Surface & Coatings Technology 228 (2013) S477–S481

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Surface & Coatings Technology

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Effects of plasma power and reaction gases on the surface properties of ePTFEmaterials during a plasma modification process

Hsi-Hsin Chien ⁎, Kung-Jeng Ma, Chien-Huang Kuo, Shu-Wei HuangCollege of Engineering, Chung Hua University, No. 707, Sec. 2, Wu Fu Rd., Hsin Chu, 30067, Taiwan

⁎ Corresponding author. Tel.: +886 3 5186467; fax:E-mail address: hhchien@chu.edu.tw (H-H. Chien).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.05.014

a b s t r a c t

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Available online 16 May 2012

Keywords:Plasma etchingePTFESuperhydrophobicPlasma power

Expanded PTFE (ePTFE) in sheet form has been widely used in various industrial environments because of itshydrophobic surface, elasticity and porous properties. To enhance its applications, the surface of sheet ePTFEhas been modified by various techniques. This study is devoted to the surface modification of ePTFE by an RFplasma system. The operating parameters including the selected gases (O2, N2, Ar) and plasma power arevaried.The samples have an obvious hydrophilic surface after Ar andN2 plasma treatment at a high RF power (50–400W).A contact angle of 22° is obtained after N2 plasma treatment at an RF power of 400W. The weakening of thecharacteristic bonds of CF3 and CF2 and the formation of a cross-linked C_N–H layer are themain reasons lead-ing to a hydrophilic surface. However, further increasing the RF power to 500W tends to produce a hydropho-bic surface due to the formation of a needle-like surface caused by severe plasma etching effects. It is observedthat the contact angle of ePTFE slightly decreases under low RF power (b200W) in O2 plasma; however, itshows a super-hydrophobic surface under a higher RF power (>400W). The main characteristic bands didn'tshow any significant change after an O2 plasma treatment even under high RF power. The needle-like structureappears on the surface, which leads to super-hydrophobic properties due to the lotus effect.

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1. Introduction

Polytetrafluoroethylene (PTFE) is known for its high thermal stabili-ty, chemical inertness, low surface tension and low coefficient of friction.These properties provide excellent performance in many applicationsincluding low friction films, seals, electronic and biomedical devices.Expanded PTFE (ePTFE) material with a porous structure and excellentelasticity further extends their application. However, the presence ofstrong C–F bonding has limited the use of PTFE substrates in microelec-tronics, where strong adhesion of contacts is required; and in medicalengineering where functional groups need to be immobilized on thesurface.

There are several approaches to improving the surface hydrophilicproperties including chemical reduction with sodium naphthalene,ion beam bombardment, flame treatment, UV or e-beam irradiationand plasma modification [1–12]. The plasma modification techniquehas advantages over other methods mainly because it avoids environ-mental contamination problems and highly efficient plasma basedtreatments are also well suited to production-scale processes andprovide good uniformity even for complex geometries. Plasma modi-fication, either cross-linking, functional group attachment, or abla-tion, can be surface-specific, leaving the bulk polymer and hence the

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mechanical properties unaffected. A wide variety of gas precursorsmake available numerous surface treatments to alter the chemicaland physical surface properties.

It has been reported that N2, NH3, H2O, C2H2, and H2O/Ar, plasmascan effectively improve the surface hydrophilic properties by cross-linking or functionalization mechanisms [13–16]. The Ar, O2 or Ar/O2

plasmas are used to improve surface adhesion of PTFE mainly by etch-ing induced surface activation and roughening effects [5,6]. The stabilityof the modified surface is another concern because the hydrophiliceffect may diminish or change after a period of time [17]. The possiblereasons are the formation of weak boundary layers or the mobility ofsurface functional groups.

In this study, the surface modification of e-PTFE material was pro-duced using an RF plasma system. The effects of process parameters,including the selected gases (O2, N2 and Ar), and plasma generationpower, on the hydrophilic properties of e-PTFE materials were inves-tigated. The related reaction mechanisms between the plasma andthe e-PTFE material are also discussed.

2. Experimental

Commercially produced ePTFE (1 mm thick, specific gravity of2.1 g cm−1, melting point of ca. 380 °C and average pore size of1.0 μm) plates were used in this study. The ePTFE plates were ultra-sonically cleaned in acetone for 5 min followed by drying at roomtemperature before plasma treatment. Plasma treatment was carried

Fig. 2. The effect of plasma power and reaction gas on the wetting angle.

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out using an RF plasma etching system with a maximum power of500 W. The O2, N2 and Ar gas flow rate was set at 25 sccm/min. Theworking pressure was kept at 20 mTorr and the duration of the plas-ma treatment was 20 min.

The wettability of the as received and the modified ePTFE sheetswas evaluated by measuring the static contact angles with distilledwater. The static contact angles were measured in air at room tem-perature using the sessile drop method and each drop was fixed at50 ml. The contour of the droplet was projected onto a screen by visiblelight and the contact anglesweremeasured on the images. High resolu-tion SEM and FTIR microscopies (Jasco FTIR-200) were employed to in-vestigate the surface morphology and structure of ePTFE plates afterplasma treatment.

3. Results and discussion

The surface morphology of the ePTFE plate is shown in Fig. 1. Themicrostructure of the porous ePTFE material consisted of nodes inter-connected by fibrils. The pore size in the ePTFE appeared smaller than0.5 μm. The measured wetting angle was 110° for the ePTFE material.The effect of plasma treatment on the PTFE surface strongly dependedon the plasma treatment conditions. Fig. 2 shows the effects of RFpower on the wetting properties of the ePTFE materials in O2, Arand N2 plasmas with a treatment duration of 20 min. In the case ofAr plasma treated ePTFE samples at a low RF power (50 W), thewetting angle only decreased to 82°. Increasing the RF power enhancedthe Ar ion bombardment, which favored the surface activation of ePTFE.The wetting angle decreased with decreasing RF power. Increasing theRF power to 300W, produced a decreased wetting angle of 36° for theePTFE material. However, increasing the RF power to over 300W, thewetting angle tended to increase with RF power. A wetting angle of98° was measured at an RF power of 500W. A similar phenomenonwas observed after N2 plasma treatment, as shown in Fig. 2. The mea-sured wetting angle of ePTFE material was higher than when treatedin Ar plasma at an RF power below 170W. As the RF power increasedto over 200W, the hydrophilic phenomena become more pronouncedcompared to samples after Ar plasma treatment. A wetting angle of22° was measured at an RF power of 400W. Further increasing the RFpower to 500W did not enhance hydrophilic behavior. The wettingangle increased to 77° at an RF power of 500W.

There was no obvious surface morphology change for ePTFE mate-rial after Ar plasma treatment at an RF power of 200 W (Fig. 3). Thestructure of nodes interconnected by fibrils still existed at low RFpower. Increasing the RF power to over 300 W, an etching phenome-na could be observed on the ePTFE surface (Fig. 4). At an RF power of400 W, the fibrils were severely broken and the interconnected nodeswere gradually etched. As a result, the surface demonstrated a porousand web-like appearance (Fig. 5(A)). A very thin modified layer could

Fig. 1. SEM surface morphology of the ePTFE material.

still be observed on the etched ePTFE surface (Fig. 5(B)). Further in-creasing the RF power to 500 W leads to a highly porous surface orneedle like structure due to the severe etching effect.

In the case of ePTFE material modified by N2 plasma, subtle differ-ences in surface morphology could be observed after treatment at alow RF power (b300 W) (Fig. 6). The surface was modified by N2 plas-ma and tended to form a thin film on the surface. As the RF power wasincreased to 400 W, the nodes and interconnected fibrils were gradu-ally etched (Fig. 7(A)), and a thicker cross-linked layer (~10 μm) wasformed on the surface as seen from the cross-sectional SEM analysis(Fig. 7(B)). Increasing the RF power to 500 W, the etching inducedporous structure became more prominent. The thickness of the cov-ered cross-linked layer was smaller than that treated under an RFpower of 400 W. The fraction of pores in ePTFE after Ar treatmentwas higher than observed in ePTFE material treated by N2 plasma atthe same RF power (Figs. 5(A) and 7(A)). It was also noted that thecross-linked layer appearing on the etched ePTFE surface was morepronounced in N2 plasma treated samples than in those treated byAr plasma.

From FTIR analysis (Fig. 8(A)), the characteristic absorption bandsof untreated ePTFE were observed at 503 cm−1 (CF2 rocking),620–640 cm−1 (CF2 wagging), 1150 cm−1 (CF2 stretching), and1240 cm−1 (CF3 stretching). The main characteristic bands showed nosignificant change after Ar plasma treatment even under a high RFpower (Fig. 8(B)). Only a slight weakening of characteristic bondscould be observed. This indicated that from FTIR analysis there was noobvious change in the surface chemistry of ePTFE material after Arplasma etching. However, in the case of ePTFE treated by N2 plasma,an obvious change in surface structurewas observed fromFTIR analysis.The main characteristic bands of CF3 and CF2 disappeared after N2 plas-ma treatment under an RF power of 400W (Fig. 8(C)). The C_N–Hbonds became the dominant bonding on ePTFE surface after N2 plasmatreatment. The main characteristic bands of CF3 and CF2 disappeareddue to the occurrence of surface cross-linking or functionalization. The

Fig. 3. SEM image of ePTFE material after Ar plasma treatment at a RF power of 200W.

Fig. 4. SEM image of ePTFE material after Ar plasma treatment at a RF power of 200 W. Fig. 6. SEM image of ePTFE material after N2 plasma treatment at a RF power of 300 W.

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formation of a thick cross-linked layer on the etched ePTFE surfacesuppressed the characteristic bands of CF3 and CF2.

The ePTFE materials modified by O2 plasma demonstrated inter-esting characteristics (Fig. 2). It was observed that the contact angleof ePTFE slightly decreased under a low RF power (b200 W) in O2

plasma; however, it showed a super-hydrophobic surface under ahigh RF power (>400 W). Wetting angles of 151° and 162° wereobtained for ePTFE after 20 min plasma treatment at an RF power of400 and 500 W respectively. The effects of RF power on the surfacemorphologies of ePTFE are shown in Fig. 9. There was no obvious sur-face morphology change for ePTFE material after plasma treatment ata low RF power of 200 W (Fig. 9(A)). On increasing the RF power to300 W a subtle morphology change was observed. As the RF power

Fig. 5. SEM images of ePTFE material after Ar plasma treatment at a RF power of 400 W:(A) surface and (B) cross-section.

was increased to over 400 W, severe surface etching occurred afterplasma treatment (Fig. 9(B)). The ion bombardment effect was en-hanced at a higher RF power, which produced a porous or needle-like surface. The PTFE surfaces which possess nonpolar characteristicsare inherently hydrophobic. The needle-like or nanostructured sur-face provided miniature convex bumps or buds, which formed a hier-archical fractal structure and reduced the number of contact points ofwater with the surface. This further increased the contact angle ofwater, thereby enhancing the surface hydrophobic properties; some-times achieving a super-hydrophobic surface. This phenomenon wasmathematically expressed by the Cassie–Baxter equation [18]. Thisequation also takes into account the complementary effect of the air

Fig. 7. SEM images of ePTFE material after N2 plasma treatment at a RF power of400 W: (A) surface and (B) cross-section.

Fig. 8. The effect of plasma gas on the FTIR spectrum of ePTFE material at an RF power of 400 W and treatment duration of 20 min. (A) Un-treatment and after (B) Ar (C) N2 (D) O2

plasma treatment.

Fig. 9. SEM images of ePTFE material after O2 plasma treatment at a RF power of(A) 200W and (B) 400W.

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entrapped between the droplets and substrate due to the texturedtopography.

The main characteristic bands didn't show any significant changeafter O2 plasma treatment even under high RF power (Fig. 8(D)).This indicated from FTIR analysis that there was no obvious changein the surface chemistry of ePTFE material after O2 plasma etching.It was observed that the introduction of oxygen leads to the forma-tion of carbonyl and hydroxyl groups on the PTFE surface at low RFpower, identified from XPS analysis (Fig. 10). Both carbonyl andhydroxyl groups are polar functional groups and are strongly hydro-philic. This explains why the contact angle slightly decreased underlow RF power.

It is believed that at low RF power, surface activation and cross-linking are the dominant processes in the plasma modification ofPTFE material [19]. As the RF power increases above a critical value,etching was also involved in the modification process [20]. In thecase of Ar plasma treatment under a low RF power, surface activationoccurred which slightly improved the hydrophilic properties of anePTFE material. High-energy Ar ion bombardment caused the PTFEsurface to be severely etched and caused decomposition of the out-most layer of the PTFE, which resulted in a porous surface. The highlyporous or needle-like surface favored hydrophobic behavior due tothe lotus effect, which explains the abrupt increase in the wettingangle following a 500W plasma treatment although surface activationoccurred at the same time. At a relatively low RF power (b400W) inan N2 plasma, a cross-linking process dominated the surface modifica-tion rather than an etching process.

The surfaces that underwent changes in surface properties due toplasma treatment had a tendency to revert back to their original state,the so called aging effect. Thus, the stability of the plasma inducedchanges on polymer surfaces over a desired time period is an impor-tant issue. It was observed that the contact angle showed a slightchange from 38° to 42° after 240 h exposure in humid air for N2 plas-ma treated samples at an RF power of 350 W. However, the contactangle significantly increased from 40° to 75° after 240 h exposure inhumid air for Ar plasma treated samples. The N2 plasma treated

Fig. 10. PS results of ePTFE after O2 plasma treatment at a RF power of 200 W.

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samples demonstrated more stable surface hydrophilic characteris-tics than those treated in Ar plasma. This means that the surface acti-vation state of ePTFE after N2 plasma treatment can last over oneweek, which is a benefit in numerous industrial applications. Thesurface super-hydrophobic property of ePTFE was very stable afterO2 plasma treatment. This indicated that the surface nanostructureinduced super-hydrophobic properties were not affected by gas ab-sorption in humid air.

4. Conclusions

At low RF power, the improvement of the wettability could beattributed to the occurrence of surface activation or cross-linking onthe ePTFE surface by Ar and N2 plasma. A contact angle of 22° wasobtained after N2 plasma treatment at an RF power of 400 W. Theweakening of the characteristic bonds of CF3 and CF2 and the forma-tion of C_N–H hydrophilic film are the main reasons leading to theproduction of a hydrophilic surface. As the RF was increased to500 W, the wetting angle abruptly increased, which implied thatthe etching induced lotus effect or cross-linking induced hydrophiliceffect occurred at a higher RF power. The main characteristic bandsdidn't show any significant change after an O2 plasma treatmenteven under high RF power. The needle-like structure appeared onthe ePTFE surface after a higher O2 plasma treatment, which reducedthe number of contact points of water with the surface and therebyenhanced the surface hydrophobic properties.

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