Design of superhydrophobic coatings fabricated by spraying for anti-icing

The icing of transmission lines has a serious impact on people’s lives and disrupts the secure and steady functioning of the power grid, causing huge economic losses. To retard the icing of transmission line glass insulators, we prepared coatings on a glass slide by spraying using epoxy resin, fluorosilicone resin, and hydrophobic silicon dioxide (SiO2). The study examined the microscopical morphology, wetting behaviour, and anti-icing and self-cleaning characteristics of coatings containing various SiO2 mass fractions. The findings indicated that the SiO2 mass fraction notably impacted the micro-nanostructure, anti-wettability, and anti-icing performance of coatings. In addition, the largest contact angle (168.2°), the smallest sliding angle (2.6°), and the longest freezing time (181.7 s) were measured for the superhydrophobic coating with SiO2 mass fraction of 34%, which was attributed to the most uniform microstructure. The superhydrophobic coatings fabricated through spraying exhibited good anti-icing and self-cleaning properties. This facilitates the anti-icing and anti-fouling of transmission line glass insulators.


Introduction
Electric transmission lines running through high-altitude mountainous areas are especially prone to ice covering.The icing of the power grid causes power outages and serious accidents such as insulator flashover, line breakage, and tower collapse.This greatly affects the safe operation of grid transmission and distribution [1].Therefore, numerous active and passive anti-icing techniques are proposed to solve the icing problem of transmission lines.Active anti-icing techniques include highcurrent ice melting, mechanical deicing, and manual deicing.Although the de-icing effect of these methods is significant, the de-icing process often consumes a lot of manpower, material, and financial resources.Taking advantage of the self-cleaning effect of lotus leaves, the coating technique is employed as a powerful passively anti-icing method.The superhydrophobic coatings have excellent water-repellent properties and exhibit good anti-icing performance, attracting widespread attention [2,3].
Superhydrophobic coatings are often prepared by building micro-nanostructures on the material surface and treating it with low-surface-energy substances.Currently, the widely used methods for superhydrophobic surface preparation include chemical vapor deposition, etching, self-assembly, and magnetron sputtering.However, these methods are limited to preparing small flat coatings in the laboratory, making it difficult to satisfy the application of large-area coatings for complex structures in industrial environments.On the contrary, spraying is a convenient and economical method to prepare large-area superhydrophobic coatings on arbitrarily shaped substrates (such as transmission line glass insulators).Qin et al. [2] utilized a step-by-step spraying method to apply PTFE/epoxy superhydrophobic coatings onto the surface of aluminum plates.The experimental findings revealed that the superhydrophobic coatings prepared at low cost could exhibit extremely small ice adhesion strength (28 kPa).Pan et al. [3] combined spraying with hard mold plate hot pressing to prepare a carbon fiber/epoxy composite superhydrophobic coating, which exhibited a long ice-delay time and a bond strength of 50 kPa to ice.By one-step spraying, Zhang et al. [4] fabricated a multi-walled carbon nanotube superhydrophobic coating on the Q235 steel surface.It was found that the coating containing a 30% MWCNT mass fraction showed the most effective delayed icing properties.It is worth noting that the spraying methods currently used to prepare superhydrophobic coatings have problems with complex solution compositions, expensive solutes [4], and cumbersome processes [2,3].These limitations may hinder the potential use in the industry.Additionally, producing superhydrophobic anti-icing coatings using one-step spraying must be studied as a potential application for transmission line glass insulators.
In the present work, we fabricated an anti-icing coating using a simple spraying.Common epoxy resins, fluorosilicone resins, and hydrophobic silicon dioxide (SiO 2 ) were selected to fabricate coatings containing various SiO 2 mass fractions on the surface of glass slides using one-step spraying.This study analyzed the impact of SiO 2 content on the micro-nanostructure, anti-wetting, and anti-icing characteristics of coatings by utilizing scanning electron microscopy, a water contact angle measuring instrument, and a semiconductor cooling platform.The relationships between microstructure, hydrophobicity, and anti-icing properties were examined.

Materials
The modified bisphenol A epoxy resin (GCC135) was purchased from Shanghai Kunshan Lvxun Chemicals Industry Co., Ltd.The Fluorosilicone resin was supplied by Chengdu Aikeda Chemical Reagent Co., Ltd.Compared with other ceramics, SiO 2 nanoparticles have the advantages of low cost, easy surface modification and controllability, facilitating the preparation of superhydrophobic coatings.As a result, SiO 2 nanoparticles were chosen to prepare superhydrophobic anti-icing coatings.Shanghai Macklin Biochemical Co., Ltd produced the SiO 2 nanoparticles.The average size and specific surface area were 30 nm and 120 m 2 /g, respectively.Absolute alcohol was supplied by Chongqing Chuandong Chemical (Group) Co., Ltd.Ethyl acetate was provided by Chengdu Kelong Chemical Co., Ltd.The glass slides (7101) were obtained from Jiangsu Feizhou Glass Plastic Co., Ltd.

Fabrication
The masses of SiO 2 (0.5 g, 0.8 g, 1.1 g, 1.4 g, 1.7 g, and 2.0 g) were weighed with an electronic balance to prepare coatings with different SiO 2 mass fractions.Among them, the mass of epoxy resin was 2 g, the mass of fluorosilicone resin was 3 g, and the mass of ethyl acetate was 20 g.The epoxy resin to curing agent mass ratio was 10:3.The epoxy resin, fluorosilicone resin, SiO 2, and ethyl acetate were mixed in a container and agitated using magnetic stirring for 10 min, and then ultrasonically dispersed by an ultrasonic cleaner for 5 min.After ultrasonic dispersion, the curing agents were applied and agitated for 5 min to provide a uniform solution.The glass slides were washed by ultrasound for 5 min in absolute ethanol and deionized water before spraying.The mixing solution was transferred to a spray bottle.After that, the spray pistol was turned toward the glass slide for spraying.Subsequently, the bottom-up spraying was performed in an even manner.The nozzle of the spray gun had a diameter of 1.5 mm.The pressure and distance of spraying were 0.4 MPa and 25 cm, respectively.After spraying, the coating was left at room temperature for 30 min, immediately followed by overnight curing in an oven.

Characterization
The micro-nanostructure of the coating was analyzed by scanning electron microscopy (SEM).The contact angle and sliding angle of coating at room temperature were obtained using a water contact angle meter.Approximately 6 μL of water droplets were utilized for testing.Five positions were chosen for the measurements.The average was calculated as the final test result.A homemade water droplet freezing instrument determined the freezing times of water droplets on the coating surface.Details of the testing process can be found in the published report [5].Furthermore, self-cleaning tests were performed with Congo red simulated dust.

Microscopic morphology
The coatings with different SiO 2 mass fractions were fabricated on the surface of the glass slides by one-step spraying.SEM was utilized to investigate the microstructure of coatings containing various SiO 2 mass fractions, as demonstrated in Figure 1.It is obvious that the SiO 2 mass fraction significantly affects the micro-nanostructure on the coating surface.At a SiO 2 content of 10%, the coating surface is observed to be smoother, with a small amount of scattered particles and protrusions (Figure 1(a)).This is mainly owing to the low amount of SiO 2 addition.The SiO 2 nanoparticles are almost completely encapsulated by epoxy resin and fluorosilicone resin.With increasing SiO 2 content, the surface of coatings progressively turns rougher.Many micron-sized particles of varying sizes are observed in the low magnification images, and numerous granular nano-SiO 2 are found on the surface of epoxy and fluorosilicone resins in the high magnification images (Figures 1(b), (c), (d), (e), and (f)).In addition, it is noted that the coating surface has more large-sized pits and holes when the SiO 2 mass fraction is 16%, 22%, 28%, and 40% (low magnification images in Figures 1(b), (c), (d), and (f)).In contrast, the coating with a SiO 2 mass fraction of 34% has a flatter microstructure (Figure 1(e)).The surface is seen to be covered with a uniform micron-sized structure (peppered with nano-SiO 2 ).

Anti-wetting characteristics
The contact angle is commonly employed to assess the hydrophobic properties of material surfaces, and the sliding angle is used to evaluate the rolling ability of water droplets.We determined the contact and sliding angles of glass and coatings, as shown in Figure 2. It can be found that the contact angle of coatings gradually improves with increasing the SiO 2 content (Figure 2(a)).As the SiO 2 content is raised to 22%, the contact angle of coatings is higher than 150°.Meanwhile, the water droplets present a rounded appearance on the surface.Compared to glass (69.2°), the coating with a SiO 2 content of 34% shows a contact angle as high as 168.2°.In contrast, the sliding angle of the coating exhibits a decreasing and then increasing trend as the SiO 2 content increases (Figure 2(b)).Notably, the glass and coatings containing SiO 2 contents of 10% and 16% have large sliding angles, and water droplets are easily adsorbed on the surface.However, the sliding angles of coatings with SiO 2 mass fractions of 34% and 40% are as low as 2.6° and 4.8°, respectively.At this point, the water droplet can roll away from the surface.It is well known that the contact angle of superhydrophobic coatings should be over 150°, and their sliding angle should be below 10°.This indicates that coatings with SiO 2 mass fractions of 34% and 40% have superhydrophobic surfaces and exhibit good superhydrophobic properties.On the other hand, it should be mentioned that the addition of excess SiO 2 results in the formation of large craters on the surface of the coating containing 40% SiO 2 (Figure 1(f)).This leads to a reduction in contact angle and an expansion in sliding angle.

Anti-icing properties
A semiconductor cooling platform performed the water droplet freezing tests to evaluate the impact of SiO 2 content on the anti-icing characteristics of coatings.The freezing processes of water droplets on glass and coatings containing various SiO 2 contents are shown in Figure 3.It is found that water droplet starts to nucleate from the bottom, and the ice crystals gradually expand to the whole droplet with time, which is a typical heterogeneous nucleation mode [1].In addition, it can be noted that the water droplet lies flat on glass and coatings containing lower SiO 2 mass fractions (10% and 16%).The water droplets are completely frozen quickly, and the contact angle of a frozen water droplets (flat laying state) is small.With increasing SiO 2 content, the hydrophobic properties of the coating surface progressively improve.This leads to a longer water droplet freezing time, and the frozen water droplet (sphere-like state) still has a large contact angle.The freezing times of glass and coatings were extracted, as shown in Figure 4. Compared to the lowest freezing time (31.3 s) of glass, the freezing times of coatings are prolonged.Furthermore, the freezing time of coatings rises and falls as the SiO 2 mass fraction gradually increases.The superhydrophobic coating with a SiO 2 mass fraction of 34% shows the longest freezing time (181.7 s), approximately 6 times more than glass.We notice a strong correlation between the micronanostructure of coatings and the hydrophobic and anti-icing properties.As the SiO 2 content rises, the structure on the coating surface progressively gets rough (Figure 1).The hydrophobicity (Figure 2) and anti-icing properties (Figure 4) also gradually improve with the SiO 2 mass fraction.However, defects such as pits and holes in the rough structure (Figure 1(b), (c), (d), and (f)) cause a decrease in the hydrophobic and anti-icing characteristics of coatings.Therefore, the coating with a SiO 2 mass fraction of 34% displays the smoothest structure (Figure 1(e)), which leads to the best water repellences and anti-icing properties (freezing time of 181.7 s).

Self-cleaning properties
The self-cleaning properties of glass and coatings were tested using Congo red powder to simulate pollutants, as shown in Figure 5. Obviously, for glass and coatings with a SiO 2 mass fraction of 10%, water drops from the dropper dissolve and accumulate with Congo red powder and cannot flow (Figures 5(a) and (b)).The primary reason is that the surface does not have a significant rough structure (Figure 1) and exhibits very poor hydrophobic properties (Figure 2).In contrast, the coating containing a 34% SiO 2 mass fraction exhibits a low sliding angle (Figure 2  Many studies have demonstrated that the rough structure on superhydrophobic surfaces contains numerous air cushions [1,2,4,6,7].It dramatically minimizes the contact area, providing stability to the water droplet and creating a Cassie condition with exceptional hydrophobic properties [1].Calculating the proportion of solid-liquid contact area of coatings can be performed using the Cassie model, assuming the intrinsic contact angle of coatings to be 108.2°.Among them, the solid-liquid contact area fraction for coating with a SiO 2 mass fraction of 34% is only 5.02%.This suggests that the superhydrophobic coatings fabricated in this study show outstanding hydrophobic properties.In this case, the water droplet on a superhydrophobic surface has a higher center of gravity, appearing globular to roll off the surface easily, thus avoiding the wetting of the coating surface and minimizing the likelihood of icing.Moreover, air is known to be a terrible heat conductor.Additionally, the very low solid-liquid contact area and substances with low surface energy on the surface of superhydrophobic coatings greatly reduce the heat transfer rate and considerably delay freezing.In

Figure 2 .
Figure 2. Wetting properties of glass and coatings.

Figure 3 .
Figure 3. Freezing morphology of water droplets on glass and coatings.

Figure 4 .
Figure 4. Freezing time of glass and coatings.The freezing times of glass and coatings were extracted, as shown in Figure4.Compared to the lowest freezing time (31.3 s) of glass, the freezing times of coatings are prolonged.Furthermore, the freezing time of coatings rises and falls as the SiO 2 mass fraction gradually increases.The superhydrophobic coating with a SiO 2 mass fraction of 34% shows the longest freezing time (181.7 s), approximately 6 times more than glass.We notice a strong correlation between the micronanostructure of coatings and the hydrophobic and anti-icing properties.As the SiO 2 content rises, the (b)) and good self-cleaning properties.This causes water droplets entrained with Congo red powder to run off the surface, resulting in a clean surface (Figure 5(c)).

Figure 5 .
Figure 5. Self-cleaning properties of glass and coatings.Many studies have demonstrated that the rough structure on superhydrophobic surfaces contains numerous air cushions[1,2,4,6,7].It dramatically minimizes the contact area, providing stability to the water droplet and creating a Cassie condition with exceptional hydrophobic properties[1].Calculating the proportion of solid-liquid contact area of coatings can be performed using the Cassie model, assuming the intrinsic contact angle of coatings to be 108.2°.Among them, the solid-liquid contact area fraction for coating with a SiO 2 mass fraction of 34% is only 5.02%.This suggests that the superhydrophobic coatings fabricated in this study show outstanding hydrophobic properties.In this case, the water droplet on a superhydrophobic surface has a higher center of gravity, appearing globular to roll off the surface easily, thus avoiding the wetting of the coating surface and minimizing the likelihood of icing.Moreover, air is known to be a terrible heat conductor.Additionally, the very low solid-liquid contact area and substances with low surface energy on the surface of superhydrophobic coatings greatly reduce the heat transfer rate and considerably delay freezing.In