Superhydrophobic Coating Based on Nano-Silica Modification for Antifog Application of Partition Glass

Abstract

In this study, vinyl triethoxysilane (VTES), KH-560 and trimethylchlorosilane (TMCS) were used to modify the surface groups of commercially available nano-silica (SiO₂, 50 nm), and ethylene vinyl acetate copolymer (EVA) was used as a film-forming agent. EVA/SiO₂, EVA/V-SiO₂, EVA/K-SiO₂ and EVA/T-SiO₂ coatings were prepared, respectively. The coatings were characterized by SEM, FTIR, TG and contact angle. It was found that when the mass percentage of SiO₂ was 66 wt%, the hydrophobicity performance of the coating could be significantly improved by silica modification. Compared to the EVA/SiO₂, the water contact angle (WCA) of the EVA/V-SiO₂, EVA/K-SiO₂ and EVA/T-SiO₂ were increased by 24.0%, 14.4% and 24.6%, respectively. The FTIR results indicated that VTES, KH-560 and TMCS could effectively replace the -OH groups on the surface of the SiO₂ after hydrolysis, resulting in the presence of water transport groups on the SiO₂ surface. The TG results certified that TMCS had the highest substitution rate (24.6%) for the -OH groups on the SiO₂ surface after the hydrolysis. Additionally, the SEM results indicated that T-SiO₂ was more easily dispersed in the EVA film-forming agent, leading to a uniform micro–nano surface rough structure, which aligned with the Cassie–Wenzel model. The durability test had demonstrated that the EVA/T-SiO₂ maintained its hydrophobic properties even after enduring 40,000 drops of water and the impact of 200 g of sand. Furthermore, it exhibited excellent resistance to acid corrosion, along with superior self-cleaning properties and an anti-fog performance. It also provided outstanding protection against high temperatures and UV radiation for outdoor applications.

1. Introduction

By thoroughly examining the surface structure of plants [1] and animals [2], superhydrophobic materials prepared through bionic methods can be utilized in various artificial superhydrophobic coatings for applications in self-cleaning [3], anti-corrosion [4], anti-icing [5] and other fields (Figure 1). Numerous methods have been proposed to create superhydrophobic coatings, such as impregnation [6] and electrodeposition [7]. These methods involve combining rough micro–nano structures with low surface energy materials to achieve superhydrophobic surfaces. A superhydrophobic surface is defined as having a water contact angle (WCA) of more than 150° and a water sliding angle (WSA) of less than 10° [8]. When designing superhydrophobic surfaces, two key parameters must be considered: low surface energy [9] and nanoscale roughness [10]. By reducing the surface energy of the solid surface and incorporating micro–nano roughness, it is possible to effectively prevent the adhesion of dust and water droplets while promoting self-cleaning. The micro–nano particle coating method involves mixing micro–nano particles with polymers and applying them to the solid surface using techniques such as spin coating, spraying, or impregnation in order to alter its rough structure to achieve the desired effect. Various types of micro–nano particles have been used for this purpose, including SiO₂ [11,12], ZrO₂ [13] and TiO₂ [14].

In the past, many researchers have demonstrated that rough surfaces could be coated with low surface energy substances to increase the WCA to achieve superhydrophobic effects, such as long-chain alkanes [15], perfluorinated functional groups [16] and silyl compounds [17]. Wang et al. [18] grafted N-(2-aminoethyl)-3-aminopropyltriethoxysilane oligomers (AEAOS) onto nano-silica and coated it on the surface of aluminum alloy to prepare a uniform superhydrophilic coating. The amino group promoted the grafting of the silane on the surface of the nano-silica. Upon the addition of 10% AEAOS, the contact angle of the modified silica film decreased to <5°, demonstrating superhydrophilicity. The AEAOS led to the aggregation of modified nano-silica particles, resulting in a reduction in surface anchor points and disordered structure that made the coating easily washed away by water. Simultaneously, the amino group in the AEAOS facilitated the formation of silicon–oxygen–silicon bonds and formed a more complete network structure. Zhang et al. [19] studied the surface modification of SiO₂ using vinyltriethoxysilane and confirmed the formation of chemical bonds between SiO₂ and silane. On the basis of this study, another study introduced hydrophobic functional groups, such as CH₂=CH- and -CH₃ in vinyltriethoxysilane, into the surface of SiO₂ by hydrothermal reaction, giving the material a lower surface energy. The treated SiO₂ provided a nano-scale rough structure and low surface energy. The WCA of the coating surface increased from 131.8° to 163.4°, and the maximum WSA was less than 3° [20]. Li et al. [16] used trimethoxy (1H, 1H, 2H, 2H-heptafluorodecyl) silane (FSCA) to modify SiO₂, carbon black and waterborne epoxy resin to prepare fluorinated superhydrophobic coatings to enhance the anti-icing and durability of cement pavements. The hydroxyl group on the surface of the SiO₂ was replaced by the hydrophobic fluoroalkyl group of FSCA, and the FSCA constructed a superhydrophobic film on the surface of the SiO₂. Zeng et al. [21] developed a nanocomposite technique for the fabrication of superhydrophobic surfaces by modifying SiO₂ particles with vinyltriethoxysilane (VETS) and perfluorooctyltriethoxysilane (PFTS). This modification effectively mitigated the agglomeration of the SiO₂ particles, leading to the formation of micropores and microgrooves that reduced the contact area between water droplets and the surface. More interestingly, zhao and colleagues [22] developed a Janus copper sheet featuring one side with superhydrophobicity (SHB) and the other with superhydrophilicity (SHL). The SHB side, treated with dodecyl mercaptan, exhibited micro-/nano-scale roughness that effectively prevented water molecules from contacting the surface. This resulted in a WCA exceeding 150°, demonstrating significant water repellency. In contrast, the SHL side retained Cu(OH)₂ nanoneedle structures formed during an oxidation treatment, providing numerous hydrophilic sites that facilitated the easy contact and spreading of water molecules. Consequently, this side displayed a WCA of less than 5°, indicating strong water adsorption capabilities. Some teams had also achieved the effect of superhydrophobic coatings by modifying epoxy resins [23]. However, these modification methods were expensive, complex and environmentally polluting.

To investigate the influence of alkyl chain length and spatial configuration on the hydrophobic properties of coatings, vinyl triethoxysilane, KH-560 and trimethylchlorosilane were selected for the modification of SiO₂. This study focused on developing superhydrophobic coatings with nanostructures utilizing modified SiO₂ in conjunction with the thermoplastic elastomer EVA, while examining how the structures and functional groups of these modifiers impacted the hydrophobic characteristics of the resulting coatings. The effects of the vinyl triethoxysilane, KH-560 and trimethylchlorosilane modifications on the SiO₂ were analyzed before dispersing them into EVA to fabricate superhydrophobic coatings. A comprehensive evaluation of various performance indicators was conducted, encompassing water contact angle (WCA), chemical stability, self-cleaning capability, mechanical durability, weather resistance and anti-fog properties. Furthermore, the influence of SiO₂ modifiers on the hydrophobicity of these coatings was also examined.

2. Experimental Section

2.1. Materials

All the materials were used as is. SiO₂ (50 nm), trimethylchlorosilane (TMCS), γ-glycidoxypropyltrimethoxysilane (KH-560), vinyltriethoxysilane (VTES), ethylene vinyl acetate copolymer (EVA), sodium chloride and tetrahydrofuran (THF) were purchased from Shanghai McLean. Ammonia water (25 wt%), acetic acid (36 wt%), soluble starch and anhydrous ethanol were purchased from Sinopharm Group Reagent Co., Ltd., Shang Hai, China. Slides of 75 mm × 25 mm × 10 mm were bought from the Sailing Company, Jiangyin, China. The deionized water was homemade.

2.2. Modification Protocol for SiO₂

2.2.1. Vinyl Triethoxysilane-Modified SiO₂

The purchased SiO₂ was dried in a 100 °C vacuum drying furnace for 48 h and then 1 g of SiO₂ was added to a beaker containing 50 mL of anhydrous ethanol, marked A. Ultrasonic mixing was then conducted for 1 h at the temperature of 40 °C (good mixing). Then, 0.1 g VTES was added to a beaker, B, which contained 20 mL anhydrous ethanol, and ammonia was added (the pH was adjusted to 9 or 10) to the hydrolysate. The mixture was ultrasonically mixed at 40 °C for 1 h. The solutions of beaker A and beaker B were poured into a three-necked flask, stirred at 90 °C for 6 h, then centrifuged, washed twice with anhydrous ethanol and once with deionized water and dried at 100 °C for 12 h. The modified nanometer SiO₂ was marked as V-SiO₂.

2.2.2. KH-560-Modified SiO₂

A total of 1 g of dried SiO₂ was placed into a two-necked flask, and 3 mL ammonia water was added to adjust pH value to 3~4. Then, 25 mL anhydrous ethanol, 2 mL deionized water and 0.05 g KH-560 were added, and the mixture was heated at 90 °C for 3 h. After the reaction was completed, the solution was filtered, washed (twice with anhydrous ethanol and once with deionized water) and dried at 100 °C. The modified SiO₂ was marked as K-SiO₂.

2.2.3. Trimethylchlorosilane-Modified SiO₂

A total of 1 g 50 nm SiO₂, 0.15 g (15 wt%) TMCS and 50 mL anhydrous ethanol were added to the two-necked flask, in turn. After magnetic stirring for 30 min, the mixture was heated at 60 °C for 1 h. After the reaction, the solution was naturally cooled to an ambient temperature, the sediment was washed (2 times with anhydrous ethanol and 1 time with deionized water) and filtered, and the obtained filter cake was dried at 100 °C for 5 h. The modified SiO₂ was marked as T-SiO₂.

2.3. Preparation of Superhydrophobic Coatings

The slides were soaked in anhydrous ethanol for 12 h and dried. First, 0.4 g SiO₂ (SiO₂, V-SiO₂, K-SiO₂ and T-SiO₂) and 0.2 g EVA were added to a three-necked flask containing 30 mL THF; the mixture was heated to 60 °C and reacted for 1 h. Then, the slides were soaked in the previous solution for 5 s. Finally, the slides were dried in a drying box at 100 °C for 1 h. The coatings were labeled as EVA/SiO₂, EVA/V-SiO₂, EVA/K-SiO₂ and EVA/T-SiO₂, respectively.

2.4. Characterization

The surface morphologies of the prepared coatings and SiO₂ were examined by operating a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-6700F, Tokyo, Japan). The slides were cut into 10 mm × 10 mm and sprayed with gold for 150 s, and then SEM was performed. The surface chemical compositions of the coatings were detected using an energy dispersive spectrometer (EDS). The surface morphology and roughness of the coatings, both prior to and following immersion at varying pH levels, were characterized using atomic force microscopy (AFM, Bruker Dimension ICON, Saarbrucken, Germany). The chemical composition of the SiO₂ before and after the modification was detected by Fourier transformed infrared spectroscopy (ATR-FTIR, Bruker, Saarbrucken, Germany). A thermogravimetric (TG) analysis was performed with a thermogravimetric analyzer at a heating rate of 10 °C/min from 25 °C to 800 °C under air atmosphere. The WCA and WSA of the different coatings were measured using an optical contact angle measuring instrument (OCA25, Dataphysics, Filderstadt, Germany) with a 5 μL liquid volume. The WCA and WSA were measured at 5 different positions on the coating, and the mathematical average of the 5 results was taken as the result. The mechanical robustness of the superhydrophobic coating was tested by grit and water droplet impact experiments. In the gravel impact test, the sand particles with an average particle size of 0.34 mm (40–60 mesh) fell freely from a height of 30 cm; the angle between the superhydrophobic coating and the horizontal plane was 30°, the gravel fell down at a mass flow rate of 8.3 g/min and the impact area was about 0.78 cm². Then, the coating was rinsed with deionized water and dried in an oven at 100 °C for 1 h before the WCA was measured. For water droplet impact experiments, the coating was placed in a glass dish with an inclination angle of 30° at a height of 30 cm below the liquid separation funnel of 500 mL, and the water droplets were released continuously at a frequency of 100 drops/min. The WCA was recorded when 500 mL of water was exhausted (the sample was dried at 100 °C for 60 min before the WCA measurement). The acid, salt and alkali resistances of the superhydrophobic coatings were characterized by measuring their WCA after immersing the coatings in acetic acid solution (pH = 3), sodium chloride solution (pH = 7, 5 wt% sodium chloride) and ammonia water solution (pH = 11), respectively. A muffle furnace (SGM·M6/10) was employed to evaluate the high-temperature resistance of the coatings. The coating was subjected to various temperatures (60 °C, 100 °C, 150 °C, 210 °C and 260 °C) for a duration of one hour before being cooled to room temperature for the subsequent assessment of its WCA and WSA. The UV protection performance of the coating was analyzed using a camera chamber UV analyzer (ZF-20D). Continuous irradiation at a wavelength of 254 nm was conducted for 420 min, with measurements of the WCA and WSA taken every 60 min. The water vapor generation device was built on the water bath, and the prepared glass slide was placed on it. The EVA/T-SiO₂ was positioned above the water bathcomprised, the glass slide was 3 cm away from the water surface, the room temperature was 25 °C and the water bath temperature was set to 60 °C. After 15 min, 30 min, 45 min, 60 min and 75 min, the surface fogging was observed and digital photos were taken for a comparative analysis. Self-cleaning experiments were carried out on the superhydrophobic coatings with different powders (sand, activated carbon powder and soluble starch).

3. Results and Discussion

3.1. Characterization of Modified SiO₂

3.1.1. SEM Analysis

Figure 2a,c,e,g display the SEM of the SiO₂ before and after the modification. The SiO₂ particles exhibited a roughly spherical shape both pre- and post-modification. The success of the SiO₂ modification was confirmed by EDS (Figure 2b,d,f,h) [24]. The unmodified SiO₂ demonstrated significant particle agglomeration due to the inter-particle interactions [25]. The use of the modifiers enhanced the dispersion of the SiO₂ [26] by binding organic groups to the particle surface, thereby reducing Van der Waals forces [27] and electrostatic interactions and weakening hydrogen bonding [28] among particles. The subsequent evaluation of the hydrophobicity of the V-SiO₂, K-SiO₂ and T-SiO₂ involved submersion in deionized water. It was evident that the V-SiO₂, K-SiO₂ and T-SiO₂ were hydrophilic, contaminating the water (Figure 2i), while the hydrophobic portions of the modified SiO₂ floated on the water surface and the hydrophilic segments sunk to the bottom, causing contamination (Figure 2j–l).

3.1.2. XRD Analysis

The XRD analysis was conducted to examine the crystal structure changes in the SiO₂ before and after the modification. The changes in the crystal structures of the SiO₂, V-SiO₂, K-SiO₂ and T-SiO₂ were analyzed by the XRD, and the results are shown in Figure 3. By observing the XRD patterns of the SiO₂, V-SiO₂, K-SiO₂ and T-SiO₂, it could be seen that the corresponding patterns of all samples had a wide peak in the diffraction angle range of 15°~28° [29]. These spectra showed that all the samples had the same structure standard of the SiO₂ amorphous peak [30], which means that the SiO₂, V-SiO₂, K-SiO₂ and T-SiO₂ were in an amorphous state and belonged to nanoscale spherical particles. The amorphous SiO₂ offered benefits such as superior stability, even dispersion and numerous active sites, facilitating homogeneous distribution in the prepared coating. These findings suggest that the structural integrity of the SiO₂ particles remained unaffected despite the variations in the modifiers [31].

3.1.3. Fourier Transformed Infrared Spectroscopy (FTIR)

FTIR spectroscopy was utilized to acquire the spectra of the samples prepared using three different modification reagents (Figure 4). From Figure 4, it can be seen that the FTIR spectrum of the SiO₂ exhibited five absorption peaks at 3333 cm⁻¹, 1673 cm⁻¹, 1075 cm⁻¹, 960 cm⁻¹ and 799 cm⁻¹, which correspond to the stretching vibration of the O-H bond, flexural expansion vibration of the H-O-H bond (surface adsorbed water), antisymmetric stretching vibration of the Si-O-Si bond, bending vibration absorption of the Si-OH bond and symmetric stretching vibration peak of the Si-O-Si bond, respectively [32].

The bending vibration peaks of the Si-OH bond exhibited a slight blue shift from 960 cm⁻¹ to 969 cm⁻¹, 974 cm⁻¹ and 967 cm⁻¹, as shown in Figure 4 and

Table 1. The corresponding wavelengths and type of peaks in FTIR spectra.

Type of Peak	Wavelength (cm−1)	Type of Peak	Wavelength (cm−1)
υ(O-H)	3333	υ(C-H)	2991, 2987, 2983, 2901, 2895, 2893
δ(H-O-H)	1637	υ(-Si-CH=CH2)	1605
δ(C-H)	1413	δs(C-H)	1409
δs(Si-CH3)	1277	υas(Si-O-Si)	1075, 1072, 1070, 1051
δ(Si-OH)	974, 969, 967, 960	υs(Si-O-Si)	799, 795, 793, 763

. This shift was attributed to the condensation between the functional groups on the surface of the modifier and the -OH groups on the surface of the SiO₂ being gradually strengthened and the -OH groups on the surface of the SiO₂ being reduced [33]. Accompanied by weakened infrared peaks, the Si-O-Si bond showed a red shift from 1075 cm⁻¹ to 1072 cm⁻¹, 1070 cm⁻¹ and 1051 cm⁻¹. At the same time, more Si-O-Si bonds were produced in the modified SiO₂ [34], which was because the organic groups on the modifier reacted with the Si-OH bonds on the surface of the SiO₂ to form Si-O-Si bonds.

The infrared peaks of the O-H and H-O-H bond of the T-SiO₂ reduced to around 3333 cm⁻¹ and 1637 cm⁻¹, respectively. Furthermore, the appearance of the C-H stretching vibration peaks at 2987 cm⁻¹ and 2901 cm⁻¹ suggested the binding of the methyl group from the TMCS onto the SiO₂ surface [35]. The modification process involved initial hydrolysis steps [36], where the Si-OH bond generated by the hydrolysis of the modifier linked with the surface Si-OH bonds of the SiO₂, transforming the hydrophilic nature of the SiO₂ into a hydrophobic one. The K-SiO₂ exhibited similar traits with the appearance of a C-H symmetric variable angle vibration peak at 1409 cm⁻¹, indicating the successful attachment of the KH-560 onto the SiO₂ surface [37]. This aligned with the conclusions drawn by Yu et al. [38]. Further, the V-SiO₂ displayed characteristic peaks at 2991 cm⁻¹, 2895 cm⁻¹, 1051 cm⁻¹, 967 cm⁻¹ and 763 cm⁻¹ in its spectrum. The characteristic peak at 1277 cm⁻¹ was associated with the symmetrical deformation vibration of Si-CH₃ [39], while the characteristic peak at 1605 cm⁻¹ indicated -Si-CH=CH₂ [40]. Additionally, the bending vibration peak of C-H in -CH₃ was observed at 1413 cm⁻¹. Zhang et al. [41] also noted similar findings regarding the -CH₃ bending vibration peak in their experimentation.

3.1.4. TG Analysis

The thermogravimetric changes in the SiO₂ before and after the organic modification were investigated and are depicted in Figure 5. Figure 6 shows the three distinct phases of the weight reduction in the SiO₂, V-SiO₂, K-SiO₂ and T-SiO₂. In the first stage (below 200 °C), the weight loss was mainly due to the water and organic impurities adsorbed on the surface of the SiO₂ [42]. In the second stage (200 to 450 °C), the weight loss could be attributed to the chemical bond between the modifier and the SiO₂ breaking during the heat treatment process [43]. In the third stage (450 to 800 °C), the weight loss was mainly caused by the fracture of the Si-O-Si bond formed by the modifier and the Si-OH group on the surface of the SiO₂. Because the Si-O-Si bond was very strong, its fracture required high energy [44]. The unmodified SiO₂ exhibited a weight loss of 5.58%. However, the V-SiO₂, K-SiO₂ and T-SiO₂ displayed weight loss rates of 7.44%, 7.34% and 7.22%, respectively. The difference between the weight loss of the SiO₂ and the V-SiO₂, K-SiO₂ and T-SiO₂ could be approximately attributed to the thermal decomposition of organic functional groups attached to the surface of the silica, and the existence of organic functional groups on the surface of silica has also been confirmed by FTIR. Meanwhile, the weight loss rate of the SiO₂ from 200 °C to 800 °C was roughly considered to be the removal of the hydroxyl groups in the surface and pore channels. In this way, the grafting rate (the ratio of the number of grafted organic functional groups to the number of hydroxyl groups on the SiO₂) and the hydroxyl substitution rate (the ratio of the replaced hydroxyl groups on the SiO₂ to the total original hydroxyl groups) could be qualitatively calculated, and the calculation formulas are shown as (1) and (2), respectively:

$$\mathrm{GR}={\displaystyle \frac{{\mathrm{n}}_{\mathrm{sub}}}{{\mathrm{n}}_{\text{-}\mathrm{OH}}}}$$

(1)

$$SR=k{\displaystyle \frac{{\mathrm{n}}_{\mathrm{sub}}}{{\mathrm{n}}_{\text{-}\mathrm{OH}}}}$$

(2)

In Equation (1), n_sub is the amount of substance (≡(O)₃SiCHCH₂, ≡(O)₃Si(CH₂)₅O₂ and -O-Si(CH₃)₃) for the V-SiO₂, K-SiO₂ and T-SiO₂, respectively) of the modified SiO₂, mol/g-SiO₂. n_-OH is the amount of -OH on the surface of the SiO₂, mol/g-SiO₂. In Equation (2), k is used to represent the number of Si-O bonds (k = 3, 3, 1 for the V-SiO₂, K-SiO₂ and T-SiO₂, respectively) formed on the surface of the SiO₂.

It was calculated that the GRs of the VTES, KH-560 and TMCS were about 23.0%, 14.4% and 25.6%, respectively, indicating that the numbers of organic functional groups introduced onto the SiO₂ were as follows: TMCS > VTES > KH-560. However, the SRs of the V-SiO₂, K-SiO₂ and T-SiO₂ were 69%, 43.2% and 25.6%, respectively, illustrating that the VTES, KH-560 and TMCS made the hydroxyl group on the surface of the SiO₂ be replaced in the order of V-SiO₂ > K-SiO₂ > T-SiO₂.

3.2. Morphology Analysis of EVA/Nano-SiO₂ Surface

It is well known that the micro/nanoscale surface structure plays a crucial role in determining the wettability of superhydrophobic materials [45]. Hence, the morphology and microstructure of the EVA/SiO₂ was initially examined. A typical SEM image of the EVA/SiO₂ surface showed a non-uniform and distinctive morphology, predominantly consisting of aggregates of small spherical SiO₂ particles, as given in Figure 6a. Upon closer SEM inspection, it was evident that these globular particles were intertwined rather than isolated (Figure 6b). It should be noted that there were numerous polyhedral nanocrystals with spherical chain-like structures on these nanoparticles, resembling “grape” structures. The WCA test results showed a WCA of 110° on the central surface of the coating because of the porous SiO₂ (Figure 6a), resulting in notable voids that could allow water infiltration into the surface indentations, ultimately causing the formation of a Wenzel model [46]. After the modification of the SiO₂, the organic groups were grafted onto the surface of the SiO₂ to enhance their hydrophobicity, resulting in a rougher surface that increased spatial resistance, as shown in Figure 6d,g,j. The “grape” gap could trap air to create an air cushion, causing water droplets to be suspended at the gas–solid–liquid interface, forming a Cassie–Wenzel model [47]. This hierarchical micro–nano “grape” surface structure significantly contributed to their superhydrophobicity. An EDS analysis of the coating revealed the chemical composition, showing the presence of C, Si and O elements, indicating the organic groups on the superhydrophobic surface were responsible for its unique properties, as shown in Figure 6c,f,i,l.

3.3. Effect of T-SiO₂ Content on Hydrophobicity

According to previous studies on the preparation of SiO₂-based superhydrophobic coatings, the content of the SiO₂ is crucial to the superhydrophobic performances of the coating [48]. Therefore, we studied the effect of the T-SiO₂ content on the preparation of superhydrophobic coatings. The T-SiO₂ mass fractions were chosen as 33 wt%, 50 wt%, 58 wt%, 66 wt% and 80 wt%. Optimal hydrophobicity, with a WCA of 157° and a WSA of only 2°, was achieved when the T-SiO₂ content was 66 wt%, as shown in Figure 7. Notably, when the content of the T-SiO₂ was increased from 33 wt% to 66 wt%, the WCA of the coating also increased gradually, indicating that the content of the SiO₂ was positively proportional to that of the WCA, which was in line with the Cassie–Wenzel theory [49]. However, when the T-SiO₂ content exceeded 66 wt%, the increase in the T-SiO₂ content led to an increase in the hardness of the coating, which made the contact surface smoother and stronger and the friction and adhesion smaller, and ultimately the WCA value declined [50]. Furthermore, this study explored the impact of the modified SiO₂ on superhydrophobic coatings by maintaining a fixed concentration of modified SiO₂ at 66 wt%. (The wettability assessment of the EVA/T-SiO₂ is presented in Video S1 and S2.)

3.4. WCA Measurement

The wettability of the samples was assessed through a WCA measurement. The corresponding data are depicted in Figure 8. The EVA was classified as a hydrophilic substance, displaying a relatively low WCA of 87° [51]. Upon the addition of SiO₂ to the EVA and its application onto the glass substrate (EVA/SiO₂), a micro–nano rough surface was created, resulting in an increased WCA of 110°. This could be attributed to the abundance of hydroxyl groups present on the SiO₂ particles’ surfaces, rendering SiO₂ hydrophilic. Through the incorporation of various modifiers to alter its surface properties, the wettability of the superhydrophobic coating was enhanced. The hydroxyl groups on the SiO₂ surface underwent reactions with the organic groups present on the modifiers, leading to the formation of a three-dimensional network structure (composed of hydrophobic organic groups) centered on the Si-O-Si. Consequently, the WCA results aligned well with the surface roughness, morphology, topography and chemical composition outcomes.

Figure 8 illustrates the wetting behavior of the superhydrophobic coatings created by using the customized SiO₂. As previously mentioned, water tended to spread easily on the smooth surface of the slide embedded with EVA, resulting in a relatively lower WCA, as depicted in Figure 8a. Conversely, the surface roughness contributed to an enhanced wetting behavior. As seen in Figure 8b, the presence of the SiO₂ enhanced the surface roughness, leading to an elevation in the WCA of the coating to 110°. The protruding structures formed trapped air at the interface between the water droplet and the coating, impeding the water droplet’s diffusion and consequently elevating the WCA. According to the Cassie–Baxter model, in cases where material surface roughness is relatively high and the intrinsic WCA is substantial, liquid infiltration into the interior of the rough structure is hindered, resulting in the entrapment of air and increased contact between the liquid and gas. The following mathematical expressions can be derived [52]:

$$\cos \theta =f_1\cos {\theta }_1+f_2\cos {\theta }_2$$

(3)

In Equation (3), θ represents the actual measured contact angle, f₁ denotes the proportion of the solid–liquid contact area and f₂ indicates the proportion of the gas–liquid contact area, with θ₁ and θ₂ representing the contact angles between the liquid–solid and gas phases, respectively. Given that f₁ + f₂ = 1 and θ₂ = 180°, while incorporating the roughness factor r for the solid surface, the equation can be transformed into [8] the following:

$$\cos \theta =r{f}_1\cos {\theta }_1+f_1-1$$

(4)

As evident from Equations (3) and (4), θ₁ is constant and reducing the value of f₁ leads to an increase in θ, implying that a reduced solid–liquid contact area results in a larger WCA of the droplet on the material surface. Consequently, a highly hydrophobic surface exhibiting WCAs of 135°, 142° and 157° could be achieved. As demonstrated in Figure 8c–e, the organic chain with low surface free energy covering the SiO₂ surface conferred hydrophobic properties, further enhancing the WCAs to 135°, 142° and 157°.

3.5. Durability and Chemical Stability Analysis

The chemical stability of the EVA/T-SiO₂ was examined by immersing the surface of the coating in solutions with pH values of 3, 7 and 11. AFM and contact angle measurement instruments were utilized to evaluate the surface morphology, average roughness and wettability of the coating subjected to the various pH environments. Surface roughness denotes the microstructural irregularities inherent to the surface of a sample. This assessment facilitated an evaluation of its resistance to acids, salts and bases, as illustrated in Figure 9 and Figure 10. The WCA of the coating was proportional to the surface roughness. Initially, the contact angle of the coating was measured at 157°, while its average roughness (Ra) was recorded as 0.851 nm (Figure 9 and Figure 10a). After a 72 h immersion in acetic acid solution, the EVA/T-SiO₂ retained its hydrophobic properties with a WCA of approximately 128° and an Ra of 0.657 nm (Figure 10b). Similarly, following immersion in ammonia solution for the same duration, the EVA/T-SiO₂ exhibited significant hydrophobicity with a WCA of around 114° and an Ra measuring as 0.628 nm (Figure 10d). These results further substantiate that the variations in the coating’s WCA correlated with the changes in roughness. Notably, the EVA/T-SiO₂ immersed in ammonia solution experienced a greater loss of hydrophobicity compared to that observed after immersion in acetic acid for an equivalent time period. This phenomenon could be ascribed to the superior resistance of the T-SiO₂ in acidic conditions, while it exhibited comparatively diminished resistance in alkaline environments. The exceptional acid resistance exhibited by the EVA/T-SiO₂ might be attributed to the substantial bonding energy linked to the long-chain hydrocarbon grafts present on the surface of SiO₂ [53]. Furthermore, when exposed to sodium chloride solution, both WCAs remained largely stable while the Ra was noted as approximately 0.689 nm (Figure 10c), likely due to the mesh structure inherent within T-SiO₂ effectively impeding sodium chloride penetration [54]. These findings confirmed that the EVA/T-SiO₂ demonstrated commendable resistance against acids, salts and alkalis.

3.6. Self-Cleaning Behavior of EVA/T-SiO₂ Surface

Considering the practical application of EVA/T-SiO₂, a large-area coating was prepared on a 75 mm × 25 mm slide and subjected to a self-cleaning test. The self-cleaning efficacy of the EVA/T-SiO₂ against various contaminants (such as sandy soil, active carbon powder and soluble starch) was evaluated, using deionized water to mimic rainfall. As illustrated in Figure 11, when the solid powder traversed an uncoated slide, it adhered to the surface. In contrast, when the solid powder encountered a slide with the EVA/T-SiO₂, water droplets swiftly transformed into spheres and rolled off the surface (Video S3). There was minimal residue left on the surface, indicating the exceptional self-cleaning capability of the coating.

3.7. Mechanical Properties of EVA/T-SiO₂ Surface

Beyond the chemical stability of the EVA/T-SiO₂, the mechanical robustness plays a crucial role in determining the longevity of the coating. To assess the mechanical wear resistance of the EVA/T-SiO₂ under specific weather conditions like dust storms in Inner Mongolia, water droplet impact and gravel impact experiments were carried out. The water droplet impact test involved placing the EVA/T-SiO₂ in a glass dish tilted at 30° with water droplets continuously released at 100 drops/min from a height of 30 cm, as shown in Figure 12a,b. The WCA was measured after 10,000 impacts (before the WCA measurement, the EVA/T-SiO₂ was dried at 100 °C for 60 min), revealing a decrease to 144° and an increase in the WSA to 15° after 60,000 drops. Despite losing superhydrophobicity, the EVA/T-SiO₂ maintained strong hydrophobicity with a WCA close to 150°. In the gravel impact test, after 200 g of impact gravel quality, the WCA decreased to 107°, confirming the good mechanical firmness of the EVA/T-SiO₂ (Figure 12c,d).

3.8. Weatherability of EVA/T-SiO₂ Coating

The EVA/T-SiO₂ coating had an excellent self-cleaning performance, chemical stability and mechanical durability, but the weather resistance was also one of the important properties of the coating. Therefore, we tested the coating against high temperatures and ultraviolet light. The EVA/T-SiO₂ coating was subjected to calcination at various temperatures, with the resulting data presented in Figure 13a. The WCA of the coating remained relatively stable up to 210 °C; however, at 260 °C, the EVA within the coating experienced thermal degradation, leading to delamination and a decrease in hydrophobicity. Subsequently, we placed the coated samples in an ultraviolet analyzer and measured their WCAs and WSAs every hour, the results are illustrated in Figure 13b. Following 420 min of UV irradiation, the coating maintained its superhydrophobic characteristics, with a measured WCA of 155°. Notably, the SiO₂ demonstrated an absorption rate exceeding 70% for ultraviolet light wavelengths below 400 nm [55]. This property effectively absorbed or blocked UV radiation, preserving the roughness and surface topography of the coating, which in turn sustained its superhydrophobic characteristics.

3.9. Application of EVA/T-SiO₂ Coating in Anti-Fog Glass

The EVA/T-SiO₂ was coated on some bathroom partition glass for an anti-fog test. It can be seen from Figure 14 that, after 75 min, the WCA of the coating reached 138° and the WSA reached 20° (Figure 14a). On the left side of the coated slide, there was no obvious water vapor retention in the water bath at 60 °C, while on the right side of the uncoated slide, there was obvious water vapor, indicating that the EVA/T-SiO₂ had a good anti-fog effect (Figure 14b) [56]. After encountering the water vapor, small water droplets were formed on the surface of the EVA/T-SiO₂, and the T-SiO₂ in the coating formed a rough structure on the surface. At the same time, the organic groups attached to the surface of the T-SiO₂ increased the space resistance, thus capturing a lot of air, which conformed to the Cassie-Wenzel model. On the one hand, there was a dipole-dipole force between the vinyl acetate monomer in the EVA and the glass surface, which made the EVA and the glass surface adsorb and adhere to each other. On the other hand, the molecular chain of the EVA could penetrate into the tiny depressions and pores on the glass surface, forming mechanical anchoring and increasing adhesion. These results showed that the EVA/T-SiO₂ could be applied to the partition glass to achieve anti-fogging effect.

3.10. Effect of Modifier on Superhydrophobic Mechanism of Coating

The formation mechanism of the EVA/modified-SiO₂ surface is shown in Figure 14. The EVA/SiO₂ conformed to the Wenzel model (Figure 15d), where the droplets are in full contact with the surface roughness, resulting in a large number of solid–liquid contact areas, thereby reducing the superhydrophobicity of the surface. Hydrophobic V-SiO₂, K-SiO₂ and T-SiO₂ were prepared, as shown in Figure 15a–c. First, VTES, KH-560 and TMCS needed to be hydrolyzed to form silanols [57], which then underwent a condensation reaction with -OH groups on the surface of the SiO₂ to form a three-dimensional network structure centered on the silicon (composed of Si-O-Si bonds) [58]. Second, on the surface of the EVA/modified-SiO₂, the modified SiO₂ particles were evenly distributed and formed a convex structure. A large number of microgrooves were formed between the convex structures, which increased the space resistance [59] and enhanced the roughness of the EVA/modified-SiO₂. As a result, the water droplets were suspended at the gas–solid–liquid interface, forming the Cassie–Wenzel model (Figure 15a–c). Third, based on the TG analysis, the grafting efficiency of the T-SiO₂, at 25.6%, surpasses that of both the V-SiO₂ (23.0%) and K-SiO₂ (14.4%). However, the hydroxyl substitution rates told a different story, with the V-SiO₂ (69.0%) and K-SiO₂ (43.2%) exhibiting higher values compared to the T-SiO₂ (25.6%). The WCA measurements revealed that the T-SiO₂ had a notably higher WCA of 157° compared to the V-SiO₂ (135°) and K-SiO₂ (142°). These data reveal that the silanol generated after the hydrolysis of the modifier could not completely replace the hydroxyl group on the surface of the SiO₂, and the hydrophobic property was related not only to the modification but also to the structure of the silanol. The modification effect of the silanol with a spatial three-dimensional structure (≡(O)₃Si(CH₂)₅O₂ and -O-Si(CH₃)₃) was significantly better than that of silanol with a linear two-dimensional structure (≡(O)₃SiCHCH₂) [57]. Although the grafting rates of the T-SiO₂ and V-SiO₂ were similar, the spatial three-dimensional structure of the organosilane chains in the T-SiO₂ provided an enhanced steric hindrance, which helped to improve the WCA. Therefore, the contact angles of the EVA/V-SiO₂, EVA/K-SiO₂ and EVA/T-SiO₂ show the rule of EVA/T-SiO₂ > EVA/K-SiO₂ > EVA/V-SiO₂. The dipole–dipole force between the EVA and the glass surface allowed the two to better adhere together (Figure 15e). These constitute a superhydrophobic coating with excellent self-cleaning properties and chemical stability, giving the glass superior anti-fogging properties.

4. Conclusions

In conclusion, this study has successfully developed a simple method for preparing SiO₂-based superhydrophobic coatings. Hydrophobic organic functional groups were effectively implanted onto the surface of SiO₂ using VTES, KH-560 and TMCS. The modified SiO₂ can be combined with EVA film to create a coating with a rough surface structure. Analyses from the SEM, XRD, TG and FTIR indicated that the T-SiO₂ was the most successful, with a substitution rate of 24.6%. When the T-SiO₂ content was 66 wt%, the EVA/T-SiO₂ exhibited excellent superhydrophobicity, with a WCA of 157° and a WSA of only 2°, which conformed to the Cassie–Wenzel model. The durability and self-cleaning tests indicated that the EVA/T-SiO₂ exhibited exceptional mechanical durability, commendable chemical stability and outstanding self-cleaning capabilities along with anti-fog properties. Furthermore, the EVA/T-SiO₂ demonstrated significant resistance to high temperatures and ultraviolet radiation, suggesting promising applications in the practical implementation of low-rise structures such as walls, roofs and vehicle bodies in Inner Mongolia. Notably, the coating preparation process is straightforward and environmentally friendly, making it an effective method for large-scale applications.