Simple Spray Preparation of Multifunctional Organic–Inorganic Hybrid Coatings for Surface Strengthening of Flat Thin-Sheet Materials

Abstract

To enhance the mechanical performance and surface hydrophobicity of flat thin-sheet materials, we have developed a facile, environmentally benign, and low-cost synthesis strategy for fabricating a robust waterborne superhydrophobic coating with excellent mechanical reinforcement, via simple spray coating using a non-fluorinated material system (waterborne silicone–acrylic copolymer and silica sol). The functional coating exhibited excellent hydrophobicity (water contact angle: 150°) regardless of the compound of the substrates, which is primarily ascribed to the presence of abundant low-surface-energy methyl groups on the coating’s surface, along with the three-dimensional hierarchical network structure formed via the cross-linked silica network. Owing to the stable cross-linked structure and strong interfacial bonding between the acrylic polymer and silica network, the composite coating exhibited exceptional mechanical reinforcement, coupled with ultrahigh mechanical and chemical stability. Specifically, the maximum flexural fracture load of the modified materials increased from 119 N to 192 N, representing a 62.7% enhancement; similarly, the moisture-induced deflection of the samples had a significant increase from −14.5 mm to −3.01 mm, which confirmed that the mechanical properties of the modified sample and its deformation resistance under high humidity conditions have been significantly enhanced. Notably, the coating retained superior hydrophobicity and mechanical performance even after 50 abrasion cycles, as well as exposure to high-intensity UV radiation and corrosive acidic/alkaline environments. Furthermore, the composite functional coating demonstrated excellent self-cleaning and anti-fouling properties. This functional composite coating offers significant potential for large-scale industrial application.

1. Introduction

Flat thin-sheet materials, such as mineral wool boards, metal plates, and polymer composite boards [1], are endowed with excellent processability and superior decorative properties. They can be engineered via composite integration with other functional materials to form multifunctional composites with synergistic effects, which can meet the multidimensional performance demands in complex operational service environments. Consequently, it exhibits substantial application potential across diverse critical sectors, e.g., construction engineering (roofing structures, interior/exterior wall decoration), transportation (high-speed rail carriage interiors, ship cabin structures), and industrial equipment manufacturing.

However, the composite of multiple materials tends to bring a significant increase in cost. Furthermore, their preparation methods are generally complex, which are not conducive to large-scale commercial production, thereby restricting the wide application of such materials. Furthermore, monolithic planar thin panel materials may exhibit diverse deformations [2,3,4], compromising their service life. This deformation is primarily induced by external factors including gravitational load, ambient humidity fluctuations, and applied external forces, ultimately leading to a shortened service life of the materials.

Therefore, to address the limitations of monolithic plate materials, such as poor mechanical properties and high-water absorbency, researchers have turned to surface strengthening technologies [5], including surface heat treatment strengthening [6], surface deformation strengthening [7,8,9], and surface coating strengthening. Among these techniques, surface coating strengthening technology has garnered significant attention from both industrial and scientific research, as it can achieve excellent mechanical reinforcement and surface hydrophobicity through a simple process and is applicable to various substrate materials [10,11]. Surface coating technology deposits functionally tailored composite coatings on substrates via techniques including dip coating [12,13,14,15], spray coating [16,17,18,19], spin coating [20,21,22,23], sol–gel process [24,25,26], chemical vapor deposition [27,28,29], and electrochemical deposition [30,31], yielding multifunctional composite materials. Inspired by the wettability of lotus leaves [10,32], the hydrophobic behavior of a solid material surface arises from the combined effect of its surface chemical composition (e.g., low surface energy fictional groups) and special surface morphologies (e.g., micro- and nano-scale hierarchical structures, porous nanostructures) [33,34,35]. Based on this mechanism, two typical strategies to achieve artificial superhydrophilicity have been proposed: either by modifying rough surfaces with low-energy materials on hydrophobic surfaces, or constructing roughness and hierarchical structures [14,36,37,38]. In recent years, studies have shown that modifying material surfaces with silica nanoparticles (SiO₂ NPs) markedly improves their wear resistance and operational stability. This improvement is attributed to the unique hierarchical structures and high hardness of NPs, rendering them among the most extensively studied and applied compounds in functional coatings [14,39,40]. Caldona et al. [41] constructed superhydrophobic and superoleophilic nanocomposite by incorporating SiO₂ NPs into the rubber-modified polybenzoxazine (PBZ) through a facile dipping and spraying technique. By spraying the polystyrene/SiO₂ core/shell NPs as a coating skeleton and the polydimethylsiloxane (PDMS) as hydrophobic interconnection, Xue et al. [42] successfully fabricated the composite coating surfaces with lasting and self-healing superhydrophobic properties. Notably, their composite coating exhibited self-healing properties, offering a novel strategy for the automatic restoration of superhydrophobicity on coating surfaces. Importantly, integrating the Stöber protocol [43] which enables the synthesis of uniform SiO₂ nanoparticles (NPs) with other techniques (e.g., modifying SiO₂ with low-surface-energy materials) has emerged as one of the most promising approaches for fabricating SiO₂-based functional NPs and coatings. However, most previous studies focused on modifying SiO₂ NPs to mimic lotus leaf surfaces have relied on costly and hazardous fluorine-based chemicals as low-surface-energy modifiers. Such chemicals pose detrimental effects on both the environment and human health during their fabrication and application processes [44,45,46]. In addition, poor mechanical, chemical and thermal stability is a major problem limiting the widespread practical applications of superhydrophobic surfaces due to their weak structures and facile chemical degradation [47,48]. Therefore, there remains an urgent need to develop a novel, scalable, and environmentally friendly approach for fabricating composite coatings with superior hydrophobicity and mechanical strength [43].

Accordingly, we tackle the aforementioned challenges through three approaches. First, we developed environmentally benign waterborne coatings from fluorine-free materials including silicon-modified acrylic copolymers (SAC) and silica sol (SS), via a facile method, as shown in Figure 1a. Specifically, methyltrimethoxysilane (MTMS) is converted into silica sol through a sol-gel reaction; this silica sol is then blended with modified acrylic polymers, enabling in situ modification of polymer groups under alkaline conditions and cross-linking of the organic-inorganic network (Figure 1b). Second, spray coating, a technique widely recognized as a scalable fabrication method, had been employed to fabricate composite coatings with superior hydrophobicity and mechanical reinforcement on a variety of substrates. Finally, the stable Si–O–Si cross-linked network within the composite coatings, together with strong interfacial bonding between the inorganic network and organic polymer matrix, endowed the modified materials with exceptional mechanical and chemical stability. Specifically, it retained outstanding hydrophobicity and mechanical performance even after 50 abrasion cycles, as well as exposure to high-intensity UV radiation and corrosive acidic/alkaline environments. Furthermore, the composite coatings demonstrated excellent self-cleaning and foul-repellent properties.

2. Materials and Methods

2.1. Materials

Methyltrimethoxysilane (MTMS, 98%) and Ammonia water solution (NH₄OH, 25%–28%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Ethanol (99%) was ordered from Beijing Tongguang Fine Chemical Co., Ltd. (Beijing, China). Silicone-modified acrylic emulsion (SAC) was supplied by Shanghai yuanye Bio-Technology Co., Ltd. (Shanghai, China), while other substrate materials were purchased locally. Deionized water (>18.2 MΩ cm) was produced with a Genle water system (Model No. S8PW121204) from Shanghai Rephile Bioscience, Ltd. (Shanghai, China).

2.2. Preparation of the Multifunctional Organic-Inorganic Hybrid Coatings

The composite coating’s aqueous deposition solution was formulated by integrating a waterborne siloxane-functionalized acrylic copolymer (SAC) with a silica sol (SS). The silica sol was generated by the hydrolysis of MTMS, followed by in situ modification and complexation with SAC under alkaline conditions. Then, the solution was directly employed to deposit the composite coating on different substrates using a spraying deposition method (Figure 1a).

Briefly, 5 mL of ammonia water solution, 50 mL of ethanol, and 10 mL of deionized water were added to a round-bottom three-necked flask equipped with a magnetic stirrer, thermocouple, heating mantle, and condenser. Then, the uniformly stirred mixed solution was heated to 60 °C. When the solution temperature had stabilized, MTMS was added dropwise. After 1.5 h of continuous heating, the heating was stopped, and the mixed solution was allowed to cool to room temperature before being aged for 16 h. A specific quantity of the silicon-containing acrylic copolymer (SAC) then was dissolved into the above dispersion by sonication for 30 min.

All substrates were cut into small pieces (3 × 6 cm²) and used directly without further pretreatment, except for being washed with ethanol and subsequently air-dried. However, since mineral wool board samples could not be cleaned with ethanol, their surface impurities were only required to be thoroughly removed using an air spray gun. After ultrasonic mixing, the solution was sprayed onto surfaces of various substrates via a spray gun at a compressed air pressure of 0.6 MPa. The gun featured a 1.5 mm diameter nozzle and was positioned 10 cm from the substrates. To ensure coating uniformity, the coating weight was controlled at 222 g m⁻². The coatings were obtained after curing at 80 °C for 1 h.

2.3. Example Characterization Technology

2.3.1. Hydrophobic Properties Test

The Water contact angle (WCA) of the prepared samples was measured using Optical Static Contact Angle Tester (JC2000D1, Shanghai Zhongchen Digital Technology Apparatus Co., Ltd., Shanghai, China). A syringe was used to deposit 5.0 μL of deionized water onto substrate surfaces; following 3 s of stabilization, images were captured and data recorded. Each sample was tested at 3 distinct locations to determine the average contact angle.

The surface free energy (SFE) was determined via the Owens method with distilled water (polar liquid) and diiodomethane (non-polar liquid) [31,32]:

$${\mathsf{\gamma }}_{\mathrm{L}}\left(1+\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\theta }\right)=2\sqrt{{\mathsf{\gamma }}_{\mathrm{S}}^{\mathrm{d}}{\mathsf{\gamma }}_{\mathrm{L}}^{\mathrm{d}}}+2\sqrt{{\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{p}}{\mathsf{\gamma }}_{\mathrm{L}}^{\mathrm{P}}}$$

(1)

where ${\mathsf{\gamma }}_{\mathrm{L}}$ represents the surface tension of the liquid; θ is the contact angle (CA) between the solid and liquid phases; ${\mathsf{\gamma }}_{\mathrm{S}}^{\mathrm{d}}$ and ${\mathsf{\gamma }}_{\mathrm{s}}^{\mathrm{p}}$ stand for the dispersion and polar components of the solid’s SFE, respectively; and ${\mathsf{\gamma }}_{\mathrm{L}}^{\mathrm{d}}$ and ${\mathsf{\gamma }}_{\mathrm{L}}^{\mathrm{P}}$ correspond to the dispersion and polar components of the liquid’s SFE, respectively.

2.3.2. Characterization of Surface Morphology and Chemical Composition

The morphologies of composite coatings on substrates were characterized via a field emission scanning electron microscope (SEM, Hitachi SU-8000, Tokyo, Japan) at an acceleration voltage of 5 kV. The elemental distribution of the samples was analyzed using an electron microscope equipped with an energy dispersive spectrometer (EDS, FEI Tecnai G2 F20 S-TWIN, Hillsboro, OR, USA). The surface roughness of the composite coating was characterized using an atomic force microscope (AFM, Bruker Dimension Icon, Berlin, Germany) operated in Tapping Mode. A RFESPA-75 probe was employed for the measurements, with the following scanning parameters set: a scan size of 5.00 μm, a scan rate of 1.00 Hz, and a resolution of 256 samples per line. All samples were sputter-coated with gold to improve surface conductivity.

Surface chemical functional groups were examined via the Fourier transform infrared spectroscopy (FTIR, Bruker Tensor, Berlin, Germany) over the range 4000–500 cm⁻¹. The ATR method was applied, with a resolution of 4 cm⁻¹ and 32 scans performed per sample. X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250Xi, Waltham, MA, USA) was used to analyze the chemical state of the elements, using C 1s 284.8 eV as the calibration standard.

2.3.3. Mechanical Strengthening Performance Test

Planar materials are prone to surface deformation during use; therefore, reinforcement treatments are typically applied to enhance their resistance to sagging. The mechanical reinforcement effects of composite organic-inorganic hybrid coatings were evaluated using flexural failure load and moisture-induced deflection. The composite solution was uniformly sprayed onto the surface of a 30 × 60 × 2 cm³ mineral wool substrate. After drying at 80 °C, the flexural failure load and moisture-induced deflection of the blank mineral wool substrate and the mineral wool board sprayed with the hybrid coating were tested in accordance with GB/T 25998-2020 [49] Mineral wool decorating and acoustic ceilings.

In addition, the adhesion strength between composite coating with different MTMS contents and the mineral wool board substrate were tested by using pull-off method with a digital display pull-off adhesion tester (BIUGED BGD-500, Guangzhou, China), in accordance with the standard GB/T 5210-2006 [50] Paints and varnishes—Pull-off test for adhesion.

2.3.4. Ultraviolet Resistance Test and Thermal Stability Test

The UV resistance of the UVA cross-linker was evaluated at 340 nm. The exposure intensity was measured at 3.3 mW·cm⁻² via the instrument’s internal calibration. The UV chamber was heated to approximately 70 °C, and exposure was cycled in 12 h intervals up to a total of 96 h. After every 12 h, hydrophilicity and mechanical performance tests on the samples were performed using water contact angle (WCA) measurements and flexural failure load tests.

The thermal stability of the composite coating was evaluated by placing the modified samples in an oven at 80 °C for a certain period, followed by testing the changes in the samples’ hydrophobic properties and mechanical properties over time. Specifically, the procedure was as follows: The modified mineral wool boards prepared in Section 2.3.3 were placed in an oven set at 80 °C and continuously held for 1–7 days. Each day, the boards were removed to test their hydrophobic properties and flexural fracture strength. Each test was repeated three times, and error bar analysis was performed.

2.3.5. Abrasive Resistance and Chemical Durability

Mechanical abrasion and chemical stability tests are widely employed to assess the mechanical and chemical durability of functional coatings. The modified sample was placed on 1200 mesh sandpaper with its coated surface facing downward, and a 50 g load was applied. The sample was moved linearly perpendicular to the coating surface over a 10 cm distance at 2.5 cm·s⁻¹, before being returned along the same path to complete one wear cycle. After each abrasion cycle, the WCA and flexural failure load were measured to evaluate abrasion resistance. To assess chemical stability, the WCA on the coatings was measured using droplets with pH values from 2 to 12. Three random positions on the coating surface were selected for WCA measurement, and error bar analysis was performed.

2.3.6. Antifouling and Self-Cleaning Properties

To evaluate the antifouling performance of the coating in practical applications, five daily common liquids of milk, coffee, orange juice, tea, and soy sauce were chosen as foulants. These liquids were deposited onto the prepared samples, and the antifouling performance was assessed by observing the droplet behavior on the coating surface.

Additionally, a 15° inclined platform was employed to evaluate the samples’ self-cleaning performance. A layer of carbon powder was evenly spread on the upward-facing test surface of the samples, followed by rinsing the surface with deionized water from a 2 cm height. The self-cleaning effect was then assessed by observing the removal of carbon powder.

3. Results and Discussion

3.1. Hydrophobic Properties of the Composite Coating

With respect to composite functional coatings, addressing their resistance to damage under humid conditions is a key priority. Consequently, the wetting behavior of these coatings was initially tested and analyzed. The hydrophobic properties of the functional coating surfaces on different substrates were characterized by water contact angle (WCA) measurements. We first tested the modified microscope glass slide samples—an inherently flat substrate. As shown in Figure 2a, the WCA of the modified glass slides was measured to be 150 ± 1°. Water droplets landing on the surface of the composite coating bounced noticeably before sliding off the surface rapidly. (Video S1). Additionally, Figure 2b depicted the contact process of a 4 μL water droplet suspended from a syringe needle with the coating surface. Even after undergoing significant deformation, the water droplet was seen to almost completely detach from the surface. These results confirm the successful fabrication of a superhydrophobic surface using the functional monomer MTMS and SAC through this straightforward method.

As shown in Figure 2c, as the amount of MTMS in the coating increased from a small amount to 1.2 mL, the WAC of the composite coating gradually increased. However, with the amount of MTMS continued to increase, the contact angle sharply decreased. Similarly, when different contents of SAC were added, the contact angle of the composite coating exhibited the same behavior as shown in Figure 2d. As the SAC dosage gradually increased from 0 to 0.2 g, the contact angle gradually increased. But as the dosage of SAC continued to increase, the contact angle decreased significantly. Based on the above results, it can be concluded that the excellent hydrophobic performance of the composite coating is correlated with the contents of MTMS and SAC in the hybrid coating. However, the two components played distinct roles in the composite coating. Specifically, the self-crosslinking waterborne silica-modified acrylic polymer served as an adhesive substrate for the hydrophobic silica particles. Polymerization could occur either between the nano-silica particles or between SAC molecules and silica particles; this enabled the formation of a composite network structure (as shown in Figure 1b) consisting of the organic polymer matrix and the Si–O–Si inorganic network, which exhibited strong bonding interactions. However, when MTMS was used in excess, it would lead to the excessive polymerization of MTMS, and a layer of gel-like substance was precipitated on the surface of the composite coating; thereby obscuring the previously formed hierarchical micro- and nano-scale structures and consequently resulting in a decrease in the coating’s surface roughness. This observation is consistent with the subsequent AFM characterization results. In addition, excessive use of SAC can also lead to its own excessive self-crosslinking, encapsulating the methyl groups produced by MTMS hydrolysis and the micro-and nano level structures formed earlier, ultimately resulting in a significant decrease in the hydrophobicity of the composite coating (Figure S1). Hence, the precise addition of MTMS and SAC is vital to achieving the ultimate hydrophobic properties of the coating.

Table 1. Surface free energy (SFE) values and WCA of modified samples with different amounts of MTMS.

Samples	SFE (mJ·m−2)	WCA (°)
0.4 MTMS-0.2 SAC	35 ± 1.6	99.5
0.6 MTMS-0.2 SAC	16 ± 1.5	118.5
1.2 MTMS-0.2 SAC	0.9 ± 0.3	150.2
1.5 MTMS-0.2 SAC	40 ± 1.0	100.1
3.0 MTMS-0.2 SAC	45 ± 1.2	94.4
5.0 MTMS-0.2 SAC	65 ± 1.5	61.6

presents the surface free energy of samples containing varying amounts of MTMS. When the MTMS content was relatively low (0.4 mL), the surface free energy stood at 35 mJ·m⁻², suggesting a limited presence of –CH₃ groups within the coating. As the MTMS content increased gradually (ranging from 0.6 to 1.2 mL), the number of introduced methyl groups rose accordingly, leading to a decrease in the coating’s surface free energy down to 0.9 mJ·m⁻². At this point, the coating exhibited optimal surface hydrophobicity, with a contact angle of 152°. However, further increasing the MTMS content (from 1.5 to 5.0 mL) resulted in a gradual increase in the coating’s surface free energy and a deterioration in its hydrophobicity. The correlation between the coating’s surface free energy and contact angle underscores the influence of the material’s surface free energy on the liquid wetting behavior of its surface.

In addition to the glass slide surfaces as discussed above, the composite coating could also be applied to various substrate surfaces. Using the method described earlier, the composite coating was successfully deposited onto several types of material surfaces: (1) paper (laboratory filter paper), (2) fabric (100% cotton fabric) (3) mineral wool board, (4) aluminum thin sheet, and (5) plastics (Poly (methyl methacrylate). As shown in Figure S2, all material surfaces coated with the composite coating exhibited excellent hydrophobic properties (water contact angles: 148–152°).

3.2. Surface Morphology and Chemical Composition of the Composite Coating

Fourier transform infrared (FTIR) spectrum of the modified sample were conducted to confirm the bonding of silica sol with SAC and to analyze the chemical structure of the composite coating. As shown in Figure 3a, the strong peaks at 1053 cm⁻¹ can be assigned to the asymmetric stretching vibration of Si–O–Si, which confirmed the formation of the inorganic cross-linked network. The absorption peak at 796 cm⁻¹ can be attributed to the Si–C bonds, whereas the strong characteristic peaks at 2935 cm⁻¹ and 2971 cm⁻¹ can be corresponded to the –CH₃ groups, which provides hydrophobic properties for the composite coating. The characteristic peaks at 1642 cm⁻¹ and 1731 cm⁻¹ correspond to the C=C bond and C=O bond unique to SAC, respectively. This confirms that in the composite coating, the organic network still maintains its integrity, while being connected to the inorganic network through Si–C bonds, thereby forming a stable three-dimensional hybrid network structure. Characteristic peaks of O 1s, C 1s, Si 2s, and Si 2p were observed in the XPS survey spectrum (Figure 3b), indicating that MTMS successfully bonded with SAC after hydrolysis and formed a microstructure on the surface of the composite coating. Si 2p high-resolution spectrum (Figure 3c) of the modified sample was divided into two characteristic peaks at 100.8 eV and 101.5 eV correspond to Si–O–Si and Si–C, which was consistent with the results of the FTIR spectrum [45,47]. It is well established that the combination of a material’s low surface energy with a suitable surface microstructure roughness exhibits exceptional surface hydrophobicity [10,32], much like that of a lotus leaf. In line with Cassie’s theory [51], surface microstructure roughness facilitates the entrapment of air, thereby exerting a significant influence on surface wettability. As shown in the cross-sectional image and surface morphology image of the sample in Figure 3d,e and Figure S3, a composite coating with a thickness of 350 nm was finally formed on the substrate surface via the spray-coating method, and a flower-like hierarchical structure, which exhibited complex banded folds and porous structures at the macro- and nano-scale, was simultaneously obtained. The size distribution of silica particles in the composite coating is shown in Figure S3. The D50 value is 0.6 μm, which is also consistent with the size of the previously analyzed micro/nano-hierarchical structure. According to the elemental analysis results of the composite coating’s microtopography (Figure 3f), the distribution range and density of Si and O highly overlap, while C is mainly distributed at the edges of protrusions and depressions. This indicates that Si and O are primarily combined to form a Si–O–Si inorganic network, and C and Si form low-surface-energy –CH₃ groups at the outer ends of the network endowing the composite coating with excellent hydrophobic properties. These results are also consistent with the microstructural analysis in Figure 1b. The resulting hierarchical microstructures at micro and nano-scales enhance the hydrophobic performance of the composite coating, which is also consistent with the previous wetting behavior.

To further characterize the surface roughness of the composite coating, AFM characterization was performed, and the results were presented in Figure 4 and

Table 2. The surface roughness Parameters of the composite coating with different amounts of MTMS (visa blank sample).

Sample	Ra (nm)	Rq (nm)	WCA (°)
Blank sample	13.6	17.4	99.5
0.6 MTMS-0.2 SAC	34.5	44.0	118.5
1.2 MTMS-0.2 SAC	88.4	109.0	150.2
1.5 MTMS-0.2 SAC	20.7	27.2	100.1

. As the MTMS dosage increased, both the arithmetic mean roughness (Ra) and root mean square roughness (Rq) values in the AFM images increased significantly, reaching their maximum values when the MTMS dosage was 1.2 mL at this dosage, the surface height difference in the images was also the largest. However, as the MTMS dosage continued to increase, Ra and Rq decreased sharply, and the surface height difference in the images also decreased notably, accompanied by a distinct reduction in water contact angle (WCA). The above results indicated that the MTMS dosage was correlated with the surface roughness of the composite coating, and selecting an appropriate MTMS dosage was crucial to achieve excellent hydrophobic performance.

3.3. Mechanical Strengthening Performance

The enhancing effect of the composite coating on the mechanical properties and hydrophobicity of mineral wool boards was verified by characterizing the flexural failure load and moisture-induced deflection of the boards before and after the composite coating was applied via spraying. As shown in Figure 5a, the flexural fracture load of the blank substrate is 118 N, while the flexural fracture loads of the boards sprayed with the composite coating are increased to varying degrees. Among these, the 1.5 MTMS-0.2 SAC sample exhibits the maximum flexural fracture load (192 N, with an increase of 62.7%), which confirmed that the composite coating can effectively enhance the mechanical properties of mineral wool boards. In addition to the enhancement of mechanical properties, the composite coating also significantly improved the surface hydrophobicity of mineral wool boards. As presented in Figure 5a, the blank mineral wool board exhibited a relatively large absolute value of moisture-induced deflection (−14.5 mm), whereas the moisture-induced deflection values of the mineral wool boards modified with the composite coating were notably reduced. Among these modified boards, the 1.2 MTMS-0.2 SAC sample showed the smallest moisture-induced deflection value (−3.01 mm), and the moisture-induced deflection values of the other modified samples were also less than 6.5 mm. This result indicated that the hybrid coating formed a dense hydrophobic layer on the surface of the mineral wool boards, thereby endowing the boards with excellent hydrophobic properties. Consequently, water vapor was prevented from penetrating into the interior of the boards, which would otherwise cause damage to the material and shorten its service life.

To further confirm the contributions of each component in the composite coating to the mechanical reinforcement effect and hydrophobic effect, the water contact angle (WCA), flexural failure load, and moisture-induced deflection of mineral wool boards modified with MTMS, SAC, and silica sol (formed by MTMS hydrolysis) were compared with those of blank samples and composite samples (Figure 5b,c). It was found that the samples modified with MTMS and silica sol exhibited superior WCA and flexural failure load compared to the SAC-modified samples, but their moisture-induced deflection was significantly poorer. This indicates that inorganic components (MTMS and silica sol) mainly contributed to the mechanical property reinforcement of the modified samples; however, due to their inability to form a dense coating, the samples showed poor long-term water resistance (reflected by moisture-induced deflection). In contrast, the organic component (SAC) formed a flexible network through cross-linking, which had limited improvement on mechanical strength but could form a relatively dense coating, thus resulting in better moisture-induced deflection performance.

Besides that, the adhesion strength between composite coatings with different MTMS contents and the mineral wool substrate was also tested (shown in Figure 5d). When the MTMS content was low, the adhesion strength between the composite coating and the substrate was relatively low; as the MTMS content increased, the adhesion strength increased correspondingly. However, when the MTMS content exceeded 1.2 mL, the adhesion strength decreased significantly.

Apparently, the above results indicate that both the self-crosslinking SAC and MTMS play a crucial role in enhancing the mechanical properties of the composite coating and facilitating the formation of a coating with excellent hydrophobic performance. The SAC provides an adhesive matrix for the hydrophobic silica particle fillers. Polycondensation reactions take place either between SAC molecules themselves or between SAC molecules and the hydroxyl groups generated via the hydrolysis of MTMS. This reaction process thereby enables the formation of strong bonding between the SAC matrix and the silica network within the composite coating [43]. It is precisely the strong interfacial bonding between SAC and silica, in conjunction with the high structural stability imparted by the Si–O–Si network [40,52], collectively endows mineral wool boards with excellent mechanical properties. In parallel, the hierarchical mesoscopic-to-microscopic surface structures and abundant surface-bonded –CH₃ groups synergistically contribute to the boards’ outstanding hydrophobic performance.

Therefore, to achieve excellent mechanical reinforcement and hydrophobic properties, it is necessary to combine the respective advantages of MTMS and SAC to finally obtain a multifunctional composite coating, which can address the drawbacks of mineral wool boards in practical applications, namely poor mechanical properties and susceptibility to moisture-induced deformation, thereby expanding their application scenarios and scale.

3.4. UV Resistance and Thermal Stability

In large-scale practical applications, the composite functional coating on the material surface is required to withstand fluctuations in temperature and prolonged exposure to sunlight. To simulate the aging behavior of the composite coating under real-world sunlight irradiation and temperature stability, an UV aging test and an oven thermal storage test were performed in this study, where a UV light source and temperature control system were employed to accelerate the natural aging process of the composite coating. As presented in Figure 6c, following continuous UV irradiation, the WCA on the material surface and the flexural fracture load underwent minimal variations. After 96 h of irradiation, the material’s WCA remained above 145°, while its flexural fracture load was sustained at over 150 N. Furthermore, the microscopic morphology of the composite coating retained a hierarchical three-dimensional structure spanning mesoscopic to microscopic scales, with no discernible differences observed when compared to its pre-irradiation state (Figure 6a,b). Similarly, the thermal stability performance of the composite coating was presented in Figure 6d. As the thermal storage time increased, the water contact angle and flexural failure load of the modified mineral wool boards decreased slightly. After 7 days of storage at 80 °C, the water contact angle of the samples was greater than 140°, and the flexural failure load was greater than 160 N, which still exhibited good performance. These results collectively confirm that the composite coating maintained excellent surface hydrophobicity and mechanical property enhancement capabilities post-aging irradiation, thereby demonstrating superior environmental stability. This remarkable environmental stability is primarily ascribed to the high structural stability of the cross-linked 3D Si–O–Si network within the composite coating, and the enhanced thermal stability conferred by the strong interfacial bonding between the polymer matrix and the inorganic network [43,53].

3.5. Abrasion and Chemical Resistance

In addition to environmental durability, mechanical stability is considered a vital criterion for surface coatings [54,55]. However, traditional hydrophobic functional coatings typically exhibit inadequate mechanical stability, primarily stemming from their inherently complex hierarchical structures and weak interfacial bonding with substrate materials [43,56]. This inherent limitation significantly restricts the widespread practical application of hydrophobic functional coating materials. Therefore, in order to evaluate the mechanical stability of the composite coating, modified mineral wool board samples were subjected to surface sandpaper abrasion (shown in Figure S5). Figure 7a shows the hydrophobicity and the mechanical strengthening performance of the coating after 50 abrasion cycles. It was noted that both the contact angle and the fracture load decrease with the abrasion cycles. Owing to the strong bonding interaction between the silicon-modified acrylic polymer resin and the silica network, the composite coating could still maintain excellent hydrophobic properties and mechanical properties even after undergoing 30 wear cycles (water contact angle of 130°and the flexural of 175 N). However, as the number of wear cycles continued to increase, the hydrophobic properties and mechanical properties of the modified samples also exhibited a significant decrease. These results demonstrated that the composite coating still maintained excellent mechanical stability and reinforcement effect despite undergoing multiple abrasion tests. The composite coating’s excellent mechanical stability and reinforcing effect stemmed from extensive, strong interactions between organic and inorganic domains—an outcome ascribed to in situ polymerization during its synthesis. Specifically, a cross-linked Si–O–Si structure with high bonding energy was formed within the coating, thereby reinforcing its structural integrity [33,48].

Beyond mechanical abrasion, coatings are also susceptible to degradation when exposed to corrosive acidic or alkaline environments. As illustrated in Figure 7b, when droplets with varying pH values were deposited onto the surface of modified glass slide samples, the droplets exhibited a near-spherical morphology. Furthermore, the contact angle values displayed minor fluctuations with changes in pH; however, all values remained consistently above 145°. Based on these data results, a schematic of the chemical stability mechanism for the hydrophobic surface in a corrosive environment is shown in Figure 8. The hierarchical structure of the composite coating, constructed at both micro- and nano-scales, facilitates the in situ formation of a protective air film on the material surface. This air film acts as a physical barrier, effectively inhibiting direct contact between the corrosive environment and the coating matrix [57]. As is well known, the degradation of hydrophobic functionality in functional coatings is predominantly driven by the desorption of surface-anchored hydrophobic molecules upon sustained exposure to corrosive environments [58]. Therefore, the excellent chemical stability of the functional coating should be mainly attributed to the high stability of the strongly cross-linked Si–O–Si framework in the composite coating, which ensure the coating’s structural integrity; and the formation of the protective air layer on the coating surface, which effectively mitigates the hydrolysis of hydrophobic Si–O–C groups, respectively.

3.6. Anti-Fouling and Self-Cleaning Performance

The other notable characteristics of functional coating surfaces are their anti-fouling and self-cleaning ability, which find utility across diverse practical applications, including architectural coatings, textile materials, and machined components. As mentioned above, the micro/nano-scale roughness of heterogeneous solid surfaces facilitates the formation of an “air cushion” at the interface between water droplets and the coating [57,59]. When the hydrophobic coating surface is immersed in water, air bubbles within the surface roughness generate a silver-mirror-like reflective effect, which arises from light reflection within the trapped air layer [60,61]. This “silver mirror” phenomenon serves as direct evidence that the surfaces of the modified samples have transitioned to the Cassie-Baxter wetting state [51].

Thus, benefiting from the presence of the “air cushion”, the composite coating exhibits excellent anti-fouling properties. As shown in Figure 9, five commonly used liquids in daily life (milk, tea, fruit juice, coffee, and soy sauce), were, respectively, deposited separately onto the surfaces of the modified glass slides with the composite coating. It was observed that, on the surfaces of the modified samples, the contact angle values exhibited slight variations within a narrow range with changes in liquid type, while all remained above 145°.

Figure 10a–d demonstrated the self-cleaning ability of the blank glass slide and the modified sample. As shown in Figure 10a,b, when deionized water was dripped dropwise onto the surface of a slightly tilted sample (tilt angle < 15°) from a height of 2 cm, it was observed that water droplets on the blank sample surface exhibited a wetted state, and residual water stains remained on the surface after dripping ceased. In contrast, for the modified samples, the impinging water droplets adopted a nearly spherical morphology and underwent “bouncing” behavior upon contact with the surface; no obvious water stains were detected on the surface after the dripping process was completed (shown in Video S1). When testing the self-cleaning performance of the samples, as illustrated in Figure 10c,d, for the blank sample, water droplets completely wetted the slightly tilted sample surface; as the volume of water gradually increased, the carbon black powder on the surface was washed away, but some powder residues remained, leaving the surface unable to be thoroughly cleaned. In contrast, for the modified glass slide sample, water droplets rolled off directly when they came into contact with the sample surface, simultaneously carrying away the carbon black powder on the surface and creating a distinct clean streak where the droplets had washed. With the continuous increase in water volume, the powder on the sample surface was completely washed away without obvious water stain residues (shown in Video S2). The above experimental results indicate that the sample modified with the composite coating exhibits excellent self-cleaning performance

3.7. Comparison of Preparation Methods and Economic Analysis

Compared with other state-of-the-art coating methods, this work used water-based silicon-modified acrylic polymer and silica inorganic network formed by the hydrolysis of MTMS as the main functional components, and fabricated a composite coating with excellent mechanical reinforcement effect and hydrophobic properties via a simple spray-coating technique, as shown in Table S1 [25,41,42,48]. In contrast, other preparation methods typically involve the combination of complex processes and multiple materials to construct rough surfaces with low surface energy, thereby achieving functional coatings; these methods are associated with cumbersome preparation procedures and inaccessible raw materials. By comparison, the preparation method adopted in this work is simple, with widely available raw materials and environmental friendliness, which is conducive to large-scale application.

The mechanical properties and hydrophobic properties of the modified mineral wool boards were compared with those of products from several commercial brands, including Alligator Paint, Asia Paint, Woods™, and Nippon Paint. The flexural failure load and water contact angle of the mineral wool boards coated with the paint developed in this work were significantly higher than those of other similar products (Table S2).

For large-scale application, the economic efficiency of the product must be considered. According to the cost accounting results, the unit price of this product is 2800 RMB per ton, while the selling prices of other similar products are all over 4500 RMB per ton, indicating that this product has a significant price advantage. In summary, the water-based multifunctional composite coating prepared in this work exhibits broad prospects for large-scale application.

4. Conclusions

In this work, we presented a facile strategy for fabricating a robust waterborne superhydrophobic coating with excellent mechanical reinforcement via simple spray coating using a low-cost, readily available, and environmentally friendly material system (waterborne silicone–acrylic copolymer and silica sol). On various solid substrates, the fabricated coating exhibited excellent hydrophobicity, with a water contact angle (WCA) as high as 150°; this performance is attributed to the low-surface-energy methyl groups and micro/nano-hierarchical surface roughness induced by MTMS. Benefiting from the stable cross-linked structure between the resin matrix (silicone–acrylic copolymer) and silica particles, the composite coating not only demonstrated excellent mechanical reinforcement but also showed ultrahigh mechanical and chemical stability. The maximum flexural fracture load of the modified materials increased from 119 N to 192 N, with an increase of 62.7%; the moisture-induced deflection of the samples increased from −14.5 mm to −3.01 mm. Specifically, it maintained outstanding hydrophobicity and mechanical performance even after 30 abrasion cycles, 7 days of thermal storage at 80 °C, exposure to high-intensity UV radiation, and immersion in corrosive acidic/alkaline environments. Additionally, the composite functional coating showed excellent self-cleaning and anti-fouling capabilities. This work provides a novel approach for the application of composite functional coatings, which is conducive to the large-scale industrial application of flat sheet materials.