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Applied Surface Science 307 (2014) 101–108

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

pplication of superhydrophobic sol gel on canvas

hing Dar Wanga,∗, Bai Jun Lina, Chien Cheng Hsiehb, Chao Chieh Linb

Institute of Materials Science, National Yunlin University of Science & Technology, 123, Sec 3, University Rd., Douliu, Yunlin, Taiwan, ROCTaiwan Textile Research Institute Yunlin branch, ROC

r t i c l e i n f o

rticle history:eceived 25 January 2014eceived in revised form 11 March 2014ccepted 25 March 2014vailable online 4 April 2014

eywords:uperhydrophobic surfaceelf-cleaning

a b s t r a c t

In this study, a superhydrophobic sol gel (SHSG) was used to promote self-cleaning ability of canvas.Four kinds of coating methods were studied and water impact test was used to assess the quality of thecoating. The scraper method rendered a relatively flat surface; and water contact angles (WCAs) of twobilayers polytetrafluoroethylene (PTFE)/SHSG-coated canvas were 153.3◦ ± 3.1◦ and 141.7◦ ± 3.3◦ beforeand after water impact of 2 h, respectively. SEM, energy dispersive x-ray spectrometry (EDX), and FTIRwere applied to characterize the PTFE/SHSG coating. Although SHSG was dropped on top of the PTFEfilm during sample preparation, the analysis of EDX and FTIR indicated that the chains of PTFE extendedupward and SHSG moved downward during the heat treatment with preheat. The combination of SHSG

ater contact angleanvaseatherability

with PTFE promoted not only hydrophobicity but also the strength and weatherability of the canvas.The tensile strength of a PTFE/SHSG-coated canvas was always superior to a PTFE-coated canvas duringaccelerated weathering test. After accelerated weathering test of 300 h, the two bilayers PTFE/SHSG-coated canvas had a tensile strength of 237 kg, static WCA of 143.7◦ ± 2.1◦, a contact angle hysteresis of8◦, and a sliding angle of 4◦, which represented an outstanding self-cleaning ability.

. Introduction

The application of nanotechnology to the traditional tex-ile industry, including natural, artificial or synthetic fibers, hasttracted considerable interest in recent years [1–20]. Due to thextraordinary photocatalytic activity, non-toxicity, high availabil-ty, biocompatibility, and low price, TiO2 nanoparticles have beenpplied extensively onto textile materials to obtain UV-protecting,elf-cleaning, superhydrophobic or superhydrophilic as well asnti-bacterial properties [1–4]. Nitrogen doped TiO2-cotton fab-ics or TiO2 combined with different metals ions (Fe, Co, Zn, andg) cotton fabrics showed significant photocatalytic activity in theegradation of methylene blue or methyl orange under visible-light

rradiation [2–4]. Since silver or silver ions are effective againstany disease-causing organisms in the body, and relatively non-

oxic for human cells, silver nanoparticles are widely applied ontoextile materials to impart antibacterial ability [5–8].

Silica sol has been used by many researchers to produceuperhydrophobic textiles [6–14]. The abundant hydroxyl groups

n the cotton fiber surface favor upload and attachment ofydrolyzed SiO2 nanoparticles on the surface. The roughness, atwo levels (micro- and nanoscales), enabling trapping of air under

∗ Corresponding author. Tel.: +886 5 5342601x3411; fax: +886 5 532 1719.E-mail address: sdwang@yuntech.edu.tw (S.D. Wang).

ttp://dx.doi.org/10.1016/j.apsusc.2014.03.173169-4332/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

water droplets can be obtained through adequate condensation ofhydrolyzed SiO2 sol. The low surface energy characteristic can beachieved by the modification of sol gel with a long chain alkyl silaneor fluorochemicals. ZnO nanorod arrays or nanoparticles, due totheir antibacterial properties and cost-effectiveness compared tosilver nanoparticles, have also been created on cotton substratesand subsequent hydrophobic modification with a long chain alkylsilane, fluoride or stearic acid [15–19]. Since the surface tensionof oil, such as hexadecane 27.5 mN/m, is lower than that of water(72.1 mN/m), the superhydrophobic surface is also superoleophilic.This characteristic can be used in fabrication of superhydropho-bic and superoleophilic textiles for the treatment of increasingindustrial oily wastewater and polluted oceanic water, as well asthe frequently occurring accidental oil spills [11–13,15,18,20]. Xueet al. [11] used tetraethoxysilicate (TEOS), 1,1,1,3,3,3-hexamethyldisilazane (HMDS), n-hexane, ammonia solution, hydrochloric acid(HCl) to fabricate superhydrophobic and superoleophilic textilesfor oil–water separation. However, n-hexane can cause neurop-athy. A superhydrophobic and oleophilic sol–gel nanocompositecoating with oil contact angle of <10◦ will improve oleophobic prop-erty through the further lowering of surface energy by the presenceof a fluorochemical [21,22].

Fluorochemicals have been employed for hydrophobic andoleophobic surface modifications in many of the reported works[10,14,15,21–23]. However, most fluorinated materials may causeserious risks to human health upon coming in contact with skin.

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n addition to fluoride, several other chemicals are also used in theabrication of the above mentioned functional textiles. It has noteen investigated extensively if these textiles trigger any allergypon skin contact. However, this may not be a case for a super-ydrophobic canvas since it does not come in contact with humanody upon use.

Canvas is used as the substrate of an oil painting. The applicationf polar organic solvents as cleaning tools for painting surfaces istill a widely adopted technique. Unfortunately, the capillary pen-tration of a liquid into the paint layers can lead to swelling andeaching of the original organic components (mainly varnishes andinding media) constituting the layered paint structure [24]. Deit al. [25] developed a new approach with gels which was intendedo minimize the time necessary to achieve the desired cleaningction, and then removed the gel by converting it to a low-viscosityuid in situ by a rapid, mild chemical perturbation. Instead of usingel with liquid cleaning agents, a superhydrophobic and super-leophilic coating can impart the canvas a self-cleaning property.

Canvas is widely used in outdoor pavilions, awnings, multi-urpose sports complexes, huge span tension membrane roofs,ents, etc. To provide a comfortable environment for users, it mustossess properties of durability, inertness, cleanability, and easy-are attributes. It should function as a self-cleaning system; andirborne dirt should be automatically washed off by rain. Afterxtended periods of outdoor exposure, it should retain a highegree of flexibility and strength. Nevertheless, the research aboutelf-cleaning of canvas is very rare.

This work investigated the application of superhydrophobicol gel (SHSG) on canvas. The SHSG was synthesized from TEOS,MDS, HCl, and ethanol. Therefore, it is fluorine free. The opti-al method of applying SHSG and polytetrafluoroethylene (PTFE)

imultaneously on the canvas was studied systemically. Watermpact test for 2 h was used to assess the endurance of the coat-ng. Contact angle tester was used to determine the water contactngle (WCA) before and after water impact test. Scanning elec-ron microscopy (SEM) and energy dispersive x-ray spectrometryEDX) were used to analyze the variation of surface morphologynd distribution of SHSG and PTFE in the coating. The 3D surfaceorphologies of the films were investigated using an atomic forceicroscope (AFM). Fourier transform infrared spectroscopy (FTIR)as used to identify chemical bonding between molecules in the

oating. Tensile test was conducted to determine the effect of sol gelddition on the strength of the canvas. Accelerated weathering testas performed to estimate the weatherability of the PTFE/SHSG-

oated canvas.

. Experimental

.1. Materials

PTFE powder of 300 nm was purchased from DU PONT. Fluo-ine surfactant S420 was purchased from AGC SEIMI CHEMICALO. The PTFE emulsion with model number Zewffl Gk570 was pur-hased from TAIWAN DAIKIN ADVANCED CHEMICALS. The mainomponent of this PTFE emulsion is PTFE. The diameter of fiberlass yarn is 4 �m. Fiber glass yarns are weaved into a fiberglassloth with a density of 24 strips of fiberglass yarns in both ver-ical and horizontal directions. The mass density of the fiberglassloth is 450 g/m2. The commercial canvas with PTFE membrane wasurchased from Chukoh Chemical Industries (Japan) with modelumber of FGT-600.

.2. Sample preparation

SHSG with tri-methyl-modified silica sol was synthesizedhrough the reaction of hexamethyldisilazane (HMDS) and

cience 307 (2014) 101–108

hydrolyzed tetraethoxysilane (TEOS) as described in detail else-where [26,27]. Four kinds of samples were prepared in this study.

Sample A:SHSG, aged for 7 days, was deposited on the commercial canvas

through a dropper and then heat treated at 200 ◦C for 2 h.Sample B:PTFE powder, fluorine surfactant S420, and SHSG in mass ratios

of 1:1:1, 1:2:2, and 1:2:4 were stirred uniformly using a homoge-nizer, deposited on the fiberglass cloth through a dropper and thenheat treated at 200 ◦C for 2 h.

Sample C:Fiberglass cloth was dipped into PTFE emulsion (Zewffl Gk570

Taiwan Daikin) and then lifted to coat with SHSG by a dropper. Afterheat treatment at 200 ◦C or 300 ◦C for 2 h, one bilayer of PTFE/SHSGwas obtained. One, two, three or five bilayers of PTFE/SHSG werecoated on the fiberglass cloth.

Sample D:The scraper with four blades was used and film thicknesses of 30,

60, 90 or 120 �m were scraped out by the four blades, respectively.The fiberglass cloth was dipped into PTFE emulsion (Zewffl Gk570Taiwan Daikin). A film thickness of 30 �m was scraped out by thescraper. The SHSG was dropped onto the PTFE film. A film thick-ness of 60 �m, containing PTFE and SHSG, was scraped out by thescraper. After heat treatment at 200 ◦C or 300 ◦C for 2 h, one bilayerof PTFE/SHSG was completed. The total thicknesses of two and threebilayers of PTFE/SHSG were 120 and 180 �m, respectively.

The fiberglass cloth coated with PTFE and SHSG of samples B, Cand D are designated as PTFE/SHSG-coated canvas.

2.3. Sample tests

2.3.1. Water impact testIn order to estimate the rain shock endurance of all the samples,

a water column with cross sectional area of 1.7 cm2 was impactedon the samples from a height of 45.5 cm for 2 h. Water impact veloc-ity was estimated to be about 2.99 m/s, and WCA was measured.

2.3.2. Tensile testSamples of 40 × 150 mm were subjected to tensile strength mea-

surements by the 8801 servohydraulic testing system of InstronEngineering Corporation. The tensile test machine worked at a rateof 100 mm/min with the initial distance between the clamps equalto 110 mm. Maximum load at break was determined for sampleswith different processing conditions.

2.3.3. Accelerated weathering testThe samples were mounted in a Global UV testing chamber

(Ci4000 Xenon Arc Weather-Ometer, ATLAS, USA) by means ofa custom-made sample holder. According to test program of ISO4892-2, the test conditions included ultraviolet irradiation of wave-length of 300–400 nm with intensity of 60 W/m2, black paneltemperature of 65 ◦C, chamber temperature of 38 ◦C, and relativehumidity of 50%. WCA of the samples was measured for every50 h, and tensile test was conducted for every 100 h of acceleratedweathering test.

2.4. Characterization of PTFE/SHSG coating

The WCA of all the samples, determined using a contact angletester, was an average of three independent measurements foreach sample. Advancing contact angle measurements were per-formed by adding liquid to the water droplets, and receding contact

angle measurements were made by removing liquid from the drops.The sliding angle was defined as the smallest tilted angle � of thePTFE/SHSG-coated canvas at which the deposited water dropletsslid off immediately after dispensing from a needle.

S.D. Wang et al. / Applied Surface Science 307 (2014) 101–108 103

Table 1Variations in WCA due to water impact test for sample C.

Number of bilayer Heat treatmenttemperature (◦C)

WCA before waterimpact (◦)

Period of waterimpact (min)

WCA after waterimpact (◦)

1 200 151.3 ± 1.6 10 105.0 ± 9.4300 138.3 ± 1.6 10 94.3 ± 2.4

2 200 153.6 ± 2.6 120 135.3 ± 3.8300 144.7 ± 5.4 120 131.3 ± 6.4

3 200 154.3 ± 1.8 120 141.7 ± 6.2300 142.3 ± 6.2 120 130.3 ± 1.2

5 200 139.3 ± 2.4 120 134.7 ± 4.3300 135.6 ± 1.5 120 128.7 ± 3.5

Table 2Variations in WCA due to water impact test for sample D.

Number of bilayer Heat treatmenttemperature (◦C)

WCA beforewater impact (◦)

Period of waterimpact (min)

WCA after waterimpact (◦)

1 200 146.0 ± 2.2 35 108.0 ± 6.5300 142.0 ± 2.2 35 106.3 ± 5.8

2 200 153.3 ± 3.1 120 141.7 ± 3.3± 2.1± 2.1

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SEM was used to analyze the morphology of PTFE/SHSG coating,nd EDX was used for surface elemental analysis. The 3D surfaceorphology and root-mean-square (RMS) roughness of the filmsere investigated using an AFM (Dimension D3100S-1) in tappingode.The bonding and distribution of PTFE and SHSG in the sam-

les were analyzed by attenuated total reflection Fourier transformnfrared (ATR-FTIR) spectroscopy (TENSOR 27 FTIR/HYPERION000, Bruker Optics) with 32 scans at a resolution of 4 cm−1. Trans-ission FTIR spectroscopy using KBr pellets with 64 scans at a

esolution of 4 cm−1 was also adopted to investigate hydrophobiconding of SHSG.

. Results and discussion

.1. Water impact test

Variations in WCA due to water impact test for samples A, B, C,nd D are summarized in Tables 1–3, and Table S4 of Supplementaryaterial, respectively.

.1.1. Water impact test of sample A:

After deposition of SHSG, WCA of the commercial canvas

as enhanced from 90.7◦ ± 4.0◦ to 144.3◦ ± 4.2◦ (Table S1 ofupplementary Material), but it decreased to 86.7◦ ± 2.1◦ afterater impact for 15 min. These results indicated that there was

able 3he effect of addition or omission of additional ethanol at molar ratio 6 after theompletion of adding process of HMDS during the preparation process of SHSG onhe tensile strength of the two bilayers PTFE/SHSG-coated canvas.

Aging periodof SHSG (day)

Additional ethanolat molar ratio 6

Heating method Tensilestrength (kg)

7 Addition Without preheat 2467 Addition With preheat 263

11 Addition Without preheat 25711 Addition With preheat 27616 Addition Without preheat 27216 Addition With preheat 29080 Addition With preheat 243

7 Omission Without preheat 2687 Omission With preheat 274

11 Omission Without preheat 28111 Omission With preheat 294

120 137.3 ± 3.6120 139.0 ± 1.7120 139.3 ± 2.3

no bonding between SHSG and the commercial canvas. SHSG wasonly physically adsorbed on the canvas, so it could not resist theimpact of water.

3.1.2. Water impact test of sample B:When PTFE powder, fluorine surfactant S420, and SHSG were

used in ratio of 1:2:4, WCA increased to 144.0◦ ± 2.9◦ after heattreatment and decreased to 88.0◦ ± 2.6◦ after water impact for10 min (Table S2 of Supplementary Material). On the other hand, ata ratio of 1:1:1, WCA was only 136.3◦ ± 1.2◦ after heat treatmentand even remained 104.3◦ ± 2.5◦ after water impact for 40 min.These results implied that when SHSG/PTFE > 1, samples had highinitial WCA, but PTFE could not seize excessive SHSG, hence WCAdecreased abruptly after the water impact. When SHSG/PTFE ≤ 1,PTFE was better able to grasp the SHSG, the sample showed betterwater impact endurance but a lower WCA.

3.1.3. Water impact test of sample C:With single bilayer of PTFE/SHSG, WCA increased to

151.3◦ ± 1.6◦ but decreased to 105.0◦ ± 9.4◦ after water impactfor only 10 min (Table 1). Five bilayers of PTFE/SHSG resulted inWCA drop only by 3.6% after water impact for 120 min. Never-theless, WCA of 139.3◦ ± 2.4◦ did not satisfy superhydrophobiccondition. With two (three) bilayers of PTFE/SHSG, the initialWCA of 153.6◦ ± 2.6◦ (154.3◦ ± 1.8◦), met the superhydrophobiccondition and WCA reduced by 12% (9%) after a water impact for120 min. However, the coating surface was not flat due to the lackof thickness control as shown in Fig. 1.

3.1.4. Water impact test of sample D:WCAs of the coatings with a heat treatment at 200 ◦C were

all higher than the corresponding coatings with a heat treatmentat 300 ◦C regardless the number of bilayers (Table 2). With onebilayer of PTFE/SHSG, WCA dropped by ∼26% after water impact for35 min. With two and three bilayers of PTFE/SHSG, WCA reachedto 153.3◦ ± 3.1◦ and 152.3◦ ± 2.1◦, and retained at 141.7◦ ± 3.3◦ and139.0◦ ± 1.7◦, respectively, after a water impact for 120 min. Theresults, shown in Tables 1 and 2, indicated that at least two bilayers

of PTFE/SHSG were essential to resist water impact. Furthermore,the coating surface of sample D obtained by scraper method (Fig. 2)was flatter than that of sample C obtained by dip-drop method(Fig. 1). Therefore, sample D and scraper method were used in thefollowing tests and analyses.

104 S.D. Wang et al. / Applied Surface Science 307 (2014) 101–108

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.2. Tensile test

Ethanol and water condensation continued to occur after thereparation process of sol gel. Aging period influenced the threeimensional structure and magnitude of the sol gel particles. Afterhe completion of adding process of HMDS, one more addition ofthanol at molar ratio 6 (ethanol/TEOS = 6) to the sol can reducehe degree of condensation and promote the transmittance of thelm spin-coated from the sol gel [27]. But coating of high transmit-ance is not required for canvas. The effect of addition or omissionf additional ethanol at molar ratio 6 after the completion of addingrocess of HMDS on the tensile strength was compared and is pre-ented in Table 3. One more addition of ethanol at molar ratio 6 afterhe addition of HMDS enhanced the tensile strength of PTFE/SHSG-oated canvas with increasing aging period of sol gel and reached

maximum of 290 kg with sol gel aged for 16 days. Sol gel agedor 80 days resulted in large particles which could not be mixedniformly with PTFE resulting in a lower tensile strength (243 kg).he tensile strength of PTFE/SHSG-coated canvas with sol gel, withne more addition of ethanol at molar ratio 6 after the addition ofMDS, aged for 11 and 16 days (276 and 290 kg) were almost equal

o PTFE/SHSG-coated canvas with sol gel, without one more addi-ion of ethanol at molar ratio 6 after the addition of HMDS, agedor 7 and 11 days (274 and 294 kg), respectively. Consequently, forberglass cloth to coat with PTFE/SHSG, no additional ethanol wasequired at the end of HMDS addition during the preparation pro-ess of SHSG. The effect of heating method on the tensile strength ofTFE/SHSG-coated canvas is also summarized in Table 3. The ten-ile strength of PTFE/SHSG-coated canvas with heating from roomemperature to 200 ◦C after scraper coating was always larger thanhat of PTFE/SHSG-coated canvas when heated at 200 ◦C but with-ut preheat after scraper coating. As the PTFE/SHSG coating waseated at 200 ◦C but without preheat, the ethanol in the coatingvaporated violently and SHSG solidified quickly. In this situation,

HSG did not have sufficient time to cross link with PTFE beforeolidification. SHSG and PTFE were more inclined to separate fromach other rendering the PTFE/SHSG-coated canvas with a lowerensile strength.

Fig. 2. A water drop on the surface of

sample C with WCA of 156.8◦ .

3.3. Accelerated weathering test

The two bilayers PTFE/SHSG-coated canvas of sample D withheat treatment at 200 ◦C were tested for accelerated weathering todetermine the variation of WCA and tensile strength at outdoors.Additionally, samples designated as PTFE-coated canvas, for whichthe fiberglass cloth was dipped into the PTFE emulsion (ZewfflGk570 Taiwan Daikin) and a film thickness of 120 �m was scrapedout by the scraper with heat treatment at 200 ◦C, were prepared forcomparison. The original tensile strength of a PTFE/SHSG-coatedcanvas (288 Kg) was stronger than a PTFE-coated canvas (256 Kg).The tensile strength of both PTFE/SHSG and PTFE-coated canvasdecreased with the increasing hours of accelerated weatheringtest, but the strength of the former was always superior to thelatter during 300 h of accelerated weathering test (Table S3 of Sup-plementary Material). Therefore, the combination of SHSG withPTFE not only increased WCA of the canvas but also promotedthe strength and weatherability of the canvas. During the 300 h ofaccelerated weathering test, WCA of the PTFE/SHSG-coated canvasdecreased slowly from 153.3◦ ± 2.1◦ to 143.7◦ ± 2.1◦ (Table S4 ofSupplementary Material), which was lower than 150.0◦. Neverthe-less, the contact angle hysteresis of the PTFE/SHSG-coated canvaswas still as low as 8.0◦ even after 300 h of accelerated weatheringtest. The sliding angles of two bilayers PTFE/SHSG-coated canvasof sample D were 3◦ and 4◦, for just prepared and after acceleratedweathering test of 300 h, respectively (see Video). Consequently,the PTFE/SHSG-coated canvas of sample D exhibited an outstandingself-cleaning ability.

3.4. SEM, EDX and AFM analyses

Since SHSG is tri-methyl-modified silica sol and the formula ofPTFE is (C2F4)n, therefore the signal of Si atom appearing in the EDXanalysis of the coating was equivalent to the detection of SHSG in

the coating. It is clear from the EDX analysis of the three bilay-ers coating (Fig. 3(c)) that the sheet in the coating was composedof PTFE and SHSG, and only PTFE but no SHSG appeared in thecrack between the sheets. A large area in one bilayer coating of

sample D with WCA of 156.4◦ .

S.D. Wang et al. / Applied Surface Science 307 (2014) 101–108 105

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Fig. 3. Photographs of SEM and EDX analysis of (a) one bilayer, (b) t

TFE/SHSG contained only PTFE (Fig. 3(a)), this was the area firstlyorroded by water during the water impact and this can explainhy one bilayer PTFE/SHSG-coated canvas of sample C and D hadoor water impact endurance (Tables 1 and 2). When the coating ofTFE/SHSG was two bilayers, the sheet structure covered the wholeoating surface (Fig. 3(b)). Consequently, WCA of the coatings, justfter heat treatment, met with the superhydrophobic condition andecreased only ∼7% after a water impact for 2 h. In case of coat-

ng of PTFE/SHSG with three bilayers, the area of one sheet in theoating was obviously larger than that in the two bilayers coating.his meant that PTFE and SHSG were more uniformly distributedn the coating of three bilayers than in the coating of two bilayers.herefore, WCA of the three bilayers coatings, just after heat treat-ent, also met with the superhydrophobic condition. It still kept139o even after water impact for 2 h, and had a smaller standardeviation than that of two bilayers coating.

The atomic percentages of F and Si in EDX analysis wereorth paying attention (Fig. 3(b) and (c)). F/Si was 58.35/5.11 and

9.99/3.99 at the two regions of a two bilayers PTFE/SHSG coatingnd was 30.66/11.2 in a large sheet of a three bilayer PTFE/SHSGoating. The SHSG was dropped onto the PTFE film in the samplereparation of sample D. Both the PTFE film and SHSG film in a sin-le bilayer, scraped out separately by the scraper, had a thickness

f 30 �m, and the SHSG film was on top of the PTFE film beforehe heat treatment. F/Si ratios of two bilayers PTFE/SHSG coatingnd three bilayers PTFE/SHSG coating indicated that PTFE movedpward and SHSG moved downward during the heat treatment.

ayers, and (c) three bilayers PTFE/SHSG-coated canvas of sample D.

Fig. 4(a and b) show 3D surface morphologies of the two bilay-ers PTFE/SHSG-coated canvas of sample D with RMS roughness of197.63 nm and a PTFE-coated canvas with the same film thicknessof 120 �m with RMS roughness of 40.30 nm. The surface of a PTFE-coated canvas was very flat compared to the steep topography ofthe two bilayers PTFE/SHSG-coated canvas.

Fig. 5 represents a picture of some water drops on a piece of twobilayers PTFE/SHSG-coated canvas showing excellent hydropho-bicity.

3.5. FTIR analysis

FTIR analysis was performed to understand why the best heattreatment temperature was 200 ◦C not 300 ◦C. SHSG was coated ona glass substrate and heat treated at 200 ◦C or 300 ◦C. The coatingwas scraped from the glass to incorporate in a thin KBr disk. Ethanolwas added during the preparation process of SHSG and was pro-duced in ethanol condensation. The strength of Si–O–Si stretchingand CH3 symmetric deformation of Si–CH3 [28] with heat treat-ment at 300 ◦C were weaker than those with heat treatment at200 ◦C (Fig. 6), it indicated that some of Si–O–Si and Si–CH3 bond-ing of SHSG was pyrolyzed with heat treatment at 300 ◦C. Therefore,SHSG had more hydrophobic bonds of –CH3 when heat treated at

200 ◦C rather than at 300 ◦C.

Fig. 7 shows the ATR-FTIR spectra of a fiberglass cloth coatedwith (a) only PTFE; (b) only SHSG; and (c) two bilayers ofPTFE/SHSG. The strength of absorption peaks at 1205 cm−1 and

106 S.D. Wang et al. / Applied Surface Science 307 (2014) 101–108

Fig. 4. AFM photograph of (a) the two bilayers PTFE/SHSG-coated canvas of sampleD; (b) a PTFE-coated canvas.

Fig. 6. Transmission FTIR spectra of a superhydrophobic film spin-coated from SH

Fig. 5. A picture of some water drops on a piece of two bilayers PTFE/SHSG-coatedcanvas.

1160 cm−1 representing CF2 groups of PTFE were quite similar inPTFE/SHSG bilayer to as those in PTFE coating. As the siloxanechains became longer or branched, the Si–O–Si absorption becamebroader and more complex. Nevertheless, the absorption peaks at1256 cm−1 and 1071 cm−1, assigned as CH3 symmetric deformationof Si–CH3 and Si–O–Si stretching [28] respectively, of SHSG coatingdisappeared or became a weaker band (1087 cm−1) in PTFE/SHSGbilayer. It should be noted again that in the preparation of two bilay-ers PTFE/SHSG-coated canvas (sample D), the SHSG was droppedonto the PTFE film and then heat treated at 200 ◦C or 300 ◦C. There-fore, the infrared spectrum implies that PTFE extended upward andSHSG moved downward during the heat treatment. PTFE entan-gled and crosslinked with SHSG after solidification. The infraredbands of PTFE were more obvious than those of SHSG in the spec-trum of two bilayers of PTFE/SHSG at 1300 cm−1–1000 cm−1, whichmeans that PTFE has a large proportion in the penetration depth ofthe ATR measurements (1–2 �m). This conclusion agrees with thatof EDX analysis. The ATR and EDX analyses can also provide evi-dence of microstructure to explain the result of Table 3. PTFE chain

and three dimensional extended structure of SHSG in PTFE/SHSGbilayer had enough time to interpenetrate and entangle with eachother as the PTFE/SHSG bilayer was heated at 200 ◦C with preheat,

SG. (a) Before heat treatment, heat treatment at (b) 200 ◦C, and (c) 300 ◦C.

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ig. 7. ATR-FTIR spectra of a glass fiber cloth coated with (a) only PTFE; (b) onlyHSG; and (c) two bilayers of PTFE/SHSG.

nd such entangling would have hindered the free stretching ofTFE chain as the PTFE/SHSG-coated canvas was tensioned. TheDX, AFM and FTIR analyses indicated that SHSG provided theoughness and hydrophobic bonds of −CH3 needed for superhy-rophobicity, and PTFE provided CF2 groups in the coating nearesto the surface to resist the water impact. Therefore, the two bilay-rs PTFE/SHSG-coated canvas exhibited excellent water impactndurance and WCA of 153.0 ± 3.0◦, which is greatly significanthan WCA (91.0 ± 3.0◦) of the commercial canvas.

. Conclusions

SHSG was applied to promote the hydrophobicity and self-leaning ability of canvas. Four kinds of coating methods, including1) dropping SHSG directly on a commercial canvas; (2) mixingTFE powder, fluorine surfactant S420, and SHSG and then drop-ing on the fiberglass cloth; (3) dipping the fiberglass cloth intoTFE emulsion (Zewffl Gk570 Taiwan Daikin) and then droppinghe SHSG onto the PTFE film; and (4) scrapping out every sub-layerf PTFE and SHSG by a scraper were studied in this work. Watermpact endurance was used to find out the best coating method.t least two bilayers of PTFE/SHSG were needed to resist the water

mpact. WCAs of two and three bilayers of PTFE/SHSG-coated can-as, obtained by scraper method and heat treatment at 200 ◦C, were53.3◦ ± 3.1◦, 152.3◦ ± 2.1◦ and 141.7◦ ± 3.3◦, 139.0◦ ± 1.7◦, beforend after water impact of 2 h, respectively. The smaller standardeviation of three bilayers coating corresponded to a larger areaf one single sheet, which contained more uniform distributionf PTFE and SHSG in the coating as revealed by SEM analysis.he tensile strength of PTFE/SHSG-coated canvas, with SHSG agedor 11 days, could reach 294 kg. Sol gel with a too long agingad too large particles which could not be mixed uniformly withTFE that resulted in a lower tensile strength. Although SHSG wasropped on the top of PTFE film, the analyses of EDX and ATReasurements indicated that the chains of PTFE extended upward

nd SHSG moved downward as the PTFE/SHSG-coated canvas waseated from room temperature. This can be interpreted in termsf entangling of PTFE and SHSG that promotes tensile strength

f the PTFE/SHSG-coated canvas. SHSG provided the roughnessnd hydrophobic bonds of –CH3 needed for superhydrophobicitynd PTFE provided CF2 groups in the coating nearest to the sur-ace to resist the water impact. Consequently, the combination of

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cience 307 (2014) 101–108 107

SHSG with PTFE promoted not only self-cleaning ability but alsothe strength and weatherability of the canvas. The tensile strengthof PTFE/SHSG-coated canvas was always superior to that of PTFE-coated canvas during weathering test of 300 h. After acceleratedweathering test of 300 h, the two bilayers PTFE/SHSG-coated can-vas had a tensile strength of 237 kg, static WCA of 143.7◦ ± 2.1◦,a contact angle hysteresis of 8◦, and a sliding angle of 4◦, whichrepresented an outstanding self-cleaning ability.

Acknowledgement

The authors would like to thank the Taiwan Textile ResearchInstitute, Republic of China, for financially supporting this researchunder the project of “Research on the compatibility and physicalproperties of sol gel and fluorine resin”.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2014.03.173.

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