Effect of Microcapsules on Mechanical, Optical, Self-Healing and Electromagnetic Wave Absorption in Waterborne Wood Paint Coatings

Abstract

A mixture of multiwalled carbon nanotubes (CNTs) and carbonyl iron powder (Iron(0) pentacarbonyl, CIP) was used as a core material, and a melamine-formaldehyde resin was used as a wall material to prepare CIP/CNTs microcapsules. A core-wall ratio, content of CNTs in the core material, stirring speed, and reaction time were carried out to explore the most significant factor affecting the coverage rate and yield of microcapsules. The most important factor affecting the preparation of CIP/CNTs microcapsules was the content of CNTs in the core material. The optimized CIP/CNTs microcapsules were mixed with shellac microcapsules, and the optimal ratio was explored by analyzing their optical, mechanical, and electromagnetic wave absorption properties in order to prepare coatings with superior performance. The lower the addition amount of CIP/CNTs microcapsules, the lower the effect on the color difference of the coating. The gloss and adhesion of waterborne wood paint coatings decreased with increasing CIP/CNTs microcapsule addition. The hardness, impact resistance and tensile properties of the coatings showed a tendency of increasing and then decreasing with the addition of CIP/CNTs microcapsules. The surface roughness of the coating basically tended to increase with the increase of CIP/CNTs microcapsule content. When the content of added CNTs in the core material was 3.0% and the content of microcapsules was 9.0%, the coating had the highest elongation at break of 12.4% and the highest repair rate of 34.3%, respectively. The mixed shellac microcapsules and CIP/CNTs microcapsules achieved a theoretical minimum reflection loss of −13.52 dB at 16.2 GHz, and the electromagnetic wave absorption band of less than −5 dB was 15.3 GHz–18.0 GHz. The results provide technical references for the preparation of self-healing composite electromagnetic wave absorption coatings on wood substrates.

1. Introduction

Wooden materials are made of wood as raw materials, which are green, light, and strong, can be processed well, and are widely used in construction, furniture, interior decoration, etc. [1,2,3]. The wooden materials have defects such as dry shrinkage and wet expansion, and are easy to crack [4]. Microencapsulation technology is the technology of encapsulating something (core material) with a continuous film (wall material) of natural or synthetic polymer compounds so that chemical characteristics and functionalities of the core material are unaffected. The advantage of microencapsulation technology is that it can achieve a slow controlled release, protection, isolation, and masking of core materials. For materials that are not easily stored and can be easily damaged, the microencapsulation technology can well protect them from an external environmental damage [5,6]. By encapsulating the core material of repair agent into microcapsules and adding them to a waterborne coating, the coatings can be self-healing. This extends a service life of the coating, which in turn better protects the wood substrate [7,8,9]. For example, the self-healing shellac microcapsules are incorporated into the coating. When the coating is damaged by an external environment, the self-healing shellac microcapsules break and the shellac repair agent flows out of the microcapsules to repair the coating.

Many electronic devices and high-powered appliances used in people’s lives have brought many electromagnetic radiation hazards while bringing convenience to people [10,11,12]. Therefore, the development of materials with electromagnetic wave absorption function is of great significance to an environmental safety and human health [13,14,15]. The electromagnetic wave-absorbing materials can absorb the electromagnetic wave energy. Its main role is to reduce the reflected and propagated energy in the process of electromagnetic wave propagation. This reduces the interference of electromagnetic waves and realizes the absorption and shielding of electromagnetic waves [16,17]. When the electromagnetic waves interact with wave-absorbing materials, there are generally three forms, that is, absorption, reflection, and transmission. Common traditional wave-absorbing materials include ferrite, carbonyl iron powder (Iron(0) pentacarbonyl, CIP), graphene, multiwalled carbon nanotubes (CNTs), etc. These wave-absorbing materials have a high dielectric loss or magnetic loss. However, due to the rapid development of technology today, the traditional materials are no longer able to meet the requirements due to their defects in the process of use [18,19,20,21]. For example, the ferrite wave-absorbing materials have the advantage of good wave-absorbing performance in the high-frequency band, at good high-temperature, and corrosion resistance, but they have a high density and poor impedance matching. The CIP has a high density, poor oxidation resistance, and unstable high-temperature wave-absorbing properties. The CNTs and other carbon materials have high dielectric constants, but their magnetic permeability is low, which leads to a poor impedance matching and poor wave absorption performance [22,23,24,25]. The absorption band of single-component carbon material is also narrow. Therefore, in order to increase the effective electromagnetic wave absorption band, to meet the requirements of “wide, thin, strong, and light” electromagnetic wave absorbing materials and improve the impedance matching of materials, it is difficult to rely on a single absorbing material to absorb electromagnetic waves. Dielectric loss-absorbing materials are often compounded with magnetic loss-absorbing materials to improve the wave-absorbing properties of the materials to meet the growing needs of people [26]. Jang et al. [27] proposed a novel method of fabricating nanohybrid particles composed of carbon nanotubes and carbonyl iron powder (CNT@CIP). The CNT@CIP nanohybrid particles can greatly improve the electrical conductivity and shielding effectiveness. Li et al. [28] successfully coated CNTs on the surface of CIP under an action of binder polypyrrole. The microwave absorption performances of composites can be well adjusted by changing the average thickness of shell. Zheng et al. [29] prepared a novel room temperature self-healing carbonyl iron powder/ polydopamine@ multi-walled carbon nanotubes (CIP/PDA@MWCNTs) composites, and enhanced the performance of microwave absorption, so that the the composites possessed dielectric and magnetic losses.

Since CNTs have the dielectric loss capability, the electromagnetic loss capability of wave-absorbing microcapsules can be further broadened by mixing CNTs into the CIP core material for the modification of wave-absorbing microcapsules [30]. In this paper, CNTs were mixed with CIP as the core material, and melamine-formaldehyde resin was used as the wall material. The biggest factor affecting the coating rate of CIP/CNTs microcapsules was investigated, and optimized experiments were conducted to obtain CIP/CNTs microcapsules with higher coverage rate. The optimized CIP/CNTs microcapsules were mixed with shellac microcapsules at different additive amounts into the waterborne coatings. Under the condition of ensuring good optical and mechanical properties of the coating, the electromagnetic wave absorption strength of the microcapsules was improved and the electromagnetic wave absorption width was enhanced in order to obtain the waterborne coatings with good comprehensive performance. By combining the self-healing shellac microcapsules with CIP/CNTs microcapsules, the waterborne wood paint coatings have certain wave absorption functions and self-healing effect, which expands an application scope of the waterborne coating on wooden materials.

2. Materials and Methods

2.1. Materials

The materials are shown in

Table 1. Materials for experiments.

Material	Molecular Mass	CAS	Manufacturer
Melamine	126.12	108-78-1	Jiangning District Wanjuyi test equipment trading company, Nanjing, China
37% Formaldehyde	30.03	50-00-0	Shandong Xinjiucheng Chemical Technology Co., Ltd., Jinan, China,
Triethanolamine	149.19	102-71-6	Nanjing Houxin Biotechnology Co., Ltd., Nanjing, China
12.5% yellow shellac solution	-	-	Hangzhou Yuhe Industrial Co., Ltd., Hangzhou, China
Rosin solution	-	-	Hangzhou Yuhe Industrial Co., Ltd., Hangzhou, China
Span-20	346.459	133-39-2	Nanjing Houxin Biotechnology Co., Ltd., Nanjing, China
Tween-20	604.813	9005-64-5	Nanjing Houxin Biotechnology Co., Ltd., Nanjing, China
Anhydrous ethanol	46.07	64-17-5	Wuxi Jingke Chemical Co., Ltd., Wuxi, China
Citric acid monohydrate	210.139	5949-29-1	Jinan xiao shi Chemical Co., Ltd., Jinan, China
CIP	55.845	13463-40-6	Nangong Xindun Alloy Welding Material Spraying Co., Ltd., Nangong, China
CNTs	12.01	308068-56-6	Suzhou CarbonFun Graphene Technology Co., Ltd., Suzhou, China
Sodium dodecylbenzene sulfonate	348.476	25155-30-0	Shandong Longhui Chemical Co., Ltd., Shandong, China
Fiberboard	-	-	Shenzhen Longgang District Buji Senbao decorative materials store, Shenzhen, China
Red ink	-	-	Shanghai Hero (Group) Co., Ltd., Shanghai, China
Detergent	-	-	Tianjin Linnet E-commerce Co., Ltd., Tianjin, China

. The fiberboard size is 100 mm × 100 mm × 5 mm. Dulux waterborne acrylic primer and topcoat were provided by Akzo Nobel Coatings (Shanghai) Co., Ltd., Shanghai, China. The main component of waterborne coatings is waterborne acrylic copolymer.

2.2. Technology Roadmap

The technology roadmap is shown in Figure 1.

2.3. Preparation of Microcapsules

(1)

Preparation of shellac microcapsules

The wall material for the shellac microcapsules was melamine-formaldehyde resin, and the core material was 12.5% yellow shellac solution and rosin solution. A core-wall ratio of shellac microcapsules was 0.80:1. Firstly, a beaker was poured with 13.52 g of 37% formaldehyde solution, 6.00 g of melamine, and 30.0 mL of deionized water at a molar ratio of formaldehyde to melamine of 3.5. The pH of the solution was adjusted to about 9.0 with triethanolamine. The solution was heated to 70 °C and reacted at 600 rpm for 0.5 h to obtain the wall prepolymer. The 0.15 g of Span-20 and 0.15 g of Tween-20 were used as emulsifiers, and they were poured into a beaker with 78.9 mL of ethanol solution. After sufficient stirring the emulsifier solution, the 4.40 g the yellow shellac and rosin solutions were added. The core material emulsion was obtained by reacting the mixture for 1.0 h at 60 °C in a constant temperature water bath at speed of 600 rpm. The prepared wall prepolymer was progressively poured into the core material emulsion at 600 rpm. The mixed emulsion was ultrasonicated for 0.5 h in a ultrasonic machine (SN-QX-100, Shaoxing Meixian Electronic Technolo-gy Co., Ltd., Shaoxing, China). The mixed emulsion was poured into a beaker and submerged in water after the ultrasonication process. The citric acid monohydrate was used to adjust the pH value of the emulsion about to 3.5–5.0. Citric acid solution was added drop by drop and the pH value was measured with a test paper. After that, the water bath was heated to 60 °C, and the reaction was carried out for 3.0 h. After the reaction, the emulsion was aged for three days, and then it was filtered after being repeatedly washed with ethanol and deionized water. After that, the solid was dried in an oven at 40 °C, and the resulting powder was the shellac microcapsules.

(2)

Preparation of CIP/CNTs microcapsules

Melamine-formaldehyde resin was used as the wall material, and CNTs mixed with CIP were used as the core material [31]. A four-factor, three-level L₉(3⁴) orthogonal test was prepared by selecting the core-wall ratio, content of CNTs in the core material, stirring speed, and reaction time as the influencing factors. The influencing factors and levels in the orthogonal test are shown in

Table 2. Influencing factors and levels in the orthogonal test.

Core-Wall Ratio	Content of CNTs in the Core Material (%)	Stirring Speed (rpm)	Reaction Time (h)
0.30:1	2.0	300	0.5
0.50:1	4.0	500	1.0
0.70:1	6.0	800	2.0

, and the orthogonal test arrangement is shown in

Table 3. Orthogonal test arrangement table.

Sample	Core-Wall Ratio	Content of CNTs in the Core Material (%)	Stirring Speed (rpm)	Reaction Time (h)
1	0.30:1	2.0	300	0.5
2	0.30:1	4.0	500	1.0
3	0.30:1	6.0	800	2.0
4	0.50:1	2.0	500	2.0
5	0.50:1	4.0	800	0.5
6	0.50:1	6.0	300	1.0
7	0.70:1	2.0	800	1.0
8	0.70:1	4.0	300	2.0
9	0.70:1	6.0	500	0.5

. By analyzing the yield and coverage rate of microcapsules, the most significant variable influencing the preparation of microcapsules was identified. The optimal solution of the other three influencing factors was fixed as the best value for microcapsule preparation, and the most significant influencing factor was chosen as the variable in the single-factor test, so as to further optimize the microcapsule preparation process.

Table 4. Material table for orthogonal experiment.

Sample	CIP/g	CNTs/g	Sodium Dodecylbenzene Sulfonate (g)	Deionized Water (g)	Melamine (g)	37.0% Formaldehyde (g)	Deionized Water (g)
1	2.53	0.05	0.04	23.22	5.00	10.00	40.00
2	2.48	0.10	0.04	23.22	5.00	10.00	40.00
3	2.43	0.15	0.04	23.22	5.00	10.00	40.00
4	4.22	0.08	0.07	38.70	5.00	10.00	40.00
5	4.13	0.17	0.07	38.70	5.00	10.00	40.00
6	4.05	0.25	0.07	38.70	5.00	10.00	40.00
7	5.90	0.12	0.10	54.18	5.00	10.00	40.00
8	5.78	0.24	0.10	54.18	5.00	10.00	40.00
9	5.66	0.36	0.10	54.18	5.00	10.00	40.00

displays the materials utilized in the orthogonal test.

Taking sample 1 (Table 4) as an example, the microcapsules were prepared as follows. Firstly, the 5.00 g of melamine, the 10.00 g of 37.0% formaldehyde, and the 40.00 g of deionized water were mixed in the beaker. The beaker was then placed in a water bath with the stirring speed at 600 rpm. After adding triethanolamine drop by drop and gradually ajusting the pH of the solution to around 9.0, the water bath was heated up to 70 °C and the reaction was carried out for 0.5 h. After the reaction was completed, a transparent liquid obtained is the melamine-formaldehyde resin prepolymer. The 0.04 g of sodium dodecylbenzene sulfonate was added to 23.22 g of deionized water, and sodium dodecylbenzene sulfonate was stirred until completely dissolved, and an emulsifying dispersant solution with a mass fraction of 0.2% were prepared. The 2.53 g of CIP and 0.05 g of CNTs were weighed separately and added to the emulsifying dispersant solution. The mixture was sonicated for 0.5 h, and the core material could be obtained. At 300 rpm, the core material was first mechanically stirred for 10 min. Then after that, the melamine-formaldehyde resin prepolymer was gradually added drop by drop to the core material. The 8.0% citric acid monohydrate was used to adjust the pH value of the above mixture to about 4.0. A constant temperature water bath was placed under the mechanical mixer. The temperature of the water bath was raised to 60 °C and the reaction was continued for 0.5 h. After 1 day of aging, the mixture was filtered and rinsed numerous times with deionized water. The product was dried for a day at 40 °C in the oven, and the CIP/CNTs microcapsules were obtained.

After the maximum influencing factor and optimal test parameter were determined by orthogonal tests, the single-factor tests were conducted with the maximum influencing factor as the variable. The core-wall ratio was set at 0.50:1, the stirring speed was 500 rpm, the reaction time was 0.5 h, and content of CNTs in the core material was set as a single variable at 3.0%, 3.4%, 3.8%, 4.2%, 4.6%, and 5.0%, respectively. Six sets of single-factor tests were conducted, and the materials used in the single-factor tests are shown in

Table 5. Material table for single factor experiment.

Content of CNTs in the Core Material (%)	CIP/g	CNTs/g	Deionized Water (g)	Sodium Dodecylbenzene Sulfonate (g)	Melamine (g)	Formaldehyde (g)	Deionized Water (g)
3.0	4.17	0.13	38.70	0.07	5.00	10.00	40.00
3.4	4.15	0.15	38.70	0.07	5.00	10.00	40.00
3.8	4.14	0.16	38.70	0.07	5.00	10.00	40.00
4.2	4.12	0.18	38.70	0.07	5.00	10.00	40.00
4.6	4.10	0.20	38.70	0.07	5.00	10.00	40.00
5.0	4.08	0.22	38.70	0.07	5.00	10.00	40.00

.

2.4. Preparation of Coating with CIP/CNTs Microcapsules and Shellac Microcapsules

When the shellac microcapsule content was 4.2%, the overall performance of coating was improved [31].The content of shellac microcapsules was 4.2%, and the amounts of CIP/CNTs microcapsules added were 3.0%, 6.0%, 9.0%, 12.0%, 15.0%, and 18.0%, respectively. The effect of CIP/CNTs microcapsules addition on the optical, mechanical, cold liquid resistance and self-healing properties of waterborne coatings containing different contents of CNTs in the core material (3.0% and 4.6%, respectively) was investigated. The fiberboard is polished smooth. The total weight of the Dulux primerwas set at 4.00 g. The thickness of the wet primer with CIP/CNTs microcapsules and shellac microcapsules on fiberboard was controlled at 25 μm using a four-sided coating preparation device (SZQ, Shanghai Qise Trading Co., Ltd., Shanghai, China). After drying at room temperature for 1 day, the Dulux topcoat was uniformly applied, and the thickness of the wet topcoat was controlled at 35 μm. The coated fiberboards were exposed to air for 20 min, and cured in a 60 °C oven until the quality no longer changed. The optical, mechanical, and cold liquid resistance properties of waterborne wood paint coatings were tested after drying at room temperature for 1 day.

2.5. Testing and Characterization

(1)

Microscopic characterization

When using an optical microscope (Zeiss Axio Scope A1, Shenzhen Zhongzheng Instrument Co., Ltd, Shenzhen, China), a small amount of sample was evenly covered on the slide. The coverslip was covered and placed on an observatory. Then the microscope was adjusted to an appropriate magnification for observation.

Scanning Electron Microscope (SEM, OLS3000, Chengjie Electronic Components Mall, Shanghai, China): the sample was glued with double-sided tape on the prepared sample disk for gold spraying production, and then placed on the sample stage for vacuuming operation. It can be observed when it is lower than 3 KPa.

The SEM images of microcapsules were also analyzed for particle size using Nano-measurer software (V 1.2.5) with a particle size sample measurement capacity of 100.

(2)

Chemical composition

The samples were analyzed by a fourier transform infrared spectroscopy (Cary630, Shenyang Jiasco Trading Co., Ltd, Shenyang, China) for chemical composition. A small amount of sample was taken out and mixed with potassium bromide. Then they were ground into powder and then poured into the mold for pressing. The resulting sample was transparent, which means the pressing was successful. The pressed sample was fixed on the observatory for testing.

(3)

Optical performance

According to ISO 7724-3:1984 “Paints and varnishes—Colorimetry—Part 3: Calculation of colour differences”, a colorimeter (3nhYS3010, Shenzhen Sanenshi Technology Co., Ltd, Shenzhen, China) was used to test a color difference of the coating. The coating was observed under the observation parameters of directional illumination of 45°, directional observation of CIE10°, and standard illuminant D₆₅ field of view. The colorimeter was turned on, and placed in two different positions on the specimen. Then the test button was pressed, the L₁, a₁, b₁ values and L₂, a₂, b₂ values at this point were recorded, separately. The total color difference ∆E between the two points was determined in Equation (1), where ∆L = L₁ − L₂, ∆a = a₁ − a₂, ∆b = b₁ − b₂. The L represents the brightness value of the coating color, and the larger L means the brighter the coating. The a represents the red-green color value of the coating. The larger a means the redder the coating, and the smaller a means the greener the coating. The b represents the yellow-blue color value of the coating. The larger b means the more yellow the coating, and the smaller b means the bluer the coating [32].

$$\mathrm{∆}E=\sqrt{{(\mathrm{∆}L)}^2+{(\mathrm{∆}a)}^2+{(\mathrm{∆}b)}^2}$$

(1)

To test the gloss of the coating, ISO 2813-1999 “Paints and varnishes—Determination of specular gloss of non-metallic paint films at 20°, 60° and 85°” was used. Using a gloss meter (3nhYG60S, Shenzhen Sanenshi Technology Co., Ltd, Shenzhen, China), the coating gloss was evaluated at incidence angles of 20°, 60°, and 85°, respectively. The gloss at a 60° incidence angle is required by the standard for all paint coatings. In order to evaluate the gloss of the coating, the gloss value at a 60° incidence angle was chosen.

(4)

Mechanical properties

A paint film adhesion tester (QFH-A, Quzhou Aipu Measuring Instrument Co., Ltd, Quzhou, China) was used to test the adhesion of the coating in accordance with ISO 2409-2007 “Paints and varnishes-Cross-cut test”. A proper distance should be cut out firstly by holding the cutting tool perpendicular to the specimen surface and exerting uniform pressure. All cuts should be slid through to the substrate surface. The above operation was then repeated and the same number of parallel cut lines were made again intersecting the original cut lines at a 90° angle to form a grid pattern. The tape was taken out and flattened on top of the grid. The paint coating was observed to peel off after being removed with force at an even speed. The paint coating adhesion is divided into six grades from high to low: 0, 1, 2, 3, 4, and 5, corresponding to shedding area of 0%, <5%, <15%, <35%, <65% and >65%, respectively. The 0 grade has the best adhesion.

The hardness of the coating was tested using a pencil hardness tester (HT-6510P, Quzhou Aipu Measuring Instrument Co., Ltd, Quzhou, China) in accordance with ISO 15184-1998 “Paints and varnishes—Determination of film hardness by pencil test”. The 9B-9H pencils were selected and one end of the pencil was sharpened by about 5 mm of wood, leaving a smooth uninjured cylindrical core. The pencil was inserted into the instrument with the pencil point in line with the paint film. The pencil was pushed at a speed of approximately 0.5 mm/s for a distance of at least 7 mm, and a damage to the paint surface was observed. The hardness of the pencil is the maximum hardness of the paint coating when there is just no damage on the paint film surface.

According to the national standard GB/T 4893.9-1992 “Furniture surface paint film impact resistance measurement method”, a paint coating impactor (QCJ-40, Quzhou Aipu Measuring Instrument Co., Ltd, Quzhou, China) was used to test the impact resistance of coating. The test piece was placed flat on the horizontal base, and the impactor was placed above the test piece. A the steel ball was raised to a specific impact height before being dropped, and was placed in the center of the impact position. Then a specimen was placed under natural light and a magnifying glass was used to see a cracking. The maximum height when the paint coating was not cracked and peeling is the maximum impact strength.

Three identical samples of each paint coating were prepared parallel to the direction of the paint coating stretching. The cracks were created on two of the paint coating samples while using a razor blade. The paint coatings before scratching, paint coatings after scratching, and paint coatings after 1 day of repair, respectively, were subjected to mechanical stretching tests using a universal mechanical testing machine (5000N, Zhejiang Wanxiong Instrument Trading Company, Ningbo, China). Equation (2) was used to determine the elongation at paint coating break in each state [33]. The repair rate of the paint coating is expressed by the elongation at the break of the paint coating, and the repair rate of the paint coating was calculated according to Equation (3) [34].

$$E=\frac{L_h-L_0}{L_0}\times 100\%$$

(2)

where E is the elongation at the paint coating break, L_h is the length between the markers after the paint coating is broken and put together again, and L₀ is the original length between the paint coating markers.

$$\eta =\frac{E_H-E_S}{E_I-E_S}\times 100\%$$

(3)

where: η is the repair rate of the paint coating, E_H is the elongation at paint coating break after 1 day of repair, E_S is the elongation at paint coating break after scratching, and E_I is the elongation at paint coating break before scratching.

(5)

Roughness

Before testing, the coated surface was wiped clean with a rag and the specimen was placed on the test bench. A diamond stylus with a radius of curvature of about 2 μm was slid slowly along the surface to be tested by a coating roughness tester (SJ-411, Dongguan Aiken Tools Co., Ltd, Dongguan, China). The arithmetic average roughness is used to assess an overall unevenness of the surface.

(6)

Cold liquid resistance performance

According to the national standard GB/T 4893.1-2005 “Determination method of cold liquid resistance on furniture surface”, the volume fraction 75.0% ethanol solution, red ink solution, 70.0% mass fraction detergent solution, and 15.0% mass fraction sodium chloride aqueous solution were chosen. Red ink indicates a dark-colored liquid, 75.0% ethanol solution is a neutral liquid, and 70.0% mass fraction detergent is a daily-use product, and 15.0% sodium chloride solution represents alkaline liquid. These four liquids essentially cover all liquid kinds that might be encountered while using furniture on a daily basis. The four soft filter papers were placed on top of the test piece and covered with a glass lid after being submerged in the matching test solution for 30 s. The specimens were removed after 1 day to dry the excess solution on the surface, and the damage was evaluated for the cold liquid resistance. The cold liquid resistance grade was divided into 1–5 levels, and the evaluation standard for cold liquid resistance level of coatings is shown in

Table 6. Evaluation standard for cold liquid resistance level of coatings.

Cold Liquid Resistant Grade/Level	Judging Criteria
1	No damage to the coating.
2	Slight discoloration can be observed when illuminated with a light source.
3	A slight mark.
4	The marks are more serious, but the overall structural appearance of the coating has not changed.
5	Severe marks and changes on the surface of the material, such as bulging, cracking, etc.

. At the same time, the gloss and color variation were evaluated.

(7)

Electromagnetic wave absorption analysis

A vector network analyzer (Agilent E8363C, Agilent Technologies Ltd., Beijing, Chi-na) was used to measure the electromagnetic wave absorption property. The Agilent E8363C has a wide frequency range of 10 MHz to 40 GHz, 110 dB of dynamic range, and less than 0.006 dB of trace noise. It has a measurement speed of less than 26 microseconds per point, 32 channels, 20,001 measurement points, and supports TRL/LRM calibration.

According to the transmission line theory, the Equations (4)–(6) for the self-impedance Z of the wave-absorbing material are as follows:

$$Z_0=\sqrt{\frac{{\mu }_0}{{\epsilon }_0}}$$

(4)

$$Z_{in}=Z_0\sqrt{\frac{{\mu }_r}{{\epsilon }_r}}\tan \mathrm{h}\left(\mathrm{j}\frac{2\pi fd}{c}\sqrt{{\mu }_r{\epsilon }_r}\right)$$

(5)

$$\mathrm{Z}=\frac{z_{in}}{Z_0}$$

(6)

Z₀ is a free-space impedance, and µ₀ and ε₀ represent the free-space permeability and dielectric constant, respectively. Z_in is a material impedance, and µ_r and ε_r represent a complex permeability and permittivity, respectively. f is a frequency, d is a thickness of the wave absorber, and c is a speed of light.

The wave absorption properties are tested by coaxial method, waveguide method, free space method and bow method. The first three are mainly used to obtain the real and imaginary parts of the magnetic permeability and dielectric constant from the parameters tested by the vector network analyzer. These three methods do not give access to the performance of the absorber in practical applications, but the reflection loss (RL) can be calculated. The RL of the samples was calculated using transmission line theory. The lower value of the theoretical reflection loss indicates the better wave absorption performance of the material. The smaller the RL, the better the electromagnetic wave absorption properties. The RL was calculated by Equation (7). The bow method measures the actual absorption of microwaves of wave-absorbing materials [35,36].

$$\mathrm{RL}=20\log \frac{Z_{in}-Z_0}{Z_{in}+Z_0}$$

(7)

Four times of the aforementioned tests were conducted with an error rate of less than 5.0%.

3. Results and Discussion

3.1. Morphological Characterization of Microcapsules Obtained from Orthogonal Tests

Figure 2 displays the morphologies of the microcapsules discovered by orthogonal tests. The CIP/CNTs microcapsules in Figure 2A–C had a particle size of approximately 5 μm–7 μm, and the morphology are round and spherical. The CIP/CNTs microcapsules in Figure 2D,F,G was less formed. In Figure 2E, the CIP/CNTs microcapsules had more well-distributed sizes, with particle sizes ranging from 5 μm–7 μm, and a small amount of agglomeration was generated. CIP/CNTs microcapsules in Figure 2I had a particle size of around 5 μm–7 μm, with agglomeration. It may be that the core-wall ratio is too high, resulting in the core not being fully encapsulated [37].

3.2. Analysis of Yield and Coverage Rate of Microcapsules Obtained from Orthogonal Tests

Table 7. Microcapsule yield and coverage rate for CIP/CNTs.

Sample	Yield (g)	Coverage Rate (%)
1	8.16	24.4
2	8.82	50.0
3	8.58	40.7
4	9.00	34.2
5	9.67	49.8
6	8.98	33.7
7	9.13	26.6
8	9.66	35.4
9	9.86	38.7

displays the yield and coverage rate of CIP/CNT microcapsules. Table 7 shows that 9.86 g was a maximum yield for the sample 9. Because the sample 9 has the largest core-wall ratio and the material used for the microcapsule wall can cover more core material. The sample 2 has the highest coverage rate of 50.0%. The results of the orthogonal test were analyzed with the microcapsule coverage rate as the index, and the results of the range are shown in

Table 8. Analysis of range for CIP/CNTs microcapsule coverage rate.

Sample	Core-Wall Ratio	Content of CNTs in the Core Material (%)	Stirring Speed (rpm)	Reaction Time (h)	Coverage Rate (%)
1	0.30:1	2.0	300	0.5	24.4
2	0.30:1	4.0	500	1.0	50.0
3	0.30:1	6.0	800	2.0	40.7
4	0.50:1	2.0	500	2.0	34.2
5	0.50:1	4.0	800	0.5	49.8
6	0.50:1	6.0	300	1.0	33.7
7	0.70:1	2.0	800	1.0	26.6
8	0.70:1	4.0	300	2.0	35.4
9	0.70:1	6.0	500	0.5	38.7
Average value 1	38.367	28.400	31.167	37.633
Average value 2	39.233	45.067	40.967	36.767
Average value 3	33.567	37.700	39.033	36.767
Range	5.666	16.667	9.800	0.866
Deviation sum of squares	55.902	418.536	161.662	1.502
Degree of freedom	2	2	2	2
F ratio	0.351	2.626	1.014	0.009
F critical value	4.460	4.460	4.460	4.460
Significance

. The coverage rate effect curve is shown in Figure 3. From Table 8 and Figure 3, it is clear that the most influential factor on coverage rate was the content of CNTs in the core material, followed by the stirring speed and core-wall ratio. The prepation of microcapsules is mostly unaffected by reaction time, and the four factors are not significant. The core-wall ratio of CIP/CNTs microcapsules was determined to be 0.50:1, the stirring speed was 500 rpm, and the reaction time was 0.5 h. Next, single-factor tests were conducted to optimize the microcapsule preparation parameters with content of CNTs in the core material as the variable.

3.3. Morphological Characterization of CIP/CNTs Microcapsules Obtained from Single-Factor Tests

The optical microscope pictures of CIP/CNTs microcapsules prepared with content of CNTs in the core material as a variable are shown in Figure 4. In Figure 4A, there is a high amount of CIP/CNTs microcapsules, and the morphology shows a rounded spherical shape. There is only a slight amount of agglomeration present in Figure 4B. The particle size of the microcapsules ranges from 0 μm–8 μm, with uneven particle size. The particle size of CIP/CNTs microcapsules in Figure 4C,D is around 0 μm–7 μm, which is more uniform, but the formation amount is less. In Figure 4E, the CIP/CNTs microcapsules have a rounded and spherical shape. In Figure 4F, although CIP/CNTs microcapsules were formed, a serious agglomeration was observed, probably due to the uneven dispersion of too many microcapsules in the core material. The overall shape and size of CIP/CNTs microcapsules with 3.0% and 4.6% CNTs in the core material were good. The microcapsules are smooth, rounded, and spherical with good shape, as evidenced by the SEM images of the microcapsules with 3.0% and 4.6% CNTs in the core material (Figure 5). Figure 6 and Figure 7 show the particle size distribution of CIP/CNTs microcapsules with CNTs with 3.0% and 4.6% content in the core material, respectively. Most of CIP/CNTs microcapsules have a particle size of 0 μm–3 μm.

3.4. Analyses of Chemical Composition for CIP/CNTs Microcapsules from the Single-Factor Tests

Figure 8 shows the infrared spectra of CIP/CNTs microcapsules with the content of CNTs in the core material of 3.0% and 4.6%, respectively. The −OH characteristic absorption peaks appear at 3349 cm⁻¹. The characteristic vibrational peaks of C=O and −NH−CO− in CNTs appears at 1350 cm⁻¹ and 2950 cm⁻¹, respectively. The characteristic peaks at 1580 cm⁻¹ and 1547 cm⁻¹ are benzene ring skeleton vibrational peaks. The −NH− characteristic vibrational peaks and the triazine ring bending vibrational characteristic peaks appear at 1151 cm⁻¹ and 806 cm⁻¹, respectively. The C−O−C vibrational peak in the melamine-formaldehyde resin appears at 1072 cm⁻¹ [38]. The Fe−O vibrational peak in the CIP appears at 638 cm⁻¹.

In conclusion, the melamine-formaldehyde resin was successfully prepared and the chemical structure of the core CIP/CNTs was not changed.

3.5. Analysis of Yield and Coverage Rate of CIP/CNTs Microcapsules from the Single-Factor Tests

The yield and coverage rates of CIP/CNTs microcapsules containing different CNTs contents are shown in

Table 9. Results of a single factor experiment on the yield and coverage rate.

Content of CNTs in the Core Material (%)	Core Material		Yield (g)	Coverage Rate (%)
	CIP (g)	CNTs (g)
3.0	4.17	0.13	9.83	53.5
3.4	4.15	0.15	9.26	40.2
3.8	4.14	0.16	9.48	45.3
4.2	4.12	0.18	9.32	41.6
4.6	4.10	0.20	9.79	52.6
5.0	4.08	0.22	9.13	37.2

. The yield of CIP/CNTs microcapsules were all above 9.00 g, and the lowest yield was achieved at 5.0% CNTs content in the core material. Since the density of CNTs is smaller than that of CIP, the higher content of CNTs in the core material, the larger the total volume of the core material will be, with the same total mass of the core material. This will lead to a lower quality of the core material covered by the same quality of wall material, which affects the yield. When the content of CNTs in the core material was 3.0%, the maximum yield of CIP/CNTs microcapsules was 9.83 g, and the maximum coverage rate reached 53.5%. When the content of CNTs in the core material was 4.6%, the yield of CIP/CNTs microcapsules was 9.79 g and the coverage rate was 52.6%.

3.6. Effect of CIP/CNTs Microcapsule Content on Optical Properties of Coatings

Table 10. Chromaticity values and color difference of waterborne wood paint coatings with different CIP/CNT microcapsule content.

Content of CNTs in Core Material (%)	CIP/CNTs Microcapsule Content (%)	L1	a1	b1	L2	a2	b2	ΔL	Δa	Δb	ΔE
Blank	0	49.60	9.12	25.32	49.65	9.19	25.60	−0.05	−0.07	−0.28	0.29 ± 0
3.0	3.0	45.06	7.43	17.95	44.77	7.51	17.33	0.29	−0.08	0.62	0.69 ± 0.01
	6.0	35.19	5.75	13.7	36.00	5.34	13.50	−0.81	0.41	0.20	0.93 ± 0.02
	9.0	31.47	1.13	11.55	31.76	1.77	10.06	−0.29	−0.64	1.49	1.65 ± 0.05
	12.0	29.89	0.29	10.41	31.27	−1.14	9.70	−1.38	1.43	0.71	2.11 ± 0.05
	15.0	27.57	−0.05	8.12	28.62	−1.47	9.57	−1.05	1.42	−1.45	2.29 ± 0.06
	18.0	24.57	−0.05	7.95	26.89	0.29	9.01	−2.32	−0.34	−1.06	2.57 ± 0.06
4.6	3.0	36.88	7.71	15.4	37.15	6.88	16.18	−0.27	0.83	−0.78	1.17 ± 0.02
	6.0	32.13	3.16	8.74	33.54	2.29	8.31	−1.41	0.87	0.43	1.71 ± 0.04
	9.0	26.90	0.92	3.82	28.67	0.07	3.66	−1.77	0.85	0.16	1.97 ± 0.06
	12.0	26.56	−0.54	3.89	27.90	0.92	2.86	−1.34	−1.46	1.03	2.23 ± 0.07
	15.0	24.3	−0.71	2.68	25.90	0.82	1.08	−1.60	−1.53	1.60	2.73 ± 0.07
	18.0	23.08	−0.39	2.72	24.95	0.92	0.83	−1.87	−1.31	1.89	2.96 ± 0.08

displays the chromaticity values and color difference of waterborne wood paint coatings. As the content of CIP/CNT microcapsules increased, the color difference values gradually grew. The maximum color difference of the coating reached 2.57 for the content of CNTs in the core material of 3.0%. The maximum color difference of the coating reached 2.96 for the CIP/CNTs microcapsules added with the content of CNTs in the core material of 4.6%. The value of L₁ and L₂ decreased gradually with increasing the content of CIP/CNTs microcapsules. This is due to the fact that CNTs are pure black powders and an increase in the CNTs content will make the coating darker. The value of a₁ and a₂ gradually decreased with the increase of CIP/CNTs microcapsule content, and changed from reddish to greenish color. The value of b₁ and b₂ gradually decreased with increasing content of CIP/CNTs microcapsules, and the yellow color gradually become lighter. Because as the content of CIP/CNTs microcapsules increased, the content of shellac microcapsules as a percentage of total microcapsules decreased, and the color of the yellow shellac solution in the shellac microcapsules becomes less obvious.

The high content of CIP/CNTs microcapsules in the waterborne wood paint coatings affects the uniformity and denseness of the coating. The filling effect will lead to uneven thickness or voids in the coating. This in turn affects the color difference of the coating. When the amount of CIP/CNT microcapsules is too large, the dispersion of the mixted microcapsule powder is worse. The agglomeration or uneven dispersion of the microcapsules will also lead to uneven color distribution of the coating, which results in color difference changes [39].

Table 11. The effect of different contents of CIP/CNTs microcapsules on coating gloss.

Content of CNTs in the Core Material (%)	CIP/CNTs Microcapsule Content (%)	20° Gloss (GU)	60° Gloss (GU)	85° Gloss (GU)
Blank	0	17.5	42.1 ± 0.8	47.4
3.0	3.0	13.0	37.1 ± 0.4	39.8
	6.0	9.8	31.4 ± 0.6	34.7
	9.0	5.1	20.6 ± 0.6	20.6
	12.0	4.3	18.5 ± 0.6	21.7
	15.0	3.7	16.8 ± 0.3	18.7
	18.0	2.0	14.4 ± 0.2	14.4
4.6	3.0	14.1	36.8 ± 0.7	35.9
	6.0	6.5	24.6 ± 0.7	27.3
	9.0	4.4	20.0 ± 0.2	25.2
	12.0	3.8	17.7 ± 0.4	19.5
	15.0	3.0	15.0 ± 0.2	18.3
	18.0	3.1	11.2 ± 0.4	17.1

displays the effect of different contents of CIP/CNTs microcapsules on coating gloss. The gloss of the coatings prepared by CIP/CNTs microcapsules with content of CNTs in the core material of 3.0% and 4.6%, respectively, both decreased gradually with the increase of the added amount. The gloss of the coating was lowest at 14.4 GU when the CIP/CNTs microcapsules with the content of CNTs in the core material of 3.0% were added at 18.0%. The gloss of the coating was lowest at 11.2 GU when the CIP/CNTs microcapsules with the content of CNTs in the core material of 4.6% were added at 18.0%. When the content of CIP/CNTs microcapsules was less than 6.0%, the gloss change of the coating was relatively flat. When the content of CIP/CNTs microcapsules is greater than 9.0%, the gloss of the coating decreases obviously.

Because CIP/CNTs microcapsules have high optical absorption, scattering, and refraction properties. The high content of CIP/CNTs microcapsules will increase the absorption and scattering of light by the coating, resulting in the decrease of light transmission of the coating. When the content of CIP/CNTs microcapsules is too high, the dispersion in the coating becomes poor. The granular bumps or depressions appear on the coating surface, and the flatness of waterborne wood paint coatings is affected. It reduces the smoothness of the waterborne wood paint coatings, which has a certain impact on the gloss of the coating. On the other hand, the CIP/CNTs microcapsules are gray-black powder. An increase in the content of CIP/CNTs microcapsules changes the color of the underside of the coating, further diminishing the gloss.

3.7. Effect of Microcapsule Content of CIP/CNTs on Mechanical Properties and Roughness of Coatings

Table 12. The impact of various CIP/CNT microcapsule contents on the mechanical characteristics and roughness of coatings.

Content of CNTs in the Core Material (%)	CIP/CNTs Microcapsule Content (%)	Adhesion/ Grade	Hardness	Impact Strength (kg∙cm)	Roughness (μm)
Blank	0	1	HB	6	0.570
3.0	3.0	1	HB	9	1.181
	6.0	2	2H	13	1.851
	9.0	2	2H	16	2.373
	12.0	2	3H	17	3.254
	15.0	2	4H	17	3.381
	18.0	3	4H	15	3.524
4.6	3.0	1	H	10	1.620
	6.0	2	H	15	1.691
	9.0	2	2H	15	1.904
	12.0	3	2H	17	1.970
	15.0	3	4H	16	2.570
	18.0	3	5H	12	2.800

shows the impact of various CIP/CNT microcapsule contents on the mechanical characteristics and roughness of coatings. The coating adhesion grade tends to increase with the increase of CIP/CNTs microcapsule content, indicating that the adhesion is getting worse, with a maximum grade of 3. As the content of CIP/CNTs microcapsules increases, the surface of the primer is rough, which reduces the bonding strength with the topcoat and decreases the structural tightness and stability of the coating. The coating cannot completely wrap the microcapsules when the microcapsule content is too high, so the coating adhesion becomes worse with the increase of CIP/CNTs microcapsule content. Coating hardness tends to increase with increasing CIP/CNTs microcapsule content. The coating hardness reached the highest level of 5H when the content of CNTs in the core material was 4.6% and the content of CIP/CNTs microcapsules was 18.0%. The CIP/CNTs core material has hard and high strength characteristics. The addition of CIP/CNTs microcapsules into waterborne coatings can enhance the mechanical properties and make them hard. The impact strength of the coatings tended to increase and then decrease with the content of CIP/CNTs microcapsules, up to 17 kg∙cm.

This is due to the fact that CNTs are characterized by a typical layered hollow structure with a fixed distance between layers. The huge aspect ratio greatly enhances the strength of CNTs and plays an important role in increasing the impact strength of the coating. Using CIP/CNTs microcapsules as a filler, when the coating is subjected to external impact, the CIP/CNTs microcapsules can undergo an elastic deformation to absorb part of the energy and reduce the impact on the coating. When the CIP/CNTs microcapsule content is low, the CIP/CNTs microcapsules can be evenly distributed throughout to create a continuous reinforced network structure. This structure can efficiently disperse and share an external impact force to increase the impact strength of waterborne wood paint coatings. As the CIP/CNTs microcapsule content increases, the dispersion performance in the coating decreases, and the performance for the impact strength of the coating is greatly weakened. The coating roughness becomes larger with the increase of CIP/CNTs microcapsule content. The roughness value of the coating prepared by CIP/CNTs microcapsules with 3.0% content of CNTs in the core material is larger.

3.8. Effect of CIP/CNTs Microcapsule Content on Elongation at Break and Self-Healing

Table 13. The effect of different contents of CIP/CNTs microcapsules on the elongation at paint coating break and repair rate of coatings.

Content of CNTs in the Core Material (%)	CIP/CNTs Microcapsule Content (%)	Elongation at Paint Coating Break (%)			Repair Rate (%)
		Original Sample	After Scratching	After Restoration
Blank	0	5.3	3.8	3.1	-
3.0	3.0	8.2	5.4	6.0	21.4
	6.0	10.6	6.1	7.2	25.6
	9.0	12.4	8.9	10.1	34.3
	12.0	11.7	8.1	8.9	22.2
	15.0	9.8	5.3	6.1	18.6
	18.0	6.2	4.7	5.0	20.0
4.6	3.0	7.5	5.1	5.5	16.7
	6.0	11.6	6.7	7.7	20.4
	9.0	17.3	10.4	12.2	26.1
	12.0	18.7	13.2	14.9	30.9
	15.0	14.1	8.4	9.7	24.6
	18.0	6.2	4.7	5.0	20.0

presents the effect of different contents of CIP/CNTs microcapsules on the elongation at paint coating break and repair rate of coatings. With an increase in the amount of CIP/CNT microcapsules, the elongation at paint coating break tended to grow and then decrease. When the content of CIP/CNTs microcapsules is gradually increased, the melamine-formaldehyde resin as wall materials increases the toughness of the coating, thus enhancing the elongation at paint coating break. However, when the content of CIP/CNT microcapsules in the coating is too high, the powder in the coating increases. The powder are not easy to be evenly dispersed. The toughness of the coating decreases, which makes the coating easy to become brittle and fracture.The highest elongation at paint coating break was 18.7% when the content of CNTs in the core material was 4.6% and the content of CIP/CNTs microcapsules was 12.0%.

As can be seen in Table 13, the coatings without microcapsules had no restorative effect. This may be due to the fact that when a scratch is produced in the coating, the absence of the repair agent in the coating leads to further enlargement of the scratch by the coating under the influence of the environment. The repair rate of coating with microcapsules showed a tendency of increasing and then decreasing with the increase of CIP/CNTs microcapsule addition. When the coating is damaged by the external environment, the self-healing shellac microcapsules breaks and the rosin-modified shellac repair liquid flows out of the microcapsule to repair the coating. At the same time, the microcapsule wall material of melamine-formaldehyde resin will increase the toughness of the coating, preventing the coating from being affected by the external environment to further expand the scratches. As the CIP/CNTs microcapsule content rises, the powder content in the coating increases and coating toughness decreases due to the fact that the shellac self-healing microcapsules are fixed in quantity. When the coating cracks due to blade scratches, the coating is too brittle, resulting in a low repair rate. The highest coating repair rate of 34.3% was achieved when the content of CNTs in the core material was 3.0% and the addition of CIP/CNTs microcapsules was 9.0%.

3.9. Effect of CIP/CNTs Microcapsule Content on Coating Morphology

The overall performance of the coatings was better with the addition of content of CNTs in the core material of 3.0% and CIP/CNTs microcapsules of 9.0%, and content of CNTs in the core material of 4.6% and CIP/CNTs microcapsules of 12.0%, respectively. Figure S1 shows the SEM images of waterborne wood paint coatings with different contents of CIP/CNTs microcapsules, where the content of CNTs in the core material is 3.0% and 4.6%, respectively. Figure 9 shows the macro-morphology of the coatings with the addition of content of CNTs in the core material of 3.0% (CIP/CNTs microcapsule of 6.0%, 9.0%, and 12.0%) and content of CNTs in the core material of 4.6% (CIP/CNTs microcapsule of 9.0%, 12.0%, and 15.0%), respectively.

By observing the changes in the coating with different microcapsule contents, the coating with the addition of two different microcapsules had the phenomenon of unevenness and smoothness. Due to the small size of the microcapsules, the dispersion in the coating deteriorates as the microcapsule content rises, leading to particle aggregation and uneven surface roughness of the coating. Additionally, the presence of microcapsules altered the rheological characteristics of the coatings, reducing the coating flowability and increasing the roughness of the waterborne wood paint coatings.

3.10. Effect of CIP/CNTs Microcapsule Content on the Cold Liquid Resistance of Coatings

Figure 10, Figure 11, Figure 12 and Figure 13 depict the changes in color difference values and gloss values after the cold liquid resistance of the coatings prepared with various content of CNTs in the core material for the four liquids, respectively.

Table 14. The cold liquid resistance level of coatings with different contents of CIP/CNTs microcapsules.

Content of CNTs in the Core Material (%)	CIP/CNTs Microcapsule Content (%)	Cold Liquid Resistant Grade/Level
		75.0% Ethanol Solution	Red Ink Solution	70.0% Detergent Solution	15.0% Sodium Chloride Aqueous Solution
Blank	0	2	5	1	2
3.0	3.0	1	5	1	1
	6.0	1	5	1	1
	9.0	1	5	1	1
	12.0	1	5	2	1
	15.0	2	5	2	2
	18.0	2	5	1	2
4.6	3.0	1	5	1	1
	6.0	1	5	1	1
	9.0	1	5	1	1
	12.0	1	5	1	1
	15.0	1	5	2	1
	18.0	2	5	1	1

lists the grades of the coatings with various content of CNTs in the core material. The coatings showed cold liquid resistance level 1–2 for 75.0% ethanol solution, 70.0% detergent solution, and 15.0% sodium chloride solution, mostly level 1. The coating has the cold liquid resistance of level 5 to red ink. The obvious color changes could be seen after cold liquid resistance, with a small amount of bulging phenomenon.

The waterborne acrylic copolymer in the waterborne coatings has good chemical stability and barrier properties. It can resist the corrosion of ethanol solution, detergent solution, and sodium chloride solution, to prevent the penetration of these solutions into the interior of the coating and the bottom substrate. Therefore, the waterborne coatings are not easy to peel, dissolve and bubble, ensuring the cold liquid resistance of the coating performance. However, the red ink contains ether and other substances that can cause the decomposition of waterborne acrylic copolymer, resulting in the red ink penetrating into the coating, and resulting in swelling and blistering inside the coating. The color difference between 75.0% ethanol solution, 70.0% detergent solution, and 15.0% sodium chloride solution had little effect on the color difference between the two types of coatings, with the overall color difference fluctuating between 2–6. The color difference of the coating after cold liquid resistance of red ink was more variable without obvious patterns. The color difference of the waterborne wood paint coatings with 4.6% content of CNTs in the core material of CIP/CNTs microcapsules fluctuated less compared to that with 3.0% content of CNTs in the core material. At a 60° incidence angle, both types of coating gloss showed a tendency to decline. When the CIP/CNTs microcapsule content was 0%–6.0%, this tendency became more pronounced.

3.11. Analysis of Electromagnetic Wave Absorption Properties of Mixed Shellac Microcapsules and CIP/CNTs Microcapsules

Based on the above results, it was found that the comprehensive performance of the coating was better when the content of shellac microcapsules was 4.2%, the content of CNTs in the core material was 3.0% and the amount of CIP/CNTs microcapsules was 9%. Therefore, in order to investigate the electromagnetic wave absorption, the mixed shellac microcapsules and CIP/CNTs microcapsules were tested for electromagnetic parameters to investigate the electromagnetic wave absorption performance. Figure 14, Figure 15 and Figure 16 show the dielectric constant, magnetic permeability, and theoretical reflection loss of the mixed microcapsule powders, respectively. At the matched thickness of 3.0 mm, the mixed microcapsules have good electromagnetic wave absorption at the high-frequency band. The theoretical minimum reflection loss of −13.52 dB is obtained at 16.2 GHz, which is an improvement in the electromagnetic wave absorption performance, as the minimum reflection loss of the powder is reduced by −6.99 GHz compared to literature [31]. The band smaller than −5 dB is 15.3 GHz–18.0 GHz, which is broadened by 1.8 GHz compared to literature [31], but the electromagnetic wave absorption band is still narrow and the electromagnetic wave absorption performance is poor at low frequencies. Comparing with the literature, the wider bandwidth of wave absorption provides a wider range of electromagnetic shielding, and the higher absorption strength can absorb more electromagnetic waves.

It may be due to the porous structure and high electrical conductivity of CNTs, which can provide larger specific surface area and voids to increase the interaction between powders and electromagnetic waves and improve the electromagnetic wave absorption effect. The good electrical conductivity can make CIP/CNTs microcapsules have higher dielectric constant, so that CIP/CNTs microcapsules have electromagnetic double-loss capability, further improving the electromagnetic wave absorption performance. However, the CIP/CNTs microcapsules still have a narrow absorption band due to the fact that the impedance matching between CIP and CNTs has not reached the optimal effect.

4. Conclusions

By using in situ polymerization, the CIP/CNT microcapsules were prepared. The orthogonal test revealed that the content of CNTs in the core material had the greatest impact on the microcapsule preparation. The optimal process for the preparation of CIP/CNTs microcapsules was determined by designing single-factor tests as the variable: the core-wall ratio of 0.50:1, the content of CNTs in the core materia of 3.0%, the stirring speed of 500 rpm, and the reaction time of 0.5 h. The variation in coating color is less affected by the addition of CIP/CNT microcapsules at lower concentrations. As more CIP/CNT microcapsules were added, the gloss and adherence of the waterborne wood paint coatings diminished. With the addition of more microcapsules, the coating hardness, impact resistance, and tensile resistance all exhibited a tendency to increase and subsequently decrease. With the increase in CIP/CNT microcapsule content, the coating roughness essentially exhibited an increasing tendency. The coating has good cold liquid resistance for 75.0% ethanol solution, 70.0% detergent solution, and 15.0% sodium chloride solution, and poor resistance to red ink solution. When the content of CNTs in the core material was 3.0% and the content of CIP/CNT microcapsules was 9.0%, the good overall performance of the coating was achieved with the color difference of 1.65, gloss of 20.6 GU, adhesion of grade 2, hardness of 2H, the impact resistance of 16 kg∙cm, and roughness of 2.373 μm. The elongation at paint coating break was 12.4% and the repair rate was 34.3%. The mixed shellac microcapsules and CIP/CNTs microcapsules achieved a theoretical minimum reflection loss of −13.52 dB at 16.2 GHz, and the absorption band of less than −5 dB was 15.3 GHz–18.0 GHz. The electromagnetic wave absorption of the mixed shellac microcapsules and CIP/CNTs microcapsules was significantly enhanced by the addition of CNTs, but the electromagnetic wave absorption band did not change much. The combination of shellac microcapsules and CIP/CNTs microcapsules gives the coating self-healing function and electromagnetic wave absorption function while maintaining the performance of the waterborne coating on the surface of fiberboard. Due to limited equipment, only the electromagnetic wave absorption properties of mixed shellac microcapsules and CIP/CNTs microcapsules were tested, the electromagnetic wave absorption properties of the coated fiberboard were not further tested, and the actual electromagnetic shielding test was not conducted. Actual electromagnetic shielding tests will be performed at a later stage and the differences with the calculated values will be compared. The electromagnetic radiation that we are exposed to in our daily life is generally below 4 GHz, and the mixed shellac microcapsules and CIP/CNTs microcapsules prepared in this paper has a high electromagnetic wave absorption at high-frequency band. The electromagnetic wave absorption at low-frequency band of the coated fiberboard needs to be further explored for better applicability on wood furniture surfaces.