Next Article in Journal
Gelatin–Sodium Alginate Composite Hydrogel for Sustained Release of Simvastatin Enabled Osteogenic Differentiation
Previous Article in Journal
Influence of Cooling Methods on Microstructure and Mechanical Properties of TiB2@Ti/AlCoCrFeNi2.1 Eutectic High-Entropy Alloy Matrix Composites
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Coatings
Volume 15
Issue 9
10.3390/coatings15091003
coatings-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effect of Shellac–Rosin Microcapsules on the Self-Healing Properties of Waterborne Primer on Wood Surfaces

by
Jingyi Hang
1,2,
Yang Han
1,2,
Xiaoxing Yan
1,2,* and
Jun Li
1,2
1
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
2
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(9), 1003; https://doi.org/10.3390/coatings15091003
Submission received: 18 July 2025 / Revised: 22 August 2025 / Accepted: 26 August 2025 / Published: 29 August 2025
(This article belongs to the Section Functional Polymer Coatings and Films)
Browse Figures
Versions Notes

Abstract

Microcapsules with self-healing properties were synthesized via emulsion polymerization, with melamine formaldehyde resin serving as the microcapsule wall and a mixture of shellac and rosin solutions encapsulated as the core. The core–wall ratio was tested as a variable, and two ratios of 0.59:1 and 0.68:1 were selected for the preparations. Microcapsules were added to the waterborne paint films at concentrations of 0%, 5.0%, 10.0%, 15.0%, 20.0%, and 25.0%, respectively, to obtain self-healing waterborne paint films with different microcapsule contents. The effects of microcapsules with different core–wall ratios and concentrations in the paint films on the optical, mechanical, and self-healing properties of the paint film were investigated. The results showed that an increase in microcapsule content led to a corresponding rise in the paint film ∆E. The coating-film glossiness exhibited a decreasing tendency with an increase in microcapsule concentration. When the content of microcapsules was below 15.0%, the coating maintained a high level of adhesion and hardness. However, once the concentration surpassed 20.0%, coating adhesion began to decline. The addition of microcapsules effectively enhanced the thermal-aging resistance of the paint film. At the core–wall ratio of 0.68:1 and a microcapsule content of 20.0%, the paint film exhibited the best aging resistance, and the repair rate reached 16.1%, indicating good self-healing performance.

1. Introduction

Wood is a natural heterogeneous polymer composite material with a porous structure and is easily affected by environmental factors [1,2,3,4]. Surface treatments such as coating are required to protect, isolate, and enhance the esthetic appearance of the substrate. However, during prolonged use, the paint film on the wood surface is often damaged due to various uncontrollable factors, leading to the formation of small cracks [5,6,7,8,9,10]. At the same time, the shrinkage and swelling characteristics of wood will also affect it. Changes in humidity will cause stress in the interior of the coating applied onto wood surfaces, potentially leading to micro-cracks [11,12]. Over time, the performance of the finish and its protective capacity toward the substrate will gradually diminish, ultimately leading to its failure and loss of protection for the wood [13,14,15]. In order to solve this problem, self-healing microcapsule materials have gradually become a hot research topic in recent years [16,17,18,19]. Chang et al. [20] reviewed the microcapsule-based wood self-healing coating technology and introduced its repair mechanism, preparation method, and performance-influencing factors. The microcapsule self-healing coating technology is inspired by the phenomenon of biological self-repair. Self-healing microcapsules can release healing agents when the paint film is damaged, filling the cracks and restoring the coating functionality, thereby prolonging the durability of the coating and enhancing its protective effect [21].
Shellac is a natural resinous substance secreted by the lac insect, which parasitizes certain trees in India and Southeast Asia [22,23,24]. It is harvested, extracted, purified, and processed to obtain the final product. The major ingredient in shellac is lac resin, accompanied by small amounts of wax, pigments, and other organic substances [25]. Its chemical composition is complex, being mainly composed of hydroxyl fatty acids and aromatic hydroxyl acid polymers [26]. Therefore, it has good film-forming properties and adhesion and excellent optical transparency. Cho et al. [27] found that shellac could form a uniform blend structure with polyacrylonitrile, which significantly improved the tensile strength and energy storage modulus of the composite film through hydrogen bonding, indicating that shellac had potential in enhancing the mechanical properties of the coating. Due to its excellent water resistance and environmentally friendly properties, shellac is commonly used as a natural coating material, showing great potential in enhancing the protective performance of furniture and handicraft surfaces [28]. Shellac is rich in hydroxyl (–OH) functional groups, has good film formation and strong adhesion, can form a protective film in damaged parts, and has a stable chemical structure, which help to delay material aging. Acarali et al. [29] used shellac as an additive to study its influence on the homogeneity and properties of waterborne acrylic organic paint films. The results indicated that shellac enhanced the homogeneity and overall performance of the paint film by improving its glossiness properties. Liu et al. [30] impregnated shellac into wood through vacuum pressure to study its effect on wood dimensional stability and thermal stability. The results showed that shellac treatment significantly improved the dimensional stability of the wood. After impregnation, the wood weight increased by 13.01%, with the measurements indicating a 20.13% decline in tangential expansion and a 24.12% decrease in radial expansion. The hygroscopicity was reduced by 38.15%, and the thermal stability was improved. Sozen [31] impregnated pine (Pinus sylvestris), beech (Fagus sylvatica), and fir (Abies alba) with shellac at different concentrations together with sodium hydroxide solution to study their effect on the physical and mechanical properties of wood. The results showed that the bending strength of pine (Pinus sylvestris) and fir (Abies alba) was highest at a shellac concentration of 5%, while that of beech (Fagus sylvatica) reached its maximum at a 1% concentration. There was no evident alteration in the wood’s chemical properties due to shellac. Wang et al. [32] proposed a novel method for imparting superhydrophobic properties to wood by impregnating it with a mixture of ethyl cellulose and shellac. Shellac promoted the dispersion of ethyl cellulose and enhanced its adhesion to the wood substrate, while stearic acid further improved the surface hydrophobicity. The treated wood surface exhibited a water contact angle of 162° and a sliding angle of 2.7°, demonstrating excellent superhydrophobicity and self-healing ability. It maintained an excellent performance even after exposure to acidic and alkaline environments, surface damage, or UV aging, making it suitable for both indoor and outdoor applications. Mohamed et al. [33] boosted the properties of shellac-based coatings via the addition of introducing functionalized nanoparticles (ZnO and ZrO2). The functionalized nanoparticles were grafted onto shellac through a bifunctional silane coupling agent, improving the light stability, antifungal properties, and hardness of the shellac. The research results indicated that the coatings containing functionalized ZnO exhibited light-protection properties, while the ZrO2-modified shellac coatings demonstrated greater hardness. In addition, compared with pure shellac, the two kinds of nanocomposites exhibited lower solubility in alcohol, demonstrating better durability.
Rosin is a resin obtained from pine trees (family Pinaceae) and is primarily composed of resin acids [34,35,36]. It is soluble in organic solvents such as ethanol and ether. Rosin can be obtained from various tree species similar to pines around the world, and it possesses excellent properties, such as moisture resistance, corrosion resistance, and insulation [37,38,39]. Rosin contains carboxyl groups (–COOH) and unsaturated carbon–carbon double bonds (C=C), exhibiting excellent fluidity and thermoplastic properties. When heated, it softens to fill cracks. The –OH groups in shellac can undergo esterification with the –COOH groups in rosin. This reaction not only delays the aging process of shellac but also enhances the fluidity and water resistance of the core material, thereby improving the stability and effectiveness of the self-healing agent. Related studies have shown that mixing rosin with shellac can improve the aging resistance and alcohol resistance of shellac-based coatings [40]. Han et al. [41] examined the role of self-healing microcapsules in enhancing the performance of wood-surface paint film by repeatedly impregnating ebony substrates with a microcapsule emulsion composed of shellac and rosin encapsulated by melamine formaldehyde (MF) resin. The findings revealed that as impregnation cycles increased, the bonding strength, durability against impact, and aging resistance all improved. The self-healing performance was also improved. When the number of impregnation cycles reached eight, the crack-width variation rate of the self-healing coating on the surface reached 28.4%, showing the best repair performance.
The Monopetalanthus spp. wood is dense and solid, with uniformly distributed catheters, thick cell walls, and a high content of hydrophobic substances, giving it strong corrosion resistance and excellent natural weatherability [42]. Its texture is delicate and beautiful, featuring a combination of straight and interlocked wavy grain patterns [43]. The surface has a soft luster, making it easy to polish or paint. Meanwhile, Monopetalanthus spp. wood has moderate hardness and high bending strength, making it suitable for precision engraving, mortise-and-tenon furniture structures, and decorative paneling processes [44]. Based on the above characteristics, Monopetalanthus spp. wood was selected as the test wood substrate. In this study, shellac–rosin microcapsules encapsulated with MF resin were prepared by emulsion polymerization. The synthesized microcapsules were incorporated into the waterborne primer to obtain paint films with varying microcapsule contents. The effects of different core–wall ratios and microcapsule contents on the optical properties, mechanical properties, and self-healing performance of the water-based coating were investigated, opening up new avenues for applying microcapsule-based self-healing technology in the design and protection of wooden furniture.

2. Materials and Methods

2.1. Materials and Equipments

The information about the materials used in the experiment is shown in Table 1, and the list of equipment required for the procedures is provided in Table 2. The Monopetalanthus spp. wood substrate used in the test had dimensions of 100 mm × 50 mm × 5 mm. The Dulux waterborne primer was supplied by Dulux Coatings Co., Ltd., Nanjing, China.

2.2. Microcapsule Preparation Method

Microcapsules were prepared by emulsion polymerization, with the MF resin as the wall material and a mixed solution of shellac and rosin as the core material. The experiment was conducted using the core–wall ratio as the primary parameter; based on the optimal core–wall ratio range (0.58–0.75) determined in a previous study [40], the two ratios of 0.59:1 and 0.68:1 were selected for the preparations.
(1) Emulsification of shellac core material: Both Span-20 and Tween-80 were weighed at an amount of 0.15 g and placed into a beaker to function as emulsifiers, followed by the addition of 78.9 mL of anhydrous ethanol. The solution was thoroughly mixed until complete dissolution was achieved. Then, 8.8 g of a uniformly mixed rosin and shellac (1:1 mass ratio) was precisely weighed and slowly added to the beaker and continuously stirred to fully emulsify. After emulsification, the mixture was transferred to a 60 °C constant-temperature water bath and continuously stirred at the preset speed for 1 h to obtain a uniform and stable core emulsion.
(2) Preparation of the wall material prepolymer: A mixture of formaldehyde solution (17.0 g) and melamine (8.7 g) for a core–wall ratio of 0.59:1 was prepared in a beaker, followed by the addition of 30 mL of distilled water. For a core–wall ratio of 0.68:1, a mixture of formaldehyde solution (14.7 g) and melamine (7.6 g) was prepared in a beaker, followed by the addition of 30 mL of distilled water. Then, triethanolamine was added dropwise until the solution reached a pH of 8–9. Subsequently, the beaker containing the mixed solution was placed in a 60 °C constant-temperature water bath and stirred at 600 rpm for 30 min, maintaining the temperature until the solution became clear and transparent, resulting in a hydroxymethyl melamine mixture.
(3) Microencapsulation: The core material was slowly mixed with the preformed wall material under continuous stirring at 600 rpm, and then the mixture was transferred to the ultrasonic emulsification disperser for 15 min of ultrasonic treatment with continuous stirring. Then, the mixture after ultrasonic treatment was placed into a water bath. While stirring at an appropriate rate, citric acid was used as the pH-adjusting agent for the solution. Once the pH dropped to around 4.5, the system temperature was gradually raised to 60 °C and kept at that temperature for 3 h to obtain the microcapsule solution.
(4) Vacuum filtration and drying: The microcapsule solution was rinsed and vacuum-filtered several times to remove residual impurities. Then, the filtered product was placed in an oven at 40 °C and dried for 24 h to obtain a light-yellow powder, which consisted of the shellac–rosin microcapsules encapsulated with MF resin.

2.3. Preparation Method for the Primer Film

The moisture content of Monopetalanthus spp. wood before application was about 10%. The boards were conditioned at room temperature and 50.0% ± 5.0% relative humidity for 7 days to reach a 15.2% equilibrium moisture content. Then, microcapsules with the two different core–wall ratios were added to the Dulux waterborne primer at mass fractions of 5.0%, 10.0%, 15.0%, 20.0%, and 25.0%. According to Table 3, the appropriate amounts of microcapsules and waterborne primer were prepared and uniformly applied to the surface of Monopetalanthus spp. wood with a brush. After air-drying at room temperature for 10 min, the coated samples were transferred to a 40 °C electric-blower drying box and dried for 20 min. The surface was sanded with 800-grit sandpaper until a smooth and even finish was achieved, followed by brushing off the dust. This process was repeated twice.

2.4. Test and Characterization

2.4.1. Microscopic Morphological Characterization

Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the microstructural features of the coatings.

2.4.2. Chemical Constituent Testing

Fourier-transform infrared (FTIR) spectroscopy was employed to analyze the chemical structure of the microcapsules and coatings.

2.4.3. Optical Performance Testing

(1) Color difference: A portable color difference meter was applied to collect the L, a, and b values of the paint surface in accordance with the reference standard GB/T 11186.3-1989 [45]. The L value represents the lightness and darkness tendency, the a value represents the red–green tendency, and the b value represents the yellow–blue tendency. A test point was selected on the film, and the test data were recorded as L0, a0, and b0. The color difference ΔE was calculated according to Formula (1). Measurements were performed in triplicate for each dataset, and the average was used for analysis.
Δ E = [ ( L − L 0 ) 2 + ( a − a 0 ) 2 + b − b 0 ) 2 1 2
(2) Glossiness: The glossiness of the paint film was tested in accordance with the standard GB/T 4893.6-2013 [46]. The glossiness of the paint film was measured and recorded using a gloss meter at three angles of incidence: 20°, 60°, and 85°.

2.4.4. Mechanical Performance Testing

(1) Hardness: In accordance with the reference standard GB/T 6739-2006, a pencil-hardness tester was employed to determine the paint film hardness [47]. Pencils ranging from 6B (softest) to 6H (hardest), with HB as the midpoint, were prepared for the test. The pencil was held at a 45° angle to the paint film surface and moved forward at a constant speed of 1 cm/s. The maximum pencil hardness that did not leave a visible mark on the paint film surface was recorded as the paint film hardness [48,49].
(2) Adhesion: In accordance with the reference standard GB/T 4893.4-2013, a QFH-HG600 paint film cross-cut tester was employed to determine the paint film adhesion [50]. A right-angle grid was cut on the paint film using the cross-cut tester in both horizontal and vertical directions. A piece of 3M adhesive tape was firmly pressed over the cut area and rubbed back and forth with a finger to ensure strong adhesion. The tape was then quickly peeled off, and the paint film adhesion was evaluated based on the amount of detachment observed on the tape [51]. The surface damage or failure was determined by evaluating whether the paint film in the test area completely or partially peeled off from the substrate after tape removal, rather than showing minor scratches or residual adhesive. The rating was based on the proportion of peeling area relative to the test area [52]. The adhesion levels were classified from levels 0 to 5, each reflecting a specific range of paint peeling: 5%, 15%, 35%, 55%, and greater than 65% [53].

2.4.5. Thermal-Aging-Resistance Testing

In order to simulate the effect of the long-term use of paint film in a natural environment, the Monopetalanthus spp. wood coated with the self-healing paint film was placed in a 60 °C oven for thermal aging for 14 h. The color difference and glossiness of the paint film were tested before and after aging.

2.4.6. Self-Healing Performance Testing

A single-edged blade with a length of 2 cm and a depth of about 100 µm was used to make a cross-cut on the self-healing paint film. OM and SEM were employed to observe the self-repairing behavior of the paint film on the surface of Monopetalanthus spp. wood, with a complete cycle of 7 days. The healing performance of the self-healing paint film was calculated by calculating the scratch-width variation rate according to Formula (2). In the formula, the DH value represents the width change rate (%), the D1 value represents the width of the scratch on the surface paint film before repair (mm), and the D2 value represents the width of the scratch on the surface paint film after repair (mm). Three independent samples were prepared and tested under each condition. The scratch-width data were calculated as the average value of these three groups.
D H = D 1 − D 2 D 1 × 100 %

2.4.7. Analysis of Variance (ANOVA)

Calculations were performed for the F, p-value, and Fcrit. F refers to the test statistic employed in the calculation of hypothesis tests. As a measure of significance, the p-value assesses whether the experimental results could reasonably occur by chance. Fcrit refers to the threshold value of the F statistic determined by a given significance level. Specifically, when F > Fcrit, it demonstrates that the variation between the two groups is significant from a statistical standpoint; when F < Fcrit, no significant difference is observed. The threshold for determining statistical significance is defined as 0.01 < p-value < 0.05, where a p-value ≤ 0.01 signifies extremely significant differences, while a p-value > 0.05 indicates no significant difference.

3. Results and Discussion

3.1. Microscopic Morphological Analysis

The OM images of the microcapsule are shown in Figure 1. The microcapsules exhibited regions of varying gray tones and bright areas, indicating that the core and wall materials were composed of different substances. In the OM images, the microcapsules were circular and evenly distributed, and chain-like aggregation of particles could be seen in some areas. SEM images of the microcapsules are shown in Figure 2. It could be observed that the microcapsules with a core–wall ratio of 0.68:1 were agglomerated with each other, and a small amount of core material could be seen to be overflowing and not being coated. The particle size distribution of the two different core–wall-ratio microcapsules is illustrated in Figure 3. The particle size of the microcapsules with a core–wall ratio of 0.59:1 was relatively uniform, and the particle size was concentrated between 4.5–5.5 µm. Microcapsules at a core–wall ratio of 0.68:1 predominantly exhibited particle sizes within the 5.5–6.5 µm range.
According to the SEM images of the two different core–wall-ratio microcapsule paint films in Figure 4, agglomeration of microcapsules was observed in both groups of paint films. Under the condition of a core–wall ratio of 0.59:1, the overall distribution of the microcapsule particles was relatively uniform. The surface was relatively smooth, and the microcapsules showed good adhesion to the matrix. This may be attributed to the relatively low core–wall ratio, which resulted in a moderate wall thickness that ensured the mechanical stability of the microcapsules while reducing the interference of interface stress upon dispersion. In contrast, under the condition of a core–wall ratio of 0.68:1, the agglomeration of microcapsules was significantly more pronounced, with increased particle distribution unevenness and the appearance of multiple protrusions or accumulation regions on the surface. Therefore, a relatively good microstructure was observed in the paint film containing microcapsules with a core–wall ratio of 0.59:1.

3.2. Chemical Composition Analysis

The FTIR spectral characteristics of MF-resin-encapsulated shellac–rosin microcapsules are displayed in Figure 5. Analysis of the shellac and shellac–rosin mixture spectra indicated that the peaks at 2937 cm−1 and 2829 cm−1 corresponded to the stretching vibrations of –CH2 groups. After mixing with rosin, this region became noticeably broader, indicating a more complex molecular environment. The absorption band observed at 1714 cm−1 corresponded to the C=O stretching vibration typical of shellac, while the peak at 1010 cm−1 corresponded to the C–O–C/C–O stretching vibration. In contrast to pure shellac, the absorption at 877 cm−1 vanished in the blended sample, suggesting that during the blending with rosin and subsequent encapsulation process, the signal of this structure was masked or weakened by the MF-resin coating layer. The spectra of microcapsule samples showed obvious absorption peaks at 3373 cm−1 and 3367 cm−1, which were attributed to the stretching vibration of the –OH groups in shellac. The peaks observed at 2957 cm−1 and 2930 cm−1 were attributed to–CH2 stretching vibrations, and the absorption peak at 1147 cm−1 was attributed to the C–O bond stretching vibration. The characteristic absorption peaks of the MF resin appeared in the infrared spectra of microcapsules with different core–wall ratios, indicating the presence of MF resin. The infrared signal at 1726 cm−1 indicated the vibrational stretching of –COOH carboxyl groups in shellac, while the spectral feature at 813 cm−1 was related to the vibrational bending of the triazine ring present in the MF wall material. The characteristic absorption peaks of rosin occurred at 1458 cm−1 and 1387 cm−1, which belong to the C–H bending vibration. The FTIR results indicated that the MF resin was able to stably and completely encapsulate the shellac–rosin mixture, with the characteristic peaks of both the core and wall materials being retained, demonstrating good structural stability of the microcapsule system. Figure 6 shows the infrared spectral features of the paint film with shellac microcapsules. The peak at 2930 cm−1 corresponded to the –CH3 group in C–H, while the peak at 1734 cm−1 represented the C=O stretching vibration in shellac, indicating that the structural features of the core material were preserved on the paint film surface. The absorption peak at 813 cm−1 was attributed to the triazine ring bending vibration in the MF resin wall material, further confirming the presence of MF resin on the paint film surface. Based on the combined spectral features, the presence of shellac microcapsules in the paint film was confirmed.

3.3. Optical Performance Analysis

3.3.1. Color Difference

According to the data presented in Table 4 and Table 5, the overall color difference increased with the rising concentration of microcapsules, indicating that higher microcapsule content led to uneven color of the paint film. Under the condition of a core–wall ratio of 0.59:1, when the microcapsule content increased from 0% to 25.0%, the color difference (ΔE) rose from 4.9 to 10.9. Similarly, at a core–wall ratio of 0.68:1, the ΔE increased from 4.9 to 10.4, indicating that a higher microcapsule content significantly enhanced the color difference. Notably, under low content conditions, the ΔE of paint films with 5.0% microcapsules was 5.1 at a 0.59:1 core–wall ratio, slightly higher than that of 0% sample, whereas the ΔE at a 0.68:1 ratio was 4.5, even lower than that of the 0% sample. This anomaly may be because the microcapsule in this proportion was uniformly dispersed in the paint film and the surface flatness and light reflection characteristics had been optimized, thereby making the color difference slightly lower. The largest color difference was detected in the paint film incorporating 25.0% microcapsules at a 0.59:1 core–wall ratio, reaching a maximum ΔE of 10.9. Conversely, at a core–wall ratio of 0.68:1, the paint film with a 5.0% microcapsule content showed the lowest ΔE value of 4.5. In addition, under high microcapsule content conditions, the increase in ΔE was slightly less pronounced at the higher core–wall ratio, suggesting that a higher ratio may help to mitigate the variation in color difference to some extent. Nevertheless, the overall trend remained consistent: a higher microcapsule content led to greater uneven color in the paint film.

3.3.2. Glossiness

According to the data presented in Table 6 and Table 7, the coating glossiness measured under a 60° light incidence showed a downward trend with increasing microcapsule content, reflecting a negative influence on surface shine. With a core–wall ratio of 0.59:1, the coating glossiness without microcapsules measured under 60° light incidence was 2.3 GU. With the addition of 5.0% microcapsules, the glossiness increased to 9.8 GU. However, as the microcapsule content continued to rise, glossiness gradually decreased, reaching 1.5 GU at a content of 25.0%, showing a trend of rising first and then falling. Under the condition of a core–wall ratio of 0.68:1, the initial glossiness was 5.2 GU. Similarly, it increased to 7.2 GU at a 5.0% microcapsule content and then decreased steadily with further increases in the content, falling to 1.5 GU at 25.0%. This phenomenon may be related to the dispersion of the microcapsules. At low content levels, the microcapsules may fill microscopic surface depressions in the paint film, improving the surface smoothness and temporarily enhancing the glossiness. In contrast, at high content levels, excess microcapsules could increase the surface roughness and light scattering, resulting in reduced glossiness. Moreover, glossiness at the 20° and 85° angles also revealed similar trends. Notably, the glossiness at 85° declined more sharply under a high microcapsule content, suggesting that excessive amounts of microcapsules increase surface diffuse reflection, further diminishing the surface glossiness. In summary, an increasing microcapsule content significantly affected the coating glossiness, and a high microcapsule concentration led to a pronounced reduction in glossiness.
The increase in microcapsule content not only led to a significant increase in the color difference of the paint film, reflecting the decline in color uniformity, but it also inhibited the glossiness, showing an overall downward trend in glossiness. At low concentrations, microcapsules were observed to fill the microscopic depressions on the surface of the paint film to a certain extent, resulting in a temporary improvement in surface smoothness and glossiness. However, with the increase in concentration, an aggregation of microcapsule particles was induced, which increased the surface roughness and enhanced the light-scattering effect, thereby leading to an increase in chromatic aberrations and a decrease in glossiness. In addition, under high microcapsule concentrations, a smaller increase in color difference was measured for the higher core–wall ratio (0.68:1) compared with the lower core–wall ratio (0.59:1), indicating that thicker wall materials played a role in adjusting microcapsule dispersion and affected the optical characteristics of the coating. Nevertheless, the overall trend was that a higher microcapsule content affected the uniformity of color and glossiness more substantially. Therefore, the dispersion and content of microcapsules were identified as the key factors affecting the optical properties of the paint film, and these should be comprehensively considered to achieve a balanced optimization of coating performance.

3.4. Mechanical Performance Analysis

According to Table 8, the effect of the addition amount of microcapsules with different core–wall ratios on the primer film adhesion showed a gradual decline. Among them, microcapsules formulated with a 0.59:1 core–wall ratio showed the best comprehensive adhesion quality in the primer film. As the core–wall ratio rose, the connection between microcapsules and the coating became stronger, making the film structure tighter. Thus, the coating anti-peeling and anti-pollution abilities of the paint film were improved. As the proportion of microcapsules increased from 0% to 5.0%, the paint film exhibited excellent adhesion, reaching levels 0–1. This may be attributed to the mechanical locking effect of the microcapsules dispersed within the paint film and its large interface contact area, which reduced the contrast in surface energy properties exhibited by the coating and the substrate and effectively reduced the interface residual stress, thereby enhancing the adhesion strength and wear resistance of the paint film. As the microcapsule addition increased to between 10.0% and 15.0%, the paint film consistently exhibited level-1 adhesion. However, when the content of microcapsules further increased to 20.0%–25.0%, the adhesion of the paint film dropped to levels 2–3, and the performance was poor, the weakest adhesion occurred in coatings incorporating microcapsules with a 0.68:1 core–wall ratio. This may result from the microcapsule size of this proportion being large and the distribution not being uniform. After application of the waterborne primer, significant differences in the contact area between the microcapsules and the wood substrate may have weakened interfacial bonding, thereby diminishing the paint film bonding capability.
As the concentration of microcapsules in the primer formulation gradually grew, the influence of microcapsules on the hardness of paint film also increased. The research results showed that the overall hardness of microcapsules formulated with a 0.59:1 core– wall ratio exhibited superior hardness compared with those with a 0.68:1 ratio, indicating that the lower core–wall ratio was more conducive to enhancing the mechanical strength of paint film. When the microcapsule content was controlled within the range of 5.0% to 10.0%, the coating hardness reached an optimal level. However, it should be noted that excessive addition of microcapsules may negatively affect the paint film glossiness. Once the microcapsules were introduced into waterborne primer, the wear resistance of paint film could be improved to a certain extent, and its enhancement mechanism mainly depended on the mechanical combination between the microcapsules and the waterborne paint film. During the process of paint film formation, microcapsules flowed with the waterborne paint film and penetrated into the vessel structures of the wood so as to improve the hardness of wood surface and create better wear resistance.

3.5. Thermal-Aging-Resistance Performance Analysis

The self-healing performance of the paint film during the thermal aging process was investigated through a comparison of color and glossiness variations before and after thermal treatment. The influence of microcapsules with different core–wall ratios on the glossiness of the paint film under 60° incident light is shown in Figure 7 and Table 9. Generally, larger differences in glossiness and color indicated poorer aging resistance. At microcapsule concentrations of 20.0% and 25.0%, the glossiness difference before and after thermal treatment remained relatively small, with both below 1.0 GU. In particular, at a microcapsule concentration of 25.0%, the glossiness difference was minimized to 0.4 GU, suggesting that this content provided good glossiness retention and exhibited strong anti-aging performance.
The color-difference change in waterborne primer films with different microcapsule contents before and after 60 °C thermal aging is shown in Figure 8. The results indicated that color difference was affected to some extent by the inclusion of microcapsules during thermal aging. In the system with a core–wall ratio of 0.59:1, the color-difference variation showed a certain volatility with the increase in microcapsule content. The color difference decreased significantly when the content of microcapsules was 5.0% and 20.0%, which may be because an appropriate amount of microcapsules was evenly incorporated into the paint layer and filled with microscopic defects on the surface, thereby improving the color uniformity and slowing down color changes caused by thermal aging. In the case of a 0.68:1 core–wall ratio, with microcapsules incorporated at 20.0%, the color difference changed the least and the color difference value was 0.9, indicating that the system could effectively slow down the color change caused by thermal aging under the condition of high microcapsule content. As the microcapsule concentration rose to 25.0%, the color difference increased to 3.2, which indicated that excessive amounts of microcapsules may affect the color stability of the paint film. This may be attributed to the uneven distribution of microcapsules at high concentrations, which could lead to aggravated local structural changes during aging, thereby increasing the color variation. Therefore, microcapsules make a positive impact on the paint film aging resistance within a specific concentration range, while too high or too low a content may weaken this effect.

3.6. Self-Healing Performance Analysis

The effect of the addition amount of microcapsules at two core–wall ratios on the repair effect of the primer film are shown in Figure 9 and Figure 10, respectively. The repair rate of waterborne primer films with different contents is displayed in Table 10. Overall, the repair efficiency showed an upward trend with the rise in core–wall ratio and microcapsule content. For the waterborne paint film without microcapsules, the scratch width did not change after 5 days of external force scratching, indicating that the crack could not be repaired by itself. After 3 days of scratching, the paint film containing microcapsules showed a certain degree of repair ability. Among them, when 5.0% microcapsules were applied under a 0.59:1 core–wall ratio, the scratch width decreased from 6.15 μm to 5.98 μm, and the repair rate was 2.7%. When the content increased to 20.0%, the scratch width decreased from 8.75 μm to 7.35 μm, and the repair rate increased to 16.0%. In the case of microcapsules with a 0.68:1 core–wall ratio, the scratch width of the paint film with a content of 5.0% was reduced from 11.91 μm to 10.91 μm, with a repair rate of 8.3%, while the width of scratches on the paint film with a content of 20.0% was reduced from 11.68 μm to 9.8 μm, with the highest repair rate of 16.1%. These results demonstrated that the paint film incorporating microcapsules showed an evident self-healing ability after 3 days of scratching, and the repair effect was further enhanced as the microcapsule incorporation increased, but the repair rate slowed down gradually after 3 days. The waterborne paint film containing shellac microcapsules possessed a certain self-healing ability, which could be utilized for repairing surface defects in the paint film. From the perspective of surface appearance, the paint film incorporating a 5.0% microcapsule addition presented a smoother texture. The paint film with a 20% microcapsule content and a core–wall ratio of 0.68:1 exhibited the highest repair rate, which might have been related to the better structural stability of microcapsules at a higher core–wall ratio. The stronger the core retention ability, the more uniform the distribution within the paint film. The optical microscopy images, along with the color difference and glossiness results, all indicated that a higher core–wall ratio could improve microcapsule dispersibility to some extent and slow the change in surface roughness, thereby facilitating sufficient release of the healing agent at the cracks and the formation of a continuous film with the substrate.
As observed from the OM images (Figure 9 and Figure 10), when the microcapsule loading was 5%, the microcapsules were relatively uniformly distributed within the coating, and the surface structure appeared smooth. However, as the loading increased to 15% and above, significant aggregation occurred (red circles in Figure 9 and Figure 10), leading to localized accumulation and defects on the coating surface. This structural variation trend was highly consistent with the nonlinear changes in the color difference observed during aging-resistance tests, indirectly supporting the conclusion that a moderate amount of microcapsules could improve structural compactness and color stability, whereas excessive microcapsule loading might cause aggregation and adversely affect the color difference.
Based on the repair rate of microcapsule waterborne paint films of different contents, a two-way ANOVA without replication was employed to analyze its statistical significance. According to the data presented in Table 11, the F value for the microcapsule content was less than Fcrit, and the p-value was less than 0.01, indicating that the microcapsule content in the paint film had a significant effect on the repair rate of the film, supporting the experimental results. The F value for the core–wall ratio of the microcapsules was greater than Fcrit, and the p-value was greater than 0.05, indicating that the core–wall ratio of the microcapsules in the coating had no significant effect on the repair rate of the film. This result may be related to the fact that microcapsule content directly affected the amount of healing agent released, whereas changes in the core–wall ratio had a relatively small impact on the release efficiency.
Figure 11 demonstrates how the self-healing microcapsule coating functions to restore the surface. When artificial scratches were created on the coating, the applied external force caused the embedded microcapsules within the coating to rupture, resulting in the release of the shellac healing agent from the core material. Upon contact with air, the solvent in shellac gradually evaporated, leading to the physical curing of shellac and the formation of a continuous and dense healing film. This healing film was found to effectively fill the cracks in the scratched area, preventing further crack propagation, while also isolating the substrate from direct exposure to the external environment to avoid further corrosion or damage. In addition, the unique chemical properties of shellac were shown to provide the healing film with good adhesion and water resistance, thereby enhancing the overall durability and protective performance of the coating. As the microcapsule proportion increased, a greater amount of healing agent was released, improving the healing effect, although the healing rate was observed to level off after a certain period, reflecting the dynamic healing process of the material.

4. Conclusions

Two different core–wall ratios of MF-resin-encapsulated shellac–rosin microcapsules were added to paint film on the Monopetalanthus spp. wood surface, and the optical performance, mechanical performance, thermal-aging resistance, and self-healing properties of the paint film were investigated. With an increase in microcapsule content, the color difference in microcapsule paint films with two different core–wall ratios showed an overall upward trend. At a 0.59:1 core–wall ratio and a 25.0% microcapsule concentration, the ΔE was maximum, reaching 10.9; At a 0.68:1 core–wall ratio and a 5.0% microcapsule concentration, the ΔE was minimal, only 4.5. The paint film glossiness showed a decreasing trend with the rise in microcapsule concentration. The higher core–wall ratio could slightly slow down the color difference change when the content of microcapsules was high, but the overall trend was still that the color uniformity of the paint film decreased as more microcapsules were incorporated. In terms of mechanical properties, the coating showed better adhesion in the case of a 5% microcapsule concentration. A microcapsule concentration of 25% resulted in enhanced paint film hardness. The paint film exhibited maximum aging resistance with a 20.0% microcapsule content at a core–wall ratio of 0.68:1, as evidenced by the minimal color difference change, with a ΔE value of just 0.9. When the content reached 25.0%, the paint film exhibited the smallest glossiness difference, measured at 0.4 GU. A maximum repair rate of 16.1% was observed at a 20.0% microcapsule content with a core–wall ratio of 0.68:1, showing good self-repair performance. Overall, at a 0.68:1 core–wall ratio with microcapsule concentrations of between 15.0% and 20.0%, the paint film showed superior comprehensive performance. The results contribute valuable technical insights for the application of MF-resin-encapsulated shellac–rosin microcapsules in wood surface paint films and offer a theoretical foundation for the further development of self-healing technology in wood paint films. Although this study characterized the microcapsule structure, providing indirect evidence supporting the formation of a core–wall structure, limitations remain due to the lack of more systematic microscopic verification. Future research will focus on optimizing the microcapsule preparation process and employing a combination of advanced structural characterization techniques to more clearly elucidate the core–wall features and the underlying mechanisms influencing their performance.

Author Contributions

Conceptualization, methodology, validation, resources, and writing—review and editing, J.H.; data management, Y.H.; formal analysis, supervision, and investigation, X.Y. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This project was partly supported by the Qing Lan Project and the Natural Science Foundation of Jiangsu Province (BK20201386).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Zelinka, S.L.; Altgen, M.; Emmerich, L.; Guigo, N.; Keplinger, T.; Kymäläinen, M.; Thybring, E.E.; Thygesen, L.G. Review of Wood Modification and Wood Functionalization Technologies. Forests 2022, 13, 1004. [Google Scholar] [CrossRef]
  2. Qi, Y.Q.; Zhou, Z.W.; Xu, R.; Dong, Y.T.; Zhang, Z.Q.; Liu, M.J. Effect of NaOH Pretreatment on Permeability and Surface Properties of Three Wood Species. ACS Omega 2023, 8, 40362–40374. [Google Scholar] [CrossRef] [PubMed]
  3. Qi, Y.Q.; Zhang, Z.Q.; Sun, Y.; Shen, L.M.; Han, J.L. Study on the Process Optimization of Peanut Coat Pigment Staining of Poplar Wood. Forests 2024, 15, 504. [Google Scholar] [CrossRef]
  4. Liu, C.; Xu, W. Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood. Coatings 2022, 12, 1857. [Google Scholar] [CrossRef]
  5. Jamalabadi, M.Y.A.; Zabari, N.; Bratasz, L. Three-dimensional numerical and experimental study of fracture saturation in panel paintings. Wood Sci. Technol. 2021, 55, 1555–1576. [Google Scholar] [CrossRef]
  6. Liu, Y.; Hu, J.; Xu, W. Surface Roughness of Wood Substrates after Grinding and Its Influence on the Modification Effect of Structural Color Layers. Forests 2023, 14, 2213. [Google Scholar] [CrossRef]
  7. Wang, C.L.; Wu, Z.H.; Wang, J.X.; Liu, Y. Theory and Method for Rapid Carbon Footprint Accounting of Solid Wood Furniture. For. Prod. J. 2025, 75, 155–163. [Google Scholar] [CrossRef]
  8. Yang, Y.Q.; Xu, W.; Liu, X.; Wang, X.D. Study on Permeability of Cunninghamia lanceolata Based on Steam Treatment and Freeze Treatment. Wood Res. 2021, 66, 1721–1731. [Google Scholar] [CrossRef]
  9. Shi, L.Y.; Liu, Y.; Hu, J.; Chen, H.; Ji, J.G. Life-Cycle Assessment of Bicycles Based on Bamboo Bending Technology. For. Prod. J. 2025, 75, 179–188. [Google Scholar] [CrossRef]
  10. Xue, J.X.; Xu, W.; Zhou, J.C.; Mao, W.G.; Wu, S.S. Effects of High-Temperature Heat Treatment Modification by Impregnation on Physical and Mechanical Properties of Poplar. Materials 2022, 15, 7334. [Google Scholar] [CrossRef]
  11. Naumovski, L.; Capuder, M.; Sandak, J.; Azinovic, B. Influence of surface preparation and surface topography on tensile shear strength of polyurethane adhesively bonded beech wood single-lap joints: A finite element method approach. Eur. J. Wood Wood Prod. 2025, 83, 139. [Google Scholar] [CrossRef]
  12. Zhang, H.Q.; Feng, X.H.; Wu, Y.; Wu, Z.H. Self-matting waterborne polyurethane acrylate wood coating by 222 nm far-UVC irradiation in ambient air. Prog. Org. Coat. 2024, 189, 108305. [Google Scholar] [CrossRef]
  13. Liu, W.J.; Zhu, L.; Varodi, A.M.; Liu, X.Y.; Lv, J.F. The Effect of Wet and Dry Cycles on the Strength and the Surface Characteristics of Coromandel Lacquer Coatings. Forests 2024, 15, 770. [Google Scholar] [CrossRef]
  14. Wang, C.; Li, J.Y.; Wang, X.W.; Chu, Q.; Wang, T.Y. Influence of Shell Structure on the Tensile Strength of Fused Filament Fabrication Models. Mater. Plast. 2024, 61, 19–26. [Google Scholar] [CrossRef]
  15. Fu, S.J.; Xiong, X.Q.; Wan, R.Y.; Zhang, M.; Xu, X.T. The Development and Future Challenges of China’s Furniture Industry. Drewno 2025, 68, 00045. [Google Scholar] [CrossRef]
  16. Chang, Y.J.; Wu, Z.H. Effect of Sandpaper Meshes on the Performance of Tilia Sp. Self-Repairing Coatings. Polymers 2023, 15, 2835. [Google Scholar] [CrossRef]
  17. Chang, Y.J.; Wu, Z.H.; Liu, E.W. Fabrication of chitosan-encapsulated microcapsules containing wood wax oil for antibacterial self-healing wood coatings. Ind. Crops Prod. 2024, 222, 119438. [Google Scholar] [CrossRef]
  18. Chen, Z.B.; Wu, Z.H. Application Prospect of Multifunctional Hydrogel Coating in Household Field. Coatings 2024, 14, 1580. [Google Scholar] [CrossRef]
  19. Chang, Y.J.; Liu, E.W.; Wu, Z.H. Constructing chitosan microcapsules using hydroxypropyl methylcellulose for self-healing antibacterial wood coating. Int. J. Biol. Macromol. 2025, 308, 142300. [Google Scholar] [CrossRef]
  20. Chang, Y.J.; Yan, X.X.; Wu, Z.H. Application and prospect of self-healing microcapsules in surface coating of wood. Colloid. Interfac. Sci. 2023, 56, 100736. [Google Scholar] [CrossRef]
  21. Ma, L.; Xu, S.Y. Investigation on the restoration properties of wood oil microcapsules in wood coatings. Prog. Org. Coat. 2024, 197, 108853. [Google Scholar] [CrossRef]
  22. Chen, Y.; Zhu, Z.; Shi, K.B.; Jiang, Z.Y.; Guan, C.R.; Zhang, L.; Yang, T.; Xie, F.W. Shellac-based materials: Structures, properties, and applications. Int. J. Biol. Macromol. 2024, 279, 135102. [Google Scholar] [CrossRef]
  23. Pekerman, P.; Yom-Tov, O.; Bar, H.; Bianco-Peled, H. Light-triggered rupture of shellac/polydopamine composite coatings. Prog. Org. Coat. 2024, 186, 107950. [Google Scholar] [CrossRef]
  24. Li, K.; Tang, B.S.; Zhang, W.W.; Tu, X.H.; Ma, J.J.; Xing, S.J.; Shao, Y.; Zhu, J.; Lei, F.H.; Zhang, H. A novel approach for authentication of shellac resin in the shellac-based edible coatings: Contain shellac or not in the fruit wax preservative coating. Food Chem. X 2022, 14, 100349. [Google Scholar] [CrossRef] [PubMed]
  25. Bar, H.; Bianco-Peled, H. The unique nanostructure of shellac films. Prog. Org. Coat. 2021, 157, 106328. [Google Scholar] [CrossRef]
  26. Yuan, Y.; He, N.; Dong, L.Y.; Guo, Q.Y.; Zhang, X.; Li, B.; Li, L. Multiscale Shellac-Based Delivery Systems: From Macro- to Nanoscale. ACS Nano 2022, 15, 18794–18821. [Google Scholar] [CrossRef]
  27. Cho, B.G.; Lee, J.E.; Jeon, S.Y.; Chae, H.G. A study on miscibility properties of polyacrylonitrile blending films with biodegradable polymer, shellac. Polym. Test. 2023, 121, 107983. [Google Scholar] [CrossRef]
  28. Liu, L.X.; Li, X.; Dong, G.; Zhang, H.; Tao, Y.F.; He, R.; Xu, J.; Ma, J.J.; Tang, B.S.; Zhou, B. Bioinspired Natural Shellac Dressing for Rapid Wound Sealing and Healing. ACS Appl. Mater. Inter. 2023, 15, 43294–43308. [Google Scholar] [CrossRef]
  29. Acarali, N.; Demir, S. Physical and chemical effects of quartet structure (bamboo/zinc borate/shellac/surfactant) on organic coatings. J. Indian. Chem. Soc. 2021, 98, 100043. [Google Scholar] [CrossRef]
  30. Liu, M.; Xu, G.L.; Wang, J.A.; Tu, X.W.; Liu, X.Y.; Wu, Z.H.; Lv, J.F.; Xu, W. Effects of Shellac Treatment on Wood Hygroscopicity, Dimensional Stability and Thermostability. Coatings 2020, 10, 881. [Google Scholar] [CrossRef]
  31. Sözen, E. Determination of Changes in the Mechanical and Color Properties of Some Wood Species Treated with Shellac. Bioresources 2022, 17, 1703–1716. [Google Scholar] [CrossRef]
  32. Wang, X.X.; Huang, Y.X.; Wang, S.D.; Lei, W.C.; Wu, J.Y.; Yu, W.J. Sustainable superhydrophobic wood with self-healing properties and enhanced durability. Chem. Eng. J. 2025, 504, 159000. [Google Scholar] [CrossRef]
  33. Mohamed, S.A.A.; Hassan, M.L.; Abd El-Sayed, E.S.; El-Sakhawy, M. Cellulose Nanofiber/Shellac Nanocomposite Films as Coatings for Packaging Paper. Cell. Chem. Technol. 2023, 57, 143–153. [Google Scholar] [CrossRef]
  34. Li, X.Q.; Chen, Y. Rosin: A comprehensive review on traditional uses, phytochemistry, and pharmacology. Fitoterapia 2024, 177, 106068. [Google Scholar] [CrossRef]
  35. Pinheiro, A.; Martins, I.; Bento, A.; Escorcio, R.; Nunes, C.; Varela, A.; Nunes, J.; Afonso, C.A.M.; Pereira, C.S. Rosin from Pinus pinaster Portuguese forests shows a regular profile of resin acids. Front. Plant Sci. 2023, 14, 1268887. [Google Scholar] [CrossRef]
  36. Rubini, M.; Sandak, J.; Robles, E.; Sandak, A.; Charrier, B. Mid-Level data fusion techniques for gum rosin discrimination according to Pinus species, tapping methods, and geographic origins. Microchem. J. 2025, 209, 112630. [Google Scholar] [CrossRef]
  37. Liu, H.C.; Wu, X.; Guo, Z.P.; Dong, P.; Ge, Q.; Wei, L.W.; Sun, Z.L. Electrical Tree Characteristics of Bisphenol A Epoxy Resin/Maleopimaric Anhydride Curing System. Polymers 2022, 14, 3867. [Google Scholar] [CrossRef]
  38. Liu, H.H.; Wu, X.; Liu, Y.P.; Guo, Z.P.; Ge, Q.; Sun, Z.L. The curing characteristics and properties of bisphenol A epoxy resin/maleopimaric acid curing system. J. Mater. Res. Technol. 2022, 21, 1655–1665. [Google Scholar] [CrossRef]
  39. Bakatovich, A.; Gaspar, F.; Boltrushevich, N. Thermal insulation material based on reed and straw fibres bonded with sodium silicate and rosin. Constr. Build. Mater. 2022, 352, 129055. [Google Scholar] [CrossRef]
  40. Tao, Y.; Yan, X.X. Influence of HLB Value of Emulsifier on the Properties of Microcapsules and Self-Healing Properties of Waterborne Coatings. Polymers 2022, 14, 1304. [Google Scholar] [CrossRef]
  41. Han, Y.; Yan, X.X.; Tao, Y. Effect of Number of Impregnations of Microberlinla sp. with Microcapsule Emulsion on the Performance of Self-Repairing Coatings on Wood Surfaces. Coatings 2022, 12, 989. [Google Scholar] [CrossRef]
  42. Zou, Y.M.; Pan, P.; Zhang, N.N.; Yan, X.X. Effect of Nano-Silver Solution Microcapsules Mixed with Rosin-Modified Shellac Microcapsules on the Performance of Water-Based Coating on Andoung Wood (Monopetalanthus spp.). Coatings 2024, 14, 286. [Google Scholar] [CrossRef]
  43. Zou, Y.M.; Pan, P.; Yan, X.X. Comparative Analysis of Performance of Water-Based Coatings Prepared by Two Kinds of Anti-Bacterial Microcapsules and Nano-Silver Solution on the Surface of Andoung Wood. Coatings 2023, 13, 1518. [Google Scholar] [CrossRef]
  44. Yu, L.; Ye, S.M.; Fang, J.; He, J.; Yang, G.C. Exploring the photochromic mechanism of Andoung wood by spectral and colorimetric analysis technology. Wood Mater. Sci. Eng. 2025, 1–15. [Google Scholar] [CrossRef]
  45. GB/T 11186.3-1989; Methods for Measuring the Color of Paint Films—Part 3: Calculation of Color Differences. Standardization Administration of the People’s Republic of China: Beijing, China, 1989.
  46. GB/T 4893.6-2013; Test of Surface Coatings of Furniture—Part 6: Determination of Gloss Value. Standardization Administration of the People’s Republic of China: Beijing, China, 2013.
  47. GB/T 6739-2006; Paint and Varnishes-Determination of Film Hardness by Pencil Test. Standardization Administration of the People’s Republic of China: Beijing, China, 1998.
  48. Wang, C.; Huang, H.Y.; Wang, X.W.; Wang, Y.B.R.; Zhu, Y. Effect of drying treatment on the physical and mechanical properties of material extrusion-based 3D-printed PETG models. BioResources 2025, 20, 7000–7009. [Google Scholar] [CrossRef]
  49. Hu, W.; Luo, M.; Liu, Y.; Xu, W.; Konukcu, A.C. Experimental and numerical studies on the mechanical properties and behaviors of a novel wood dowel reinforced dovetail joint. Eng. Fail. Anal. 2023, 152, 107440. [Google Scholar] [CrossRef]
  50. GB/T 4893.4-2013; Test of Surface Coatings of Furniture. Part IV: Determination of Adhesion-Cross Cut. Standardization Administration of the People’s Republic of China: Beijing, China, 2013.
  51. Xu, X.T.; Zhang, M.; Yue, X.Y.; Xiong, X.Q. Design of furniture mortise-and-tenon joints: A review of mechanical properties and design recommendations. Wood Mater. Sci. Eng. 2025, 1–15. [Google Scholar] [CrossRef]
  52. Hu, W.; Yu, R. Mechanical and acoustic characteristics of four wood species subjected to bending load. Maderas-Cienc. Tecnol. 2023, 25, 39. [Google Scholar] [CrossRef]
  53. Liu, Y.; Hu, W.; Kasal, A.; Erdil, Y.Z. The state of the art of biomechanics applied in ergonomic furniture design. Appl. Sci. 2023, 13, 12120. [Google Scholar] [CrossRef]
Coatings 15 01003 g001
Figure 1. OM images of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Figure 1. OM images of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Coatings 15 01003 g001
Coatings 15 01003 g002
Figure 2. SEM images of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Figure 2. SEM images of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Coatings 15 01003 g002
Coatings 15 01003 g003
Figure 3. Particle size distribution of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Figure 3. Particle size distribution of microcapsules with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Coatings 15 01003 g003
Coatings 15 01003 g004
Figure 4. SEM images of microcapsule paint films with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Figure 4. SEM images of microcapsule paint films with two varying core–wall ratios: (A) 0.59:1; (B) 0.68:1.
Coatings 15 01003 g004
Coatings 15 01003 g005
Figure 5. The FTIR spectra of microcapsules synthesized at two core–wall ratios.
Figure 5. The FTIR spectra of microcapsules synthesized at two core–wall ratios.
Coatings 15 01003 g005
Coatings 15 01003 g006
Figure 6. The FTIR spectra of microcapsule paint films synthesized at two core–wall ratios.
Figure 6. The FTIR spectra of microcapsule paint films synthesized at two core–wall ratios.
Coatings 15 01003 g006
Coatings 15 01003 g007
Figure 7. Paint film glossiness with different microcapsule concentrations under 60° incident light before and after thermal aging.
Figure 7. Paint film glossiness with different microcapsule concentrations under 60° incident light before and after thermal aging.
Coatings 15 01003 g007
Coatings 15 01003 g008
Figure 8. Paint film ΔE with different microcapsule concentrations before and after thermal aging.
Figure 8. Paint film ΔE with different microcapsule concentrations before and after thermal aging.
Coatings 15 01003 g008
Coatings 15 01003 g009
Figure 9. Effect of microcapsule content with a core–wall ratio of 0.59:1 on the primer film repair effect. Before repair: (A) 5.0%; (B) 10.0%; (C) 15.0%; (D) 20%; (E) 25%. After repair: (F) 5.0%; (G) 10.0%; (H) 15.0%; (I) 20%; (J) 25%. Red circles mean the aggregation occurred, and red square lines mean the scratch width.
Figure 9. Effect of microcapsule content with a core–wall ratio of 0.59:1 on the primer film repair effect. Before repair: (A) 5.0%; (B) 10.0%; (C) 15.0%; (D) 20%; (E) 25%. After repair: (F) 5.0%; (G) 10.0%; (H) 15.0%; (I) 20%; (J) 25%. Red circles mean the aggregation occurred, and red square lines mean the scratch width.
Coatings 15 01003 g009
Coatings 15 01003 g010
Figure 10. Effect of microcapsule content with a core–wall ratio of 0.68:1 on the primer film repair effect. Before repair: (A) 5.0%; (B) 10.0%; (C) 15.0%; (D) 20%; (E) 25%. After repair: (F) 5.0%; (G) 10.0%; (H) 15.0%; (I) 20%; (J) 25%. Red circles mean the aggregation occurred, and red square lines mean the scratch width.
Figure 10. Effect of microcapsule content with a core–wall ratio of 0.68:1 on the primer film repair effect. Before repair: (A) 5.0%; (B) 10.0%; (C) 15.0%; (D) 20%; (E) 25%. After repair: (F) 5.0%; (G) 10.0%; (H) 15.0%; (I) 20%; (J) 25%. Red circles mean the aggregation occurred, and red square lines mean the scratch width.
Coatings 15 01003 g010
Coatings 15 01003 g011
Figure 11. Repair mechanism diagram of a microcapsule self-repairing coating on Monopetalanthus spp. wood.
Figure 11. Repair mechanism diagram of a microcapsule self-repairing coating on Monopetalanthus spp. wood.
Coatings 15 01003 g011
Table 1. Test materials.
Table 1. Test materials.
Test MaterialPurityManufacturer
37% FormaldehydeARShanghai Biotechnology Co., Ltd., Shanghai, China
MelamineARShandong Yousuo Chemical Technology Co., Ltd., Linyi, China
TriethanolamineARGuangzhou Jiale Chemical Co., Ltd., Guangzhou, China
Span-20ARShandong Yousuo Chemical Technology Co., Ltd., Linyi, China
Tween-80ARShandong Yousuo Chemical Technology Co., Ltd., Linyi, China
12.5% Shellac solution-Shanghai Yuyan Building Materials Co., Ltd., Shanghai, China
Liquid dust-free rosin-Suzhou Guyue Musical Instruments Co., Ltd., Suzhou, China
Monohydrate citric acidARNanjing Quallong Biotechnology Co., Ltd., Nanjing, China
Anhydrous ethanolARWuxi Jingke Chemical Co., Ltd., Wuxi, China
Ethyl acetateARXi’an Tianmao Chemical Co., Ltd., Xi’an, China
Table 2. Test equipment.
Table 2. Test equipment.
EquipmentModelManufacturer
Electronic balanceJCS-WHarbin Zhonghui Weighing Apparatus Co., Ltd., Harbin, China
Constant-temperature heating magnetic stirrerDF-101SGongyi Yuhua Instrument Co., Ltd., Gongyi, China
Ultrasonic material emulsification disperserBILON-500Shanghai Bilang Instrument Co., Ltd., Shanghai, China
Electric heating constant-temperature blower drying boxDHG-9643BS-ⅢShanghai Xinmiao Medical Instrument Co., Ltd., Shanghai, China
Optical microscopeZeiss Axio Scope A1Carl Zeiss AG., Baden, Germany
Scanning electron microscopeQuanta-200Thermo Fisher Technology Co., Ltd., Massachusetts, USA
Fourier-transform infrared spectrometerVERTEX 80VBruker Co., Karlsruhe, Germany
Portable colorimeterSEGT-JZhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China
Intelligent gloss meterHG268Shenzhen 3nh Technology Co., Ltd., Shenzhen, China
Ten-thousand-abilities test machineAGS-XShimadzu Corporation, Kyoto, Japan
Pencil-hardness meterHT-6510PQuzhou Aipu Metrology Instrument Co., Ltd., Quzhou, China
Paint film adhesion testerQFH-AQuzhou Aipu Metrology Instrument Co., Ltd., Quzhou, China
Table 3. Ingredient list for the primer film with microcapsules added.
Table 3. Ingredient list for the primer film with microcapsules added.
Microcapsule Content (%)Microcapsules (g)Waterborne Primer (g)Self-Healing Waterborne Primer (g)
0.00.04.04.0
5.00.23.84.0
10.00.43.64.0
15.00.63.44.0
20.00.83.24.0
25.01.03.04.0
Table 4. Influence of microcapsule concentration at a 0.59:1 core–wall ratio on the paint film ΔE.
Table 4. Influence of microcapsule concentration at a 0.59:1 core–wall ratio on the paint film ΔE.
Microcapsule Content (%)L0a0b0Lab∆E
068.77.412.969.311.116.14.9
5.074.19.215.472.78.220.25.1
10.067.410.629.171.29.224.95.8
15.070.57.810.869.311.116.16.3
20.074.47.41370.510.620.79.2
25.077.75.710.673.17.920.310.9
Table 5. Influence of microcapsule concentration at a 0.68:1 core–wall ratio on the paint film ΔE.
Table 5. Influence of microcapsule concentration at a 0.68:1 core–wall ratio on the paint film ΔE.
Microcapsule Content (%)L0a0b0Lab∆E
068.77.412.969.311.116.14.9
5.073.710.821.671.29.224.94.5
10.074.27.915.369.311.116.15.9
15.062.315.929.864.513.622.77.8
20.080.65.611.476.55.319.38.9
25.076.88.39.884.22.94.810.4
Table 6. Influence of microcapsule concentration at a 0.59:1 core–wall ratio on the paint film glossiness.
Table 6. Influence of microcapsule concentration at a 0.59:1 core–wall ratio on the paint film glossiness.
Microcapsule Content (%)20° (GU)60° (GU)85° (GU)
00.62.31.2
5.01.49.85.3
10.00.95.71.1
15.00.63.10.5
20.00.82.70.4
25.00.61.50.2
Table 7. Influence of microcapsule concentration at a 0.68:1 core–wall ratio on the paint film glossiness.
Table 7. Influence of microcapsule concentration at a 0.68:1 core–wall ratio on the paint film glossiness.
Microcapsule Content (%)20° (GU)60° (GU)85° (GU)
01.25.22.7
5.01.37.22.1
10.00.83.80.8
15.00.72.60.4
20.00.71.90.3
25.00.81.50.1
Table 8. Effect of microcapsule content on the mechanical properties of paint film.
Table 8. Effect of microcapsule content on the mechanical properties of paint film.
Core–Wall RatioMicrocapsule Content (%)Adhesion (Level)Hardness
0.59:100HB
5.012B
10.012B
15.02B
20.02B
25.02HB
0.68:100HB
5.01HB
10.02B
15.022B
20.032B
25.03HB
Table 9. Paint film glossiness under 60° incident light for different microcapsule contents before and after thermal aging.
Table 9. Paint film glossiness under 60° incident light for different microcapsule contents before and after thermal aging.
Core–Wall RatioMicrocapsule Content
(%)		Glossiness Before Thermal Aging
(GU)		Glossiness After Thermal Aging
(GU)		Glossiness Difference
(GU)
0.59:102.33.31.0
5.09.811.51.7
10.05.76.71.0
15.03.14.11.0
20.02.73.40.7
25.01.52.00.5
0.68:105.27.01.8
5.07.28.91.7
10.03.84.81.0
15.02.63.30.7
20.01.92.50.6
25.01.51.90.4
Table 10. Repair rate of paint film with different contents of microcapsules.
Table 10. Repair rate of paint film with different contents of microcapsules.
Microcapsule Content
(%)		Core–Wall Ratio of 0.59:1Core–Wall Ratio of 0.68:1
Before Repair
(μm)		After Repair
(μm)		Repair Rate
(%)		Before Repair
(μm)		After Repair
(μm)		Repair Rate
(%)
05.625.620.010.2510.250.0
5.06.15 5.98 2.70 ± 0.1011.9110.918.30 ± 0.29
10.07.026.753.00 ± 0.0811.3010.1510.20 ± 0.17
15.07.106.387.90 ± 0.1011.5510.1512.10 ± 0.33
20.08.757.3516.00 ± 0.3311.689.8016.10 ± 0.14
25.08.197.0414.00 ± 0.3612.4210.6714.10 ± 0.30
Table 11. Significance analysis of the repair rate.
Table 11. Significance analysis of the repair rate.
Difference SourceSSdfMSFp-ValueFcrit
Content of microcapsules349.35569.8713.5529550.00623555.0503291
Core–wall ratio24.653333124.6533334.78210270.08042026.607891
Error25.77666755.1553333
Total399.7811
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Hang, J.; Han, Y.; Yan, X.; Li, J. Effect of Shellac–Rosin Microcapsules on the Self-Healing Properties of Waterborne Primer on Wood Surfaces. Coatings 2025, 15, 1003. https://doi.org/10.3390/coatings15091003

AMA Style

Hang J, Han Y, Yan X, Li J. Effect of Shellac–Rosin Microcapsules on the Self-Healing Properties of Waterborne Primer on Wood Surfaces. Coatings. 2025; 15(9):1003. https://doi.org/10.3390/coatings15091003

Chicago/Turabian Style

Hang, Jingyi, Yang Han, Xiaoxing Yan, and Jun Li. 2025. "Effect of Shellac–Rosin Microcapsules on the Self-Healing Properties of Waterborne Primer on Wood Surfaces" Coatings 15, no. 9: 1003. https://doi.org/10.3390/coatings15091003

APA Style

Hang, J., Han, Y., Yan, X., & Li, J. (2025). Effect of Shellac–Rosin Microcapsules on the Self-Healing Properties of Waterborne Primer on Wood Surfaces. Coatings, 15(9), 1003. https://doi.org/10.3390/coatings15091003

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop