition. The UV coatings containing UF@TS had better impact resistance than those containing only UF@TS-R. The impact resistance of coating 2-4# showed an abnormal decrease, which may have been due to excessive addition of UF@TS-R leading to an overall performance decline. When the addition of UF@TS-R was too high, the agglomerated microcapsules acted as stress concentration points and were prone to initiating crack propagation under external impact. This phenomenon confirmed that there is an optimal threshold for the addition of microcapsules. Beyond this threshold, the synergistic interactions among the multiple components within the system transform into competitive effects. The impact resistance performance of coatings 1-1# to 1-5# showed significant fluctuations, with a decline at medium addition levels. This may have been due to agglomeration of microcapsules within this range, leading to stress concentration. However, the performance of coatings 1-4# and 1-5# improved at higher addition levels. This could be because the increased number of microcapsules, acting as new components, replaced some of the original components of the UV coating, fundamentally altering the coating’s impact resistance. In comparison, the performance of coatings 2-1# to 2-5# was more stable, achieving the optimal impact resistance grade of 3 at addition levels of 15% and 25%. This indicates that UF@TS had better compatibility with the matrix than UF@TS-R, maintaining good dispersion even at high addition levels.

The roughness of the coatings generally increased. The roughness of coatings 2-1# to 2-5# was lower than that of coatings 1-1# to 1-5#. This was because the poor microscopic morphology of the UF@TS-R coatings led to further compounding of the roughness of the topcoat and primer, resulting in increased roughness. Therefore, considering all factors, the mechanical properties of coatings 2-1#, 2-2#, and 2-3#, with addition levels of 15% or less, were optimal.

3.5. Analysis of the Color–Changing Performance of Coatings on the Surface of Bleached Poplar Wood

As can be seen from the color–difference data for the coatings shown in

Table 7. Chromaticity values and ΔE of coating with temperature with different coating processes.

Sample (#)	Chromaticity Parameters	−20 °C	−10 °C	0 °C	10 °C	20 °C	30 °C	40 °C	50 °C	Color–Changing Temperature (°C)
0	L	81.200	80.175	82.650	82.250	83.650	83.325	82.000	83.200	-
		±1.961	±2.815	±2.461	±2.200	±3.455	±2.937	±3.041	±2.516
	a	1.375	−0.500	0.650	−0.475	0	−0.750	−0.550	−0.800
		±0.054	±0.021	±0.023	±0.016	±0	±0.026	±0.011	±0.03
	b	8.325	10.800	9.475	9.600	9.675	9.475	8.050	8.675
		±0.192	±0.22	±0.341	±0.300	±0.205	±0.405	±0.184	±0.319
	ΔE	-	3.270	1.988	2.480	3.1170	3.218	2.103	2.975
1-1	L	66.575	65.275	64.475	66.725	65.850	66.975	66.925	66.350	-
		±1.817	±1.267	±1.707	±2.082	±1.939	±3.511	±2.186	±1.811
	a	20.225	21.200	21.000	18.375	20.100	18.850	17.775	17.300
		±0.236	±0.511	±0.170	±0.384	±1.011	±0.703	±0.455	±0.307
	b	2.625	2.100	2.300	3.100	1.200	1.950	1.300	0.925
		±0.080	±0.055	±0.058	±0.126	±0.043	±0.007	±0.041	±0.033
	ΔE	-	1.708	2.262	1.916	1.604	1.583	2.807	3.391
1-2	L	65.275	63.975	63.000	65.975	63.450	66.225	63.650	61.975	-
		±1.874	±2.350	±2.170	±2.609	±2.672	±2.413	±0.829	±2.662
	a	24.100	25.250	25.550	20.850	23.975	20.800	21.650	23.900
		±0.446	±0.524	±0.959	±0.709	±0.769	±0.581	±0.876	±0.298
	b	2.750	1.725	2.200	3.900	0.775	2.025	0.775	−0.775
		±0.045	±0.069	±0.065	±0.087	±0.033	±0.056	±0.019	±0.029
	ΔE	-	2.016	2.753	3.518	2.692	3.510	3.542	4.833
1-3	L	59.500	58.475	59.300	58.250	59.450	59.125	57.250	56.925	30
		±2.661	±2.142	±1.126	±2.305	±1.604	±2.51	±1.268	±1.620
	a	32.600	32.650	31.800	31.700	29.600	28.150	29.550	28.050
		±1.035	±0.341	±0.375	±0.836	±1.036	±1.088	±1.144	±1.028
	b	−0.525	−1.200	−0.825	−1.625	−1.600	−1.925	−3.525	−3.075
		±0.023	±0.030	±0.020	±0.079	±0.074	±0.065	±0.117	±0.090
	ΔE	-	1.228	0.877	1.893	3.187	4.68	4.834	5.817
1-4	L	55.600	56.250	56.725	57.075	58.125	58.075	55.700	55.300	30
		±1.642	±1.156	±1.757	±1.341	±2.548	±1.725	±2.007	±1.755
	a	37.500	37.275	36.725	35.250	35.425	34.275	34.000	34.025
		±1.201	±1.437	±1.287	±1.476	±0.602	±1.103	±0.998	±1.004
	b	−3.675	−5.125	−4.650	−4.550	−5.775	−6.700	−6.975	−7.150
		±0.149	±0.117	±0.148	±0.171	±0.07	±0.181	±0.125	±0.277
	ΔE	-	1.605	1.678	2.829	3.885	5.067	4.811	4.924
1-5	L	49.200	48.325	48.000	49.350	48.475	47.875	48.125	47.600	30
		±1.243	±1.808	±2.224	±1.450	±1.373	±2.33	±2.153	±2.186
	a	40.925	40.050	39.025	36.500	36.800	35.925	34.675	32.925
		±1.271	±1.126	±0.693	±1.463	±1.186	±0.739	±0.993	±1.004
	b	−4.025	−4.425	−4.300	−4.600	−5.525	−9.250	−7.075	−7.275
		±0.073	±0.194	±0.159	±0.161	±0.210	±0.408	±0.237	±0.236
	ΔE	-	1.300	2.264	4.465	4.449	7.352	7.037	8.782
2-1	L	73.675	72.775	73.675	73.600	74.225	75.475	74.125	73.875	-
		±2.724	±2.689	±2.617	±2.844	±1.971	±2.450	±2.805	±3.125
	a	14.150	13.525	13.000	12.200	11.150	9.675	10.975	11.825
		±0.204	±0.415	±0.422	±0.444	±0.463	±0.382	±0.369	±0.248
	b	1.000	1.500	1.700	0.075	1.700	2.425	0.825	0.425
		±0.037	±0.035	±0.063	±0	±0.037	±0.148	±0.036	±0.017
	ΔE	-	1.204	1.346	2.160	3.129	5.030	3.212	2.403
2-2	L	73.200	72.325	72.750	71.025	73.100	72.275	70.325	70.100	20
		±3.057	±1.773	±2.517	±2.134	±2.229	±2.082	±2.129	±1.459
	a	18.525	16.375	16.550	17.525	14.100	15.875	14.025	14.400
		±0.320	±0.432	±0.500	±0.355	±0.354	±0.480	±0.437	±0.415
	b	−0.150	−1.975	−0.575	−1.200	−0.625	−1.625	−2.350	−2.450
		±0.004	±0.069	±0.009	±0.016	±0.023	±0.063	±0.067	±0.076
	ΔE	-	2.953	2.070	2.614	4.452	3.171	5.775	5.649
2-3	L	66.900	64.150	62.275	64.950	65.325	65.950	62.125	61.125	30
		±1.520	±1.963	±2.113	±1.844	±2.382	±1.963	±1.239	±1.198
	a	19.250	21.050	20.825	16.200	16.650	15.850	15.500	17.200
		±0.537	±0.890	±0.800	±0.486	±0.530	±0.389	±0.727	±0.596
	b	−2.125	−3.525	−5.350	−3.400	−4.200	−4.125	−5.800	−6.325
		±0.070	±0.151	±0.143	±0.138	±0.150	±0.127	±0.207	±0.122
	ΔE	-	3.572	5.854	3.838	3.681	4.057	7.097	7.429
2-4	L	63.050	61.30	61.725	65.500	61.850	63.325	62.075	61.950	40
		±1.845	±1.186	±2.670	±1.976	±2.010	±1.869	±2.067	±2.288
	a	24.850	23.950	24.700	21.650	22.975	21.425	20.775	19.800
		±0.660	±0.648	±0.571	±0.713	±0.720	±0.817	±0.995	±0.838
	b	−1.675	−1.650	−4.475	−2.150	−4.600	−4.700	−5.425	−5.375
		±0.072	±0.042	±0.143	±0.105	±0.156	±0.061	±0.054	±0.159
	ΔE	-	1.968	3.101	4.058	3.676	4.578	5.623	6.356
2-5	L	57.650	55.550	56.975	56.325	56.400	59.100	56.725	57.275	30
		±1.784	±2.108	±1.268	±1.786	±1.518	±0.858	±1.736	±1.178
	a	28.150	28.325	26.700	23.775	22.450	20.925	21.175	20.625
		±0.971	±0.597	±0.665	±0.250	±0.599	±0.507	±0.514	±0.676
	b	−5.250	−7.450	−6.550	−7.050	−7.950	−7.125	−8.400	−7.600
		±0.083	±0.399	±0.190	±0.191	±0.226	±0.198	±0.282	±0.204
	ΔE	-	3.046	2.061	4.913	6.430	7.604	7.709	7.892

, coatings 2-1# to 2-5# exhibited more pronounced color–changing properties in the low–temperature range. In contrast, for coatings 1-1# to 1-5#, the red base color of the cationic red dye, because of its poor dispersion, became more layered after multiple coatings. The accurate measurement of color–difference changes with temperature was further complicated by the nonuniform distribution of microcapsules, and the color–difference changes became increasingly inconspicuous. It could be concluded that coatings 1-1# to 1-3# did not possess color–changing properties. The ΔE value of coating 2-3# increased rapidly in the temperature range from −20 °C to 0 °C, reaching 3.572 at −10 °C and further increasing to 5.854 at 0 °C. This indicated that the color–changing starting temperature of coating 2-3# was below −20 °C and that its low–temperature color–changing efficiency was significantly better than that of coatings 1-1# to 1-5#. The ΔE of coating 2-5# reached 7.892 at 50 °C, which was lower than the 8.782 of coating 1-5#. However, the main increase in color difference of coating 2-5# was distributed in the low-temperature range from −20 °C to 20 °C, which better met the practical application requirements of low–temperature–sensitive decorative materials. Excessive addition of microcapsules led to reduced color–changing sensitivity. This was because in coatings with an overabundance of microcapsules, the temperature response speed of the microcapsules in the inner layers is slower than that of the surface–layer microcapsules during temperature changes. Taking all factors into account, coating 2-3# achieved concentrated color–changing in the range of −20 °C to 30 °C, with a significant increase in ΔE in the core low–temperature range of −10 °C to 10 °C. The color–changing completion temperature was 30 °C, which is suitable for the dynamic color–regulation needs of furniture surfaces in environments ranging from normal– to low–temperature conditions. The results of the significance analyses for the tabulated data are presented in Tables S10–S17. As shown in Tables S10–S16, ΔE values measured at various temperatures did not differ significantly among the coatings.

3.6. Analysis of the Aging Performance of Coatings on the Surface of Bleached Poplar Wood

Color Difference

As shown in Figure 6, after a relatively short period of high-temperature aging, the coating was generally damaged, with partial delamination still occurring between the topcoat and primer because of the significant difference in thermal expansion coefficients between them. The morphology of coatings 2-6# to 2-10# was relatively well–maintained. The color–changing ΔE of the coatings after aging is shown in

Table 8. ΔE of high– and low–temperature coatings with different coating processes after aging.

Sample (#)	Low–Temperature Colorimetric Value (−20 °C)			High–Temperature Colorimetric Value (50 °C)			ΔE After Aging	ΔE Before Aging
	L1	a1	b1	L2	a2	b2
1-1	59.525	18.350	28.550	61.775	16.550	27.275	3.151	3.391
1-2	62.700	19.875	28.400	62.025	18.525	25.950	2.878	4.833
1-3	56.275	23.475	28.575	56.175	23.225	27.575	1.036	5.817
1-4	57.425	26.575	27.725	58.675	25.450	24.975	3.223	4.924
1-5	52.525	26.125	32.375	54.100	24.525	28.075	4.851	8.782
2-1	71.250	13.225	29.050	73.400	12.175	27.200	3.024	2.403
2-2	63.300	15.900	26.900	71.300	14.475	27.400	8.141	5.649
2-3	65.875	17.600	31.625	68.625	12.175	26.075	8.234	7.429
2-4	62.250	21.675	34.525	63.475	18.475	31.425	4.621	6.356
2-5	59.900	20.600	28.675	62.950	16.100	25.250	6.425	7.892

. Table S17 shows no significant difference in the high- to low-temperature color difference of coatings subjected to different finishing processes after aging. Although the coating damage caused by the aging process brought certain difficulties to the color–difference test and resulted in some unrealistic color–difference data, the color–changing performance after aging still showed certain patterns. Coatings 1-1# to 1-3# were severely cracked and completely lost their color–changing properties. The delamination of the topcoat was more severe at lower microcapsule addition levels, which means that the aging performance of the topcoat first deteriorated and then improved with increasing UF@TS-R addition. This was because the red–colored coating had a lower reflectance and absorbed more heat at high temperatures than the colorless coating. The unevenly distributed microcapsules may have also led to uneven heating of the coating, causing the topcoat to expand, squeeze, deform, and crack. At the same time, the color–difference changes after aging for coatings 1-4# and 1-5# were still relatively low. This was because, compared with the light blue and white base colors of UF@TS (the color of the urea–formaldehyde resin wall material) at high temperatures, the red base color of UF@TS-R, due to the cationic red dye, absorbed more heat. The volatilization of the solvent decanol was further accelerated, resulting in a significant reduction in color–changing performance. At higher microcapsule addition levels, the coating had lower reflectance and absorbed more heat. If the heat–absorption rate is too fast, delamination can occur rapidly in a short period of time. The morphology of coatings 2-1# to 2-5# was better, which may have been due to the synergistic effect between the heat–absorption rate and the color change with depth caused by the color difference of the coatings during the heat–conduction process from the surface layer to the deep layer. This resulted in smaller expansion differences and lower stress between different coatings during the temperature–rising process, thus avoiding delamination. Therefore, the coating process of coatings 2-1# to 2-5#, that is, adding UF@TS to the topcoat and UF@TS-R to the primer resulted in better aging resistance. At lower addition levels, these coatings had better macroscopic morphology and color–changing properties after aging.

It can be seen from

Table 9. Glossiness of coatings with different coating processes after aging.

Sample (#)	Gloss After Aging (GU)			Aging Gloss Loss Rate (%)	60° Gloss Before Aging (GU)
	20°	60°	85°
1-1	1.1	1.7	1.5	29	2.4
1-2	0.8	1.5	0.5	11	1.7
1-3	0.7	1.4	1.8	12	1.6
1-4	0.7	1.2	1.7	7	1.3
1-5	0.7	1.2	0.8	25	1.6
2-1	1.4	2.3	1.9	28	3.2
2-2	1.2	1.9	2.4	5	2.0
2-3	1.1	2.0	3.2	28	2.8
2-4	1.1	1.9	1.7	38	3.1
2-5	0.8	1.8	2.4	35	2.8

that coatings 2-1# to 2-5# had better gloss stability. Tables S18–S20 indicate that, after aging, the coatings exhibited no significant differences in gloss at an incident angle of 85°. Table S21 demonstrates that the gloss loss rate of the coatings after aging did not differ significantly. The aging gloss loss rate of coating 2-3# was 28%, which may have been due to the higher heat-absorption efficiency of UF@TS-R, with a dark red base color, compared with UF@TS leading to accelerated coating aging and surface powdering. At the same time, the topcoat was a matte paint with low gloss. The slight decrease in gloss after aging also resulted in a higher gloss loss rate.

3.7. Thermochromic Mechanism of Coatings

The mechanism of the color changing of the coating is shown in Figure 7, which illustrates the color response characteristics of UF@TS in the topcoat and UF@TS-R in the primer of coatings 2-1# to 2-5# with temperature changes. UF@TS exhibited a deep blue color in low–temperature environments. As the temperature rose from −20 °C to 10 °C, the color maintained a stable blue hue. When the temperature continued to increase to 50 °C, the microcapsules gradually faded to a light blue or even close to colorless. UF@TS-R, containing a red dye, always retained a warm–colored base. At low temperatures, the thermochromic components interacted with the dye to produce a synergistic color–displaying effect, resulting in a deep purple appearance. At 50 °C, as the thermochromic components continued to decolorize, the material mainly exhibited the pink hue of the dye. Both types of materials underwent significant color changes around 30 °C, and the color–changing process was completely reversible, with the initial color restored upon cooling.

The microcapsules distributed in the depths of the coating exhibited a certain delay in color–changing response due to the lag effect of heat conduction. During the temperature–rising process, the surface–layer microcapsules reached the color–changing threshold temperature first and began to decolorize, while the deep-layer microcapsules needed a longer time to complete the same color change. This response difference led to a gradual color–changing effect from the surface to the interior of the coating during temperature changes. Especially under high-addition conditions, this heat–conduction lag effect was more significant.

4. Conclusions

The effects of composites of two types of microcapsules, UF@TS and UF@TS-R, on the optical, mechanical, thermochromic, and aging properties of UV coatings were analyzed and compared with those of thermochromic UV coating with UF@TS-R added alone. The results showed that as the amount of microcapsule addition increased, the gloss of the coating initially decreased and then increased. The reflectance continuously declined, the adhesion and hardness decreased, the impact resistance rating initially decreased and then increased, and the roughness continuously increased. With increasing amounts of microcapsule addition, the color-changing performance and the color difference of the color change across the entire temperature range increased. However, the color–changing efficiency decreased in the low–temperature range. After aging, the topcoat of the coating deteriorated severely, and the gloss loss rate of the coating due to aging was high. Coating 2-3#, synthesized by adding UF@TS to UV topcoat and UF@TS-R to UV primer at a mass concentration of 15%, showed the best overall performance. The 60° gloss of the coating was measured at 2.8 GU, with a reflectance of 57.18%. The adhesion was rated at Grade 3, the hardness was 6H, the impact resistance was classified as Grade 3, and the surface roughness was 3.289 μm. The color difference between high and low temperatures was 7.429, and the gloss loss rate due to aging was 28%. While UF microcapsules enable effective thermochromism, their formaldehyde release potential warrants caution. Future studies should assess formaldehyde emissions under service conditions and explore bio-based alternatives for ecofriendly applications.