sa

Applied Surface Science 255 (2009) 44794483

Hydrophobically modied fumed silica

Water contact angle

ce w

e u

is in

Fm

ease

cop

to b

or p

2008 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surfa

.e ls1. Introduction

In recent times, superhydrophobic surfaces (surfaces withwater contact angle (WCA) higher than 1508) have attractedattention because of their self-cleaning, anti-icing, anti-stickingand anti-contamination properties [122]. A perfect example ofsuperhydrophobic surface from nature is the lotus leaf, on which awater droplet forms a sphere and rolls off. This is called lotuseffect which is accomplished by a surface topography with microand nano-structures [23]. By mimicking this effect, surfaces can bemodied to develop articial superhydrophobic coatings. Eventhough several methods are reported in literature to obtainsuperhydrophobic surfaces, many of them may not be feasible forlarge area application.

Many methods have been reported in literature for thepreparation of hydrophobic and superhydrophobic surfaces [122]. Recent reviews focus on the progress in the variousmethods ofpreparation, theoretical modelling and applications of super-hydrophobic surfaces in the last decade [15]. The preparationmethods are categorized as bottom-up, top-down, and combina-tion approaches. These methods are mainly based on twoapproaches: either make a rough surface from a low surfaceenergy material or modify a rough surface with a material of low

surface energy [6]. However, some of these methods are substrate-selective and some other methods involve complicated multi-stepprocesses or expensive instruments.

Fluorinated polymers are of special interest in the creation ofsuperhydrophobic surfaces due to their extremely low surfaceenergies [711]. Roughening these polymers in different waysleads to superhydrophobicity directly. For example, Peng et al.have obtained a highly hydrophobic porous PVDF by using amodied phase inversion method [7]. It is reported thatperuoroalkyl polymer can provide a surface with very lowsurface energy and displays good hydrophobicity [8]. Many suchuorinated materials have not been used directly but linked orblended with other materials to make superhydrophobic surfaces.PVDF is a commercially available uoropolymer with low surfaceenergy (25 dynes/cm) and good physical, chemical, and mechan-ical properties. Also, being an engineering thermoplastic, PVDF hasbetter stiffness and strength than those of most uorine-contain-ing polymers. It has exceptional chemical stability and excellentresistance to aging and durability. Many superhydrophobicsurfaces reported in the literature aremade by using a combinationof very low surface energy materials and very high surfaceroughness characteristics.

A superhydrophobic surface can also be made by introducingroughness to a hydrophobic surface with a contact angle largerthan 908. It has been reported in the literature that incorporation ofnanoparticles and microparticles in solgel matrix or hybridpolymer matrix can lead to superhydrophobicity [916]. Li et al.

* Corresponding author. Tel.: +91 80 25086251; fax: +91 80 25210113.

E-mail address: bharathi@css.nal.res.in (B.B.J. Basu).

0169-4332/$ see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2008.11.065A simple method for the preparation ofhybrid composite coatings

Bharathi Bai J. Basu *, Ashok Kumar Paranthaman

Surface Engineering Division, National Aerospace Laboratories, Bangalore 560017, Indi

A R T I C L E I N F O

Article history:

Received 18 August 2008

Received in revised form 7 October 2008

Accepted 18 November 2008

Available online 3 December 2008

Keywords:

Superhydrophobic

Polyvinylidene uoride

PVDF

Coatings

A B S T R A C T

A superhydrophobic surfa

particles in polyvinyliden

hybrid composite coating

silica concentration in PVD

and the sliding angle decr

scanning electron micros

nanolaments was found

effective and can be used f

substrates.

journa l homepage: wwwuperhydrophobic PVDFHMFS

as obtained by embedding hydrophobically modied fumed silica (HMFS)

oride (PVDF) matrix. The water contact angle (WCA) on the PVDFHMFS

uenced by the content and nature of silica particles in the coating. As the

atrix was increased from 33.3% to 71.4%,WCA increased from 1178 to 1688d from 908 to

amount of HMFS silica particles were dispersed in toluene byultrasonication, mixed with PVDF solution while magneticallystirring for about 30 min to 1 h till a homogenous mixture wasobtained. Then the precursor mixture was transferred to a spraygun and sprayed on to clean dry glass slides or other substrates.The coating was dried at room temperature.

3. Results and discussion

TheWCA of a blank PVDF coating without HMFS was about 908.HMFS silica particleswere used as ller for the preparation of PVDF

B.B.J. Basu, A.K. Paranthaman / Applied Surface Science 255 (2009) 447944834480obtained a superhydrophobic surface on a PVDF macroporous lmthrough self-assembled silica colloidal templates [9]. Chang et al.prepared organic superhydrophobic lms by utilizing TA-Nuoroalkylate and methyl methacrylate copolymer as water-repellent materials and inorganic silica powder as surface rough-ness material [10]. Superhydrophobic properties of polymer basednanocomposite coatings prepared by dispersing nanoCaCO3 inPVDF matrix have been reported [11]. The coating surface ischaracterized by microspheres with bumpy surface similar to thetopological structure of lotus leaf and a water contact angle (WCA)of 1538 was obtained for this coating. Su et al. have embeddednanosilica particles on epoxy-coated glass surface, but super-hydrophobicity is achieved by subsequent uoroalkyl silanecoating [12]. Fluoroalkyl silanes are either incorporated in thehybrid mixture or applied as a top-coat in some of the reportedwork in order to decrease the surface energy [1113]. Qu et al. haveprepared a hybrid lm by emulsion polymerization but theprocedure is tedious and involves multi-steps [14]. Superhydro-phobic nanocomposite coating was obtained by using a mixture ofsmall anatase and large boehmite particles and its hydrophobicitydepended on the surface roughness and micropore fraction [15].

A superhydrophobic surface of PEEK/PTFE composite coatinghas been reported very recently [17]. The microstructure iscontrolled by varying the curing temperature and superhydro-phobic surface is obtained when the coating is cured at 300 8C.Though PTFE has lower surface energy than PVDF, it is difcult toprepare coatings with it due to its limited solubility in solvents.

We found that surface roughness can be created in PVDF-basedcoating matrix by embedding hydrophobically modied silica(HMFS) particles in the matrix so that the surface becomessuperhydrophobic. Li et al. have reported that superhydrophobicnanosilica can be prepared by surface modication usinghexamethyldisilazane [18]. In this study, we have used commer-cially available hydrophobically modied fumed silica particles asller to prepare superhydrophobic composite coatings. In aprevious work by Yan et al., CaCO3 nanoparticles were added asller to PVDF matrix to create superhydrophobic compositecoatings. Since CaCO3 is hydrophilic, uoroalkyl was added torender it hydrophobic and prolonged stirring of the mixture forabout 13 days was required. In this study, there was no need toinclude expensive reagents like uoroalkyl silanes in the coatingmixture as the silica particles were highly hydrophobic and thestirring timewas about 30 min to 1 h. Spray coating was employedbecause of its advantages for preparing coatings of desiredthickness with minimum wastage of precursor solution. Henceit is a simple and cost-effective approach for creating super-hydrophobic and self-cleaning surfaces.

2. Experimental

Commercial grade PVDF powder was procured from M/sPragathi Chemicals, India. HMFS silica was procured from M/sABCR GmbH, Germany. Dimethylformamide (DMF) and toluenewere purchased from SigmaAldrich. Static contact angles weremeasured by using Contact Angle Goniometer instrument,Ramehart, Inc. The water droplet used for measurements was10 mL. The average of ve measurements made at differentpositions on the coating surface was adopted as the value of WCA.Sliding angles were measured using home-built tilting table with aprotractor using a 10 mL water drop. The reported sliding angle isthe average of ve measurements. All measurements wereperformed at ambient conditions. Surface morphology of thecomposite coatings was examined using Leo 440I Scanningelectron microscope.

PVDF solution (5%) was prepared by dissolving PVDF powder inDMF and heating to about 50 8C for complete dissolution. A knowncomposite coating. These particles are highly hydrophobic innature. It is amorphous hexamethyldisilazane treated fumed silicawith a surface area of 200 m2/g and an ultimate particle size of0.02 mm (as per the specications given in the Gelest catalog).However SEM study of these silica particles dispersed in toluenehas shown particles of 1mm along with agglomerates of 25mm.

The effect of the concentration of HMFS on the hydrophobicityof PVDFHMFS composite coating was studied. The results areshown in Table 1. It was found that WCA increased and slidingangle decreased with increase in HMFS content in the coating.When HMFS concentration was 50%, WCA increased above 1508and sliding angle (SA) decreased to about 38 so that super-hydrophobicity was obtained. Due to experimental difculties formeasuring advancing and receding contact angles for thesecoatings, contact angle hysteresis could not be obtained andhence sliding angles were measured. In the case of PVDFHMFScoatings with silica content 50% and above, water drops actuallyrolled off from the coating surfaces. When the silica content was66.7% and above, water drops rolled off from the coating surfaceseven before tilting the surface. Hence the sliding angle for thesecoatings was nearly zero or 90

33.3 2:1 117 90

50 1:1 160 3

60 2:3 164 2

66.7 1:2 167

conditions. It was found that the PVDFHMFS (1:1) compositecoating prepared from a precursor aged for 3 days exhibited a

micropores enhance superhydrophobicity [19]. Thus the micro-structure of the coatingswas found to depend on the silica content,

Fig. 1. Images of water drop on PVDFHMFS coatings with different silica concentrations: (a) 33.37%, (b) 50%, (c) 60%, (d) 66.67% and (e) 71.42%.

B.B.J. Basu, A.K. Paranthaman / Applied Surface Science 255 (2009) 44794483 4481porous structure with long nanolaments (Fig. 2c and d). Thecomposite coating with PVDF:HMFS (1:2) was more rough anddisplayed porous structure. When the precursor was ultrasoni-cated before spraying, SEM images showed micro-scale bumpsinterspersed with honeycomb-like structure as shown in Fig. 3cand d. The areas surrounding the honeycomb contained silicaparticles. The aggregates of particle were smaller for coatingprepared with ultrasonicated precursor. The bumps also had amicroporous structure. In all the four cases, the composite coatingsexhibited superhydrophobicity irrespective of the differences intheir microstructure. Hsieh et al. have proposed that waterdroplets cannot penetrate easily into micropores on a repellentsurface and therefore rough surfaces with a high fraction ofFig. 2. SEM images of PVDFHMFS (1:1) composite coatings prepared under differemagnication of (a); (c) after aging the precursor, magnication 500; (d) higher magaging time and spraying parameters of the precursor mixture.It was found that solvent (DMF)nonsolvent (toluene) ratio is

an important parameter. If the amount of toluene used fordispersing silica particles is high, PVDF gets precipitated. Hence anoptimum amount of toluene should be used to dilute the PVDFsolution since excess toluene precipitates PVDF. The precipitatedPVDF can be redissolved by adding more DMF to the solution andby warming.

The usefulness of a superhydrophobic coating is determined byits stability. The surface should retain its water-repellent proper-ties after immersion in water. It was found that the coatingappeared as a silver mirror when it was immersed in water andviewed at a glancing angle as shown in Fig. 4. Similar opticalnt conditions (a) with freshly mixed precursor, magnication 500; (b) highernication of (c).

fere

) hi

B.B.J. Basu, A.K. Paranthaman / Applied Surface Science 255 (2009) 447944834482property has been reported earlier for superhydrophobic surfaces[20,21]. The mirror-like appearance is due to an air layer betweenwater and superhydrophobic surface. It was found that the PVDFHMFS (1:1) coating was not wetted after 6 h of immersion in waterand there was no change inWCA but an increase in SA to about 158was observed in the immersed area. The coating regained itssuperhydrophobic property after drying at room temperature.PVDFHMFS (1:2) coating was not wetted after 30 h of immersionin water. But SA increased to 108 while there was no change inWCA. Drying at room temperature resulted in complete regaining

Fig. 3. SEM images of PVDFHMFS (1:2) composite coatings prepared under difmagnication of (a); (c) after ultrasonicating the precursor, magnication 500; (dof the superhydrophobic property. Composite coatingswith highersilica concentration showed better water repellency than coatingswith lower silica concentration. Zimmerman et al. also have notedthe increase in sliding angles of silicone-based superhydrophobiccoatings [22]. They have proposed that increased sliding angle isrelated to the pinning of the contact due to surface inhomogene-ities. It is presumed that hydrophilic defects on the surface aregenerated during immersion that lead to an increase in slidingangle without inuencing the static contact angle.

Fig. 4. Total internal reection of PVDFHMFS coatings on glass substrate whenimmersed in water.These composite coatings exhibited self-cleaning property. Itwas found that the water droplets removed any dust particlessprinkled on the coating. The coatingswere stable for long periods ifstored under ambient conditions. The rst batch of our coatingsprepared one year ago still continue to retain their superhydro-phobic property. Coatings were exposed to sunlight and wind tostudy the effect of weather on the performance of coatings. Evenafter continuous exposure for several days, there was no deteriora-tion in performance and the coatings were superhydrophobic.

Generally superhydrophobic surfaces are easily damaged by

nt conditions (a) with freshly mixed precursor, magnication 500; (b) highergher magnication of (c).scratching and abrading due to their high surface roughness [22]. Toour knowledge, no superhydrophobic coating with high abrasionresistance has been reported so far. The coating integritywas testedqualitatively by cotton swab abrasion test [24]. The ratings criteriawere as follows: Rating 1the substrate was exposed; Rating 2signicant amount ofmaterialwas removed, substrate not exposed,thick abrasion line seen; Rating 3less material was removed, theabrasion line was less thick; Rating 4abrasion line was very thin;Rating5noabrasion lineobserved.Thecompositecoatings showedrating 3. The rubbed area was not wetted bywater but displayed anincrease in SA of about 208. After annealing at 100 8C for 1 h coatingexhibited rating 4. Surprisingly, mechanical property of the coatingwas goodevenat higher silica concentrations.On the contrary, PVDFblank coating showed poorer adhesion and abrasion resistance thanthe composite coatings.

The condensation of water droplets on the composite coatingsurface was studied. Recent studies have shown that many of thesuperhydrophobic surfaces, both natural and articial may not betruly superhydrophobic depending on how water gets on to theirsurfaces [25,26]. It was shown that if water vapor inltrates intothe microvoids on the surface of lotus leaf, water drops can stick totheir surface and they do not roll off even after tilting the surface atany angle. Superhydrophobic behaviour of such surfaces is termedto be metastable [26]. Hence it is important to consider howwatergets on to and evolves on the surface. We have carried out a simplewater condensation test on the PVDFHMFS composite coating byexposing it to fog by keeping outside overnight. An epoxy paintcoating was also kept for comparison. The temperature was about

cavities and nanobers was found to be responsible for thesuperhydrophobicity. The method is simple and cost-effective andcan be used for preparing self-cleaning superhydrophobic coatingon large areas on different kinds of substrates for practicalapplications.

Acknowledgement

The authors are grateful to Dr. A.R. Upadhya, Director, NAL, Dr.Kota Harinarayana, Raja Ramanna Fellow, NAL and Dr. K.S. Rajam,Head, Surface Engineering Division, NAL for their constant supportand encouragement for this work. We wish to thank Prof. A.Venkateswara Rao, Shivaji University, Kolhapur, India for staticcontact angle measurements, and Raghavendra, Materials ScienceDivision, NAL for technical assistance with SEM studies.

References

[1] X.-M. Li, D. Reinhoudt, M. Crego-Calama, Chem. Soc. Rev. 36 (2007) 13501368.[2] X. Zhang, F. Shi, J. Niu, Y. Jiang, Z. Wang, J. Mater. Chem. 18 (2008) 621633.

B.B.J. Basu, A.K. Paranthaman / Applied Surface Science 255 (2009) 44794483 44831520 8C. Fig. 5 shows photographs of the coatings aftercondensation of water drops on their surfaces. Water drops onthe composite coating were spherical whereas water dropscondensed on the epoxy coating were wetting the surfacecompletely. It was found that water drops on the compositecoating rolled down without wetting the surface on tilting slightlywhereas the epoxy coating remained wetted by water even aftertilting to 908.

4. Conclusions

Fig. 5. Photographs of condensed water drops on (a) epoxy paint coating; and (b)PVDFHMFS composite coating.Creation of surface roughness by embedding hydrophobicallymodied fumed silica particles in PVDF matrix resulted insuperhydrophobic coatings. An increase in HMFS silica concentra-tion in PVDFmatrix allowed a substantial increase inwater contactangle from 1178 to 1688 and the sliding angle decreased from908 to