polytetrafluoroethylene hybrid superhydrophobic coating

Applied Surface Science 258 (2012) 9859–9863

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Formation of SiO2 /polytetrafluoroethylene hybrid superhydrophobic coating Yansheng Zheng a , Yi He b,∗ , Yongquan Qing b , Zhihao Zhuo b , Qian Mo b a b

Lushan College of Guangxi University of Technology, Luzhou 545616, Guangxi, P.R. China College of Biological and Chemical Engineering, Guangxi University of Technology, Liuzhou 545006, Guangxi, P.R. China

a r t i c l e

i n f o

Article history: Received 29 March 2012 Received in revised form 11 June 2012 Accepted 12 June 2012 Available online 22 June 2012 Key words: Superhydrophobic coating Micro- and nanoscale structures Water contact angle SiO2 sol

a b s t r a c t Superhydrophobic coating has been fabricated on the glass substrates with modified SiO2 sol and polytetrafluoroethylene emulsion through a sol–gel process. SiO2 sol was modified with ␥-glycidoxypropyl trimethoxysilane. The coatings were characterized by water contact angle measurement, Scanning electron microscope, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy and thermal synthetic analysis. The experimental results show that coatings exhibited superhydrophobic and heatresistant property with a water average contact angle of 156◦ and sliding angle of 6◦ , coating has a rough surface with both micro- and nanoscale structures, ␥-glycidoxypropyl trimethoxysilane enhanced the hydrophobicity of the coatings. Low surface energy of polymer and special structure of the coatings were responsible for the hydrophobic of the surfaces. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Superhydrophobic surfaces have gained considerable attention due to their many potential applications, such as self-cleaning coatings [1], microfluidics [2], and impermeable textiles [3,4]. The wetting behavior of thin film is determined by the surface energy of constitutive materials as well as the topological roughness of the surface [5]. Generally speaking, to create a superhydrophobic surface (water contact angle, WCA > 150◦ ), nanoscaled roughness and hydrophobic surface (smooth surface contact angle is >90◦ ) are two base requirements. Up to now, various methods have been explored to fabricate nano- and micro rough surfaces, such as chemical vapor deposition [6], lithographic patterning [7], chemical etching [8], and electrochemical deposition [9]. However, most of the preparations involve strict conditions such as harsh chemical treatments, expensive materials (e.g., fluoroalkylsilanes [10–12] and nanotubes [13]). Reducing the cost, simplifying the fabrication process, increasing the durability of the final products, and using nontoxic materials are critical to large-scale manufacturing of superhydrophobic surfaces. Fluorinated polymers are of special interest in the creation of superhydrophobic surfaces due to their extremely low surface energies [14,15]. Polytetrafluoroethylene (PTFE) is widely used as coating for antiadhesion agents and chemical isolators. PTFE surfaces show hydrophobic and oleophobic characteristics. Water contact angles on smooth PTFE are about 100–110◦ . Various approaches were tried to prepare superhydrophobic PTFE

∗ Corresponding author. Tel.: +86 077 23516078; fax: +86 077 23516078. E-mail address: [email protected] (Y. He). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.043

surfaces [16,17]. However, PTFE cannot be used directly but linked or blended with others materials to fabricate superhydrophobic surfaces. For example, Jiang and co-workers [18] prepared super-hydrophobic and super-oleophilic coatings by mixing PTFE emulsion, polyvinyl acetate polyvinyl alcohol and surfactant (sodium dodecyl benzene sulfonate, SDBS) in proportion. The sol–gel process allows the mixing of organic and inorganic components at the nanometric scale, leading to the so-called organic/inorganic hybrid nanocomposites. It has drawn much interest in recent years and is now being widely used to prepare superhydrophobic surfaces, Very recently, Su et al. [19] fabricated superhydrophobic coatings by a novel sol–gel which is made from hydrolysis and condensation of the by-product of polymethylhydrosiloxane reacting with ␥-aminopropyltriethoxysilane, whose WCA is about 157◦ . Shang et al. [20] prepared the durable superhydrophobic cellulose fabric from water glass and n-octade-cyltriethoxysilane (ODTES) with 3-glycidyloxypropyl trimethoxysilane (GPTMS) as crosslinker by sol–gel method. Basu et al. [21] have compared superhydrophobic surface by applying precursor mixtures containing hydrophobically modified silica (HMS) nanoparticles dispersed in sol–gel matrices on different substrates. In the work of Xiu et al. [22], inorganic superhydrophobic, low surface energy silica coatings were prepared using sol–gel processing with tetraethoxysilane and trifluoropropyltrimethoxysilane as precursors. A contact angle of 172◦ and a contact angle hysteresis of 2◦ were obtained. SiO2 are one of the most popular nanomaterials that are being used in organic/inorganic materials. However, in the application of nano-technique some problems often exist, such as dispersion of nanoparticles and their compatibility with other material [23–25].

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OH OH

OCH3 H3CO Si

HO O

OCH3

O

+ HO HO

OH

SiO

OH

OH

OH

OH

H+ H2O

SiO

OH

O

Si

O O O Si CCH2CH2OCH2CHCH2

O

O Si OH

Modified silicon sol

O CH2CH2CH2OCH2CHCH2

Modified SiO2

SiO2

KH-560

O CH2CH2CH2OCH2CHCH2

Silicon network

PTFE Aging Curing

PTFE

glass

additives Fig. 1. Scheme of preparing coating.

␥-Glycidoxypropyl trimethoxysilane (GPTS) is a useful precursor for sol–gel preparation of inorganic-organic hybrid materials, the silica network is formed by hydrolysis and condensation of a methoxy radical in the presence of water [26]. GPTS was used as coupling agent for the preparation of the composites. In this letter, GPTS was used to increase the compatibility between the organic (PTFE emulsion) and inorganic phases (silica network) thereby reduce the phase separation. In this study, SiO2 nanoparticles surface were modified with GPTS, silane coupling agent had reacted with hydroxyl group, dispersibility and surface hydrophobicity of silica were improved well. We created superhydrophobic coating on glass slides by dip-coating from modified silica sol and PTFE emulsion suspensions, the coating had excellent superhydrophobicity. 2. Experimental details

instrument at room temperature, the average value of 5 measurements, made at different positions of the same sample. A slide of the sample was supported on two stages at two ends. One of them was fixed, and the other one could be raised. The angle at which the drop slides can be measured by means of a protractor attached to the instrument. The accuracy of the contact angle measurement system was ±1◦ . 2.3. Characterization The surface morphology of the coatings was characterized by digital scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) spectrum was obtained (Kratos, UK) with an AlK␣ X-ray source. Fourier transform infrared (FT-IR) spectra were obtained on an Avatar Fourier transform infrared spectrophotometer, and cured films were tested with thermo gravimetric analysis (TG) and differential scanning calorimetry (DSC).

2.1. Sample preparation 20 g of silica sol (average particle size 7–9 nm, 1.2 g/mL, 40 wt.% colloidal silica in water purchased from Guangzhou Lanshan Chemical Company,) was added to 10 mL of 1:1 mixture (by weight) of water and ethanol. The pH was adjusted to 4.0 by the addition of 2 M acetic acid, 2 mL of GPTS was added to the mixture with vigorous stirring at 40 ◦ C for1 h. This GPTS modified solution was called solution A. Solution B consisted of 9 g PTFE emulsion (60 wt.%, 1.2 g/mL, average particle size 0.2–0.3 um, purchased from Guangzhou Songpo Chemical Company), dimethyl formamide (DMF), and deionized water. Then solution A was added slowly into solution B, the pH was adjusted to 5.0 by the addition of acetic acid. The reaction mixture was stirred for 3 h at 30 ◦ C and aged at room temperature for 24 h. Thus, homogeneous white viscous liquid was obtained. The mixture was directly coated on glass via a dip-coating process, the cleaned substrate was immersed into the sol for 5 min before withdrawal; the withdrawing speed was kept at 4–10 cm/min. After each dip-coating, the substrate was dried for 2 min at room temperature (25 ◦ C). Then the coating was heated at 120 ◦ C for 30 min to remove residual solvent and solidify the coating. Uniform coatings were formed after air drying and heat curing step. Fig. 1 shows the schematic illustration of preparing coating. 2.2. Water contract angle and sliding angle measurements Water contact angle was measured with a 10 ␮L deionized water droplet on a Dataphysics OCA 20 (Dataphysics, Germany)

3. Results and discussion Polymer/SiO2 nanocomposites have received much attention in recent years and employed in a variety of applications. SiO2 nanoparticles are an ideal raw material to make organic/inorganic hybrid material, and because of SiO2 particles have a lot of silanol ( SiOH) groups on the surfaces, particles can form aggregation during drying step. So it is difficult to well disperse in the polymer. The dispersion of nanometer-sized particles in the polymer matrix has a significant impact on the properties of nanocomposites. A good dispersion may be achieved by surface chemical modification of the nanoparticles or physical methods such as a high-energy ballmilling process and ultrasonic treatment. The most frequently used method is to modify the surface of silica nanoparticles, which can improve the dispersion of nanosilica in the polymer matrix. In general, surface modification of nanosilica can be carried out by either chemical [27,28] or physical methods. In the present work, SiO2 particles were reacted with GPTS in an acidic water/ethanol, GPTS is hydrophobically modified nanosilica in which the surface hydroxyl groups are replaced by hydrophobic hydrocarbon chains, the surface-capping layer can effectively prevent the uncontrolled agglomeration. Moreover, the surface hydrophobicity of nanocomposites can also be greatly promoted. ATR-FTIR was used to confirm successful modification of the SiO2 particles with GPTS. Fig. 2 shows the ATR-FTIR spectra of bare and GPTS functionalized SiO2 coating. The peak at around 1600 cm−1 and the broad absorption band at around 3400 cm−1

Y. Zheng et al. / Applied Surface Science 258 (2012) 9859–9863

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Fig. 2. The infrared spectrograms of samples (a) modified by silane coupling agent; (b) unmodified.

are due to the Si OH groups [29]. After the reaction with GPTS, the peak area of Si OH band became smaller, new bands at 2916 cm−1 appear, indicating the presence of CH2 groups [30], and they confirmed the existence of GPTS on the surface of modified nanosilicas. The siloxane network, as indicated by the Si O Si bands at around 800–1150 cm−1 [31], the strong band observed at 1710 cm−1 correspond to CF2 CF2 stretching vibration [32]. Not only GPTS modified the properties of silica but it improved obviously the adhesion between coating and substrate. The flexible interlayer reduces surface tension and stress concentration, no chip-off or crack occurred during operation process. PTFE emulsion properly scattered in modified silica sol, PTFE particles and modified silica particles spread and twines each other, forming a uniform mixed materials. As the solvents evaporation and the heat-treatment, hybrid SiO2 sol formed Si O Si network, a high degree of cross-linking gives rise to space network polymers, creating an organic/inorganic hybrid gel coating in the formation of an appropriately rough surface. It is well known that PTFE has a low surface energy. PTFE particles were embedded, stuck and attached in the matrix. In order to study the influence of the weight contents of SiO2 on the surface coverage, a series of composite coatings were fabricated under different weight ratios. After it was found that the optimum SiO2 –PTFE weight ratio was 20:9. Reacting process of the coating with temperature is understood through determining its TG and DSC curve. Heat treatment also has strong influence on the microstructure of coating; microstructure determines roughness of coating layer [33]. Fig. 3 shows the effect of heat treatment on the wettability of the SiO2 /PTFE hybrid coatings. The samples were heated at deferent temperatures, 100, 200, 250,

Fig. 4. Thermal analysis of coating.

and 300 ◦ C for1 h, and WCA and sliding angle values were measured at room temperature after each heat treatment. It is discovered that coatings attained best water-repellent property (WCA = 156◦ ) after heating up to 250 ◦ C. However, WCA reduced apparently as temperatures exceed 300 ◦ C, the experimental results indicated that coating began to damage thermally. Fig. 4 presents TG and DSC curves, exothermic peaks and weight loss appeared in the DSC–TG curves when the temperature is above 300 ◦ C, which meant coatings suffering thermal damage. Alkyl groups began to oxidative decomposition at 350 ◦ C, organofluorine began to decompose at 480 ◦ C, there is an obvious exothermic peak existing at 554 ◦ C due to organic material decomposition

WCA

160

20

140 o

Sliding angle ( )

100

0

WCA( )

120 80 60 40

sliding angle

15 10 5

20 0

0 100

150

200 0

Temperature( C)

250

300

100

150

200

250 o

Temperture( C)

Fig. 3. Effect of heat treatment on WCA and sliding angle of the coaating.

300

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Fig. 5. XPS elemental analysis spectra for the coating surface.

completely. Therefore, the optimized heat treatment condition was 250 ◦ C 1 h. To confirm the existence of PTFE on the surface layer of the complex coating as prepared, X-ray photoelectron spectroscopy (XPS) spectrum was obtained. XPS measurements of hybrid coatings were performed to determine the film coverage on the template. The area of a XPS peak is proportional to the number of atoms sampled. Therefore, the peak intensities of XPS spectra may be used to get information of relative concentration of different elements in the film as well as the film coverage. Fig. 5 shows results that the intensities of the F is electrons at 686.2 eV and to the Si 2p peak at 103.3 eV for SiO2 . From the spectrum of the complex coating surface, indicating the aggregation of PTFE near the outmost surface layer. It has been reported that void fraction and surface roughness would significantly affect the wettability of a surface with a water droplet. [34], the micro-morphology of the coating samples was observed with SEM. The effects of GPTS on the surface microstructure and morphology were investigated, the (a) samples were prepared by GPTS-modified SiO2 sol and the (b) samples were prepared by as-received SiO2 sol. The superhydrophobic

Fig. 6. SEM images of coating surface.

Y. Zheng et al. / Applied Surface Science 258 (2012) 9859–9863

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Fig. 7. Pictures of the water droplets on the coating surface (a) modified by silane coupling agent; (b) unmodified.

coating surfaces were composed of nanoparticles and many bumps, micrometer-sized and nanometer-sized hierarchical roughness is present in Fig. 6c and d. The coating exhibits distinct superhydrophobic properties, showing the WCA of coating reached as high as 156◦ with a sliding angle of 6◦ . However, Fig. 6b shows flat morphology, and no evident hierarchical structure, otherwise, cracking of the coating can be found, the water WCA of the coating decreased to 123◦ and sliding angle >30◦ . SiO2 /PTFE hybrid product results in the generation of two distinctive scales of roughness, corresponding to GPTS-modified SiO2 nanoparticles and PTFE particles. The Cassie–Baxter model [35] is used to describe the relationship between surface roughness and apparent WCA. According to theoretical analysis, the thermodynamic contact angle depends on the fractions of liquid area in contact. The nanometer and micrometer scale structures on the hierarchically rough surfaces entrap air below water droplets and thus create the liquid–solid–air composite interface that is required for superhydrophobicity [36]. This configuration is described by Eq. (1) cosr = f1 cos − f2 f1 + f2 = 1

(1)

where  r is the apparent contact angle, f1 is the liquid–solid surface fraction; f2 is air surface fraction with water. If f1 is very small, the surface contact area available to water is very low. The solid surface fraction or surface roughness f only depends on the surface structure [37]. An increased surface roughness results in a reduced contact area between solid and water due to an increase in air bubbles trapped at the interface. As easily understood, the hierarchical structure is very important for the fabrication of a superhydrophobic surface with self-cleaning function. The digital images of glass substrates with superhydrophobic coatings are shown in Fig. 7. 4. Conclusions In this article, a simple and cost-effective procedure for nanocomposite coating was prepared from modified SiO2 and PTFE emulsion; the superhydrophobicity can be attributed to both the surface microstructure and the low surface energy of polymer. The coating showed average WCA of 156◦ and sliding angle 6◦ . It was found that unmodified SiO2 coatings had cracks whereas GPTSmodified SiO2 coating was free of cracks; the presence of GPTS is indispensable in the preparation of superhydrophobic coating in this process. It can be employed in a wide variety of large-scale both industrial and scientific applications. Acknowledgement The research reported in this paper was supported by Educational Commission of Guangxi Zhuang Autonomous Region of China (grant no. 2010MS123).

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