Anti-icing application of superhydrophobic coating on glass insulator

Insulators, as important components in transmission lines, are prone to disturb the safe running of the power system by the ice accumulation on surfaces. Traditional anti-icing coatings are difficult to practically apply to insulators. Here, superhydrophobic (SHP) coatings were fabricated on the glass insulator surface by spraying. We studied microstructure, wettability, water droplet bouncing behavior, and anti-glaze icing properties of SHP coating. The results demonstrated that SHP coatings had micro-nano rough structures. The excellent superhydrophobicity (contact angle of 165.2° and sliding angle of 3.7°) was achieved. The water droplet was easily adhered to the surface of the glass insulators. At the same time, individual water droplets could bounce away from the surface after impacting the SHP coating. In the glaze environment, water droplets sprayed onto the SHP coating merged with each other and slid off the surface. These significantly reduce the likelihood of freezing. Furthermore, the SHP coating could dramatically delay the glaze icing and decrease the icing area. The icing weight and icicle length were smaller than glass insulators. The SHP coatings prepared in this work display great potential for the anti-icing of glass insulators.


Introduction
Icing can easily cause accidents such as insulator flashover and even result in the loss of electrical insulation properties.These incidents negatively impact the stable functioning of power systems.A large number of scholars have carried out a lot of work for this purpose.Active deicing techniques such as mechanical deicing, current melting, and artificial deicing are used to solve the problem of ice accumulation on transmission lines.However, it should be pointed out that the active deicing technology has the defects of low efficiency and huge consumption, and it is difficult to realize the application on insulators [1].In addition, the widely used water-repellent coatings (RTV and PRTV) have some antifouling effect, but their surfaces still suffer from the problem of ice cover.
In recent years, superhydrophobic (SHP) coating on the basis of the lotus leaf effect has shown outstanding anti-icing properties, becoming a hot research topic in the field [2].In brief, by creating rough structures and modifying them with low-surface-energy substances, SHP coatings can be fabricated.Liao et al. [3] prepared ZnO/SiO 2 /PTFE SHP coatings by RF magnetron sputtering.The contact angle reached a maximum of 167.2°, while the sliding angle was less than 1°.Within 120 min, only 17.9 % of the area (only the edge) of the film surface was frozen.Cao et al. [4] prepared metalbased SHP coatings by template and chemical vapor deposition.In an indoor environment with low temperature and high humidity, SHP coating displayed excellent anti-icing properties.The freezing time was significantly increased (about 300 s).Moreover, the ice adhesion strength (~14.8 kPa) decreased by 90% with respect to the substrate.Wan et al. [5] employed nanosecond laser processing to construct SHP surfaces on aluminum alloys.In the −15℃ temperature environment, the SHP surface exhibited low adhesion (sliding angle below 6.3°).This essentially prevented ice formation, allowing water droplets to slide off the surface effortlessly.The delay time for icing measurement exceeded 700 s, causing a noteworthy decline in the icing amount.A durable SHP aluminum surface was fabricated through anodizing by Liu et al. [6].The surface not only demonstrated excellent antifrosting performance but also effectively retarded the glaze icing.In addition, due to the hard porous alumina and self-healing properties, the SHP aluminum surface maintained outstanding superhydrophobicity after 64 abrasion-damage cycles.Although the reported SHP surfaces have good anti-icing properties, most of the materials used above are metals [3−6], and there is a lack of relevant studies for glass insulators.On the other hand, the preparation of the aforementioned SHP coatings is limited by expensive and complicated equipment.The preparation process is cumbersome and only suitable for preliminary research in the laboratory, which makes it difficult to meet industrial mass production [4,5].At this point, SHP anti-icing coatings can be conveniently and quickly prepared on specially shaped surfaces using simple and efficient spraying.Nevertheless, it is noteworthy that SHP coatings produced by spraying still focus on small areas of block specimens.Currently, there are limited reports on preparing SHP coatings by directly spraying them onto the glass insulator surface.The studies on water droplet bouncing and anti-glaze icing of SHP glass insulators are lacking.
In this work, SHP coatings were developed directly on the glass insulator surface by spraying.The micro-morphology and elemental content of SHP coatings were characterized with scanning electron microscopy and energy spectroscopy.A water contact angle meter and a high-speed camera were utilized to assess the wettability and water droplet bouncing properties of SHP coatings.Glaze icing tests were performed in an artificial climate chamber.The ice-covering morphology was captured with a cell phone camera.The glaze icing processes of SHP coatings were analyzed by measuring the icing weight and icicle length.

Materials
Modified Bisphenol A Epoxy Resin (GCC135) was supplied by China Kunshan Lvxun Chemicals Industry Co., Ltd.Polytetrafluoroethylene (PTFE) particles (average size of 200 nm) were bought from Shanghai Macklin Biochemical Co., Ltd.Fluorosilicone resin was provided by Chengdu Aikeda Chemical Reagent Co., Ltd.Ethyl acetate was offered by Chengdu Kelong Chemical Co., Ltd.Anhydrous ethanol was supplied by Chongqing Chuandong Chemical (Group) Co., Ltd.Ordinary glass insulators (FC70/146) were bought from Taobao.com.All reagents used in this work were analytical grade and applied directly without further treatment.

Preparation
Ethyl acetate (25 g), fluorosilicone resin (3 g), and epoxy resin (2 g) were placed in a beaker.The mixture was magnetically agitated for 30 min.Next, PTFE powders (5 g) were applied, and magnetic stirring and ultrasonic shaking were conducted for 10 min.Afterward, epoxy resin hardener (0.6 g) was blended under magnetic agitation for 10 min to prepare a homogeneous solution.The mixture was placed in a spray bottle.The gun was targeted at the horizontally positioned glass insulator to spray.Before spraying, it was necessary to clean the glass insulators with anhydrous ethanol and deionized water and then dry them using hot air for later use.The nozzle of the spray gun had a diameter of 1.5 mm.The pressure and distance of spraying were 0.3 MPa and 15 cm, respectively.After spraying, the glass insulator was left at room temperature for 30 min, immediately followed by overnight curing in a 70℃ oven.

Characterization
The microstructure and elemental composition were characterized by scanning electron microscopy (SEM, Zeiss Auriga) and energy spectroscopy (EDS).The three-dimensional structure and the roughness were evaluated by a laser confocal microscope (LEXT OLS4000).The contact angle and sliding angle were measured by a water contact angle meter (SINDIN SDC-100).Water droplets of 6 μL were used for testing, and three positions were selected for measurement and averaged.The water droplet bouncing behaviors were assessed by a high-speed camera (M220, Revealer).In addition, glaze icing tests were performed in the artificial climate chamber at three temperatures (−5℃, −8℃, and −12℃).The ambient humidity was 90%, and the wind speed was controlled at 1.2 m/s.Water droplets, measuring about 40 μm in diameter, were sprayed onto the glass insulator surface with a spray nozzle.The test lasted for 80 min.A cell phone camera was used to take pictures of the ice cover's appearance.After the test, the weight of the glass insulator was weighed, and the length of the icicle was measured.

Macrophoto and microstructure
SHP coating was prepared on the ordinary glass insulator surface by spraying, as displayed in Figure 1.It is observed that the glass insulator surface is relatively smooth.The contact angle is only 41.7°, and the sliding angle is over 90°.This makes it simpler for water droplets to stick to the surface and spread on the surface.In contrast, the SHP coating has a uniform white film layer distributed on the surface.The contact angle is significantly increased to 165.2°.At the same time, the sliding angle is also decreased to 3.7°.As a result, the surface of SHP coating displays outstanding water repellency and self-cleaning properties, allowing water droplets to quickly slide off the surface.The microscopic morphology, elemental composition, and three-dimensional structure on the SHP coating surface were characterized, as presented in Figure 2. The SHP coating surface is filled with spherical PTFE nanoparticles, forming a typical micro-nano roughness structure.This meets the basic conditions for the construction of the SHP surface (Figure 2a).It is widely recognized that the presence of C−F groups results in low surface energy, which facilitates the achievement of superhydrophobicity.The F element with a content of up to 51.3% is detected on the SHP coating surface (Figure 2b), which originates from the fluorosilicone resin and PTFE particles.The large amount of C−F groups brought by the added fluorosilicone resin and PTFE particles effectively reduces the surface energy.Furthermore, the three-dimensional structure (Figure 2c) also verifies the establishment of a micro-nanoroughness structure with a surface roughness of 3.67 μm.Therefore, the SHP coating surface shows excellent hydrophobicity owing to the joint influence of micronanoroughness structures and low-surface-energy groups (Figure 1).

Water droplet bouncing behavior
A large number of water droplet bouncing tests on flat plates have been reported.There is insufficient research on the bouncing behavior displayed by water droplets on glass insulators.Figure 3 exhibits the bouncing characteristics of single water droplets on glass and SHP insulator surfaces.During the test, a single water droplet is released from a distance of about 30 cm to collide with insulators.The observation reveals that individual water droplet impacting the glass insulator surface first spreads and then contract and finally spread flat on the surface.This is due to the large adhesion force on the glass insulator surface, which can easily absorb the water droplets.On the contrary, when a single water droplet hits the SHP insulator surface, the water droplet first rebounds significantly and then bounces off the surface.This suggests that there is a small adhesion force and energy loss between the SHP surface and the water droplet, enabling the water droplet to retain enough kinetic energy to rebound away from the surface.This is also attributed to the micro-nano-scale roughened structures (Figure 2) and excellent hydrophobicity (Figure 1) on the surface of the SHP insulator.In order to simulate the glaze, the water droplet bouncing tests in a glaze-freezing environment were performed in an artificial climate chamber.Figure 4 illustrates the behaviors of water droplets bouncing on both glass and SHP insulators in the glaze icing environment.Different from a single water droplet, the spray nozzle forms small water droplets, similar to rain, that are blasted on the surface as the glaze freezes.Consistent with the bouncing behavior of a single water droplet, numerous small water droplets immediately adhere to the glass insulator and spread out on the surface upon impact.Subsequently, the small water droplets gradually expand to form a water film that completely covers the surface.In the case of the SHP insulator, it is evident that some of water droplets bounce off the surface immediately after impact, while others merge with each other and grow up before rolling away from the surface.As a result, the majority of water droplets in the glaze environment can roll off the surface of the SHP insulator.With excellent superhydrophobicity (Figures 1, 2, and 3), the SHP insulator surface retains a large dry area, which dramatically minimizes the likelihood of water droplet freezing and is conducive to the realization of glaze icing resistance of glass insulator.

Anti-glaze icing performance
To investigate the anti-glaze icing properties of the SHP insulator, the glaze icing tests under three low-temperature environments were performed in an artificial climate chamber.The ice-covering morphology is displayed in Figure 5.It is noted that as the ice-covering time extends, the glass insulator surface is first obscured by a film of apparent ice.Then, the ice grows downward along the ice layer to form a long icicle.The superior hydrophobicity (Figure 1) and water droplet bouncing properties (Figures 3 and 4) remarkably decrease the possibility of water droplet freezing [4,6].A large number of small water droplets that do not have time to tumble away from the SHP insulator surface will freeze.However, the surface still maintains large un-iced areas, and the icicles formed at the edges are significantly reduced.In addition, the ambient temperature greatly affects the ice cover on the insulator surface.As the ambient temperature decreases, the icing on both glass and SHP insulator surfaces is gradually increasing.In particular, the icing on the glass insulator surface is more serious.At this point, the icing has completely wrapped around the insulator.The ice thickness and icicle length increase significantly.In three low-temperature environments, the surface of the SHP insulator can remain a certain dry area, and the ice-covered area and the length of the icicle are decreased.
Figure 5. Ice-covering morphology of glass insulator and SHP insulator in three low-temperature environments.On the other hand, the icing weight and icicle length of the insulators were measured after the icecovering test, as presented in Figure 6.As the ambient temperature decreases, the icing weight (Figure 6a) and icicle length (Figure 6b) of the glass and SHP insulators gradually increase, which agrees with the results in Figure 5.The icing weight of the SHP insulator drops by more than 35% compared to the glass insulator in the three low-temperature environments.It is noticed that the SHP insulator has no icicles at the edges in the low-temperature environment of −5℃ (Figure 5).In contrast, the icicle length of the glass insulator has grown to 2.31 cm.As the ambient temperature decreases further, the icicle length of the SHP insulator increases rapidly due to localized areas of icing.However, SHP insulator still has significantly smaller icicle length than glass insulator.In summary, the prepared SHP insulator has excellent superhydrophobicity and water droplet bouncing performance, showing superior anti-glaze icing properties, and offers promising application on glass insulators for transmission lines.

Conclusion
A SHP coating was successfully fabricated on the glass insulator surface by spraying.Typical micronano structures and low-surface-energy nanoparticles were generated on the SHP coating surface.Contact angles of up to 165.2° and sliding angles down to 3.7° were measured.The excellent superhydrophobicity and low adhesion properties allowed a single water droplet to bounce away from the surface rapidly upon striking the SHP coating.In a glaze icing environment, numerous small water droplets gradually merged and rolled off the SHP coating surface.As a result, the majority of the area on the SHP insulator remained relatively dry compared to the extensive water layer present on the glass insulator surface.In three low-temperature environments, the surface of the glass insulator experienced more severe glaze icing.However, the icing area, icing weight, and icicle length on the SHP insulator surface were considerably reduced.The prepared SHP insulator shows good anti-glaze icing performance, which offers a valuable reference for applying anti-icing coatings to glass insulators in practical applications.

Figure 1 .
Figure 1.Macroscopic morphology and wettability of glass insulator and SHP insulator.The microscopic morphology, elemental composition, and three-dimensional structure on the SHP coating surface were characterized, as presented in Figure2.The SHP coating surface is filled with spherical PTFE nanoparticles, forming a typical micro-nano roughness structure.This meets the basic conditions for the construction of the SHP surface (Figure2a).It is widely recognized that the presence of C−F groups results in low surface energy, which facilitates the achievement of superhydrophobicity.The F element with a content of up to 51.3% is detected on the SHP coating surface (Figure2b), which originates from the fluorosilicone resin and PTFE particles.The large amount of C−F groups brought by the added fluorosilicone resin and PTFE particles effectively reduces the surface energy.Furthermore, the three-dimensional structure (Figure2c) also verifies the

Figure 3 .
Figure 3. Bouncing behavior of single water droplet on surface of glass insulator and SHP insulator.

Figure 4 .
Figure 4. Bouncing behavior of water droplet on glass insulator and SHP insulator in glaze icing environment.

Figure 6 .
Figure 6.Icing weight (a) and icicle length (b) of glass insulator and SHP insulator.