Effect of Number of Impregnations of Microberlinla sp with Microcapsule Emulsion on the Performance of Self-Repairing Coatings on Wood Surfaces
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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(7), 989; https://doi.org/10.3390/coatings12070989
Submission received: 31 May 2022
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Revised: 29 June 2022
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Accepted: 9 July 2022
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Published: 14 July 2022
Abstract
:Embedding melamine-formaldehyde (MF) resin-coated shellac microcapsules in waterborne coatings can extend the service longevity of waterborne coatings on a wood surface to a certain extent. Due to the content limitation of self-repairing microcapsules in waterborne coatings, the effective self-healing performance time is short. With the aim of improving the self-repairing properties of self-repairing coatings on the surface of a Microberlinla sp substrate, a more effective self-healing mechanism was achieved by impregnating the ebony wood substrate several times with an MF resin-coated transparent shellac-rosin microcapsule emulsion. After the impregnation of the ebony boards with microcapsules, a waterborne acrylic resin coating containing 3.0 wt.% transparent shellac microcapsules was applied to the surface of the wood boards. The influence of the number impregnations on the surface coating’s physical properties, chemical properties, and self-repairing properties was explored. The results showed that the hardness of the surface coating on the ebony boards changed little under different numbers of impregnations. With the increasing number of impregnations, the surface coatings’ adhesion and impact strength slowly increased, the chromatic difference value was increased, and the roughness first increased and then decreased. Impregnating ebony boards with the microcapsule emulsion contributes to enhancing the aging resistance and repair performance of surface coatings on the ebony boards. When the number of impregnations was eight, the width change rate of cracks on surface self-healing coatings was 28.4%, which suggested the best repair performance among all samples. By impregnating the wood substrate with the self-healing microcapsule emulsion, the effect of the interaction between microcapsules and wood on the self-repairing properties of the surface coating was studied, contributing to the theory for further improving the self-repairing properties of waterborne coatings on wood surfaces and promoting the application and development of self-healing microcapsules.
1. Introduction
Nowadays, wooden constructions [1,2] and products [3,4,5] are increasingly popular in people’s daily lives because of their excellent mechanical properties [6,7] and decorative characteristics [8,9,10]. To a certain extent, the application of coatings to wood surfaces can isolate moisture in the air and protect the wood substrate from mildew [11] and decay [12]. However, due to natural shortcomings such as dimensional instability [13] and poor durability [14,15], micro-cracks often inevitably appear on the surface coatings during the use of wooden products [16,17], which negatively impacts the coating’s appearance, firmness, and longevity, resulting in the service life of the coating being greatly shortened [18]. When the coating loses its efficacy, the unprotected wood becomes more vulnerable to damage, resulting in unnecessary economic losses. By embedding self-healing microcapsules in the surface coating on the wood, micro-cracks in the coating can be filled and repaired automatically, which enhances the stability of the surface coating [19]. However, due to the limitation of the content of self-healing microcapsules in the coating, the surface coating will still have some defects, such as a short effective duration of self-healing performance [20], and insufficient repair effect [21]. Therefore, to improve the effectiveness and durability of self-healing coatings, based on the porosity and permeability of the wood substrate, the wood substrate was impregnated with a self-repairing microcapsule emulsion , and then the self-healing coating with microcapsules was applied to the wood surface to enhance the combination of the microcapsules and the wood substrate. Because of the low cost of the raw materials and impregnation process, this process is commercially feasible for improving the repair efficiency of coatings and protecting the wood substrate.
Yan et al. [22] synthesized salicylic acid/silica microcapsules (SSM) using sol–gel technology and used them to impregnate poplar (Populus nigra L.) woods using a vacuum-pressure treatment. Compared with poplar wood without the SSM treatment, the poplar wood impregnated with SSM exhibited significantly improved decay resistance. Can et al. [23] used an interfacial polymerization method to achieve the microencapsulation of di-ammonium hydrogen phosphate in polyurethane. The vacuum-pressure treatment was used to impregnated poplar (Populus euramericana) boards with the microcapsules. The results indicated that the impregnated poplar boards showed higher fire resistance. Mathis et al. [24] impregnated sugar maple (Acer saccharum Marsh.) and red oak (Quercus rubra L.) boards with microcapsules with phase change material to prepare engineered wood flooring with high thermal mass. The results showed that the sugar maple was harder to impregnate, while the heating storage of the red oak wood impregnated with microcapsules was enhanced. These studies effectively endowed or improved the properties of corrosion resistance, flame retardancy, and heating storage effect in wood substrates. However, there are still gaps in the research regarding improving the self-repair ability of wood surface coatings by means of microcapsule impregnation [25].
Previous studies [26,27,28] have shown that melamine-formaldehyde (MF) resin-coated shellac microcapsules show good self-healing performance in the waterborne acrylic resin coating on wood surfaces. In addition, the physical properties, chemical properties, and self-repairing ability of the self-repairing coatings were improved when the microcapsule content in the coatings was 3.0% and the curing agent in the microcapsules was transparent shellac [29]. Shellac has the disadvantages of brittleness, poor water resistance, and heat resistance [30]. The addition of rosin in a mass ratio of 1:1 can delay the aging of shellac, improve the water resistance of shellac, and enhance the resistance of the core repair agent [31]. Therefore, in this paper, a transparent shellac microcapsule emulsion was synthesized and used to impregnate ebony boards several times. Then, the surface of the impregnated ebony was coated with the waterborne acrylic resin coating containing 3.0% microcapsules. The optical properties, mechanical properties, cold liquid resistance, and other properties of the surface coating on the ebony board were characterized and analyzed. The effect of the number of impregnations on the physical and chemical properties of the self-healing coatings on the ebony boards was explored. The high-temperature aging resistance of the surface coatings was tested, and the effect of the number of impregnations on the self-repairing ability of surface coatings was analyzed. Additionally, by comparing the scratch repair performance of a coating containing 3.0% transparent shellac microcapsules on the ebony surface without impregnation treatment and that of coating containing 3.0% transparent shellac microcapsules on the ebony surface with impregnation treatment, the influence of the interaction between microcapsules and wood on the self-repairing ability of the surface coatings on the ebony was analyzed. This contributes to further improvement of the self-repairing ability of waterborne coatings on a wood substrate.
2. Experimental Materials and Methods
2.1. Experimental Materials
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 obtained from Shandong Yousuo Chemical Technology Co., Ltd., Linyi, China. Transparent shellac with a purity of 12.5 wt.% was purchased from Shanghai Yuyan building materials Co., Ltd., Shanghai, China. Rosin (Mw: 302.46 g/mol, CAS No.: 8050-09-7) was purchased from Suzhou Guyue musical instrument Co., Ltd., Suzhou, China. The ethyl acetate (Mw: 88.11 g/mol, CAS No.: 141-78-6) and 37 wt.% formaldehyde solution (Mw: 30.03 g/mol, CAS No.: 50-00-0) were offered by Xi’an Tianmao Chemical Co., Ltd., Xi’an, China. Triethanolamine (analytical pure, Mw: 149.19 g/mol, CAS No.: 102-71-6) was obtained from Guangzhou Jiale Chemical Co., Ltd., Guangzhou, China. 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. Distilled water was purchased from Guangzhou Watsons Food and Beverage Co., Ltd., Guangzhou, China. Absolute ethanol (Mw: 46.07 g/mol, CAS No.: 64-17-5) was purchased from Wuxi Jingke Chemical Co., Ltd., Wuxi, China. The waterborne coating (waterborne acrylic acid copolymer dispersion, matting agent, additive, and water) was purchased from Dulux Coatings Co., Ltd., Shanghai, China. Ebony (Microberlinla sp) boards (50 mm × 50 mm × 10 mm, length × width × thickness) were provided by Beijing Tiantan Furniture Co., Ltd., Beijing, China.
2.2. Preparation Method of Microcapsules Emulsion and Impregnated Ebony Boards with Self-Repairing Films
- (1)
- Synthesis of microcapsule emulsion and microcapsule powders
The overall schematic diagram of the change in the emulsion system built during the reaction is shown in Figure 1. First, 27.04 g 37 wt.% formaldehyde solution and 12.00 g melamine were weighed and then mixed with 30 mL distilled water in a beaker. After the pH value of the mixture was adjusted to 8–9 with several drops of triethanolamine, the mixture was stirred at 600 rpm for 30 min in a 60 °C DF-101s constant temperature water bath (Gongyi Yuhua Instrument Co., Ltd., Zhengzhou, China) in order to obtain the MF prepolymer solution. Then, 0.3 g Span-20, 0.3 g Tween-20, and 157.8 mL absolute ethanol were fully stirred and mixed in another beaker. Additionally, 8.8 g transparent shellac resin and 8.8 g rosin were mixed evenly in the beaker. The mixture was put in a 60 °C water bath at 600 rpm for 60 min to obtain the core material emulsion. Finally, the core material emulsion was slowly added to the MF prepolymer solution at 600 rpm. The BILON-500 ultrasonic material emulsion disperser (Shanghai Bilang Instrument Co., Ltd., Shanghai, China) was used to disperse the sample for 15 min. Then, the sample was transferred into the water bath. At 600 rpm, the pH value was controlled to about 4.5 using citric acid. Then, the water bath was heated to 60 °C to conduct a constant temperature reaction for 2 h, resulting in the microcapsule emulsion being obtained. After standing for 3 d, the prepared microcapsule emulsion was washed many 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 to obtain the microcapsule powders.
- (2)
- Impregnation treatment of ebony boards
The ebony boards were immersed in the prepared microcapsule emulsion for 5 min and then taken out to dry naturally. This process was recorded as 1 imprenation. The above steps were repeated to obtain ebony boards that had been impregnated several times.
- (3)
- Preparation of waterborne films on the ebony boards
First, 0.12 g MF resin microcapsule-coated transparent shellac was embedded in the 3.88 g waterborne coating. After stirring and mixing evenly, self-repairing waterborne coating with 3.0% microcapsules was obtained. After polishing the impregnated ebony wood boards with sandpaper, the prepared coatings were evenly applied to the surface of the boards with a brush. Then, they were placed in a cool and ventilated place for 3 h for drying, and then the surface of the films was polished using fine sandpaper of 600 mesh (23 μm). These operation steps were repeated twice to obtain coated boards for subsequent testing.
To explore the effects of the number of impregnations, the microcapsule content in waterborne coating, and film thickness on the self-healing ability of the coating on the ebony surface, an L4 (23) orthogonal test was designed. The orthogonal test arrangement is shown in Table 1 and Table 2. Then, the most influential factor was chosen as the variable in order to carry out single-factor tests to explore the effect of the interaction mechanism of microcapsules and wood on the self-repairing performance of the surface coating.
2.3. Testing and Characterization
According to the Chinese standard GB/T 4893.6-2013 [32], the gloss of the surface coating on the ebony was measured using an HG268 gloss meter (Shenzhen Threenh Technology Co., Ltd., Shenzhen, China). According to GB/T 11186.3-1989 [33], the SEGT-J portable chromatic difference instrument (Zhuhai Tianchuang Instrument Co., Ltd., Zhuhai, China) was used to measure the chromaticity value of the surface coating on the ebony. A point of the water-based waterborne film was randomly selected for the test, and the obtained chromaticity values were recorded as L1, a1, and b1, respectively. Then, a second point of the water-based waterborne film was randomly selected for testing, and the obtained chromaticity values were recorded as L2, a2, and b2, respectively. ΔL* indicates the lightness difference, Δa* indicates the difference between red and green, and Δb* indicates the difference between yellow and blue. The chromatic difference value of the waterborne film was calculated according to CIELAB chromatic difference, as shown in Formula (1):
ΔE* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
According to the Chinese standard GB/T 6739-2006 [34], the hardness of the waterborne film was measured using 6H-6B pencils. 6B is the softest, 6H is the hardest, and HB is the intermediate value. The angle between the pencil and the coating was 45°. When there was no indentation on the coating surface, the maximum hardness of the pencil was recorded as the hardness of the coating. Using the QFH-HG600 film scribing instrument (Shanghai Le’ao Test Instrument Co., Ltd., Shanghai, China), the adhesion grade of the coating was analyzed. There are six grades, of which grade 0 represents the best coating adhesion and grade 5 represents the worst coating adhesion. According to GB/T 1731-1993 [35], the impact strength of the coating was characterized using a QCJ paint film impactor tester (Shanghai Le’ao Test Instrument Co., Ltd., Shanghai, China). The roughness of the coating [36,37] was measured using a JB-4C precision roughness tester, which was provided by Shanghai Taiming Optical Instrument Co., Ltd., Shanghai, China.
The Zeiss Axio Scope A1 optical microscope (OM, Carl Zeiss AG, Oberkochen, Germany) was used to characterize the appearance of transparent shellac microcapsules. The Quanta-200 scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA) was selected to characterize the micromorphology of microcapsules and waterborne coatings on the surface of the ebony. Using a VERTEX 80V Fourier transform infrared spectrometer (FTIR, Shanghai Smio Analytical Instrument Co., Ltd., Shanghai, China), the chemical composition of the transparent shellac microcapsules, the core material, the wall material, and the waterborne coatings on the surface of the ebony were analyzed by means of the ATR tablet pressing method.
The ebony boards coated with the self-healing coating were put in a DHG-9643BS-III electric heating constant temperature blast drying oven (Shanghai Xinmiao Medical Instrument Co., Ltd., Shanghai, China) at 120 °C to perform the high-temperature aging test. The chromaticity value and gloss value of the surface coating were tested every 6 h. The chromatic difference values before and after the aging of the surface coating on the ebony were calculated according to the chromatic difference, as described in Formula (1). The light loss rate before and after the aging of the surface coating on the ebony was calculated according to Formula (2). GL indicates the light loss rate. G1 indicates the gloss of the surface coating on the ebony before aging, %. G2 indicates the gloss of the surface coating on the ebony after aging, %.
GL = (G1 − G2)/G1 × 100%
After performing the high-temperature aging test for 36 h, the hardness, adhesion, impact strength, and roughness of the surface coating on the ebony were measured. The method used was the same as for the mechanical property tests of the surface coating on the ebony before aging.
The self-healing coating was damaged in the direction parallel to the tensile direction using a Q/YSMMDE 3 single-sided blade with a length of 2 cm and a depth of about 100 µm. OM and SEM were used to observe the repair changes at the scratch of the coating before and after standing for 5 d. The change rate of scratch width was calculated according to Formula (3) in order to characterize the repair performance of the self-healing coating. DH indicates width change rate, %. D1 indicates the scratch width of the surface coating on the ebony before repair, μm. D2 indicates the scratch width of the surface coating on the ebony after repair, μm.
DH = (D1 − D2)/D1 × 100%
All tests were repeated 4 times with an error of less than 5%.
3. Results and Discussion
3.1. Micromorphology, Particle Size Distribution, and Chemical Composition Analysis of Transparent Shellac Microcapsules
Figure 2 shows the OM image, SEM image, and particle size distribution of MF resin microcapsules containing transparent shellac-rosin as the curing agent. The appearance of the microcapsules is spherical, smooth, and has a uniform size distribution. The average particle size of the transparent shellac microcapsules is 5.72 μm.
Figure 3 shows the infrared spectra of the transparent shellac microcapsules, the MF resin, and the transparent shellac resin. The triazine ring bending vibration absorption peak at 813 cm−1 belongs to the MF resin. The peak at 1558 cm−1 is the stretching vibration absorption peak of N–H. The peak at 1000 cm−1 is the absorption peak of C–O. These peaks indicating MF resin are also present on the curves of the transparent microcapsules. The characteristic peak at 2829 cm−1 belongs to −CH3 in C–H, and the wide peak at 1716 cm−1 is responsible for the stretching vibration of C=O. These peaks, which indicate shellac, can also be found on the curves of the microcapsules. As a result, combined with the micromorphological analysis, it can be concluded that the MF resin microcapsules containing transparent shellac were successfully synthesized.
3.2. Analysis of Orthogonal Test Results
The repair effect of the surface coating on the ebony determined by means of the orthogonal test is shown in Figure 4. By comparing the width of the stretch before and after standing for 5 d, and calculating the change rate of the width of the stretch before and after standing for 5 d according to Formula (3), the self-repairing performance of the surface coating on the ebony was quantified. The results of the stretch width change rate are shown in Table 3. To explore this factor, which has the greatest impact on the scratch repair effect among the three factors, the results of the width change rate were analyzed on the basis of range and variance, as shown in Table 4.
The range is the difference in the average value of each factor. It indicates the influence of each factor on the test results. The larger the range is, the greater the influence of this factor on the test results. According to the range results of the scratch width change rate, it can be found that the range varied the most with number of impregnations, indicating that the number of impregnations had the greatest influence on the test results, followed by the microcapsule content in the waterborne coatings. According to the variance results, the Fratio for number of impregnations was 290.181, while the Fcritical value was 161.000, which is less than the Fratio. Therefore, the number of impregnations significantly affected the change in the scratch width of the surface coating on the ebony. The results of range and variance showed that the number of impregnations of the ebony played a crucial role in the repair performance of its surface coating.
To explore the relationship between the number of impregnations and the coating properties, by taking number of impregnations as a single factor variable, and fixing the microcapsule content (3.0 wt.%) and coating thickness (10.0 μm), the single factor independent test was carried out according to the single factor test schedule as shown in Table 5.
3.3. Effect of Number of Impregnations on the Microstructure and Chemical Composition of Surface Coatings
Photographs of the blank board (impregnation zero times, after coating) and ebony boards coated with the waterborne coating containing 3.0% microcapsules with different numbers of impregnations are shown in Figure 5. The morphology images of the end faces of the bony boards coated with the waterborne coating containing 3.0% microcapsules with different numbers of impregnations are shown in Figure 6. Figure 6A shows the interface between the unimpregnated ebony and the waterborne coating, indicating that the interface between the coating and the wood was well demarcated. According to Figure 6A–D, with increasing number of impregnations, the microcapsule agglomeration at the end face of the ebony was more obvious. Following impregnation with microcapsules, its end face was no longer flat, and exhibited irregular granules, and the microcapsules were attached to the end face of the ebony. According to Figure 6B–D, it can be observed that microcapsules gathered at the wood vessels, while for the wood fiber cells and other wood pores with small diameters, the microcapsules were found it difficult to enter. When the number of impregnations was three, some microcapsules adhered to the inner wall of the vessels, as shown in Figure 6B. When the number of impregnations was eight, the number of microcapsules adhering to the inner wall of the vessels increased significantly, as shown in Figure 6C. When the number of impregnations was increased to 15, there were lots of microcapsules forming agglomerations, as shown in Figure 6D. With increasing number of impregnations with microcapsule emulsion, the content of microcapsules in the vessels also increased, and many microcapsules agglomerated together. Therefore, the greater the number of impregnations, the greater the microcapsule content, and the greater the inevitable agglomeration, which can easily cause adverse effects such as blocking.
Table 6 and Figure 7 show the FTIR characteristic peaks of the end faces of sampe 7 sample (microcapsule emulsion impregnated eight times), sample 5 (microcapsule emulsion impregnated zero times), and the blank sample. The absorption peak at 1558 cm−1 is the stretching vibration absorption peak of N–H, and that at 813 cm−1 is the bending vibration absorption peak of the triazine ring in MF resin, indicating that for the microcapsules, the MF resin shell structure was successfully prepared. According to the infrared curve of the blank sample, the absorption peak at 3340 cm−1 is the stretching vibration of –OH on the cell wall of ebony. According to the infrared curve of sample 7, the absorption peak at 2900 cm−1 is the characteristic peak of C–CH3 in C–H. The peak at 1695 cm−1 represents the stretching vibration peak of C=O, indicating the existence of shellac microcapsules in the ebony boards after eight impregnation treatments. The infrared spectrum curves show that shellac microcapsules were successfully impregnated in the ebony boards.
3.4. Effect of Number of Impregnations on the Mechanical Properties of Surface Coatings
The mechanical properties of the surface coating on the ebony for different numbers of impregnations are shown in Table 7. The hardness of the surface coating on the ebony treated with different numbers of impregnations remained unchanged, and was H. With increasing number of impregnations, both the adhesion grade and the impact strength of the surface coating on the ebony increased slowly. This may be because the increasing number of microcapsules attached to the surface of the ebony resulted in increased surface roughness of the wood and decreased adhesion of the coating. The roughness of the ebony surface coating first increased and then decreased with increasing number of impregnations. The impact strength of the surface coating on the ebony without impregnation treatment was 8 kg·cm. After eight impregnation treatments, the impact strength of the surface coating on the ebony increased to 9 kg·cm, indicating that after impregnation with the microcapsules, the microcapsules gathered on the wood cell wall to increase the strength of the impregnated ebony.
3.5. Influence of Number of Impregnations on the Chromatic Difference and Gloss of the Surface Coatings on the Ebony
The chromatic difference is crucial for measuring the optical performance of coatings. The chromaticity values of the surface coatings on the ebony for different numbers of impregnations are shown in Table 8. The chromaticity parameters of the surface coating on the unimpregnated ebony were compared with those of the surface coating on the ebony after impregnation, and the chromatic difference results were calculated to examine the influence of number of impregnations on the color difference in the surface coatings on the ebony. It is obvious from Table 8 that, with increasing number of impregnations, the brightness value L* of the surface coating on the ebony increased, and the chromatic difference value ΔE* also increased. This may be because microcapsules were impregnated on the wood surface, and after the solvent was volatilized, the color of the microcapsule powders affects the chromaticity parameters of the coating, increasing the chromatic difference value of the surface coatings.
The gloss values of the surface coatings on the ebony for different numbers of impregnations at different incident angles are shown in Table 9. At an incident angle of 85°, the gloss of the coating decreased gradually with increasing number of impregnations. At an incident angle of 60°, the gloss of the ebony surface showed a downward wave trend. It can be seen that the number of impregnations of the ebony had an impact on the gloss of its surface coating. When the number of impregnations was eight, the gloss of the surface coating on the ebony was relatively high, at 31.9%.
3.6. Effect of Number of Impregnations on Aging Repair Performance of Surface Coatings on the Ebony
3.6.1. Microstructure and Chemical Composition of Coatings before and after Aging Tests
The SEM images of the surface coatings on the ebony for different numbers of impregnations before and after aging are shown in Figure 8. Figure 8A–D present the SEM images of the surface coatings of ebony for different numbers of impregnations before aging. It can be found that the micromorphology of the coatings was consistent. Figure 8E–H present the SEM images of the surface coatings of the ebony for different numbers of impregnations after aging. After a high-temperature treatment for a long time, the coatings were aged. Obvious round hole cracks can be observed on the coating surface from the SEM images. The surface coating on the ebony without microcapsule emulsion impregnation had a larger area of round hole cracking, with a diameter of about 30 μm. With increasing number of microcapsule impregnations, the cracking area of the surface coatings on the ebony exhibited a decreasing trend. It can be observed that when the number of impregnations was eight, the cracking area of the coating was the smallest. It can be seen that the impregnation treatment of the ebony with the microcapsule emulsion can enhance the aging resistance of the surface coatings on the ebony and effectively reduce the loss.
The infrared spectrum information of the self-repairing coating on the surface of the ebony after eight impregnations before and after aging is shown in Table 10 and Figure 9. The peak at 2952 cm−1 was the characteristic peak of –CH3 in C–H. The peak at 1728 cm−1 was the characteristic peak of the C=O group in the waterborne acrylic resin coating. The infrared curves of the coating before and after aging were consistent, with only the peak intensity being enhanced and weakened, and there was no appearance or disappearance of peaks, indicating that the chemical composition of the self-healing coating on the surface of the ebony after impregnation treatment changed little before and after high-temperature aging.
3.6.2. Mechanical Properties of Coating before and after Aging
The changes in the mechanical properties of the surface coating on the ebony with different numbers of impregnations before and after the 120 °C aging test are shown in Table 11. The hardness of the surface coating changed little before and after aging. The change in the adhesion of the surface coating on unimpregnated ebony before and after aging was consistent with the change in the adhesion of the surface coating on impregnated ebony. The number of impregnations had little effect on the hardness and adhesion of the coating before and after aging. According to Table 11, the coating roughness changed irregularly before and after aging. The impact strength of the surface coating on the ebony without impregnation treatment decreased from 8 to 6 kg·cm, while after three impregnation treatments, the impact strength of the surface coating on the ebony only decreased from 8 to 7 kg·cm. After eight impregnation treatments, the impact strength of the surface coating on the ebony did not decrease significantly after aging. The aging process of the surface coatings on the ebony after impregnation treatment slowed down, and the rate of the downward trend of the mechanical strength of ebony and its surface coating also slowed down.
3.6.3. Chromatic Difference and Gloss of Coatings before and after Aging
Table 12 shows the change in the chromatic difference value of the surface coating on ebony with different numbers of impregnations as a function of aging time in a 120 °C high-temperature aging environment. With increasing aging time, the chromatic difference of the surface coatings on the ebony increased to varying extents under different numbers of impregnations. When the aging time was 6–18 h, the chromatic difference value of the surface coating on the ebony without impregnation treatment was small. When the aging time exceeded 18 h, the chromatic difference value of the surface coating on the ebony with the impregnation treatments was small. The chromatic difference in the surface coating on the ebony before and after aging was jointly determined by the coating and the ebony substrate. This is because after long-term high-temperature aging, ebony carbonizes, and the dual effect of internal and external stress causes the surface coating to age.
When the number of impregnations was eight, the chromatic difference of the coating changed little after high-temperature aging for 18 h. After aging for 36 h, the chromatic difference ΔE* of the surface coating on the ebony without impregnation treatment was 10.44. The chromatic difference ΔE* of the surface coating on the ebony with three impregnations was 9.01. The chromatic difference ΔE* of the surface coating on the ebony with eight impregnations was 8.96. The chromatic difference ΔE* of the surface coating on the ebony with 15 impregnations was 8.99. These results show that the color of ebony after impregnation treatment was more stable in the high-temperature aging environment. The impregnation treatment can improve the aging resistance of wood substrates to some extent. When the number of impregnations was eight, the color stability of the surface coating on the ebony under the high-temperature aging environment was better.
In the high-temperature aging environment at 120 °C, the changes in the gloss of the surface coating on the ebony with different numbers of impregnations with aging time are shown in Table 13. The light loss rate of the coating at a 60° incident angle is shown in Figure 10. According to Table 13, it can be found that under different numbers of impregnations, the variation in the gloss of the surface coating on the ebony with aging time was not obvious, but showed a downward trend. According to Figure 10, when the aging time was 0–6 h, the light loss rate of the surface coating on the ebony increased significantly. After aging for 6 h, the light loss rate increased slowly and remained stable. With number of number of impregnations of the ebony, the gloss loss of the surface coating first increased and then decreased. The number of impregnations had little effect on improving the gloss of the aged surface coating on the ebony.
3.7. Effect of the Interaction Mechanism between Microcapsules and the Wood Substrate on the Repair Performance of the Surface Coating
The scratch repair test results of the self-healing coating on the ebony surface under different numbers of impregnations are shown in Figure 11. By using a Q/YSMMDE 3 single-sided blade to artificially damage the coating, resulting in uniform scratches on the surface, the change in scratch width on the surface coating after standing for 5 d was observed. The change rate of scratch width on the coating under different numbers of impregnations is shown in Figure 12.
In the prepared microcapsule core material, shellac is dissolved in ethanol solution. After the microcapsule is broken, the core material flows out into the coating microcrack gap. After the ethanol evaporates, the shellac gradually separates out and can be physically cured at room temperature to fill the coating crack. The function of the microcapsule structure is to prevent the evaporation of ethanol to keep the shellac in a liquid state in order to achieve a self-healing effect. The scratch width on the self-repairing coating on the surface of the ebony without impregnation treatment decreased from 29.56 to 22.17 μm, with a scratch width change rate of 25.0%. The scratch on the self-repairing coating on the surface of the ebony with three impregnation treatments decreased from 26.51 to 19.64 μm, with a scratch width change rate of 25.9%. The scratch on the self-repairing coating on the surface of the ebony with eight impregnation treatments decreased from 22.89 to 16.4 μm, with a scratch width change rate of 28.4%. The scratch on the self-repairing coating on the surface of the ebony with 15 impregnation treatments decreased from 22.87 to 17.59 μm, with a scratch width change rate of 23.1%. With increasing number of impregnations, the change in the scratch width on the self-healing coating on the ebony surface first increased and then decreased. When the number of impregnations was eight, the change rate of scratch width on the self-healing coating on the ebony surface was the highest among all samples.
The impregnation treatment enhanced the interaction mechanism between the microcapsules and the wood, based on the above analysis. The self-healing surface coating on the ebony treated by microcapsule emulsion impregnation improved the repair performance. When the number of impregnations was eight times, the surface self-healing coating exhibited better repair performance.
4. Conclusions
In this paper, the physicochemical properties and self-healing properties of self-healing coatings on the surface of the ebony treated by means of microcapsule emulsion impregnation were investigated. The results of physical and chemical property tests showed that the hardness of the surface coating on the ebony under different numbers of impregnations changed little, which was H. With increasing number of impregnations, the adhesion and impact strength of the surface coating on the ebony increased slowly, and the roughness first increased and then decreased. After receiving eight impregnation treatments, the impact strength of the surface coating on the ebony increased to 9 kg·cm. At the same time, with increasing number of impregnations, the lightness value L* of the surface coating on the ebony increased, and the chromatic difference value ΔE* also increased. With number of impregnations with the microcapsule emulsion, the cracking area of the surface coating on the ebony exhibited a decreasing trend. It can be observed that when the number of impregnations was eight, the cracking area of the coating was the smallest. Following the impregnation treatment, the aging process of the ebony surface coating slowed down, and the declining trend of mechanical strength following high-temperature aging slowed down. After performing the impregnation treatment eight times, the impact strength of the surface coating on the ebony did not decrease following high-temperature aging, and was still 9 kg·cm. After microencapsulation emulsion impregnation treatment, the repair performance of the self-healing coating on the surface of the ebony was improved, and the impregnation treatment widened the response mechanism between the microcapsules and the wood. When the number of impregnations was eight, the change rate of scratch width in the self-healing coating on the surface of the ebony was the highest, and was 28.4%, and the repair performance was the best. By exploring the influence on the performance of a wood surface coating, a new way of achieving the function of self-healing coatings can be provided. In further research, the interaction mechanisms among coatings, microcapsules, and wood substrates, as well as the cause of the change in the FTIR characteristic peaks, still need to be explored and clarified.
Author Contributions
Conceptualization, methodology, validation, resources, data management, and supervision, Y.H.; formal analysis, investigation, X.Y.; and 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 & 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 that there is no conflict of interest.
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Figure 1.
The overall schematic diagram of the change in the built emulsion system during the reaction.
Figure 1.
The overall schematic diagram of the change in the built emulsion system during the reaction.
Figure 2.
OM image (A), SEM image (B), and particle size distribution (C) of transparent shellac microcapsules.
Figure 2.
OM image (A), SEM image (B), and particle size distribution (C) of transparent shellac microcapsules.
Figure 3.
FTIR image of transparent shellac microcapsules.
Figure 4.
Results of width change of the surface coating on the ebony after scratch: (A) sample 1 before repair, (B) sample 2 before repair, (C) sample 3 before repair, (D) sample 4 before repair, (E) sample # after repair, (F) sample 2 after repair, (G) sample 3 after repair, (H) sample 4 after repair.
Figure 4.
Results of width change of the surface coating on the ebony after scratch: (A) sample 1 before repair, (B) sample 2 before repair, (C) sample 3 before repair, (D) sample 4 before repair, (E) sample # after repair, (F) sample 2 after repair, (G) sample 3 after repair, (H) sample 4 after repair.
Figure 5.
Photographs of ebony boards: (A) impregnation 0 times, before coating, (B) impregnation 0 times, after coating, (C) impregnation 3 times, after coating, (D) impregnation eight times, after coating, and (E) impregnation 15 times, after coating.
Figure 5.
Photographs of ebony boards: (A) impregnation 0 times, before coating, (B) impregnation 0 times, after coating, (C) impregnation 3 times, after coating, (D) impregnation eight times, after coating, and (E) impregnation 15 times, after coating.
Figure 6.
SEM images of the end face of the ebony: (A) impregnation 1 time, (B) impregnation 3 times, (C) impregnation 8 times, and (D) impregnation 15 times.
Figure 6.
SEM images of the end face of the ebony: (A) impregnation 1 time, (B) impregnation 3 times, (C) impregnation 8 times, and (D) impregnation 15 times.
Figure 7.
FTIR of the end face of the ebony.
Figure 8.
SEM images of coatings on the ebony before and after aging under different deposition times: (A) impregnation 0 times before repair, (B) impregnation 3 times before repair, (C) impregnation 5 times before repair, (D) impregnation 15 times before repair, (E) impregnation 0 times after repair, (F) impregnation 3 times after repair, (G) impregnation 5 times after repair, and (H) impregnation 15 times after repair.
Figure 8.
SEM images of coatings on the ebony before and after aging under different deposition times: (A) impregnation 0 times before repair, (B) impregnation 3 times before repair, (C) impregnation 5 times before repair, (D) impregnation 15 times before repair, (E) impregnation 0 times after repair, (F) impregnation 3 times after repair, (G) impregnation 5 times after repair, and (H) impregnation 15 times after repair.
Figure 9.
FTIR of the surface coating on impregnated ebony before and after aging.
Figure 10.
The light loss rate of the surface coating on the ebony under different numbers of impregnations before and after aging.
Figure 10.
The light loss rate of the surface coating on the ebony under different numbers of impregnations before and after aging.
Figure 11.
OM of the surface coating on the ebony under different numbers of impregnations before and after aging: (A) impregnation 0 times before repair, (B) impregnation 3 times before repair, (C) impregnation 5 times before repair, (D) impregnation 15 times before repair, (E) impregnation 0 times after repair, (F) impregnation 3 times after repair, (G) impregnation 5 times after repair, and (H) impregnation 15 times after repair.
Figure 11.
OM of the surface coating on the ebony under different numbers of impregnations before and after aging: (A) impregnation 0 times before repair, (B) impregnation 3 times before repair, (C) impregnation 5 times before repair, (D) impregnation 15 times before repair, (E) impregnation 0 times after repair, (F) impregnation 3 times after repair, (G) impregnation 5 times after repair, and (H) impregnation 15 times after repair.
Figure 12.
Changes in the scratch width of the surface coating on the ebony under different numbers of impregnations after aging.
Figure 12.
Changes in the scratch width of the surface coating on the ebony under different numbers of impregnations after aging.
Table 1.
Orthogonal test factors and levels.
| Level | Impregnation Times (Time) | Microcapsule Content in the Coating (%) | Film Thickness (μm) |
|---|---|---|---|
| 1 | 3 | 0 | 10 |
| 2 | 15 | 3 | 20 |
Table 2.
Orthogonal test schedule.
| Sample (#) | Number of Impregnations (Impregnations) | Microcapsule Content in the Coating (%) | Film Thickness (μm) |
|---|---|---|---|
| 1 | 3 | 0 | 10 |
| 2 | 3 | 3 | 20 |
| 3 | 15 | 0 | 20 |
| 4 | 15 | 3 | 10 |
Table 3.
Scratch width change rate results.
| Sample (#) | Instant Scratch Width (μm) | Scratch Width after Repair for 5 d (μm) | Scratch Width Change Rate (%) |
|---|---|---|---|
| 1 | 22.74 | 25.69 | 11.48 |
| 2 | 30.99 | 26.47 | 14.59 |
| 3 | 31.65 | 24.99 | 21.04 |
| 4 | 22.87 | 17.59 | 23.09 |
Table 4.
Range and variance analysis of scratch width change rate.
| Range and Variance | Sample (#) | Number of Impregnations (Impregnations) | Microcapsule Content in the Coating (%) | Film Thickness (μm) | Scratch Width Change Rate (%) |
|---|---|---|---|---|---|
| Range | 1 | 3 | 0 | 10 | 11.48 |
| 2 | 3 | 3.0 | 20 | 14.59 | |
| 3 | 15 | 0 | 20 | 21.04 | |
| 4 | 15 | 3.0 | 10 | 23.09 | |
| Mean 1 | 13.035 | 16.260 | 17.285 | - | |
| Mean 2 | 22.065 | 18.840 | 17.815 | - | |
| R | 9.030 | 2.580 | 0.530 | - | |
| Variance | Error Square Sum | 81.541 | 6.656 | 0.281 | - |
| Degree of Freedom (df) | 1 | 1 | 1 | - | |
| Fratio | 290.181 | 23.687 | 1.000 | - | |
| Fcritical value | 161.000 | 161.000 | 161.000 | - | |
| Significance | * | - | - | - |
* Indicates significance.
Table 5.
Single-factor experiment schedule.
| Sample (#) | Number of Impregnations (Impregnations) | Microcapsules Content (%) | Film Thickness (μm) |
|---|---|---|---|
| 5 | 0 | 3.0 | 10 |
| 6 | 3 | 3.0 | 10 |
| 7 | 8 | 3.0 | 10 |
| 8 | 15 | 3.0 | 10 |
Table 6.
FTIR characteristic peak of end faces for sample 7, sample 5, and the blank sample.
| Wavenumber (cm−1) | Bond | Substance |
|---|---|---|
| 3340 | –OH | ebony cell |
| 1695 | C=O | Shellac |
| 2900 | –CH3 | Shellac |
| 1725 | –COOH | Shellac |
| 1650 | C=N | MF resin |
| 1558 | N–H | MF resin |
| 1380 | C–N | MF resin |
| 1151 | C–O–C | MF resin |
| 1001 | C–H | MF resin |
| 813 | Triazine ring | MF resin |
Table 7.
Mechanical properties of coatings on the ebony under different numbers of impregnations.
| Number of Impregnations (Impregnations) | Hardness | Adhesion (Grade) | Impact Strength (kg·cm) | Roughness (μm) |
|---|---|---|---|---|
| 0 | H | 2 | 8 | 1.31 |
| 3 | H | 2 | 8 | 1.39 |
| 8 | H | 3 | 9 | 0.88 |
| 15 | H | 3 | 9 | 1.53 |
Table 8.
Chromatic difference values of coatings on the ebony under different numbers of impregnations.
Table 8.
Chromatic difference values of coatings on the ebony under different numbers of impregnations.
| Number of Impregnations (Impregnations) | ΔL* | Δa* | Δb* | ΔE* |
|---|---|---|---|---|
| 0 | 51.15 | 13.48 | 23.83 | - |
| 3 | 51.18 | 14.35 | 29.38 | 5.62 |
| 8 | 58.35 | 12.08 | 23.53 | 7.34 |
| 15 | 60.88 | 11.88 | 24.33 | 9.87 |
Table 9.
Gloss values of coatings on ebony under different numbers of impregnations.
| Number of Impregnations (Impregnatnions) | Gloss (%) | ||
|---|---|---|---|
| 20° | 60° | 85° | |
| 0 | 11.7 | 47.0 | 44.3 |
| 3 | 7.9 | 30.7 | 32.4 |
| 8 | 9.2 | 31.9 | 31.8 |
| 15 | 4.6 | 20.9 | 23.8 |
Table 10.
FTIR characteristic peaks of the surface coating on impregnated ebony before and after aging.
Table 10.
FTIR characteristic peaks of the surface coating on impregnated ebony before and after aging.
| Wavenumber (cm−1) | Bond | Substance |
|---|---|---|
| 2952 | –CH3 | Shellac |
| 1728 | –COOH | Shellac |
| 1540 | N–H | MF resin |
| 1384 | C–N | MF resin |
| 1141 | C–O–C | MF resin |
| 1020 | C–H | MF resin |
| 817 | Triazine ring | MF resin |
Table 11.
Mechanical properties of the surface coating on the ebony under different numbers of impregnations before and after aging.
Table 11.
Mechanical properties of the surface coating on the ebony under different numbers of impregnations before and after aging.
| Number of Impregnations (Impregnations) | Aging Time (h) | Hardness | Adhesion (Grade) | Impact Strength (kg·cm) | Roughness (μm) |
|---|---|---|---|---|---|
| 0 | 0 | H | 2 | 8 | 1.31 |
| 36 | 2H | 3 | 6 | 0.98 | |
| 3 | 0 | H | 2 | 8 | 1.39 |
| 36 | H | 3 | 7 | 1.14 | |
| 8 | 0 | H | 3 | 9 | 0.88 |
| 36 | 2H | 4 | 9 | 0.96 | |
| 15 | 0 | H | 3 | 9 | 1.53 |
| 36 | H | 4 | 8 | 2.11 |
Table 12.
Chromatic difference of the surface coating on the ebony under different numbers of impregnations before and after aging.
Table 12.
Chromatic difference of the surface coating on the ebony under different numbers of impregnations before and after aging.
| Number of Impregnations (Impregnations) | Aging Time (h) | ΔL* | Δa* | Δb* | ΔE* |
|---|---|---|---|---|---|
| 0 | 0 | 51.15 | 13.48 | 23.83 | - |
| 6 | 52.93 | 12.70 | 25.28 | 2.42 | |
| 12 | 52.45 | 11.47 | 27.50 | 4.40 | |
| 18 | 51.40 | 11.85 | 28.20 | 4.67 | |
| 24 | 53.83 | 11.65 | 32.55 | 9.31 | |
| 30 | 52.40 | 10.38 | 33.03 | 9.79 | |
| 36 | 53.45 | 10.55 | 33.58 | 10.44 | |
| 3 | 0 | 53.18 | 14.35 | 29.38 | - |
| 6 | 52.00 | 9.65 | 28.33 | 4.96 | |
| 12 | 51.45 | 9.73 | 25.85 | 6.07 | |
| 18 | 51.95 | 9.33 | 26.05 | 6.15 | |
| 24 | 49.25 | 9.40 | 26.73 | 6.85 | |
| 30 | 52.35 | 7.68 | 25.83 | 7.61 | |
| 36 | 53.30 | 9.60 | 21.73 | 9.01 | |
| 8 | 0 | 58.35 | 12.08 | 23.53 | - |
| 6 | 61.00 | 8.40 | 25.90 | 5.12 | |
| 12 | 58.43 | 8.20 | 27.90 | 5.84 | |
| 18 | 59.23 | 9.35 | 28.88 | 6.08 | |
| 24 | 59.93 | 8.58 | 28.75 | 6.48 | |
| 30 | 58.40 | 7.85 | 29.70 | 7.48 | |
| 36 | 58.75 | 10.90 | 32.40 | 8.96 | |
| 15 | 0 | 60.88 | 11.88 | 24.33 | - |
| 6 | 59.80 | 6.85 | 27.05 | 5.89 | |
| 12 | 62.10 | 6.45 | 26.28 | 5.82 | |
| 18 | 60.63 | 6.85 | 29.40 | 7.15 | |
| 24 | 61.23 | 5.23 | 28.35 | 7.78 | |
| 30 | 60.15 | 8.15 | 31.38 | 8.01 | |
| 36 | 62.30 | 6.43 | 31.33 | 8.99 |
Table 13.
The gloss of the surface coating on the ebony under different numbers of impregnations before and after aging.
Table 13.
The gloss of the surface coating on the ebony under different numbers of impregnations before and after aging.
| Number of Impregnations (Impregnations) | Aging Time (h) | Gloss (%) | ||
|---|---|---|---|---|
| 20° | 60° | 85° | ||
| 0 | 0 | 11.70 | 46.98 | 44.25 |
| 6 | 10.03 | 36.55 | 34.63 | |
| 12 | 10.23 | 36.18 | 32.03 | |
| 18 | 10.23 | 35.73 | 33.45 | |
| 24 | 9.48 | 35.48 | 31.98 | |
| 30 | 8.38 | 34.98 | 34.10 | |
| 36 | 9.68 | 33.43 | 34.68 | |
| 3 | 0 | 7.88 | 30.65 | 32.35 |
| 6 | 3.65 | 14.05 | 8.98 | |
| 12 | 4.00 | 13.90 | 8.33 | |
| 18 | 3.75 | 13.78 | 7.23 | |
| 24 | 3.38 | 13.65 | 7.40 | |
| 30 | 2.93 | 13.55 | 8.40 | |
| 36 | 3.28 | 12.90 | 6.18 | |
| 8 | 0 | 9.15 | 31.93 | 31.78 |
| 6 | 4.63 | 16.70 | 15.65 | |
| 12 | 3.90 | 16.63 | 13.75 | |
| 18 | 4.20 | 15.80 | 14.95 | |
| 24 | 3.75 | 15.68 | 13.35 | |
| 30 | 3.88 | 15.13 | 12.90 | |
| 36 | 4.35 | 14.88 | 13.93 | |
| 15 | 0 | 4.55 | 20.85 | 23.80 |
| 6 | 3.48 | 13.45 | 10.88 | |
| 12 | 3.48 | 13.40 | 9.40 | |
| 18 | 3.60 | 13.20 | 10.93 | |
| 24 | 3.23 | 12.80 | 8.48 | |
| 30 | 3.15 | 12.55 | 8.55 | |
| 36 | 3.35 | 12.30 | 8.63 | |
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Han, Y.; Yan, X.; Tao, Y. Effect of Number of Impregnations of Microberlinla sp with Microcapsule Emulsion on the Performance of Self-Repairing Coatings on Wood Surfaces. Coatings 2022, 12, 989. https://doi.org/10.3390/coatings12070989
AMA Style
Han Y, Yan X, Tao Y. Effect of Number of Impregnations of Microberlinla sp with Microcapsule Emulsion on the Performance of Self-Repairing Coatings on Wood Surfaces. Coatings. 2022; 12(7):989. https://doi.org/10.3390/coatings12070989
Chicago/Turabian StyleHan, Yan, Xiaoxing Yan, and Yu Tao. 2022. "Effect of Number of Impregnations of Microberlinla sp with Microcapsule Emulsion on the Performance of Self-Repairing Coatings on Wood Surfaces" Coatings 12, no. 7: 989. https://doi.org/10.3390/coatings12070989
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