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Article

Effect of Two Pomelo Peel Flavonoid Microcapsules on the Performance of Waterborne Coatings on the Surface of Poplar Boards

by
Jinzhe Deng
1,2,
Tingting Ding
1,2 and
Xiaoxing Yan
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 2024, 14(8), 937; https://doi.org/10.3390/coatings14080937
Submission received: 27 June 2024 / Revised: 18 July 2024 / Accepted: 22 July 2024 / Published: 26 July 2024
(This article belongs to the Section Functional Polymer Coatings and Films)
Browse Figures
Versions Notes

Abstract

:
Two types of microcapsules were added to the coating separately. The specifications of the poplar board were 50 mm × 50 mm × 8 mm. The antibacterial rate of the poplar board surface gradually increased with the increase in the microcapsule content, and the antibacterial activity for Staphylococcus aureus was slightly higher than that against Escherichia coli. Influenced by the change in the wood grain’s color on the poplar board itself, both microcapsules had no significant effect on the chromaticity value and color difference of the poplar board surface, as well as the reflectance of the visible light band. The glossiness decreased with the increase in the microcapsule content, and the gloss loss rate increased with the increase in the microcapsule content. With the increase in the microcapsule content, the hardness of the poplar board surface in both groups increased slightly, and the roughness increased gradually. The adhesion of the poplar board surface coating with melamine-resin-coated pomelo peel flavonoid microcapsules was slightly reduced, and the impact resistance was not significantly affected. Chitosan-coated pomelo peel flavonoid microcapsules had no significant effect on the adhesion of the poplar board surface coating, and the impact resistance increased slightly when the content of microcapsules was higher. Comprehensively, the poplar board coating with 9.0% chitosan-coated pomelo peel flavonoid microcapsules had a better overall performance, with antibacterial activities for Escherichia coli and Staphylococcus aureus of 70.6% and 77.6%, respectively. The color difference was 6.70, the gloss loss rate was 53.9%, the reflectivity was 50.60%, the hardness was H, the adhesion was grade 1, the impact resistance was grade 2, and the roughness was 2.10 μm. The results provide technical references for the application of antibacterial microcapsules of pomelo peel flavonoids on the surface of wood materials.

1. Introduction

Furniture, as an essential product in daily life, has never ceased to be explored by people [1,2,3,4,5,6]. Wood plays an important role in furniture manufacturing [7,8,9]. The production speed of poplar wood is fast, the material is moderate, the wood is tough and corrosion-resistant, and it has strong stability, making it a commonly used raw material for wood furniture [10,11,12]. However, poplar wood has problems such as a loose fiber structure and difficulty drying, leading to bacterial growth [13,14,15]. These issues result in higher requirements for an antibacterial effect on the surface of poplar boards. Therefore, poplar wood is often modified or surface-coated to improve its properties [16,17,18]. Waterborne coatings have been widely used in the market because of their low volatile organic compound contents, convenient construction, and good weather resistance [19,20,21]. Surface coatings are important carriers of bacterial transmissions, so improving the antibacterial properties of coatings can effectively block bacterial transmission [22,23,24]. The use of special microcapsule technology can encapsulate some natural antibacterial agents or antibacterial substances to make antibacterial microcapsules, improve the processing performance of antibacterial agents, enhance the stability of natural antibacterial agents, and improve their performance, expanding their application range [25].
Melamine resin is a white powder monoclinic crystal with good interfacial compatibility with waterborne coatings [26,27], and it is therefore often used as a wall material for microcapsules. Melamine resin has higher thermal stability, higher hardness, and good mechanical properties, but there is the possibility of releasing formaldehyde [28,29]. This will cause certain harm to the environment, and the antibacterial rate of microcapsules prepared is limited. Therefore, in addition to the commonly used melamine resin, chitosan, with antibacterial and nontoxic properties, is chosen as the wall material for microcapsules. Chitosan is a kind of product mainly extracted from the chitin of shrimp, crab, and other shells, and it has received widespread attention due to its applications in various fields [30,31,32]. Kumar et al. [33] developed boric acid cross-linked chitosan microcapsules loaded with frankincense oil and applied them to cotton using a rolling drying method. The release rate, encapsulation efficiency, microcapsule yield, and functional characteristics of the finished fabric were studied. The experimental results show that the finished fabric had antibacterial activity, with an antibacterial rate of 88.69% against Escherichia coli and 94.5% against Staphylococcus aureus. The flavonoid compounds selected for the core material are widely distributed in plants, and most of them exist in the form of sugar ligands when combined with glycosides. Zhu et al. [34] established effective parameters for extracting flavonoids from eucommia ulmoides pollen, characterized the flavonoid compositions of the extraction material, and explored their biological activity. The results indicate that the extract has strong inhibitory effects on both Escherichia coli and Staphylococcus aureus. Pomelo peel is a rich and inexpensive fruit waste that contains abundant flavonoids and is an ideal raw material for extracting flavonoids [35]. The flavonoids in pomelo peel have broad-spectrum antibacterial effects and can effectively inhibit the growth of various bacteria and fungi. The production process is simple and has broad application prospects [36]. Because of the fact that pomelo peel flavonoids are, generally, in irregularly shaped yellow crystalline states and have large particles, they are not suitable for direct addition to coatings. Therefore, pomelo peel flavonoids can be prepared into microcapsules and added to waterborne coatings through microcapsule technology.
The application of melamine-resin-coated pomelo peel flavonoid microcapsules and chitosan-coated pomelo peel flavonoid microcapsules in waterborne coatings, using poplar boards as substrates, was explored. By controlling the content of the microcapsules in the surface coating on the poplar boards, the influence of different microcapsule contents on the morphology, chemical composition, antibacterial performance, optical properties, and mechanical properties of the surface of a poplar board was analyzed to ensure that the surface coating on the poplar board had good properties while increasing the antibacterial performance. In order to solve the problem of flavonoids extracted from pomelo peel being crystalline solids and difficult to directly add to waterborne coatings, microcapsules were prepared by microcapsule technology, and their original antibacterial properties were successfully retained. Increasing the antibacterial properties of waterborne coatings used on wood substrates has potential application value.

2. Materials and Methods

2.1. Experimental Materials

The required materials for this experiment are shown in Table 1. The degree of deacetylation of chitosan ranges from 80.0% to 95.0%. The pomelo used in the experiment was Shatian pomelo, from Yulin, China. The specifications of the poplar boards were 50 mm × 50 mm × 8 mm, and they were purchased from Yong’an He’an Industry and Trade Co., Ltd., Sanming, China. The primer and topcoat were waterborne acrylic coatings provided by Jiangsu Haitian Technology Co., Ltd., Nanjing, China. The nonvolatile content of the waterborne acrylic coatings was 30.0%–35.0%.

2.2. Preparation of Microcapsules

(1)
Preparation of melamine-resin-coated pomelo peel flavonoid microcapsules
Preparation of microcapsule wall materials from melamine resin: First, 19.30 g of a 37% formaldehyde solution and 10.0 g of melamine were mixed homogeneously according to the molar ratio of 3:1 and then added into a beaker with deionized water, and the pH was adjusted to 8 with triethanolamine; a magnetic stirrer was placed in the beaker, and then the beaker was put into an 80 °C constant temperature water bath with constant stirring at 800 rpm for 40 min to obtain the prepolymerization of the wall materials of the melamine resin and then cooled down to the room temperature to standby.
Preparation of core emulsion from pomelo peel extract: Fresh pomelo was washed and peeled, dried in an oven until constant weight, and then ground into powder. The powder was extracted by adding anhydrous ethanol at a material–liquid ratio of 1:10 for 48 h and then heated in a constant temperature water bath at 60 °C for 6 h. The pomelo peel extract was filtered using a vacuum filtration machine and then evaporated and concentrated by a rotary evaporator. The extract was freeze-dried in a freeze-dryer, and the crude flavonoids extracted from the pomelo peel were obtained as pomelo flavonoids. A certain mass of deionized water was weighed and added to a certain mass of sodium dodecylbenzene sulfonate to prepare an emulsifier solution. Then, 2.06 g of pomelo peel flavonoid was dissolved in a certain mass of ethanol to obtain a core solution with a concentration of 10%. The core solution was added to the beaker containing an emulsifier solution; a magnetic stirrer was added, and the reaction was continuously stirred for 40 min at 600 rpm in the constant temperature water bath at 60 °C to obtain the core emulsion.
Microcapsule encapsulation: The prepolymerized wall materials of melamine resin were slowly added drop by drop to the beaker with the core emulsion. Citric acid monohydrate was used to adjust the pH value of the solution to 3, and the solution was heated in a constant temperature water bath for 3 h. The resulting solution was left at room temperature for 36 h. It was rinsed by filtration through a vacuum filtration machine, using ethanol and water. The obtained product was put into an oven at 50 °C, dried for 24 h, and then ground, and the resulting powder was melamine-resin-coated pomelo peel flavonoid microcapsules.
(2)
Preparation of chitosan-coated pomelo peel flavonoid microcapsules
The ratio of the oil phase to the water phase in the mixed system of microcapsule preparation was 2:1.
Preparation of water-phase solution: A certain mass of acetic acid was added to deionized water to formulate a 1% acetic acid solution. Then, 0.80 g of chitosan was dissolved in 39.20 g of 1% acetic acid solution in a beaker. The beaker was placed in a constant temperature water bath at 60 °C. A magnetic stirrer was added to the beaker, and the mixture was stirred continuously at 600 rpm for 1 h. The resultant solution was a 2 wt% chitosan solution, which was set aside as a wall solution for microcapsules.
Preparation of oil-phase solution: A certain mass of Tween-80 was weighed and added to deionized water to prepare an emulsifier solution. Then, 0.80 g of pomelo peel flavonoids was dissolved in anhydrous ethanol to formulate a core solution with a concentration of 10%. The core solution was added to the beaker containing the emulsifier solution. The beaker was put into a magnetic stirrer, and the reaction was continuously stirred for 40 min at 600 rpm in a 60 °C constant temperature water bath to obtain the core emulsion.
The cross-linking reaction: The core solution was added to the wall emulsion. A 0.5 mol/L NaOH standard solution was used to adjust the pH of the mixed solution. A certain mass of sodium tripolyphosphate was added to a beaker for a physical cross-linking reaction with chitosan wall materials. The beaker was placed in a constant temperature water bath at 60 °C and the mixed solution was stirred continuously at 600 rpm for 3 h. The solution precipitated from the mixing reaction was filtered and placed in a freeze dryer, freeze-dried for 48 h, and then milled. The resulting powder was the chitosan-coated pomelo peel flavonoid microcapsule powder.

2.3. Coating Preparation Method

A hand-finishing method was used to finish the poplar boards. Two layers of primer and two layers of topcoat were applied. A coating preparer was used to control the thickness of each layer of coating to 80 μm. The total amount of coating was 320 g/m2, and each layer was 80 g/m2. According to the area of the poplar board, the average total amount of the coating used was 1.44 g per poplar board and 0.36 g per layer. Since bacteria usually attach to the coating surface first, microcapsules were added to the topcoat, and no microcapsules were added to the primer. The materials used for the different microcapsule contents of coatings are shown in Table 2.
The total mass of the waterborne coating was controlled to be constant, and two types of microcapsules were added into the waterborne topcoat at different contents (0%, 1.0%, 3.0%, 5.0%, 7.0%, and 9.0%). The surface of the poplar board was first sanded with 500 grit sandpaper to keep the surface of the substrate clean, smooth, and flat. Then, the primer was evenly applied to the surface of the poplar boards with a brush. After the first coat of primer was applied, the boards were left at room temperature for 30 min to allow the primer to level off, and then the boards were placed in an oven at 50 °C to cure the coating. After the coating was fully cured, it was sanded with sandpaper, and then the second coat of primer was applied again with the same drying and sanding steps. The topcoat was then applied, and topcoats with different contents of microcapsules were applied twice with the same steps as described above. After the second topcoat was dried and completed, the poplar boards were taken out of the oven and placed at room temperature to dry for 12 h, and the substrate was finished with the coating.

2.4. Testing and Characterization

2.4.1. Morphology and Chemical Composition Testing

A scanning electron microscope (SEM) was used to characterize the microscopic morphology of the coating. A Fourier transform infrared spectrometer (FTIR) was used to test the chemical composition of the microcapsules and the coating. For testing microcapsules, a powder compactor was used to make thin slices of microcapsule powder.

2.4.2. Antibacterial Testing

The antibacterial properties of the waterborne coatings were tested according to GB/T 21866-2008 [37]. For antibacterial tests, Escherichia coli (ATCC25922) and Staphylococcus aureus (ACTT6538) were selected. A nutrient agar medium powder was added into purified water, heated at 121 °C, dissolved, and sterilized for 30 min. The mixture was poured into a Petri dish and cooled to make a flat nutrient agar medium for use. Fresh strains stored on an inclined plane were transferred to the flat nutrient agar medium using inoculation rings and cultured in a constant temperature and humidity chamber with a temperature of 37 °C and relative humidity of 95% for 18 h.
Nutrient broth powder was added into purified water, sterilized at 121 °C for 30 min, heated, and dissolved, and a broth culture liquid was prepared for use. Sodium chloride was used to prepare an eluent with a concentration of 0.85%. According to GB/T 4789.2-2022 [38], 1–2 rings of fresh bacteria were scraped from the flat nutrient agar medium using the inoculation ring and added to the broth culture liquid. Then, a bacterial suspension was diluted with water to a concentration of 106 CFU/mL for use.
About 0.5 mL of the bacterial suspension was added to the surface coating on a poplar board, and a sterilized plastic film was applied on the surface of the poplar board without bubbles. The treated sample was placed in a sterilized Petri dish and placed in a constant temperature and humidity chamber at 37 °C and a relative humidity of 95% for 24 h. The sample was taken out and rinsed with 20 mL eluent. The rinsed solution was inoculated in the flat nutrient agar medium and incubated in a constant temperature and humidity chamber for 48 h. Three sets of parallel tests were performed for each sample. Finally, a live bacteria count was carried out, and the flat nutrient agar medium was put onto the colony counter for the counting of the colonies. An average value of the three parallel tests was recorded as the number of colonies in the sample. The antibacterial rate R of the coating was calculated according to Formula (1). Here, B represents the average number of recovered colonies after 48 h of coating without microcapsules, and C represents the average number of recovered colonies after 48 h of coating with microcapsules, in CFU/piece.
R = ( B − C ) / B × 100 %

2.4.3. Optical Performance Testing

The color difference of the coating was tested by a portable color difference meter according to GB/T 11186.3-1989 [39]. After the color difference meter calibration, three points were randomly selected for each sample to determine the L, a, and b colorimetric values, and the average value was recorded as the color of the film. The L value represents the brightness of the measured sample, the a value represents the red and green color change, and the b value represents the yellow and blue color change. The values of the sample without microcapsules were denoted as L1, a1, and b1, and the values of the coating containing microcapsules were denoted as L2, a2, and b2; the color difference ΔE of the coating was calculated by Formula (2). Here, ΔL = L2 − L1, Δa = a2 − a1, and Δb = b2 − b1.
Δ E = Δ L 2 + Δ a 2 + Δ b 2 1 / 2
A gloss meter was used to test the glossiness of the coating according to GB/T 4893.6-2013 [40]. The glossiness of the coating was recorded at incidence angles of 20°, 60°, and 85°. The glossiness of the coating without microcapsules is denoted as G0, the glossiness of the coating with microcapsules is denoted as G1, and the gloss loss rate of the coating at a 60° incidence angle is denoted as GL. The calculation formula is shown in Formula (3).
G L = ( G 0 − G 1 ) / G 0 × 100 %
An ultraviolet spectrophotometer was used to measure the light transmittance of coating in the visible band with a wavelength range of 380–780 nm. When a beam passed through the coating, the ratio of the remaining light intensity to the incident light intensity was the transmittance.

2.4.4. Mechanical Performance Testing

The hardness of the coating was tested according to standard GB/T 6739-2022 [41] using a portable coating hardness tester. A pencil was inserted into the portable coating hardness tester at a 45° angle, and when the pencil scratched the sample and created a permanent indentation, the surface hardness of the coating was recorded.
The adhesion of the substrate surface was tested using a scriber according to the standard GB/T 4893.4-2013 [42]. The sample was placed on an operating table, a hand held the handle of the scriber, the scriber was perpendicular to the surface of the substrate to cut the surface of the substrate, and then the substrate was rotated 90° for cutting. A piece of tape was attached to the gripper and torn off, and the adhesion grade of the coating was assessed according to the situation of the coating falling off.
The impact resistance of the surface coating was tested by a coating impactor according to the standard GB/T 4893.9-2013 [43], and the impact resistance grade was evaluated. The sample was placed on a horizontal base, and the impact height was 50 mm from the upper surface of the steel ball to the lower surface of the impact block. Under natural light, a cracking of the coating was observed with a magnifying glass. Each sample was impacted with 5 parts. Firstly, the numerical grade of each impact part with the same impact height was evaluated, and then the integer closest to the arithmetic average was taken as the final evaluation result.
A roughness tester was used to test the roughness of the coating. The measured coating sample was placed on a testing table, a diamond stylus was adjusted to contact the sample surface, and the surface roughness value of the coating was recorded. The roughness unit was µm.

3. Results and Discussion

3.1. Morphology Analysis of Surface Coating on Poplar Boards

The microscopic morphology of the melamine-resin-coated pomelo peel flavonoid microcapsules and the chitosan-coated pomelo peel flavonoid microcapsules is shown in Figure 1. The melamine-resin-coated pomelo peel flavonoid microcapsules were uniform and smooth spheres, and each microcapsule sphere was independent of the others. The chitosan-coated pomelo peel flavonoid microcapsules also formed spheres, but the adhesion between spheres was observed obviously.
As shown in Figure 2, the particle size of the melamine-resin-coated pomelo peel flavonoid microcapsules was mainly 1–6 μm, and that of the chitosan-coated pomelo peel flavonoid microcapsules was mainly 1–7 μm. The particle size distribution of the two microcapsules was compared. The particle size distribution of the microcapsules coated with melamine resin was more concentrated, and the particle size of the microcapsules coated with chitosan was more dispersed.
The macroscopic morphology of the waterborne coating with two types of microcapsules with different contents on the poplar board surface is shown in Figure 3 and Figure 4, respectively. The transparent waterborne coating without the microcapsules has no effect on the color of the poplar board surface. The poplar color itself is lighter, and the poplar board surface with the addition of melamine-resin-coated pomelo peel flavonoid microcapsules in Figure 3 shows obvious granular microcapsules. When the microcapsule content is 7.0% and 9.0%, the microcapsule particles tend to agglomerate more. These results indicated that the dispersion of the melamine-resin-coated pomelo peel flavonoid microcapsules in the surface coating on the poplar boards was poor and uneven, and the higher content of the melamine-resin-coated pomelo peel flavonoid microcapsules would affect the aesthetics of the poplar boards in actual use. The poplar board surface coated with the chitosan-coated pomelo peel flavonoid microcapsules was flat, and there were no obvious particles, which indicated that the chitosan-coated pomelo peel flavonoid microcapsules had good dispersion in the coating on the poplar board and would not affect the appearance on the poplar board surface. This is because chitosan has good hydrophilicity; the microcapsules with chitosan as the wall material aggregate slowly and have good dispersibility in the waterborne coatings using water as the solvent. The melamine-resin-coated pomelo peel flavonoid microcapsules use melamine resin as the wall material. Compared to the chitosan, the melamine resin has lower water absorption, so it aggregates more significantly in the waterborne coatings than the microcapsules with chitosan as the wall material.
Figure 5 shows the microscopic morphology on the poplar board surface coating without microcapsules, with 7.0% melamine-resin-coated pomelo peel flavonoid microcapsules, and with 7.0% chitosan-coated pomelo peel flavonoid microcapsules. The poplar board surface without and with microcapsules is wrinkled, and the poplar board surface without microcapsules is less wrinkled, which is due to the roughness of the poplar board itself and the scratches left by the brush. The poplar board surface with melamine-resin-coated pomelo peel flavonoid microcapsules was the most uneven; the convex and wrinkled feeling was obvious. This indicates that the dispersion of the melamine-resin-coated pomelo peel flavonoid microcapsules on the surface of the poplar board is poor. The surface coating with the chitosan-coated pomelo peel flavonoid microcapsules was relatively flat, and the feeling of concave and convex folds was low. The results indicated that the chitosan-coated pomelo peel flavonoid microcapsules had better dispersion on the surface of the poplar board and were more suitable for practical application.
Figure 6 shows the microscopic morphology of the cross-section between the poplar board surface coating and the poplar board interface. The coating of poplar board surface primer has a certain sealing and isolation effect on the wood surface. There is no obvious penetration of a large number of microcapsules. The microcapsules are added to the topcoat, which can be used to inhibit the bacteria attached to the surface of wood products and exert a certain antibacterial effect on the surface of wood products.

3.2. Chemical Composition Analysis of Surface Coating on Poplar Boards

Figure 7 shows the infrared spectra of the poplar board surface coating without microcapsules and the poplar board surface coating with two different microcapsules. Absorption at 1726 cm−1 in all three infrared spectra is the characteristic peak of C=O in the waterborne acrylic resin, and the vibration peak of C–O is around 1144 cm−1. In the coating with melamine-resin-coated pomelo peel flavonoid microcapsules, the bending vibration absorption peak of the triazine ring in the melamine resin appeared at 813 cm−1 [44]. In the coating with chitosan-coated pomelo peel flavonoid microcapsules, the absorption peak of C–O–C in the chitosan structure appeared around 1088 cm−1. The absorption peak of CH2– was at 2932 cm−1, and the absorption peak at about 3390 cm−1 was formed by hydroxyl association [45]. The occurrence of these peaks proves that after the two microcapsules are added to the waterborne coatings, they do not react with the components of the waterborne coatings to form new substances, so they can be used in waterborne coatings.

3.3. Antibacterial Property Analysis of Surface Coating on Poplar Boards

Figure 8 shows the trend of the antibacterial rate on the poplar board surface coating against Escherichia coli and Staphylococcus aureus. The antibacterial rate on the poplar board surface coating against Staphylococcus aureus was higher than that of Escherichia coli, and the antibacterial rate gradually increased with the increase in microcapsule content. When the content of the melamine-resin-coated pomelo peel flavonoid microcapsules reached 9.0%, the antibacterial rates on the poplar board surface coating against Escherichia coli and Staphylococcus aureus could reach 66.3% and 71.0%, respectively. When the content of the chitosan-coated pomelo peel flavonoid microcapsules reached 9.0%, the antibacterial rate on the poplar board surface coating against Escherichia coli was 70.6%, and the antibacterial rate against Staphylococcus aureus was 77.6%. These results proved that the two microcapsules could exert antibacterial effects on the poplar board surface coating and improve the antibacterial rate of the coating, and the antibacterial effect of the microcapsules prepared with chitosan as the wall material on the surface coating on poplar boards was better than that of the microcapsules prepared with melamine resin.

3.4. Optical Property Analysis of Surface Coating on Poplar Boards

Table 3 shows the chromaticity values and the color differences on the poplar broad surfaces with different contents of the two types of microcapsules added. The poplar board was made of natural wood, and the color of the wood grain itself had a big difference. The waterborne coating used was transparent and colorless and had no effect on the color of the poplar board. The color of the microcapsules is closer to the color of the poplar board, so there is no obvious rule for the effect of the content of microcapsules on the surface coatings of the poplar board for the chromaticity difference between the two groups of poplar board surfaces. The a-value representing the red–green value and the b-value representing the yellow–blue value of the surface coatings on the poplar boards in both groups are positive, indicating that the color of the poplar board surface is reddish and yellowish. However, affected by the color change in the wood grain itself, the effect of the content of the microcapsules on the chromaticity value of the poplar board surface also has no obvious law, and the overall value change is small. Therefore, the different contents of the two types of microcapsules have no significant effect on the chromaticity value and color difference of the poplar board surface.
Table 4 shows the glossiness and gloss loss rate of poplar board surfaces with different contents of two types of microcapsules. The gloss of poplar board surface in both groups under different incidence angles decreases with the increase in microcapsule content, while the gloss loss rate of poplar board surface under 60° incidence angle increases gradually with the increase in microcapsule content. This is because the increase in microcapsule content increases the number of granular microcapsules on the poplar board surface, which makes the surface of the poplar board rougher and exacerbates the diffuse reflection phenomenon. With the increase in microcapsule content, the light loss rate of the poplar board surface coating with the addition of melamine-resin-coated pomelo peel flavonoid microcapsules increased and varied greatly. This is due to the poor dispersibility of the melamine-resin-coated pomelo peel flavonoid microcapsules, which are unevenly distributed and agglomerate significantly in the surface coating on poplar boards. The surface of the poplar board is relatively rough, and the diffuse reflection phenomenon is more severe. Therefore, the increase in microcapsule content causes a significant change in the surface gloss loss rate of the poplar board. The increase in the gloss loss rate of poplar board surface with the addition of chitosan-coated pomelo peel flavonoid microcapsules is smaller. This is because the chitosan-coated pomelo peel flavonoid microcapsules have better dispersion. The poplar board surface is flatter, with no obvious particles, and the diffuse reflection phenomenon is smaller, so the gloss loss rate is smaller.
Table 5 and Figure 9 show the reflectivity values and reflectivity curves of the visible light band on the surface of poplar boards with two types of microcapsules added at different concentrations, respectively. The reflectivity of the visible light band of the poplar board surface in both groups decreased slightly with the increase in microcapsule content. But the spacing between the two groups of curves is very close, and the difference in value is small. This indicates that the effect of the two types of microcapsules on the reflectivity of poplar board surface coating is not significant. This is because of the influence of the color on the poplar board. The poplar board has different stripe color changes, which will affect the visible light band reflectivity on the poplar board. Therefore, the different contents of the two microcapsules do not have a significant effect on the visible light reflectivity of the poplar board.
The optical properties of poplar board surfaces with different contents of the two microcapsules were compared. Influenced by the wood grain color of the poplar board itself, the different contents of the two microcapsules had no significant effect on the chromaticity value and color difference of the poplar board surface as well as the reflectivity in the visible light band. The glossiness decreased with the increase in microcapsule content, while the gloss loss rate increased with the increase in microcapsule content. The surface of the poplar boards with added chitosan-coated pomelo peel flavonoid microcapsules was flatter, with less diffuse reflection, and the increase in the value of the gloss loss rate of the poplar board surface had a smaller change.

3.5. Mechanical Property Analysis of Surface Coating on Poplar Boards

Table 6 shows the effect of different contents of two types of microcapsules on the mechanical properties of poplar boards. The hardness of the poplar board surface without microcapsules was lower for B. The hardness of the poplar board surface in both groups increased slightly with the increase in microcapsule content. This is because the addition of the microcapsules increased the density of the waterborne coating and improved the coating hardness. When the content of melamine-resin-coated pomelo peel flavonoid microcapsules reached 7.0%, the highest hardness was H. When the content of chitosan-coated pomelo peel flavonoid microcapsules reached 9.0%, the highest hardness was H. The hardness of the poplar board surface in both groups of waterborne coatings increased slightly with the addition of microcapsules.
The adhesion of the poplar board surface coating was grade 1 when the content of melamine-resin-coated pomelo peel flavonoid microcapsules did not exceed 5.0%. The adhesion grade of the poplar board surface coating was reduced to grade 2 when the content of microcapsules exceeded 5.0%. This is because the melamine-resin-coated pomelo peel flavonoid microcapsules have poor dispersibility. When the microcapsule content is too high, the microcapsules will form more serious agglomeration in the coating. The poplar board itself is relatively rough, and the addition of microcapsules reduces the adhesion between the coating and the poplar board. The adhesion grades of the poplar board surface coatings with different contents of the chitosan-coated pomelo peel flavonoid microcapsules were all grade 1, which indicated that the content of the chitosan-coated pomelo peel flavonoid microcapsules of 9.0% and below would not affect the adhesion of the poplar board surface coatings, and it was suitable for application in practice.
The impact resistance of the poplar board surfaces with different contents of the melamine-resin-coated pomelo peel flavonoid microcapsules was the same as that of the poplar board surfaces without microcapsules, and there were 1–2 circles of slight cracks on the coated surfaces. This suggests that the coatings with 9.0% or less of the melamine-resin-coated pomelo peel flavonoid microcapsules do not significantly affect the impact resistance of poplar board surfaces. When the content of the chitosan-coated pomelo peel flavonoid microcapsules was 1.0%–5.0%, the impact resistance on the poplar board surface was grade 3. When the content of microcapsules was 7.0% and 9.0%, the impact resistance on the poplar board surface was grade 2, there were no cracks on the surface of the coating, but impact marks could be seen, which was because the chitosan-coated pomelo peel flavonoid microcapsules were less agglomerated, and it was easier to add them to the aqueous coatings. Uniformly dispersed, when the poplar board surface is impacted by an external force, the microcapsule particles in the coating can disperse part of the impact force, so the impact resistance of the poplar board surface is improved.
The roughness of the poplar board surface without microcapsules was 0.37 μm. The roughness on the poplar board surface increased gradually with the increase in microcapsule content. The roughness values of the surface coatings on the poplar boards with the addition of melamine-resin-coated pomelo peel flavonoid microcapsules were higher than those with chitosan-coated pomelo peel flavonoid microcapsules. This is due to the poor dispersibility of the melamine-resin-coated pomelo peel flavonoid microcapsules, which are more likely to form large agglomerations in the poplar board surface coating, resulting in an increase in the roughness of the poplar board surface.

4. Conclusions

As the content of the microcapsules increased, the antibacterial rates of both groups of poplar board surface coatings against two types of bacteria gradually increased. The antibacterial rate against Staphylococcus aureus was higher than that against Escherichia coli. Because of the influence of the wood grain color of the poplar board itself, the different contents of the two microcapsules have no significant effect on the chromaticity value and color difference of the surface of the poplar board, as well as the reflectivity in the visible light band. The glossiness decreases with the increase in microcapsule content, while the gloss loss rate gradually increases. The surface hardness of both groups of poplar boards slightly increased, with the highest being H. When the content of the melamine-resin-coated pomelo peel flavonoid microcapsules exceeded 5.0%, it slightly reduced the adhesion of the poplar board surface coating, while the chitosan-coated pomelo peel flavonoid microcapsules had no significant effect on the adhesion of the surface coating on the poplar board. When the content of the chitosan-coated pomelo peel flavonoid microcapsules was high, the impact resistance grade of the surface on the poplar board was raised to grade 2. The surface roughness of both groups of poplar boards gradually increased with the increase in microcapsule content. When the content of the melamine-resin-coated pomelo peel flavonoid microcapsules was 5.0%, the surface coating on the poplar board showed excellent comprehensive performance: the antibacterial rate of the coating against Escherichia coli was 32.2%, the antibacterial rate against Staphylococcus aureus was 41.9%, the color difference was 3.69%, the gloss loss rate was 64.1%, the reflectivity was 60.13%, the adhesion grade was 1, the hardness was HB, the impact resistance grade was 3, and the roughness was 2.07 μm. When the content of the chitosan-coated pomelo peel flavonoid microcapsules was 9.0%, the comprehensive performance of the surface coating on the poplar board was better: the antibacterial rate of the coating against Escherichia coli was 70.6%, the antibacterial rate against Staphylococcus aureus was 77.6%, the color difference was 6.70, the gloss loss rate was 53.9%, the reflectivity was 50.60%, the adhesion grade was 1, the hardness was H, the impact resistance grade was 2, and the roughness was 2.10 μm. Comparing the effects of two types of microcapsules on the surface properties of poplar boards, it can be concluded that chitosan-coated pomelo peel flavonoid microcapsules with 9.0% content exhibit better comprehensive performance in the coating. While enhancing the antibacterial performance of the poplar board surface coating, they can also ensure that the coating has good mechanical and optical properties.

Author Contributions

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

Funding

This project was partly supported by Postgraduate Research and Practice Innovation Program of Jiangsu Province (SJCX24_0399) and by the Natural Science Foundation of Jiangsu Province (BK20201386).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that no conflicts of interest.

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Coatings 14 00937 g001
Figure 1. SEM images of the microcapsules: (A) melamine-resin-coated pomelo peel flavonoid microcapsules and (B) chitosan-coated pomelo peel flavonoid microcapsules.
Figure 1. SEM images of the microcapsules: (A) melamine-resin-coated pomelo peel flavonoid microcapsules and (B) chitosan-coated pomelo peel flavonoid microcapsules.
Coatings 14 00937 g001
Coatings 14 00937 g002
Figure 2. The particle size of the microcapsules: (A) melamine-resin-coated pomelo peel flavonoid microcapsules and (B) chitosan-coated pomelo peel flavonoid microcapsules.
Figure 2. The particle size of the microcapsules: (A) melamine-resin-coated pomelo peel flavonoid microcapsules and (B) chitosan-coated pomelo peel flavonoid microcapsules.
Coatings 14 00937 g002
Coatings 14 00937 g003
Figure 3. Macroscopic morphology on the poplar board surface with different contents of melamine-resin-coated pomelo peel flavonoid microcapsules added: (A) without microcapsules, (B) with 1.0% microcapsules, (C) with 3.0% microcapsules, (D) with 5.0% microcapsules, (E) with 7.0% microcapsules, and (F) with 9.0% microcapsules.
Figure 3. Macroscopic morphology on the poplar board surface with different contents of melamine-resin-coated pomelo peel flavonoid microcapsules added: (A) without microcapsules, (B) with 1.0% microcapsules, (C) with 3.0% microcapsules, (D) with 5.0% microcapsules, (E) with 7.0% microcapsules, and (F) with 9.0% microcapsules.
Coatings 14 00937 g003
Coatings 14 00937 g004
Figure 4. Macroscopic morphology on the poplar board surface with different contents of chitosan-coated pomelo peel flavonoid microcapsules added: (A) without microcapsules, (B) with 1.0% microcapsules, (C) with 3.0% microcapsules, (D) with 5.0% microcapsules, (E) with 7.0% microcapsules, and (F) with 9.0% microcapsules.
Figure 4. Macroscopic morphology on the poplar board surface with different contents of chitosan-coated pomelo peel flavonoid microcapsules added: (A) without microcapsules, (B) with 1.0% microcapsules, (C) with 3.0% microcapsules, (D) with 5.0% microcapsules, (E) with 7.0% microcapsules, and (F) with 9.0% microcapsules.
Coatings 14 00937 g004
Coatings 14 00937 g005
Figure 5. SEM images of poplar board surface: (A) without microcapsules, (B) with 7.0% melamine-resin-coated pomelo peel flavonoid microcapsules, and (C) with 7.0% chitosan-coated pomelo peel flavonoid microcapsules.
Figure 5. SEM images of poplar board surface: (A) without microcapsules, (B) with 7.0% melamine-resin-coated pomelo peel flavonoid microcapsules, and (C) with 7.0% chitosan-coated pomelo peel flavonoid microcapsules.
Coatings 14 00937 g005
Coatings 14 00937 g006
Figure 6. SEM images of the cross-section of the interface between the surface coating and the poplar board: (A) without microcapsules, (B) with 7.0% melamine-resin-coated pomelo peel flavonoid microcapsules added to the topcoat, and (C) with 7.0% chitosan-coated pomelo peel flavonoid microcapsules added to the topcoat.
Figure 6. SEM images of the cross-section of the interface between the surface coating and the poplar board: (A) without microcapsules, (B) with 7.0% melamine-resin-coated pomelo peel flavonoid microcapsules added to the topcoat, and (C) with 7.0% chitosan-coated pomelo peel flavonoid microcapsules added to the topcoat.
Coatings 14 00937 g006
Coatings 14 00937 g007
Figure 7. FTIR image of the coatings on the poplar board surface.
Figure 7. FTIR image of the coatings on the poplar board surface.
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Coatings 14 00937 g008
Figure 8. The antibacterial rate on the poplar board surface: (A) antibacterial rate against Escherichia coli and (B) antibacterial rate against Staphylococcus aureus.
Figure 8. The antibacterial rate on the poplar board surface: (A) antibacterial rate against Escherichia coli and (B) antibacterial rate against Staphylococcus aureus.
Coatings 14 00937 g008
Coatings 14 00937 g009
Figure 9. Reflectivity curves of visible light band of the coating: (A) with melamine-resin-coated pomelo peel flavonoid microcapsules and (B) with chitosan-coated pomelo peel flavonoid microcapsules.
Figure 9. Reflectivity curves of visible light band of the coating: (A) with melamine-resin-coated pomelo peel flavonoid microcapsules and (B) with chitosan-coated pomelo peel flavonoid microcapsules.
Coatings 14 00937 g009
Table 1. The materials required for the experiment.
Table 1. The materials required for the experiment.
MaterialMolecular FormulaCAS No.Reagent Purity LevelManufacturer
MelamineC3H6N6108-78-1Analytical ReagentShandong Yousuo Chemical Technology Co., Ltd., Linyi, China
37% Formaldehyde solution ---Xi’an Tianmao Chemical Co., Ltd., Xi’an, China
Triethanolamine C6H15NO3102-71-6Analytical ReagentGuangzhou Jiale Chemical Co., Ltd., Guangzhou, China
Chitosan(C6H11NO4)n9012-76-4Biological ReagentSinopharm Chemical Reagent Co., Ltd., Shanghai, China
Acetic acidCH3COOH64-19-7Analytical ReagentWuxi Jingke Chemical Co., Ltd., Wuxi, China
Tween-80C24H44O69005-65-6Analytical ReagentTianjin Beichen District Fangzheng Reagent Factory, Tianjin, China
Sodium tripolyphosphateNa5P3O107758-29-4Analytical ReagentSinopharm Chemical Reagent Co., Ltd., Shanghai, China
0.5 mol/L NaOH standard solution---Phygene Biotechnology Co. Ltd., Fuzhou, China
Citric acid monohydrateC6H10O85949-29-1Analytical ReagentSuzhou Changjiu Chemical Technology Co., Ltd., Suzhou, China
Sodium dodecyl benzene sulfonateC18H29NaO3S25155-30-0Analytical ReagentTianjin Beichen District Fangzheng Reagent Factory, Tianjin, China
Anhydrous ethanolC2H6O64-17-5Analytical ReagentWuxi Jingke Chemical Co., Ltd., Wuxi, China
Escherichia coli---Beijing Pharma and Biotech Center, Beijing, China
Staphylococcus aureus---Beijing Pharma and Biotech Center, Beijing, China
Table 2. The dosage of the coating with different contents of microcapsules.
Table 2. The dosage of the coating with different contents of microcapsules.
Content of Microcapsules
(%)		Mass of Primer
(g)		Mass of Microcapsules
(g)		Mass of Topcoat
(g)
00.72000.720
1.00.7200.0070.713
3.00.7200.0220.698
5.00.7200.0360.684
7.00.7200.0500.670
9.00.7200.0650.655
Table 3. Chromaticity and color difference of poplar board surface with different contents of microcapsules.
Table 3. Chromaticity and color difference of poplar board surface with different contents of microcapsules.
TypesContent of Microcapsules (%)LabΔE
Coating with melamine-resin-coated pomelo peel flavonoid microcapsules073.878.4327.23-
1.074.938.7329.072.14
3.079.235.6027.876.09
5.076.376.6725.173.69
7.072.909.4322.604.84
9.074.5710.1023.134.48
Coating with chitosan-coated pomelo peel flavonoid microcapsules073.878.4327.23-
1.079.775.2730.207.32
3.075.078.1728.932.10
5.075.307.0328.202.22
7.070.0111.1328.374.85
9.071.4010.3033.176.70
Table 4. Glossiness and gloss loss rate on the poplar board surface with different contents of the microcapsules.
Table 4. Glossiness and gloss loss rate on the poplar board surface with different contents of the microcapsules.
TypesContent of Microcapsules (%)20°60°85°Gloss Loss Rate (%)
Coating with melamine-resin-coated pomelo peel flavonoid microcapsules02.3313.3744.23-
1.02.1711.9021.2011.0
3.01.738.107.8339.4
5.01.034.801.9764.1
7.00.634.031.4769.9
9.01.003.901.1370.8
Coating with chitosan-coated pomelo peel flavonoid microcapsules02.3313.3744.23-
1.01.779.0012.6732.7
3.01.708.5012.7736.4
5.01.307.2712.2045.6
7.01.276.438.2751.9
9.01.206.177.3353.9
Table 5. The influence of microcapsules on the reflectivity of poplar board surface in the visible light band.
Table 5. The influence of microcapsules on the reflectivity of poplar board surface in the visible light band.
TypesContent of Microcapsules (%)Reflectivity (%)
Coating with melamine-resin-coated pomelo peel flavonoid microcapsules061.30
1.067.19
3.064.55
5.060.13
7.058.90
9.055.57
Coating with chitosan-coated pomelo peel flavonoid microcapsules061.30
1.064.93
3.059.78
5.062.23
7.056.18
9.050.60
Table 6. The influence of the microcapsules on mechanical properties of the poplar board surface.
Table 6. The influence of the microcapsules on mechanical properties of the poplar board surface.
TypesContent of Microcapsules
(%)		HardnessAdhesion GradeImpact Resistance GradeRoughness
(μm)
Coating with melamine-resin-coated pomelo peel flavonoid microcapsules0B130.37
1.0B130.83
3.0HB131.20
5.0HB132.07
7.0H232.58
9.0H233.98
Coating with chitosan-coated pomelo peel flavonoid microcapsules0B130.37
1.0HB130.55
3.0HB131.23
5.0HB131.69
7.0HB121.98
9.0H122.10
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Deng, J.; Ding, T.; Yan, X. Effect of Two Pomelo Peel Flavonoid Microcapsules on the Performance of Waterborne Coatings on the Surface of Poplar Boards. Coatings 2024, 14, 937. https://doi.org/10.3390/coatings14080937

AMA Style

Deng J, Ding T, Yan X. Effect of Two Pomelo Peel Flavonoid Microcapsules on the Performance of Waterborne Coatings on the Surface of Poplar Boards. Coatings. 2024; 14(8):937. https://doi.org/10.3390/coatings14080937

Chicago/Turabian Style

Deng, Jinzhe, Tingting Ding, and Xiaoxing Yan. 2024. "Effect of Two Pomelo Peel Flavonoid Microcapsules on the Performance of Waterborne Coatings on the Surface of Poplar Boards" Coatings 14, no. 8: 937. https://doi.org/10.3390/coatings14080937

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