Effect of Microcapsule Concentration with Different Core-Shell Ratios on Waterborne Topcoat Film Properties for Tilia europaea

: The effects of the core-shell ratio and concentration of urea formaldehyde (UF) resin-coated waterborne acrylic resin microcapsules on the optical properties, mechanical properties and liquid resistance of waterborne topcoat coatings on the surface of Tilia europaea were investigated. With the increase of microcapsule concentration, the color difference and hardness of the paint ﬁlm gradually increased, the gloss and adhesion of the paint ﬁlm gradually decreased, and the impact resistance and elongation at break of the paint ﬁlm increased ﬁrst and then decreased. With the increase of the core-shell ratio, the hardness and impact resistance of the paint ﬁlm increased ﬁrst and then decreased, and the adhesion of the paint ﬁlm decreased gradually. Red ink had a great inﬂuence on the liquid resistance of paint ﬁlm. When the core-shell ratio of UF-coated waterborne acrylic resin microcapsule was 0.58:1 and the microcapsule concentration was 10.0%, the comprehensive performance of paint ﬁlm on Tilia europaea was better. The prepared self-healing microcapsules applied to the waterborne coatings committed to prolonging the service life of the paint ﬁlm. content the color difference ascended at the same core-shell The color difference of the paint ﬁlm with the microcapsule concentration of 0–10.0% small.


Introduction
As a natural polymer composite material, wood material has unique texture and color [1]. It is a common green resource in nature [2]. As environmental awareness is growing, wood materials have become the preferred object of use because of their high strength to weight ratio, low energy consumption, natural degradation and recyclable utilization, and they are widely used in construction, indoor, packaging, railway transportation (as hardwood sleepers) and other production activities [3,4]. However, the cellulose, the main component of such kind of materials, contains many hydrophilic groups, which leads to its wet expansion and the drying of the wood, with a size variation which limits its application scopes and further affects its service life [5][6][7][8]. In general, wood materials are subjected to surface treatments before being put into actual use [9][10][11][12][13]. The rheology and stabilizers are very important for the properties of coatings [14]. Waterborne wood coatings are very popular in wood surface finishing because of their advantages regarding environmental protection, pollution resistance and fast drying [15]. However, there are also some disadvantages, such as easy aging and microcracks in daily use, which impinge on the beauty and service life of these products [16].
The service life of paint film can be prolonged by embedding a repair agent in the coating [17,18]. This specific implementation includes a liquid core fiber method and a microcapsule method [19]. Based on the application of microcapsule technology in composite materials, some progress has been made. Litina et al. [20] first applied the polymer microcapsules, including liquid sodium silicate, to the self-repair of industrial repair mortar, and the best self-healing additives were selected. Accordingly, it turned out that both the crack width and depth of the new composite cement-based repair material were reduced, and the permeability was restored. Njoku et al. [21] fabricated a novel pH-responsive permeabilityswitching polyurea-formaldehyde-coated 8-hydroxyquinoline microcapsule, which was used to prepare intelligent epoxy film for the permanent protection of Al-based alloys. The results indicated that the modified epoxy-based smart film has a good barrier and is active in improving the self-healing and corrosion inhibition performances. Wang et al. [22] synthesized fluorescent-labeled self-healing microcapsules (FLSM) by ultrasonic dispersion in-situ polymerization. Fluorescein sodium-labeled epoxy resin served as the restorative agent and UF resin as the cladding material. The results confirmed that FLSM can detect the position of microcracks and repair them and also indicates the width of microcracks on the cement surface. The above studies showed that the internal and external damage of the matrix material could be automatically repaired by mimicking the self-healing function of organisms and the participation of external environmental factors, to prolong the service life of the material. But it is worth noting that these studies focused on readily available commercial repair mortars, Al alloys and cement paste, and there are few reviews on the self-healing of waterborne wood paint films.
In our early work, epoxy resins laden with UF microcapsules were fabricated, and the effects of microcapsules and the coating process on gloss, color difference and the toughness of waterborne paint films for F. mandshurica veneers were studied [23]. As a common wood material, Tilia europaea has moderate hardness and a uniform and fine texture [24]. It is easy to process and possesses strong toughness, good corrosion resistance, wear resistance and resistance to crack deformation [25]. Nowadays, the demands of waterborne film-forming wood coatings are more and more extensive, so there is a need for a variety of wood substrates to meet the public aesthetic and to create application opportunities for self-healing wood products in a smart manner. Epoxy resin as a restorative agent needs to contain a hardener or a crosslinking agent and to be heated to achieve curing, which is not realistic for wood products, especially for furniture and floor products. Generally speaking, a waterborne primer is mainly used to fill pores and improve film thickness. On the other hand, the main function of a topcoat is to play the role of decoration and protection, such as keeping light and color, improving hardness and adhesion and leveling. Primer and topcoat have different functions, and their performances may be different. In this respect, the effect of microcapsules with different core-shell ratios on the performance of a waterborne primer for Tilia europaea was recently investigated [26]. Therefore, it is necessary to explore the presence of microcapsules on the properties of a waterborne topcoat on wood.
In this work, the safe and non-toxic water-based acrylic resin was used as the core material, and the hard and dense UF resin was used as the shell material to prepare microcapsules with seven different core-shell ratios. The microcapsules were added to the waterborne topcoat and spread on the Tilia europaea substrate to study the effect of the core-shell ratio and also the content of the microcapsules on the performance of the waterborne paint film. The waterborne acrylic resin with the same material as the paint base is selected as the core material to explore the effects of the addition of microcapsules on the optical properties, mechanical properties and liquid resistance of the waterborne topcoat. The repair agent can be cured at room temperature without introducing other media, which is conducive to interface bonding. In the future, the goal is to improve the repair rate of microcapsules. The results so obtained may serve as useful prerequisites in the preparation of self-healing waterborne wood coatings.

Experimental Materials
The materials required for the experiment are displayed in Table 1. The Tilia europaea with the size of 90 mm × 45 mm × 4 mm has been processed until its surface color was uniform, and its moisture content reached the local equilibrium, respectively 14.9%. The main components of waterborne topcoat were waterborne acrylic emulsion, polyurethane emulsion, additives and water.

Preparation of Microcapsules
Microcapsules with waterborne acrylic resin as the core material and UF resin as the shell material were prepared by in situ polymerization with the following values of the core-shell ratio as an independent variable: 0.42:1, 0.50:1, 0.58:1, 0.67:1, 0.75:1, 0.83:1 and 0.92:1 [27]. This paper mainly studied the effects of the core-shell ratio and the concentration of microcapsules on the properties of paint film. In later work, the stabilizer will be used to modify the microcapsules. The detailed compositions of the seven systems are shown in Table 2. To exemplify, the preparation process of microcapsules with a core-shell ratio of 0.42:1 was as follows: Table 2. Material list of microcapsules with different core-shell ratios. The 40.0 g of urea and 54.0 g of formaldehyde solution were uniformly stirred with a magnetic stirrer. A corresponding amount of triethanolamine was added to adjust the pH to 8.0. The shell material was obtained after stirring the mixture for 70 min at 350 rpm and at a constant temperature of 75 • C. The emulsion of the core material was obtained by adding 25.0 g of waterborne acrylic resin into a mixture made of 1.8 g of sodium dodecyl benzene sulfonate (SDBS) and 193.0 g of distilled water (well-stirred previously) under vigorous stirring (1200 rpm) for 35 min at 60 • C. Then, the shell material solution was added into the core material emulsion, and the pH was adjusted to 2.0 by adding CAM under a stirring speed of 300 rpm for 2.5 h at 65 • C. The reaction mixture was kept at rest for 2 days at room temperature and then filtered, washed and dried to obtain the waterborne acrylic resin microcapsules with a core-shell ratio of 0.42:1.

Preparation of Waterborne Coatings
Seven types of microcapsules with different core-shell ratios (0.42:1, 0.50:1, 0.58:1, 0.67:1, 0.75:1, 0.83:1, 0.92:1) were prepared. Six kinds of coating films were prepared for each type of microcapsule. The mass fraction of microcapsules in the paint films was 0%, 5.0%, 10.0%, 15.0%, 20.0% and 25.0%, respectively. The materials required for the preparation of different core-shell ratio paint films are collected in Table 3. The microcapsules and waterborne coatings were mixed evenly and then spread on Tilia europaea substrate with a ZBQ tetrahedral preparator (Pushen Chemical Machinery Co., Ltd., Shanghai, China). Then the Tilia europaea, coated with water-based paint, was dried in a ventilated place for 25 min, then lightly polished with sandpaper until the paint film surface was smooth [28]. The above steps were repeated twice to obtain the paint film samples of about 60.0 microns thick required for further testing. Table 3. Composition of self-healing waterborne topcoat.

Testing and Characterization
The gloss of the paint film at 60 • was tested by the WG-60G glossmeter (Shenzhen Baoan Sannuo Instrument Supplier, Shenzhen, China); the color difference of the paint film was tested by the WR-10 portable colorimeter (Shenzhen Baoan Sannuo Instrument Supplier, Shenzhen, China), and the chromaticity values of two points on the paint film were comparatively assessed. The chromaticity values of one point on the paint film were denoted as L 1 , a 1 , b 1 , c 1 and H 1 , and the chromaticity values of the second point were denoted as L 2 , a 2 , b 2 , c 2 and H 2 [29]. The color difference (∆E) was calculated according to The hardness of paint films was tested by QHQ pencil hardness tester (Shenzhen Baoan Sannuo Instrument Supplier, Shenzhen, China), according to the hardness sequence of 6B-6H. The tip of the pencil was ground flat; the paint film was fixed on the stage, and the pencil was inserted into the hardness tester, so that the pencil abutted on the paint film. The pencil hardness tester was moved slowly to observe the marks left behind by the pencil on the paint film. The hardness of the pencil with no scratching signs on the tested surface was considered the hardness of paint film.
The adhesive force was measured by a BYK cross-cut tester (Shenzhen Baoan Sannuo Instrument Supplier, Shenzhen, China). The paint film was drawn horizontally and vertically at the same place with the cross-cut tester, then pasted with adhesive tape. The adhesive force of the paint film was measured according to the peeling area of the paint film.
The impact resistance was tested by a QCJ-120 film impactor (Shenzhen Baoan Sannuo Instrument Supplier, Shenzhen, China). The paint film was placed at the bottom of the impactor, and the impactor impacted the paint film from progressively increasing heights. The height of the impactor at which microcracks appeared on the surface was considered the impact resistance of the paint film. Sodium chloride, alcohol, detergent and red ink were used to test the liquid resistance of the paint film [30]. A filter paper soaked with these liquids taken individually was placed on the surface of the paint film and covered with a glass slide. One day later, the filter paper was removed, and the residual reagents on the surface of the paint film were wiped off. The change of the paint film was taken into account, and the color difference and gloss in the tested area were evaluated. According to the pollution of the paint film surface, the liquid resistance grade of paint film was determined, and the effects of these four solutions on paint film were compared. Liquid resistance grade 1 indicated that there was no obvious change on the surface of the paint film. Grade 2 indicated that there was slight discoloration or discontinuous marks on the surface of the paint film. Grade 3 indicated that there were obvious marks on the surface of the paint film. Grade 4 indicated that there were serious marks on the surface of the paint film, but the structure of the paint film had not been changed. Grade 5 indicated that the paint film was broken and the structure of the paint film was changed.
The surface morphology of prepared microcapsules and paint films was investigated by an EVO18 scanning electron microscope (Changzhou Jonah Electromechanical Technology Co., Ltd., Changzhou, China), while the chemical composition of the paint film was studied by a BFH-960 Fourier transform spectrometer (Guangzhou Biaoji Packaging Equipment Co., Ltd., Guangzhou, China). All the experiments were carried out four times, and the experimental error was within 5.0%.

Effect of Microcapsule on Optical Properties of Paint Films
The results of color difference of waterborne topcoats with different core-shell ratios are plotted in Figure 1. The color differences of paint films with different core-shell ratios were basically the same, except for the color difference of paint film with a 0.92:1 core-shell ratio, which was significantly greater than those corresponding to other ratios. However, as the microcapsule content increased, the color difference ascended at the same core-shell ratio. The color difference of the paint film with the microcapsule concentration of 0-10.0% was small. The effect of the concentration of microcapsules with different core-sh the gloss of the paint film at 60° incident angle is shown in Figure 2. The pai was negatively correlated with the concentration of microcapsules. When th  The effect of the concentration of microcapsules with different core-shell ratios on the gloss of the paint film at 60 • incident angle is shown in Figure 2. The paint film gloss was negatively correlated with the concentration of microcapsules. When the concentration increased from 0% to 5.0%, the gloss decreased steeply. When the concentration continued to increase, the gloss also showed a downward trend but with a lower decreasing rate. The reason was that the surface of paint film without microcapsules was smooth. After the addition of microcapsules, the number of particles increased, resulting in the enhancement of diffuse reflection on the surface of the paint film [31]. There was no significant difference in the overall gloss of the paint films with different core-shell ratios over the entire range of microcapsule concentration. The effect of the concentration of microcapsules with different core-sh the gloss of the paint film at 60° incident angle is shown in Figure 2. The pai was negatively correlated with the concentration of microcapsules. When th tion increased from 0% to 5.0%, the gloss decreased steeply. When the co continued to increase, the gloss also showed a downward trend but with creasing rate. The reason was that the surface of paint film without microc smooth. After the addition of microcapsules, the number of particles increas in the enhancement of diffuse reflection on the surface of the paint film [31 no significant difference in the overall gloss of the paint films with differe ratios over the entire range of microcapsule concentration.

Effect of Microcapsule on Mechanical Properties of Paint Films
The hardness of paint films with different core-shell ratios of micr shown in Table 4. This quantity increased with the increase of microcapsul tion in a certain range at the same core-shell ratio. The overall hardness of th at the 0.42:1, 0.50:1, 0.58:1 and 0.67:1 core-shell ratio was higher than that of t core-shell ratios. At 0.75:1 and 0.92:1 core-shell ratios, as the microcapsule co

Effect of Microcapsule on Mechanical Properties of Paint Films
The hardness of paint films with different core-shell ratios of microcapsules is shown in Table 4. This quantity increased with the increase of microcapsule concentration in a certain range at the same core-shell ratio. The overall hardness of the paint films at the 0.42:1, 0.50:1, 0.58:1 and 0.67:1 core-shell ratio was higher than that of the last three core-shell ratios. At 0.75:1 and 0.92:1 core-shell ratios, as the microcapsule concentration reached 20.0-25.0%, the paint film hardness decreased. The main reason could have been that the microcapsules were agglomerated in the coatings, which reduces the interfacial bonding between the paint film and the microcapsules. Based on the results of gloss and color difference, the hardness of the paint film was better when the core-shell ratio was small and the microcapsule concentration ranged within 10.0%-15.0%.  Table 5 shows the adhesion test results of waterborne topcoats with variable percentages of microcapsules deposited on Tilia europaea. The data proved that the adhesion of the paint film decreased with microcapsule concentration, which was due to the poor bonding interaction acting at the interface between the paint film and Tilia europaea. The adhesion was good enough at the core-shell ratio of 0.42:1, 0.50:1, 0.58:1 and 0.67:1. When the core-shell ratio was relatively larger, the adhesion became obviously poorer. This is because the high core-shell ratio of the microcapsules (0.75:1, 0.83:1, 0.92:1) induced the formation of microcapsule aggregates, which made spreading the topcoat onto substrate into a uniform film very difficult. Comparatively, when the microcapsule concentration was 0-10.0%, the adhesion of the paint film towards the substrate was the best. The impact resistance results of the paint films are displayed in Table 6. The data indicate that the paint films with 0.58:1 and 0.67:1 core-shell ratios of microcapsules had better impact resistance. At the same core-shell ratios, the impact resistance increased at first and then decreased with the increase in microcapsule content. The main reason for this behavior lies in the weak adherence of the film onto the substrate when the mass fraction of microcapsules is low and the ability of the paint film surface to maintain its integrity under external impact is reduced [32]. Practically, the best impact resistance data were obtained for the paint films characterized by a microcapsule concentration of 10.0-15.0% at the same core-shell ratios of 0.58:1 and 0.67:1. Table 6. Effect of microcapsule concentration and core-shell ratio on the impact resistance of paint films. Based on the fact that the best data of hardness, adhesion and impact resistance have been obtained for the paint films with core-shell ratios of 0.42:1-0.67:1, irrespective of the mass fraction of microcapsules, the systems with the same compositional characteristics have been tested via elongation at break. It can be seen from Figure 3 that, at the same core-shell ratio, with the growing concentration of microcapsule, the elongation at break of the paint film first increased and then decreased, but the overall change range was small. However, the best data of the elongation at break were obtained for the paint films with a core-shell ratio of 0.58:1 and a microcapsule concentration of 10.0-15.0%.

Mass Fraction
teristics have been tested via elongation at break. It can be seen from Figure 3 that, at same core-shell ratio, with the growing concentration of microcapsule, the elongatio break of the paint film first increased and then decreased, but the overall change ra was small. However, the best data of the elongation at break were obtained for the p films with a core-shell ratio of 0.58:1 and a microcapsule concentration of 10.0-15.0%.

Effect of Microcapsule on the Liquid Resistance of Paint Films
The color difference values obtained after liquid resistance tests performed on p films shown in Table 7 were quite small, except for the red ink test. In this partic case, the color difference of the paint film markedly increased when the mass fractio microcapsules reached 25.0%. The color difference changed more greatly under larger core-shell ratio conditions. On the other hand, the paint film gloss essentially no change before and after the four kinds of liquid resistance experiments (see Table  Overall, the gloss decreased slightly after the red ink by comparison with the g measured after the other three types of liquid resistance tests. The results of paint liquid resistance level were determined and are displayed in Table 9. In the case of

Effect of Microcapsule on the Liquid Resistance of Paint Films
The color difference values obtained after liquid resistance tests performed on paint films shown in Table 7 were quite small, except for the red ink test. In this particular case, the color difference of the paint film markedly increased when the mass fraction of microcapsules reached 25.0%. The color difference changed more greatly under the larger core-shell ratio conditions. On the other hand, the paint film gloss essentially had no change before and after the four kinds of liquid resistance experiments (see Table 8). Overall, the gloss decreased slightly after the red ink by comparison with the gloss measured after the other three types of liquid resistance tests. The results of paint film liquid resistance level were determined and are displayed in Table 9. In the case of the 0.42:1 to 0.67:1 core-shell ratio, the liquid resistance level of the paint film to NaCl, ethanol and detergent was level 1 without any mark. When the core-shell ratio was above 0.75:1 and the microcapsule concentration was high, the liquid resistance level of the paint film to these three liquids was poor. With the increase in microcapsule content, the liquid resistance level of the paint film to NaCl, ethanol and detergent was almost the same, while the liquid resistance to red ink gradually deteriorated. Generally, when the core-shell ratio was small and the microcapsule concentration was 0-15.0%, the liquid resistance of the paint film was better. Table 7. Effect of microcapsule concentration and core-shell ratio on the color difference of painted surfaces after exposure to different liquids.   Table 9. Effect of microcapsule concentration and core-shell ratio on liquid resistance level.

Morphology Characterization and Chemical Composition Analysis
According to the above results of the optical properties, mechanical properties and liquid resistance of the paint films, the paint films with the microcapsules of 0.58:1 coreshell ratio have the best performance. The morphology of microcapsules with a core-shell ratio of 0.58:1 revealed a spherical shape and a relatively uniform particle size of 5 microns, as can be seen in Figure 4. core-shell ratio have the best performance. The morphology of microcapsules with a core-shell ratio of 0.58:1 revealed a spherical shape and a relatively uniform particle size of 5 microns, as can be seen in Figure 4. SEM images of the paint film with different microcapsule concentrations and the same core-shell ratio of 0.58:1 is inserted in Figure 5. The surface of the paint film without microcapsules was smooth ( Figure 5A). Instead, at low particle concentration, the film appearance indicates microcapsules are evenly distributed, but, at a high particle fraction of 25.0%, an aggregation phenomenon tends to prevail on the surface of the investigated paint films ( Figure 5B-D).  Figure 6 shows the FTIR spectra of the paint films with microcapsules of different concentrations and a core-shell ratio of 0.58:1. There was O-H and N-H antisymmetric tensile vibration and symmetric tensile vibration at 3360 cm −1 [33]. The stretching vibration absorption peak of saturated C-H was mainly near 2929 cm −1 and 2865 cm −1 [27]. The double peak stretching vibration peak of the C-N secondary amide bond was near 1639 cm −1 and 1447 cm −1 , and the bending vibration absorption peak of N-CO-N was at wave number 646 cm −1 , indicating that the wall material of microcapsule was urea formalde- SEM images of the paint film with different microcapsule concentrations and the same core-shell ratio of 0.58:1 is inserted in Figure 5. The surface of the paint film without microcapsules was smooth ( Figure 5A). Instead, at low particle concentration, the film appearance indicates microcapsules are evenly distributed, but, at a high particle fraction of 25.0%, an aggregation phenomenon tends to prevail on the surface of the investigated paint films ( Figure 5B-D). core-shell ratio have the best performance. The morphology of microcapsules with a core-shell ratio of 0.58:1 revealed a spherical shape and a relatively uniform particle size of 5 microns, as can be seen in Figure 4. SEM images of the paint film with different microcapsule concentrations and the same core-shell ratio of 0.58:1 is inserted in Figure 5. The surface of the paint film without microcapsules was smooth ( Figure 5A). Instead, at low particle concentration, the film appearance indicates microcapsules are evenly distributed, but, at a high particle fraction of 25.0%, an aggregation phenomenon tends to prevail on the surface of the investigated paint films ( Figure 5B-D).  Figure 6 shows the FTIR spectra of the paint films with microcapsules of different concentrations and a core-shell ratio of 0.58:1. There was O-H and N-H antisymmetric tensile vibration and symmetric tensile vibration at 3360 cm −1 [33]. The stretching vibration absorption peak of saturated C-H was mainly near 2929 cm −1 and 2865 cm −1 [27]. The double peak stretching vibration peak of the C-N secondary amide bond was near 1639 cm −1 and 1447 cm −1 , and the bending vibration absorption peak of N-CO-N was at wave number 646 cm −1 , indicating that the wall material of microcapsule was urea formalde-  Figure 6 shows the FTIR spectra of the paint films with microcapsules of different concentrations and a core-shell ratio of 0.58:1. There was O-H and N-H antisymmetric tensile vibration and symmetric tensile vibration at 3360 cm −1 [33]. The stretching vibration absorption peak of saturated C-H was mainly near 2929 cm −1 and 2865 cm −1 [27]. The double peak stretching vibration peak of the C-N secondary amide bond was near 1639 cm −1 and 1447 cm −1 , and the bending vibration absorption peak of N-CO-N was at wave number 646 cm −1 , indicating that the wall material of microcapsule was urea formaldehyde resin. The 1730 cm −1 was the characteristic peak of C=O of water-based acrylic resin. As the concentration changed, the peaks did not disappear, indicating that there was no difference in paint film composition with different microcapsule concentrations. hyde resin. The 1730 cm −1 was the characteristic peak of C=O of water-based acrylic As the concentration changed, the peaks did not disappear, indicating that there w difference in paint film composition with different microcapsule concentrations.

Conclusions
The UF-coated waterborne acrylic resin microcapsule with the different core ratios and the same concentration had little effect on the overall color difference gloss of the paint film. When the core-shell ratio was 0.58:1-0.67:1 and the microca concentration was 10.0-15.0%, the hardness of the paint films was 2H-4H and th pact resistance was 12.0-15.0 kg·cm. The adhesion of the paint films was high a core-shell ratio of 0.42:1-0.67:1. When the core-shell ratio of microcapsules was and the microcapsule concentration was 10.0%, the color difference of the paint film 1.8, the gloss was 5.1, the hardness was 2H, the adhesion was grade 1, the impa sistance was 15.0 kg·cm, the elongation at break of the paint film was 16.7%, and th sistance to NaCl, ethanol and detergent was grade 1; at this time, the comprehe performance of waterborne paint film on Tilia europaea surface was better. The r provided the technical assistance for the development of self-healing waterborne coatings.
Author Contributions: Conceptualization, methodology, validation, resources, data manage supervision, X.Y.; Formal analysis, investigation, writing-review and editing, W.Z., Investi and writing-review, L.W. and X.Q. All authors have read and agreed to the published vers the manuscript.

Conclusions
The UF-coated waterborne acrylic resin microcapsule with the different core-shell ratios and the same concentration had little effect on the overall color difference and gloss of the paint film. When the core-shell ratio was 0.58:1-0.67:1 and the microcapsule concentration was 10.0-15.0%, the hardness of the paint films was 2H-4H and the impact resistance was 12.0-15.0 kg·cm. The adhesion of the paint films was high at the coreshell ratio of 0.42:1-0.67:1. When the core-shell ratio of microcapsules was 0.58:1 and the microcapsule concentration was 10.0%, the color difference of the paint film was 1.8, the gloss was 5.1, the hardness was 2H, the adhesion was grade 1, the impact resistance was 15.0 kg·cm, the elongation at break of the paint film was 16.7%, and the resistance to NaCl, ethanol and detergent was grade 1; at this time, the comprehensive performance of waterborne paint film on Tilia europaea surface was better. The results provided the technical assistance for the development of self-healing waterborne wood coatings.
Author Contributions: Conceptualization, methodology, validation, resources, data management, supervision, X.Y.; Formal analysis, investigation, writing-review and editing, W.Z., Investigation and writing-review, L.W. and X.Q. All authors have read and agreed to the published version of the manuscript.
Funding: This project was partly supported by the Natural Science Foundation of Jiangsu Province (BK20201386).