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Article

Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood

by
Cheng Liu
1,2 and
Wei Xu
1,2,*
1
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
2
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(12), 1857; https://doi.org/10.3390/coatings12121857
Submission received: 8 November 2022 / Revised: 27 November 2022 / Accepted: 29 November 2022 / Published: 30 November 2022
(This article belongs to the Special Issue Sustainable Coatings for Functional Textile and Packaging Materials)
Browse Figures
Versions Notes

Abstract

:
Acrylic acid has good environmental weather resistance, water resistance, alcohol resistance, dirt resistance, and other properties. An acrylic acid dispersion with a core–shell structure was prepared and reacted with a polyisocyanate-type curing agent to prepare a waterborne polyurethane topcoat suitable for wood. The prepared two-component polyurethane topcoat was used in combination with a waterborne primer and a waterborne sealing primer and applied to Pine (Pinus strobus) boards to carry out gloss, wear resistance, and adhesion experiments. The effects of different coating amounts and spraying times on the comprehensive properties of the wood coatings were studied. The experimental results showed that when the self-made two-component polyurethane topcoat for wood was matched with the primer and the sealing primer, the coating amount of the sealing primer had little effect on improving the adhesion and wear resistance but had a significant effect on the gloss. The spraying times of the primer and the topcoat greatly impacted the adhesion, but they had no decisive impact on the improvement of the gloss. When the coating amount was 60 g/m2 for the waterborne sealing primer, 100 g/m2 for waterborne primer, and 120 g/m2 for the self-made topcoat and the coating was sprayed twice, the comprehensive performance of the obtained film on the wood was the best. This research on the preparation and coating process optimization of the two-component waterborne polyurethane coatings for wood can provide a technical basis for the application of two-component waterborne polyurethane topcoats for wood.

1. Introduction

Because of the excellent environmental performance of waterborne coatings [1,2,3], they have gradually become the mainstream of wood surface coatings [4,5,6]. Most waterborne wood coatings are polyurethane coatings [7]. However, the physical and chemical properties of one-component polyurethane products, such as its water resistance [8,9] and chemical resistance [10,11], are vastly inferior to those of two-component products [12]. Therefore, two-component waterborne polyurethane coatings have become the main substitute of solvent coatings, and research into two-component waterborne polyurethane coatings has gradually become an industry hotspot.
The wet adhesion and salt spray resistance of two-component waterborne polyurethane coatings are influenced by the resin type, chemical structure, and NCO/OH according to research by Xu et al. [13]. They found that coatings based on resins with more hydroxyl groups typically have a stronger wet adhesion and salt spray resistance. According to Liu et al. [14], the performance of waterborne polyurethane was affected by various formulations of the two components as well as its curing agent, wetting dispersant, defoamer, and wetting agent. The prepared two-component waterborne polyurethane topcoat had a high gloss and high weather resistance. By employing water-dispersible isocyanate as a curing agent to create a waterborne two-component polyurethane with good mechanical properties and corrosion resistance, Li et al. [15] further clarified the effect of the ratio of polyester polyol and polyether polyol in the soft segment on the coating performance. By mixing an amphiphilic linear bendritic carbosilane block surfactant with a hardener, Wang et al. [16] physically separated a polyisocyanate emulsion from water. With Fourier transform infrared spectroscopy, the amphiphilic linear bendritic carbosilane block surfactant’s effectiveness in preventing undesirable side reactions changed.
In addition, due to the characteristics of wood shrinkage and swelling [17], the coating process will affect the original moisture balance of the wood, resulting in the wood coating being easy to crack [18], thus affecting the overall quality of the film. However, two-component waterborne polyurethane coatings prepared by the existing research methods are mostly used on the surface of steel substrates [19]. At present, there are still gaps in the preparation theory and coating process of two-component waterborne polyurethane coatings for wood. Therefore, it is of great significance to explore the preparation method of two-component waterborne polyurethane coatings for wood and to evaluate the effect of the coating process in promoting the development of waterborne wood coatings.
In this paper, Pine (Pinus strobus) boards were used as the substrate. Firstly, a two-component polyurethane coating, which met the usage standard of wood coatings, was prepared and characterized. Subsequently, the coating process of the waterborne primer, the waterborne sealing primer, and the self-made coating for wood was designed and studied, and the most suitable coating process for the self-made waterborne two-component polyurethane coating (topcoat) was explored. By investigating and contrasting the influence of the parameters of coating amount and spraying times of sealing primer, primer, and self-made topcoat on the wear resistance, adhesion, and gloss of the film on the wood surface, the optimal coating process was determined, which can provide a technical basis for the application of two-component waterborne polyurethane coatings for wood.

2. Materials and Methods

2.1. Experimental Materials

Table 1 shows the list of experimental materials.

2.2. Experimental Methods

2.2.1. Preparation of the Two-Component Polyurethane Wood Coating

(1)
Preparation of secondary dispersion of acrylic resin
First, 51.70 g deionized water and 8.75 g acrylate phosphate emulsifier were added to a beaker, and the temperature was raised to 50 °C by heating and stirring in a water bath environment at a speed of 400 r/min. The obtained solution was transferred to a four-necked flask, and 8.90 g MMA, 5.30 g HEMA, 6.56 g BA, 2.90 g ST, 3.50 g AA, 0.37 g EHA, and 8.20 g BMA were added. The mixture was heated to 60 °C at a speed of 500 r/min to obtain a pre-emulsion. The pre-emulsion was mixed with 1.50 g of DTBP initiator and 1.30 g of DMEA chain transfer agent. The mixture was heated to 85 °C and stirred at a constant speed of 200 r/min to obtain an emulsion, and then it was maintained for 0.5 h. After that, the remaining pre-emulsion was added to the prepared emulsion drop by drop within 2–3 h, and the stirring speed was controlled at 200–250 r/min for mixing. The purpose of adding the pre-emulsion into the system in batches was to control the reaction speed and to obtain a suitable lotion with a core–shell structure. Finally, a blue translucent emulsion was obtained. After holding for 0.5 h, it was cooled down to 70 °C, mixed with 0.12 g ammonia water as a neutralizing agent for neutralization, filtered, and discharged to obtain the hydroxyl acrylic acid secondary dispersion emulsion.
(2)
Mixing of dispersion and curing agent
After uniformly mixing the self-made secondary acrylic acid dispersion and isocyanate curing agent according to a 1:4 molar ratio of NCO/OH, TEGO-842 defoamer and other additives were added under mechanical stirring, and, finally, the two-component polyurethane coating was synthesized as the topcoat.

2.2.2. Coating Amount Tests

(1)
Coating amount scheme
Nine coating amount schemes were applied for testing to determine whether the amount of the coating influenced the film’s performance. In Table 2, the precise coating amount is displayed.
(2)
Coating steps
After the Pine wood boards (30 × 30 × 10 mm3 (length × width × thickness)) were first sanded with 300-mesh sandpaper in the direction of the wood grain, the waterborne styrene acrylic sealing primer was evenly sprayed on their surface in accordance with the intended coating amount. The coated wood boards were placed in a drying room with controlled humidity and temperature for drying. After taking them out, they were sanded along the wood grain with 400-mesh sandpaper. Then, a second spray was repeated on the coated substrate and dried. The waterborne primer was sprayed in accordance with the operating stages mentioned above. The prepared wood boards were sanded twice with 600-mesh sandpaper. The self-made two-component polyurethane topcoat was applied to the samples after they had dried in accordance with the same procedure. The samples were sanded with 800-mesh sandpaper and then placed in the drying room to dry. Then, they were removed so that the second layer of topcoat could be sprayed. The performance of the coating on the wood boards was tested after drying.

2.2.3. Test of Spraying Times

Different coating processes have different effects on the performance of the entire film. To study the compatibility between the self-made topcoat and the primer, the matching coating process was explored by using the optimal coating amount scheme. Table 3 exhibits the topcoat, primer, and sealing primer’s supporting scheme design. Figure 1 is a process flow chart of the spraying times experiment.

2.2.4. Testing and Characterization

A KTF630 dynamic light scattering laser particle size analyzer (Taiwan Kelitai Industrial Co., Ltd., Hsinchu, China) was used to determine the size of the hydroxy waterborne acrylic acid dispersion. By using an EVO18 transmission electron microscopy device (TEM, Carl Zeiss AG, Oberkochen, Germany), the morphology of the hydroxy waterborne acrylic acid dispersion was identified. The chemical composition of the secondary dispersion and hydroxyl waterborne acrylic acid dispersion were both analyzed using 6700 Fourier transform infrared spectroscopy (Spike Analytical Instruments, Kleve, Germany). A DZ3320A differential scanning calorimeter (DSC, Nanjing Dazhan Instrument Co., Ltd., Nanjing, China) was used to carry out the differential thermal analysis of the dispersion and determine the glass transition temperature (Tg). According to GB/T 4893.8-1985 [20], the wear resistance of the film was tested by an abrasion tester (Fischer Instrumentation Ltd., Nantong, China). According to GB/T 4893.4-1985 [21], a PIG dry-film tester (Foshan Nanchao E-commerce Co., Ltd., Foshan, China) was used to carry out the adhesion tests. The coating structure and adhesion test method are shown in Figure 2. Under a microscope, peeling of the film on the wood boards was observed. From the overall statistics of the peeling of the film, the film sample that had the best adherence was identified. According to GB/T 4893.6-2013 [22,23], the gloss of the film was measured with a gloss meter (Beijing Dana Experimental Technology Co., Ltd., Beijing, China). According to GB/T 4893.1-1985 [24,25], deionized water was used to test the water resistance of the film for 24 h. According to GB/T 4893.7-1985 [26], the resistance of the film to temperature difference changes was tested. According to GB/T 4893.2-2020 [27], the humidity and heat resistance of the film was measured by setting the temperature at 70 °C for 15 min. According to GB/T 4893.3-2020 [28], the dry heat resistance of the film was measured by setting the temperature at 70 °C for 15 min.
All the above tests were carried out 4 times with an error within 5%.

3. Results and Discussion

3.1. Characterization of the Hydroxyl-Type Waterborne Acrylic Acid Dispersion

3.1.1. Particle Size Analysis of Hydroxyl-Type Waterborne Acrylic Acid Dispersion

Figure 3 shows the laser particle size analysis of the acrylic acid dispersion. The average particle size of the acrylic acid dispersion was 136.2 nm, and the particle size distribution was quite uniform.

3.1.2. Transmission Electron Microscopy Examination of the Hydroxyl-Type Waterborne Acrylic Acid Dispersion

Figure 4 displays the transmission electron microscope analysis diagram for the dispersion particles. The hydroxyl-type waterborne acrylic acid dispersion’s particles were spherical, and their average particle diameter was about 120 nm, which was about consistent with 136 nm.
The primary synthetic form of the polymer used in the monomer polymerization process to create the emulsion was a core–shell structure. As the hydrophilic monomers underwent branching polymerization on the surface of the colloidal particles, the core layer monomers transformed into core–shell polymers, which had molecular weights larger than the core polymers and had hydrophilic polymers inside and hydrophobic polymers outside [29,30]. The addition of the AA monomer formed a hydrophilic polymer as the core of the core–shell structure. As the polymer of HEMA was insoluble in water and its particle size was larger than that of the polymer formed by the AA monomer, it acted as a hydrophobic shell, forming a stable core–shell structure, as shown in Figure 4 and Figure 5. Because the experiment’s chosen copolymers had strong compatibility and could further enhance the overall performance of the film through mutual penetration, the line between the core and shell in the figure is hazy.

3.1.3. Infrared Spectrum Analysis of Hydroxyl-Type Waterborne Acrylic Acid Dispersion

Figure 6 depicts the infrared spectrum analysis diagram of the secondary dispersion created prior to the reaction and the two-component polyurethane coating that resulted from it. From the infrared spectrum curve of the secondary dispersion of acrylic acid, it can be seen that the absorption peak at 3391 cm−1 was the extended vibration absorption peak of -OH [31], the absorption peaks at 2988 cm−1 and 2872 cm−1 were the extended vibration absorption peaks of -CH3 and -CH2 [32], respectively, the characteristic absorption peaks of ketosyl and ester acyl groups were observed at 1722 cm−1, and the absorption peaks at 1453 cm−1 and 1384 cm−1 were the in-plane deformation vibration of -CH2 [33]. The symmetric stretching vibration absorption peak of C-O-C in the methyl methacrylate polymer was observed at 1146 cm−1 [34]. The absorption peak at 1072 cm−1 was produced by the vibration of the C-O and -COOH of the acrylic polymer. The characteristic absorption peak of BA was observed at 962 cm−1. The synthesis of the target product was finished in the experimental process. In contrast, there was no vibration absorption peak of the hydroxyl group at 3391 cm−1 in the curve of the two-component polyurethane coating. The peak near 1538 cm−1 was the N-H deformation vibration absorption peak. Because the carbamate bonds were generated during curing, this indicated that the dispersion and the curing agent interacted chemically after curing.

3.1.4. Differential Thermal Analysis of Hydroxyl-Type Waterborne Acrylic Acid Dispersion

The working temperature for glass transition is defined as the temperature at which a polymer transitions from a high-elastic state to a glass state. It also refers to the temperature at which a non-basically shaped polymer (including the non-crystalline part of a crystalline polymer) transitions from a glass state to a high-elastic state or from the latter to the former [35,36]. It is the limit working temperature at which a non-basically shaped polymer monomer moves independently in the polymer chain segment. Molecular chain movement has a significant impact on the glass transition temperature of polymer molecules. Therefore, it is necessary to measure and examine the glass transition temperature of polymer molecules. Figure 7 shows that there was a distinct glass transition temperature curve. The value of Tg of the hydroxyl-type waterborne acrylic acid dispersion was about 50 °C.

3.2. Influence of Different Coating Amount on the Film’s Performance

3.2.1. Wear Resistance Analysis

The maximum degree of wear that a film surface can withstand is referred to as its wear resistance. The wear resistance test is mainly applied to topcoat layers [37]. The variance in coating thickness is mostly responsible for the variation in wear resistance of the same wooden products. To assess the benefits and drawbacks of the coating amount, the weight loss of the film at 500 revolutions was used to determine the wear resistance of the film.
As seen in Figure 8, the orthogonal curve exhibited an upward trend as the coating amount increased, indicating that the film weight loss of the sealing primer increased. As a result, the wear resistance was proportional to the coating amount of the sealing primer under the weight loss conditions of 500 revolutions. When the coating amount of the primer was 80–100 g/m2, the orthogonal curve showed a downward trend with the increase in the coating amount, which indicated that the film weight loss of the waterborne primer was decreasing, and the wear resistance was inversely proportional to the coating amount of the primer. When the coating amount was 100–120 g/m2, the orthogonal curve showed an upward trend with the increase in the coating amount, which indicated that the film weight loss of the primer increased, and the wear resistance was inversely proportional to the coating amount of the primer at 100–120 g/m2. For the weight loss of the film at 500 revolutions, when the coating amount of the self-made topcoat was 100–120 g/m2, the orthogonal curve exhibited a downward trend as the coating amount increased, indicating that the weight loss of the waterborne topcoat film decreased. Accordingly, the wear resistance was inversely proportional to the coating amount of the self-made waterborne topcoat. When the coating amount of the self-made topcoat was controlled at 120–140 g/m2, the orthogonal curve showed an upward trend with the increase in the coating amount, indicating that the weight loss of the topcoat film increased, and the wear resistance was inversely proportional to the coating amount of the self-made topcoat. The wear resistance and coating amount were not inversely related. When the coating was too thick, there may have been microcracks inside, and there would likely have been more internal stress, which will have affected the coating’s ability to resist wear.
For weight loss of the film at 500 revolutions, the range of the coating amount of the sealing primer was 127.9, the range of the coating amount of the primer was 66.3, and the range of the coating amount of the sealing primer was 48.4. The main reason for the weight loss of the film was the maximum range in the coating amount of the sealing primer. The range of the weight loss data of the topcoat amount was the smallest, which indicated that the topcoat amount had the smallest influence on the film.
According to the analysis of the factors affecting the weight loss of the film at 500 revolutions in Table 4, the coating amounts of sealing primer, primer, and self-made topcoat had no significant effect on the weight loss of the film. The variance results of these factors were consistent with the results of the range analysis. If the film was slightly worn during use for a short time, the wear resistance of the film was closely related to the coating amount of the sealing primer and the topcoat. This was because the topcoat itself had very good wear resistance, and the adhesion of the sealing primer also helped to improve the wear resistance. When the adhesion force is small, the connection between the film and the substrate is unstable, which causes the film to be worn more significantly. On the contrary, if the adhesion is strong, the degree of firmness between the film and the wood substrate increases, and the degree of wear of the film decreases. Since the hardness and adhesion of the sealing primer and the primer have a direct impact on abrasion resistance, the coating amount of the sealing primer and the primer would affect the abrasion resistance of the film when the wooden products are used for a long time. Wear resistance is thus a crucial criterion, regardless of how much time has passed, in assessing the qualification of the film quality.
To further improve the wear resistance of the film, more thorough attention to the performance of the film itself needed to be given according to the variance analysis results, which showed that there was no direct correlation between the wear resistance and the coating amount.

3.2.2. Adhesion Analysis

Adhesion is the capacity of a film to adhere to a substrate through a certain action. It is also the first requirement and fundamental capability for the decoration and enhancement of a film’s performance. For adhesion, the interface between the coating and the substrate should be chosen to test and discuss [38]. The peeling rate of a film at the intersection of cut marks is an important parameter for measuring the adhesion of a film. The adherence is better when the peeling rate is lower.
According to Figure 9, the coating amount of the sealing primer was inversely related to adhesion when the coating amount was regulated at 60–100 g/m2. The coating amount of the waterborne primer was proportional to adhesion when the coating amount was adjusted between 80–120 g/m2. The topcoat amount was inversely proportional to adhesion when the coating amount was kept within the range of 100–140 g/m2.
The values of F in Table 5 identify the variables influencing adhesion. The coating amounts of sealing primer, primer, and self-made topcoat had no significant effect on the adhesion of the film. This showed that the primer had the least impact on adhesion, followed by the topcoat. In addition, the coating quantity of sealing primer had the largest impact. This was primarily due to the variations in the film’s composition between each scheme. Because each layer’s amount and thickness vary, so too does their adhesion to the film. The greater the amount of each layer, the thicker the film, and the smaller the interaction force between the films, and the smaller the adhesion. Mechanical adhesion theory and adsorption theory make up the core of adhesion theory. The amount of substrate surface penetration and the development of small grooves after the coating is sprayed on the blank will significantly affect the adhesion. As a result, to a certain extent, the amount of coating and surface roughness are related. If there is an insufficient coating, the coating cannot form grooves on the substrate and there will be little adhesion. Accordingly, if the coating is applied in excess, additional coating will accumulate on the substrate’s surface rather than penetrate the interior, which will cause a significant decrease in adhesion. Additionally, the volatilization of water and solvents might also result in a decrease in adhesion. As a result, an excessively thick coating will negatively impact adherence. The topcoat in a film has a bigger impact on adherence than the primer because the interface between the two is different. The primer must be in simultaneous contact with the sealing primer and topcoat, while the topcoat just interacts with the primer. It is first important to increase the adhesion of a film to further optimize the coating amount. Moreover, it is still important to consider the surface and paint’s roughness to significantly increase the film’s adhesion.

3.2.3. Gloss Analysis

A key factor in determining whether a film has a bright or matte appearance is gloss. The gloss test is mainly applied to a topcoat layer [39]. The gloss value needs to be close to 50 to produce the desired gloss or the “half gloss” look.
According to Figure 10, the coating amount of the sealing primer was inversely proportional to the gloss when the coating amount was 60–100 g/m2. The coating amount of the primer was inversely related to the gloss when the coating amount was adjusted within the range of 80–100 g/m2. The coating amount of the primer was proportionate to the gloss when it was applied in amounts more than 100 g/m2. The amount of self-made topcoat was proportionate to the gloss when the coating amount was kept within the range from 100 g/m2 to 140 g/m2.
According to Table 6, the coating amounts of the sealing primer, primer, and self-made topcoat had no significant effect on the gloss of the film. The coating amount of the topcoat was the main factor influencing the gloss followed by the coating amount of the primer, and the coating amount of the sealing primer had the least impact, as can be seen from the F-ratio in Table 6. The topcoat, which was positioned in the top layer of the multi-layer paint film, had a greater impact on the gloss of the overall paint film than the primer, middle layer, sealing primer, or bottom layer, respectively. The topcoat’s function is to increase gloss and fullness.
The film leveling generated by the temperature difference between the surface tension and the film thickness is the most important link in the film-formation process. It is only necessary to maintain a sufficient film thickness to improve the leveling property of the film in order to form a smooth and flat surface of the paint film. On the other hand, if the thickness of the film is insufficient for supporting a high film leveling, the surface of the film will eventually result in uneven or low-gloss defects. Additionally, it is important to guarantee uniform leveling with the aid of film-forming aids in order to enhance the leveling property of the film. As a result, substances with a high leveling property can aid in the development of a smooth coating film. In addition, the film thickness is the factor that affects the film’s leveling property. The amount of coating, which directly affects the gloss, is what determines the film thickness.
Through a variance analysis, it was determined that the gloss was significantly influenced by how much topcoat was applied. Therefore, it was necessary to first modify the coating amount appropriately in order to change the gloss of the film, which is quite helpful in terms of coating techniques.

3.3. Process Analysis for the Optimal Coating Thickness

The benchmark for determining an excellent coating quantity technique is to evaluate all aspects of the film’s quality. Among all the judgment indices, the film quality is the most crucial judgment criterion. Additionally, there are many criteria for judging the film quality. All variables, particularly the coating amount, which may have an impact on the film quality, should be taken into account while examining it. Table 7 displays the results of the coating scheme’s physical and chemical performance tests.
Samples 4 and 6 had poor resistance to cold and hot temperature differences and were unable to generate a flat and smooth film surface. The film’s property requirements could not be met by samples 7–9 due to their inadequate adhesion performance. Regarding the film peeling, samples 3 and 5 need to be enhanced and innovated. Sample 1 was less heat resistant than sample 2, in addition to it having a higher incidence of film peeling. Therefore, the coating technology of sample 2 (the coating amounts of the three coatings of 60 g/m2, 100 g/m2, and 120 g/m2, respectively) was the best among all the samples.

3.4. Effect of Spraying Times on the Performance of Waterborne Two-Component Polyurethane Coating

3.4.1. Adhesion

The adhesion of the film was directly impacted by the amount of the three different types of coatings and the various coating technologies. Table 8 demonstrates the peeling rate of the film under different spraying processes. The overall paint film peeling rate was 0.7% when the sealing primer, primer, and self-made topcoat were applied twice, indicating the best adherence. The film samples with two applications of sealing primer had a much higher peeling rate than the samples with one application of sealing primer.
The coating numbers of the sealing primers greatly affected the peeling rate of the film, which was a result of the sealing primers’ impact. The sealing primer had a very low solid content and viscosity, making it very easy to penetrate the wood and perform the functions of moisture absorption and moisture dissipation. Additionally, it could successfully stop moisture, grease, and other chemical components from seeping out of the wood, enhancing the adhesion of the entire film. The coating numbers of the sealing primer had a significant impact on adhesion, particularly when the surface roughness and coating amounts of the blank wood boards were fixed. The amount of coating penetration into the wood and the number of fine grooves created when the coating was sprayed on the blank boards directly affected the adhesion that resulted when the sealing primer came into contact with the wood. Small coating numbers resulted in fewer grooves between the coating and the substrate, which resulted in less adhesion. Grooves between the coating and the wood could only be sufficiently generated when the number of coatings was correct. The adhesion was much better due to the increase in the number of grooves. Additionally, adhesion may be more impacted by the number of topcoat applications than the number of primer coats. Compared to the primer, the self-made topcoat had a significantly stronger adhesion to the wood in terms of the resin and coating composition itself.

3.4.2. Gloss

Table 9 indicates the films’ gloss under different spraying processes. According to value of 55.4% for the best scheme for gloss in reference [40], the glossiness of the paint film prepared by this coating process was higher. The film with the highest gloss (78.8%) contained two coats of sealing primer, two coats of primer, and one coat of self-made topcoat. There was minimal correlation between the number of topcoat layers and gloss, as seen in Table 9, where the gloss values of the film ultimately created by samples 2′, 4′, 6′, and 8′ were lower than those of samples 1′, 3′, 5′, and 7′.
The primary purpose of a primer, which makes up the majority of a coating film, is to create a coating layer on the substrate. Following the application of the primer, the finish could be kept shiny without absorbing into the wood. There was a small amount of filler that easily reached the wood pores in the synthetic raw materials of the primer. The film was simple to sand after drying because of the high solid content of the primer, which could help the topcoat achieve a smooth surface effect on its surface. The smoothness of the primer was completely increased by increasing the coating numbers, thus enhancing the gloss of the topcoat applied to its surface. Waterborne coatings typically have a low solid component amount, which results in a low coating fullness. The sealing primer’s function is to moisten the substrate and to get into the wood’s pores in order to stop the wood’s oil from transferring to the primer and topcoat. In addition to significantly increasing the coating film’s fullness, this can also significantly reduce expenditure.
The peeling rate of the same film must be less than 5%, as per the applicable provisions of indoor waterborne wood coatings. The film peeling rates of samples 6′, 7′, and 8′ in Table 8 were less than 5%, which satisfied the specifications. In this study, scheme 8′ was chosen as the optimum matching technique for the preparation of two-component waterborne polyurethane coatings based on the gloss value of 50% being the qualified standard and taking the benefits of adhesion and gloss into consideration. The process flow was as follows: (1) the blank boards were sanded with 300-mesh sandpaper, evenly sprayed according to the coating amount of the sealing primer of 60 g/m2, and placed in a constant-temperature chamber for drying. (2) The dried wood boards were sanded with 400-mesh sandpaper, uniformly sprayed according to the coating amount of the sealing primer of 60 g/m2, and placed in a constant-temperature chamber for drying. (3) The dried samples were sanded with 600-mesh sandpaper, sprayed evenly according to the coating amount of primer of 100 g/m2, and then put into a constant-temperature chamber for drying. This process was repeated twice. (4) The prepared samples were sanded with 800-mesh sandpaper, sprayed evenly according to the coating amount of 120 g/m2 of self-made topcoat, and put into a constant-temperature chamber for drying. This process was also repeated twice. Compared with reference [40], the adhesion of the optimal scheme (peeling rate 0.7%) in this paper was good, and the gloss (56.9%) was a bit higher than the value of 55.4% shown in the reference [40].

4. Conclusions

A dispersion with stable core–shell structures was prepared by semi-continuous solution polymerization, and a waterborne acrylic two-component polyurethane coating for wood was successfully prepared by adding a polyisocyanate curing agent. The self-made two-component polyurethane coating for wood was qualified in terms of overall gloss, adhesion, and wear resistance when it was used as a topcoat and matched with a primer and a sealing primer. The most influential factor for adhesion and wear resistance was the coating amount of the sealing primer, but the optimization of the coating amount had little effect on improving the adhesion and wear resistance of the film. The thickness of the self-made topcoat was the main variable influencing the gloss. The topcoat’s coating quantity could be changed to achieve various surface effects. The adherence was greatly affected by the coating numbers of the primer and the topcoat. Since there was little correlation between the gloss and the number of the coatings, increasing the number of coatings did not significantly improve gloss. As a result, the performance of the film itself must be taken into account in order to successfully improve the gloss of the film. In order to achieve the best results, the sealing primer should be coated at 60 g/m2, the primer at 100 g/m2, and the topcoat at 120 g/m2. This coating strategy resulted in a gloss value of 75%, water resistance that did not deteriorate after 48 h, and good adhesion. This study and preparation of a new core–shell structure topcoat enriches the types of waterborne wood coatings available. Examining how coating technology affects the overall performance of two-component polyurethane coating is beneficial for its technical application and encourages the use and advancement of waterborne coatings for wood.

Author Contributions

Conceptualization, C.L.; methodology, C.L.; validation, C.L.; resources, C.L.; data curation, C.L.; writing—original draft preparation, C.L.; supervision, C.L.; data analysis, C.L.; investigation, C.L.; writing—review and editing, W.X. All authors have read and agreed to the published version of the manuscript.

Funding

This project was sponsored by the Qing Lan Project and the Natural Science Foundation of Jiangsu Province (BK20201386).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 12 01857 g001 550
Figure 1. Process flow chart of spraying times experiment.
Figure 1. Process flow chart of spraying times experiment.
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Coatings 12 01857 g002 550
Figure 2. Coating structure and adhesion test method.
Figure 2. Coating structure and adhesion test method.
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Coatings 12 01857 g003 550
Figure 3. Laser particle size analysis of acrylic acid dispersion.
Figure 3. Laser particle size analysis of acrylic acid dispersion.
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Coatings 12 01857 g004 550
Figure 4. Transmission electron microscope analysis diagram for the dispersion particles.
Figure 4. Transmission electron microscope analysis diagram for the dispersion particles.
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Coatings 12 01857 g005 550
Figure 5. Core–shell structure of the hydroxyl-type waterborne acrylic acid dispersion particles.
Figure 5. Core–shell structure of the hydroxyl-type waterborne acrylic acid dispersion particles.
Coatings 12 01857 g005
Coatings 12 01857 g006 550
Figure 6. FTIR spectrum of the secondary dispersion (blue line) and the two-component polyurethane coating (orange line).
Figure 6. FTIR spectrum of the secondary dispersion (blue line) and the two-component polyurethane coating (orange line).
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Coatings 12 01857 g007 550
Figure 7. DSC of the hydroxyl-type waterborne acrylic acid dispersion.
Figure 7. DSC of the hydroxyl-type waterborne acrylic acid dispersion.
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Coatings 12 01857 g008 550
Figure 8. Relationship between coating amount and weight loss.
Figure 8. Relationship between coating amount and weight loss.
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Coatings 12 01857 g009 550
Figure 9. Visual analysis of the film’s peeling rate.
Figure 9. Visual analysis of the film’s peeling rate.
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Coatings 12 01857 g010 550
Figure 10. Visual analysis of the film’s gloss.
Figure 10. Visual analysis of the film’s gloss.
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Table
Table 1. List of experimental materials.
Table 1. List of experimental materials.
Experimental MaterialsMolecular Mass (g/mol)CASManufacturer
Methyl methacrylate (MMA)100.11680-62-6Jilin Shenhua Group Zhangjiagang Chemical Industry Co., Ltd., Suzhou, China
2-hydroxyethyl methacrylate (HEMA)130.1418868-77-9Suzhou Senfida Chemical Co., Ltd., Suzhou, China
Butyl acrylate (BA)128.169141-32-2Shandong Xiaoqing New Chemical Co., Ltd., Jinan, China
Styrene (ST)104.15100-42-5Wanhua Chemical Group Co., Ltd., Yantai, China
Acrylic acid (AA)72.06379-10-7Wanhua Chemical Group Co., Ltd., Yantai, China
2-ethylhexyl acrylate (EHA)184.28103-11-7Suzhou Senfida Chemical Co., Ltd., Suzhou, China
Butyl methacrylate (BMA)142.19697-88-1Jinan Yuanxiang Chemical Co., Ltd., Jinan, China
N,N-Dimethylethanolamine (DMEA)89.136108-01-0Xindian chemical materials Co., Ltd., Shanghai, China
Propylene glycol butyl ether (PNB)238.321129387-86-8Guangzhou Kangyang Chemical Co., Ltd., Guangzhou, China
Acrylic phosphate167.034021-Suzhou Senfida Chemical Co., Ltd., Suzhou, China
Di-tert-butyl peroxide (DTBP)146.227110-05-4Shandong Shengqi New Material Co., Ltd., Jinan, China
2,2,3,4,4,4-hexafluorobutyl methacrylate250.1436405-47-7Hubei Zhenbo Chemical Co., Ltd., Wuhan, China
Ammonia--Jining Anping Chemical Co., Ltd., Jining, China
Bayhydur 304 isocyanate curing agent--Kostron (Shanghai) Investment Co., Ltd., Shanghai, China
TEGO-842 (defoamer)--Kostron (Shanghai) Investment Co., Ltd., Shanghai, China
Propylene glycol methyl ether acetate (PMA)--Nanjing Xingsha Chemical Co., Ltd., Nanjing, China
RM2020--Nantong Yongle Chemical Co., Ltd., Nantong, China
Waterborne styrene acrylic sealing primer--Lanzhou Ketian Water Technology Co., Ltd., Lanzhou, China
Waterborne acrylic primer--Hebei Chenyang Waterborne Coatings Co., Ltd., Baoding, China
Pinus strobus boards--Xuzhou Zhonghao Furniture Co., Ltd., Xuzhou, China
Table
Table 2. Coating amount scheme.
Table 2. Coating amount scheme.
SampleSealing Primer Amount (g/m2)Primer Amount (g/m2)Self-Made Topcoat Amount (g/m2)
16080100
260100120
360120140
48080120
580100140
680120100
710080140
8100100100
9100120120
Table
Table 3. Topcoat, primer, and sealing primer’s supporting scheme design.
Table 3. Topcoat, primer, and sealing primer’s supporting scheme design.
SampleSpraying Times (Times)
Sealing Primer PrimerSelf-Made Topcoat
1′111
2′112
3′121
4′122
5′211
6′212
7′221
8′222
Table
Table 4. Variance analysis of the weight loss of the film.
Table 4. Variance analysis of the weight loss of the film.
Variance SourceCoating Amount of the Sealing PrimerCoating Amount of PrimerCoating Amount of Self-Made TopcoatError
SS0.0290.0070.0030.03
df2222
F-ratio1.0000.2410.103
Fcrit19.00019.00019.000
SignificanceNo significant effect
Table
Table 5. Variance analysis of the adhesion of the film.
Table 5. Variance analysis of the adhesion of the film.
Variance SourceCoating Amount of the Sealing PrimerCoating Amount of PrimerCoating Amount of Self-Made TopcoatError
SS2752.0563813.7221265.3892752.06
df2222
F-ratio1.0001.3860.460
Fcrit19.00019.00019.000
SignificanceNo significant effect
Table
Table 6. Variance analysis of the gloss.
Table 6. Variance analysis of the gloss.
Variance SourceCoating Amount of the Sealing PrimerCoating Amount of the PrimerCoating Amount of the Self-Made TopcoatError
SS45.54982.642261.24245.55
df2222
F-ratio1.0001.8145.735
Fcrit19.00019.00019.000
SignificanceNo significant effect
Table
Table 7. Physical and chemical properties of different coating schemes.
Table 7. Physical and chemical properties of different coating schemes.
Variance SourcePeeling Rate (%)Weight Loss of the Film at 500 Revolutions (g)Cold and Hot Temperature Difference ResistanceWater Resistance (Grade)Heat Resistance (Grade)Gloss (%)
113.00.052Qualified2471.7
211.00.086Qualified2275.9
313.00.153Qualified2480.1
476.00.167Unqualified1375.8
517.00.169Qualified2473.2
68.00.274Unqualified3467.4
799.50.281Qualified2579.7
830.00.187Qualified2456.9
936.00.216Qualified2475.0
Table
Table 8. Films’ peeling rates under different spraying processes.
Table 8. Films’ peeling rates under different spraying processes.
SampleSpraying Times of Sealing Primer (Times)Spraying Times of Primer (Times)Spraying Times of Self-Made Topcoat (Times)Peeling Rate (%)
1′11158.9
2′11214.5
3′12179.4
4′12214.3
5′21111.4
6′2121.7
7′2212.4
8′2220.7
Table
Table 9. Films’ gloss under different spraying processes.
Table 9. Films’ gloss under different spraying processes.
SampleSpraying Times of Sealing Primer (Times)Spraying Times of Primer (Times)Spraying Times of Self-Made Topcoat (Times)Gloss (%)
1′11150.7
2′11248.0
3′12169.0
4′12261.0
5′21162.6
6′21257.8
7′22178.8
8′22256.9
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Liu, C.; Xu, W. Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood. Coatings 2022, 12, 1857. https://doi.org/10.3390/coatings12121857

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Liu C, Xu W. Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood. Coatings. 2022; 12(12):1857. https://doi.org/10.3390/coatings12121857

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Liu, Cheng, and Wei Xu. 2022. "Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood" Coatings 12, no. 12: 1857. https://doi.org/10.3390/coatings12121857

APA Style

Liu, C., & Xu, W. (2022). Effect of Coating Process on Properties of Two-Component Waterborne Polyurethane Coatings for Wood. Coatings, 12(12), 1857. https://doi.org/10.3390/coatings12121857

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