Keywords

1 Introduction

Water-based coatings are eco-friendly because they reduce the emission of environmentally unfriendly volatile organic compounds. Waterborne coatings are usually made by physically blending and emulsifying the binder, pigments, and additives with water. Copolymers of acrylic, vinyl, and styrene compounds are typically used as the binder in this type of coating. Of all the types of polymers, acrylic copolymer is one of most commonly used resins in waterborne coatings. In general, acrylic resins are synthetic copolymers of acrylic and methacrylic acids or their corresponding esters created by free-radical polymerization [1]. Acrylic resin is a low-cost material compared with other resins such as epoxy resins and polyurethane, and acrylic copolymers can be stable in water to form aqueous dispersions for application in eco-friendly coatings [2, 3]. They show excellent performance in film formation, high gloss, good adhesion, fast drying, outdoor durability, high transparency, and so forth [4,5,6,7,8,9,10,11]. Acrylic resins have good compatibility with other components and can be modified by a variety of components, such as polymers, silica nanoparticles, mucin gel, fiber, etc., to form coatings with improved performance [12,13,14,15]. In general, acrylic resin is a popular base polymer for developing coatings with desired new properties and for applications in adhesives, construction, automotive and additives, etc. [16,17,18,19,20,21]. Unfortunately, acrylic-based waterborne coatings have some weaknesses such as low water and corrosion resistance, poor thermal stability and weak chemical resistance, and waterborne acrylic-based coatings are considered inappropriate for application in corrosive environments. If these weaknesses of waterborne acrylic coatings can be overcome, their applicability could significantly broaden.

The monomers used to synthesize acrylic copolymers are all with vinyl groups, which makes the structure of acrylic copolymers easy to manipulate. In this regard, acrylic copolymers can easily react with other components and form covalent bonds between different components. The acrylic-based copolymers can be mixed at a molecular scale and can exhibit new functionalities. For instance, anti-icing performance was gained by the acrylic–silicone copolymer used for wind turbine blades [22]. The introduction of indole derivative groups or tertiary amines into acrylic resins can produce copolymers with anti-fouling performance [23, 24]. A biocompatible copolymer was copolymerized by acrylic copolymer and polyhedraloligosilsesquioxane [25]. These acrylic-based copolymers exhibit biocompatible stability and could be developed as a denture resin. An acrylic–poly(dimethyl siloxane) copolymer with improved gas permselectivity was synthesized by an atom transfer radical polymerization technique [26].

Conventionally, acrylic-based copolymers are prepared with a variety of materials, including organic polymers (polyurethane, polystyrene and epoxy), organic silicone, inorganic nanoparticles (clay, silica), etc. In this study we intended to extend this to the designing of waterborne coatings with intended functionalities through manipulation of acrylic-based copolymers with different nanostructures. The objective was to show that the acrylic copolymer can be an excellent base polymer for developing waterborne coatings by manipulating its nanostructure through the introduction of different components into the acrylic base. Experiments were carried out to demonstrate this approach by synthesizing several acrylic-based waterborne coatings with different nanostructures, including homogeneous, worm-like, and spherical-like nanostructures, by introducing three different components.

2 Methods

2.1 Materials

Acrylic acid (AA), 1,1’-azobis(cyclohexanecarbonitrile) (ACHN), 2-dimethylaminoethanol (DMEA), 1-methoxy-2-propanol, 2-mercaptoethanol, 3-methacryloxypropyltrimethoxysilane (MPS), titanium (IV) oxide (TiO2), methacryloyl chloride, bis(3-aminopropyl) terminated PDMS and 1-methoxy-2-propanol obtained from Sigma-Aldrich were used directly without further purification. Styrene (St), n-butyl acrylate (BA), methyl methacrylate (MMA), 2-hydroxypropyl acrylate (HPA) were supplied by Sigma-Aldrich and were all purified by Al2O3 (Sigma-Aldrich) chromatographic column. Alkyd was synthesized as described in our previous study [27]. Curing agent amino resin Resimene 717, a type of melamine resin with alkoxy groups, was supplied by Jiangsu Shisong New Materials Technology Co., Ltd.

2.2 Preparation of Waterborne Acrylic Copolymer, Acrylic-Based Coatings and Coating Films

TiO2 was modified by MPS to introduce reactivity [28]. Bis(3-aminopropyl) terminated PDMS and methacryloyl chloride were used to prepare vinyl-terminated PDMS. The copolymerization process of acrylic and acrylic-based coatings was as follows. The MPS-modified TiO2 or alkyd or vinyl-terminated PDMS was dispersed in 1-methoxy-2-propanol and added to a 4-necked flask. A mixture of acrylic monomers (BA (50 g), HPA (15 g), AA (15 g), MMA (10 g) and St (10 g)), initiator and solvent was then dropped into the flask and the reaction temperature was 88 °C. The content of MPS-modified TiO2 or alkyd or vinyl-terminated PDMS was 5 wt% of the acrylic monomers. The reaction was maintained for 3 h. After copolymerization, the solvent was removed by nitrogen gas. DMEA was used to neutralize part of the carboxyl groups at 50–60 °C, and water was added to prepare copolymer aqueous dispersions under mechanical stirring to obtain completely homogeneous dispersions. The copolymers synthesized were named acrylic, acrylic-TiO2 coating, acrylic–alkyd coating and acrylic–PDMS coating according to the additional component added to the acrylic system. The samples that were a physical blending of acrylic copolymer aqueous dispersion with MPS-modified TiO2, alkyd and vinyl-terminated PDMS separately were also prepared for comparison, and the loading content of other components was 5%wt of acrylic copolymer. The samples were named PB-acrylic/TiO2 coating, PB-acrylic/alkyd coating and PB-acrylic/PDMS coating. The coating films were prepared from copolymer aqueous dispersions, which were mixed with the curing agent. After mixing, the mixture was poured onto the substrate and a film adaptor was used to get a wet film of 200 µm. The wet films were dried at room temperature for 1 h, and then cured at 140 °C for 30 min to get dry films of 20 ± 2 µm.

2.3 Characterization

The absorption performance of the coating films was measured using an ultraviolet−visible spectrophotometer (Cary Series UV–Vis spectrophotometer, Agilent, USA) in the range of 200–800 nm. The ultraviolet protection factor (UPF) value of the coating films was measured by a UPF and UV penetration/projection measurement system (Model: YG902) from 280 to 400 nm. The mean UPF was calculated automatically by the test system from eight sets of data obtained from different test areas. Transmission electron microscopy (TEM) was used to study the morphologies with a JEOL-2100 microscope under 120 kV. To obtain clear morphologies of micelles, the acrylic and acrylic–alkyd coating samples were negatively stained with a uranyl acetate substitute (UAR-EMS stain) for 30 min at room temperature. Atomic force microscopy (AFM) was used to evaluate the surface morphology and microphases of the coating films. The AFM measurements of the films were performed with a Cypher AFM microscope (Asylum Research). Electrochemical impedance spectroscopy (EIS) tests were conducted with a Bio-Logic electrochemical workstation at ambient temperature (25 ± 2 °C). A three-electrode cell arrangement with 3.5%wt (w/v) NaCl solution was used to conduct the tests. Carbon steel substrates coated with the films were set as the working electrode with a circular tested area of ~ 1 cm2. An Ag/AgCl (Sat. KCl) electrode was used as the reference electrode and a Pt-coated Ti mesh was used as the counter electrode. The amplitude of the sinusoidal voltage was 10 mV, and the frequency range was 100 kHz to 10 MHz. The EIS data were acquired when the samples were immersed in salt solution after 2 days. Immersion test was performed in 3.5 wt% (w/v) NaCl solution at room temperature. The edges and back sides of the samples used for the immersion test were all covered with epoxy resin. The surface morphologies of the coating films were detected by scanning electron microscopy (SEM) with a Zeiss Supra 55 VP under 5 kV. The samples were coated with 5-nm Au film for good conductivity. The contact angle of the coating films was evaluated by Tensiometer KSV CAM 101 (KSV Instruments Ltd, Finland) at room temperature and water was used as the test liquid. The dirt-picking performance was measured by placing black carbon particles on the coating film and using water to clean the films.

3 Results and Discussion

3.1 Enhanced UV Protection Through the Formation of an Acrylic-Based Nanocomposite

An example of creating a functional waterborne coating with desirable nanostructure is the incorporation of MPS-modified nanoparticle TiO2 in an acrylic base to enable the formation of a homogeneous nanostructure with nanoparticles uniformly distributed in the acrylic base. UV protection is an important property for waterborne coatings that are exposed to intense UV environmental conditions. It is well known that incorporating UV absorbers into the polymer matrix is a method of improving the UV protection of polymers. Nanoparticle TiO2 is a commonly used UV absorber for improving the protection performance of acrylics. In the present study, the UV protection performance of the acrylic base coating and the coating with nanoparticle TiO2 was evaluated by UV absorbance and UPF value tests, as shown in Fig. 1a, b. The absorbance of the PB-acrylic/TiO2 coating film slightly increased compared with acrylic film, which we believed to be due physical blending being unable to achieve a coating with uniformly distributed nanoparticle TiO2. The distribution of nanoparticles and compatibility between nanoparticles and polymer matrix need to be improved to obtain well-dispersed mixer for high efficiency of UV protection [29, 30]. This was achieved by the acrylic–TiO2 coating film that incorporated nanoparticle TiO2 in the acrylic base by copolymerization, which enabled the formation of a nanocomposite with a homogeneous nanostructure of uniformly distributed nanoparticles in the acrylic base. As shown in Fig. 1a, the coating film with the acrylic–TiO2 nanocomposite showed significantly better UV absorption performance than the coating film made by physical blending, especially in the wavelength between 315 and 400 nm (UV-A). In addition, the UPF value of the acrylic–TiO2 coating film was more than threefold higher than that of the PB-acrylic/TiO2 coating film (Fig. 1b). The remarkable improvements in UV absorbance and UPF value of the acrylic–TiO2 coating can be explained by the uniformity of the coating film. The appearance of the PB-acrylic/TiO2 and acrylic–TiO2 coating films is shown in Fig. 1c. It is obvious that the acrylic–TiO2 coating film has a homogeneous distribution of nanoparticles.

Fig. 1
3 illustrations. A, a spectra graph of absorbance versus wavelength plots 3 downward lines. B, a spectra graph of U P F value versus different films plots an upward lines. C, a set of 2 photographs of P B acrylic or T i O 2 coating film and acrylic T i O 2 coating film.

a UV absorption spectra of acrylic base coating film and acrylic-based coating films by UV–vis, b UPF value of coating films in the wavelength of 280–400 nm, c images of the PB-acrylic/TiO2 and acrylic-TiO2 coating films

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The difference in UV absorbance can be explained by the acrylic–TiO2 coating prepared through copolymerization having improved compatibility between the acrylic base and the nanoparticle TiO2. In this case, the copolymerization of acrylic and MPS-modified nanoparticle TiO2 achieved the uniform distribution. From the TEM image in Fig. 2b, it is obvious that the TiO2 nanoparticles are wrapped by the acrylic copolymer. Moreover, from the AFM images in Fig. 3, the acrylic–TiO2 coating film has a relatively homogeneous nanostructure, therefore the filling of TiO2 nanoparticles by copolymerization leads to a uniform coating film compared with physical blending. In general, the enhanced UV absorbance and UPF value of the acrylic–TiO2 coating film suggest that the uniform distribution of nanoparticle TiO2 in the acrylic base plays an important role in improving UV protection.

Fig. 2
A set of 4 micrographic images. Images a and c represent fine surfaces with a few dots. Images b and d represent dark shaded spots and circular dark spots, respectively.

Transmission electron microscopy images of copolymer water dispersions, a acrylic copolymer, b acrylic-TiO2 copolymer c acrylic–alkyd copolymer, and d acrylic–PDMS copolymer

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Fig. 3
A set of 4 microscopic images of different topography is at the top, and 4 microscopic images of different phases are at the bottom, of an acrylic co-polymer film and different acrylic coating films. A set of 4 illustrations depicts worm-like nano-structures that are tightly packed.

Atomic force microscopy topographic and phase images of acrylic copolymer film and acrylic-based coating films

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3.2 Enhanced Corrosion Resistance of Acrylic–Alkyd Coating with Worm-Like Nanostructure

Acrylic waterborne coatings often have weak corrosion resistance in humid environments [31, 32]. A possibility method of enhancing their corrosion resistance is to incorporate other components with high hydrophobicity such as alkyd in order to produce anti-corrosion waterborne coatings [33]. Unfortunately, waterborne coatings made by physical blending of acrylic and alkyd polymer failed to show increased anti-corrosion performance. As shown in Fig. 4, the corrosion resistance of PB-acrylic/alkyd coating film, as indicated by the impedance values from the Nyquist plots and impedance spectrum of the coating, actually reduced compared with the acrylic coating. This result suggested that physical blending of acrylic with alkyd cannot improve the corrosion resistance of the acrylic base, possibly because physical blending of acrylic and alkyd cannot obtain a uniform coating film with a dense film surface due to incompatibility. In contrast, as shown in Fig. 4, the anti-corrosion performance of the acrylic–alkyd coating film was significantly enhanced, which we believe was due to the uniform distribution of alkyd in the acrylic base, better compatibility and the formation of a dense coating film. For the acrylic–alkyd coating film, the aqueous dispersion formed nanoparticles of ≈100 nm as shown in Fig. 2c. In addition, the acrylic–alkyd coating film formed a nanocomposite with a uniform and worm-like nanostructure shown in Fig. 3. The worm-like nanostructure was more closely packed, which created a better barrier that reduced the penetration of corrosive salt solution into the coating film, thereby enhancing the anti-corrosion performance of the acrylic base. The dense film surface of the acrylic–alkyd coating film was confirmed in the SEM image shown in Fig. 4f as compared with the loose surface of the acrylic film shown in Fig. 4e. Moreover, the immersion test confirmed the EIS result.

Fig. 4
6 illustrations. Graphs a and b plot negative z i m versus z r e. Graph c plots mod z versus frequency. A has 1 upward and 2 downward trends. B and C have 3 downward trends. D presents 6 scanned images of different co-polymer and coating film types. Micrographs E and F represent fine surfaces.

Impedance spectra of the acrylic copolymer film and acrylic-based coated carbon steel in 3.5%wt NaCl aqueous solution: a Nyquist plots, b enlarged part of the Nyquist plots, c Bode plots, d immersion tests of coating films in 3.5 wt% NaCl solution at room temperature; scanning electron microscopy images of e acrylic film, and f acrylic–alkyd coating film

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3.3 Multifunctional Acrylic–PDMS Coating with Spherical-Like Nanostructure

Acrylic waterborne coatings are also commonly used to protect surfaces from dirt and graffiti. PDMS has low surface energy and high hydrophobicity, which is often used to creating easy-clean coatings. As another example of creating a desirable waterborne coating through manipulation of acrylic-based nanocomposites with different nanostructures, the PDMS component was incorporated to prepare an acrylic-based copolymer with a high dirt resistance. The surface performance of the coating films was evaluated, as shown in Fig. 5a. The contact angle of the acrylic–PDMS coating film increased compared with acrylic film and the PB-acrylic/PDMS coating film formed by blending the hydrophobic PDMS component. In addition, the dirt-resisting performance of the coating films was tested and shown in Fig. 5b. From the images of the PB-acrylic/PDMS coating film, it is obvious that the black carbon particles were not washed off completely, whereas the acrylic–PDMS coating film was totally cleaned by water washing.

Fig. 5
2 illustrations. A, a error line graph of contact angle versus different acrylic coating films plots an increasing error line. B, 6 scanned images of different acrylic coating films. The images before washing have spots on the surface. The images after washing have a clear surface.

a Water contact angle of acrylic and acrylic-based coating films, and b images of dirt-picking property of the coating films

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The dirt-resistance performance of the PB-acrylic/PDMS coating film was unsatisfactory because the PDMS component is hydrophobic and cannot be uniformly distributed in the acrylic base by physical blending due to the poor compatibility between acrylic and PDMS. In contrast, copolymerization improved the compatibility between the acrylic and PDMS components and contributed to the formation of a uniform and spherical-like nanostructure, which can explain the excellent dirt-resistant property of the acrylic–PDMS copolymer. From Fig. 2d, it is clear that the acrylic–PDMS copolymer formed a nanocomposite with particles at the nanoscale. Figure 3 shows spherical-like nanostructure of the acrylic–PDMS coating. In this nanostructure, the PDMS component tended to migrated to the top of the surface due to its low surface energy and hydrophobicity. In this case, the acrylic–PDMS film surface is a PDMS-rich phase that can show similar properties to the PDMS component. These properties contributed to the improvement in the dirt-picking performance of the acrylic–PDMS coating film.

4 Conclusion

We carried out experiments to demonstrate the design of multifunctional waterborne coatings through manipulation of acrylic-based copolymers with different nanostructures. Three components were chosen to study–an inorganic nanoparticle (TiO2), an organic short polymer (alkyd) and an inorganic polymer (PDMS)–to manipulate the acrylic nanostructure in order to create waterborne coatings with desirable functionalities and improve the compatibility between different components. The copolymerization of MPS-modified TiO2 and acrylic monomers achieved a uniform acrylic–TiO2 coating film. The uniform distribution of nanoparticles led to better UV protection property compared with the physical blended sample. Due to thermodynamic incompatibility of acrylic and alkyd, the acrylic–alkyd coating film formed a worm-like nanostructure, which contributed to its dense film surface that had better barrier performance and enhanced the corrosion resistance. The acrylic–PDMS coating film formed a spherical-like nanostructure and the film surface was a PDMS-rich phase, which showed similar performance to the PDMS component compared with PB-acrylic/PDMS. Therefore, the properties of the acrylic–PDMS coating changed significantly: the contact angle increased to 103° and the coating film showed enhanced resistance to dirt-picking. Generally, by copolymerizing acrylic with other components, it is easy to manipulate the nanostructure of acrylic-based copolymers and improve compatibility between different components. Moreover, the nanostructure of acrylic-based copolymers plays an important role in the coating’s performance. Acrylic-based copolymers with an organized nanostructure can exhibit desirable functions. This approach to designing acrylic-based copolymers with an organized nanostructure could open opportunities for developing waterborne acrylic-based copolymers with multifunctionality.

5 Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.