Design and Synthesis of Low Surface Energy Coating with Functionalized Al2O3 Nanoparticles

In a high-moisture environment where dust and coastal saltwater are prevalent, the stability of power equipment can be adversely affected. This issue can result in equipment downtime, particularly for transformers, severely disrupting the continuous operation of DC transmission systems. To address this challenge, a superhydrophobic modified fluorosilicone coating was developed, incorporating anti-stain properties. To tackle this issue comprehensively, an orthogonal experiment was conducted, involving six factors and three levels. The study focused particularly on assessing the impact of water-repellent recovery agents, nanofillers, antistatic agents, anti-mold agents, leveling agents, as well as wetting and dispersing agents on the coating’s surface tension. The results demonstrate that selecting an appropriate base resin and incorporating well-matched functional additives played a central role in effectively reducing the surface tension of the coating. Consequently, optimized coatings exhibited exceptional resistance to stains and displayed strong corrosion resistance.


Introduction
Fluoropolymers have found extensive application in functional organic superhydrophobic coatings due to their exceptional attributes, including remarkable thermal oxidation stability, resistance to harsh weather conditions, imperviousness to solvents, electrical insulating properties, and exceptional water and oil repellency, along with effective anti-fouling characteristics [1][2][3].These materials exhibit low surface energy primarily due to the small size of fluorine atoms, which efficiently shield fluorocarbon atoms without causing any spatial resistance.Furthermore, the carbon-fluorine bonds resist polarization, resulting in diminished intermolecular forces and, consequently, lower surface tension [4,5].Some reports even indicate that the surface tension of fluoropolymers can be as low as 6 mN/m [6].Consequently, the synthesis of low-energy fluoropolymers and the development of superhydrophobic coatings or coatings based on them have garnered significant attention in recent years.
Silicone resins possess outstanding thermal oxidation stability, cold resistance, resilience against adverse weather conditions, water repellency, electrical insulation, and anti-fouling properties [7,8].Recent research has focused on incorporating silicone polymers into fluoroacrylate copolymers to create fluorosilicone block copolymers [9][10][11].However, fluorosilicone exhibits suboptimal adhesion to substrates due to its limited polarity.Typically, organic resins are employed to modify fluorosilicone, combining the excellent attributes of fluorosilicone resin with those of other organic resins, thus broadening its range of applications [12,13].
The amalgamation of fluorosilicone with polyester polymers enhances the overall properties of the resultant material.Polyester resin offers high strength and corrosion resistance [14,15], imparting a synergistic effect that reinforces and toughens the hybrid material.Moreover, the high cross-linking density and favorable compatibility of the blended polymer contribute to the uniformity of the film's chemical and physical structure, preventing the buildup of stains and enhancing interfacial adhesion [16].To achieve this goal, various additives can be employed.Notably, polysiloxane additives are known to reduce surface energy by facilitating the migration of polysiloxanes to the surface, with their structure primarily featuring -(CH 3 ) 2 -SiO-groups [17,18].The chemical structure, side chains, and end groups of these additives play pivotal roles in determining their properties and their impact on film performance.While conventional silicone additives have been regarded as versatile agents for many years, their limited effectiveness under outdoor conditions has led to the emergence of various functional additives that form strong covalent bonds with the coating matrix [19,20].
In this study, an orthogonal test design method was employed to investigate multiple factors and levels.This approach involves selecting a representative set of data points from a full-scale experiment, ensuring orthogonality.Specifically, three different fluorosilicones and five types of polyester were blended to assess their compatibility, adhesion, and surface tension in a hybrid clearcoat.Subsequently, an orthogonal test was conducted to compare the effects of six distinct functional additives on the surface tension of the fluorosilicone/polyester base clear coat.The functional nanofiller of aluminum oxide (Al 2 O 3 ) was modified through the sol-gel method.Silane, as an interface compatibilizer, can significantly enhance the dispersion of alumina within the fluorosilicone coating, thereby further enhancing the mechanical stability of the coating.

Surface Modification of Al 2 O 3
The process of surface modification of Al 2 O 3 nanoparticles was conducted through a sol-gel chemical functionalization.This involved the in situ hydrolysis of two key compounds, namely ethyl orthosilicate (also known as Tetraethoxysilane, TEOS) and aminopropyltriethoxysilane (APTES).Initially, a specific amount of Al 2 O 3 was dispersed into a solution consisting of TEOS and APTES in a ratio of 7:3 by weight.This mixture was mechanically stirred for 1 h and subsequently subjected to ultrasonication for 2 h.The pH was adjusted to 4.5 using glacial acetic acid, and the resulting mixture was allowed to sit for 48 h.Following this, a solution of NaOH with a mass fraction of 2.5% was utilized to raise the pH to 8.5, and gelation was initiated under water bath conditions at 65 • C. The Al 2 O 3 was finally centrifuged and washed several times with a mixture of alcohol and water after the water bath.The resulting product functional Al 2 O 3 was dried in freeze vacuum for further characterization.

Screening of Resin
The combination of three fluorosilicone resins (HLR-Si, AG-2AF, and AG-109F) and five polyester resins (1265XL, YP2224, FS-2970B, FS-4660, and FS-4470) was listed as one by one to measure the surface tension after film formation.The mass ratios of polyester resin and fluorosilicone resin (W polyester :W fluoro ) were 1:11, 1.5:11, 2:11, 2.5:11, and 3:11.Polyester resin (1 g) and fluorosilicone resins (11 g) were dispersed in a mixed solution of PMA (19 g), butyl acetate (15 g), and xylene (1.5 g) by stirring for 30 min.Afterward, a 0.62 g curing agent was added to the mixture and stirred for a certain time.The resulting solution was coated on a clean glass slide and dried at 333 K for 7 h.All resins were prepared according to the mass ratios and methods mentioned above.The synthesis of fluorosilicone/polyester resins was performed following the same procedure reported above.

Sample Preparation
The coating sample was based on fluorosilicone/polyester resins.Then, the additives were known as surface active reagents enabling the reduction in the surface tension.In this work, additives including water-repellent recovery reagent, nanofiller, antistatic reagent, anti-mold and anti-algae reagent, levelling reagent, and wetting and dispersing reagent were added to the resin mixture at various concentrations.These six additives chosen are listed in Table 1, whose chemical structures are differently functionalized and the effects have been reported [21].0.9 0.9 3 0.9 0.9 0.9 According to the orthogonal, an orthogonal test is a method to analyze factors and levels of the representative experiment.In this study, the orthogonal experiment of six factors and three levels was used to fix the amount of film-forming resin and reaction conditions and change the concentration of additives, namely, the six factors are, respectively, the amount of water-repellent recovery reagent, nanofiller, antistatic reagent, anti-mold and anti-algae reagent, levelling reagent, and wetting and dispersing reagent.Each factor has three levels, and each level is shown in Table 1.The orthogonal experiment scheme is shown in Table 2.
The first step for coating was to synthesize additives ultrasonically dissolved in butyl acetate/PMA solution together and stirred at 600 r/min for 30 min.The mass ratio of butyl acetate: PMA was 1:1.5.Then, fluorosilicone resin and polyester resin were successively added into the mixed solution and stirred for 30 min.Afterward, 0.5 wt% curing agent was slowly dispersed in the mixture and stirred for 20 min.Finally, the finished product was filtered through an 80-mesh net.The resulting product was applied to a clean glass slide, and the obtained specimen was placed in an oven at 333 K for 10 h.

Testing and Characterization
The surface wettability, assessed through contact angle (CA), was determined using a contact angle goniometer (SL200KB, KINO Industry Co., Ltd., Boston, MA, USA).For each sample, the average value of measurements is obtained from three different locations.
The contact angle method of measuring the surface tension of material is based on the development of Young's equation and is, by far, the most common way.
Van Oss et al. [22] suggested that the surface tension consists of the Lifshitz-van der Waals component γ LW and the Lewis acid-base component γ AB : Here, γ AB is separated into the acid parameter (γ + ) and base parameter (γ − ).It can be expressed as: The test liquids would be at least one apolar and at least two polar, whose surface tension components γ LW , γ + , and γ − are known as listed in Table 3.The relation between the contact angle θ of test liquids and the solid surface tension is as follows: Good [23] suggests measurement with suitable pairs, such as water/formamide and water/ethylene.
The morphologies of Al 2 O 3 were characterized by scanning electron microscopy (SEM, Hitachi, Tokyo, Japan).The chemical bonding states of the functionalized Al 2 O 3 were analyzed by Fourier-transformed infrared spectroscopy (FTIR, Thermo Fisher Nicolet, Waltham, MA, USA).
According to GB/T 9286-1998 Paints and varnisher-Cross cut test [24]-Part 7, the cutting tool was held so that it was perpendicular to the surface of the sample cutting tool with uniform force and using an appropriate spacing guide, forming a specified number of cuts on the coating with a uniform cutting rate.The samples were coated on the sample substrate and cut by hand.After the cutting was finished, the samples were carefully examined with a visual magnifier in a well-lit environment and the test surfaces were graded by comparing them with the diagram.
The chemical resistance of the coating was tested by immersing the samples in NaOH (pH~12) and HCl (pH~4) for 7 days.
To demonstrate the anti-stain performance of the fabricated surfaces, the coated glass was placed in a Petri dish with a tilting angle of 15 • .Both surfaces were contaminated with graphite particles that acted as artificial dirt.Water droplets were then dripped onto both contaminated surfaces.The anti-stain capability was examined by assessing whether different liquids such as dye, ink, milk, Coca-Cola, and slurry could flow down the surface without leaving any stains.

Analysis of Surface Tension of Fluorosilicone/Polyester Resins
The fluorosilicone/polyester resin coatings were investigated by measuring surfacefree energy and assessing compatibility and adhesion.Figure 1  Conversely, other pairings encounter compatibility issues, leading to the separation of resin phases.Consequently, the adhesion of the clearcoat is compromised due to the incomplete cross-linking of the matrix.According to GB/T 9286-1998, the first three levels are considered satisfactory for general use, with higher grades indicating better adhesion between the coating and substrate.Notably, HLR-Si/FS-4470 demonstrates superior adhesion capabilities.As a result, the HLR-Si/FS-4470 coating matrix enables lower surface tension with exceptional compatibility and adhesion.As anticipated based on the wettability, the coatings exhibit a notably low surface energy (γ) owing to the presence of fluorine [25,26].As illustrated in Figure 1, the inclusion of fluorosilicone/polyester resins indeed impacts surface tension, depending on the type of resin.The results indicate that the surface tension of AG-2AF/polyester resins (Figure 1b) ranges from 34.699 to 47.928 mJ/m 2 .Notably, HLR-Si/polyester resins (Figure 1a) display lower surface tension than AG-2AF and AG-109F.There exists an optimal ratio of fluorosilicone and polyester resins that results in reduced surface tension.
The compatibility of the resins and the adhesion of the clearcoat are presented in Table 4.It is evident from Table 4 that only certain combinations exhibit good compatibility, namely HLR-Si/FS-2970B, HLR-Si/FS-4660, HLR-Si/FS-4470, and AG-109F/FS-4660.Conversely, other pairings encounter compatibility issues, leading to the separation of resin phases.Consequently, the adhesion of the clearcoat is compromised due to the incomplete cross-linking of the matrix.According to GB/T 9286-1998, the first three levels are considered satisfactory for general use, with higher grades indicating better adhesion between the coating and substrate.Notably, HLR-Si/FS-4470 demonstrates superior adhesion capabilities.As a result, the HLR-Si/FS-4470 coating matrix enables lower surface tension with exceptional compatibility and adhesion.

Analysis of Surface Tension from Orthogonal Experiment Results
According to the results of the above discussion in Section 3.1, a clearcoat of HLR-Si/FS-4470 resins is selected for orthogonal design.The surface tension values of the coatings loaded with different concentrations of additives were measured.Table 5 shows the results of the surface tension of the coatings obtained by the orthogonal test.The various concentrations of additives resulted in the coating surface tension change.Results show that the surface tension reduction is more marked in sample 2. For further analysis, Table 6 shows the mean and range of the levels of each factor obtained from the calculations corresponding to Nos. 1-18 in Table 5.The range is the different value between the maximum and minimum of the mean value.The range of nanofiller is large, so the nanofiller has the most significant influence on the surface tension.The heightened hydrophobicity with greater nanofiller proportions can primarily be attributed to the increased surface roughness resulting from increased nanoparticles.According to the results, it can be seen that the factors affecting the surface tension of the coating are ranked B > D > E > A > F > C. The preferred formulation for the low surface tension coating is A2B3C3D2E2F2 (waterrepellent recovery reagent 0.6 g, nanofiller 4.5 g, antistatic reagent 3 g, anti-mold and anti-algae reagent 0.6 g, levelling reagent 0.6 g, and wetting and dispersing reagent 0.6 g).The surface tension of the optimal sample A2B3C3D2E2F2 is 10.400 mJ•m −2 .This is attributed to the incorporation of a fluorosilicone resin possessing low surface energy characteristics and a precise blend of six additives.

Analysis of Surface Characteristics of Low Surface Energy Coating
Based on the surface energy results obtained from orthogonal experiments, we conducted a single-factor experimental analysis with constant 0.6 g water-repellent recovery reagent, 3 g antistatic reagent, 0.6 g anti-mold and anti-algae reagent, 0.6 g levelling reagent, and 0.6 g wetting and dispersing reagent and varying quantities of f-Al 2 O 3 nanoparticles to obtain SEM images of fluorosilicone/polyester coatings with different nano-filler levels.
As shown in Figure 2a, the paint film without f-Al 2 O 3 nanoparticles exhibited a smooth and uniform appearance.The introduction of f-Al 2 O 3 nanoparticles led to an increase in coating surface roughness.With the addition of 1.5 g of f-Al 2 O 3 (Figure 2a), the paint film contained fewer particles, resulting in a relatively even and structurally uniform surface.With a gradual increase in f-Al 2 O 3 , particle distribution on the paint film surface became more prominent.The surface morphology transitioned from relatively loose to denser.When f-Al 2 O 3 addition reached 4.5 g, a layered micro-nanostructure was observed on the coating surface constructed by numerous cross-linked particles, which enhanced the air-trapping capability of the coating.However, excessive nanofiller significantly degraded both the surface quality and aesthetics of the paint film.Therefore, appropriate f-Al 2 O 3 promoted surface roughness to form trapped air cushions at the gas-liquid-solid three-phase interface, which eventually decreases the surface energy of the coating.In Figure 3d, the alterations in surface wettability between the pristine Al2O3 and the modified Al2O3 are depicted.As a comparison, hydrophobic modified Al2O3 exhibited non-wetting properties, confirming the hydrophobicity of f-Al2O3 nanoparticles.The exposed Al2O3 surface was relatively smooth with a low water contact angle value (71.2 ± 3.9°).In contrast, the silane-modified Al2O3 surface had some irregular particles with hydrophobicity and the contact angle value increased to 102.3 ± 2.0°.Resistance to acid, alkali, and corrosion, as well as anti-stain properties, are additional requirements for protective coatings.In Figure 4a, for better observation, hydrochloric acid solution (pH = 4) and sodium hydroxide solution (pH = 12) were prepared with blue and pink dyes, respectively.The prepared low-surface-energy coatings were immersed in acidic and alkaline solutions for 7 days.The results indicate that, after one week of immersion, the surface remained in its previous state with no color adhesion.The droplets on the coating surface in both corrosive solutions maintained a spherical shape, indicating that the coating exhibited good long-term mechanical stability under corrosive conditions.
Figure 4b corroborates the anti-stain performance of the low-surface-energy coating against various stains.When dyes, ink, cola, and slurries were dropped onto the FS surface, the droplets maintained nearly spherical shapes on the surface, readily sliding apart without leaving any stains.The exceptional non-wettability of the coating is attributed to the presence of abundant air gaps between the modified nanoparticles on the coating's surface and the liquid, coupled with lower surface energy, resulting in reduced adhesion to the surface.FTIR analysis was conducted on Al 2 O 3 and f-Al 2 O 3 nanoparticles to assess their modification status, as shown in Figure 3c.The peaks appear at 595 and 656 cm −1 , indicating the Al-O stretching mode, respectively [27].A wide-ranging band near 3449 cm −1 is attributed to O-H stretching vibration [28].Additionally, at 1650 cm −1 , a band emerges as a result of the O-H bending of absorption of moisture [29].Furthermore, a wide band within the 1200-1000 cm −1 range, along with a band at approximately 800 cm −1 , can be attributed to the asymmetric and symmetric stretching vibrations of Si-O-Si bonds, respectively [30,31].The observed peak at approximately ~2925 cm −1 may be attributed to the C-H stretching vibrations originating from the CH 3 and CH 2 groups present in the silane coupling agents [32].Combining these results with the SEM images, it can be inferred that, after modification, some porous nanoscale organosilicon particles have coated the surface of Al 2 O 3 .Therefore, the presence of methyl and amino groups further confirms the successful grafting of APTES onto the surface of Al 2 O 3 .
In Figure 3d, the alterations in surface wettability between the pristine Al 2 O 3 and the modified Al 2 O 3 are depicted.As a comparison, hydrophobic modified Al 2 O 3 exhibited non-wetting properties, confirming the hydrophobicity of f-Al 2 O 3 nanoparticles.The exposed Al 2 O 3 surface was relatively smooth with a low water contact angle value (71.2 ± 3.9 • ).In contrast, the silane-modified Al 2 O 3 surface had some irregular particles with hydrophobicity and the contact angle value increased to 102.3 ± 2.0 • .
Resistance to acid, alkali, and corrosion, as well as anti-stain properties, are additional requirements for protective coatings.In Figure 4a, for better observation, hydrochloric acid solution (pH = 4) and sodium hydroxide solution (pH = 12) were prepared with blue and pink dyes, respectively.The prepared low-surface-energy coatings were immersed in acidic and alkaline solutions for 7 days.The results indicate that, after one week of immersion, the surface remained in its previous state with no color adhesion.The droplets on the coating surface in both corrosive solutions maintained a spherical shape, indicating that the coating exhibited good long-term mechanical stability under corrosive conditions.

Conclusions
In this study, we investigated surface tension, compatibility, and adhesion when blending fluorosilicone/polyester polymers with different resins.Notably, the compatibility of these two types of resins exhibited significant variations, directly impacting their interfacial adhesion performance.Among all the resins, HLR-Si/FS-4470 demonstrated superior overall properties.Furthermore, the functional additives within the HLR-Si/FS-4470 coating played a crucial role in determining its properties.Orthogonal experiments revealed that the nanofiller had the most significant impact on surface tension, followed by anti-mold and anti-algae reagent, levelling agent, water-repellent recovery reagent, wetting and dispersing reagent, and antistatic reagent.The optimal formulation for achieving a low-surface-tension coating included 0.6 g of water-repellent recovery reagent, 4.5 g of nanofiller, 3 g of antistatic reagent, 0.6 g of anti-mold and anti-algae reagent, 0.6 g of leveling reagent, and 0.6 g of wetting and dispersing reagent.Additionally, the composite coating exhibited a degree of resistance to staining.This research provides valuable insights into the development of low-surface-tension surfaces with hydrophobic characteristics and corrosion resistance.
depicts the changes in surface tension concerning different resins.Materials 2023, 16, x FOR PEER REVIEW 6 of 13

Figure 2 .
Figure 2. SEM images of fluorosilicone/polyester coating with nanofillers of different quantities of f-Al 2 O 3 nanoparticles: (a) 0 g; (b) 1.5 g; (c) 3 g; (d) 4.5 g.SEM analysis in Figure3reveals the morphological changes on the Al 2 O 3 surface before and after modification.In Figure3a, unmodified Al 2 O 3 nanoparticles exhibit substantial aggregation due to van der Waals forces.As shown in Figure3b, after surface modification, the swollen appearance and organic hydrolysis of the functionalized Al 2 O 3 surface can be observed, both induced by the grafting of silane onto the alumina surface.This silane treatment, following sol-gel processing, effectively prevents aggregation and further improves the interface compatibility between Al 2 O 3 and organic solvents.

Figure 4 .
Figure 4. (a) Acid, alkali, and corrosive resistance test and (b) anti-stain test of the low-surfaceenergy coating.

Table 1 .
Influencing factors and levels of orthogonal experiment.

Table 4 .
The compatibility and adhesion of coatings without additives.

Table 5 .
CA and surface tension.

Table 6 .
Range results of the surface tension in the orthogonal experiment.