Influence of Chitosan–Sodium Tripolyphosphate-Coated Tung Oil Microcapsules on the UV Coating Performance of Cherry Wood Surface

Abstract

By incorporating microcapsules with self-healing properties into the coating, a self-healing coating can be obtained, which can repair cracks or damage. In this study, chitosan–sodium tripolyphosphate-coated tung oil microcapsules 1# and 2# with a high encapsulation efficiency were incorporated into a UV-cured topcoat on cherry wood surfaces at different ratios. The results showed that as the microcapsule content increased, the coating’s reflectivity and gloss loss increased, while its impact resistance improved. However, the coating’s adhesion and hardness decreased. The coating containing 6% microcapsule 1# exhibited optimal performance on cherry wood board. The reflectance of the ultraviolet–visible light of the coating was 41.14%, the lightness value was 58.35, the red-green value was 13.96, the yellow-blue value was 25.32, the color difference was 4.47, the gloss reduction rate was 66.84%, the roughness was 1.11 μm, the impact resistance grade was level 4, the adhesion was level 1, the hardness was 3H, and the recovery rate was 17.06%. Comparative analysis revealed that both the chitosan/arabic gum-encapsulated tung oil microcapsules and chitosan–sodium tripolyphosphate-coated tung oil microcapsules could impart self-healing functionality to UV-cured coatings when incorporated into the finish. Notably, the coating system containing 6% chitosan/arabic gum-encapsulated tung oil microcapsules demonstrated optimal performance characteristics when applied to cherry wood substrates. The research findings demonstrate the technical feasibility of achieving self-healing functionality in UV-cured coatings for cherry wood surfaces.

1. Introduction

Nowadays, wood-based architectural structures and their manufactured products are gaining increasing popularity [1,2,3,4] due to their excellent decorative characteristics and mechanical properties [5]. However, wooden products are prone to mold and deformation due to long-term weight-bearing [6] and other environmental factors as well as their own materials [7,8], dimensions [9,10], mortise and tenon joints [11,12,13], and manufacturing processes [14,15], which lead to a decline in their performance [16,17]. Consequently, protective measures for wood products are of critical importance [18,19]. The coating not only enhances the aesthetic appeal of wood products [20], but also serves as an economical, effective, and straightforward protective measure [21]. But their long-term performance is inevitably compromised by external environmental factors such as temperature fluctuations, frictional wear, and mechanical impacts. These stressors ultimately lead to the formation of microcracks and consequent degradation of protective capabilities. Meanwhile, intrinsic wood transformations [22] may initiate microcracks and facilitate their propagation, thereby compromising coating integrity, diminishing protective efficacy, and ultimately reducing product service life [23,24]. Moreover, since microcracks are often subvisual to the naked eye, their formation can lead to significant economic losses in product maintenance, inspection, and upkeep [25]. This necessitates proactive measures to enhance the coating quality through advanced technologies, making the development of self-healing coatings an imperative research direction.

The concept of self-healing originates from biological systems [26]. Scientists have successfully introduced this mechanism into coating technology, resulting in self-healing coatings [27,28]. These advanced coatings can autonomously detect and repair microcracks, thereby effectively restoring both functional performance and structural integrity after damage occurs. According to different repair mechanisms, self-healing coatings can be mainly divided into two categories [29,30]: one relies on externally added repair materials (extrinsic type), and the other is based on the inherent properties of the coating resin itself (intrinsic type). Extrinsic self-healing further includes hollow fiber self-repair, nanoparticle self-repair, microcapsule self-repair, etc. Among them, microcapsule-based self-healing systems have become particularly prominent due to their fast response to cracks, simple synthesis, low cost, and excellent dispersibility in polymer matrices [31,32,33,34]. Pioneering the industrial application of self-healing microcapsules by emulating biological self-repair mechanisms, White et al. successfully developed dicyclopentadiene microcapsules with urea–formaldehyde resin shells through in situ polymerization [35]. This groundbreaking work has since catalyzed growing research interest in self-healing microcapsule technology. Chang et al. prepared self-healing microcapsules using melamine–formaldehyde resin as the wall material and a mixture of shellac solution and water-based coating as the core material through emulsion polymerization [36]. The effects of four parameters—the mass ratio between the core and wall materials, the mass ratio between the emulsifier and core material, the stirring rate, and the mass ratio between the two core materials—were investigated with respect to the microcapsule yield and encapsulation efficiency. Among these factors, the stirring rate was found to exert the most significant influence on the preparation of melamine–formaldehyde resin-encapsulated shellac/water-based coating microcapsules. Lv et al. employed phycocyanin as the core material and chitosan/alginate/calcium chloride as the wall materials to prepare microcapsules, which achieved an encapsulation efficiency of 70.92% for the core substance [37]. The prepared microcapsules were demonstrated to enhance both the stability of phycocyanin and its anti-allergic efficacy.

Among various microcapsule wall materials, chitosan stands out as a promising candidate due to its natural origin, non-toxicity, excellent biocompatibility, and biodegradability [38]. This study aims to investigate the effects of chitosan–sodium tripolyphosphate (CS-TPP)-coated tung oil microcapsules on the performance of UV-curable coatings applied on cherry wood surfaces. Two CS-TPP-coated tung oil microcapsules with higher encapsulation efficiencies were incorporated into the UV topcoat of cherry wood panels at varying ratios. The mechanical properties, self-healing performance, and micromorphology of each coating system were systematically evaluated to identify the optimal formulation. This study demonstrates the technical feasibility of self-healing UV coatings for cherry wood surfaces.

2. Test Materials and Methods

2.1. Experimental Equipment and Materials

The experimental apparatus employed in this study are summarized in

Table 1. Test equipment.

Equipment Name	Model	Manufacturer
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
Forced convection drying oven	DHG-9240A	Shanghai Aozhen Instrument Manufacturing Co., Ltd., Shanghai, China
UV curing machine	620#	Huzhou Tongxu Machinery Equipment Co., Ltd., Nanxun, China
Optical microscope	AX-10	Carl Zeiss Co., Ltd., Baden-Württemberg, Germany
Fourier transform infrared (FTIR) spectrometer	VERTEX 80V	Bruker Technology Co., Ltd., Hamburg, Germany
Scanning electron microscope (SEM)	QUANTA-200	Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA
Precision roughness tester	J8-4C	Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China
Gloss meter	HG268	Shenzhen 3nh Technology Co., Ltd., Shenzhen, China
Colorimeter	CR7	Shenzhen 3nh Technology Co., Ltd., Shenzhen, China
Universal mechanical testing machine	AGS-X	Shimadzu Seisakusho, Kyoto, Japan
UV–Vis spectrophotometer	U-3900	Hitachi Instruments (Suzhou) Co., Ltd., Suzhou, China
Heat-collecting constant-temperature heating magnetic stirrer	DF-101Z	Shanghai Yixin Scientific Instrument Co., Ltd., Shanghai, China
Ultrasonic emulsifying disperser	BILONG-500	Shanghai Bilang Instrument Co., Ltd., Shanghai, China
Small-scale spray dryer	JA-PWGZ100	Shenyang Jingao Instrument Technology Co., Ltd., Shenyang, China
Circulating water vacuum pump	SHZ-D	Shanghai Simate Instrument & Equipment Co., Ltd., Shanghai, China

. The cherry wood boards used in this experiment, with dimensions of 50 mm × 50 mm × 8 mm, were procured from Hongchangsheng Timber Factory in Linyi, Shandong Province. Both the UV primer and UV topcoat used in this study were supplied by Jiangsu Haitian Company Co., Ltd., Nantong, China. Both the UV primer and UV topcoat had a solid content greater than 98.0%. The UV primer primarily consisted of epoxy acrylate resin, polyester acrylate resin, trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTMA), photoinitiators, defoamers, leveling agents, and other additives. The experimental materials are listed in

Table 2. Test materials.

Name	Molecular Formula	CAS No.	Manufacturer
Polyoxyethylene sorbitan monooleate (T-80)	C24H44O6	9005-65-6	Fangzheng Reagent Factory, Beichen District, Tianjin, China
Sodium dodecylbenzene sulfonate (SDBS)	CH3(CH2)11C6H4SO3Na	25155-30-0	Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China
Sodium tripolyphosphate (STPP)	Na5P3O10	7758-29-4	Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China
Chitosan	(C6H11NO4)n	9012-76-4	Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China
Acetic acid	C2H4O2	64-19-7	Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China
Tung oil	C65H100O14	-	Shanghai Shenmeng Home Furnishing Co., Ltd., Shanghai, China
Tannic acid	C76H52O46	1401-55-4	Tianjin Zhonglian Chemical Reagent Co., Ltd., Tianjin, China

.

2.2. The Preparation of Tung Oil Microcapsules Coated with Chitosan and Sodium Tripolyphosphate

Two different types of microcapsules were prepared by adjusting the parameters of the preparation conditions [39], with the specific experimental material dosages listed in

Table 3. Schedule of test material dosage.

Sample	Chitosan (g)	1% Acetic Acid (g)	Tung Oil (g)	T-80 (g)	SDBS (g)	Use Pure Water for Emulsification (g)	STPP (g)	STPP Used Pure Water (g)
1#	0.800	79.200	1.600	0.940	1.410	76.050	0.400	9.600
2#	0.800	79.200	1.600	1.058	1.292	76.050	0.400	9.600

.

Preparation steps: Taking the preparation of microencapsulated sample 1# as an example:

The 0.940 g of T-80 and the 1.410 g of SDBS were weighed and poured into a beaker containing deionized water. The mixture was stirred thoroughly to prepare a 3% emulsifier solution. An amount of 1.600 g of tung oil was weighed and added to the beaker containing the emulsifier solution. A magnetic stir bar was placed in the beaker, which was then positioned in a thermostatic magnetic stirrer. The stirring speed was adjusted to 1800 rpm with the temperature set at 50 °C, and the mixture was stirred continuously for 1 h. The beaker was then transferred to an ultrasonic emulsification disperser, where it was ultrasonically treated for 5 min under intermittent conditions (1 s pulse followed by 2 s pause), resulting in a milky-white liquid that served as the core material emulsion.

The 0.800 g of chitosan was weighed and added to a beaker along with 79.200 g of 1% acetic acid solution. The beaker was placed in a thermostatic magnetic stirrer and heated at 50 °C while stirring at 1000 rpm for 1 h until complete dissolution of chitosan was achieved, resulting in a 1% chitosan solution. The 1% chitosan solution was added dropwise into the beaker containing the core material emulsion. The pH of the mixture was adjusted to 4.5, followed by heating and stirring at 50 °C and 1000 rpm in a water bath for 1 h to ensure thorough mixing between the core material emulsion and wall material solution. An amount of 0.400 g of STPP was weighed and dissolved in 9.600 g of deionized water to prepare a 4% STPP solution. This solution was then added dropwise to the previously prepared mixture of core material emulsion and wall material to induce a cross-linking reaction with the chitosan wall material. The cross-linking reaction was carried out at 50 °C with stirring at 1000 rpm for 2 h. The resulting mixture was then allowed to stand at room temperature for 24 h before proceeding to spray drying. The spray drying was performed with an inlet temperature of 120 °C and a feeding rate of 150 mL/h. The dried product obtained was the microcapsule powder.

2.3. Coating Method

The cherry wood boards were hand-brushed using the two-base–two-top method, with two coats of primer and two coats of topcoat applied. The substrate surface was pre-sanded with 1000-grit sandpaper prior to finishing to remove burrs and improve paint film adhesion [40,41]. Subsequently, 0.400 g of primer was weighed and brush-coated onto the cherry wood board surface. After leveling for 10 min, the coated board was placed into a UV curing machine and cured for 60 s. The coated surface was then sanded with abrasive paper, after which the second primer coat was applied and cured following the identical procedure, thereby completing the primer application process. Two types of chitosan–sodium tripolyphosphate-coated tung oil microcapsules were incorporated into the UV topcoat at varying concentrations of 3%, 6%, 9%, 12%, and 15% (w/w). The microcapsule-modified UV topcoat formulations were then applied onto the primed wood substrates following the identical coating procedure used for the primer application (brush coating of precisely measured 0.400 g aliquots, 10 min leveling, and 60 s UV curing). The cherry wood boards coated with microcapsule-free UV topcoat served as blank controls. The specific material formulations used in the coating process are detailed in

Table 4. Details of materials used for cherry wood board surface coating.

Coating	The Types of Added Microcapsules	Microcapsule Content (%)	Quality of UV Primer (g)	Quality of UV Topcoat (g)	Microcapsule Quality (g)
0	no	0	0.800	0.800	0.000
1	Microcapsule 1#	3	0.800	0.776	0.024
2		6	0.800	0.752	0.048
3		9	0.800	0.728	0.072
4		12	0.800	0.704	0.096
5		15	0.800	0.680	0.120
6	Microcapsule 2#	3	0.800	0.776	0.024
7		6	0.800	0.752	0.048
8		9	0.800	0.728	0.072
9		12	0.800	0.704	0.096
10		15	0.800	0.680	0.120

.

2.4. Test and Characterization

2.4.1. Optical Performance Test

Gloss measurement of the coating film: The glossiness of the coating film was tested in accordance with GB/T 4893.6-2013 [42]. The gloss meter can be used to measure the glossiness of coatings when illuminated by light sources at different angles. During gloss measurement using a gloss meter, completely cover the instrument’s measurement window with the coated surface, then press the measurement button and record the data. The glossiness of the coating surface containing microcapsules was recorded as G₁, while that of the microcapsule-free coating film was denoted as G₀. The gloss loss rate G was calculated according to Equation (1).

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

(1)

Color difference measurement of coating films: The surface color difference in the coating films was measured using a colorimeter. During testing, the color values of the microcapsule-free UV topcoat film were measured first, followed by measurement of the microcapsule-incorporated coating film, with the instrument automatically calculating the color difference ΔE [43]. The L value represents lightness, where higher L values indicate brighter sample colors, while lower values correspond to darker appearances. The a value measures the red-green chromaticity, where negative a values indicate greenish hues and positive a values denote reddish tones. The b value quantifies the yellow-blue chromaticity, where negative b values indicate bluish tones and positive b values represent yellowish hues.

The visible light transmittance of the paint film was tested: Visible light transmittance was defined as the ability of light to pass through a sample. The percentage of the luminous flux transmitted through the sample relative to the incident luminous flux was recorded as the transmittance of the prepared paint film. The visible light transmittance of coatings with and without microcapsules was tested using a UV–Vis spectrophotometer (U3900). The transmittance (τ) of each sample was calculated according to Formula (2). ${\mathsf{\tau }}_{\mathrm{t}}\left(\mathsf{\lambda }\right)$ represents the measured transmittance of the sample at a specific wavelength obtained by the UV–Vis spectrophotometer; $\mathrm{v}\left(\mathsf{\lambda }\right)$ denotes the CIE standard photopic luminosity function; $\mathrm{d}\left(\mathsf{\lambda }\right)$ indicates the relative spectral power distribution of the light source.

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

(2)

2.4.2. Self-Healing Performance Test

Self-healing performance test of the paint film: The self-healing capability of the paint film was evaluated using a scratch test. The coating surface was scratched using a blade, and the scratch width was observed and recorded using an optical microscope, with the initial width documented as W₁. The scratch at the same location was re-examined after 48 h, and its width was recorded as W₂. The self-healing rate (H) was calculated according to Equation (3) [1].

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

(3)

2.4.3. Microstructural Morphology of Coatings

The microscopic morphology of both the microcapsules and the paint film was characterized using a scanning electron microscope (SEM). When characterizing the sample morphology using the SEM, the specimens were secured to the sample stage using conductive adhesive. The samples were mounted on the sample holder in accordance with the experimental requirements. The SEM airlock was opened under safe venting conditions. The prepared sample stub was loaded onto the 5-axis motorized stage (XYZRZ configuration) and properly seated. Following chamber closure, the turbo-molecular pump engaged to achieve the manufacturer-specified operating vacuum (typically ≤ 5 × 10⁻³ Pa). At stabilized vacuum conditions, secondary electron imaging was initiated through the SEM operating system (SmartSEM V05.06), with digital image acquisition performed at calibrated working distances.

2.4.4. Chemical Composition Characterization of Paint Film

The chemical composition of the paint film was analyzed and characterized using a Fourier transform infrared (FTIR) spectrometer. Prior to testing the paint film samples, the ATR crystal surface was carefully cleaned. The test specimens were then prepared to the appropriate dimensions. The experimental parameters were adjusted according to the sample requirements. The test sample was placed on the ATR crystal surface, ensuring proper contact, and the sampling clamp was rotated to secure the specimen. All spectral data were acquired and archived through the instrument control software interface (OMNIC 9.2) [1].

2.4.5. Mechanical Performance Testing

Paint film roughness testing: The surface roughness of the samples was measured using a precision roughness tester. During the test, the sample was placed on the sample stage, and the probe position was adjusted using the knob. When the probe made contact with the sample surface, the test was initiated and the data were recorded.

Hardness test: The hardness of the cherry wood surface coating was tested according to the national standard GB/T 6739-2022 [44]. During the test, the pencil was secured in the pencil hardness tester, ensuring a 45° angle between the pencil and the coating surface. A constant load of 750 g was applied to the coating surface, and the pencil was pushed to slide across the coating at a uniform speed over a specified distance. The coating surface was examined for any scratches or penetration marks. If no scratches were observed on the coating, a harder pencil was selected for repeated testing. If scratches or penetration occurred, the hardness grade of the current pencil was recorded. The hardness rating of the coating was determined as the highest pencil grade that failed to scratch the coating surface. A higher hardness grade indicates better scratch resistance of the coating.

Adhesion test: The coating adhesion was evaluated in accordance with GB/T 4893.4-2023 [45]. The coating adhesion was tested by making a cross-cut pattern with 2 mm spacing using a multi-blade cutter on the paint film surface, followed by tape peeling to evaluate the adhesion between the paint film and substrate. The adhesion performance was evaluated by examining the paint film peeling condition in the cross-cut area under magnification, with the adhesion grade determined according to GB/T 4893.4-2023 based on the observed degree of film detachment.

Impact resistance test: According to GB/T 4893.9-2013 [46], the test panel was securely mounted on the impact tester platform, ensuring the surface was perfectly flat and perpendicular to the impact plunger. The impact plunger was released to free-fall along the vertical guide tube, striking the steel ball on the test panel surface. The coating surface was then examined under magnification to detect any cracks, peeling, or other damage. The impact resistance grade of the paint film was assessed according to the degree of surface damage observed.

3. Results and Discussion

3.1. Chemical Composition of UV-Cured Coating on Cherry Wood Board Incorporated with Chitosan–Sodium Tripolyphosphate-Coated Tung Oil Microcapsules

The FTIR spectra of coatings with varying microcapsule concentrations are presented in Figure 1. The characteristic absorption peaks at 1159 cm⁻¹ (C−O stretching vibration), 1724 cm⁻¹ (C=O stretching vibration), 2925 cm⁻¹ (C−H stretching vibration), and 1036 cm⁻¹ (C−O stretching vibration) were consistently observed in all spectra. These vibrational signatures were confirmed to originate from both the UV primer and UV topcoat layers of the coating system applied to the cherry wood substrate. The characteristic absorption peaks at 3449 cm⁻¹ (attributed to the -OH stretching vibration of chitosan), 1647 cm⁻¹ (corresponding to the -NH₂ bending vibration of chitosan), and 1168 cm⁻¹ (assigned to the P-O stretching vibration of tripolyphosphate) were clearly observed in the FTIR spectra of the microcapsule-containing coatings. Additionally, the peak at 1575 cm⁻¹, resulting from the electrostatic interaction between the amino groups of chitosan and phosphate groups of tripolyphosphate, was detected. These spectral features collectively confirmed the successful incorporation of the chitosan–sodium tripolyphosphate-coated tung oil microcapsules within the coating matrix.

3.2. Optical Properties of UV Coating on Cherry Wood Surface

As shown in Figure 2 and

Table 5. Reflectivity of cherry wood surface coatings.

Coating	Add the Types of Microcapsules	Microcapsule Content (%)	Reflectance (%)
0	no	0	44.96 ± 1.21
1	Microcapsule 1#	3	42.94 ± 1.27
2		6	41.14 ± 0.84
3		9	42.74 ± 0.81
4		12	44.03 ± 0.82
5		15	45.19 ± 1.44
6	Microcapsule 2#	3	42.64 ± 1.39
7		6	41.77 ± 0.61
8		9	43.33 ± 0.81
9		12	45.24 ± 1.48
10		15	42.42 ± 0.39

, these are the reflectance values when the coating on the cherry wood surface contains microcapsules. When no microcapsules were added to the UV coating on the cherry wood surface, the coating exhibited a reflectance of 44.96%. However, with the introduction of a small amount of microcapsules, the reflectance of the coating decreased. Specifically, when 3% of microcapsule type 1# was incorporated into the coating, the reflectance dropped to 42.94%. However, when the content of microcapsule 1# in the coating was increased to 15%, the reflectance of the coating rose to 45.19%. Although no clear pattern emerged in the reflectance among the samples with the increasing microcapsule content, an upward trend became apparent through the data fluctuations. This phenomenon may be attributed to the refractive index mismatch between the microcapsules and the UV topcoat, causing light scattering at their interfaces, which ultimately reduces the overall reflectance of the microcapsule-incorporated coating compared to the pure UV coating system. With the increase in microcapsule content in the coating, the reflectance of each coating showed an upward trend again. This could be attributed to the fact that the addition of a large number of microcapsules caused aggregation on the coating surface, which resulted in a relatively smoother surface, thereby enhancing the reflectance [47].

It is shown in Table S1 that the effects of both the microcapsule content (F = 2.285, p = 0.222) and microcapsule type (F = 0.033, p = 0.864) on the coating reflectivity were not statistically significant (p > 0.05). Although the sum of squares (SS = 11.29984) of the microcapsule content was relatively large, its F-value was lower than the critical value (F-crit = 6.388), indicating that the differences in reflectivity among different contents were not significant. Meanwhile, the SS of the microcapsule type was only 0.04096, and its influence was considered negligible.

As presented in

Table 6. Chromaticity and color difference values of cherry wood surface coating.

Coating	Add the Types of Microcapsules	Microcapsule Content (%)	Chromaticity Value			Color Difference Value ΔE
			Lightness Value L	Red and Green Values a	Yellow-Blue Value b
0	no	0	62.35	15.60	24.15	-
1	Microcapsule 1#	3	59.91	14.17	24.43	2.84 ± 0.08
2		6	58.35	13.96	25.32	4.47 ± 0.09
3		9	59.24	12.63	24.94	4.37 ± 0.12
4		12	58.32	13.63	26.85	5.21 ± 0.11
5		15	57.46	11.44	28.90	7.98 ± 0.07
6	Microcapsule 2#	3	61.83	14.71	23.31	1.34 ± 0.02
7		6	59.31	13.70	25.47	3.80 ± 0.06
8		9	58.27	13.93	25.15	4.51 ± 0.09
9		12	57.62	12.68	24.60	5.58 ± 0.16
10		15	57.34	11.13	27.44	7.46 ± 0.10

, the chromaticity values of and color differences in cherry wood surface coatings containing different microcapsules are shown. For the coating without microcapsules, the lightness (L), red-green (a), and yellow-blue (b) values were measured at 62.35, 15.60, and 24.15, respectively. The incorporation of microcapsules led to varying degrees of lightness (L) reduction in the coatings. When both types of microcapsules were added at 15% loading, the L values decreased to 57.46 and 57.34, respectively. With decreasing microcapsule content in the coating, the a showed a corresponding reduction, while the yellow-blue value (b) progressively increased. This phenomenon was attributed to the light-scattering and absorption properties of the white powdery microcapsules composed of chitosan–sodium tripolyphosphate-coated tung oil. Their incorporation into the coating system significantly altered the optical pathways, leading to the observed reduction in lightness (L). The introduction of white chitosan–sodium tripolyphosphate-coated tung oil microcapsules neutralized the inherent redness of the cherry wood substrate, reducing the reflection of red wavelengths and consequently shifting the a* value toward the negative axis (green direction) in the CIELAB color space. The increase in the yellow-blue (b) value indicated a progressive enhancement in yellow components in the coating with the addition of microcapsules, primarily attributed to the tung oil core material. The inherent yellowness of the tung oil core material (a yellow liquid) combined with the microporous structure of the microcapsule shells resulted in a slightly yellowish-white appearance of the chitosan–sodium tripolyphosphate-coated tung oil microcapsules. As the microcapsule concentration increased, cumulative yellow components led to progressive enhancement in the b (yellow-blue) value.

As shown in Table 6, the color difference (ΔE) in the cherry wood surface coatings exhibited a progressive increase with a higher microcapsule content. Among all the tested samples, sample 5 containing 15% microcapsule 1# demonstrated the most significant color variation, reaching a maximum ΔE value of 7.98. Sample 6, containing 3% microcapsule 2#, exhibited the smallest color difference (ΔE) of 1.34 among all tested formulations. Comparative analysis of the two microcapsule types revealed a consistent positive correlation between the microcapsule concentration and color difference (ΔE) for both formulations. When the microcapsule content in the coating was 3%, 6%, and 15%, the color influence on the cherry wood surface coating was found to be smaller for coatings containing microcapsule 2# compared to those containing microcapsule 1#, and a significant difference in color deviation was observed between the two. When the microcapsule content was 9% and 12%, coatings with microcapsule 1# exhibited less color influence on the cherry wood surface coating than those with microcapsule 2#, though the difference in color deviation between them was smaller. Therefore, it was concluded that, overall, coatings containing microcapsule 2# had a smaller impact on the color of the cherry wood surface coating compared to those containing microcapsule 1#.

As shown in Table S2, the microcapsule content was found to have a highly significant effect on the color difference in the coating (F = 30.87, p = 0.0029), indicating that different addition levels (3−15%) significantly altered the color performance. In contrast, the influence of the microcapsule type was not significant (F = 1.75, p = 0.257), suggesting that the difference between 1# and 2# microcapsules could be neglected in terms of color variation.

As shown in

Table 7. Glossiness and gloss loss rate of cherry wood board surface coating.

Coating	The Types of Added Microcapsules	Microcapsule Content (%)	Glossiness (GU)			Loss of Light Rate (%)
			Incident Angle 20°	Incident Angle 60°	Incident Angle 85°
0	no	0	22.10	57.30	58.50	-
1	Microcapsule 1#	3	9.50	32.90	30.90	42.58 ± 0.32
2		6	5.10	19.00	14.80	66.84 ± 0.33
3		9	4.40	9.30	2.50	83.77 ± 1.00
4		12	2.50	10.30	4.20	82.02 ± 1.08
5		15	1.60	5.70	1.50	90.05 ± 0.84
6	Microcapsule 2#	3	10.60	38.40	37.10	32.98 ± 0.22
7		6	5.40	18.10	16.40	68.41 ± 0.76
8		9	2.80	11.80	7.50	79.41 ± 0.86
9		12	1.90	6.20	2.80	89.18 ± 0.74
10		15	1.20	4.80	1.70	91.62 ± 0.91

, the gloss values and gloss loss rates of the cherry wood surface coatings are presented. The coating without microcapsules exhibited the highest gloss levels, with measured values of 22.10 GU (20°), 57.30 GU (60°), and 58.50 GU (85°) under the three standard measurement angles. Among all the microcapsule-modified samples, sample 6 containing 3% microcapsule 2# exhibited the highest gloss levels with measurements of 10.60 GU (20°), 38.40 GU (60°), and 37.10 GU (85°), representing a 32.98% reduction in 60° gloss compared to the unmodified control sample, indicating that even at optimal loading, microcapsule incorporation significantly impacts surface reflectivity while maintaining functional gloss properties for practical applications. With the increasing microcapsule content in the coating, the progressive accumulation of microcapsules led to a sharp decline in the glossiness of the cherry wood panel coating. Notably, when the microcapsule concentration reached 15%, the coating exhibited extreme gloss loss rates of 90.05% and 91.62% under different measurement conditions. This is likely caused by the uneven dispersion of microcapsules in the coating, which resulted in localized accumulation of microcapsules and the formation of coarser surface structures. Consequently, diffuse reflection increased while glossiness decreased, giving the coating a visually matte appearance.

As presented in Table S3, the microcapsule content was observed to exert a highly significant influence on the gloss loss rate of the coating (F = 44.19, p = 0.0014), demonstrating that different addition levels (3−15%) significantly affected the surface gloss. However, the effect of the microcapsule type was not found to be significant (F = 0.065, p = 0.811), indicating that no difference was observed between 1# and 2# microcapsules in terms of gloss impact.

Compared with the coating containing chitosan/arabic gum-encapsulated tung oil microcapsules [48], the coating with chitosan–sodium tripolyphosphate-coated tung oil microcapsules exhibited a similar reflectance trend, which is attributed to the minor difference in particle size between the two types of microcapsules. However, lower color difference values were consistently observed in the coating containing chitosan–sodium tripolyphosphate-coated tung oil microcapsules when compared to that with chitosan/arabic gum microcapsules. This phenomenon is attributed to the yellowish powder characteristic of arabic gum, which was used as one of the wall materials. Consequently, microcapsules prepared with chitosan/arabic gum as the wall material were found to exhibit a yellowish coloration. As a result, a more significant effect on the coating’s color difference was caused by these microcapsules than by those employing chitosan–sodium tripolyphosphate as the wall material. However, higher gloss values were consistently observed in the coating containing chitosan/arabic gum-encapsulated tung oil microcapsules compared to that with chitosan–sodium tripolyphosphate-coated tung oil microcapsules. This result suggested that better compatibility with the UV coating system was achieved by the microcapsules using chitosan/arabic gum as the wall material. Consequently, superior optical performance was demonstrated when these microcapsules were incorporated into the UV coating formulation.

3.3. Mechanical Properties of UV Coatings on Cherry Wood Surfaces

Table 8. Roughness of cherry wood surface coating.

Coating	The Types of Added Microcapsules	Microcapsule Content (%)	Roughness (μm)
0	no	0	0.42 ± 0.01
1	Microcapsule 1#	3	0.58 ± 0.01
2		6	1.11 ± 0.02
3		9	1.91 ± 0.02
4		12	3.22 ± 0.03
5		15	4.00 ± 0.05
6	Microcapsule 2#	3	0.57 ± 0.01
7		6	1.32 ± 0.01
8		9	2.21 ± 0.02
9		12	3.25 ± 0.04
10		15	4.27 ± 0.02

presents the surface roughness test results of the coatings applied to cherry wood substrates. When only UV primer and UV topcoat were applied to the cherry wood surface, the surface roughness of the coating was measured to be 0.42 μm. The surface roughness of the coating increased with a higher microcapsule content. When the coating contained 3% of microcapsule 1# and 2#, the roughness values reached 0.58 μm and 0.57 μm, respectively. Notably, the maximum roughness of 4.27 μm was observed when 15% of microcapsule 2# was incorporated into the coating. The surface roughness of the coating progressively increased with a higher microcapsule content, which was consistent with the morphological characteristics observed in the SEM images. The coating containing chitosan–sodium tripolyphosphate-coated tung oil microcapsules exhibited higher surface roughness compared to the chitosan/arabic gum microcapsule-incorporated coating [48], consequently resulting in the lower glossiness of the former system.

As shown in Table S4, the microcapsule content was demonstrated to have an extremely significant effect on the coating roughness (F = 421.811, p = 1.68 × 10⁻⁵), indicating that different addition levels (3−15%) significantly modified the surface roughness. In contrast, the influence of the microcapsule type was found to be marginally significant (F = 6.4, p = 0.065), approaching but not falling below the 0.05 significance threshold, which suggested that a slight difference in roughness might exist between the 1# and 2# microcapsules.

Table 9. Mechanical performance testing results of cherry wood surface coating.

Coating	The Types of Added Microcapsules	Microcapsule Content (%)	Impact Resistance Grade	Adhesion Grade	Hardness
0	no	0	5	1	4H
1	Microcapsule 1#	3	5	1	3H
2		6	4	1	3H
3		9	3	2	2H
4		12	3	3	H
5		15	3	3	H
6	Microcapsule 2#	3	4	1	4H
7		6	4	2	3H
8		9	3	2	3H
9		12	3	3	2H
10		15	3	3	H

presents the mechanical property test results for the cherry wood surface coatings. The coating system consisting of a UV primer and microcapsule-free UV topcoat on cherry wood substrates exhibited optimal mechanical performance, with an impact resistance rating of Grade 5, excellent adhesion classification of Class 1, and superior surface hardness measuring 4H. The impact resistance of the coating improved to Grade 4 when the content of microcapsule 1# exceeded 6% and microcapsule 2# surpassed 3%. Further enhancement to Grade 3 was achieved with increasing microcapsule loading, demonstrating a clear concentration-dependent reinforcement effect. The coating’s adhesion strength demonstrated an inverse relationship with the microcapsule content, showing measurable degradation when the concentrations surpassed critical thresholds: the adhesion performance decreased from Class 1 to Class 2 upon exceeding 6% loading for both microcapsule 1# and the microcapsule 2# series. The relationship between the coating hardness and microcapsule content revealed a negative correlation, where increasing microcapsule loading adversely affected the hardness performance. Notably, at the maximum tested concentration of 15%, the coating exhibited its lowest hardness value of merely H grade.

Compared to the mechanical performance observed with the chitosan/arabic gum-encapsulated tung oil microcapsules [48], the coatings incorporating both types of microcapsules demonstrated comparable results in the impact resistance and adhesion tests. When containing equal microcapsule concentrations, coatings with these two distinct wall material systems showed negligible performance differences in standardized evaluations. The coating containing chitosan–sodium tripolyphosphate-coated tung oil microcapsules exhibited more significant hardness variation compared to the chitosan/arabic gum system. At 15% loading of either microcapsule 1# or 2#, the coating hardness decreased to H grade. In contrast, the coating maintained 2H hardness when incorporating 15% chitosan/arabic gum-encapsulated tung oil microcapsules, demonstrating superior stability. This clearly demonstrates that the microcapsules prepared using chitosan/arabic gum as the wall material possess superior mechanical properties.

3.4. Self-Healing Performance of UV Coating on Surface of Cherry Wood Boards

The scratch repair test results for coatings containing different microcapsules are shown in

Table 10. Self-repairing test of cherry wood board surface coating.

Coating	The Types of Added Microcapsules	Microcapsule Content (%)	The Width of the Scratch Before Repair (μm)	The Width of the Scratch After Repair (μm)	Repair Rate (%)
0	no	0	12.99	12.99	-
1	Microcapsule 1#	3	15.05	13.65	9.30 ± 0.11
2		6	12.84	10.65	17.06 ± 0.12
3		9	12.38	10.02	19.06 ± 0.37
4		12	15.05	12.95	13.95 ± 0.28
5		15	13.65	11.90	12.82 ± 0.13
6	Microcapsule 2#	3	12.65	11.47	9.33 ± 0.16
7		6	12.95	10.85	16.22 ± 0.15
8		9	15.25	12.62	17.25 ± 0.31
9		12	15.06	13.65	9.36 ± 0.19
10		15	18.02	15.75	12.60 ± 0.40

. Figure 3 shows the scratch repair performance of coatings containing different concentrations of microcapsule 1# on cherry wood surfaces. The coating containing 3% microcapsules achieved a scratch-healing efficiency of 9.30%. The scratch-healing efficiency of the coating initially increased and then decreased with the increasing microcapsule content, reaching a maximum repair rate of 19.06% at 9% microcapsule loading. Figure 4 demonstrates the scratch-healing performance of coatings with varying concentrations of microcapsule 2#. The relationship between the microcapsule content and healing efficiency in these coatings showed a similar trend to that observed with microcapsule 1#, with both demonstrating an initial increase followed by a decrease in the repair rate. The coating demonstrated optimal scratch-healing performance with a 17.25% repair rate at 9% microcapsule 2# loading. As shown in Table S5, the microcapsule content was found to have a significant effect on the self-healing efficiency (F = 15.046, p = 0.011), while the influence of the microcapsule type was not observed to be significant (F = 3.142, p = 0.151). This indicated that the self-healing performance was significantly affected by different microcapsule loading levels.

Similar to the coatings incorporated with chitosan/arabic gum microcapsules [48], both types of microcapsule-modified coatings were demonstrated to acquire certain self-healing capabilities. A similar trend was observed in the relationship between the content and repair rate, where the self-healing performance of the coatings initially increased and subsequently decreased with the increasing microcapsule concentration. The highest self-healing efficiency was achieved at a microcapsule content of 9% in the coating system. The self-healing performance evaluation revealed that cherry wood coatings containing 9% chitosan/arabic gum-encapsulated tung oil microcapsules achieved a scratch repair rate of 31.32%, whereas those incorporating 9% chitosan–sodium tripolyphosphate-coated tung oil microcapsules (designated as Type 1#) exhibited a lower repair efficiency of 19.06%. The superior surface smoothness was presumably caused by the enhanced compatibility between the UV topcoat and the microcapsules with chitosan/arabic gum walls. A surface roughness of 1.20 μm was recorded for the coating containing 9% of these microcapsules. The coating containing 9% of microcapsule 1# exhibited a higher surface roughness of 1.91 μm, which was attributed to the accumulation and agglomeration of microcapsules on the coating surface that hindered the release of healing agents, consequently impairing the scratch repair capability.

The performance of various samples was compared. When the content of microcapsules did not exceed 9%, the coatings with both types of microcapsules were observed to exhibit an upward trend. Among them, the better repair effects were achieved as follows: a repair rate of 17.25% was recorded for sample 8 (with 9% microcapsule 2# added), a repair rate of 19.06% was obtained for sample 3 (with 9% microcapsule 1# added), and a repair rate of 17.06% was achieved by coating 2 (with 6% microcapsule 1# added). However, when the microcapsule content in the coating reached 9%, a higher surface roughness was observed. The roughness of sample 8 and sample 3 was measured at 1.91 μm and 2.21 μm, respectively, while that of coating 2 was 1.11 μm. Additionally, coating 2 demonstrated superior impact resistance, adhesion, and hardness compared to samples 3 and 8. Compared to the coating without microcapsules, sample 2 exhibited a smaller color difference and lower gloss loss. Based on the comprehensive test results, it was found that coating 2—the cherry wood coating with 6% microcapsule 1#—demonstrated the best overall performance in this study.

3.5. Microscopic Morphology of UV Coating on Surface of Cherry Wood Boards

As shown in Figure 5, the microscopic morphology of the UV coating on the cherry wood surface is presented. The UV coating without microcapsules exhibited a relatively smooth surface with minor depressions. The incorporation of microcapsules was found to mitigate these surface depressions; however, particulate protrusions were observed due to the non-uniform dispersion of microcapsules within the coating. As the microcapsule content increased, these aggregates became more numerous and larger in size. This phenomenon was determined to contribute to the reduction in both the glossiness and surface smoothness of the coating.

Figure 6 presents the cross-sectional SEM images of the cherry wood panels. The coating was observed to adhere relatively uniformly to the wood surface. This indicates that the UV coating application provided certain sealing effects to the substrate. UV coating material was observed within the vessel pores of cherry wood, demonstrating successful penetration of the UV primer into the wood structure.

3.6. Comparison of Surface Coating Properties of Cherry Wood Boards with Two Types of Microcapsules Added and Self-Healing Mechanism of Coatings

The comparative performance results of surface coatings containing two types of microcapsules on cherry wood substrates are presented in

Table 11. Properties of cherry wood surface coatings.

Coating Name	Self-Healing Performance	Optical Performance			Mechanical Properties
	Repair Rate (%)	Visible Light Reflectance (%)	Loss of Light Rate (%)	Color Difference Value	Impact Resistance Grade	Adhesion Grade	Hardness	Roughness (μm)
Cherry wood coating containing 6% chitosan/arabic gum-encapsulated tung oil microcapsules	22.61	45.25	13.44	1.31	4	2	4H	0.71
Cherry wood coating containing 6% chitosan–sodium tripolyphosphate-coated tung oil microcapsules 1#	17.06	41.14	66.84	4.47	4	1	3H	1.11

. The cherry wood panel coated with 6% chitosan/arabic gum-encapsulated tung oil microcapsules demonstrated optimal coating performance. Coating 2 of the cherry wood panels containing 6% microcapsule 1# exhibited superior performance characteristics. Comparative analysis revealed that the cherry wood coating containing 6% chitosan/arabic gum-encapsulated tung oil microcapsules demonstrated significantly superior performance in the self-healing rate, glossiness, color difference, and surface roughness when compared to coating 2 containing 6% chitosan–sodium tripolyphosphate-coated tung oil microcapsules 1#. The two coatings exhibited comparable results in visible light reflectivity, impact resistance, adhesion, and hardness tests. Comprehensive evaluation of all the test results indicated that the cherry wood panel coated with 6% chitosan/arabic gum-encapsulated tung oil microcapsules exhibited optimal overall coating performance. Under these conditions, the coating exhibited a UV–visible reflectance of 45.25%, with measured color parameters of L = 61.62 (lightness), a = 15.20 (red-green value), and b = 25.16 (yellow-blue value). The total color difference (ΔE) was recorded as 1.31, while the gloss loss reached 13.44% at a 60° incident light angle. The coating exhibited the following performance characteristics: surface roughness of 0.71 μm was recorded, with an impact resistance rating of Grade 4, adhesion strength of Grade 2, and pencil hardness of 4H. Additionally, a scratch self-healing efficiency of 22.61% was achieved.

The self-healing mechanism of the paint film is illustrated in Figure 7. When the coating is damaged and cracks appear, the microcapsules rupture due to stress as cracking occurs in the coating, thereby releasing the core repair agent. Upon rupture of the microcapsules, the tung oil flows into the crack regions. Due to the abundant unsaturated conjugated double bonds in its molecular structure, the tung oil reacts with atmospheric oxygen and subsequently undergoes curing to form a film, thereby repairing the cracks. Consequently, the incorporation of self-healing microcapsules into the coating imparts autonomous repair capability to the coating system.

4. Conclusions

The effect of chitosan–sodium tripolyphosphate-coated tung oil microcapsules on the performance of UV coatings on cherry wood surfaces was investigated by incorporating the microcapsules into the surface UV varnish at different ratios. As the content of microcapsules in the coating increased, the reflectance of the coating showed an upward trend. When the coating contained 15% of microcapsule 1#, the reflectance reached 45.19%. When the coating contained 15% of microcapsule 1#, the color difference reached 7.92, while the coating with 3% microcapsule 2# exhibited the smallest color difference of 1.34. Gloss loss showed a positive correlation with the microcapsule concentration. The minimum gloss loss (32.98% at 60°) was observed in the coating containing 3% microcapsule 2#, while higher contents resulted in increased gloss loss. A direct proportionality was observed between the coating surface roughness and microcapsule concentration. As the microcapsule content increased, the coating showed enhanced impact resistance but reduced adhesion and hardness. The 3% microcapsule 2# formulation exhibited good balanced properties with Grade 4 impact resistance, Grade 1 adhesion, and 4H hardness. At 9% loading of microcapsule 1#, the coating achieved its peak self-repairing performance with the healing efficiency reaching 19.06%. Based on the comprehensive test results, UV coating 2 (containing 6% microcapsule 1#) demonstrated optimal performance characteristics on cherry wood substrates. The coating exhibited the following comprehensive performance characteristics: a UV–visible reflectance of 41.14%, with a total color difference of 4.47. The surface roughness was measured at 1.11 μm. The mechanical properties included Grade 4 impact resistance, Grade 1 adhesion, and 3H pencil hardness, while demonstrating a 17.06% self-repair rate. Comparative analysis between the chitosan/arabic gum-encapsulated tung oil microcapsules and chitosan–sodium tripolyphosphate-coated tung oil microcapsules revealed that the former demonstrated superior performance in UV coatings on cherry wood substrates, exhibiting effective crack repair capability. The optimal coating formulation containing 6% chitosan/arabic gum-encapsulated tung oil microcapsules achieved a self-healing efficiency of 22.61%, representing a 5.55% enhancement over coating 2 with the chitosan–sodium tripolyphosphate-coated tung oil microcapsules.