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Article

Effect of Transparent, Purple, and Yellow Shellac Microcapsules on the Optical Properties and Self-Healing Performance of Waterborne Coatings

by
Yan Han
1,2,
Xiaoxing Yan
1,2,* and
Yu Tao
1,2
1
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
2
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(8), 1056; https://doi.org/10.3390/coatings12081056
Submission received: 30 May 2022 / Revised: 1 July 2022 / Accepted: 20 July 2022 / Published: 25 July 2022
Browse Figures
Versions Notes

Abstract

:
Three kinds of melamine-formaldehyde (MF) microcapsules, containing transparent shellac, purple shellac, and yellow shellac as core curing agents, were synthesized via in situ polymerization, and then were embedded into the water-based acrylic resin coatings according to the concentrations of 0, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%, respectively, to obtain waterborne films with different microcapsule contents. The color of different shellacs was relevant to the color parameters of the self-healing waterborne film. The content of microcapsules was negatively correlated with the chromatic aberration of the surface of waterborne films. When the content of microcapsules was 0–6.0%, the chromatic aberration of waterborne films was relatively low. The content of microcapsules and the color of the different shellacs would affect the light transmittance of waterborne films. Among all samples, the light transmittance of the waterborne film containing 3.0% transparent shellac microcapsules was the highest. The microcapsules with different colors of shellac in waterborne films had different self-repairing effects. When the content of microcapsules did not exceed 6.0%, the tensile repair rate of the waterborne film containing yellow shellac encapsulated microcapsules was the highest, at 47.19%. The scratch experiment illustrated that the scratch width of the waterborne coating with yellow shellac microcapsules decreased most significantly, and the width change rate was 73.0% after 5 days. The coating containing the 3.0% yellow shellac microcapsule has the best comprehensive performance on optical and self-healing properties. Exploring the influence of shellac resin’s color and the microcapsules’ content on the waterborne film provides technical references for the application of shellac in waterborne coatings and contribute to the further development of the preparation process of self-healing coatings.

1. Introduction

In addition to the function of decoration [1,2], the coating on the surface of products is crucial for separation and protection [3,4]. Nevertheless, under the action of natural or human factors such as ultraviolet rays [5], erosion [6,7], and wear and tear [8], the coating will inevitably be damaged, causing some unforeseen cracks during its service life, which will weaken or even deprive its decorative effects and functional reliability.
In order to prevent the coating from cracking and maintain the original performance of the coating, embedding microcapsules containing curing agents in the coating matrix as common means to prolong the durability of the coating has obtained widespread attention over several decades [9,10]. Zhang et al. [11] synthesized poly urea-formaldehyde (PUF) coated tall oil fatty acid-based epoxy ester microcapsules via in situ polymerization. The epoxy coatings based on steel panels were obtained by embedding 10 wt.% PUF-epoxy resin microcapsules, which performed well on corrosion resistance and self-repairing properties. Thakur et al. [12] synthesized a kind of self-healing epoxy resin coating containing microcapsules which were both a rosin-based epoxy and an imidoamine curing agent encapsulated by MF resin on the surface of carbon steel plates. The artificial scratch experiment showed that the scratch could be healed within 24 h. Zotiadis et al. [13] encapsulated epoxy resin in PUF microcapsules by one-stage in situ polymerization and successfully dispersed them into the alkyd resin coatings coated on the steel surface, which verified the self-healing effect of PUF-epoxy microcapsules. Li et al. [14] embedded the urea-formaldehyde coated tung oil microcapsule prepared by in situ polymerization to the epoxy matrix on the AA2024 aluminum alloy substrate. By electrochemical impedance spectroscopy and micromorphology tests, the self-healing effect of urea-formaldehyde coated tung oil microcapsules was successfully proven.
Melamine-formaldehyde (MF) resin, formed by polycondensation of melamine and formaldehyde, is a thermosetting resin with the advantages of a simple synthesis process, low preparation cost, and good thermal stability [15]. Compared with urea formaldehyde resin, MF resin has a higher thermal stability [16,17,18]. In addition, because of its excellent interfacial compatibility with waterborne coatings [19], it is often used as the wall material of microcapsules based on waterborne coatings. Shellac, as a biodegradable natural colloid composed of shellac resin, shellac wax, and shellac pigment, has been used for the storage and protection of wooden handicrafts [20] such as musical instruments, furniture, and daily ornaments. It is non-toxic and harmless [21]. Existing studies [22,23] have shown that by embedding MF resin-coated shellac microcapsules in waterborne coatings, the core solution can flow to the micro-cracks and physically solidify in the micro gaps at room temperature to repair the coating after the coating was damaged by the environment or man-made factors. Additionally, it had been verified that the film with MF resin-coated shellac microcapsules after repairing had high gloss and toughness and good acid resistance. However, the shellac has the disadvantages of high brittleness, poor water resistance, and bad heat resistance, which limits its application in self-healing fields [24]. According to the results of thermogravimetric analysis (TGA) and Fourier transform infrared radiation (FTIR) in existing research [25,26], it indicated that the reaction between the reactive groups of shellac and rosin improved the mixture at a higher melting temperature. Additionally, there is an esterification reaction between the -OH of shellac and -COOH group of rosin. These changes cause the shellac to delay the aging, enhance the fluidity, and improve the water-resistance abilities to enhance the tolerance and effect of core repair agents.
According to the different raw material geographic origins and treatment methods [27], the shellac resin products on the market mainly include transparent shellac, purple shellac, and yellow shellac. However, the influence of shellac resin products with different colors on the optical properties and self-repairing effect of waterborne coatings has not been clear. Therefore, in this paper, three different shellac liquids (transparent shellac, purple shellac, and yellow shellac), were used as core curing agents to prepare self-healing microcapsules with different core material colors. Then, the three kinds of shellac core microcapsules were embedded in the waterborne acrylic resin coating matrix with concentration gradients of 0, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%, respectively, to obtain self-healing waterborne films with different microcapsules contents. By studying the optical properties and self-repairing performance of the surface of self-repairing coatings, the relationship between the content of microcapsules containing shellac in different colors and the comprehensive performance of the coatings has been analyzed to accumulate the positive theoretical results for the subsequent application of self-healing waterborne coatings.

2. Materials and Methods

2.1. Experimental Materials

The 37 wt.% formaldehyde solution (Mw: 30.03 g/mol, CAS No.: 50-00-0) and ethyl acetate (Mw: 88.11 g/mol, CAS No.: 141-78-6) was purchased from Xi’an Tianmao Chemical Co., Ltd., Xi’an, China. Melamine (Mw: 126.15 g/mol, CAS No.: 108-78-1), Span-20 (emulsifier, Mw: 346.459 g/mol, CAS No.: 1338-39-2), and Tween-20 (emulsifier, Mw: 1227.5 g/mol, CAS No.: 9005-64-5) were supplied by Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China. Triethanolamine (analytical pure, Mw: 149.19 g/mol, CAS No.: 102-71-6) was obtained from Guangzhou Jiale Chemical Co., Ltd., Guangzhou, China. Rosin (Mw: 302.46 g/mol, CAS No.: 8050-09-7) was purchased from Suzhou Guyue Musical Instrument Co., Ltd., Suzhou, China. The citric acid monohydrate (Mw: 210.14 g/mol, CAS No.: 5949-29-1) was purchased from Nanjing Quanlong Biological Hydration Technology Co., Ltd., Nanjing, China. The absolute ethanol (Mw: 46.07 g/mol, CAS No.: 64-17-5) was purchased from Wuxi Jingke Chemical Co., Ltd., Wuxi, China. The above reagents were analytically pure. The waterborne coating, the main components of which included 90.0 wt.% waterborne acrylic acid copolymer dispersion, 6.0 wt.% water, 2.0 wt.% matting agent, and 2.0 wt.% additive, was obtained from Dulux Coatings Co., Ltd., Shanghai, China. Three kinds of different shellacs with a purity of 12.5 wt.%, including transparent shellac, purple shellac, and yellow shellac, were supplied by Shanghai Yuyan Building Materials Co., Ltd., Shanghai, China. The images of the three different colors of shellac are shown in Figure 1. The left one is transparent shellac, the middle one is purple shellac, and the right one is yellow shellac. The different color of three different shellac products [28] is due to the different decolorization process, while the chemical composition of active ingredients is the same. Additionally, the transparent shellac is synthesized with a more thorough decolorization.

2.2. Preparation Method of Microcapsules

The microcapsules containing transparent shellac liquids in the core materials are used as an example.
(1)
Synthesis of the prepolymer
In a molar ratio of 3.5:1, the 27.04 g 37 wt.% formaldehyde solution and 12.0 g melamine were mixed evenly, and then 60 mL distilled water was added to make the mixture dissolve in a baker. To achieve the pH value of the mixture of 8–9, the triethanolamine was dropwise added. Then, it was stirred in a 70 °C DF-101s constant temperature water bath (Gongyi Yuhua Instrument Co., Ltd., Zhengzhou, China) at a speed of 600 rpm for 30 min, to synthesize the prepolymer.
(2)
Emulsion of core materials
The 8.8 g shellac liquid and 8.8 g rosin liquid were weighed and mixed evenly for standby. The 0.3 g Span-20, 0.3 g Tween-20, and 157.8 mL absolute ethanol were fully stirred in the beaker. The mixed liquid of shellac and rosin was dropped into the mixture. After stirring evenly, the prepared liquid was stirred at 600 rpm in a 60 °C constant temperature water bath for 60 min to synthesize the core material emulsion.
(3)
Encapsulation of microcapsules
The prepared emulsion was slowly added to the melamine-formaldehyde prepolymer solution at the stirred speed of 600 rpm. Then, the mixed liquid was carried out for ultrasonic concussion in the BILON-500 ultrasonic material emulsion disperser (Shanghai Bilang Instrument Co., Ltd., Shanghai, China) for 15 min. Subsequently, the sample was transferred into a water bath pot and stirred at 600 rpm. Adding several drops of citric acid can obtain a mixture with a pH value of 4.5. After raising the water temperature in the water bath pot to 60 °C, the constant temperature reaction lasted for 3 h. The samples were then left under room conditions for 3 d. Finally, the obtained sample was washed several times with deionized water and absolute ethanol, filtered with the SHZ-D circulating water multipurpose vacuum pump (Henan Yuhua Instrument Co., Ltd., Zhengzhou, China), and dried.
Microcapsules with purple shellac solution and yellow shellac solution were prepared according to the same preparation process and amount.

2.3. Preparation Method of Films

Three kinds of MF resin microcapsules encapsuling transparent shellac, purple shellac, and yellow shellac were embedded in the waterborne coating containing the contents of 0, 3.0%, 6.0%, 9.0%, 12.0%, and 15.0%, respectively. After stirring and mixing evenly, the self-repairing waterborne coatings were obtained. The material details of the self-repairing waterborne coatings containing different microcapsules are shown in Table 1. The prepared coatings were applied to the silica gel molds (6 mm × 2 mm × 2 mm, length × width × thickness). After being exposed to the air and dried for 20 min, all the samples were put into a DHG-9643BS-III electric heating constant temperature blast drying oven (Shanghai Xinmiao Medical Instrument Co., Ltd., Shanghai, China) at 60 °C for drying until the quality of the samples did not change. Then, the coating samples were gently peeled off from silica gel molds for subsequent testing.

2.4. Testing and Characterization

By the Quanta-200 scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA) and Zeiss Axio Scope A1 optical microscope (OM, Carl Zeiss AG, Oberkochen, Germany), the micromorphology of microcapsules was observed. By the software “Nano Measurer”, the particle sizes of shellac microcapsules with different colors were analyzed, and the size sample had a measurement capacity of 100. The chemical components of microcapsules, waterborne coatings, and shellac solution were characterized by the VERTEX 80V Fourier transform infrared spectrometer (Shanghai Smio Analytical Instrument Co., Ltd., Shanghai, China).
The coating rate of microcapsules was characterized by the weighing method. The m1 g microcapsules were weighed and grinded. Then, they were soaked in ethyl acetate for 24 h. After filtering out the ethyl acetate, they were soaked in ethanol for 48 h. After being washed by deionized water, the residue was put into the oven for drying. The mass of the residue (wall material) is m2, and the coating rate (c) of the microcapsule is calculated according to Formula (1) [29]:
c = (m1 − m2)/m1 × 100%
By the SEGT-J portable chromatic aberration instrument (Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China), the chromaticity values of the waterborne films without microcapsules, which were recorded as L1, a1, and b1, were measured. Additionally, the chromaticity values of the waterborne films with microcapsules were recorded as L2, a2, and b2. By taking four points from each coating sample, the average value was calculated as the chromaticity value of the waterborne film. The chromaticity values of the waterborne films at the cracks before and after self-repairing for 24 h were measured with the chromatic aberration meter, and the average value was gauged after multiple measurements. The measured data of the chromaticity values of the waterborne coating at the crack before self-repairing for 24 h were recorded as L3, a3, and b3, and the measured data of the chromaticity values of the waterborne coating at the crack after self-repairing for 24 h were recorded as L4, a4, and b4. ΔL* indicates the lightness difference, Δa* suggests the difference between red and green, and Δb* suggests the difference between yellow and blue. The chromatic aberration value ΔE* of the waterborne coating with microcapsules was calculated according to the CIELAB chromatic aberration Formula (2) [30]:
ΔE = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
The U-3900 HITACHI UV spectrophotometer (Hitachi Scientific Instruments Beijing Co., Ltd., Beijing, China) was used to measure the light transmittance of waterborne films containing different shellac microcapsules. The measurement range was 300–800 nm, and the scanning speed was 600 nm/min.
The cracks, which were parallel to the tensile direction by a single-sided blade with a depth of about 100 µm, were generated on the prepared self-healing waterborne coating samples. The AGS-X universal mechanical testing machine (Shimadzu production Institute, Kyoto, Japan) was used to carry out the mechanical tensile test on the three kinds of waterborne coatings (original coatings, coatings after being damaged, and coatings after self-repairing for 24 h). The self-healing rate of the waterborne coating was expressed by the elongation at break, which was calculated according to Formula (3) [31]:
ƞ = (EH − ES)/(EI − ES) × 100%
ƞ, repair rate, %;
EH, elongation at break of self-repairing films after self-repairing for 24 h, %;
ES, elongation at break of self-repairing films after being damaged, %;
EI, elongation at break of self-repairing films before being damaged, %.
Similarly, a single-sided blade was used to scratch the self-healing coating. The width changes in the cracks after generating and standing for 5 days were observed by OM. The change rate of scratch width was calculated according to Formula (4) [32]:
DH = (D1 − D2)/D1 × 100%
DH, width change rate, %;
D1, scratch width of waterborne films before self-healing, mm;
D2, scratch width of waterborne films after self-healing, mm;
All the above tests were carried out four times with an error of less than 5%.

3. Results and Discussion

3.1. Micromorphology, Particle Size Distribution, Coating Rate, and Chemical Composition Analysis of Transparent, Purple, and Yellow Shellac Microcapsules

The images of micromorphology and particle size distribution of MF resin-coated transparent shellac, purple shellac, and yellow shellac microcapsules are shown in Figure 2 and Figure 3. According to Figure 2A–C, it can be observed that the microcapsules are regular, spherical, and basically smooth. According to Figure 3A–C, the three kinds of microcapsules have a uniform size distribution, respectively. Among them, the average particle size of transparent shellac microcapsules is 5.7 μm (standard deviation: 0.1). The average particle size of purple shellac microcapsules is 7.4 μm (standard deviation: 0.2). The average particle size of yellow shellac microcapsules is 5.3 μm (standard deviation: 0.1). Combined with Figure 2D–F, it is obvious that there are two parts in the prepared microcapsules. The shell-core structure has been formed. In Figure 2C,F, there are more agglomerates than in the other images. These agglomerates are presumed to be the MF resin, which does not encapsulate core materials successfully and easily adheres to the wall material of microcapsules, resulting in the microcapsule agglomeration.
Coating rate refers to the mass proportion of core material in microcapsules. The content of core material has a crucial influence on the repair performance of microcapsules. The results of microcapsule coating rate are shown in Table 2. The coating rate of microcapsules with different color shellac does not differ greatly. The coating rate of yellow shellac microcapsules is the highest (42.0%), followed by that of transparent shellac microcapsules (38.0%) and purple shellac microcapsules (36.0%).
The infrared spectra of transparent shellac microcapsules, purple shellac microcapsules, yellow shellac microcapsules, wall material MF resin, and core material (transparent shellac) are shown in Figure 4. At 813 cm−1, the triazine ring bending vibration absorption peak of wall material is recorded. The stretching vibration absorption peak of N-H is at 1558 cm−1. The absorption peak at 1000 cm−1 belongs to C-O. These corresponding peaks [33] that belong to MF resin appear on the infrared spectra curves of different microcapsules. Additionally, the absorption peak at 2829 cm−1 is the characteristic peak of -CH3 in C-H. The peak at 1716 cm−1 is responsible for the stretching vibration of C=O. Similarly, these characteristic peaks that belong to shellac [34] appear again on the infrared spectra curves of different microcapsules. Therefore, combined with the analysis of micromorphology, MF resin-coated transparent shellac, purple shellac, and yellow shellac microcapsules were successfully prepared.

3.2. Chromatic Aberration Analysis of Waterborne Coatings Containing Different Microcapsules

3.2.1. Effect of Shellac Microcapsule Content with Different Core Materials on the Chromatic Aberration of Waterborne Coatings

The optical photographs of waterborne films containing the three different microcapsules are shown in Figure 5. With the increase in microcapsule content, the agglomeration of microcapsules in the coating is more obvious. Figure 6 shows the relationship between the content of shellac microcapsules and different colors and the color parameters of water-based coatings. When the lightness difference value ΔL* is negative, it indicates that the waterborne film’s color becomes dark. When ΔL* is positive, it indicates that the waterborne film’s color becomes light [35]. With the increasing content of microcapsules, the color of the waterborne film containing microcapsules with transparent shellac and yellow shellac first became darker and then lighter. The ΔL* of the waterborne film with purple shellac microcapsules had a positive correlation with the content of microcapsules, and the value of ΔL* was in negative territory. This might be due to the darker color of microcapsules containing purple shellac as core material. When the red–green difference Δa* is positively increasing, it indicates that the waterborne film’s color is redder and redder [36]. When the red–green difference Δa* is negatively increasing, it indicates that the waterborne film’s color is greener and greener. The Δa* of waterborne coatings containing microcapsules with purple shellac and yellow shellac increased with the increase in microcapsules content, and the Δa* had been consistently positive and positively increasing. Compared with the Δa* change range of waterborne coatings containing microcapsules with purple shellac, that of waterborne film containing yellow shellac microcapsules was smaller. On the contrary, the Δa* of waterborne coatings containing transparent shellac microcapsules had been negatively increasing. The positive yellow–blue difference value Δb* indicates that the waterborne coating is yellow. For three kinds of waterborne coatings with different microcapsules, all Δb* values were positive and increased with the increase in microcapsules content. Among them, the change range of the waterborne coatings containing transparent shellac microcapsules was the smallest, followed by that containing yellow shellac and purple shellac. The changing trend of overall chromatic aberration ΔE* was consistent with the changing trend of yellow–blue difference Δb*. The above results showed that the color of the core material affects the color of the prepared self-healing waterborne coating. The content of microcapsules in the waterborne coating was negative to the chromatic aberration of the prepared self-healing waterborne coating.

3.2.2. Effect of Core Material Color on the Chromatic Aberration of Waterborne Coatings before and after Self-Repairing

The changes in color parameters of waterborne coatings containing shellac microcapsules with different colors before and after self-repairing for 24 h are shown in Table 3. After self-repairing for 24 h, the chromatic aberration of the waterborne coating with microcapsules at the crack was smaller than that of the waterborne coating without microcapsules at the crack. The chromatic aberration value had a tendency of first decreasing and then increasing. After self-repairing for 24 h, the ΔE* of waterborne coating with 3.0% transparent shellac microcapsules was 1.12, the ΔE* of waterborne coating with 3.0% purple shellac microcapsules was 0.74, and the ΔE* of waterborne coating with 3.0% yellow shellac microcapsules was 0.61. All above were less than 3.17, which was the ΔE* of waterborne coating without microcapsule. After the waterborne film was damaged, the self-healing microcapsules in the waterborne coating broke, and then the core repair agent flowed out and solidified into the cracks of the waterborne coating to fill the micro gaps and reduce the chromatic aberration at the cracks of the waterborne film. The color parameters of waterborne films containing shellac encapsulated microcapsules with different colors were significantly different at the cracks after repairing for 24 h, which may be due to the obvious chromatic aberration of the core material. For shellac microcapsules with different colors that were broken, the color parameters of the core material (curing agent) after flowing out and filling into the micro gaps in waterborne coatings were significantly different. It can be seen that the addition of core repair agents with different colors will affect the change in color parameters after self-repairing. Additionally, the core repair agents with different colors can be used as a color indicator for whether the waterborne film produces microcracks.

3.2.3. Analysis of Light Transmittance of Waterborne Coatings with Different Microcapsules

The light transmittance of polymers has an important influence on their application in optical coating. Figure 7 shows the light transmittance curves of self-healing waterborne coatings containing shellac microcapsules with different colors. It can be found that the light transmittance of waterborne coatings decreased with the increase in microcapsule contents in the coatings. With the raising content of microcapsules to 6.0%, the light transmittance of the water-based coating decreased greatly. This may be due to the raising content of microcapsules, resulting in the accumulation of phase separation on the surface of the waterborne coating. This contributed to causing the partly rough areas on the waterborne film surface, which could strengthen the refraction of the waterborne film surface so that the light transmittance of the waterborne film decreased greatly. When the microcapsule content exceeded 6.0%, the light transmittance of the waterborne film tended to decline gently.
When the content of microcapsules was 3.0%, the transmittance of the waterborne film with transparent shellac microcapsules was about 76.8% under 600–800 nm visible light. The transmittance of the waterborne film with purple shellac microcapsules was about 67.2% under 650–800 nm visible light. The transmittance of the waterborne film with yellow shellac microcapsules was about 65.8% under 600–800 nm visible light. When the microcapsule content was fixed, the light transmittance of the waterborne film with transparent shellac microcapsules was higher than the light transmittance of the waterborne film with purple shellac microcapsules, and the light transmittance of the waterborne film with yellow shellac microcapsules. This may be because the color of purple shellac and yellow shellac microcapsules was darker, resulting in the decrease in light transmittance of the waterborne film. Additionally, for the visible wavelength segment maintaining stable light transmittance, the waterborne film containing transparent shellac microcapsules was the widest, and the waterborne film containing purple shellac microcapsules was the shortest. The content of microcapsules and the color of the core material of microcapsules will affect the light transmittance of the waterborne films. The light transmittance of the waterborne film with 3.0% transparent shellac microcapsules was the highest.

3.3. Tensile Repair Effect Analysis of Waterborne Films Containing Different Shellac Microcapsules

3.3.1. Analysis of Tensile Properties of Waterborne Films

The tensile strength of waterborne films with three different shellac microcapsules in three situations is shown in Table 4. When the transparent shellac microcapsules or yellow shellac microcapsules were embedded in the waterborne coatings, the maximum tensile strength of the self-healing waterborne films occurred at 3.0 wt.% microcapsules and then decreased with the increasing content of microcapsules. When the purple shellac microcapsules were embedded in the waterborne coatings, the maximum tensile strength of the waterborne films decreased continuously with the increase in microcapsules content. This may be due to the significant difference between the modulus of waterborne acrylic resin coatings and shellac microcapsules. When the microcapsules were embedded in the waterborne coatings, there would be a volume gap between them. As a result, under the action of tensile load, each part of the waterborne film would quickly exceed the strength limit which caused the waterborne film to be damaged due to uneven stress. With the increasing content of microcapsules, the microcapsules were easy to form agglomeration in the waterborne film (Figure 5), resulting in more obvious stress concentration. When the 3.0% transparent shellac microcapsules were added, the dispersion of microcapsules in the waterborne film was better and the defects were less. Under this condition, the tensile strength of the self-healing waterborne film was the largest. For the original coating containing 15% transparent shellac microcapsules, the maximum tensile strength was enhanced and was close to the original coating without microcapsules. This may be because when the content of transparent shellac microcapsules reached 15%, there was enough shellac making the repair effect of microcapsules compensate for the damage of paint film. When the microcapsule content was fixed at 3.0%, the stress–strain curves of waterborne films with three kinds of shellac microcapsules are shown in Figure 8. This figure illustrated that the tensile resistance of the waterborne film with transparent shellac microcapsules was better than that of the waterborne films with purple shellac microcapsules and that of the waterborne films with yellow shellac microcapsules. The excellent tension resistance of transparent shellac microcapsules may be due to the more advanced decolorization of transparent shellac. The shellac solution with fewer impurities performs better on the tension resistance of waterborne films.

3.3.2. Analysis of Repair Rate of Waterborne Films

The stress–strain curves and elongation at break results of self-repairing waterborne films containing three different shellac microcapsules under three modes (original waterborne film, waterborne film after being damaged, and the waterborne film after self-repairing for 24 h) are shown in Figure 9 and Table 5. According to Figure 9, it can be found that for the tensile resistance, the waterborne film after self-repairing for 24 h was better than the waterborne film after being damaged. The microcapsules in the waterborne film performed well on a self-repairing effect on the damage to the waterborne film. The elongation at break of the self-healing waterborne films with transparent shellac microcapsules and yellow shellac microcapsules increased first and then decreased with the increasing content of microcapsules. It can be seen that the toughness of the waterborne film can be enhanced by microcapsules. The waterborne film repair rate calculated according to the elongation at break of the self-healing waterborne film under three modes is shown in Table 6. With the increasing content of microcapsules, the repair rate of the self-healing waterborne film increased first and then decreased. This may be because when the content of microcapsules was low, microcapsules could be evenly dispersed in the matrix of the self-repairing waterborne film. When the waterborne film was damaged by an external force, the microcapsule was ruptured causing the core material shellac to flow out and physically solidify in the micro gaps to achieve the self-repairing effect. However, when the content of microcapsules exceeded 6.0%, the toughness of the waterborne film itself decreased significantly, and it also greatly affected the self-repairing rate of the waterborne film. By adding less than 6.0% microcapsules, the maximum repair rate of the waterborne film containing yellow shellac microcapsules was 47.19%, followed by the waterborne film containing transparent shellac microcapsules with a repair rate of 43.79%, and that containing purple shellac microcapsules with a repair rate of 27.46%.

3.4. Self-Repairing Effect Analysis of the Waterborne Film Containing Microcapsules

According to the above analysis, when the content of microcapsules was 3.0%, the optical performance was the best. Therefore, the coating samples with microcapsules with different colors with a content of 3.0% were selected for width change rate analysis. The scratch repair effect of self-healing waterborne films with microcapsules loaded with different core materials on the surface of silica gel is shown in Figure 10. For the waterborne coating without microcapsules after being scratched by external force for 5 days, the scratch width did not change, which indicated that the crack could not be repaired by itself. For the waterborne film containing transparent shellac microcapsules after being scratched by an external force for 5 days, the scratch width reduced from the original 22.05 μm to 13.30 μm, with a width change rate of 39.7%. For the waterborne film loaded with purple shellac microcapsules after being scratched by external force for 5 days, the scratch width reduced from the original 20.3 μm to 14.35 μm, with a change rate of 29.3%. The crack width of the waterborne film loaded with shellac microcapsules after being scratched by external force for 5 days, changed from 22.05 μm to 5.95 μm, with a width change rate of 73.0%.
For the core of microcapsules, the shellac is dissolved in ethanol solution. When the microcapsules are broken, the core material flows out into the microcrack gap of the coating. After the ethanol is evaporated, the shellac gradually separates out and is physically cured at room temperature to fill the coating cracks. The microcapsule structure can prevent the evaporation of ethanol to keep shellac in a liquid state with a self-repairing function. Therefore, mixing microcapsules into waterborne coatings will be conducive to achieving the self-repairing performance when cracks occur, so as to avoid further propagation of cracks and more serious damage to the coating. Different colors of shellac core materials had different effects on the self-repairing effect of microcapsules in the coating. The repair effect of yellow shellac microcapsules was the best, followed by transparent shellac microcapsules. The higher self-healing effect of yellow shellac microcapsules may be due to the obvious agglomeration of the prepared yellow shellac microcapsules (Figure 2C). There are more microcapsules gathered around the scratch and there are more core materials flowing out. Therefore, the scratch repair effect of the coating with yellow shellac microcapsules is more obvious.

4. Conclusions

The optical properties and self-healing performance of water-based coatings containing transparent, purple, and yellow shellac microcapsules with different concentrations were studied. Compared with the previous studies using only transparent shellac as a repair agent [37], it is proven that the color of different core materials in microcapsules affected the color parameters and self-healing effect of the prepared self-healing waterborne films. The change range of color parameters of the waterborne film with transparent shellac microcapsules was the smallest. The content of microcapsules in the waterborne film was negatively correlated with the chromatic aberration of the waterborne film. When the content of microcapsules was 0–6.0%, the chromatic aberration of the waterborne film was relatively low. The core materials with different colors affected the change in color parameters at the crack after the waterborne film was repaired. The content of microcapsules and the color of the core material of microcapsules would affect the light transmittance of the waterborne film. When the content of microcapsules was raised to 6.0%, the light transmittance of waterborne films decreased greatly, and the light transmittance of the waterborne film containing 3.0% transparent shellac microcapsules was the highest. When the content of microcapsules in the waterborne film exceeded 6.0%, the toughness of the waterborne film decreased. When the content was 3.0%, the tensile properties of the waterborne film with transparent shellac microcapsules performed significantly better than those of waterborne films with purple shellac microcapsules and yellow shellac microcapsules. Shellac microcapsules with different colors in the waterborne film had different repair effects. When the content of microcapsules did not exceed 6.0%, the maximum self-repairing rate of waterborne film containing yellow shellac microcapsules was 47.19%, followed by the waterborne film containing transparent shellac microcapsules with a repair rate of 43.79%, and the waterborne film containing purple shellac microcapsules with a repair rate of 27.46%. The scratch experiment illustrated that the scratch width of the waterborne film containing yellow shellac microcapsules decreased most significantly, and the width change rate was 73.0% after 5 days. Considering both optical properties and self-healing effect, the waterborne coating containing 3.0% yellow microcapsules performed better. It offers a technological reference for the application of shellac in the water-based coating.

Author Contributions

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

Funding

This project was partly supported by the Postgraduate Research and Practice Innovation Program of Jiangsu Province (KYCX22_1098) and the Natural Science Foundation of Jiangsu Province (BK20201386).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Transparent, purple, and yellow shellac liquid.
Figure 1. Transparent, purple, and yellow shellac liquid.
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Figure 2. The micromorphology of transparent, purple, and yellow shellac microcapsules: (A) SEM image of transparent shellac microcapsules, (B) SEM image of purple shellac microcapsules, (C) SEM image of yellow shellac microcapsules, (D) OM image of transparent shellac microcapsules, (E) OM image of purple shellac microcapsules, (F) OM image of yellow shellac microcapsules.
Figure 2. The micromorphology of transparent, purple, and yellow shellac microcapsules: (A) SEM image of transparent shellac microcapsules, (B) SEM image of purple shellac microcapsules, (C) SEM image of yellow shellac microcapsules, (D) OM image of transparent shellac microcapsules, (E) OM image of purple shellac microcapsules, (F) OM image of yellow shellac microcapsules.
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Figure 3. Particle size distribution of different shellac microcapsules: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
Figure 3. Particle size distribution of different shellac microcapsules: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
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Figure 4. FTIR of core material, wall material, transparent, purple, and yellow shellac microcapsules.
Figure 4. FTIR of core material, wall material, transparent, purple, and yellow shellac microcapsules.
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Figure 5. Waterborne films containing different content of microcapsules: (A) no microcapsules, (B) 3.0 wt.% transparent shellac microcapsules, (C) 6.0 wt.% transparent shellac microcapsules, (D) 9.0 wt.% transparent shellac microcapsules, (E) 12.0 wt.% transparent shellac microcapsules, (F) 15.0 wt.% transparent shellac microcapsules, (G) 3.0 wt.% purple shellac microcapsules, (H) 6.0 wt.% purple shellac microcapsules, (I) 9.0 wt.% purple shellac microcapsules, (J) 12.0 wt.% purple shellac microcapsules, (K) 15.0 wt.% purple shellac microcapsules, (L) 3.0 wt.% yellow shellac microcapsules, (M) 6.0 wt.% yellow shellac microcapsules, (N) 9.0 wt.% yellow shellac microcapsules, (O) 12.0 wt.% yellow shellac microcapsules, (P) 15.0 wt.% yellow shellac microcapsules.
Figure 5. Waterborne films containing different content of microcapsules: (A) no microcapsules, (B) 3.0 wt.% transparent shellac microcapsules, (C) 6.0 wt.% transparent shellac microcapsules, (D) 9.0 wt.% transparent shellac microcapsules, (E) 12.0 wt.% transparent shellac microcapsules, (F) 15.0 wt.% transparent shellac microcapsules, (G) 3.0 wt.% purple shellac microcapsules, (H) 6.0 wt.% purple shellac microcapsules, (I) 9.0 wt.% purple shellac microcapsules, (J) 12.0 wt.% purple shellac microcapsules, (K) 15.0 wt.% purple shellac microcapsules, (L) 3.0 wt.% yellow shellac microcapsules, (M) 6.0 wt.% yellow shellac microcapsules, (N) 9.0 wt.% yellow shellac microcapsules, (O) 12.0 wt.% yellow shellac microcapsules, (P) 15.0 wt.% yellow shellac microcapsules.
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Figure 6. Relationship between loading amount of shellac microcapsules with different core materials and coating color parameters: (A) changes in ΔL*, (B) changes in Δa*, (C) changes in Δb*, (D) changes in ΔE*.
Figure 6. Relationship between loading amount of shellac microcapsules with different core materials and coating color parameters: (A) changes in ΔL*, (B) changes in Δa*, (C) changes in Δb*, (D) changes in ΔE*.
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Figure 7. UV transmittance curves of microcapsule coatings with different shellac: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
Figure 7. UV transmittance curves of microcapsule coatings with different shellac: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
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Figure 8. Stress–strain curves of microcapsule coatings with different shellac core materials.
Figure 8. Stress–strain curves of microcapsule coatings with different shellac core materials.
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Figure 9. Stress–strain curves of the waterborne film with 3.0% microcapsule with different shellac: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
Figure 9. Stress–strain curves of the waterborne film with 3.0% microcapsule with different shellac: (A) transparent shellac microcapsules, (B) purple shellac microcapsules, (C) yellow shellac microcapsules.
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Figure 10. OM of scratch test of waterborne films with shellac microcapsules: (A) damaged pure waterborne coating, (B) damaged waterborne coating containing transparent shellac microcapsules, (C) damaged waterborne coating containing purple shellac microcapsules and (D) damaged waterborne coating containing yellow shellac microcapsules before self-repairing for 5 days; (E) damaged pure waterborne coating, (F) damaged waterborne coating containing transparent shellac microcapsules, (G) damaged waterborne coating containing purple shellac microcapsules and (H) damaged waterborne coating containing yellow shellac microcapsules after self-repairing for 5 days.
Figure 10. OM of scratch test of waterborne films with shellac microcapsules: (A) damaged pure waterborne coating, (B) damaged waterborne coating containing transparent shellac microcapsules, (C) damaged waterborne coating containing purple shellac microcapsules and (D) damaged waterborne coating containing yellow shellac microcapsules before self-repairing for 5 days; (E) damaged pure waterborne coating, (F) damaged waterborne coating containing transparent shellac microcapsules, (G) damaged waterborne coating containing purple shellac microcapsules and (H) damaged waterborne coating containing yellow shellac microcapsules after self-repairing for 5 days.
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Table
Table 1. Material details of waterborne coatings containing different microcapsules.
Table 1. Material details of waterborne coatings containing different microcapsules.
Microcapsule Content
(%)		Mass of Microcapsules
(g)		Mass of Waterborne Coatings (g)Total Mass
(g)
004.004.0
3.00.123.884.0
6.00.243.764.0
9.00.363.644.0
12.00.483.524.0
15.00.603.404.0
Table
Table 2. Coating rate results of different microcapsules.
Table 2. Coating rate results of different microcapsules.
Core Materialm1 (g)m2 (g)Coating Rate (%)
Transparent shellac1.000.6438.0
Puple shellac1.000.6236.0
Yellow shellac1.000.5842.0
Table
Table 3. Effects of different shellac materials on coating color parameters before and after self-repairing.
Table 3. Effects of different shellac materials on coating color parameters before and after self-repairing.
Microcapsule Content (%)Transparent Shellac MicrocapsulesPurple Shellac MicrocapsulesYellow Shellac Microcapsules
ΔL*Δa*Δb*ΔE*ΔL*Δa*Δb*ΔE*ΔL*Δa*Δb*ΔE*
0−2.150.60−2.253.17−2.150.60−2.253.17−2.150.60−2.253.17
3.0−0.750.85−0.031.12−0.330.130.650.74−0.50−0.08−0.350.61
6.0−0.450.400.600.85−0.050.33−0.430.54−1.150.35−1.501.92
9.00.15−0.600.330.70−0.55−0.38−0.150.680.530.500.450.85
12.0−1.27−0.75−1.201.900.70−0.05−0.080.71−0.03−0.45−0.780.90
15.0−1.08−1.15−1.031.880.38−0.30−0.680.830.450.33−0.350.67
Table
Table 4. Tensile properties of self-healing waterborne coatings with different microcapsules in three situations.
Table 4. Tensile properties of self-healing waterborne coatings with different microcapsules in three situations.
Microcapsule TypesMicrocapsule Content (%)Maximum Tensile Strength (MPa)
Original Waterborne FilmWaterborne Film after Being DamagedWaterborne Film after Self-Repairing for 24 h
Transparent shellac microcapsules015.86 ± 0.338.89 ± 0.128.78 ± 0.19
3.016.67 ± 0.606.93 ± 0.1412.65 ± 0.21
6.013.58 ± 0.4011.94 ± 0.2613.92 ± 0.35
9.012.21 ± 0.3913.83 ± 0.1611.58 ± 0.32
12.09.77 ± 0.239.51 ± 0.179.25 ± 0.22
15.013.97 ± 0.1711.07 ± 0.206.46 ± 0.20
Purple shellac microcapsules3.015.71 ± 0.4210.30 ± 0.289.58 ± 0.12
6.014.67 ± 0.2712.65 ± 0.2910.61 ± 0.26
9.09.70 ± 0.228.49 ± 0.2111.52 ± 0.36
12.010.45 ± 0.4012.62 ± 0.445.78 ± 0.14
15.010.30 ± 0.2310.13 ± 0.1610.64 ± 0.26
Yellow shellac microcapsules3.016.06 ± 0.2314.65 ± 0.4415.71 ± 0.33
6.014.22 ± 0.3520.43 ± 0.489.57 ± 0.22
9.011.95 ± 0.2814.14 ± 0.3311.15 ± 0.30
12.09.33 ± 0.2512.07 ± 0.2618.09 ± 0.30
15.010.51 ± 0.298.95 ± 0.1418.83 ± 0.35
Table
Table 5. Elongation at break results in waterborne coatings with three kinds of shellac microcapsules in three situations.
Table 5. Elongation at break results in waterborne coatings with three kinds of shellac microcapsules in three situations.
Microcapsule TypesMicrocapsule Content (%)Elongation at Break (%)
Original Waterborne FilmWaterborne Film after Being DamagedWaterborne Film after Self-Repairing for 24 h
Transparent shellac mi-crocapsules058.14 ± 0.7940.07 ± 0.7243.77 ± 0.68
3.065.58 ± 0.7132.58 ± 0.5347.03 ± 0.98
6.044.60 ± 0.9421.59 ± 0.3631.47 ± 0.99
9.027.86 ± 0.6416.28 ± 0.4624.64 ± 0.36
12.021.57 ± 0.2913.67 ± 0.2920.08 ± 0.42
15.018.08 ± 0.4612.22 ± 0.3415.32 ± 0.34
Purple shellac microcapsules3.047.84 ± 0.7533.99 ± 0.9336.97 ± 0.88
6.036.35 ± 0.7216.79 ± 0.4322.16 ± 0.62
9.020.56 ± 0.4314.44 ± 0.4418.70 ± 0.57
12.018.52 ± 0.5210.99 ± 0.3213.71 ± 0.40
15.018.16 ± 0.119.71 ± 0.1311.70 ± 0.31
Yellow shellac microcapsules3.094.71 ± 0.5719.85 ± 0.6382.26 ± 2.06
6.026.35 ± 0.719.06 ± 0.1822.14 ± 0.58
9.021.13 ± 0.555.87 ± 0.1314.65 ± 0.45
12.020.52 ± 0.353.74 ± 0.116.12 ± 0.16
15.014.84 ± 0.180.92 ± 0.024.40 ± 0.13
Table
Table 6. The results of the repair rate of waterborne coatings with three kinds of shellac microcapsules.
Table 6. The results of the repair rate of waterborne coatings with three kinds of shellac microcapsules.
Microcapsule Content (%)Repair Rate of Waterborne Coatings (%)
Transparent Shellac MicrocapsulesPurple Shellac MicrocapsulesYellow Shellac Microcapsules
020.4920.4920.49
3.043.7921.4736.48
6.042.9427.4647.19
9.072.2269.6072.66
12.081.1736.0825.32
15.052.9623.5070.70
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Han, Y.; Yan, X.; Tao, Y. Effect of Transparent, Purple, and Yellow Shellac Microcapsules on the Optical Properties and Self-Healing Performance of Waterborne Coatings. Coatings 2022, 12, 1056. https://doi.org/10.3390/coatings12081056

AMA Style

Han Y, Yan X, Tao Y. Effect of Transparent, Purple, and Yellow Shellac Microcapsules on the Optical Properties and Self-Healing Performance of Waterborne Coatings. Coatings. 2022; 12(8):1056. https://doi.org/10.3390/coatings12081056

Chicago/Turabian Style

Han, Yan, Xiaoxing Yan, and Yu Tao. 2022. "Effect of Transparent, Purple, and Yellow Shellac Microcapsules on the Optical Properties and Self-Healing Performance of Waterborne Coatings" Coatings 12, no. 8: 1056. https://doi.org/10.3390/coatings12081056

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