Totally Waterborne, Nonfluorinated, Mechanically Robust, and Self

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Totally Waterborne, Nonfluorinated, Mechanically Robust and Self-Healing Superhydrophobic Coatings for Actual Anti-Icing Yabin Li, Bucheng Li, Xia Zhao, Ning Tian, and Junping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15061 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Totally Waterborne, Nonfluorinated, Mechanically Robust and Self-Healing Superhydrophobic Coatings for Actual Anti-Icing

Yabin Li, †,‡ Bucheng Li, † Xia Zhao, ‡ Ning Tian†,§ and Junping Zhang†* †Key

Laboratory of Clay Mineral Applied Research of Gansu Province, Lanzhou Institute of

Chemical Physics, Chinese Academy of Sciences, Lanzhou730000, P.R. China ‡Department

of Chemical Engineering, College of Petrochemical Engineering, Lanzhou

University of Technology, Lanzhou 730050, P.R. China §University

of Chinese Academy of Sciences, Beijing 100049, P. R. China

* Address correspondence to [email protected]

ABSTRACT: Bioinspired superhydrophobic coatings are of great interest in academic and industrial areas. However, their real world applications are hindered by some main bottlenecks, especially the pollutive preparation methods (e.g., organic solvents and fluorinated compounds) and poor mechanical stability. Here, we report for the first time totally waterborne, nonfluorinated, mechanically robust and self-healing superhydrophobic coatings. The coatings were fabricated by spray-coating polyurethane (PU) aqueous solution and a hexadecyl polysiloxane-modified SiO2 (SiO2@HD-POS) aqueous suspension onto substrates using PU as 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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the adhesive. The SiO2@HD-POS suspension was synthesized by HCl-catalyzed reactions among hexadecyltrimethoxysilane,

tetraethoxysilane

and

SiO2

nanoparticles.

Besides

high

superhydrophobicity, the coatings exhibit exceptional mechanical stability against sandpaper abrasion for 200 cycles at 9.8 KPa and tape-peeling for 200 cycles at 90.5 KPa, because of high durability and unique hierarchical macro-/nanostructure of the coating as well as solid lubricating of the SiO2@HD-POS nanoparticles fallen off from the coatings. The coatings also show fast and stable self-healing capability owing to migration of the healing agent (HD-POS) to the damaged surface. Moreover, the coatings exhibit good static and dynamic anti-icing performance in outdoor environment (-15 °C, relative humidity = 54%). The superhydrophobic coatings may be used in various areas, as the main bottlenecks have been successfully broken. Keywords: superhydrophobic, waterborne, self-healing, surface chemistry, silanes

INTRODUCTION Natural competition and selection have donated many interesting abilities to animals and plants in responding and adapting the changes of environment around them.1-2 For example, although living in a muddy environment, lotus leaves never get dirty owing to their extraordinary water repellency and unique self-cleaning property.3-7 Inspired by the lotus effect,8-9 superhydrophobic coatings featured by high contact angles (CA > 150°) and low sliding angles (SA) of water have drawn significant attention.9-13 Superhydrophobic coatings are very promising materials in various areas including self-cleaning,14-15 oil/water separation,4, 16 anti-icing,17-19 anti-corrosion,20 and anti-biofouling.21-22 Various approaches have been developed to fabricate superhydrophobic coatings, e.g., sol-gel, chemical vapor deposition, electrospinning and chemical etching.22-24 However, their application is restricted by some main bottlenecks, especially the pollutive

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preparation methods (e.g., organic solvents and fluorinated compounds) and poor mechanical stability. Volatile organic compounds including alcohols, alkanes, arenes and ketones are often employed as solvents for preparation of superhydrophobic coatings.4,

15, 25

Finally, the volatile

organic compounds completely volatilized from the superhydrophobic coatings. The use of volatile organic compounds adds production cost and results environment pollution and safety issues. These problems make superhydrophobic coatings far from large-scale production and real world applications. As we all know, water is a green and ideal solvent for chemical reactions. Waterborne superhydrophobic coatings are highly desirable in academia and industry.26 At present, there remains a great challenge to prepare waterborne superhydrophobic surfaces because the low-surface-energy materials, indispensable for preparation of superhydrophobic coatings, often cannot be dissolved or dispersed very well in water. Chen et al. prepared an aqueous dispersion using waterborne silicone-acrylic copolymer and silica sol, which could be used to prepare superhydrophobic coatings, but n-butanol was still used as the solvent in certain step.16 In fact, almost all the so-called waterborne superhydrophobic coatings are just waterborne to some extent and organic solvents are inevitably employed. Up to now, totally waterborne superhydrophobic coatings are very rare. Additionally, fluorinated compounds are frequently used to enhance superhydrophobicity owing to their ultralow surface energy. Pradeep et al. fabricated waterborne superhydrophobic materials by modification of cellulose nanofibers and kaolinite with 1H,1H,2H,2Hperfluorooctyltriethoxysilane.22,

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Lin et al. prepared superamphiphobic coatings using an

aqueous dispersion based on fluorinated nanoparticles, silane and surfactant.25 However, the fluorinated materials together with the degraded products have harmmful environmental and biological impacts, causing increasing safety concerns.28-29 3 ACS Paragon Plus Environment


Moreover, both natural and artificial superhydrophobic coatings are generally mechanically weak because their micro-/nanostructures can be easily destroyed. For example, slight finger touch can fatally destroy the micro-/nanostructures of most of superhydrophobic surfaces, not to mention heavy-duty friction. The damage of micro-/nanostructures enhances the solid-liquid contact area, leading to adhesion of water droplets on the coatings.22 This will result in increase in the SA and CA hysteresis, and finally loss of superhydrophobicity.30 Much attention has been paid to enhance mechanical durability of superhydrophobic coatings. For instance, the “paint + adhesive” strategies have been invented.26, 31-37 Parkin et al. used adhesives to promote robustness of superhydrophobic surfaces.37 Guo et al. employed inorganic materials to improve mechanical robustness of superhydrophobic surfaces.33, 38 Unfortunately, these coatings are not waterborne and their preparation methods are complicated. On the other hand, self-healing superhydrophobic coatings have been developed in order to offset their poor mechanical stability and to maintain the superhydrophobicity.39-41 However, almost all of the self-healing superhydrophobic coatings are based on fluorinated compounds, and none of them are waterborne. The application of superhydrophobic coatings for anti-icing has been in a spotlight of research. The pioneer work is inspiring, and many superhydrophobic coatings have been developed for anti-icing.18, 42-44 However, a large part of the coatings showed obvious deterioration in the antiicing performance during repeated icing/melting cycles, which is attributed to their poor stability. Moreover, the anti-icing performance was in most cases studied in simulated environments. For example, Vasileiou et al. studied impact of droplets on substrates in an -15 °C insulated chamber.45 Wang et al. investigated anti-icing properties of superhydrophobic coatings in a 20 °C chamber.46 There is no doubt that huge differences exist between simulated and actual environments. Anti-icing performance of superhydrophobic coatings in actual environment is greatly necessary to be investigated. 4 ACS Paragon Plus Environment

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Here, we report totally waterborne, nonfluorinated, mechanically robust and self-healing superhydrophobic coatings for anti-icing in actual environment. First, a homogeneous aqueous suspension of hexadecyl polysiloxane-modified SiO2 (SiO2@HD-POS) was prepared. Then, a polyurethane (PU) aqueous solution and the SiO2@HD-POS suspension were in turn spraycoated onto substrates. The as-prepared PU/SiO2@HD-POS superhydrophobic coatings show excellent superhydrophobicity, very high mechanical stability and self-healing capability. Moreover, the coatings also exhibit good static and dynamic anti-icing performance in outdoor environment (-15 °C, relative humidity = 54%).

RESULTS AND DISCUSSION Preparation of SiO2@HD-POS Suspensions. The suspensions were fabricated via HClcatalyzed reactions among hexadecyltrimethoxysilane (HDTMS), tetraethoxysilane (TEOS) and SiO2 nanoparticles (Figure 1a). After reaction for 24 h, the SiO2@HD-POS suspensions were formed. TEOS is helpful to enhance dispersion stability of the SiO2@HD-POS aqueous suspensions. The SiO2 nanoparticles are ca. 15-20 nm in diameter, and are in the form of aggregates with network structure as shown in Figure 1b. The polycondensation of HDTMS and TEOS formed film-like HD-POS, which linked the SiO2 particles together (Figure 1c). Consequently, the suspension changed from semitransparent to milky (Figure S1). The formation of SiO2@HD-POS is proved according to the Fourier Transform Infrared (FTIR) spectra in Figure S2. For SiO2, the broad absorption band at 3427 cm-1 is owing to the stretching vibration of -OH groups.47 The absorption bands at 1105 and 801 cm-1 are ascribed to the stretching vibrations of Si-O-Si.32 The band at 474 cm-1 is attributed to the bending vibration of Si-O.48 After modification of the SiO2 nanoparticles with HDTMS and TEOS, the new bands 5 ACS Paragon Plus Environment


at 2920, 2851 and 1467 cm-1 were detected according to the FTIR spectrum of SiO2@HD-POS, which are ascribed to the C-H stretching and bending of the methyl and methylene groups originated from the hexadecyl groups of HD-POS. The absence of the band at 1080 cm-1 (Si-O-C groups) means complete hydrolysis of HDTMS and TEOS.49 Meanwhile, there is no obvious change of the broad absorption band at 3427 cm-1 (-OH groups), indicating condensation of the Si-OH groups to Si-O-Si between the hydrolyzed silanes and the SiO2 nanoparticles.

Figure 1. (a) Fabrication of the SiO2@HD-POS suspensions. TEM photographs of (b) SiO2 nanoparticles and (c) SiO2@HD-POS. Fabrication of PU/SiO2@HD-POS Superhydrophobic Surfaces. The superhydrophobic surfaces were fabricated via in turn spray-coating the PU aqueous solution and the SiO2@HDPOS aqueous suspension onto glass slides (Figure 2a). Different from conventional superhydrophobic surfaces, the PU/SiO2@HD-POS coatings have a lot of large protuberances (100-200 μm) on the surface, forming macroscale roughness (Figure 2b). The SEM images showed that the coating is smooth in the microscale but rough in the nanoscale (Figure 2c-d). The 6 ACS Paragon Plus Environment

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SiO2 nanoparticles are connected with each other via HD-POS, generating a compact nanostructure. Thus, the PU/SiO2@HD-POS coating has a unique hierarchical macro/nanostructure. The cross-section SEM image clearly showed that the SiO2@HD-POS layer was linked to the surface of glass slide by the PU layer (Figure 2e). The SiO2@HD-POS layer partly permeated into the PU layer, as both of them are waterborne and the SiO2@HD-POS layer was deposited onto the PU layer before complete evaporation of water from the PU layer. Thus, the PU layer could strengthen the chemical bonding between the SiO2@HD-POS layer and the substrate (glass slide).32 This is beneficial to mechanical stability of the surfaces.

Figure 2. (a) Fabrication of the PU/SiO2@HD-POS surfaces by spray-coating. (b) Photograph, (c, d) SEM photographs and (e) cross-section SEM photograph of the PU/SiO2@HD-POS coating, (f) XPS spectrum, and (g) C 1s and (h) Si 2p spectra of the PU/SiO2@HD-POS coating.



From the X-ray photoelectron spectrum (XPS) of the PU/SiO2@HD-POS coating, the O 1s (542.3 eV), C 1s (293.4 eV) and Si 2p (110.8 eV) peaks were observed (Figure 2f). The C/O/Si atomic ratio is 2.79/1.40/1, and the C content is as high as 53.71% (Table S1), demonstrating abundant hexadecyl groups at surface. This result also means successfully modification of the SiO2 nanoparticles by HD-POS. According to the C 1s spectrum (Figure 2g), the peaks at 284.8 eV and 284.0 eV are ascribed to C-C (C-H) and C-Si of the -Si(CH2)15CH3 groups.50 Notablely, the intensity of the C-C (C-H) peak is much higher than that of the C-Si peak, which is consistent with composition of the -Si(CH2)15CH3 groups. The Si 2p spectrum of the coating is composed of three peaks at 104.5 eV (Si-OH), 103.4 eV (Si-O-Si) and 102.0 eV (Si-C) (Figure 2h).16, 50 The absence of the C-O peak in the C 1s spectrum and the Si-O-C peak in the Si 2p spectrum further confirmed complete hydrolysis of the silanes. The EDS elemental maps of the coating demonstrate uniform distribution of Si, C and O (Figure S3).

Figure 3. Photographs of the PU/SiO2@HD-POS coatings (a, b) with spherical water drops, (c) in water, (d) with water bouncing off, and (e) on the surface of diverse substrates. The synergistic effect of the macro-/nanostructure and the hexadecyl groups makes the PU/SiO2@HD-POS coatings highly superhydrophobic. Water drops are in the stable CassieBaxter state, and have a water CA of 163.9° and a SA of 3.7° (Figure 3a-b). Once immersed in 8 ACS Paragon Plus Environment

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water, the coating was surrounded by an air cushion with strong light reflection (Figure 3c). Additionally, a continuous water jet dyed by methylene blue can completely bounce off, i.e., without any impalement of water (Figure 3d). An 8 μL water drop with 1 cm release height can bounce 14 times before setting down (Movie S1). Besides glass slides, the PU/SiO2@HD-POS coatings are applicable to diverse surfaces including polyester fabric, PTFE plate, wood plate, stainless steel plate, aluminum plate and PU sponge (Figure 3e). All of the surfaces are superhydrophobic with water CA > 157° and SA < 8° (Table 1). The resulted waterborne superhydrophobic materials may be used for various applications. For example, the superhydrophobic PU sponge could be used to absorb both light oils and heavy oils from water with the absorption capacity of 23.94-84.78 g g-1 (Figures S4-S5). Table 1. Water CA and SA on the surface of the PU/SiO2@HD-POS coatings on diverse substrates. Substrates

CA / °

SA / °

Glass slide

163.9±0.8

3.3±1.0

Polyester fabric

157.5±2.2

2.5±0.6

PTFE plate

160.2±0.4

7.8±1.3

Wood plate

159.4±1.4

5.5±0.6

Steel plate

161.5±2.3

4.8±2.2

Aluminum plate

160.2±0.9

5.0±0.8

PU sponge

157.6±1.7

7.3±1.5

Mechanical Stability of PU/SiO2@HD-POS Superhydrophobic Coatings. Reciprocating friction against sandpaper was adopted to assess mechanical stability of the PU/SiO2@HD-POS superhydrophobic coatings (Figure S6). The coatings were placed face-down to sandpaper (2000 9 ACS Paragon Plus Environment


meshes), and then were horizontally moved along the sandpaper for a few cycles under certain pressure (2.3, 4.5 or 9.8 KPa). As the abrasion cycles increased, the superhydrophobicity decreased gradually (Figure 4a). Increasing the pressure to 9.8 KPa has no influence on the CA, but resulted in slightly higher SA. After 150 cycles (60 m) under 9.8 KPa, the coating retained its superhydrophobicity (CA = 150.1°, SA = 35.8°). Compared with most of the reported stable superhydrophobic coatings,33, 35, 51-52 the PU/SiO2@HD-POS coating showed superior mechanical stability and maintained its superhydrophobicity even after 150 cycles. The stability of the coating is comparable to the robust superhydrophobic coatings prepared via the “paint + adhesive” strategy.37 After 200 cycles, the CA decreased to 146.9° and the SA increased to 48.5°, which is about two folds improvement compared with the all-organic superhydrophobic coatings recently reported by Tiwari et al.53 The gradual deterioration of the superhydrophobicity is because some protuberances on the surface of the coating were destroyed and the abrasion marks became increasingly evident with increasing the abrasion cycles (Figure S7). Also, the surface of the coating became more flat (Figure 4b-c). To assess abrasion resistance of the coating under mild conditions that may occur in daily life, the coating was also rubbed against A4 paper. The water CA and SA were 150.5° and 22.2°, respectively, after 200 cycles under 9.8 KPa (Figure S8).


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Figure 4. (a) Changes of superhydrophobicity in the abrasion test against sandpaper and (b, c) SEM photographs of the coatings after 200 cycles (40 cm per cycle) under 9.8 KPa. (d) Changes of superhydrophobicity with tape-peeling cycles and (e, f) SEM photographs of the coatings after 200 cycles under 90.5 KPa. (g) Interaction between a water drop and the coating after tapepeeling under 90.5 KPa for 200 cycles. (h) Self-cleaning property of the heavily scratched coating. The exceptional mechanical stability of the surfaces was further demonstrated by the tapepeeling test. As shown in Figure 4d, the CA gradually decreased and the SA gradually increased with increasing the peeling cycles. A higher pressure caused more evident change of the 11 ACS Paragon Plus Environment


superhydrophobicity. According to the photographs after tape-peeling (Figure S9), the damaged region gradually became evident with increasing the abrasion cycles. After tape-peeling under 90.5 KPa for 200 cycles, the surface morphology of the coating was also observed. Compared with the original coating (Figure 2b-c), no change could be seen at low magnification (Figure 4e), but the surface became slightly more flat at high magnification (Figure 4f). This is mainly because the weakly attached SiO2@HD-POS was peeled off by the tape, which is also responsible for gradual deterioration of the superhydrophobicity. After tape-peeling under very high pressure (90.5 KPa) for 200 cycles, the coating still retained its superhydrophobicity (CA = 151.9°, SA = 28.6°). A 4 μL water drop can completely detach from the damaged region without leaving any trace even after repeatedly squeezing the water drop (Figure 4g). As far as we know, it is rare for superhydrophobic coatings to withstand such a high pressure in the tape-peeling test. Compared with most of the reported superhydrophobic coatings,29, 46, 54 the PU/SiO2@HD-POS coating showed superior stability against tape-peeling, which is comparable to the all-organic superhydrophobic coatings developed by Tiwari et al.53 Besides reciprocating friction and tape-peeling tests, the exceptional mechanical stability of the PU/SiO2@HD-POS coatings was also confirmed by the scratching test with a knife (Figure S10). The sand microparticles as model dirts on the surface of the heavily scratched coating were easily swept away by water drops, and the surface became very clean (Figure 4h). These phenomena suggest that the heavily scratched coating is still superhydrophobic and self-cleaning. We proposed mechanisms for the exceptional mechanical stability of the PU/SiO2@HD-POS coatings. First, the coating is durable, as the SiO2@HD-POS layer firmly bound with the substrate via the PU layer and the SiO2 nanoparticles in the SiO2@HD-POS layer firmly bound with each other via HD-POS. Moreover, the coating has a unique hierarchical macro/nanostructure (Figure 5a). Different from conventional superhydrophobic surfaces, the 12 ACS Paragon Plus Environment

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PU/SiO2@HD-POS coatings have a lot of large protuberances on the surface, forming macroscale roughness. During the process of abrasion or tape-peeling, the protuberances could prevent direct contact between the nanoscale roughness of the coating and sandpaper or adhesive tape, efficiently reducing damage of the coating. Although some of the protuberances were partly damaged or even removed during reciprocating friction or tape-peeling, the coating still maintained the partially destroyed macro-/nanostructure. Additionally, the nanoparticles dropped from the coating could reduce mechanical wear of the coating (Figure 5b).55-56 Therefore, the synergistic effect of these three aspects endows the PU/SiO2@HD-POS coatings with exceptional mechanical stability.

Figure 5. Schematic illustrations of (a) micro-/nanostructure of the PU/SiO2@HD-POS coating and (b) the SiO2@HD-POS nanoparticles acting as the solid lubricants in the abrasion against sandpaper. Chemical Stability of PU/SiO2@HD-POS Superhydrophobic Coatings. The chemical stability was investigated by immersing the coatings in organic solvents, aqueous solutions of pH 1-14, and 1 M NaCl(aq) solution for 24 h. The coating is highly stable against immersion in 13 ACS Paragon Plus Environment


various organic solvents, aqueous solutions with pH ≤ 8 and 1 M NaCl(aq) solution (Figure S11ab). The changes of the water CA and water SA are negligible. The sample is also stable against immersion in alkaline solutions with pH up to 14 regarding the CA. The increase in the SA in alkaline solutions is because the OH- ions lead to partial cleavage of the Si-O-Si bonds,57 resulting in higher surface energy compared with the original coating. In the process of UV irradiation (HSX-UV300, 6.6 W) for 120 h, the CA showed small fluctuation (160.9°-163.5°), and no observable change in the SA was observed (Figure S11c). This means the coating is able to resist long-term intensive UV irradiation.

Figure 6. (a) Schematic illustrations of the O2 plasma/self-healing process of the PU/SiO2@HDPOS coating, the self-healing mechanism and the corresponding photographs of water drops on the coatings. (b) Variation of water CA with plasma/self-healing cycles. (c) Water CA and SA of the coatings before and after self-healing from other damages: i) damaged by immersion in strong alkaline solution (pH 14) for 24 h, and ii) damaged by 60 icing/melting cycles. (d) XPS survey spectra of the coating after O2 plasma damage and after self-healing.


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Self-Healing Capability of PU/SiO2@HD-POS Superhydrophobic Coatings. O2 plasma damage, a common approach to chemically destroy superhydrophobic surfaces, was used to prove self-healing capability of the PU/SiO2@HD-POS superhydrophobic coating. Figure 6a shows schematic illustrations of the O2 plasma/self-healing process and the corresponding photographs of water drops on the coating. After O2 plasma damage for 5 s (PDC-002, 7.16 W), the coating became superhydrophilic with a water CA of ca. 0°. Fortunately, the coating has selfhealing capability against O2 plasma damage by heating at 150 °C for 1 h (Figure 6a). In fact, the damaged coating became superhydrophobic in 10 min with a CA of 156.9° and a SA of 21° (Figure S12), demonstrating fast self-healing capability. The superhydrophobicity was further restored by increasing the heat treatment time to 1 h (CA = 163.1°, SA = 5°). of The coating was still superhydrophobic after nine cycles (Figure 6b), exhibiting stable self-healing capability. Besides O2 plasma damage, the coating is also self-healing from other damages, e.g., immersion in strong alkaline solution (pH 14) for 24 h and 60 simulated icing/melting cycles (Figure 6c and Table S2). The self-healing mechanism is proposed as below. The outermost hexadecyl groups on the coating were bombarded by O2 plasma, generating abundant hydrophilic oxygen-containing groups (e.g., -OH, -COOH and C=O groups),39 which resulted in complete reversal of the wettability to superhydrophilic. During heat treatment, the healing agent (HD-POS) with low surface energy migrated to the oxidized surface and the hydrophilic groups were embedded in the coating. This is a spontaneous process at high temperature (150 °C) in order to minimize the surface energy.30 The XPS analysis showed that the O 1s peak became very strong after O2 plasma damage compared with the original coating, and then was evidently weakened after selfhealing at 150 °C (Figure 6d). Also, the O content on the surface of the coating was 40.37% after O2 plasma damage (Table S3), which is much higher than that of the original coating (26.96%). 15 ACS Paragon Plus Environment


Then, the O content decreased to 29.90% after self-healing, which is similar to the original coating. The XPS results confirmed our proposal about the self-healing mechanism.

Figure 7. (a) Photographs of static icing process of water drops on the PU/SiO2@HD-POS coating and bare glass slide. (b) SEM photograph of the coating after the iced water drops being peeled off. (c) Photographs of dynamic icing process of water drops on bare glass slide and the coating. (d) Changes of superhydrophobicity with icing/melting cycles. (e, f) SEM photographs of the coating after 60 icing/melting cycles. Anti-Icing Performance of PU/SiO2@HD-POS Superhydrophobic Coatings. The actual anti-icing performance of the PU/SiO2@HD-POS superhydrophobic coating was studied via static and dynamic experiments in outdoor environment (-15 °C, relative humidity = 54%). In the static anti-icing experiment, a 60 μL water drop dyed with methylene blue was put on the coated glass slide with bare glass slide for comparison. The freezing process was observed and recorded by a video camera. On bare glass slide, the water drop began to freeze at 5 min 28 s, and was 16 ACS Paragon Plus Environment

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completely frozen at 9 min 58 s (Figure 7a). On the coated glass slide, the water drop began to freeze with a hazy appearance at 34 min 16 s, and was completely frozen at 40 min 37 s. This means the PU/SiO2@HD-POS coating can delay icing by ca. 30 min in an extremely freezing weather. There are two possible reasons for the freezing delay: i) the small contact area of water drop with the coating provides much less area for ice nucleation, constraining expansion of icing film; ii) the air cushion between water and the coating hinders the heat transfer or increases the barrier for heat transfer between the water drop and the coating.43, 58 We also found that the ice drop could be lightly removed from the superhydrophobic coating by slight poking using a pipette (Movie S2), indicating low adhesion strength of the ice drop on the coating. Compared with the original coating, there was no observable change of the surface morphology after removal of the ice drop (Figure 7b). The dynamic anti-icing experiment was carried out by continuously dropping water with 1 cm release height onto the 4° tilted coating and bare glass slide. On bare glass slide, ice gradually formed at 0 min 23 s (Figure 7c), and a large quantity of ice formed after 33 min. By contrast, no ice was formed on the coating during the whole process of the experiment (Movie S3), demonstrating excellent dynamic anti-icing property of the PU/SiO2@HD-POS coating. Different from the static experiment, water drops bounced off quickly once dropped on the coating surface in the dynamic experiment. Figure S13 presents a representative rolling water drop on the coating. The water drop had rolled off from the coating before its temperature decreased to the freezing point. Cylic icing/melting experiment was performed to study lifespan of the PU/SiO2@HD-POS superhydrophobic coating for anti-icing. Although the superhydrophobicity gradually decreased with the increase of icing/melting cycles (Figure 7d), the coating was still superhydrophobic after 60 icing/melting cycles. Compared with some reported anti-icing coatings,15, 17 ACS Paragon Plus Environment

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the


PU/SiO2@HD-POS coating showed a longer lifespan and maintained its superhydrophobicity after 60 icing/melting cycles, and is comparable with the icephobic coating on stainless steel prepared using perfluorosilane.42 After 60 icing/melting cycles, there was no change in surface morphology (Figure 7e-f). Therefore, the possible reason for decrease of the superhydrophobicity is partial hydrolysis of Si-O-Si to Si-OH groups owing to long time contacting with water.

CONCLUSIONS In summary, we have fabricated totally waterborne, nonfluorinated, mechanically robust and selfhealing PU/SiO2@HD-POS superhydrophobic coatings by the combination of PU adhesive and SiO2@HD-POS. Besides high superhydrophobicity and high chemical stability against immersion in corrosive liquids and UV irradiation, the coatings feature exceptional mechanical stability and self-healing capability against diverse damages. The synergistic effects of the high durability and unique hierarchical macro-/nanostructure of the coatings as well as solid lubricating of the SiO2@HD-POS nanoparticles fallen off from the coatings endow the coatings with exceptional mechanical robustness. The migration of the healing agent (HD-POS) to the damaged surface endows the coatings with fast and stable self-healing capability. Moreover, the coatings exhibit good static and dynamic anti-icing performance in an extremely freezing weather (outdoor environment, -15 °C, relative humidity = 54%), and a long lifespan in cyclic icing/melting test. Since the anti-icing performance is based on superhydrophobicity of the coating, the coating could restore its anti-icing performance after healing of diverse chemical damages. Additionally, the preparation method is totally green, and the safety concerns are eliminated. The coatings are cost-effective and applicable onto various substrates. The superhydrophobic coatings may be applicable to many fields, as the main bottlenecks have been


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successfully broken. This study sheds a new light on preparation of green, mechanically robust and self-healing superhydrophobic coatings for actual anti-icing.

MATERIALS AND METHODS Fabrication of SiO2@HD-POS Suspensions. The suspensions were fabricated by HClcatalyzed polycondensation of HDTMS and TEOS in the existence of SiO2 nanoparticles. First, 0.1 g of SiO2 was added in an HCl aqueous solution (0.01 M, 50 mL), which was vigorously stirred at 600 rpm for 10 min, and then ultrasonicated for 10 min. Subsequently, appropriate volumes of HDTMS and TEOS were added. After reaction at room temperature for 24 h, the uniform SiO2@HD-POS suspension was formed. Fabrication of PU/SiO2@HD-POS Superhydrophobic Coatings. The coatings were prepared according to our previously modified method with some modification.26 Glass slides were washed with ethanol and deionized water, and then dried using nitrogen flow. The PU/SiO2@HD-POS coatings were fabricated by in turn spray-coating the PU solution and the SiO2@HD-POS suspension onto glass slides. The glass slide was vertically attached on a heating plate of 150 ºC. Then, the PU solution (1.0 mL) was spray-coated on the glass slide using an airbrush with a 0.4 mm spray nozzle. Likewise, the SiO2@HD-POS aqueous suspension (4.0 mL) was spray-coated on the surface of the PU adhesive layer. The as-prepared PU/SiO2@HD-POS coatings were cured at room temperature for 2 h before various tests. Mechanical Stability Tests. For the reciprocating friction test, the PU/SiO2@HD-POS coatings were placed face-down to sandpaper (2000 meshes), and then horizontally moved along the sandpaper for several cycles (40 cm per cycle) under a certain pressure (2.3, 4.5 or 9.8 KPa). For the tape-peeling test, the adhesive tape (3M, Scotch 600) was paved onto the surface of the



PU/SiO2@HD-POS coatings. A certain pressure (2.3, 16.6 or 90.5 KPa) was applied on the tape, and then peeled off after 5 s. Afterwards, water CA and SA were measured. Anti-Icing Tests. The actual anti-icing properties of the PU/SiO2@HD-POS coating was studied via static and dynamic experiments in outdoor environment (-15 °C, relative humidity = 54%) in Lanzhou, China. The cyclic icing/melting test was performed in an -15 °C refrigerator. The PU/SiO2@HD-POS coating was placed face-down on the water surface in a beaker. After complete icing, the sample was taken out of the refrigerator. After melting of the ice, the water CA and SA on the coating were recorded.

Conflict of Interest: The authors declare no competing financial interest. Acknowledgements: This work is supported by the National Natural Science Foundation of China (21667017, 51503212 and 51873220), the "Hundred Talents Program" of the Chinese Academy of Sciences, the Major Basis Research Projects of Gansu Provincial Science and Technology Department, China (18JR5RA001), the Funds for Creative Research Groups of Gansu, China (17JR5RA306) and the Talents of Innovation and Entrepreneurship Project of Lanzhou, China (2016-RC-77). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

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Totally Waterborne, Nonfluorinated, Mechanically Robust, and Self

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