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General Water-Based Strategy for the Preparation of Superhydrophobic Coatings on Smooth Substrates Minhuan Liu, Taoyan Mao, Yichun Zhang, Xu Wu, Fanghui Liu, Hui Yang, Jinben Wang, Cheng Zheng, Xiaozhen Zhao, and Zhengping Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04105 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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General Water-Based Strategy for the Preparation of Superhydrophobic Coatings on Smooth Substrates

Minhuan Liu,a Taoyan Mao,a Yichun Zhang,a Xu Wu,a,∗ Fanghui Liu,b Hui Yang,b Jinben Wang,b Cheng Zheng,a Xiaozhen Zhao,a Zhengping Wanga,∗

a

Department of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006,

China b

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and

Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

* Corresponding authors. E-mail address: [email protected] (X. Wu), [email protected] (Z. Wang)


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ABSTRACT The development of water-based coating strategies have led to safe and affordable techniques with applications in various fields, while the design of water-based superhydrophobic coatings still needs to be improved to facilitate their large-scale production and widespread adoption.

Herein, we report a novel

water-based strategy that can endow smooth surfaces with superhydrophobicity by designing a multifunctional polymer and incorporating a hydrophobic nanomaterial and a selective solvent.

The superhydrophobic coatings show outstanding flexibility

and can be made in various colors using either hydrophobic or hydrophilic dyes. The fabrication process is scalable, and it may open up many possibilities for the preparation of water-based coatings using hydrophobic nanomaterials.

Keywords: Superhydrophobic coatings, Water-based coatings, Smooth substrates, Hydrophobic nanomaterials, Flexibility



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1. INTRODUCTION Superhydrophobic surfaces have water contact angles exceeding 150° and sliding angles that are smaller than 10°.1

The extreme repellency to water is due to the

micro/nanoscale roughness of the surface in combination with a low surface energy component such as silicon or fluorine.2-3

While many applications are envisioned

for these surfaces, only a few of these functional materials can be employed in practice due to the tedious fabrication process, excessive cost and hazards associated with fluorinated compounds and solvents.4-6 Among the fabrication techniques, coating strategies are relatively facile and economical especially for large-scale manufacturing and for endowing various substrates with the desirable water repellency.7,8 Substrates onto which coatings are applied can be categorized as rough and smooth materials.

Substrates exhibiting inherent roughness (such as the micro-sized

fibers found in cotton and paper) can be readily rendered superhydrophobic or superoleophobic without the need for additional modification of their surface texture,9,10 although the further incorporation of nanoparticles could enhance their liquid repellency.11

In the case of the smooth substrates such as metals, glass, and

polymers, the stringent requirement of constructing surface roughness is necessary in order to achieve a superhydrophobic surface.

The etching pretreatment of these

smooth substrates,12-14 as well as melting and molding coating precursors into fibers have been used to obtain the desired roughness.15

An easier method is to design a

filler component that exhibits intrinsic roughness, so that the superhydrophobicity can


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be achieved by a one-step coating strategy. Various hydrophobic fillers such as fluorinated SiO2,16 TiO2,17 ZnO,18 multi-walled carbon nanotubes19 or raspberry-like polymer particles20 have been incorporated

into

the

solvent-based

superhydrophobic coatings.

components

for

the

preparation

of

The dispersion of these hydrophobic components relies

on the organic solvents such as α,α,α,-trifluorotoluene, N, N-dimethylacetamide, and tetrahydrofuran.

Although most of the reported water-based components are not

fully waterborne, they can be diluted with water to minimize their cost and safety risks, and thus are of great technological interest, especially in large-scale industrial processes.

In the case of these water-based components, the fillers (with rare

exceptions21) generally are water-dispersible, with some examples including modified exfoliated graphite nanoplatelets7 or hydrophilic SiO2 nanoparticles.22-24

It is

noteworthy that an impressive range of hydrophobic and difficult to be functionalized nanomaterials that have not been employed as water-based coating components.25,26 In our previous work, we had designed and prepared superhydrophobic coatings covering

rough

substrates.28,29

substrates27

and

anti-smudge

coatings

protecting

smooth

In this article, a conceptually different design based on an emulsion

system is reported.

This strategy employs the hydrophobic filler in a water-based

component, comprising a multifunctional polymer (MFP), water as well as a selective solvent.

It provides a convenient method for applying superhydrophobic coatings

onto various smooth substrates and will have much relevance to the coatings field.



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2. EXPERIMENTAL SECTION 2.1. Materials. Methyl methacrylate (MMA), butyl acrylate (BA), 2-(dimethylamino)ethyl methacrylate

(DMAEMA),

2-hydroxyethyl

methacrylate

(HEMA),

2,2′-azobis(2-methylpropionitrile) (AIBN), 2-hydroxypropanoic acid, and toluene were supplied by Hersbit Chemical Co. Ltd., and were of analytical reagent grade. Tri(isopropoxy)vinylsilane (TIVS, >99%) was provided by Ark (FoGang) Chemicals Industry Co. Ltd.

The hydrophobic silica nanoparticles (~20 nm particle diameter)

were purchased from Degussa AG.

The hydrophilic dye rhodamine 6G and the

hydrophobic dye benzidine yellow G were purchased from Guangdong Wengjiang Chemical Reagent Co. Ltd. and were used as received. 2.2. Synthesis of the MFP. The MFP was synthesized via free radical polymerization using various monomers.

In a typical procedure, a 250 mL flask containing toluene (44.00 g) was

purged with nitrogen for 30 min at room temperature and subsequently heated to 100 °C.

A mixture of monomers including MMA (15.02 g, 0.1500 mol), BA (25.63

g, 0.2000 mol), DMAEMA (4.716 g, 0.03000 mol), HEMA (6.507 g, 0.05000 mol), TIVS (6.972 g, 0.03000 mol), and AIBN (0.5000 g, 3.045 mmol) were added dropwise into the flask under mechanical stirring over a period of ~2 h. Subsequently, AIBN (0.1000 g, 0.6090 mmol) was added into the above mixture, and the reaction was allowed to continue for another 12 h.

The resulting mixture was

then cooled to room temperature, and the DMAEMA units were neutralized using


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2-hydroxypropanoic acid to yield the MFP. 2.3. Preparation of the Emulsion. The hydrophobic silica nanoparticles (0.5000 g) were dispersed into toluene (9.500 g) and mixed with the MFP (1.000 g, 50 wt%).

After this mixture had been

stirred for 0.5 h, 40.00/80.00/120.0 mL of water was added dropwise to the as-prepared mixture over a period of ~0.5 h, and the stirring was continued for another 1 h to obtain a homogeneous emulsion. 2.4. Preparation of the Coatings. Tin plates were immersed into the emulsion for approximately 10 s, and the dip-coated plates were then dried at room temperature for 10 min.

The coatings

were finally cured at 160 °C for 1 h. 2.5. Characterization Methods 1

H NMR spectra of the MFP were recorded using a Bruker Avance 500

spectrometer and acetone-d6 was employed as the solvent. Size-exclusion chromatography (SEC) analysis of the MFP was performed using a Waters 515 system that was equipped with a Waters 2410 refractive index detector. The SEC system was equipped with a guard column and calibrated with monodispersed polystyrene standards.

THF was employed as the eluent at a flow

rate of 1.0 mL min-1. The diameters of the silica nanoparticles and the emulsion droplets were determined via dynamic light scattering (DLS).

DLS analyses were performed at a

scattering angle of 90° at 25.0 ± 0.1 °C using a laser light scattering (LLS)



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spectrometer (ALV/SP-125) equipped with a solid-state He-Ne laser (22 mW) and a multi-τ digital time correlator (ALV-5000).

The incident beam was provided by the

light (λ = 632.8 nm) from the He-Ne laser.

The CONTIN method30 was used to

analyze the correlation function. The contact angles (CAs) and the sliding angles (SAs) were measured using an optical CA measuring device (JC2000A, Shanghai Zhongchen Powereach, China). Each reported CA represented the average value of five measurements of 2 µL water droplets at different positions on the coating.

The SA was determined as the angle at

which 20 µL water droplets began to roll off the coating when its inclination was incrementally increased.

Each reported SA also represented the mean of five

measurements. The morphology of the pristine silica nanoparticles and the polymer-based coatings were characterized using a scanning electron microscopy (SEM).

The SEM

characterization was performed using a Hitachi S-530 scanning electron microscope and the samples were sputter-coated with palladium and gold. The thickness of the coatings was measured using a CM-8825FN digital coating thickness gauge (Landtex, China). The elemental analysis of the coating surfaces was determined via X-ray photoelectron spectroscopy (XPS, ESCALAB250XI, Thermo VG Scientific, USA) before and after the abrasion treatment.

The XPS characterization was implemented

using an electron take-off angle of 45° with a sampling depth of ~6.6 nm. Microphotographs were recorded using a MD30 optical microscope (Mshot,


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China). The abrasion resistance was evaluated by a sand dropping method.31

A coated

tin plate was fixed onto a sample holder that was tilted at 45°, and sand with diameters of 100~300 µm was allowed to flow at a constant rate from a 1 m height down onto the coating.

The CAs and SAs of water were measured on the coatings

that were subjected to this abrasion treatment for various lengths of time. The aggregates of the pristine silica nanoparticles and the polymer-based were observed using a transmission electron microscope (TEM, JEOL JEM-2100F, Japan). The samples were added dropwise onto a 400-mesh carbon-coated copper grid without subjecting them to dilution, and cured at 160 °C for 1 h after drying at room temperature for 10 min. The anticorrosion behavior of the coating was examined using an electrochemical workstation (CHI860D, China) at 25 °C in a 3.5 wt% NaCl solution. The three-electrode cell used in the test was equipped with a saturated calomel electrode (SCE) as a reference electrode and a platinum sheet serving as a counter electrode.

The bare and coated tin plates, as the working electrodes, were covered

with epoxy resin leaving an area of 1.0 cm2 exposed to the electrolyte.32

The steady

state of the cells was detected by the open-circuit potential (OCP), and the potentiodynamic polarization curves were measured from -1.5 to +1.0 V at a scan rate of 10 mV/s.



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3. RESULTS AND DISCUSSION A schematic depiction of the coating procedure as well as the structure of MFP is shown in Figure 1.

The structural characterization of MFP is provided in Figures S1

and S2 in the Supplementary Information (SI).

The MFP structure was designed by

carefully choosing specific monomers that served various roles. used as the main film-forming material.33,34 the MFP with water solubility.

MMA and BA were

DMAEMA was incorporated to provide

Meanwhile, HEMA was selected to form a

crosslinked film via the attack of MMA, BA and HEMA ester bonds by the HEMA hydroxyl groups to eliminate methanol, butanol and ethylene glycol.35,36

TIVS was

utilized as an environmentally-friendly composite to impart the coating with a low surface tension and the desired liquid repellency.

Figure 1. Schematic illustration of the procedures for preparing the emulsion and coating.


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The hydrophobic silica nanoparticles were used as the filler, and toluene was selected as the solvent to disperse these silica particles.

The toluene dispersion of

silica particles was subsequently mixed and emulsified with the MFP and water to obtain the designed emulsion.

The silica particles were found to be dispersible in

toluene as well as in water-miscible solvents such as acetone.

However, the silica

particles would precipitate when their acetone solution was mixed with water, thus inhibiting the fabrication of a water-based system.

The photographs of silica

particles dispersed into toluene at a weight ratio of 1:20 (silica particles:toluene) and the emulsion of silica particles, toluene, MFP and water at a respective weight ratio of 1:20:1:80 are shown in the Figures 2a and b, and the diameters of the silica particles and emulsion droplets were determined via DLS.

The diameter of the silica particles

obtained via DLS was 22 ± 5 nm, which was consistent with that observed in the SEM image.

The diameter of the emulsion droplets was 1.3 ± 0.6 µm with a narrow

distribution.

The MFP bearing both hydrophobic and hydrophilic units was found to

synergistically interact with the silica particles and toluene to form a stable emulsion (Figure S3), which is readily applicable for a wet-chemical coating technique such as dip-coating.



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Figure 2. Photograph of the silica particles dispersed into toluene and the DLS plots of the silica particles (a). Photograph and the DLS plots of the emulsion of silica particles, toluene, MFP and water at a weight ratio of 1:20:1:80 (b) and a microphotograph of this emulsion (c). Microphotographs of the stained emulsion of silica particles, toluene, MFP and water at weight ratios of 1:20:1:80 (d), 1:20:1:160 (e), and 1:20:1:240 (f), respectively.

The microphotographs of the emulsions containing varying water contents are shown in Figures 2c, d, e and f.

To aid with the viewing of the micron-sized droplets

in the emulsion, the water was stained with rhodamine 6G (Figures 2d, e and f).

In

comparison with the pristine emulsion (Figure 2c), it is readily apparent from the stained emulsion in Figure 2d that the red water droplets were surrounded by a colorless toluene layer, thus providing an indication of the water-in-oil mode of the emulsion.

As the water:toluene weight ratio was further increased from 4:1 to 8:1

and to 12:1, the diameters of the stained water droplets in the emulsion increased dramatically (Figures 2e and f).

The emulsion was found to become unstable and


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phase separation was observed when the water:toluene ratio was increased excessively to ~14:1. As the emulsion of silica particles, toluene, MFP and water with the respective weight ratio of 1:20:1:80 was used to coat the substrate, the subsequent thermal curing caused the water and toluene to evaporate, thus yielding a coating with a thickness of 8.0 ± 0.5 µm.

The SEM images of the pristine silica nanoparticles and of the coating

are shown in Figures 3a and b.

The MFP and silica particles were found to

aggregate into blocks, thus forming an even and compact coating with a micro-/nano-scaled texture that can provide both necessary roughness for liquid repellency and facilitate bonding between the particles as well as the substrate.

In

contrast, coatings consisting solely of silica particles became loosely deposited onto the substrate and they were easily damaged when they were touched with a finger (Movie S1) or subjected to strong wind.

The bonding between the MFP and the

silica particles was further testified by TEM.

For the coating consisting solely of

silica particles (Figure 3c), although the silica particles were deposited in bulk, the outline of the particles was clearly visible with good clarity.

In the case of the

coating consisting of MFP and silica particles (Figure 3d), the polymer bound the silica particles tightly in a block, so that the profile of silica particles could not be easily distinguished and thus the clarity became poor.

Notably, the fibrous polymers

can be clearly observed in Figure 3d (see the dash frame), indicating that the silica nanoparticles were bound together by the polymers.

An XPS spectrum of the

coating is shown in Figure 3e, and the atomic abundance of the coating surface is



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provided in the Table S1 in the SI.

The Si atoms accounted for 18.23% of the

coating surface composition, thus providing the material with the desired low-surface energy for liquid repellency.

Figure 3.

SEM images of the pristine silica nanoparticles (a) and the coating prepared using the

emulsion of silica particles, toluene, MFP and water at a weight ratio of 1:20:1:80 (b). TEM images of the pristine silica nanoparticles (c) and the coating prepared using an emulsion of silica particles, toluene, MFP and water at a weight ratio of 1:20:1:80 (d). An XPS spectrum of the coating surface (e).

The CA and image of a water droplet sitting on the coating (f).

Superhydrophobic coatings prepared in the absence of any dye (left, g), with a hydrophobic yellow dye (middle, g), and with a hydrophilic red dye (right, g). Photographs of a stained water droplet sliding off the coating recorded at 0.1 s intervals (h).


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Water droplets sitting on the surface of the coating exhibited a CA of 151.5 ± 0.8° (Figure 3f), and a SA of 8.6 ± 0.3°.

The stained water droplets rapidly rolled off the

coating without wetting or contaminating the surface along their paths (Figure 3h), and the sliding process is shown in Movie S2 in the SI.

The observance of a CA

exceeding 150° along with a SA of less than 10° suggested that the coating was superhydrophobic.

Importantly, the superhydrophobicity of the coatings was also

observed when they were applied onto other substrates such as filter paper, wood, polytetrafluoroethylene (PTFE) plates, polyethylene terephthalate (PET) films and glass slides (Figure S4), thus demonstrating that the coating can be used to endow various materials with self-cleaning surface properties. The emulsion contained both water and toluene phases, so that either hydrophobic or hydrophilic dyes could be used to prepare coatings with various colors. A photograph of the emulsions prepared with a hydrophobic yellow dye and a hydrophilic red dye is provided in Figure S5 in the SI, and the resultant coatings are shown in Figure 3g.

Strikingly, the presence of dyes with varying solubility did not

influence the coating’s superhydrophobicity. A sand dropping method was used to evaluate the abrasion resistance of the coating and the water repellency of the coating surface when the outer surface was worn down and the internal matrix became freshly exposed.37-39

Liquid repellent

coatings bearing intricate surface structures are mechanically weak, and therefore these coatings are normally prone to wearing.40-42

The continuous abrasion with

falling sand for 5 min would consume ~1 µm of the coating layer, and the CAs



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decreased along with the abrasion time (Figure 4a).

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In addition, the abrasion

affected the SA dramatically and 5 min of abrasion treatment caused the test liquid to remain on the newly exposed surface without rolling off. A decrease in liquid repellency with abrasion had been observed by other researchers, although this phenomenon has not been fully explained.21

The Si

contents of the initial and the newly exposed coating surfaces, as well as the topography of these surfaces were compared in order to gain insight into this phenomenon.

The Si content of the newly exposed surface was found to decrease to

10.65%, and the XPS results are given in Figure 4b and Table S1 in the SI.

The

decreased Si content could be attributed to the upward migration of the low surface energy components during the curing process.43,44

However, this loss of low surface

energy content alone would not be sufficient to cause the liquid droplets to adhere to the surface according to our previous work.29 The changes of surface structures resulting from the abrasion were further investigated via SEM and AFM (Figures 4c-f).

The texture of the coating surfaces was damaged by the abrasion, and the

height of the roughness was found to decrease dramatically from ~200 nm to the scale of tens of nanometers. The roll-off superhydrophobic behavior of the coating indicated a Cassie state with air pockets existed due to a desirable degree of surface roughness.45 During the curing process, the evaporation of water and toluene, as well as the subsequent shrinkage and subsidence of the MFP, would cause the primary surface to become rough, and this curing-induced roughness was apparently lost after the abrasion


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treatment.

When we compared the effect of the ratios between the silica particles

and the MFP in our efforts to optimize the coating preparation process, we found that a ratio of 1:1 rather than a ratio of 1:2 (with more MFP) was required to generate a coating with sufficient surface roughness to achieve superhydrophobicity.

This

suggested that the destruction of the surface roughness is the main reason for the loss of superhydrophobicity.

Figure 4. Changes in the CAs with abrasion time (a). XPS analysis of the coating after the abrasion test (b).

SEM images of the coating after the abrasion test (c) and (d).

Three-dimensional AFM topography images of the coatings recorded before (e) and after (f) the abrasion treatment.

Flexibility is one of the intrinsic advantages of organic polymers over inorganic materials, and materials capable of reversible bending are of great value for many synthetic devices and systems. seldom been reported.

However, flexible superhydrophobic materials have

It was noteworthy that our coatings were found to have

excellent flexibility on the bendable substrates such as PET (Figures 5a-c and Movie



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S3) and tin plate (Figure 5d). without

showing

noticeable

Page 18 of 24

The coatings can strongly adhere to the substrates signs

of

rupturing,

and

can

retain

their

superhydrophobicity after they were subjected to severe bending.

Figure 5. Photographs of the superhydrophobic coating on PET that had been subjected to bending tests (a), (b) and (c). Photograph of a coated tin plate with bending deformation (d).

A

tin plate bearing coated and uncoated sections after immersion in a 10 wt% NaCl solution for 48 (e) and 120 h (f). Curves of the open circuit potential over time for the bare and coated tin plate (shown in black and red, respectively, g). Tafel polarization curves of the bare and coated tin plate (shown in black and red, respectively, h).

As we had anticipated, the liquid repellent coating provided excellent protection against corrosion.

Photographs of a tin plate bearing coated and uncoated regions

after immersion in a 10 wt% NaCl solution for various times are shown in Figures 5e


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and f.

The rust on the uncoated region was visible only several hours after the

immersion had begun, and this deterioration continued as the immersion treatment was prolonged.

However, in the case of the coated region, the tin plate exhibited

minimal signs of corrosion even when it was exposed to the salt solution for more than 100 h.

After exposure to the 10 wt% NaCl solution for 48 h, the coating

remained superhydrophobic and the water droplets still could slide off the coating surface, as shown in Movie S4.

However, a few subtle etched pits grew on the

coating surface with prolonged immersion time, and the water droplet was easily pinned on the surface in the pit-etching region due to the high surface energy of the rust.46

The anticorrosion performances of the bare tin plate and coated tin plate were

further examined by an electrochemical corrosion test.

As shown in Figure 5g, the

OCP of the above two samples reached a steady state within 1000 s, and the coated tin plate exhibited a higher potential, indicating that the coating resisted corrosion.47 Moreover, Tafel polarization curves were employed to investigate the corrosion resistance of the coating (Figure 5h).

In comparison with the bare tin plate, the

superhydrophobic coating exhibited a positive shift in the corrosion potential (Ecorr) and a reduction in the corrosion current density (Icorr).

Generally, a higher Ecorr value

and a lower corrosion Icorr value indicates that the coating provides better corrosion protection and is able to slow down the dissolution rate of the substrate, which thus demonstrates that this coating imparts corrosion resistance.47



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4. CONCLUSIONS A water-based strategy that can be readily used for the preparation of superhydrophobic coatings has been developed.

The superhydrophobic coatings can

be applied to various substrates and these coatings can also be prepared with either hydrophobic or hydrophilic dyes in various colors without compromising the performance.

In addition, the coating exhibited excellent flexibility and protected

the substrate against corrosion.

As demonstrated herein, hydrophobic nanomaterials

can be used in water-based superhydrophobic coatings with the multifunctional polymer, and their applicability will shed new light on finding novel hydrophobic precursors for the preparation of water-based components.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at

. (Figures S1 and S2) An 1H NMR spectrum and an SEC trace of the polymer, (Figure S3) time sequence photographs indicating the emulsion stability, (Figure S4) photographs comparing the superhydrophobicity of the coatings on various substrates, (Figure S5) photographs of emulsions prepared with and without dyes, (Table S1) atomic compositions of the coating surfaces before and after the abrasion test, (Movie S1) finger touching test being performed on the coatings, (Movie S2) the sliding process of water droplets on the coating, (Movie S3) the bending test being performed on the coating, (Movie S4) the sliding process of water droplets on the coating that had been exposed to an NaCl solution.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X. Wu). *E-mail: [email protected] (Z. Wang).

ACKNOWLEDGEMENTS We gratefully thank the Natural Science Foundations of China (21406040 and 21676061), the Science and Technology Project of Guangdong Province (2015A050502052 and 2017A050501040), and the Science and Technology Project of Guangzhou City (201610010018, 201607010285 and 201607010080) for sponsoring this research.

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