Preparation and Performance Characterization of Melamine-Formaldehyde-Microencapsulated Waterborne Topcoat–Brass Powder–Waterborne Acrylic Coating

Abstract

A novel self-healing brass powder/waterborne acrylic decorative coating for wooden substrates was developed, in which γ-methacryloxypropyltrimethoxysilane (KH570)-modified brass powder (with a coupling agent concentration of 6% and reaction solution pH of 5) was employed as the filler, and melamine-formaldehyde (MF) resin-encapsulated water-based paint microcapsules were utilized as the healing agent. The brass powder content and the core–wall ratio of the topcoat microcapsules were identified as the predominant factors affecting both the optical and mechanical properties of the self-healing brass powder/waterborne acrylic coating on Basswood surfaces. Therefore, the brass powder content was selected as the primary influencing factor. With concentration gradients of 0.5%, 1%, 3%, 5%, 7%, 9%, and 10%, and under constant conditions of 3% microcapsule content and room temperature curing, the effect of brass powder content on the properties of self-healing microcapsule coatings with different core–wall ratios was investigated. The waterborne acrylic wood coating containing 3% brass powder and 3% microcapsules with a core–wall ratio of 0.58:1 exhibited superior overall performance. This optimized formulation not only maintained excellent optical properties but also significantly enhanced mechanical performance, while preserving outstanding aging resistance, liquid resistance, and self-healing capability. The coating demonstrated the following comprehensive performance metrics: a glossiness of 24.0 GU, color difference (ΔE) of 2.13, chromatic aberration (ΔE*) of 13.68, visible light reflectance of 0.5879, dominant wavelength of 587.47 nm, visible light transmittance of 74.33%, pencil hardness of H grade, impact resistance of 2 kg·cm, adhesion rating of class 2, surface roughness of 2.600 μm, along with excellent aging resistance and liquid resistance properties, while achieving a self-healing efficiency of 19.62%. The coating also exhibited a smooth and uniform microscopic morphology, with the chemical bonds of both the modified brass powder and microcapsules remaining intact within the coating matrix.

1. Introduction

Nowadays, wooden structures and their products have become increasingly popular [1,2,3] due to their excellent decorative characteristics and mechanical properties [4,5,6,7,8,9]. However, wood products are highly susceptible to mold, deformation, and performance degradation [10] due to long-term load-bearing [11], environmental factors, as well as material properties [12,13,14], dimensions [15], and manufacturing techniques [16]. Therefore, wood coatings are crucial—they not only enhance the aesthetic appeal of wood products but also serve as an economical, effective, and straightforward protective measure [17,18].

Metal-filled decorative coatings can be prepared by incorporating metal fillers into coatings. Compared to conventional colored coatings, metal-powder composite coatings exhibit superior physicochemical properties and application potential. These coatings not only provide protective functions and decorative effects on object surfaces but also enhance product value, making them widely researched and applied in recent years. Regarding fillers, metal powders such as copper, aluminum, iron, zinc, and tungsten have been the focus of research due to their high electrical conductivity, thermal conductivity, strength, and hardness. They are used to improve the original performance of coatings or even impart new properties. For coating materials, waterborne polyurethane, acrylic, epoxy, and polyester have been employed to prepare metal-powder composite coatings, which typically demonstrate excellent corrosion resistance, wear resistance, impact resistance, and aesthetic appeal.

Brass powder, known for its bright, shimmering color and strong decorative effect, is cost-effective and readily available. When blended with water-based coatings, the resulting brass-powder aqueous coating can be applied to wood surfaces, imparting a reflective golden luster that significantly enhances the visual appeal of wooden materials [19,20,21]. However, pure metal powder fillers demonstrate poor compatibility with aqueous coatings and are prone to agglomeration within the coating system. This phenomenon not only severely compromises the optical properties of the coating but also exacerbates the adverse effects on the mechanical performance of water-based coatings. The resultant coatings exhibit increased brittleness, greater susceptibility to cracking, and reduced adhesion strength. These deficiencies render them inadequate for commercial wood coating applications, thereby significantly constraining the practical implementation and development of metal powder-modified water-based coatings [22,23]. Han et al. successfully prepared a water-based acrylic coating with brass powder that exhibits excellent decorative effects [24]. However, the addition of brass powder significantly reduced the coating’s glossiness and adhesion. Therefore, this study attempts to incorporate microcapsules into the coating to mitigate the negative impact of brass powder on these properties.

Microencapsulation technology refers to a technique that utilizes natural or synthetic film-forming materials to encapsulate solid, liquid, or even gaseous substances, forming microcapsules with core@shell structures. Due to their unique capability of completely isolating core materials from external environments while maintaining their original properties, functional microcapsules with special characteristics such as corrosion resistance, flame retardancy, phase change, color change, antibacterial performance [25], and self-healing properties continue to be extensively researched and applied today. Zhang et al. employed tall oil fatty acid epoxy ester as the core material and successfully prepared microcapsules with poly (urea-formaldehyde) (PUF) shells via in situ polymerization [26]. When the epoxy resin@PUF microcapsules were incorporated into epoxy coatings, the resulting coatings demonstrated excellent self-healing properties and corrosion resistance. Zotiadis et al. adopted a one-step in situ polymerization method to encapsulate epoxy resin with PUF, forming microcapsules that were subsequently dispersed in alkyd resin coatings applied on steel substrates [27]. The self-healing effectiveness of the epoxy resin@PUF microcapsules was successfully verified. Li et al. utilized in situ polymerization to fabricate urea-formaldehyde resin-encapsulated tung oil microcapsules [28]. After incorporating these microcapsules into epoxy coatings, their self-healing capability was conclusively demonstrated through scanning electron microscopy observations and electrochemical impedance spectroscopy measurements.

However, existing research has primarily focused on incorporating microcapsules into non-metallic powder-coating blended systems, such as epoxy coatings or UV-curable coatings, while the dispersion stability issues between microcapsules and metallic powders—particularly in wood coating applications—remain unexplored. For instance, although Thakur et al. successfully developed self-healing microcapsules with melamine resin shells and rosin-based epoxy resin/rosin-derived imine hardener cores, these microcapsules were incorporated into an epoxy resin coating rather than a metallic powder-coating blended system [29]. Similarly, Han et al. merely fabricated a brass powder-modified water-based acrylic coating without introducing microcapsules or investigating their effects on coating performance; moreover, the addition of brass powder was found to compromise the mechanical properties of the coating [24]. Consequently, investigating the compatibility of self-healing microcapsules in metallic powder-coating hybrid systems and elucidating their repair mechanisms could pioneer novel pathways for developing next-generation self-healing decorative coatings.

Melamine-formaldehyde (MF) resin, a thermosetting polymer synthesized through polycondensation of melamine and formaldehyde [30], offers relatively low production costs. Compared with urea-formaldehyde resin (UF), MF resin exhibits significantly lower formaldehyde emission and superior thermal stability [31,32,33]. Due to its excellent interfacial compatibility with water-based coatings, it has been widely employed as a shell material for microencapsulation [34]. Current research demonstrates that when MF-encapsulated water-based coating microcapsules are incorporated into aqueous coatings, the core material can autonomously cure at room temperature following environmental or mechanical damage to the coating, thereby achieving self-repair functionality.

A novel self-healing brass powder-water-based acrylic decorative coating for woodware was developed using KH570-modified brass powder (with reaction solution pH of 5 and coupling agent concentration of 6%) as filler and MF resin-encapsulated water-based topcoat microcapsules as healing agent. Orthogonal experiments were conducted with optical and mechanical properties as evaluation indices to investigate the effects of four factors—brass powder content, core–wall ratio of microcapsules, microcapsule content, and curing temperature—on the coating’s gloss and elongation at break on wood surfaces. After the most influential factor was identified, single-factor tests were performed to evaluate the coating’s optical properties, mechanical performance, aging resistance, cold liquid resistance, and self-healing capability. The optimal preparation process for the self-healing microcapsule-modified brass powder-waterborne acrylic decorative coating for woodware was ultimately determined. This process not only preserved the decorative characteristics of brass powder but also enabled damage repair through microcapsules. Compared with traditional single-functional coatings (e.g., those containing only metal powder or microcapsules), the present system achieved dual functional integration of decoration and self-healing, thereby establishing a technical foundation for the development of wood decorative coatings.

2. Materials and Methods

2.1. Materials

The brass powder (1000 mesh, ~13 μm diameter) was supplied by Nangong Xindun Alloy Welding Material Spraying Co., Ltd., Xingtai, China. γ-Methacryloxypropyltrimethoxysilane (KH570, C₁₀H₂₀O₅Si, M_w: 248.35 g/mol, CAS: 2530-85-0) was provided by Hangzhou Jessica Chemicals Co., Ltd., Hangzhou, China. Water-based acrylic primer and topcoat (Dulux brand) were purchased from AkzoNobel Paints Co., Ltd., Shanghai, China. Formaldehyde solution (37 wt%, CH₂O, M_w: 30.03 g/mol, CAS: 50-00-0) was obtained from Xi’an Tianmao Chemical Co., Ltd., Xi’an, China. Melamine (C₃H₆N₆, M_w: 126.15 g/mol, CAS: 108-78-1) was acquired from Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China. Triethanolamine (C₆H₁₅NO₃, M_w: 149.19 g/mol, CAS: 102-71-6, analytical grade) was sourced from Guangzhou Jiale Chemical Co., Ltd., Guangzhou, China. Sodium dodecylbenzenesulfonate (C₁₈H₂₉NaO₃S, M_w: 348.476 g/mol, CAS: 25155-30-0) was supplied by Tianjin Beichen Fangzheng Reagent Factory, Tianjin, China. Citric acid monohydrate (C₆H₁₀O₈, M_w: 210.14 g/mol, CAS: 5949-29-1) was purchased from Suzhou Changjiu Chemical Technology Co., Ltd., Suzhou, China. Absolute ethanol (C₂H₆O, M_w: 46.07 g/mol, CAS: 64-17-5) was procured from Wuxi Jingke Chemical Co., Ltd., Wuxi, China. Deionized water was prepared in the laboratory. Basswood panels (Tilia europaea, 100 × 50 × 5 mm³) were bought from Beijing Yidimei Model Co., Ltd., Beijing, China. Diaopai detergent was provided by Nice Group Co., Ltd., Hangzhou, China, and instant coffee was obtained from UCC Ueshima Coffee Co., Ltd., Kobe, Japan.

2.2. Modification Methods of Brass Powder

A homogeneous solvent mixture was prepared by combining anhydrous ethanol (80.0 g) and deionized water (20.0 g) at a 4:1 mass ratio in a glass beaker under ambient conditions (25 ± 1 °C). The mixture was thoroughly stirred with a glass rod to ensure homogeneity and then reserved as the solvent for subsequent silane hydrolysis reactions. A measured quantity of citric acid monohydrate was added to the solution to adjust the pH to approximately 5.0, as verified by a calibrated pH meter. The silane-modified solution was prepared by controlled dropwise addition of 0.6 g KH570 coupling agent under continuous stirring. Subsequently, 10.0 g brass powder was introduced into the modification solution and reacted for 3 h under mechanical stirring (600 rpm) in a thermostated water bath (30 ± 0.5 °C). The product was sequentially washed via vacuum filtration using deionized water and anhydrous ethanol to remove unreacted silanes, followed by drying in a convection oven at 60 °C for 12 h to yield surface-modified brass powder.

2.3. Preparation of Water-Based Topcoat Microcapsules Coated with MF Resin

(1)

Preparation of hydroxymethylol melamine (HMM) mixture

The following steps were performed according to

Table 1. List of raw material consumption of MF resin-coated waterborne topcoat microcapsules with different core–wall ratios.

Core–Wall Feed Ratio	Melamine (g)	37% Formaldehyde (g)	Deionized Water (g)
0.58:1	10.06	19.52	50.29
0.67:1	8.71	16.81	43.53
0.75:1	7.78	15.02	38.89

. A certain mass of melamine and 37% formaldehyde solution was weighed at a molar ratio of 1:3, and a specific amount of deionized water was added. The mixture was rapidly stirred with a glass rod at room temperature. A small amount of triethanolamine was added dropwise using a rubber-tipped dropper to adjust the pH of the mixture. After the pH reached 8–9, the beaker was placed in a constant-temperature water bath and stirred at 70 °C and 600 rpm for 30 min to obtain a water-soluble HMM mixture.

(2)

Emulsification of core material substances

First, 89.7 g of deionized water was weighed and added into a beaker, followed by the dropwise addition of 0.3 g of sodium dodecyl benzene sulfonate (SDBS). The mixture was stirred uniformly with a glass rod to obtain an emulsifier solution. Then, 10.0 g of water-based topcoat was weighed and added dropwise into the emulsifier solution. The beaker was placed in a constant-temperature water bath and stirred at 70 °C and 600 rpm for 60 min to obtain a well-dispersed core material emulsion.

(3)

The forming of microcapsules

The prepolymer solution was slowly added dropwise into the core material emulsion at a stirring speed of 600 rpm. Subsequently, the mixture was transferred to a BILON-500 ultrasonic material emulsification disperser (Shanghai Bilang Instrument Co., Ltd., Shanghai, China) and subjected to ultrasonic treatment for 15 min. After ultrasonic dispersion, the pH of the solution was adjusted using a 20 wt% citric acid monohydrate solution. When the pH reached approximately 3, the water bath temperature was gradually increased to 60 °C, and the reaction was maintained at this temperature for 3 h under constant conditions. The resulting product was allowed to stand at room temperature for 3 days, followed by repeated washing and suction filtration using deionized water and anhydrous ethanol. Finally, the product was dried in an oven at 60 °C for 1 day to obtain the microcapsule powder. Figure 1 shows a schematic diagram of the microcapsule preparation process.

2.4. Preparation of Self-Healing Brass Powder—Waterborne Acrylic Coating

First, the modified brass powder and prepared water-based topcoat microcapsules were added to a water-based acrylic topcoat at specified mass concentrations. The mixture was thoroughly stirred with a glass rod to achieve homogeneity, yielding 2.0 g of topcoat for subsequent use. Next, 2.0 g of water-based primer was weighed and evenly divided into three portions. Each portion was uniformly brush-coated onto the surface of a basswood substrate that had been pre-smoothed with 800-grit sandpaper. After each primer application, the sample was placed in an oven at a controlled temperature for curing and drying. Following the application of each primer layer, the surface was smoothed using 1000-grit sandpaper. Finally, the prepared topcoat was applied in two even layers over the third primer coating. The coated sample was then transferred to an oven at a set temperature for final curing. The sample was removed only after its mass stabilized, indicating complete drying.

2.5. The Design of Orthogonal Test Schemes

Based on the above conditions, an L₉(3⁴) orthogonal array test (four factors at three levels) was designed considering the following four factors: content of KH570-modified brass powder (3%, 6%, 9%), core–wall ratio of microcapsules (0.58:1, 0.67:1, 0.75:1), microcapsule content (3%, 6%, 9%), and curing temperature (room temperature, 40 °C, 60 °C) (Tables S1 and S2). The formulation details of the coating systems for orthogonal testing are presented in Table S3.

2.6. Design of Single-Factor Experimental Protocol

The prepared coating samples were orthogonally analyzed using glossiness and elongation at break as evaluation criteria to identify the most significant influencing factor. Experimental process optimization was then conducted focusing on this primary factor to investigate the effects of self-healing microcapsules with different core–wall ratios on the performance of brass powder-waterborne acrylic coatings. With brass powder content determined as the dominant factor, concentration gradients of 0.5%, 1%, 3%, 5%, 7%, 9%, and 10% were tested under constant conditions of 3% microcapsule content and room temperature curing, to examine the influence of brass powder content on coating performance with varying core–wall ratio microcapsules (

Table 2. Single-factor test schedule.

Sample	The Feeding Ratio of the Microcapsule Core Wall	Brass Powder Quality (g)	Microcapsule Quality (g)	Topcoat Quality (g)	Primer Quality (g)
10	0.58:1	0.00	0.06	1.94	2.00
11		0.01	0.06	1.93	2.00
12		0.02	0.06	1.92	2.00
13		0.06	0.06	1.88	2.00
14		0.10	0.06	1.84	2.00
15		0.14	0.06	1.80	2.00
16		0.18	0.06	1.76	2.00
17		0.20	0.06	1.74	2.00
18	0.67:1	0.00	0.06	1.94	2.00
19		0.01	0.06	1.93	2.00
20		0.02	0.06	1.92	2.00
21		0.06	0.06	1.88	2.00
22		0.10	0.06	1.84	2.00
23		0.14	0.06	1.80	2.00
24		0.18	0.06	1.76	2.00
25		0.20	0.06	1.74	2.00
26	0.75:1	0.00	0.06	1.94	2.00
27		0.01	0.06	1.93	2.00
28		0.02	0.06	1.92	2.00
29		0.06	0.06	1.88	2.00
30		0.10	0.06	1.84	2.00
31		0.14	0.06	1.80	2.00
32		0.18	0.06	1.76	2.00
33		0.20	0.06	1.74	2.00

).

All samples were coated following the standardized protocol: three coats of primer followed by two coats of topcoat, with both the brass powder and microcapsules exclusively incorporated into the topcoat layers.

3. Test and Characterization

3.1. Characterization of Macroscopic and Microscopic Morphology and Chemical Composition

The coated basswood surfaces were documented macroscopically using a SONY Alpha 7C camera (Sony (China) Co., Ltd., Beijing, China). Microcapsule morphology was examined with an Axio Lab. A1 optical microscope (Beijing Precise Instrument Co., Ltd., Beijing, China), while both microcapsules and coatings were characterized microscopically using scanning electron microscopy (SEM). Particle size distribution of microcapsules was determined using Nano Measurer software 1.2.5 with a sample size of 100 [35]. Chemical composition analysis was performed through Fourier-transform infrared spectroscopy (FT-IR), employing the pellet method for microcapsules and attenuated total reflectance (ATR) mode for coating samples.

3.2. Coating Optical Performance Test

The coating glossiness was measured in compliance with Chinese National Standard GB/T 4893.6-2013 [36] using an NHG268 touchscreen triple-angle glossmeter (Shenzhen 3nh Technology Co., Ltd., Shenzhen, China). Gloss unit (GU) values were recorded at three incident angles (20°, 60°, and 85°) following the standardized testing protocol.

The color difference in the coatings was measured using an SEGT-J 3NH triple-angle colorimeter (Shenzhen 3nh Technology Co., Ltd., Shenzhen, China). A random point on the coating surface was first selected and measured with the colorimeter to obtain the initial set of colorimetric data (L₁, a₁, b₁). Subsequently, another random point was selected and measured to acquire the second set of data (L₂, a₂, b₂). The color difference value (ΔE) of the coating was then calculated according to Formula (1), where ΔL = L₁ − L₂ represented the lightness difference, Δa = a₁ − a₂ indicated the red/green chromaticity difference, and Δb = b₁ − b₂ denoted the yellow/blue chromaticity difference [37,38].

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

(1)

Additionally, the chromaticity values of waterborne coatings containing different brass powder concentrations were measured using a colorimeter. For each coated sample, four measurement points were taken, and the average values were calculated and recorded as L*, a*, and b* values representing the coating’s chromaticity. These three chromaticity parameters were then subtracted from those of the pure waterborne coating (without brass powder) to obtain ΔL*, Δa*, and Δb* values. The color difference (ΔE*) for coatings with varying brass powder content was finally calculated according to color difference Formula (1) [39].

The visible light wavelength-reflectance curves, dominant wavelength data, and color purity of the Basswood surface coatings were obtained using a Hitachi U-3900/3900H ultraviolet-visible spectrophotometer (Hitachi Instruments (Suzhou) Co., Ltd., Suzhou, China) across the visible spectrum (780–380 nm). In accordance with ASTM G173-03 standard [40], the solar reflectance (R) of the coatings within the visible light range was calculated using Formula (2).

$$R={\displaystyle \frac{{\int }_{380}^{780}r\left(\lambda \right)i\left(\lambda \right)d\left(\lambda \right)}{{\int }_{380}^{780}i\left(\lambda \right)d\left(\lambda \right)}}$$

(2)

The standard solar irradiance intensity was denoted as i(λ) (unit: W·m⁻²·nm⁻¹), while the reflectance value obtained through testing was represented by r(λ).

The wavelength-transmittance curves of Basswood surface coatings within the visible light spectrum (780–380 nm) were acquired using a Hitachi U-3900/3900H UV-Vis spectrophotometer (Hitachi Instruments Co., Ltd., Suzhou, China). The solar transmittance (τ) in the visible range was subsequently calculated through Formula (3) to characterize the coating’s transparency [41].

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

(3)

The weighting factor was derived from the spectral distribution of CIE standard illuminant D65 and the CIE photopic luminosity function, denoted as d(λ)v(λ), while the transmittance at each wavelength was expressed as τt (unit: %).

3.3. Coating Hardness Test

The hardness test was conducted using the QHQ-A portable pencil hardness tester (Liangchuang Instruments (Suzhou) Co., Ltd., Suzhou, China) with pencils ranging from 6B to 6H. The pencil was inserted at 45° into the hardness tester under a load of 750 g. Starting from 6B, the pencil hardness was gradually increased until a permanent indentation appeared on the coated sample. The pencil hardness at this point was recorded as the coating hardness.

3.4. Coating Impact Resistance Test

The impact resistance of the coating was evaluated using the BEVS 1601 impact tester (Beijing Times Summit Technology Co., Ltd., Beijing, China). The finished Basswood panel was placed on the impact tester, with the steel ball positioned directly above the test surface. The steel ball was then raised to a specific height and allowed to fall freely onto the panel. The minimum height at which the coating was damaged was recorded as the impact resistance strength of the coating, with the unit expressed in kg·cm.

3.5. Coating Adhesion Test

The adhesion of the coating was tested using the QFH-HG600 cross-cut tester (Dongguan Zhongte Automation Technology Co., Ltd., Dongguan, China). The blade of the cross-cut tester was held perpendicular to the coating surface, and uniform manual pressure was applied to create a set of parallel cuts. A second set of cuts was then made at a 90° angle to the first set, forming a grid pattern on the coating surface. A piece of adhesive tape of appropriate length was firmly applied over the grid and left in place for 5 min. Afterward, the tape was steadily peeled off by hand. The adhesion grade of the coating was determined based on the amount of coating removed by the tape. The adhesion grades were classified into Levels 1–5, with Level 1 representing the best adhesion (minimal coating removal) and higher numbers indicating progressively poorer adhesion.

3.6. Coating Roughness Test

The surface roughness of the finished basswood panel was measured using a precision roughness tester J8-4C (manufactured by Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China). The coated panel was placed on the testing platform, and the probe was carefully positioned to make contact with the wood surface. After adjusting the probe to stabilize at the zero-reference position, the measurement was initiated, and the roughness value (Ra) was recorded in micrometers (µm).

3.7. Coating Aging Resistance Test

The coating samples were subjected to dry heat resistance testing by placing them in an oven at 160 °C for accelerated thermal aging. Additionally, in accordance with ASTM D4587-2011 [42], the prepared coating specimens underwent UV photo-oxidation resistance testing using UVA-340 fluorescent lamps, with three cycles of 4 h irradiation (0.89 W·m⁻²·nm⁻¹) followed by 20 h condensation. The color difference (ΔE*) of the coating was calculated using Formula (1) to evaluate its aging resistance.

3.8. Coating Resistant to Cold Fluid Performance Test

The test solutions were selected as follows: a 10 wt% aqueous citric acid solution, 96 vol% undenatured ethanol, detergent, and coffee (prepared by dissolving 40 g of freeze-dried instant coffee in 1 L of boiling water). Filter papers with a diameter of 25 mm were saturated with each test solution and then placed on the coated surface. The specimens were stored in a sealed environment. After 24 h of exposure, the filter papers were removed, and the samples were allowed to stand for 16 h. The coating damage was then observed and rated according to

Table 3. Grading table of cold liquid resistance of the film.

Grade	Explanation
1	No change; the test area cannot be distinguished from the adjacent area.
2	Slight change. Only when the light source is projected onto the test surface and reflected into the observer’s eyes can the test area be distinguished from the adjacent areas, such as fading, glossiness and color change. The surface structure of the test remained unchanged. Such as swelling, fiber protrusion, cracking, and bubbling.
3	Moderate change. It can be seen in several directions, and the test area can be distinguished from the adjacent areas, such as fading, glossiness and color change. There was no change in the surface structure of the test, such as expansion, fiber protrusion, cracking or bubbling.
4	Obvious change; it is visible in all visible directions, and the test area can be clearly distinguished from the adjacent areas, such as fading, glossiness and color change. And/or there are slight changes in the surface structure of the test, such as expansion, fiber protrusion, cracking, and bubbling.
5	Serious change; the surface structure of the test has changed significantly; And/or fade, become lustrous and change color; And/or all or part of the surface material is removed; And/or the filter paper sticks to the surface.

. The chromaticity values of the Basswood surface coating were measured before and after the liquid resistance test using a chroma meter. The color difference (ΔE*) was calculated using Formula (1).

3.9. Coating Self-Healing Performance Test

The variation rate of scratch width was commonly adopted as an evaluation method for coating self-healing performance [43]. The coating with a uniform thickness of 60 µm and consistent chemical composition was prepared using a ZBQ four-sided coating applicator (Pushine Inspection Instrument (Shanghai) Co., Ltd., Shanghai, China). A precisely controlled 60 µm-deep scratch was created on the self-healing coating using a Flying Eagle brand 74-C single-edge razor blade (Shanghai Gillette Co., Ltd., Shanghai, China), which completely penetrated the coating layer. The scratch morphology was observed by optical microscopy (OM) immediately after generation and after 3 days of static placement in an LT-TH-80 constant temperature and humidity chamber (Dongguan Lituo Testing Instrument Co., Ltd., Dongguan, China) maintained at 25 °C with 45% relative humidity. The scratch width change rate (D_H), calculated according to Formula (4), was used to characterize the self-healing performance of the coating.

$$D_H={\displaystyle \frac{D_1-D_2}{D_1}}\times 100\%$$

(4)

In this formula, D_H represented the width variation rate (expressed in %), D₁ corresponded to the initial scratch width on the Basswood surface coating (measured in µm), while D₂ reflected the scratch width after a 3-day recovery period (recorded in µm).

All the aforementioned tests were repeated four times with an error margin of less than 5%.

4. Results and Discussion

4.1. Microscopic Morphology and Infrared Spectroscopy Analysis of MF Resin-Water-Based Topcoat Microcapsules with Different Core–Wall Feed Ratios

(1)

Microscopic morphology analysis

The microscopic morphologies of three microcapsules with different core–wall feed ratios are shown in Figure 2 and Figure 3. In Figure 2, two distinct media are clearly observed within the successfully prepared microcapsules. Based on combined SEM and OM analyses, the self-healing microcapsules with core–wall feed ratios of 0.58:1, 0.67:1, and 0.75:1 were all confirmed to exhibit spherical morphology. The microcapsules with a core–wall feed ratio of 0.58:1 were observed to possess well-defined morphology with smooth surfaces and plump spherical shapes. In contrast, significant interparticle adhesion was exhibited by the 0.67:1 ratio of microcapsules, along with less uniform spherical morphology and more pronounced agglomeration phenomena. The presence of small flocculent structures was detected in the 0.75:1 ratio of microcapsules, which was found to further intensify their aggregation effect. The particle size distributions of the three microcapsule formulations with different core–wall ratios are presented in Figure 4. The particle sizes of the 0.58:1 ratio of microcapsules were predominantly distributed within the 5.0–10.0 µm range. A greater concentration of particles was observed in the 5.0–7.5 µm range for both the 0.67:1 and 0.75:1 ratios of microcapsules, which were generally determined to have smaller particle sizes compared to the 0.58:1 ratio specimens.

(2)

Infrared spectroscopy analysis

As shown in Figure 5, the infrared spectral transmittance curves of the three microcapsules were found to exhibit essentially identical fluctuation trends, indicating that the chemical bonds contained in these samples were fundamentally identical. In the MF resin wall material, the characteristic absorption peak at 813 cm⁻¹ was attributed to the bending vibration of triazine rings [44], while the stretching vibration absorption peak of N-H was observed at 1558 cm⁻¹. The absorption peak around 1000 cm⁻¹ was identified as corresponding to C-O bonds. These characteristic peaks specific to MF resin were consistently detected in the FTIR spectra of all microcapsule variants [45]. The characteristic peak of C=O bonds in waterborne acrylic resin was located at approximately 1726 cm⁻¹ [46]. The stretching vibration peak of C-H bonds in C-CH₃ was recorded at 2920 cm⁻¹ [47]. When combined with the microscopic morphological analysis of the samples, the microcapsules were confirmed to be successfully prepared.

4.2. The Orthogonal Experiment Result Analysis

In the brass powder/waterborne acrylic decorative coating system, the content of metallic fillers significantly influences the optical and mechanical properties of the coating. The concentration of brass powder in the coating should be maintained within 0%–10% [24]. Therefore, three levels (3%, 6%, and 9%) were selected in the orthogonal experiments to investigate the effect of brass powder content on the comprehensive performance of the coating. The incorporation of self-healing microcapsules can introduce self-repairing functionality to the brass powder/waterborne acrylic decorative coating system. For self-healing coatings, factors such as the core–wall mass ratio [48] and content of microcapsules may lead to variations in both the fundamental properties and self-healing efficiency of the coating. Typical core–wall mass ratios for self-healing microcapsules include 0.58:1, 0.67:1, and 0.75:1. Microcapsules prepared under these ratios generally exhibit favorable morphology, high yield, and excellent healing performance.

Preliminary experiments indicated that the coating’s properties were constrained by the maximum loading capacity of microcapsules. Research by Li et al. demonstrated that excessive microcapsule content (particularly above 9%) may cause increased color deviation, reduced gloss, diminished self-healing efficiency, and surface roughness due to uneven microcapsule distribution [49]. To ensure optimal physicochemical properties and self-healing performance, the microcapsule content should also be controlled below 10%. Consequently, three levels (3%, 6%, and 9%) were chosen in the orthogonal experiments to examine the influence of microcapsule content on coating performance.

Simultaneously, as a common factor affecting coating properties, curing temperature should also be considered in the process optimization of brass powder/waterborne acrylic decorative coatings for wooden substrates [50]. Since wood-based coatings in thermal curing systems must account for the thermal expansion/contraction characteristics of wood itself, the curing temperature for wood surface coatings generally should not exceed 60 °C. Temperatures below 60 °C represent a balanced compromise considering film-forming quality, substrate protection, energy efficiency, and reaction controllability, making them suitable for most waterborne acrylic coating applications. Accordingly, three curing conditions—room temperature (approximately 25–30 °C), 40 °C, and 60 °C—were selected for the orthogonal experiments.

(1)

Glossiness

The gloss measurement results of the modified brass powder-waterborne acrylic decorative coatings containing self-healing microcapsules prepared through orthogonal experiments are presented in Table S4. Among these, Sample 1 was identified as exhibiting the optimal gloss performance, with measured values of 9.7 GU at 20° incidence angle, 23.6 GU at 60° incidence angle, and 12.8 GU at 85° incidence angle, all of which were determined to be superior to those of other orthogonal samples. The intuitive analysis of coating gloss at 60° incidence angle was provided in Table S5. At this measurement angle, in addition to Sample 1 showing the highest gloss value, Sample 2 was also found to demonstrate relatively excellent performance with a gloss value of 15.3 GU. The gloss effect curve of decorative coatings at 60° incidence angle was illustrated in Figure 6. Based on the range analysis results, the brass powder content was revealed to be the most influential factor affecting coating gloss, followed sequentially by the core–wall ratio of microcapsules, curing temperature, and microcapsule content. According to the variance analysis results of gloss presented in

Table 4. Analysis of variance of gloss.

Factor	Bias Squares	Degree of Freedom	F Ratio	Critical Value of F	Significance
Brass powder content	293.482	2	19.137	19.000	*
Core–wall feed ratio	65.936	2	4.299	19.000
Microcapsule content	15.336	2	1.000	19.000
Curing temperature	18.682	2	1.218	19.000
Error	15.34	2

A single asterisk (*) represents statistical significance at the 0.05 probability level.

, the significance ranking of these four factors was confirmed to be consistent with the range analysis results. The brass powder content was statistically proven to have the most significant impact on coating gloss, with the microcapsule core–wall ratio being the secondary influential factor. Through comprehensive analysis of the range results, the optimal process parameters for preparing self-healing brass powder-waterborne acrylic decorative coatings with maximum gloss were determined as follows: brass powder content of 3%, microcapsule core–wall ratio of 0.58:1, microcapsule content of 3%, and room temperature curing condition.

(2)

Elongation at break

The mechanical properties of coatings were scientifically evaluated since they are frequently subjected to various forces during service. For decorative coatings, the elongation at break was considered an effective indicator for assessing mechanical performance. When external tensile forces were applied, coatings with higher elongation at break values were demonstrated to possess better ductility, enabling them to resist stretching forces and consequently delay or prevent cracking and delamination. In the orthogonal experiments, the elongation at break results of modified brass powder-waterborne acrylic decorative coatings containing self-healing microcapsules were analyzed as shown in Table S6. Sample 1 was identified as exhibiting the optimal performance with 7.12% elongation, followed by Sample 3 (5.15%) and Sample 2 (4.59%). The effect curve of elongation at break for the prepared decorative coatings is presented in Figure 7. Through range analysis, the brass powder content was determined to be the most influential factor affecting elongation at break, succeeded by the microcapsule core–wall ratio, microcapsule content, and curing temperature. The variance analysis results of coating elongation at break (

Table 5. Analysis of variance of elongation at break.

Factor	Bias Squares	Degree of Freedom	F Ratio	Critical Value of F	Significance
Brass powder content	17.137	2	133.883	19.000	*
Core–wall feed ratio	4.251	2	33.211	19.000	*
Microcapsule content	2.710	2	21.172	19.000	*
Curing temperature	0.128	2	1.000	19.000
Error	0.13	2

A single asterisk (*) represents statistical significance at the 0.05 probability level.

) were found to be consistent with the range analysis findings. Regarding elongation at break, three factors—brass powder content, microcapsule core–wall ratio, and microcapsule content—were all confirmed to have significant impacts, with brass powder content being the most dominant. The curing temperature was shown to have the least effect on elongation at break. Based on comprehensive range analysis, the optimal parameters for achieving maximum elongation at break in self-healing brass powder-waterborne acrylic decorative coatings were established as: 3% brass powder content, 0.58:1 microcapsule core–wall ratio, 3% microcapsule content, and room temperature curing. These parameters were verified to be identical to those obtained from the gloss analysis, indicating consistency between the two performance evaluations.

4.3. Single-Factor Test Result Analysis

Decorative coatings primarily emphasize optical and mechanical properties, with self-healing performance being merely an auxiliary function. Moreover, the healing efficiency showed minimal variation when the microcapsule content was altered. For instance, Zou et al. successfully incorporated three types of UV-curable microcapsules into UV coatings and investigated their healing properties [51]. Their study revealed that although the microcapsule content varied within the range of 2.0%–10.0%, the self-healing rates exhibited only minor differences (fluctuating between 25% and 30%) without continuous improvement as microcapsule content increased. Consequently, the orthogonal experimental results were primarily analyzed based on optical and mechanical properties to identify the most influential factors affecting the brass powder/waterborne acrylic coating performance. Through analysis of variance and range analysis of coating gloss and elongation at break, it was determined that brass powder content exerted the most significant influence on both properties. The optimal performance was achieved when the coating contained 3% brass powder, microcapsules with a core–wall mass ratio of 0.58:1, 3% microcapsule content, and was cured at room temperature. Therefore, in subsequent single-factor experiments, brass powder content was selected as the primary variable. A concentration gradient of 0.5%, 1%, 3%, 5%, 7%, 9%, and 10% was tested under fixed conditions of 3% microcapsule content and room temperature curing, to systematically evaluate the effect of brass powder content on the performance of self-healing microcapsule-modified coatings with different core–wall ratios.

4.3.1. Optical Performance Analysis

(1)

Macroscopic morphology

The specimens prepared through single-factor experiments were presented in Figure 8, Figure 9 and Figure 10.

(2)

Glossiness

The influence of brass powder content on the gloss of decorative coatings containing microcapsules with different core–wall ratios was demonstrated in Table S7 and Figure 11. The coatings with core–wall ratios of 0.58:1 and 0.75:1 demonstrated superior gloss performance compared to the 0.67:1 group, which was primarily attributed to their smoother surface morphology and better dispersibility of microcapsules. Experimental data revealed that coating gloss exhibited a significant negative correlation (R² > 0.92) with brass powder content at all measured angles (20°, 60° and 85°). This correlation originated from the inherent surface roughness and agglomeration tendency of modified brass powder, which increased the coating surface roughness and enhanced light scattering.

Within the 0%–0.5% brass powder content range, the most significant gloss reduction effect was observed, with an average 25.2% decrease recorded across all three microcapsule formulations. Among these, Sample 11 was identified as exhibiting the most rapid gloss reduction at 36.1%. Beyond 0.5% brass powder content (up to 10%), the gloss-diminishing effect was demonstrated to become more gradual.

(3)

Color difference

The effect of brass powder content on the ΔE of decorative coatings containing microcapsules with different core–wall ratios is illustrated in Figure 12 and Table S8. For all brass powder decorative coatings containing microcapsules with varying core–wall ratios, the increase in brass powder content was found to compromise the color uniformity of the coating surfaces. Generally, a positive correlation was observed between brass powder content and color difference magnitude. When the brass powder content was maintained within the 0%–1% range, the minimum color difference (ΔE = 0.00) was recorded for all three coating formulations. This phenomenon was attributed to the uniform dispersion of brass powder achieved at these low concentrations. Beyond 3% brass powder content, the surface color uniformity was demonstrated to deteriorate. Among the tested formulations, coatings containing microcapsules with 0.58:1 and 0.75:1 core–wall ratios were measured to have relatively smaller color differences (2.13 and 2.38, respectively). Within the 3%–7% brass powder range, these two coatings maintained ΔE values below 3.00, indicating their superior color retention capability. The most significant color difference impact was observed in coatings containing 0.67:1 ratio of microcapsules. At 7% brass powder content, this formulation exhibited a ΔE value of 7.42, which was calculated to be 5.29 and 5.37 units higher than the 0.58:1 and 0.75:1 ratios of counterparts, respectively. When the brass powder content reached 10%, the maximum ΔE values were recorded for all three coating types. This effect was explained by the increased surface roughness caused by brass powder agglomeration at higher concentrations, which was shown to amplify color difference perception. The observed phenomenon was attributed to the inferior dispersibility of brass powder at higher concentrations, where particle agglomeration was frequently induced. This resulted in elevated surface roughness of the coating, which consequently led to more significant color differences being manifested.

(4)

Chromaticity change

The influence of brass powder content on the chromatic aberration (ΔE*) of decorative coatings containing microcapsules with different core–wall ratios is presented in Table S9 and Figure 13. For all three coating formulations with varying core–wall ratios, the chromatic aberration was observed to become progressively more pronounced with increasing brass powder content, though the specific core–wall ratio was not found to significantly affect the ΔE* values. A consistent trend was demonstrated where the chromatic aberration was amplified with higher brass powder concentrations. Under 9%–10% brass powder content, the most substantial color changes were recorded, with all three coatings exceeding ΔE* values of 10.00. Notably, for coatings containing 0.58:1 and 0.75:1 core–wall ratio microcapsules, ΔE* values above 10.00 were already achieved when brass powder content exceeded 3%. The observed chromatic variation was primarily caused by the inherent yellow chromogenic characteristics of brass powder and its covering effect on the basswood substrate. The yellow covering effect was enhanced with increased brass powder content, resulting in greater ΔE values and consequently more pronounced chromatic changes in the coating. Additionally, the influence of microcapsule core–wall ratio on chromatic variation was found to be minimal, a phenomenon that was strongly correlated with the observed ΔE* change patterns.

(5)

Visible light reflectance

The effects of varying brass powder content on the visible light reflectance (R) of decorative coatings containing microcapsules with different core–wall ratios are reflected in Figure 14 and Figure 15, Table S10. No significant influence of microcapsule core–wall ratios on the visible light reflectance of brass powder coatings was observed. For coatings containing microcapsules with fixed core–wall ratios, an inverse correlation between brass powder content and R values was established. This phenomenon was attributed to the light reflection and scattering caused by metallic fillers in the coating system. When higher concentrations of metallic fillers were incorporated, the increased density of metal particles was shown to enhance light-particle interactions, resulting in greater light scattering and reflection losses. Additionally, in the visible light spectrum, partial light absorption by metallic fillers with subsequent conversion to thermal energy was demonstrated to contribute to reduced reflectance. Furthermore, the presence of metallic fillers was found to increase surface roughness, thereby amplifying light scattering and reflection losses at the coating surface. Within the 0%–10% brass powder content range, a progressive decrease in R values was recorded with increasing brass powder content. Among all formulations, coatings containing 0.58:1 core–wall ratio microcapsules were measured to exhibit the highest reflectance values.

Similarly, the color saturation of coating samples is characterized in Figure 14. The brass powder decorative coatings containing microcapsules with different core–wall ratios were observed to exhibit identical trends in color saturation variation—a progressive decrease in color saturation was recorded with increasing brass powder content, where lower saturation values corresponded to less vibrant coloration. Among all samples, the coating containing 0.5% brass powder was determined to maintain optimal color vibrancy.

The dominant wavelengths of surface colors for brass powder decorative coatings containing microcapsules with different core–wall ratios are presented in Table S11. As a critical parameter for evaluating the overall visual color of coating samples, the dominant wavelength was demonstrated to effectively characterize the color appearance. All samples were measured to exhibit dominant wavelengths within the range of 590–586 nm, which falls within the yellow spectrum (597–577 nm) of visible light, confirming that all coatings displayed yellow surface coloration regardless of microcapsule core–wall ratios. An inverse correlation between brass powder content and dominant wavelength values was established. Lower brass powder concentrations were found to produce wavelength closer to 589 nm, indicating a visual color shift toward orange. Conversely, higher brass powder content was associated with wavelengths approaching 586 nm, representing a purer yellow coloration centered around 580 nm.

(6)

Visible light transmittance

The light transmittance of coatings had a significant impact on their application prospects in decorative coatings. Figure 16 shows the transmittance curves of brass powder decorative coatings containing microcapsules with different core–wall feed ratios in the visible light range. Table S12 presents the transmittance of brass powder decorative coatings containing microcapsules with different core–wall feed ratios in the visible light range. Based on the figures and tables, it was found that the light transmittance of brass powder decorative coatings containing microcapsules with different core–wall feed ratios all decreased continuously as the brass powder content in the coatings increased. For the brass powder decorative coating containing microcapsules with a 0.58:1 core–wall feed ratio, the transmittance decreased from 91.58% to 56.85%, with a difference of 34.73%. For the coating containing microcapsules with a 0.67:1 core–wall feed ratio, the transmittance decreased from 85.83% to 56.23%, with a difference of 29.60%. For the coating containing microcapsules with a 0.75:1 core–wall feed ratio, the transmittance decreased from 85.26% to 54.43%, with a difference of 30.83%. When the brass powder content was increased to 10%, the most significant decrease in transmittance was observed in the three types of brass powder decorative coatings containing microcapsules with different core–wall feed ratios. This is because, in the visible light range, the transmittance of light was influenced by the refractive index and absorption coefficient of the material. As a metallic filler, brass powder was characterized by its high refractive index and light absorption capacity. The degree of incident light absorption and scattering was enhanced with increasing brass powder content, resulting in reduced light transmittance. Furthermore, at lower concentrations, the brass powder was uniformly dispersed, yielding higher transmittance. However, when the content was increased (e.g., to 10%), particle agglomeration was frequently observed, leading to the formation of high-concentration zones and micro-defects. These structural irregularities consequently intensified light scattering and significantly diminished the transmittance.

The experimental results demonstrated that when the brass powder content was maintained at or below 3%, uniform particle dispersion was achieved, and the coating was characterized by high gloss (with low surface roughness), minimal color difference (ΔE < 3.0), excellent light transmittance (>85%), and a vibrant orange-yellow hue (dominant wavelength approximately 589 nm). However, when the concentration exceeded 3%, particle agglomeration was significantly intensified, resulting in increased surface roughness and the formation of micro-defects. Consequently, a sharp 25.2% reduction in gloss was observed in the low-concentration range, along with expanded color difference (ΔE reaching 7.42 in the 0.67:1 ratio group) and simultaneous deterioration of both transmittance (maximum reduction of 34.73%) and reflectivity. Additionally, a color shift toward pure yellow was detected, with the dominant wavelength decreased to 586 nm accompanied by reduced saturation.

4.3.2. Coating Hardness Analysis

The hardness of coatings containing microcapsules with different core–wall feed ratios on basswood surfaces, as influenced by the brass powder content in the coatings, is shown in

Table 6. Effect of microcapsules with different core–wall ratios on the hardness of brass powder film.

Brass Powder Content (%)	Hardness
	0.58:1	0.67:1	0.75:1
0	H	H	H
0.5	H	H	H
1	H	H	H
3	H	H	H
5	H	H	H
7	H	H	H
9	H	H	H
10	3H	H	3H

. The hardness of the basswood coating containing microcapsules without brass powder addition reached grade H, demonstrating significant improvement compared to the pure coating (grade 4B). This enhancement was attributed to the microcapsules functioning as high-hardness fillers that effectively reinforced the system’s mechanical strength.

For the three types of brass powder coatings containing microcapsules with different core–wall feed ratios, the hardness of the coatings showed no significant improvement at brass powder contents ranging from 0% to 9%. However, for the two self-healing water-based coatings containing microcapsules with core–wall feed ratios of 0.58:1 and 0.75:1, a notable enhancement in hardness was observed when the brass powder content increased from 9% to 10%. The hardness grade jumped from H to 3H, indicating a substantial change. This suggests that when the brass powder content reached 10%, the microscopic defects and voids in the coating were more effectively filled by the brass powder, reducing the porosity of the coating. Additionally, sufficient chemical bonds were formed in the brass powder-KH570-waterborne acrylic system, leading to stronger intermolecular interactions.

When the brass powder content in the coating was fixed, the influence of microcapsules with different core–wall feed ratios on the coating hardness was generally consistent. However, the coating containing microcapsules with a core–wall feed ratio of 0.67:1 was found to exhibit slightly lower hardness, which remained unchanged at grade H. This phenomenon is attributed to the surface morphology of the self-healing microcapsules. Under core–wall feed ratios of 0.58:1 and 0.75:1, the prepared microcapsules were observed to possess a more spherical shape. When subjected to external forces, the spherical particles were characterized by uniform stress distribution in all directions, resulting in isotropic deformation resistance. In contrast, the microcapsules prepared at a core–wall feed ratio of 0.67:1 tended to form ellipsoidal structures. Due to their anisotropic nature, the ellipsoidal particles exhibited uneven stress distribution—lower stress along the long axis and higher stress along the short axis—making them more susceptible to deformation in the short-axis direction. Consequently, when tested with an H-grade pencil, the water-based coating containing 10% microcapsules with a 0.67:1 core–wall ratio was observed to undergo significant deformation. In comparison, coatings containing 10% microcapsules with 0.58:1 or 0.75:1 ratios only showed noticeable deformation under the higher pressure of a 3H-grade pencil. These findings demonstrated that the spherical microcapsules contributed more effectively to enhancing the mechanical stability of the coating system.

4.3.3. Coating Resistance to Impact Analysis

The impact resistance of water-based coatings containing microcapsules with different core–wall feed ratios on Basswood surfaces, as influenced by the brass powder content in the coatings, is presented in

Table 7. Effect of microcapsules with different core–wall ratios on the impact resistance of brass powder film.

Brass Powder Content (%)	Impact Resistance (kg·cm)
	0.58:1	0.67:1	0.75:1
0	2	1	1
0.5	2	1	1
1	2	1	1
3	2	1	1
5	3	1	3
7	4	2	3
9	7	3	4
10	8	3	5

. The impact strength of the coatings was found to be affected by both the brass powder content and the type of microcapsules incorporated. For all three types of water-based coatings containing microcapsules with different core–wall feed ratios, the impact resistance was observed to increase steadily with higher brass powder content. When the brass powder content was increased, the solid powder filler was shown to enhance the coating density. Higher density resulted in improved resistance to external impacts. Specifically, the impact resistance of the coating containing 0.58:1 core–wall ratio microcapsules was measured to increase from 2 kg·cm to 8 kg·cm, representing the highest impact strength among all tested samples. The coating with 0.67:1 core–wall ratio microcapsules demonstrated an increase from 1 kg·cm to 3 kg·cm, while the coating containing 0.75:1 ratio microcapsules showed improvement from 1 kg·cm to 5 kg·cm. These results indicated that the coating with 0.58:1 core–wall ratio microcapsules exhibited the strongest impact resistance, followed by the 0.75:1 ratio formulation, while the 0.67:1 ratio coating displayed the weakest performance. This difference was attributed to the inferior spherical morphology of the 0.67:1 ratio microcapsules. When subjected to external impact forces, these microcapsules were more prone to deformation, thereby reducing the overall impact resistance of the coating.

4.3.4. Coating Adhesion Analysis

The adhesion performance of water-based acrylic coatings containing microcapsules with different core–wall feed ratios on Basswood surfaces, as influenced by brass powder content variations, is presented in

Table 8. Effect of microcapsules with different core–wall ratios on the adhesion of brass powder film.

Brass Powder Content (%)	Adhesion (Grade)
	0.58:1	0.67:1	0.75:1
0	2	2	2
0.5	2	2	2
1	2	2	2
3	2	2	2
5	2	3	3
7	3	3	3
9	3	4	4
10	3	4	4

. When the brass powder content ranged from 0% to 3%, the coating adhesion was consistently rated as excellent (Grade 2). However, when the brass powder content was further increased, the adhesion properties were observed to deteriorate to varying degrees. For coatings containing microcapsules with 0.67:1 and 0.75:1 core–wall feed ratios, the adhesion grade was found to gradually decline from Grade 2 to Grade 4. The coating with 0.58:1 ratio microcapsules showed a less severe reduction, decreasing from Grade 2 to Grade 3. These results indicated that for all three coating formulations with different microcapsule ratios, an inverse correlation was established between brass powder content and coating adhesion performance. In Peng et al.’s study, the adhesion grades of waterborne coatings containing microcapsules were predominantly classified as 1–2, whereas the brass powder coatings incorporating microcapsules in this work exhibited relatively lower adhesion performance, with most samples rated at grade 2–3 [52].

This phenomenon is attributed to the disruption of organic matrix homogeneity when inorganic solid powder is incorporated. The originally dense coating structure was compromised, leading to reduced internal cohesion. Furthermore, the particle size and morphology of brass powder were found to affect its distribution within the coating matrix. Excessive filler content was shown to cause uneven distribution, thereby interfering with substrate–coating bonding. When comparing coatings with identical brass powder content, the core–wall feed ratio of microcapsules was demonstrated to have limited influence on adhesion performance. However, microcapsules with 0.67:1 and 0.75:1 ratios were observed to have more negative impacts on adhesion than those with 0.58:1 ratio.

4.3.5. Analysis of Coating Roughness

The surface roughness of water-based coatings containing microcapsules with different core–wall feed ratios on Basswood surfaces, as influenced by brass powder content variations, is presented in

Table 9. Effect of microcapsules with different core–wall ratios on the roughness of brass powder film.

Brass Powder Content (%)	Roughness (µm)
	0.58:1	0.67:1	0.75:1
0	1.147 ± 0.026	1.147 ± 0.026	1.147 ± 0.026
0.5	1.570 ± 0.036	1.924 ± 0.046	1.726 ± 0.043
1	2.192 ± 0.056	2.523 ± 0.068	2.249 ± 0.050
3	2.600 ± 0.065	2.742 ± 0.066	2.643 ± 0.068
5	2.816 ± 0.075	3.360 ± 0.088	3.230 ± 0.081
7	3.339 ± 0.086	3.888 ± 0.100	3.696 ± 0.093
9	4.991 ± 0.126	5.407 ± 0.136	5.098 ± 0.135
10	5.540 ± 0.151	6.138 ± 0.155	5.874 ± 0.152

. With increasing brass powder content, the roughness was observed to rise from 1.570 µm to 5.540 µm for coatings containing 0.58:1 ratio of microcapsules, from 1.924 µm to 6.138 µm for 0.67:1 ratio microcapsule coatings, and from 1.726 µm to 5.874 µm for 0.75:1 ratio microcapsule coatings. The introduction of brass powder was found to significantly increase the surface roughness of the self-healing water-based coatings. This phenomenon is attributed to the particulate nature of brass powder filler, whose solid particles were shown to disrupt the uniform and dense surface characteristics of the coatings. Furthermore, the core–wall feed ratio of microcapsules was demonstrated to have a significant impact on coating roughness. The coating containing 0.58:1 ratio of microcapsules exhibited the lowest roughness and best surface smoothness, followed by the 0.75:1 ratio formulation, while the 0.67:1 ratio coating showed the highest roughness values. These differences were correlated with the morphological characteristics of the microcapsules. The 0.58:1 ratio microcapsules were characterized by uniform spherical morphology with smaller particle sizes, whereas both 0.75:1 and 0.67:1 ratio microcapsules displayed inferior surface morphology and were observed to exhibit varying degrees of agglomeration, which contributed to increased coating roughness.

4.3.6. Coating Aging Resistant Performance Analysis

(1)

High temperature accelerates aging

After undergoing high-temperature accelerated aging tests, the ΔE* values of brass powder decorative coatings containing microcapsules with different core–wall ratios were analyzed in relation to the brass powder content, as shown in Figure 17. As the aging time increased, the color change in all coating samples became more pronounced. Among them, for the three types of coatings containing microcapsules with different core–wall ratios, the most significant color change was observed during the 0–5 h stage of high-temperature accelerated aging. When the core–wall ratio of the microcapsules was 0.67:1, the coating exhibited the most noticeable color difference, with a ΔE* value of 53.87 in the absence of brass powder. This was followed by coatings containing microcapsules with ratios of 0.75:1 and 0.58:1, which showed ΔE* values of 51.07 and 50.56, respectively. Additionally, the brass powder content in the coating was found to be positively correlated with the coating’s resistance to high-temperature aging. The higher the brass powder content, the less pronounced the color change induced by high-temperature exposure. This is attributed to the inherent yellow color of brass powder, which resulted in a darker initial appearance for coatings with higher brass powder content. Since transparent coatings also darkened after aging, the color change was less noticeable in coatings with higher brass powder content compared to those with lower brass powder content. For the coating containing microcapsules with a core–wall ratio of 0.58:1, the ΔE* value after 20 h of high-temperature accelerated aging decreased from 57.71 to 50.84 as the brass powder content increased from 0% to 10%. Similarly, for the coating containing microcapsules with a core–wall ratio of 0.67:1, the ΔE* value decreased from 60.17 to 51.37 under the same conditions. For the coating containing microcapsules with a core–wall ratio of 0.75:1, the ΔE* value decreased from 59.60 to 51.01. The core–wall ratio of the microcapsules was found to significantly influence the color change in the brass powder coatings after high-temperature aging. The coating containing microcapsules with a core–wall ratio of 0.58:1 exhibited the least color change, followed by the coating with a ratio of 0.75:1, while the coating with a ratio of 0.67:1 showed the most severe color degradation under high-temperature conditions.

(2)

Resistant to ultraviolet (UV) aging

After three cycles of UV aging resistance testing, the ΔE* values of brass powder decorative coatings containing microcapsules with different core–wall ratios were analyzed in relation to the brass powder content, as shown in Figure 18. The brass powder-waterborne acrylic coatings embedded with microcapsules of different core–wall ratios exhibited a consistent trend in color change under UV aging: as the brass powder content increased, the ΔE* value after UV aging decreased. This phenomenon is attributed to the ability of brass powder to absorb and reflect UV radiation, thereby reducing the energy of UV exposure on the coating surface and mitigating coating degradation and discoloration. The influence of the microcapsules’ core–wall ratio on the color change in the brass powder coatings after UV aging was also found to follow a clear pattern. Consistent with the high-temperature aging results, the coating containing microcapsules with a core–wall ratio of 0.58:1 displayed the least discoloration, followed by the coating with a ratio of 0.75:1. By combining the results of high-temperature aging resistance, it was observed that the brass powder-waterborne acrylic coating containing microcapsules with a core–wall ratio of 0.58:1 exhibited the best performance in both heat and UV resistance. This indicated that when the core–wall ratio of the waterborne topcoat microcapsules was set at 0.58:1, the prepared microcapsules demonstrated superior resistance to both high-temperature and UV aging.

4.3.7. Coating Resistance to Cold Fluid Performance Analysis

(1)

Liquid resistance and gloss

After the liquid resistance test, the changes in glossiness of different types of self-healing brass powder-waterborne acrylic decorative coatings on Basswood surface with varying brass powder content were shown in Table S13. The glossiness of all coatings was reduced to varying degrees. The lower the brass powder content, the greater the decrease in glossiness after liquid resistance treatment. Among the four tested liquids, coffee had the most significant impact on the glossiness of the brass powder coatings, followed by cleaner and citric acid. This is because the colored substances in coffee could darken the coating color. Meanwhile, coffee, cleaner, and citric acid all contained acidic components, which reacted with the chemical substances in the coatings, resulting in a rougher surface and thus reduced glossiness. Ethanol, as a solvent, could dissolve part of the coating, making it thinner and causing a loss of glossiness. The brass powder-waterborne acrylic coating containing microcapsules with a core–wall feed ratio of 0.58:1 exhibited the most noticeable decrease in glossiness after the liquid resistance test. When the coating contained 3% brass powder by mass, the glossiness of the brass powder-waterborne acrylic coating with microcapsules (Sample 13) decreased by 8.5 GU after citric acid treatment, by 7.1 GU after ethanol treatment, by 11.5 GU after cleaner treatment, and by 12.5 GU after coffee treatment.

(2)

Resistance to changes in liquid color

The ΔE* of different types of self-healing brass powder-waterborne acrylic decorative coatings on Basswood surface before and after the liquid resistance test were presented in Table S14, showing variations with the brass powder content in the coatings. The brass powder content was found to be directly proportional to the chromaticity change after liquid exposure. The higher the brass powder content in the coating, the more pronounced the chromaticity change observed after the test. This is attributed to the presence of metallic elements such as copper in the brass powder, which could be dissolved in acidic liquid environments, leading to discoloration of the coating. Among the four tested liquids, coffee and cleaner were identified as having the most significant impact on the coating’s color change. Overall, the brass powder-waterborne acrylic coating containing microcapsules with a core–wall feed ratio of 0.58:1 exhibited the smallest chromaticity change after citric acid treatment, demonstrating the best performance.

(3)

Liquid resistance grade

The grading results of the liquid resistance test for the self-healing brass powder-waterborne acrylic decorative coatings on Basswood surface were presented in Table S15. Overall, as the brass powder content increased, a gradual decline in the liquid resistance of the self-healing decorative coatings was observed. The core–wall feed ratio of the microcapsules was found to have little effect on the liquid resistance of the brass powder coatings. Ethanol was shown to have almost no impact on all coatings. When the brass powder content ranged from 0% to 7%, the liquid resistance grade for coffee remained at level 2, with only slight discoloration marks appearing on the coating surface. However, when the brass powder content reached 9% and 10%, the liquid resistance grade for coffee dropped to level 3. This was attributed to the increased surface roughness of the coatings at higher brass powder content, allowing the dark brown coffee to penetrate and fill the voids, making the coatings more susceptible to staining. For the cleaner resistance test, coatings with a brass powder content of no more than 1% achieved a grade of level 1. Once the brass powder content exceeded 1%, the grade decreased to level 2. Based on the data in the table, the optimal liquid resistance performance of the brass powder-acrylic coatings was achieved when the brass powder content did not exceed 3%.

4.3.8. Coating Self-Healing Performance Analysis

The self-healing effects of the brass powder-waterborne acrylic decorative coatings on Basswood surface in scratch tests were demonstrated in Figures S1–S3, and

Table 10. Scratch repair of brass powder decorative coating containing microcapsules with different core–wall ratios.

Brass Powder Content (%)	DH (%)
	0.58:1	0.67:1	0.75:1
0	25.04 ± 0.58	21.65 ± 0.54	24.92 ± 0.60
0.5	23.09 ± 0.53	20.09 ± 0.48	22.77 ± 0.57
1	22.99 ± 0.59	18.73 ± 0.50	22.29 ± 0.51
3	19.62 ± 0.49	17.41 ± 0.42	19.55 ± 0.50
5	15.73 ± 0.43	14.25 ± 0.37	14.20 ± 0.36
7	13.28 ± 0.34	12.10 ± 0.31	12.69 ± 0.32
9	11.53 ± 0.29	10.73 ± 0.27	11.35 ± 0.30
10	10.76 ± 0.29	9.32 ± 0.24	10.42 ± 0.27

. When the brass powder content was 0%, the coating exhibited the best self-healing performance. For coatings containing microcapsules with a core–wall feed ratio of 0.58:1, the healing rate decreased from 25.04% to 10.76%, with a difference of 14.28%. Similarly, coatings with microcapsules at a 0.67:1 ratio showed a healing rate reduction from 21.65% to 9.32%, a difference of 12.33%. Meanwhile, those with a 0.75:1 ratio experienced a decline from 24.92% to 10.42%, corresponding to a difference of 14.50%. As the brass powder content increased, the self-healing performance of the brass powder coatings was observed to gradually deteriorate. In Wu et al.’s study, the maximum self-healing efficiency of 34.3% was achieved (with 3.0% CNTs and 9.0% microcapsule content), while the self-healing rate in the current work ranged between 10% and 25% [53]. This discrepancy may be attributed to several mechanisms: First, the hardness and particle size of brass powder could potentially cause premature wear or fracture of microcapsules even before coating damage occurred, thereby compromising the integrity of the self-healing system. Second, brass powder particles might accumulate on microcapsule surface, forming a physical barrier that isolates the microcapsules from the coating matrix and further diminishes self-healing performance. Additionally, brass powder accumulation at crack sites could create flow barriers that not only restrict the penetration and mobility of healing agents but also prolong their transport pathways. Although Wu’s coating demonstrated superior healing efficiency, both systems remain limited to repairing minor damage or non-critical areas, indicating that further optimization is still required for practical daily applications.

The results further demonstrated that varying core–wall mass ratios of microcapsules induced significant differences in the self-healing performance of brass powder coatings. Specifically, the coating containing microcapsules with a 0.58:1 core–wall ratio exhibited optimal self-healing capability among all tested formulations.

4.3.9. Coating Microstructure and IR Spectrum Analysis

(1)

Microscopic morphology of the coating

From the above results, it was concluded that the brass powder-waterborne acrylic coating containing 3% MF resin-encapsulated water-based topcoat microcapsules with a core–wall feed ratio of 0.58:1 (Sample 13) on Basswood surface exhibited the most excellent comprehensive performance. A high glossiness of 24.0 GU was achieved, along with a small color difference (ΔE*) of 2.13. Significant chromaticity variation and high color saturation were observed. The hardness was rated as H, impact resistance reached 2 kg·cm, adhesion was classified as grade 2, and surface roughness measured 2.600 µm, indicating outstanding mechanical properties. Moreover, superior performance was maintained after high-temperature aging, UV aging, and cold liquid resistance tests, with optimal self-healing capability demonstrated. Therefore, microscopic morphology characterization was conducted on this optimized coating, as shown in Figure 19. Compared to the pure microcapsule-waterborne acrylic coating without brass powder (Sample 10), the 3% brass powder-containing coating was found to maintain relatively smooth and clean surface morphology, which was consistent with the roughness analysis results, confirming its favorable surface characteristics.

(2)

Coating infrared spectrum analysis

Figure 20 shows the infrared spectrum of the brass powder-waterborne acrylic coating containing 3% self-healing microcapsules (Sample 13) on the Basswood surface. The characteristic peaks of KH570-modified brass powder filler were clearly observed in the spectrum of the modified coating. The characteristic peak of methylene was detected at 2857 cm⁻¹. The peak appearing at 1144 cm⁻¹ was attributed to the asymmetric stretching mode of Si-O-M (M=Si, Cu, Zn). Strong characteristic peaks of silicon-carbon bonds (Si-C) were identified at 842 cm⁻¹ and 753 cm⁻¹. The in-plane bending vibration peak of C-H in C=CH₂ was observed at 988 cm⁻¹, while the asymmetric stretching vibration peak of siloxane was found at 1039 cm⁻¹. Additionally, the bending vibration peak of the triazine ring in melamine resin wall material was detected at 813 cm⁻¹. The characteristic peak of N-H bonds in melamine resin was present at 1385 cm⁻¹. At 1600 cm⁻¹, a characteristic peak caused by the stretching vibration of C=N in melamine resin was identified [54]. The characteristic peak of C=O in waterborne acrylic resin was located around 1727 cm⁻¹, and the stretching vibration peak of C-H in C-CH₃ was observed at 2924 cm⁻¹.

5. Conclusions

The orthogonal experiment identified that the content of brass powder and the core–wall ratio of the microcapsules in the topcoat were the most significant factors affecting the optical and mechanical properties of the self-healing brass powder-waterborne acrylic coating on basswood surfaces. Single-factor experimental analysis showed that when the brass powder content did not exceed 3%, the particles were uniformly dispersed, providing the coating with excellent optical properties, including high gloss, low color difference, high light transmittance, and vibrant orange-yellow hue. Additionally, the coating exhibited optimal liquid resistance (e.g., against coffee and detergents) under acidic conditions. However, when the content exceeded 3%, brass powder agglomeration led to a significant increase in surface roughness and microdefects, causing a sharp decline in optical performance—reduced transmittance, increased color difference, and a shift toward pure yellow. The inorganic filler disrupted the uniformity of the organic system, reducing adhesion (from grade 2 to grade 4). The self-healing performance was also inhibited due to particle abrasion on the microcapsules and hindered repair agent flow (healing rate decreased from 25.04% to 10.76%). Furthermore, when the content reached 10% or higher, mechanical properties were significantly enhanced: hardness increased due to the filling of microdefects by brass powder and enhanced chemical bonding. The coating with a core–wall ratio of 0.58:1 exhibited the best impact resistance, as its spherical microcapsule structure effectively distributed stress uniformly. Higher brass powder content resulted in smaller color changes after high-temperature and UV exposure, but liquid resistance and self-healing performance deteriorated. Considering all indicators, the wood finish waterborne acrylic coating containing 3% brass powder and 3% microcapsules with a core–wall ratio of 0.58:1 demonstrated superior overall performance, balancing excellent optical properties with enhanced mechanical properties while maintaining good aging resistance, liquid resistance, and self-healing capability. The coating exhibited a gloss of 24.0 GU, color difference (ΔE) of 2.13, chromaticity change (ΔE*) of 13.68, visible light reflectance (R) of 0.5879, dominant wavelength of 587.47 nm, visible light transmittance of 74.33%, hardness of H, impact resistance of 2 kg·cm, adhesion of grade 2, roughness of 2.600 µm, excellent aging and liquid resistance, and a healing rate of 19.62%. The microscopic morphology of the coating was smooth and flat, and FT-IR spectroscopy of the modified brass powder and microcapsules showed no new peaks or disappearance of characteristic peaks, indicating no significant chemical structural changes. In practical applications, the optimized melamine resin-encapsulated waterborne topcoat microcapsule-brass powder-waterborne acrylic coating developed in this study provides wood manufacturers with a dual-functional solution combining decorative appeal and self-healing properties. The 3% brass powder content imparted a distinct golden decorative effect, enhancing the coating aesthetic value. The incorporation of microcapsules introduced self-healing functionality, enabling the repair of microcracks and extending the coating service life while reducing maintenance costs. However, the healing rate remained limited, and excessive brass powder content compromised performance. Future research should focus on core–shell microcapsule design and brass powder surface modification to overcome content-performance constraints and advance the development of dual-functional wood coatings with both decorative and self-healing properties.