Preparation of Melamine-Formaldehyde Resin/Rice Husk Powder Coated Epoxy Resin Microcapsules and Effects of Different Microcapsule Contents on the Properties of Waterborne Coatings on Tilia europaea Surface
1
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China
2
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(8), 1213; https://doi.org/10.3390/coatings12081213
Submission received: 25 July 2022
/
Revised: 12 August 2022
/
Accepted: 14 August 2022
/
Published: 19 August 2022
Abstract
:With the development of economy and science and technology, people put forward higher standards for the performance of the surface coating of wood products, which requires us to carry out innovative research on the coating. In this work, a kind of microcapsule was prepared with melamine-formaldehyde resin/rice husk powder as wall material and epoxy resin as core material. The microcapsules were added to the waterborne acrylic resin coating according to the contents of 0%, 1.0%, 4.0%, 7.0%, 10.0%, 13.0%, 16.0% and 20.0% respectively, and were coated on the surface of the Tilia europaea boards in the form of topcoat. The effects of different contents of microcapsules on the optical properties, mechanical properties and aging resistance of the coating were explored, and the optimal content that can effectively improve the properties of the coating was analyzed. Test results indicated that when the microcapsule content is 7.0%, the comprehensive properties of the coating is optimal. At this time, the color difference of the coating is 6.96, the gloss at 60° is 13.4%, the hardness is 2H, the adhesion grade is 1, and the impact resistance is 12.0 kg·cm. After the aging test, the gloss loss rate decreases, the color difference is 5.69, and the gloss at 60° is 11.6%. The results of aging test show that the coating with epoxy microcapsules has a certain self-healing function. In this study, the microcapsules which can optimize the mechanical properties waterborne coating and prolong the service life of wood were prepared. This can meet the diverse needs of consumers, supply a theoretical reference for the preparation of functional microcapsules, and provide reference value for the functional research of the coating on wood furniture surface.
1. Introduction
Wood material is one of the most common materials in furniture manufacturing. It is widely loved because of its green environmental protection, good elasticity, impact resistance, rich and beautiful texture and tone, simple construction and other characteristics. However, as a natural polymer heterogeneous composite, wood materials are vulnerable to environmental impact, prone to cracking, decay, aging and other problems, which greatly limits the application and development of this green material in reality [1,2,3,4,5,6,7,8]. Painting on its surface can not only effectively protect the wood from being scratched by hard objects, isolate the direct contact between the wood and the outside world to inhibit the development of decay process, but also delay the aging problem of wood caused by long-term sunlight and greatly extend its service life [9,10,11,12,13,14].
Traditional wood coatings take organic solvents as the dispersion medium, which has high production cost and wastes energy, and will cause serious harm to the environment. Therefore, the research, development, improvement and application of new water-based coatings have become a hot topic in recent years. Waterborne coating is a green product with water as solvent. It does not contain formaldehyde, has no irritating smell, and is environmentally friendly and safe [15,16,17,18,19,20,21]. However, in the process of actual use, the coating is affected by the environment and external forces, which will inevitably produce the use traces and small cracks, and shorten the service life of wood products. This poses a new challenge to improve the mechanical properties of waterborne coating. Microcapsule is a technology that completely wraps the core material with wall material to protect the chemical properties of the core material. Many studies have shown that if the microcapsules are added to the coating, when the cracks occur in the coating, the outer wall material of the microcapsule will crack, and the inner core material will slowly release to fill and repair the cracks [22,23,24,25,26]. It can be seen that using microcapsule technology to modify waterborne coating can not only enhance the mechanical performance of the coating, but also repair the micro cracks of the coatings and achieve the self-healing function of the coatings.
As the main component of woody organisms, the cellulose has nice mechanical properties and has been widely studied and applied in the application field of enhancing the toughness of polymers. Rice husk is rich in cellulose, which is a good raw material for wood plastic composites [27,28,29,30]. Melamine-formaldehyde resin is a kind of resin with good mechanical properties, which has the characteristics of low price and simple preparation process. Therefore, when preparing microcapsules, it is usually selected as the wall material [31,32,33]. Epoxy resin materials have the high chemical resistance, good adhesion, curing performance, impact resistance, and meet the requirements of environmental protection [34,35,36]. Using the mixture of melamine-formaldehyde resin and rice hull powder as the wall material of microcapsules can have the nice interfacial compatibility with water-based coating. Taking epoxy resin as the core material can give full play to the good performance of epoxy resin and improve the toughness of the coating.
In this work, the melamine-formaldehyde resin/rice husk powder coated epoxy resin microcapsules were prepared by situ-polymerization. The selected paint is waterborne acrylic resin paint, and the selected base material is Tilia europaea boards. The microcapsules were not added to the primer, but were added to the topcoat according to different contents [37]. The micro morphology, chemical composition, optical properties, mechanical properties and aging resistance of the coatings were tested and characterized, and the effect of microcapsule content on the performance of waterborne coating is explored, which provides a certain theoretical basis for the performance optimization of water-based coating applicable to the surface of wood substrate.
2. Experimental Materials and Methods
2.1. Experimental Materials
The materials used in this work are shown in Table 1. The wood base material is Tilia europaea boards, and the size is 60 mm × 80 mm × 5 mm. The topcoat and primer are Nippon waterborne acrylic paint.
2.2. Preparation Method of the Epoxy Resin Microcapsules
The microcapsules prepared in this test take melamine-formaldehyde resin/rice husk powder as the wall material and epoxy resin as the core material. The preparation process is as follows:
(1) Preparation of rice husk powder: the 10.0 g of the 100 mesh rice husk powder was weighed, and the hydrogen peroxide was poured in a ratio of 1:20 by volume of solid-liquid. The aqueous sodium hydroxide solution was slowly poured in until the pH of the solution was 11.5. The obtained solution was placed in a constant temperature water bath at 70 °C for 24 h, then placed in a centrifuge for centrifugation, and the clarified liquid on it was poured out. The remaining substances were poured back into a beaker and dried in baking oven at 60 °C for 3 h. As shown in Figure 1, the dried rice husk powder was ground in the grinder to reduce the particle size to the micron level, which can better cover the core material.
(2) Preparation of wall material: the 5.0 g of melamine, the 10.0 g of 37.0% formaldehyde and the 10.0 g of water were added to the beaker, and the 1.0 g of pretreated rice husk powder was added, and then mixed evenly. The triethanolamine was slowly added until the pH value was adjusted to 8.59. The obtained solution was placed in a 70 °C constant temperature magnetic stirrer and reacted at 800 r/min speed. When the solution became transparent, the 15.0 g of water was added, and the prepolymer was obtained after reaction for 30 min.
(3) Preparation of core material: the 1.0 g epoxy resin and the 1.0 g benzyl alcohol as diluent were weighed and stirred until fully mixed. The 100.0 g water and 1.0 g sodium dodecyl benzene sulfonate powder were weighed to another beaker, which was stirred evenly to become an emulsifier solution. The diluted epoxy resin solution was poured in, which was fully stirred evenly and then put into a 60 °C constant temperature magnetic stirrer at a speed of 1200 r/min. After 30 min, the n-octanol was dripped for defoaming. The obtained solution was the core material solution.
(4) Microencapsulation: the wall material was slowly poured to the core material, and the citric acid monohydrate was added one by one at the speed of 300 r/min. After being stirred until completely dissolved, the pH test paper was used to test the pH value until the value was 2.5–3.0. Then the temperature was slowly raised to 70 °C, and the reaction lasted for 3 h. The resulting substance was filtered by suction and then put into a drying oven at 80 °C for 4 h and then ground. As shown in Figure 2, the obtained powder were the prepared microcapsules, which were light yellow white.
2.3. Preparation of the Coatings
The 600 mesh sandpaper was used to gently polish the Tilia europaea boards to remove burrs, the brushes were used to remove debris, and the rags were used to wipe dirt off the wood surface to make the wood surface smooth and clean. The corresponding weights of microcapsules and coatings are shown in Table 2. The microcapsules were added to the topcoat, and the microcapsules were not added to the primer. The microcapsules in the topcoat were 0%, 1.0%, 4.0%, 7.0%, 10.0%, 13.0%, 16.0% and 20.0% respectively. First, the film preparer was used to apply the primer evenly on the surface of the boards. After drying at room temperature for 12 h, the 800 mesh sandpaper was used to polish carefully. Then the boards were coated for the second time, and then dried in the oven at 30 °C for 24 h, and the sandpaper was used to polish it again. After the primer was coated, the topcoat with microcapsules was evenly painted on the surface of the boards twice, and the coating process was the same as that of the primer. The thickness of the coating was 60 μm.
2.4. Testing and Characterization
The test instruments used in this paper are shown in Table 3. The micro morphology of the coating was analyzed by scanning electron microscope. Infrared spectroscopy was used to analyze the coating.
According to the standard of GB/T 11186.3-1989 [38], a portable colorimeter was used to test the color difference of the coatings. First, the colorimeter was calibrated, the test hole was aligned with the plate to be tested, and two groups of the values of L*, a* and b* were recorded. The L* indicates brightness, and a positive value indicates that the surface color of the measured object is bright. The a* indicates the change of color from red to green, and a positive value indicates that the color is red. The b* indicates the change of color from yellow to blue, and a positive value indicates that the surface color of the tested object is yellow. The first group of data tested was recorded as L1*, a1*, b1*. The second group of data tested was recorded as L2*, a2*, b2*. The color difference was represented by ΔE, calculated by the Formula (1). In this formula, Δa* = a2* − a1*, ΔL* = L2* − L1*, Δb* = b2* − b1*.
ΔE* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
As a surface optical property of an object, the gloss depends on the specular reflection ability of the object surface. According to the standard of GB/T 4893.4-2013 [39], the gloss meter was used to record the gloss of the coatings at three different angles of 20°, 60° and 85°. The gloss loss of the coatings with microcapsules was calculated compared with that without microcapsules at an incident angle of 60°. The G0 is the coating without microcapsules, the G1 is the coating with microcapsules, and the GL is the light loss rate. The light loss rate was calculated according to the Formula (2).
GL = (G0 − G1)/G0 × 100%
According to the standard of GB/T 6739-2006 [40], the hardness of the paint film was tested with a portable paint film hardness tester. The wooden rod part of the pencil top was cut off, and the pencil lead was exposed for about 3 mm and placed for use. The boards were placed flat on the console, and the pencil was at an angle of 45° to the surface of the boards. The pencil was pushed on the surface of the paint film, and the pencil hardness that cut the surface of the paint film was the surface hardness of the paint film. By repeating the above experiments, the surface hardness of the paint films with different microcapsule content was measured respectively.
The adhesion of the paint film was tested with a paint film gridding instrument according to the standard of GB/T 9286-1998 [41]. The tooth pitch of this instrument is 2 mm. The board was placed on the operating platform, and the handle of the scriber was held so that the knife was perpendicular to the surface of the board, and then the surface was cut with uniform force and fixed speed. After that, the board was rotated by 90°, and the previous operation was repeated at the cut previously, so that a mesh pattern appeared on the surface. All cuts in this operation should penetrate the coating. After that, the adhesive tape with a width of 25.0 mm and an adhesion of 10 ± 1 N was pasted on the whole grid, and then it was gently torn off. The adhesion was judged on the basis of the peeling off of the coatings. The adhesion grade of the coating was divided into 0, 1, 2, 3, 4 and 5 from high to low. The falling off areas of the paint film correspond to 5.0%, 15.0%, 35.0%, 55.0% and more than 60.0% respectively. The higher the grade, the worse the adhesion of the paint film.
Impact resistance of paint film refers to the ability of the paint film applied to wood to deform rapidly without cracking under the action of high-speed gravity. According to the standard of GB/T 1732-1993 [42], the paint film impactor tester was used to test the impact resistance of the film. After the impact block was raised to a certain height, it falls freely to impact the specimen. The surface of the coatings was observed for cracks and spalling, and the maximum height without cracking was the impact strength.
According to the standard of GB/T 1740-2007 [43], the aging test in this paper was to create a simulated environment to accelerate the aging of the boards. The coated wood boards were placed in a 120 °C electric blast drying oven for high-temperature aging test, and the coating surface was observed after 12 h. The effect of microcapsules on the high temperature aging resistance of the coating was analyzed.
3. Results and Discussion
3.1. Morphology Analysis of the Microcapsules and Coatings
Figure 3 shows the SEM morphology of microcapsules. The prepared microcapsules are well coated, spherical and uniform in particle size, about 5–7 μm. Figure 4 shows the SEM morphology of the coatings with different content of the microcapsules on the wood substrate. The surface of the coating without microcapsules is smooth and flat, and the surface smoothness of the coating with a small amount of microcapsules is second. With the increase of microcapsule content, the agglomeration in the coating increases. When the content reaches 13.0%, the agglomeration is obvious and the surface becomes uneven. This is because microcapsules are granular. With the increase of content, microcapsules cannot be evenly distributed in the paint, so the particles agglomerate when coated on wood.
3.2. Infrared Spectrum Analysis of the Microcapsules and Coatings
Figure 5 is the infrared spectrum of the microcapsules and coatings added with different content of the microcapsules. The peaks at 1148 and 862 cm−1 represent the vibration peaks of C–O and O–H in aqueous acrylic acid respectively. The peak at 1726 cm−1 represents the characteristic peak of C=O in aqueous acrylic acid. These characteristic peaks are found in both the waterborne coating without microcapsules and the waterborne coatings with microcapsules, but they are not found in the microcapsules.
The vibration peak of C–N in melamine-formaldehyde resin appears at 1344 cm−1, and the bending vibration peak of triazine ring in melamine-formaldehyde polycondensation system appears at 813 cm−1. These peaks are characteristic peaks of melamine-formaldehyde resin, which exist in the microcapsules and the waterborne coatings with microcapsules, proving that the wall material in the microcapsules have not been damaged. The peak at 3418 cm−1 is the vibration peak of N–H in melamine-formaldehyde resin. Because the microcapsule content is small, this peak is not obvious in the coatings. At 2872 cm−1, it is the characteristic absorption peak of –CH2 of polyether amine, and at 939 cm−1, it is the stretching vibration peak of C–O–C of ether bond. These are the characteristic peaks of epoxy resin, which exist in microcapsules and coatings with microcapsules, proving that the core material in microcapsules has not been damaged. These peaks proved that there was no chemical reaction between the wall and core materials of microcapsules, and the microcapsules were successfully prepared. The microcapsules have no chemical reaction with the waterborne coating, and exist in the waterborne coating.
3.3. Effect of Microcapsule Content on Optical Properties of the Coatings
The chromaticity value and color difference of the coatings with different content of the microcapsules on the wood substrate are shown in Table 4 and Figure 6. It can be seen that the color difference of the coating without microcapsules is small. The color difference of the coatings added with the microcapsules gradually increases with the increase of content, showing an upward trend. When the microcapsule content is 0%–7.0%, the color difference increases significantly. When the microcapsule content is 7.0%–13.0%, the change of color difference is relatively gentle, which is lower than 8.0. When the microcapsule content is 20.0%, the color difference reaches the maximum. This is because the microcapsule itself is a light yellowish powder, which will affect the color of the coating when added to the coating. When the content is too high, the microcapsules are unevenly distributed in the coating, resulting in a large color difference.
The effect of microcapsules with different content on the gloss of the coatings is shown in Table 5 and Figure 7. With the increase of microcapsule content, the gloss of the coatings gradually decreases and the gloss loss rate gradually increases. When the microcapsule content is 0%–4.0%, the gloss is high. When the microcapsule content is 4.0%–10.0%, the gloss of the coating decreases significantly. When the content of microcapsules is 13.0%–20.0%, the decrease of the gloss tends to be gentle. This is because the smoothness and roughness of the coating surface will affect the gloss of the coating. When there are more microcapsules in the coating, the microcapsule particles will reduce the surface flatness of the coating, thereby reducing the light reflection ability of the coating, so the gloss of the coating will decrease and the light loss rate will increase.
3.4. Effect of Microcapsule Content on Mechanical Properties of the Coatings
The effect of microcapsules with different content on the hardness of the coatings on wood is shown in Figure 8. With the increase of microcapsule content, the hardness of the coatings slowly increases. The hardness of the coating without microcapsules is H. When the content of the microcapsules is 1.0%–7.0%, the hardness of the coating increases to 2H. When the content of the microcapsules is 10.0%–16.0%, the hardness of the coating increases to 3H. When the content of the microcapsules is 20.0%, the hardness reaches 4H. This may be because the coating hardness is mainly determined by the material properties of the coating itself. The microcapsules act as fillers in the coating. The increase of filler content has a positive effect on the coating hardness, so the coating hardness increases slightly.
The coating adhesion test results are shown in Figure 9. When the content of the microcapsules is 0%–13.0%, the adhesion of the coatings is basically unchanged. When the content increases to 16.0%–20.0%, the adhesion of the coatings decreases significantly and the adhesion grade increases. This is because when there are many microencapsulated particles in the coating, the binder content in the coating decreases, and the adhesion to wood decreases, resulting in the reduction of adhesion.
The impact resistance test result of the coatings is shown in Figure 10. The impact resistance of the coating is directly proportional to the content of the microcapsules, which indicates that the impact resistance of the coating surface can be increased by adding microcapsules into the waterborne coating. The impact resistance of the coating without microcapsules is 4.0 kg·cm. When the microcapsules were added to 7.0%, the impact resistance was significantly enhanced, which is 12.0 kg·cm. When the microcapsule content is 20.0%, the impact resistance of the coating is the best, which is 18.0 kg·cm. This is because the composition of the microcapsules is that the wall material covers the core material, and the wall material can play a certain protective function on the core material. When the coating surface is subjected to external force, the microcapsules can form a force layer to disperse the force on the buffer coating, so as to improve the impact resistance of the coating.
3.5. Effect of Microcapsule Content on Aging Resistance of the Coatings
In the actual use of wood furniture, the coating will be affected by the external environment for a long time, which will produce cracks, bubbles and other phenomena. This will cause the wall material of microcapsules in the coating to crack, and the core material epoxy resin can fill the cracks, playing a certain repair function [44,45]. Therefore, in this paper, the situation of coating damage in the external environment was simulated. The coating without microcapsules and the coatings with microcapsules were destroyed through the aging test, and the influence of microcapsules on the self-healing function of the coating was explored by comparing the aging test of the coatings. The coated wood boards were put into an oven at 120 °C for aging test. After 12 h, the condition of the coating surface was observed, and the color difference and gloss of the coatings were recorded. The surface damage of the coating without microcapsules and the coating with different content of microcapsules was compared under high temperature aging environment.
The color difference and test results of the coating are shown in Table 6. The color difference comparison before and after aging of the coating is shown in Figure 11. By observing the results of the aging resistance test of the coating, it can be found that under the same environment and after the same aging time, when the content of microcapsules is 1.0%–16.0%, the color difference of the coatings with microcapsules is less than that of the coating without microcapsules. This may be because the coating is affected by the high temperature environment of the oven, which will produce microcracks or bubbles. The microcapsules can play a protective role in the coating, and the epoxy resin can play a repair function, filling some cracks, so the color difference is small. When the microcapsule content is 16.0%–20.0%, the color difference of the coatings with microcapsules is larger than that of the coating without microcapsules. This may be because when the content is too high, the microcapsules cannot be evenly distributed in the coating, resulting in the reduction of the adhesion between the coatings and between the coatings and wood. Therefore, the toughness of the coatings is reduced, and the protective effect of the coatings on wood is also reduced, so the cracks and bubbles are more likely to occur at high temperatures, resulting in larger color differences.
By comparing the color difference before and after aging, it can be found that when the content of the microcapsules is less than 7.0%, the color difference of the coating without microcapsules and the coatings with microcapsules increases after aging. This is because the color of wood and coatings will change greatly under high temperature environment, and the content of the microcapsules is too low to effectively play the role of protective coating and aging resistance, so the color difference of the coating is large. When the content of the microcapsules is more than 7.0%, the color difference after aging is less than that before aging. This shows that microcapsules need to reach a certain content in order to have an obvious anti-aging effect. The above results show that the microcapsules can repair the coatings and improve the aging resistance of the coatings. When the content is 7.0%–16.0%, the aging resistance is the best.
The coating gloss test result is shown in Table 7. The comparison of the gloss loss of the coating before and after color aging is shown in Figure 12. It is obvious that with the increase of microcapsule content, the gloss shows a downward trend. However, under 60° incident light, the light loss rate of the coating with microcapsules after aging test is significantly lower than that before aging. This is because the core material in the microcapsule fills the cracks and bubbles of the coating during aging to a certain extent, so the light loss rate is reduced.
4. Conclusions
In this paper, the microcapsules were successfully prepared with melamine-formaldehyde resin/rice husk powder as wall material and epoxy resin as core material. The Tilia europaea boards were selected as the base material, and the waterborne acrylic coating was used as the paint base. The microcapsules were added to the topcoat according to different contents. The optical properties, mechanical properties and aging resistance of the coatings were tested and analyzed. The results showed that with the increase of microcapsule content, the color difference of the coating increased, the gloss decreased, the hardness increased, the adhesion decreased, and the impact resistance increased. The mechanical properties of the coatings have been enhanced. The results of aging test showed that the microcapsules can effectively improve the aging resistance of the coatings. When the microcapsule content is 7.0%, the comprehensive performance of the coating is the best. At this time, the color difference of the coating is 6.96, the gloss at 60° is 13.4%, the hardness is 2H, the adhesion grade is 1, the impact resistance is 12.0 kg·cm, the color difference after aging test is 5.69, and the gloss at 60° is 11.6%. This study provides a certain reference for the expansion of the optical properties, mechanical properties and self-healing function of waterborne coatings.
Author Contributions
Conceptualization, methodology, validation, resources, data management, and supervision, N.H.; formal analysis, investigation, X.Y.; and writing—review and editing, Y.H. All authors have read and agreed to the published version of the manuscript.
Funding
This project was partly supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_1098) and the Natural Science Foundation of Jiangsu Province (BK20201386).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Acknowledgments
The authors are grateful for financial support from the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_1098) and the Natural Science Foundation of Jiangsu Province (BK20201386).
Conflicts of Interest
The authors declare that there is no conflict of interest.
References
- Kanbayashi, T.; Ishikawa, A.; Matsunaga, M.; Kobayashi, M.; Kataoka, Y. Application of Confocal Raman Microscopy for the Analysis of the Distribution of Wood Preservative Coatings. Coatings 2019, 9, 621. [Google Scholar] [CrossRef]
- Mai, C.; Schmitt, U.; Niemz, P. A brief overview on the development of wood research. Holzforschung 2021, 76, 102–119. [Google Scholar] [CrossRef]
- Rowell, R.; Bongers, F. Role of Moisture in the Failure of Coatings on Wood. Coatings 2018, 7, 219. [Google Scholar] [CrossRef]
- Zhao, Y.; Ren, J.; Zheng, X.; Pan, B.; Leng, W. Effects of three kinds of fungi on color, chemical composition and route of infection of Picea sitchensis. J. For. Eng. 2021, 6, 88–93. [Google Scholar]
- Hu, W.G.; Liu, N.; Guan, H.Y. Optimal design of a furniture frame by reducing the volume of wood. Drewno 2019, 62, 85–97. [Google Scholar]
- Xu, W.; Fang, X.; Han, J.; Wu, Z.; Zhang, J. Effect of Coating Thickness on Sound Absorption Property of Four Wood Species Commonly Used for Piano Soundboards. Wood Fiber Sci. 2020, 52, 28–43. [Google Scholar] [CrossRef]
- Li, R.; He, C.; Wang, X. Evaluation and Modeling of Processability of Laser Removal Technique for Bamboo Outer Layer. JOM 2021, 73, 2423–2430. [Google Scholar] [CrossRef]
- Liu, Y.; Hu, J. Investigation of Polystyrene-Based Microspheres from Different Copolymers and Their Structural Color Coatings on Wood Surface. Coatings 2021, 11, 14. [Google Scholar] [CrossRef]
- Bahchevandziev, K.; Mihajlovski, N. Surface retention of polyurethane and acrylic coatings on impregnated spruce wood (Picea abies Karst) and comparison with some wood preservatives. Balt. For. 2022, 27, 266–270. [Google Scholar] [CrossRef]
- Hu, W.; Zhang, J. Bolt-Bearing Yield Strength of Three-Layered Cross-Laminated Timber Treated with Phenol Formaldehyde Resin. Forests 2020, 11, 551. [Google Scholar] [CrossRef]
- Fu, R.; Zhang, W.; Li, D.; Zhang, H. Analyses on chemical composition of ancient wood structural component by using near infrared spectroscopy. J. For. Eng. 2021, 6, 114–119. [Google Scholar]
- Liu, Y.; Hu, J.; Wu, Z. Fabrication of Coatings with Structural Color on a Wood Surface. Coatings 2020, 10, 32. [Google Scholar] [CrossRef]
- Žigon, J.; Kovač, J.; Petrič, M. The influence of mechanical, physical and chemical pre-treatment processes of wood surface on the relationships of wood with a waterborne opaque coating. Prog. Org. Coatings 2021, 162, 106574. [Google Scholar] [CrossRef]
- Davis, K.; Leavengood, S.; Morrell, J. Performance of Exterior Wood Coatings in Temperate Climates. Coatings 2021, 11, 325. [Google Scholar] [CrossRef]
- Zhao, Z.; Niu, Y.; Chen, F. Development and finishing technology of waterborne UV lacquer-coated wooden flooring. BioResources 2021, 16, 1101–1114. [Google Scholar] [CrossRef]
- Park, J.; Im, D.; Jin, Y.; Jang, S.-H.; Lim, K.T.; Lee, W.-K. Synthesis and properties of eco-friendly coatings using waterborne polyaspartic and blocked isocyanate. Mol. Cryst. Liq. Cryst. 2019, 688, 1–6. [Google Scholar] [CrossRef]
- Liu, Q.; Gao, D.; Xu, W. Effect of Paint Process on the Performance of Modified Poplar Wood Antique. Coatings 2021, 11, 1174. [Google Scholar] [CrossRef]
- Liu, Q.; Gao, D.; Xu, W. Influence of the Bottom Color Modification and Material Color Modification Process on the Performance of Modified Poplar. Coatings 2021, 11, 660. [Google Scholar] [CrossRef]
- Hu, J.; Liu, Y.; Wu, Z.; Pang, X. Construction of bionic structural color coating on wood surface based on polystyrene microspheres. J. For. Eng. 2021, 6, 35–42. [Google Scholar]
- Wu, Y.; Sun, Y.; Yang, F.; Zhang, H.; Wang, Y. The Implication of Benzene–Ethanol Extractive on Mechanical Properties of Waterborne Coating and Wood Cell Wall by Nanoindentation. Coatings 2019, 9, 449. [Google Scholar] [CrossRef]
- Lu, Z.; Hu, J.; Zhang, M.; Chang, S.; Liu, Y.; Zheng, L.; Li, X. Research on wettability of wood surface with waterborne-resin priming paint. J. For. Eng. 2021, 6, 178–183. [Google Scholar]
- Yan, X.; Zhao, W.; Wang, L. Mechanism of Thermochromic and Self-Repairing of Waterborne Wood Coatings by Synergistic Action of Waterborne Acrylic Microcapsules and Fluorane Microcapsules. Polymers 2022, 14, 56. [Google Scholar] [CrossRef] [PubMed]
- Yan, X.; Wang, L.; Qian, X. Effect of Microcapsules with Different Core–wall Ratios on Properties of Waterborne Primer Coating for European Linden. Coatings 2020, 10, 826. [Google Scholar] [CrossRef]
- Yan, X.; Zhao, W.; Qian, X. Effect of Water-Based Emulsion Core Microcapsules on Aging Resistance and Self-Repairing Properties of Water-Based Coatings on Linden. Appl. Sci. 2021, 11, 4662. [Google Scholar] [CrossRef]
- Feng, X.; Chen, J.; Wu, Z.; Wu, Y.; Gan, J. The formation mechanism of “skin-tactile” coating and its application and trends in furniture. J. For. Eng. 2021, 6, 167–175. [Google Scholar]
- Petrulis, D.; Petrulyte, S. Potential use of microcapsules in manufacture of fibrous products: A review. J. Appl. Polym. Sci. 2019, 136, 47066. [Google Scholar] [CrossRef]
- Brand, M.A.; Jacinto, R.C.; Antunes, R.; da Cunha, A.B. Production of briquettes as a tool to optimize the use of waste from rice cultivation and industrial processing. Renew. Energy 2017, 111, 116–123. [Google Scholar] [CrossRef]
- Guilbert-Garcia, E.; Salgado-Delgado, R.; Rangel-Vazquez, N.A.; Garcia-Hernandez, E.; Rubio-Rosas, E.; Salgado-Rodriguez, R. Modification of rice husk to improve the interface in isotactic polypropylene composites. Lat. Am. Appl. Res. 2012, 42, 83–87. [Google Scholar]
- Zeng, Z.Q.; Liu, H.C.; Yu, H.P. Structure and properties of natural rubber filled with epoxidised natural rubber modified rice husk ash. Mater. Res. Innov. 2013, 17, S251–S254. [Google Scholar] [CrossRef]
- Yan, X.; Li, W.; Han, Y.; Yin, T. Preparation of Melamine/Rice Husk Powder Coated Shellac Microcapsules and Effect of Different Rice Husk Powder Content in Wall Material on Properties of Wood Waterborne Primer. Polymers 2022, 14, 72. [Google Scholar] [CrossRef]
- Mao, A.; Hassan, E.B.; Kim, M.G. The Effects of Adding Melamine at Different Resin Synthesis Points of Low Mole Ratio Urea-Melamine-Formaldehyde (UMF) Resins. BioResources 2014, 8, 5733–5748. [Google Scholar] [CrossRef]
- Altun, S.; Tokdemir, V. Modification with Melamine Formaldehyde and Melamine-Urea Formaldehyde Resin to Improve the Physical and Mechanical Properties of Wood. BioResources 2017, 12, 586–596. [Google Scholar] [CrossRef]
- Santos, J.; Pereira, J.; Paiva, N.; Ferra, J.; Magalhães, F.; Martins, J.; de Carvalho, L. Water resistance evaluation of a MFU resins with different molar ratio catalyzed with citric acid. Int. J. Adhes. Adhes. 2022, 117, 103020. [Google Scholar] [CrossRef]
- Kakichi, Y.; Hashimoto, T.; Yamaguchi, A.; Urushisaki, M.; Sakaguchi, T.; Kawabe, K.; Kondo, K.; Iyo, H. Synthesis of Phenolic Resin-Based Epoxy Resins Containing Acetal Linkages and Degradability of Their Cured Epoxy Resins. Kobunshi Ronbunshu 2016, 73, 108–115. [Google Scholar] [CrossRef]
- Raj, M.; Maheta, J.; Raj, L. Synthesis characterization and application of hexafunctional epoxy resin and comparison against commercial epoxy resin. Polym. Polym. Compos. 2022, 30, 09673911221076721. [Google Scholar] [CrossRef]
- Zhao, Y.; Xu, R.; Xiao, Y.; Wang, H.; Zhang, W.; Zhang, G. Mechanical Performances of Phenolic Modified Epoxy Resins at Room and High Temperatures. Coatings 2022, 12, 643. [Google Scholar] [CrossRef]
- Lagaňa, R.; Svocák, J.; Kúdela, J. A study of the surface properties of a solid wood coating system with self-healing microcapsules. In 9th Hardwood Proceedings-Part II; University of Sopron: Sopron, Hungary, 2021; Volume 9, pp. 61–67. [Google Scholar]
- GB/T 11186.3-1989; Methods for Measuring the Colour of Paint Films. Part III: Calculation of Colour Differences. Standardization Administration of the People’s Republic of China: Beijing, China, 1990.
- GB/T 4893.4-2013; Test of Surface Coatings of Furniture. Standardization Administration of the People’s Republic of China: Beijing, China, 2013.
- GB/T 6739-2006; Paint and Varnishes-Determination of Film Hardness by Pencil Test. Standardization Administration of the People’s Republic of China: Beijing, China, 1998.
- GB/T 9286-1998; Paint and Varnishes-Cross Cut Test for Films. Standardization Administration of the People’s Republic of China: Beijing, China, 1998.
- GB/T 1732-1993; Determination of Impact Resistance of Film. Standardization Administration of the People’s Republic of China: Beijing, China, 1993.
- GB/T 1740-2007; Determination of Resistance to Heat and Humidity of Paint Films. Standardization Administration of the People’s Republic of China: Beijing, China, 2007.
- Matsuo, M.; Yamamoto, H.; Umemura, K.; Kawai, S. Color Changes of Wood during Natural Aging and Heat Treatment. Mokuzai Gakkaishi 2016, 62, 266–274. [Google Scholar] [CrossRef]
- Kránitz, K.; Sonderegger, W.; Bues, C.-T.; Niemz, P. Effects of aging on wood: A literature review. Wood Sci. Technol. 2016, 50, 7–22. [Google Scholar] [CrossRef]
Figure 1.
Pretreated rice husk powder.
Figure 2.
Macro morphology of the microcapsules: (A) the microcapsules before grinding, (B) the microcapsules after grinding.
Figure 2.
Macro morphology of the microcapsules: (A) the microcapsules before grinding, (B) the microcapsules after grinding.
Figure 3.
Morphology of the microcapsules.
Figure 4.
Morphology of the coatings with different content of the microcapsules: (A) 0%, (B) 1.0%, (C) 7.0%, (D) 13.0%.
Figure 4.
Morphology of the coatings with different content of the microcapsules: (A) 0%, (B) 1.0%, (C) 7.0%, (D) 13.0%.
Figure 5.
FTIR image of the microcapsules and coatings.
Figure 6.
Effect of the microcapsule content on color difference of the coatings.
Figure 7.
Effect of the microcapsule content on gloss of the coatings.
Figure 8.
Hardness of the coatings.
Figure 9.
Adhesion grade of coatings.
Figure 10.
Impact resistance of coatings.
Figure 11.
Effect of the microcapsule content on color difference of the coatings before and after aging.
Figure 11.
Effect of the microcapsule content on color difference of the coatings before and after aging.
Figure 12.
Effect of the microcapsule content on gloss loss of the coatings before and after aging.
Table 1.
List of experimental materials.
| Material | Molecular Formula | MW (g/mol) | CAS No. | Concentration | Producer |
|---|---|---|---|---|---|
| Tilia europaea | - | - | - | - | Shanghai Jiangda Economic and Trade Co., Ltd., Shanghai, China |
| waterborne acrylic resin | - | - | 9003-01-4 | - | Jiangsu Changjiang Paint Co., Ltd., Nanjing, China |
| 100 mesh rice husk powder | - | - | - | - | Xi’an Ruihe Bioengineering Technology Co., Ltd., Xi’an, China |
| hydrogen peroxide | H2O2 | 34.00 | 7722-84-1 | 5.0% | Shandong Xiekang Medical Technology Co., Ltd., Shandong, China |
| sodium hydroxide | NaOH | 40.00 | 1310-73-2 | - | Xilong Science Co., Ltd., Shantou, China |
| bisphenol A epoxy resin | C21H24O4 | 340.41 | 25085-99-8 | - | Nantong Xingchen Synthetic Material Co., Ltd., Nantong, China |
| T-31 curing agent | - | - | - | - | Nantong Xingchen Synthetic Material Co., Ltd., Nantong, China |
| melamine | C3H6N6 | 126.12 | 108-78-1 | 99.8% | Nantong Xingchen Synthetic Material Co., Ltd., Nantong, China |
| formaldehyde solution | CH2O | 30.03 | 50-00-0 | 37.0% | Xilong Science Co., Ltd., Shantou, China |
| triethanolamine | C6H15NO3 | 149.19 | 102-71-6 | 99.9% | Guangzhou Jieya Chemical Co., Ltd., Guangzhou, China |
| benzyl alcohol | C7H8O | 108.14 | 100-51-6 | 99.9% | Xilong Science Co., Ltd., Shantou, China |
| sodium dodecyl benzene sulfonate | C18H29NaO3S | 348.48 | 25155-30-0 | 99.9% | Tianjin Zhiyuan Chemical Reagent Co., Ltd., Tianjin, China |
| citric acid monohydrate | C6H10O8 | 210.14 | 5949-29-1 | 99.9% | Xilong Science Co., Ltd., Shantou, China |
| n-octanol | C8H18O | 130.29 | 111-87-5 | 99.9% | Xilong Science Co., Ltd., Shantou, China |
| ethyl acetate | C4H8O2 | 88.11 | 141-78-6 | 99.9% | Nanjing Chemical Reagent Co., Ltd., Nanjing, China |
| anhydrous ethanol | C2H6O | 46.07 | 64-17-5 | 99.9% | Wuxi Zhanwang Chemical Reagent Co., Ltd., Tianjin, China |
Table 2.
Experimental material list.
| Content of Microcapsules (%) | Weight of Microcapsules (g) | Weight of Primer (g) | Weight of Topcoat (g) |
|---|---|---|---|
| 0 | 0 | 3.00 | 3.00 |
| 1.0 | 0.03 | 3.00 | 2.97 |
| 4.0 | 0.12 | 3.00 | 2.88 |
| 7.0 | 0.21 | 3.00 | 2.79 |
| 10.0 | 0.30 | 3.00 | 2.70 |
| 13.0 | 0.39 | 3.00 | 2.61 |
| 16.0 | 0.48 | 3.00 | 2.52 |
| 20.0 | 0.60 | 3.00 | 2.40 |
Table 3.
Experimental instrument list.
| Instrument | Model | Producer |
|---|---|---|
| constant temperature heating magnetic stirrer | DF-101Z | Zhengzhou Tengyue Instrument Equipment Co., Ltd., Zhengzhou, China |
| four side film applicator | BEVS1803 | Guangzhou Keyu New Material Technology Co., Ltd., Guangzhou, China |
| scanning electron microscope (SEM) | Quanta-200 | FEI Company, Hillsboro, OR, USA |
| Fourier transform infrared spectrometer (FTIR) | VERTEX 80V | Germany Bruker Co., Ltd., Karlsruhe, Germany |
| portable colorimeter | SC-10 | Shanghai Hechen Energy Technology Co., Ltd., Shanghai, China |
| gloss meter | X-rite ci60 | Shenzhen Laite Instrument Equipment Co., Ltd., Shenzhen, China |
| portable paint film hardness tester | QHQ-A | Aikosi Electronic Technology Co., Ltd., Changzhou, China |
| paint film gridding instrument | QFH-A | Aikosi Electronic Technology Co., Ltd., Changzhou, China |
| paint film impactor tester | BEVS1601 | Guangzhou Xinyi laboratory equipment Co., Ltd., Guangzhou, China |
| electric blast drying oven | DHG-9031A | Shanghai Jinghong Experimental Equipment Co., Ltd., Shanghai, China |
Table 4.
Chromaticity value and color difference of the coatings with different microcapsule content.
Table 4.
Chromaticity value and color difference of the coatings with different microcapsule content.
| Microcapsules Content (%) | L1* | a1* | b1* | L2* | a2* | b2* | ΔL* | Δa* | Δb* | ΔE* |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 61.90 | 15.62 | 41.82 | 62.61 | 15.5 | 41.44 | 0.71 | 0.12 | 0.38 | 0.81 |
| 1.0 | 53.41 | 14.34 | 40.97 | 55.12 | 12.67 | 38.09 | 1.71 | 1.67 | 2.88 | 3.74 |
| 4.0 | 55.30 | 12.15 | 34.35 | 57.92 | 9.81 | 36.77 | 2.62 | 2.34 | 2.42 | 4.27 |
| 7.0 | 43.32 | 15.14 | 35.38 | 37.45 | 18.82 | 34.69 | 5.87 | 3.68 | 0.69 | 6.96 |
| 10.0 | 42.74 | 7.56 | 37.59 | 37.18 | 11.77 | 36.76 | 5.56 | 4.21 | 0.83 | 7.02 |
| 13.0 | 38.11 | 9.59 | 25.66 | 31.49 | 6.38 | 22.77 | 6.62 | 3.21 | 2.89 | 7.90 |
| 16.0 | 39.26 | 16.2 | 19.10 | 30.84 | 13.05 | 23.22 | 8.42 | 3.18 | 4.12 | 9.90 |
| 20.0 | 21.68 | 11.15 | 12.10 | 12.7 | 14.61 | 17.02 | 8.98 | 3.46 | 4.92 | 10.81 |
Table 5.
Gloss of the coatings with different microcapsule content.
| Microcapsules Content (%) | 20° | 60° | 85° | Light Loss Rate (%) |
|---|---|---|---|---|
| 0 | 7.2 | 31.2 | 48.5 | - |
| 1.0 | 6.5 | 23.3 | 28.1 | 25.32 |
| 4.0 | 3.6 | 14.7 | 6.4 | 52.88 |
| 7.0 | 3.1 | 13.4 | 6.0 | 57.05 |
| 10.0 | 1.6 | 7.4 | 4.4 | 76.28 |
| 13.0 | 1.4 | 3.1 | 2.7 | 90.06 |
| 16.0 | 1.6 | 3.0 | 2.5 | 90.38 |
| 20.0 | 1.4 | 2.7 | 2.3 | 91.35 |
Table 6.
Chromaticity value and color difference of the coatings before and after aging.
| Microcapsules Content (%) | State | L* | a* | b* | ΔL* | Δa* | Δb* | ΔE* |
|---|---|---|---|---|---|---|---|---|
| 0 | before aging | 62.25 | 15.56 | 41.63 | 5.57 | 4.68 | 0.77 | 7.31 |
| after aging | 56.68 | 10.88 | 40.86 | |||||
| 1.0 | before aging | 54.26 | 13.50 | 39.53 | 5.87 | 3.32 | 1.36 | 6.88 |
| after aging | 48.39 | 10.18 | 38.17 | |||||
| 4.0 | before aging | 56.61 | 10.98 | 33.40 | 5.32 | 2.38 | 2.16 | 6.22 |
| after aging | 51.29 | 8.60 | 35.56 | |||||
| 7.0 | before aging | 35.49 | 16.98 | 35.03 | 4.89 | 2.67 | 1.17 | 5.69 |
| after aging | 40.38 | 14.31 | 33.86 | |||||
| 10.0 | before aging | 39.96 | 9.66 | 37.17 | 5.63 | 2.40 | 0.98 | 6.20 |
| after aging | 34.33 | 7.26 | 36.19 | |||||
| 13.0 | before aging | 34.8 | 7.98 | 22.30 | 6.21 | 3.78 | 1.91 | 7.52 |
| after aging | 28.59 | 4.20 | 24.21 | |||||
| 16.0 | before aging | 35.05 | 14.64 | 21.16 | 7.14 | 2.80 | 2.51 | 8.07 |
| after aging | 27.91 | 11.84 | 18.65 | |||||
| 20.0 | before aging | 17.19 | 10.22 | 14.46 | 8.23 | 2.66 | 3.13 | 9.20 |
| after aging | 8.96 | 12.88 | 11.33 |
Table 7.
Gloss loss of the coatings before and after aging.
| Microcapsules Content (%) | 20° | 60° | 85° | Light Loss Rate after Aging (%) |
|---|---|---|---|---|
| 0 | 5.3 | 26.8 | 43.0 | - |
| 1.0 | 7.4 | 28.5 | 45.2 | −6.34 |
| 4.0 | 3.3 | 14.4 | 14.0 | 46.27 |
| 7.0 | 3.2 | 14.4 | 13.9 | 47.76 |
| 10.0 | 2.2 | 10.9 | 13.6 | 59.33 |
| 13.0 | 2.7 | 10.6 | 12.2 | 60.45 |
| 16.0 | 2.1 | 10.0 | 5.0 | 62.69 |
| 20.0 | 2.0 | 8.1 | 3.5 | 69.78 |
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |
© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Share and Cite
MDPI and ACS Style
Huang, N.; Yan, X.; Han, Y. Preparation of Melamine-Formaldehyde Resin/Rice Husk Powder Coated Epoxy Resin Microcapsules and Effects of Different Microcapsule Contents on the Properties of Waterborne Coatings on Tilia europaea Surface. Coatings 2022, 12, 1213. https://doi.org/10.3390/coatings12081213
AMA Style
Huang N, Yan X, Han Y. Preparation of Melamine-Formaldehyde Resin/Rice Husk Powder Coated Epoxy Resin Microcapsules and Effects of Different Microcapsule Contents on the Properties of Waterborne Coatings on Tilia europaea Surface. Coatings. 2022; 12(8):1213. https://doi.org/10.3390/coatings12081213
Chicago/Turabian StyleHuang, Nan, Xiaoxing Yan, and Yan Han. 2022. "Preparation of Melamine-Formaldehyde Resin/Rice Husk Powder Coated Epoxy Resin Microcapsules and Effects of Different Microcapsule Contents on the Properties of Waterborne Coatings on Tilia europaea Surface" Coatings 12, no. 8: 1213. https://doi.org/10.3390/coatings12081213
Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.
