Penetration depth of atmospheric pressure plasma … eight-layer stack of woven polyester fabrics was exposed to a helium/oxygen atmospheric pressure plasma jet. Water-absorption time

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gy 202 (2007) 77–83www.elsevier.com/locate/surfcoat

Surface & Coatings Technolo

Penetration depth of atmospheric pressure plasma surface modificationinto multiple layers of polyester fabrics

C.X. Wang a,b,c,d, Y. Ren a,b,c,e, Y.P. Qiu a,b,c,⁎

a State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Chinab Key Laboratory of Textile Science and Technology, Ministry of Education, China

c College of Textiles, Donghua University, Shanghai 201620, Chinad College of Textiles and Clothing, Yancheng Institute of Technology, Jiangsu 224003, China

e School of Textile and Clothing, Nantong University, Nantong 226007, China

Received 11 January 2007; accepted in revised form 18 April 2007Available online 3 May 2007

Abstract

Penetration depth of plasma surface modification of polyester fabrics was investigated. An eight-layer stack of woven polyester fabrics wasexposed to a helium/oxygen atmospheric pressure plasma jet. Water-absorption time was used to evaluate surface hydrophilicity on the top and thebottom sides of each fabric layer and water capillary rise height was recorded as a measure of modification effectiveness for each fabric layer.Surface morphology and chemical compositions of each fabric layer in the stack were analyzed by atomic force microscopy (AFM) and X-rayphotoelectron spectroscopy (XPS). After atmospheric pressure plasma jet treatment, the top side of the polyester fabric became more hydrophilic.The penetration of plasma surface modification into the fabric layers was deeper for fabrics with larger average pore sizes. It was found thathelium/oxygen atmospheric pressure plasma jet was able to penetrate 8 layers of polyester fabrics with pore sizes of 200 μm.© 2007 Elsevier B.V. All rights reserved.

Keywords: Atmospheric pressure plasma jet; Penetration depth; Polyester fabric; Pore size; AFM; XPS

1. Introduction

Plasma surface treatment of various polymeric materialsstarted in 1960s [1] and is recently becoming more and morepopular as a surface modification technology. In the textile in-dustry, plasma treatments have many advantages over conven-tional wet chemical treatments. It does not involve large quantitiesof chemicals and water and thus is dry and environmentallyfriendly [2–6].

For a nonporous substrate, plasma surface modification iseffective in a depth of several nanometers on one side of thesubstrate and changes the outermost layer of the materialwithout affecting its bulk properties [7]. However, for porousmaterials, the picture gets more complicated. Regions of aporous structure that could be exposed to plasma are outersurfaces, interior surfaces and the bulk phase [8]. Especially for

⁎ Corresponding author. College of Textiles, Donghua University, Shanghai201620, China. Tel.: +86 021 67792744; fax: +86 021 67792627.

E-mail address: [email protected] (Y.P. Qiu).

0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2007.04.077

porous structured fabrics, there is a difference between thevisible surface and the actual inner surface to be modifiedcompared with films or foils. Fabrics are complex structurescomposed of single fibers and yarns distributed over the totalfabric thickness up to several millimeters which have to betreated reasonably homogenously throughout the entire thick-ness [9]. To ensure a uniform plasma treatment effect on thesurfaces of all fibers in a textile structure, the chemically activespecies must be able to penetrate the textile structure in anacceptable length of treatment time. Good penetration of plasmaeffects into textile structures is crucial for successful plasmamodification of textile materials [2].

There are some reports in literature about the penetration ofplasmas into porous materials. It has been suggested that plasmatreatments can create a large number of chemically activespecies (ions or radicals) that permeate through pores [10].Krentsel and coworkers [11,12] studied the penetration ofplasma surface modification into porous media using a lowtemperature cascade arc torch. It was shown that surface fluo-rination was detected inside the porous matrix on the surface of

Fig. 1. Images of polyester fabrics with the average pore sizes of (a) 200 μm and(b) 100 μm.

Fig. 2. Schematic of the atmospheric pressure plasma treatment system and eightlayers of stacked polyester fabrics.

78 C.X. Wang et al. / Surface & Coatings Technology 202 (2007) 77–83

the inner layers that were not directly exposed to CF4 and C2F4species from the torch. Masuka et al. [13] studied lowtemperature plasma initiated graft copolymerization of N,N-dimethylacrylamide onto 0.45 μm polypropylene membranes.They found grafted material not only on the interior and theouter surfaces of the membrane but also in the bulk region of thesubstrate. Mukhopadhyay et al. [14,15] indicated that RFplasma-induced hydrophobic coatings can be deposited on astack of five porous filter papers and the extent of permeation ofthe plasma into the inner layers varied with monomer structureand plasma parameters. Geyter et al. [9] treated three layers ofnonwoven with dielectric barrier discharge plasma at mediumpressure and proved that process pressure had an importanteffect on the penetration of plasma through the textile layers.Vatuna et al. [16] treated different textile layers with anatmospheric pressure plasma and observed penetration after atreatment of 20 min. Holländer [17] investigated the surfaceoxidation inside porous polymeric materials and found that thepore size seemed to be important for better treatment results.

Most of the porous materials discussed in the literatures weremembranes [8,13,18–20] or nonwoven [9,11,12] or filter papers[14,15]. Woven fabrics are one of the largest categories oftextile materials which could potentially be plasma-treated to

reduce the amount of chemicals used in wet processing of thesematerials. For atmospheric pressure plasma jet, the nozzle ejectsactive species on only one side of the substrate and its pene-tration depth into woven fabrics has not been investigated.

The purpose of this research is to study the penetration depthof atmospheric pressure plasma jet into a stack of woven poly-ester fabrics with various pore sizes. The surface modificationeffect of the plasma treatment was characterized by the changesin water-absorption time on the top and bottom sides of eachfabric layer and in capillary rise height of each fabric layer in thestack. Atomic force microscopy (AFM) and X-ray photoelec-tron spectroscopy (XPS) were employed to determine the sur-face morphological and chemical changes after the plasmatreatments.

2. Experimental

2.1. Materials

The fabric used in this study was woven polyester fabrics(shown in Fig. 1) with the average pore sizes of 100 μm and200 μm and a thickness around 0.25 mm. Before the plasmatreatments, the polyester fabrics were scoured with acetone for30 min to clean the fabric and then dried in vacuum at roomtemperature for 12 h. The cleaned fabrics were cut into20 mm×300 mm pieces. Eight layers of woven polyesterfabrics were glued together at their periphery with white glue toprevent any plasma penetration through edges and then laterplaced on the substrate conveyor belt. The fabric layers werelabeled as shown in Fig. 2.

Fig. 3. AFM 3-D profiles of (a) Layer 1, (b), Layer 2 and (c) Layer 8 of the stacked fabrics with the pore size of 200 μm and (d) Control.

Fig. 4.AFManalysis of surface roughness for the control and each treated fabric layer.

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2.2. Plasma treatments

Plasma treatments of the stacked samples were carried out in anatmospheric pressure plasma jet apparatus manufactured by SurfxTechnologies (California, USA). This device employs a capaci-tively coupled electrode design and produces a stable discharge atatmospheric pressurewith 13.56MHz radio frequency power. Theplasma jet system was equipped with a round nozzle of an activearea of 3.14×1 cm2 mounted above the conveyor belt (Fig. 2)moving underneath the plasma jet at a speed of 3mm/s resulting inplasma exposure time of 2.67 s. The distance between the nozzleand the substrate surface was about 2–3 mm. The carrier gas washelium (99.99% purity) with a flow rate of 20 liter/min (LPM) and0.2 LPM oxygen was added. The power was set at 100 W.

2.3. Wettability measurements

The wettability of the top and the bottom sides of each fabriclayer was measured according to the BS4554: 1970. Amicrolitersyringe was used to place a distilled water droplet of 3 μl on thefabric surface. The time for the droplets to be completelyabsorbed into the fabric was taken as the water-absorption time[15]. Five measurements were taken for each sample.

The capillary rise method was employed to evaluate thewettability improvement for each fabric layer [21,22], as theseporous fabrics were made of textured yarns that could absorbthe liquid droplet too quickly and were not compact and flatenough to allow a realistic contact angle measurement. Thefabric strip was suspended vertically with the lower end dipped

Table 1Relative chemical composition and atomic ratios determined by XPS forpolyester fabrics untreated and treated with APPJ

Sample Chemical composition(%)

Atomic ratio

C1s O1s O/C

Layer 1 60 40 0.66Layer 2 62 38 0.62Layer 3 64 36 0.58Layer 4 65 35 0.55Layer 5 66 34 0.51Layer 6 67 33 0.48Layer 7 69 31 0.45Layer 8 74 26 0.35Control 77 23 0.31

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into a 0.5% potassium chromate aqueous solution. Spontaneouswicking occurred due to capillary forces. The yellow colorationof chromate solution on the white fabric clearly indicated thecapillary rise height and a ruler marked in mm assembled alongthe strip was used to make the height measurements. Thecapillary rise height readings were made after 30 min. Theheights of three specimens for each sample were recorded tocalculate the average height value within the measurement error(±1 mm).

2.4. Surface morphology analysis

The topographical studies of the fiber surfaces were carriedout using an Ultra-high Vacuum Scanning Tunneling Micro-scope (Omicron, Germany). Tapping mode was used to preventsignificant deterioration of the fiber surfaces. Each AFM image

Fig. 5. Deconvolution analyses of XPS core level C1s spectra of (a) Layer 1, (b) Layer 2

was analyzed in terms of surface average roughness (Ra) androot mean square roughness (Rms) of 0.25×0.25 μm2 area onthe surface of the samples [23].

2.5. Surface chemical composition analysis

The surface chemical composition of the fabrics wasanalyzed by XPS measurements on a Thermo ESCALAB 250system equipped with a Mg Ka X-ray source with a pass energyof 1253.6 eV. The analysis was carried out under ultra highvacuum conditions (10−9–10−10 Torr). The power was set at300 W and the spectra were taken at 90°.

3. Results and discussion

3.1. AFM analysis

The AFM images of 10×10 μm2 area on the fiber surfaces inthe control and each treated fabric layer in the stacked samplesare presented in Fig. 3. The control fiber has a relatively smoothsurface as shown in Fig. 3(d). However, after atmosphericpressure plasma jet treatment, an increased number of micro-pits were formed on the fabric surfaces in different layers asshowed in Fig. 3(a–c). Much more micro-pits were formed onthe first several layers (Layers 1 to 5) indicating that the activespecies mainly reacted with the substrate surfaces in the firstfew layers. It is interesting to note that even for several layersaway from the top layer (Layers 6 to 8), some small micro-pitswere still present, indicating deep penetration of chemicallyactive species into the fabric layers. AFM images of the treatedfabrics showed that atmospheric pressure plasma jet treatment

and (c) Layer 8 of the stacked fabrics with the pore size of 200 μm and (d) Control.

Table 2Results of deconvolution of C1s peaks for polyester fibers untreated and treatedwith APPJ

Sample Relative area of different chemical bonds (%)

C–C C–O C_O

Layer 1 46.6 36.7 16.7Layer 2 50.1 32.8 17.1Layer 3 55.6 29.5 14.9Layer 4 60.1 24.6 15.3Layer 5 65.3 24.2 10.5Layer 6 66.6 24.6 8.8Layer 7 66.4 24.7 8.9Layer 8 67.6 23.1 9.3Control 70.9 20.0 9.1

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created micro-pits whose density, depth and size decreased withincreasing fabric layers as shown quantitatively in Fig. 4,similar to what has been reported by others [24].

The formation of micro-pits on the treated fabric surface wascaused by the etching reactions, which resulted in polymerdegradation through ion and electron bombardment and theoxidative reactions with activated oxygen atoms as well as free

Fig. 6. Water-absorption times for the control and the two sides of each treatedfabric layer in the stacked fabrics with the pore size of (a) 100 μm and (b) 200 μm.

radicals. The penetration depths of different activated species aredifferent due to the difference in energy and lifetime of thesespecies. In general, activated particles with short lifetime such asions, electrons, and atomic oxygen could hardly reach deep intothe fabric layers while long lasting free radicals could. Thereforethe top few fabric layers had more surface etching while thebottom few layers had little etching but mainly surface chemicalmodification. In additionUVradiationmay also play an importantrole as helium was used as the plasma gas due to the strongemission in the UV region. The surface of plasma-treatedpolyester fiber became rougher resulting in improved wettability[25,26].

3.2. XPS analysis

Detailed XPS analysis revealed the surface chemistrychanges for top sides of the control and the treated fabric inthe stack. Table 1 shows the atomic concentration for the controland the treated fabrics. The O/C photoelectron peak ratio mayimply the surface modification extent. The carbon contentdecreased while the content of O1s increased suggesting thatoxidation occurred on the surfaces of all layers of the stacked

Fig. 7. Capillary rise heights of control and each treated fabric layer in thestacked fabrics with the pore size of (a) 100 μm and (b) 200 μm.

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fabrics exposed to the helium/oxygen atmospheric pressureplasma jet [27,28]. The O/C ratio on the surface of Layer 1 wasdoubled compared to the untreated surface. The O/C ratio ofeach fabric layer decreased sequentially from Layer 1 to Layer8. However, even for Layer 8 the O/C ratio was a little largerthan that of the control indicating that the surface modificationof atmospheric pressure plasma jet could penetrate as deep asthe eighth layer of the fabric with the pore size of 200 μm.

Atmospheric pressure plasmas can generate a wide range ofactive species including atomic oxygen, ozone, neutral andmetastable molecules, radicals and UV radiation [9]. Oxygen-containing plasmas increase the surface energy of materials byintroducing oxygen-containing polar groups onto the materialsurface [29–33]. In order to identify what were the chemicalfunctional groups introduced to the surface of each fabric layerafter atmospheric pressure plasma jet treatment, deconvolutionanalyses were performed to C1s spectra using XPSPEAKsoftware as shown in Fig. 5(a–d) and the results are shown inTable 2. As well documented in literature [25,26,34,35], the C1speak for untreated polyester mainly contains three distinct sub-peaks corresponding to C–C/C–H (285 eV), C–O (286.1 eV)and C_O (288.6 eV). After plasma treatment, it is evident thatthe sub-peak at 285 eV decreased markedly and the sub-peak at286.1 eV drastically increased due to increased methylenecarbons singly bonded to oxygen. Some of the C–C bonds inpolymer surface may be scissioned by the plasma treatments.The carbon radicals, formed by the abstraction of hydrogenatoms from the polymer chains will recombine with oxygenatoms generated in plasma jet and air by the electron impactdissociation [21,26], resulting in formation of the oxygen-containing polar groups on the fabric surface. In addition, thereis a small peak at 291.4 eV due to the π–π shake up satellite inphenyl groups [9]. It almost disappeared after the plasmatreatments indicating the breakage of benzene rings resultedfrom the impact of active species in the plasmas.

It can be seen that the helium/oxygen atmospheric pressureplasma jet mainly modified CH2 or phenyl rings in the polyesterpolymer chains to form C–O and C_O groups. The introductionof oxygen-containing polar groups on fabric surface leads toincreased hydrophilicity as reported in literature [36–38].

3.3. Wettability improvements

Fig. 6 shows the water-absorption times for the control andeach treated fabric layer with different pore sizes. Fig. 7 presentsthe capillary rise height of each treated fabric layer withdifferent pore sizes. After plasma treatment, the water-absorption time for both sides of the fabrics was reduced andthe capillary rise height of each fabric layer was increased. Itindicates that the atmospheric pressure plasma jet treatment iseffective not only on the first layer but also on several layersbelow the surface. In general the bottom side of the each fabriclayer had a longer water-absorption time and the fabric in thedeeper layer had a lower capillary rise height. The decrease ofwater-absorption time and the increase of capillary rise heightcan be attributed to the increased roughness of the fiber surfacesdue to plasma etching and the introduction of more polar groups

due to plasma chemical modification. This is consistent with theresults of AFM and XPS which showed that the first severallayers (Layers 1 to 5) had the increased surface roughness andincreased number of polar groups while the other three layers(Layers 6 to 8) had mainly surface functionalization.

Increasing pore size results in improved hydrophilicity of thetreated fabric layers and a deeper penetration of hydrophilisa-tion. It can be assumed that the active species in the plasma jetshit the surface of the fibers and reacted with polyestermolecules, making the fabrics more wettable. Obviously, largerpore sizes make the reactive species less likely to collide withthe fibers before they can penetrate through layers of fabrics.

Decreased water-absorption time on both sides of eachplasma-treated fabric layer and increased capillary flow heightsconfirmed that the atmospheric pressure plasma jet treatmenttook place not only on the top fabric sides facing the plasma jet,but also more or less on bottom sides of the fabric layers [36].

4. Conclusion

Penetration of surface modification of multiple layers offabrics exposed to helium/oxygen atmospheric pressure plasmajet was possible. The pore size of the fabric affected plasmatreatment effect on each fabric layer and its penetration depthinto the textile structure. After atmospheric pressure plasma jettreatment, the top side of the first fabric layer of the stackedpolyester samples was substantially more wettable than thecontrol fabric. The plasma treatment effect on the fabric sur-face was gradually reduced as the fabric layer got deeper sincethe extent of plasma modification of other layers was de-pendent on the degree of penetration of chemically activespecies in the plasma jet, which in turn depended on theaverage pore size of the treated fabric. For fabrics with thelarger pores, it should be less likely for the active species tocollide with the fabric and more likely for them to movethrough the pores and to reach subsequent fabric layers. Thesurface modification of atmospheric pressure plasma jet couldpenetrate into at least eight layers of fabrics with an averagepore size of 200 μm and the thickness of 0.25 mm while it wasonly able to diffuse through six layers of the fabrics with anaverage pore size of 100 μm.

Acknowledgements

This study was sponsored by the Program for ChangjiangScholars and Innovative Research Team in University(No. IRT0526) and Shanghai Pujiang Program (No. 06PJ14011).

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