recent approaches to highly hydrophobic textile surfaces

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Recent Approaches to Highly HydrophobicTextile SurfacesThomas Bahners a , Torsten Textor b , Klaus Opwis c & EckhardSchollmeyer da Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798Krefeld, Germany;, Email: [email protected] Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798Krefeld, Germanyc Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798Krefeld, Germanyd Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798Krefeld, Germany

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To cite this article: Thomas Bahners, Torsten Textor, Klaus Opwis & Eckhard Schollmeyer (2008): RecentApproaches to Highly Hydrophobic Textile Surfaces, Journal of Adhesion Science and Technology, 22:3-4,285-309

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Journal of Adhesion Science and Technology 22 (2008) 285–309www.brill.nl/jast

Recent Approaches to Highly Hydrophobic Textile Surfaces

Thomas Bahners ∗, Torsten Textor, Klaus Opwis and Eckhard Schollmeyer

Deutsches Textilforschungszentrum Nord-West e.V., Adlerstr. 1, D-47798 Krefeld, Germany

AbstractA super-hydrophobic character is increasingly required for high-performance technical textiles in order toattain effective liquid repellence, self-cleaning, uni-directional liquid transport, or to create barrier coatingson fiber surfaces. Accordingly, numerous novel approaches to decrease the surface free energy of fibershave been studied in the last years, either employing wet-chemical finishes based on modern chemicaldevelopments such as silane chemistry, nanocomposite structures, or physically applied thin layers. Similarto other branches, textile researchers have also tried to mimic the extreme water repellence of several plantand animal surfaces according to the understanding by Cassie and Baxter.

The scope of this paper is to give a critical overview of the principles, advantages and disadvantages ofseveral concepts.

While leading to high water or even oil repellence, chemical finishes applied using conventional meth-ods — i.e. dipping or padding and ensuing thermal fixation — mostly fail to withstand influences such asmechanical stress — e.g. abrasion, high tensile forces —, climate, aggressive chemical environments, andhigh temperatures, to which technical textiles are subjected to in use. This is true for conventional fluo-rocarbons or novel finishes such as silicones. Here, cross-linked layers of non-polar character prove to besuperior. These can either be obtained by deposition of inorganic–organic nanocomposites, e.g. using thesol–gel technique, or by deposition of thin layers by physical methods. With regard to the effects of micro-rough fiber surfaces, present knowledge indicates an inferior durability due to the destruction of the delicatetopography in use. In natural systems such as plants, this effect is overcome by self-healing mechanismswhich technical products do not possess. Koninklijke Brill NV, Leiden, 2008

KeywordsSuperhydrophobic, textile, fiber, sol–gel, thin layers, microroughness

1. Introduction

In contrast to common clothing and home textiles, so-called technical textiles area growing and important market for high performance products. Technical tex-tiles find application in fields as diverse as medical engineering, aerospace andautomotive industry, modern architecture and construction, filtration, and transport

* To whom correspondence should be addressed. Tel.: +49 (0)2151 843-156; Fax: +49 (0)2151 843-143;e-mail: [email protected]

Koninklijke Brill NV, Leiden, 2008 DOI:10.1163/156856108X295437

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286 T. Bahners et al. / Journal of Adhesion Science and Technology 22 (2008) 285–309

systems. Increasingly super-hydrophobic character is required for these products toattain effective liquid repellence, self-cleaning, uni-directional liquid transport, orto create barrier coatings on fiber surfaces.

Today, the general requirements for effective finishes — hydrophobic or others— include minimum application of auxiliaries, reduction or avoidance of wastewater or gases, and durability. While a finish on clothing and home textiles hasto withstand mainly a certain number of washing/cleaning procedures over theirlifecycle, technical textiles are subjected to influences such as mechanical stress —e.g. abrasion, high tensile forces, climate, aggressive chemical environments, andhigh temperatures, to name a few. Accordingly, a main feature of an effective finishbesides the actual effect has to be its capability to withstand these factors.

As a consequence, numerous approaches to create super-hydrophobic textile sur-faces have been published in the last years from a number of research groups aimingto replace the conventional wet-chemical finish using fluorocarbons. The scope ofthis paper is to give an introduction to several novel approaches to create durablehydrophobic finishes on textile substrates, which make use of developments in mod-ern materials science, which either have been investigated in fundamental researchor are already applied in various industrial branches. Experimental data from recentstudies by the authors are used to detail several of the concepts.

2. The Wetting Behavior of a Droplet on a Planar Surface

The wetting behavior of a surface, namely the spreading of a droplet of a givenliquid, is basically determined by the relation of the interfacial energies betweenthe solid substrate and liquid, γsl, between the substrate and gaseous atmosphere,e.g. vapor, γsv, and between the liquid and atmosphere, γlv. The relation betweenthese quantities and the (static) contact angle �Y of a droplet residing on top of thesurface is described by Young’s equation:

cos�Y = (γsv − γsl)

γlv. (1)

The important factor is the energy loss following the increase in surface area ofthe droplet (i.e. spreading) related to the energy gain following adsorption. Thesystem reaches equilibrium when the total energy has reached a minimum. Detailedtheoretical background can be found in [1, 2].

Besides the reduction of the substrate surface energy through the introductionof non-polar groups, the hydrophobicity can also be increased by increasing thesurface roughness [3–6]. Here, two potential cases have to be considered. If theliquid is able to penetrate the micro-rough structure, e.g. due to shallow surfaceprofile and/or sufficient wetting, a closed contact area between the droplet and thesubstrate is preserved. This is called the ‘Wenzel state’. If the liquid, on the otherhand, sits on top of the surface structure without penetrating the ‘valleys’, air willbe enclosed between the droplet and the substrate and the interface between the

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T. Bahners et al. / Journal of Adhesion Science and Technology 22 (2008) 285–309 287

Figure 1. Behavior of a droplet on a perfectly flat surface (a), on a rough surface according to the‘Wenzel state’ (b) and on a rough surface according to the ‘Cassie–Baxter state’ (c).

liquid and the atmosphere is increased. This is called the ‘Cassie–Baxter state’. Alldiscussed states are sketched in Fig. 1.

In the Wenzel (W) state the topographic properties of the surface are described bya roughness factor r [4], which gives the ratio of the effective area of actual, roughsurface to the ideal flat surface, i.e. r � 1. The apparent contact angle is given by

cos�W = r · cos�Y. (2)

In the case of a flat surface, r will be 1 and, accordingly, �W = �Y. Besides thestatic contact angle, the roughness factor r will geometrically enhance also theso-called contact angle hysteresis according to Wenzel’s model. The contact anglehysteresis is the difference between the advancing angle �A and the receding an-gle �R which are observed in a dynamic measurement, i.e. �� (�� = �A − �R).In the case of a high hysteresis, a droplet will stick to the surface in spite of a highstatic contact angle.

When the roughness factor r exceeds a certain level, the Cassie–Baxter (CB)state is given (cf. [5, 6]). The liquid/air interface increases, while the solid/liquidinterface approaches a minimum. The liquid can only gain very little adsorptionenergy and the increase of surface area (i.e. spreading of the droplet) is hindered forenergetic reasons [6]. In the Cassie–Baxter state the apparent contact angle is givenby

cos�CB = −1 + �s(1 + cos�Y), (3)

where �s is the ratio of the actual liquid–solid interface area to the apparent contactarea (�s � 1). Again, given a perfectly flat surface, i.e. �s = 1, the contact angleis equal to the Young angle, �CB = �Y. In the transition from the Wenzel to theCassie–Baxter (CB) state, the hysteresis starts to decrease due to air trapped beneaththe droplet, finally allowing a complete roll-off of the droplet.

One can easily take from equations (2) and (3) that micro-roughening will inboth states lead to increased hydrophobicity only if the ideal flat surface is alreadyhydrophobic, i.e. if �Y � 90◦.

Surfaces of certain plants — such as the leaf of the Lotus plant — have a surfacetopography with two scales of roughness in the form of a base profile with peak-to-peak distances of the order of several micrometers and a superposed fine structure

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with peak-to-peak distances significantly below one micrometer [7–11]. Given this,the Lotus leaf follows the Cassie–Baxter state as sketched in Fig. 1c. Surfaces withthese characteristics are termed super-repellent because of a complete roll-off ofa liquid droplet without any residues.

3. Peculiarities of Textile Substrates

The characteristic and often complex geometry of textile substrates — significantlydiffering from the more or less planar surfaces discussed before — has notableeffects on the initial wetting behavior of a droplet and the dynamics of wetting aswell as the appropriate method to characterize the wettability of a textile.

A textile fabric may be constructed as a non-woven structure of fibers — usuallyfibers are directly spun on a moving belt and form an irregular web — or in a regulargeometry by weaving or knitting. In the latter cases, the fabric is formed by threads,which might be a single, rather thick, endless fiber — commonly termed monofil,a multifilament yarn made of a number of endless fibers, or a spun yarn made ofa number of short fibers. While monofils have diameters of the order of severalhundred micrometers, fibers may have diameters from less than 1 to 20 µm and —in the case of natural fibers — have rough surfaces. Typical examples are shownin Fig. 2. Accordingly, three factors influence the wetting behavior of a droplet ona textile surface.

(I) Macroscopically, a textile has a coarse, textured surface, which may have sim-ilar effects on the wetting behavior of a sessile droplet as was discussed forthe Wenzel or Cassie–Baxter cases.

(II) The cylindrical geometry of a synthetic fiber has the strange consequencethat a droplet will not spread even along an ideally hydrophilic surface, i.e.�Y → 0 [12, 13]. As has been discussed by de Gennes [12], the main factor isthat on a cylindrical substrate, the solid–liquid interface will always be smaller

Figure 2. SEM micrographs of a woven fabric made of PET monofilaments (left) and of a wovenfabric made of polyamide multifilament yarn (right).

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Figure 3. The wetting of a droplet on a hydrophobic or partially wetting fiber (left) and on an ideallywetting fiber where the water contact angle �Y → 0 (right).

than the liquid–vapor interface, which prohibits the droplets to spread totally.In reality, a thin film of approx. 20 nm thickness will form on the sides of thedroplet as is sketched in Fig. 3. The effect can be observed, e.g., on spiderwebs, where water forms droplets in spite of the hydrophilic fibers.

(III) The capillary system, especially of fabrics made of multifilament yarn, pro-vokes the penetration of a sessile droplet. Especially in the case of substrateswhich are not very hydrophobic, there will not be a real equilibrium!

The wetting behavior of a liquid on a textile substrate has consequences onthe experimental procedures for characterization. While many researchers employcontact angle measurements for the characterization of the wetting properties oftextiles, the value of this method is questionable. It is easily taken from the consid-erations made before that the measurement of the apparent contact angle of a sessiledrop will not give a true measure of the wettability — or even surface energy —of the fiber surfaces. The apparent contact angle will always be determined by thecombined effect of microscopic surface property (i.e. fiber), macroscopic surfacegeometry (fabric), and capillary phenomena. In case of hydrophilic substrates the‘sessile’ droplet will penetrate the porous textile typically in seconds and effec-tively prohibit the measurement. On a hydrophobic surface, the apparent contactangle will always differ from the ‘true’ contact angle on the fiber surface. Capillaryeffects occur even on hydrophobic substrates and compete with evaporation of theliquid.

Given this background, the authors employed in the framework of their studiesmethods common in textile industry, basically relying on the penetration behaviorof the sessile droplet.

A quantitative measurement is based on the measurement of the time elapsed un-til a defined droplet totally penetrates the fabric. In the so-called ‘TEGEWA-test’,the droplet has a volume of 0.05 ml and is dropped from a height of 40 mm ontothe sample. Mostly, an aqueous dyestuff solution is used instead of water for bettervisualization. Often, the spreading geometry is measured also. The determinationof penetration time has considerable experimental errors, if the sample is highly

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Table 1.Test liquids used for the so-called DuPont wetta-bility test

Liquid no. Water content (Vol%)

1 982 953 904 805 706 607 508 40

hydrophilic or highly hydrophobic. In the former case, penetration might be toofast to determine the precise moment of complete spreading. In the latter case, thepenetration is in competition with evaporation of the droplet. In most reported stud-ies, therefore, measurements on hydrophobic samples are stopped after pre-definedmeasurement times, e.g. 15 minutes, and the penetration time is specified as ‘greaterthan 15 min’.

In order to overcome the long duration of a drop penetration time measurement,the wettability, especially of highly hydrophobic textile samples, may be charac-terized following a procedure known as the DuPont test. For the test 8 mixtures ofwater and iso-propanol have to be prepared as detailed in Table 1. Beginning withliquid no. 1, three droplets of the liquid (approx. 2 µl) are applied. In the case that2 of the 3 droplets do not penetrate the textile, one continues with the next liquid.The no. of the liquid which just penetrates the samples gives a grading (between0 and 8). Accordingly, the grading ‘0’ characterizes an extremely hydrophilic sam-ple and ‘8’ a (super)hydrophobic sample.

4. Novel Chemical Finishes of Non-polar Character

Conventionally, fluorocarbons are applied as hydrophobizing agents in a wet-chemical process, i.e. by padding followed by thermal stabilization. While thegeneral performance of these products with regard to water and oil repellence isvery good, disadvantages have to be considered in relation to the durability ofthe effect in use as well as waste water and environmental problems associatedwith the use of long-chain fluorocarbons in the finishing step. The fluorocarbonsmostly employed are based on perfluorooctanoic acid (PFOA) derivatives that areknown to be bio-persistent. Recent developments in the field of wet-chemical fin-ishing processes, predominantly by suppliers of auxiliaries for the textile industry,have concentrated on innovative fluorocarbon products [14, 15] and/or stabilizingprocesses [16] in order to increase the wash fastness of the finish.

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As an interesting alternative to fluorocarbons, silicone-based finishes have beenincreasingly discussed in the last years. Reviews on silicones and their applica-tions on textiles are given for example by Gokulnathan [17] and Menezes et al.[18]. A rather recent paper by Gao and McCarthy [19] reports the exclusive useof the (existing) geometric structure of the fabric accompanied by a hydrophobicsilicone finish. While the finish produces a contact angle of 110◦ on a flat surface,the authors report contact angles of up to 170◦ on a fabric made from poly(ethyleneterephthalate) (PET) microfiber yarn. No further roughening of the fiber surfaceswas incorporated (see also Section 6.1).

Inorganic–organic hybrid polymers deposited on the fiber surface through a sol–gel process combine qualities of ceramics and synthetic polymers and offer an im-mense potential for creative modifications of surface properties with a low technicaleffort at moderate temperatures [20]. Compared to common textile finishing strate-gies, finishing with inorganic–organic hybrid polymers allows to define/modifya variety of properties by an appropriate composition of the sol. The actual coatingprocess can be carried out with a comparatively low technical effort, i.e. by simpledipping or padding (dipping followed by squeezing between two rollers) processeswhich are common techniques in the textile industry. The sol–gel technique can becarried out yielding very thin layers with only a slight increase in weight which isimportant, e.g., for architectural applications where light weight is much desired(cf. Fig. 4 [23, 24]).

Daoud et al. [21] applied transparent and durable superhydrophobic silica coat-ing films on cotton fabrics. The coatings were produced via cohydrolysis andpolycondensation of hexadecyltrimethoxysilane (HDTMS), tetraethyl orthosilicate(TEOS), as well as 3-glycidoxypropyltrimethoxysilane (GPTMS). Water contactangles on the coated substrates were found to be as high as 140◦. Also for the ex-ample of cotton, a superhydrophobic complex coating for cotton fabrics based onsilica nanoparticles and perfluorooctylated quaternary ammonium silane couplingagent (PFSC) was reported by Yu et al. [22]. The water contact angle increased upto 145◦. The oil repellence was also improved.

The application of the sol–gel technique to increase the water repellence oftechnical textiles made of glass fibers as well as synthetic high-performancefibers was reported by Textor et al. (see, e.g. [23, 24]). The inorganic–organichybrid polymers considered by Textor et al. were derived from alkoxysilanesmodified with an additional organic group. This group consists of a hydrocarbonchain with functional epoxy, methacrylic or thiol groups. Typical examples are3-glycidoxypropyltrimethoxysilane (GPTMS), 3-chloropropyltrimethoxysilane,and methacryloxypropyltrimethoxysilane (MPTS). In the presence of certainamounts of water, under both basic or acidic conditions, the alkoxy groups undergohydrolysis and condensation to form a sol, which is then applied to the textile sub-strate. In a following curing step the condensation of the hydrolyzed alkoxysilanesis completed by simultaneous cross-linking of the functional groups [25].

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Figure 4. SEM micrographs of ‘as-received’ polyamide fabric (top) and the same fabric coated witha GPTMS-based hybrid-polymer (bottom) with different magnifications (left and right).

While the hybrid polymer principally forms a ‘coating’ on the textile fibers inthe described concept, it should be noted that additional mechanisms may increasethe coating adhesion to certain polymers. In the case of synthetic polymers, suchas poly(ethylene terephthalate) (PET), these might be modified before applicationof the sol, e.g., by introducing amino groups similar to disperse dying. Also, thephoto-grafting of unsaturated alkoxysilanes was described by Textor et al. [26] asa pre-treatment for polyolefins yielding good adhesion of the hybrid polymers tothese polymers.

The hybrid polymer defines the surface properties of the textile fiber. It is im-portant to note that functionalities can be controlled by the choice and/or modifi-cation of the inorganic and/or organic part of the precursor [23, 24, 27–29]. Theexample shown in Fig. 5 sketches how a highly hydrophobic coating can be cre-ated by the addition of a fluorinated alkoxysilane to a basic sol, e.g., made of3-glycidoxypropyltrimethoxysilane (GPTMS). Extreme water repellence could al-ready be effected by only small concentrations of the fluorinated silane (Fig. 6). Forthe example of technical fabrics made of p-aramide, Textor and co-workers [30, 31]studied the effect of the modification of a basic sol (pre-hydrolyzed alkoxysilane,crosslinking agent, ethanol) with n-propyltrialkoxysilane, n-octyltrialkoxysilane

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Figure 5. Modification of (partially hydrolyzed) GPTMS with 1,1,2,2-tetrahydro-perfluorooctyltriethoxysilane to create highly hydrophobic hybrid polymers.

Figure 6. Influence of the amount of 1,1,2,2-tetrahydroperfluorooctyltriethoxysilane added toa GPTMS-based hybrid polymer on the contact angle of water on a flat surface (PET film).

and a fluorinated silane on water repellence. The experimental data indicate thatextreme water repellence could be achieved (Table 2). Again, very small amountsof the hydrophobizing agents were added. Note that while the DuPont grading stillgives a rating of some sort, drop penetration times were so long that no differentia-tion was possible.

5. Physical Deposition of Thin Layers of Non-polar Character

A number of physical polymer surface modifications have been described in theliterature, examples of which can be found, e.g., in [32, 33]. All these processes are

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Table 2.Water repellence of fabrics made of p-aramide, which were coated with a basic sol (pre-hydrolyzedalkoxysilane, crosslinking agent, ethanol) modified with an additional hydrophobizing agent

Sol modification Drop penetration time DuPont grading*

As-received – 0 0Sol–gel coated 1.6 Vol% n-propyltrialkoxysilane >1 h 4

0.7 Vol% fluorinated silane ∼1 h 89 Vol% n-octyltrialkoxysilane >1 h 3–4“pure” n-propyltrialkoxysilane >1 h 4

* DuPont gradings range from 0 (highly hydrophilic) to 8 (highly hydrophobic).

based on activation of the polymer surface with energetic particles. With respectto achievable surface modification and the lack of disposal of chemicals remainingfrom the process, gas discharge based processes and photo-induced surface mod-ifications seem to offer a great potential. An interesting perspective, with regardto consumption of auxiliaries, waste water or gases, as well as durability, is givenby using these physical processes to deposit functional thin layers. The followingSections cover the developments aiming at this kind of surface modification.

5.1. Surface Modification Using Dielectric Barrier Discharge (DBD)

Gas discharge processes are well known in materials science. The activation of thesubstrate surface by energetic particles initiates radical reactions, which may leadto oxidation, etching, grafting, as well as plasma polymerization. In the last case,the plasma gas needs to contain radicals, which allows cross-linking and depositionof a thin layer.

With regard to achieving a hydrophobic or even superhydrophobic finish, graft-ing or polymerization processes have to be considered. Both process designs havebeen intensively studied in the last years by various research groups. Far from beinga comprehensive overview of the numerous papers, the works of Tsafack et al. [34],Burtovyy et al. [35], Hegemann [36] and Zhang et al. [37] shall be mentioned here.

Tsafack et al. as well as Burtovyy et al. studied the potential of plasma-based grafting of hydrophobic groups onto fabrics made of poly(acrylonitrile)(PAN), polyamide and PET. The plasma pre-treatment is usually done in anair or inert gas plasma. To increase water repellence, they offered substancesas diverse as perfluoroalkyl acrylate, (meth)acrylate phosphates, phosphonates,poly(pentafluorostyrene) (PPFS) to the plasma-activated fabrics. In general, toachieve high water contact angles, the grafting step had to last several minutes atsubstrate temperatures in excess of 100◦C. Given this background, Burtovyy et al.assumed that grafted layers of about 2 to 3 nm thickness were necessary in order todeposit an efficient and durable finish [35].

Hegemann [36] reviewed plasma polymerization processes as a means to achievehydrophobic surface properties of textiles, discussing the possibilities of using hy-

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Figure 7. Definitions and characteristics of technical plasma processes used for material processing.

drocarbon, organosilicon, fluorocarbon process gases. A typical example of thisprocess design based on the use of a fluorocarbon is described by Zhang et al. [37].It should be mentioned here that in contrast to a photo-chemical process as is dis-cussed in Section 5.2, the co-polymerization of the layer on the surface, i.e. withactual bonding between the substrate and the polymerized layer, is in competitionwith homo- or gas-phase polymerization. Due to the bonding to the substrate onlyco-polymerization can be expected to produce durable thin layers.

Gas discharge processes, as exemplified by the papers discussed above, arebroadly classified as sketched in Fig. 7. Low temperature, low pressure plasmaprocesses are widely employed in various industrial applications, but pose prob-lems when applied to large width products such as technical textiles, which maybe as wide as 10 meters. Given this background, Hegemann [36] discussed the per-spectives of atmospheric plasma processes as compared to low pressure plasmasand considered their effectiveness and efficiency.

In general, it can be stated that novel technical (and scientific) approaches forthe treatment of textiles favor more and more the atmospheric pressure plasma.A promising process design is the dielectric barrier discharge (DBD), which re-quires rather simple machinery without the need for low-pressure reactors andoffers a continuous treatment. A recent development has been the use of oxygen-free gases for long-lasting hydrophilic as well as hydrophobic finishes. A study bythe authors investigated the potential of an atmospheric plasma treatment to deposita low-energy thin layer by plasma polymerization from fluorocarbon process gaseson textile substrates in order to establish high water as well as oil repellence [38,39]. The results shall be discussed in some detail here.

A major factor in the process design is the choice of the process gas. Here, theF/C-ratio is a prime parameter, as the occurrence of CF2-radicals in the plasma isstrongly dependent on this parameter [40] (Fig. 8). Mainly CF2-radicals promotepolymerization. In addition, since competitive reactions with oxygen may workagainst the growth of a hydrophobic thin layer, appropriate measures have to betaken to avoid diffusion of air and water and/or their transport into the reactor by the

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Figure 8. Influence of F/C ratio of a fluorocarbon process gas on the occurrence of CF2- andCF3-radicals in the plasma [40]. According to their spectroscopic properties, CF2-radicals can best bedetected in the plasma by UV absorption, while CF3-radicals are detected by UV emission.

porous textile. With regard to the latter, it is also well known that certain materials,e.g. polyamides or cellulose, have rather high water content. Thus, the laboratoryexperiments reported here were performed in a closed reactor — although at at-mospheric pressure. It should be noted here that atmospheric plasma treatments ofmoving textile fabrics were performed in ‘open’ reactors with an effective oxygencontent of less than 0.5% (cf. Bahners et al. [39]).

X-ray photoelectron spectroscopy (XPS) analysis of a large number of sampleswhich were treated under variations of the plasma parameters, i.e. process gas —e.g. C3HF7 or perfluorocyclobutane (c−C4F8) —, mixture of plasma and carriergas, and duration of the treatment, revealed that there was a correlation betweenthe wettability and the surface content of carbon-bonded oxygen, i.e. C−O andC−OOR (Fig. 9). Depending on the process gas, the water repellence characterizedby the DuPont grading could be increased from 2 up to 7 in the case of PET fabricsand from 3 to 6 in the case of fabrics made of PA (superhydrophobic samples wouldbe characterized by a grading of 8).

The experiments showed that a general problem was the low resistance of thethin layers against mechanical stress (abrasion) and extraction. Two reasons wereconsidered by the authors [39]: (1) all processes based on plasma polymerizationinclude (desired) co-polymerization — i.e. covalent bonding — as well as compet-itive homo-polymerization in the gas phase, where the latter will not promote highthin layer adhesion; (2) in case of technical samples, residual finishes with low ad-hesion might cover the surface and prohibit covalent bonding. The latter assumption(2) was extensively investigated. XPS data showed that the surface chemistry of the‘as-received’ sample clearly deviated from the stoichiometry of PET, indicating thepresence of finishing agents from the production process.

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Figure 9. Correlation of the change in wettability of monofilfabrics made of PET (O) and PA (2) andthe change in carbon-bonded oxygen content (sum of C−O and C−OOR) following plasma treatmentunder various conditions. Reference to each sample is the untreated counterpart. The wettability ischaracterized by the DuPont grading; carbon-bonded oxygen content was determined by XPS.

Table 3.The effect of thermal treatment at 200◦C on the water repellence of (a) plasma treated PET fabricsand (b) plasma treated PET fabrics, which were subjected to intense washing. The water repellence ischaracterized by the DuPont grading*

Duration of thermal (a) Immediately after (b) Following intensivetreatment (min) the plasma treatment washing

– 5 4–51 6–7 42 8 44 8 5–66 8 58 8 5

10 8 5

* DuPont gradings range from 0 (highly hydrophilic) to 8 (highly hydrophobic).

It is known from conventional (wet-chemical) fluorocarbon finishes that thermalpost-treatments can enhance the effect of the finish. In view of the problems en-countered with respect to effecting durability, similar post-treatments of the plasmatreated samples were studied by Bahners et al. [38]. As is shown by the data givenin Table 3, the water repellence could actually be enhanced by a thermo-settingprocess immediately after the plasma treatment. Effectively, super-hydrophobicproperties are created. At the same time, the effect of the plasma treatment, which

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decreased, e.g., after intensive washing or extraction, could be recovered by thethermal treatment. Similar effects were reported by Zhang et al. [37].

It should be added here that the samples treated by the fluorocarbon plasmashowed a high oil repellence as well, with a linear correlation between the resultantoil repellence of the samples and surface fluorine content which was determined byXPS. In the case of samples with high oil repellence, the XPS spectra also showedCF2- and CF3-signals. The best effects were achieved when c−C4F8 was employed.

5.2. Surface Modification by Irradiation With Monochromatic UV Excimer Lamps

The irradiation of a fibrous polymer using UV lamps can effect photochemical sur-face modifications, if the photons are sufficiently absorbed. Given the high photonenergies at wavelengths below 250 nm and the usually high absorption of UV pho-tons by the relevant polymers, bond breakage occurs in the outermost surface layerof fibers and radicals are generated with a rather high quantum yield. If performedin ambient air, the irradiation will effectively lead to photo-oxidation and increasedhydrophilicity [41, 42].

Similar to plasma-based processes, this process can lead to more radical surfacemodifications, especially of hydrophobic character, if the irradiation is performedin reactive atmospheres other than air [43–51] (see also [26, 39]). In general, thephotochemical process can be regarded as an activation of the interface of a bi-layersystem (Fig. 10). The condition to initiate such reactions is a marked difference inthe absorbances of the reactive atmosphere (low or non-absorbing) and the substrate(strongly absorbing).

The use of monochromatic lamps has the advantage of not inducing cross-linkingin the atmosphere itself, as it is non-absorbing. Radical bonding and cross-linkingmay then be initiated by activation of the (highly absorbing) polymer surface. Inaddition, the use of a monochromatic light source allows to optimize the processwith regard to the absorption properties of the fiber polymer. As a typical example,aromatic polymers such as PET absorb wavelengths of the order of 230 nm as wellas below 200 nm strongly with absorption coefficients of the order of 104 cm−1.Choosing a UV source emitting in these spectral ranges — e.g. a KrCl∗ excimerlamp emitting at 222 nm — will markedly increase the quantum yield of radicalgeneration.

Figure 10. Photo-chemically induced generation of a cross-linked layer on the surface of a stronglyabsorbing substrate. The reactive ‘atmosphere’ does not absorb the ultraviolet radiation, which thusreaches and activates the substrate initiating radical cross-linking at the interface.

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Table 4.Possible reactions following UV lamp irradiation

Reaction type Reaction scheme Effect

I Recombination A∗1 + ∗A1 → A1–A1 None

II Reaction with radical(s) of A∗1 + ∗A2 → A1–A2 Cross-linking

a neighboring chainIII Reaction with reactive A∗

1 + Z → A1–Z∗ Additionatmosphere

IV Reaction with A∗1 + Z → A1–Z∗ Cross-linking, thin

a bi-functional material A1–Z∗ + ∗A2 → A1–Z–A2 layer deposition

Table 5.Advancing contact angles of water on PET film irradiated in presence of reactive atmospheres (contactangle on the untreated film was 76◦)

Reactive atmosphere Irradiation time (min) Contact angle (◦)

1,5-hexadiene (gaseous) 5 9115 92

Perfluoro-4-methylpent-2-ene (gaseous) 10 116Silicon tetrahydride (SiH4) in N2 (gaseous) 10 96

Basically, four different types of reactions are possible (Table 4): (I) recombi-nation of radicals, (II) cross-linking of polymer chains, (III) addition of radicalsfrom the reactive atmosphere, and (IV) addition of bi-functional molecules withensuing cross-linking between the functional groups, which can induce thin layerdeposition.

In a number of papers, the authors studied fundamental effects of the photochem-ical process designed for an increase of hydrophobicity [39, 48, 49]. By choosingthe appropriate substance serving as the reactive atmosphere during irradiation, thewater contact angle could be changed significantly, as shown in Table 5 for treatedPET films, and in Table 6 for various high-performance polymers. The FT-IRspectrum (PAS) of the 1,5-hexadiene treated PET sample showed new hydrocar-bon peaks at 2927 and 2854 cm−1 caused by aliphatic C–H stretching vibrations(Fig. 11). There was no evidence of unsaturated carbon bonds at wavenumbersaround 1600 cm−1.

While the hydrophobic modification was extremely resistant against mechani-cal strain, e.g. abrasion, in all stated examples, it was only the modification dueto irradiation under gaseous 1,5-hexadiene which showed a remarkable resistanceagainst chemical attacks. The achieved wetting behavior of the modified polymerswas maintained even after immersing the material in concentrated sodium hydrox-ide for several days. The modification achieved cannot be removed by wiping andprotects the PET against concentrated sodium hydroxide solution. After 72 h stor-

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Table 6.Advancing contact angles of water (◦) on films made of several polymers which were irradiated in thepresence of 1,5-hexadiene, 1-hexene, and cyclohexane (gaseous)

Polymer Untreated After irradiation in the presence of

1,5-hexadiene 1-hexene cyclohexane

Meta-aramide 65 >90 89 89Poly(etherimide) (PEI) 70 95 92 Not measuredPoly(etheretherketone) (PEEK) 75 95 93 Not measured

Figure 11. FT-IR spectra (PAS) of PET film untreated and after irradiation in 1,5-hexadiene. Arrowsindicate bands related to aliphatic C−H stretching vibrations.

age in concentrated sodium hydroxide solution, the PET film becomes slightly dulland the contact angle of distilled water on this surface decreases from 76◦ to 53◦,while after applying the polyolefinic layer it stays clear and the contact angle doesnot drop below 66◦.

Unlike in gas-discharge processes (plasma treatments), the ‘atmosphere’ in thephoto-chemical process might be gaseous, a liquid, or even a melt. As exam-ples of photochemical reaction type III (cf. Table 4), irradiations of various filmswere performed in the presence of (bi-functional) 1,5-hexadiene and perfluoro-4-methylpent-2-ene in gaseous as well as liquid forms. From XPS analysis it couldbe shown that in both forms of atmosphere, the reactive substances were bound tothe substrate surface after irradiation (Fig. 12). The spectra of PET films irradiatedin the presence of 1,5-hexadiene showed an increase in the signal at the bindingenergy 285 eV (C−H and C−C bonds), which was up to nearly 70% in the case

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Figure 12. Comparative XPS studies of PET surfaces after irradiation (10 min) in 1,5-hexadiene andperfluoro-4-methyl-pent-2-ene atmospheres (in both liquid and gaseous forms).

of the liquid medium. In case of perfluoro-4-methylpent-2-ene as the medium, theXPS analysis revealed signals related to fluorine (690 eV) and to −CF3 (294 eV)carbons. The F/C-ratio increased to 0.30 when using the liquid medium. The com-parison of the spectra recorded from PET films which were irradiated in liquid andgaseous atmospheres of perfluoro-4-methylpent-2-ene and 1,5-hexadiene showedthat the surface modification was stronger in case of a liquid atmosphere.

Recent experiments by the authors studied the water repellence of PET fabrics(technical fabrics), photochemically treated in the presence of, e.g., 1,5-hexadiene,1,7-octadiene, diallylphthalate (DAP) and 1H ,1H ,2H ,2H -perfluorodecyl acrylate(PFDA). Exemplary experimental data are summarized in Fig. 13 showing droppenetration times in excess of 1 hour (measurements were stopped after this time)and DuPont grading of up to 8. The relevant values for the untreated fabrics weredrop penetration time approx. 20 s and a DuPont grading 0. Based on the well-known effect of heat treatments on long-chain fluoro compounds (cf. Sections4 and 5.1), the samples treated with PFDA were also characterized following a fur-ther heat treatment. As was found in the case of wet-chemical finishes and plasma-deposited fluorocarbon thin layers, the water repellence of the samples could befurther enhanced by heat treatment in this case also.

6. Introducing Micro-roughness

6.1. Introducing Micro-roughness to Fibers

Given the background for the self-cleaning properties known for the micro-rough and highly hydrophobic surfaces of a number of plants and animals (see

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(a)

(b)

Figure 13. Water repellence of photochemically treated PET fabrics (technical fabrics) characterizedby the drop penetration time (a) and the DuPont grading (b). The fabrics were irradiated in the presenceof diallylphthalate (DAP) and 1H ,1H ,2H ,2H -perfluorodecyl acrylate (PFDA) with varying irradia-tion time. Samples treated with PFDA were also characterized following a further heat treatment. Therelated values of the untreated fabrics were drop penetration time approx. 20 s and DuPont grading 0,respectively.

Refs [4–11]), different approaches to the creation of super-hydrophobic surfaces ontextiles have been discussed.

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In principle, the studies by Gao and McCarthy as well as Textor et al. (Refs[19] and [23, 24]) have shown that the underlying texture of the textile alreadyenhances the water repellence. Nevertheless, a number of research groups have putlarge efforts into creating a multi-scaled topography in order to mimic surfaces asfound on the leaf of the Lotus plant, watercress as well as various chafers, to givea few examples.

One favored concept today is the deposition of nano-sized particles dispersedin a hydrophobic binder [52–56]. While the hydrophobic binder imparts basic hy-drophobicity to the fiber surfaces, i.e. �Y � 90◦, the inclusion of particles createsa multi-scaled surface topography in combination with the texture of yarn and fab-ric.

In general, the concept has been shown to create super-hydrophobic textile sur-faces. Ramaratnam et al. [55] have shown that artificial ‘Lotus leaf’ structures canbe fabricated on cotton substrates via the controlled assembly of carbon nanotubes.Water contact angles greater than 150◦ were measured. Liu et al. [56] used thecombination of a polystyrene grafted layer as low surface energy component andnanoparticles. Present knowledge, however, indicates an inferior durability due tothe destruction of the delicate topography in use. In natural systems such as plants,this effect is overcome by self-healing mechanisms not available on the technicalproducts.

Another way to create a micro-roughness, proposed by the authors in the 1980s,is the irradiation of fiber surfaces with pulsed, highly absorbed laser light. Funda-mental work has shown that a characteristic modification of the surface topographyof highly absorbing and oriented synthetic fibers such as PET, PA, or aromaticpolyamides is observed after irradiation with pulsed UV excimer lasers [57–62].The originally smooth surface of these fibers changes to a rather regular roll-likestructure on the micrometer scale, perpendicular to the fiber axis, after irradiation.Typical peak-to-peak roughness, Sy, is of the order of 2 to 3 µm (Fig. 14).

The micro-rough fiber surfaces can be expected to affect the basic wettabilityof the fiber. Experimental results by Bahners and co-workers [49, 63] have shownthat this treatment, especially in combination with a suitable hydrophobic finish,creates effects that are in agreement with the fundamental considerations by Wenzeland Cassie–Baxter. It is interesting to note that Bahners and co-workers employeda photochemical surface modification as described in Section 5.2 after the lasertreatment to avoid effects on surface chemistry by laser ablation. The exemplarydata given in Table 7 indicate the potential of a treatment combining the aboveprocesses for the creation of highly water- and/or oil-repellent PET fabrics. Similarexperiments using a multifilament fabric made of p-aramide show the effects ofthe complex, capillary geometry of textile: An applied droplet will stand on thefabric for several minutes before penetrating it spontaneously. At no time the dropletspreads on the surface [63].

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Figure 14. PET monofilaments ‘as received’ (left) and UV laser treated (right). The laser irradiationwas done using a pulsed KrF excimer laser emitting at 248 nm (10 pulses with a pulse energy of60 mJ/cm2).

Table 7.Penetration time of a water droplet into a PET fabric following combined laser and photochemi-cal treatments

Untreated Laser treated (10 pulses of90 mJ/cm2 each) on

One side Both sides

No lamp treatment 50 s 3 min 3 minUV excimer lamp irradiation (222 nm) 20 min 43 min No penetrationin an atmosphere ofperfluoro-4-methylpent-2-ene (5 min)

6.2. Coated Fabrics

Many technical textiles are finished with a heavy polymeric coating of, e.g.,poly(vinyl chloride) (PVC), polyurethane, or silicone, which effectively masks thetextile fabric with a smooth surface. One exemplary application of coated textiles,where easy or self-attained cleanability is highly required, is the construction oftextile roofs in modern architecture. The present solution to decrease dirt take-up isto apply hydrophobic topcoats, i.e. layers of lacquer of approx. 5 µm thickness, tothe coated fabrics. However, the performance of these conventional topcoats withregard to cleanability is not sufficient. The approaches to the creation of super-hydrophobic surfaces by means of micro-rough surfaces as discussed in Section 6.1refer to the texture of textile fabrics as well as the actual fiber surfaces, and are notnecessarily applicable to coated textiles.

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A further important aspect is that the textiles considered here are subject tohigh, especially mechanical, stress. Textile roofs face not only weathering — UVaging, hail, etc. —, and high tensile forces, but also improper handling on the con-struction site. Micro-structured surfaces obtained by techniques such as, e.g., theapplication of nano-sized particles [51–56] or laser treatment [49, 63], but also bymicro-lithography are not likely to withstand these influences effectively.

In a recent study [64], the potential of hydrophobic topcoats exhibiting a micro-rough topography was studied. The topcoats were based on UV curable, mainlyacrylated, lacquers applied using a novel two-step curing process. In this processUV irradiation with a wavelength of 126 or 172 nm is used for the cross-linkingof an ultra-thin surface layer, while the bulk of the lacquer is cured subsequentlyat longer wavelengths. Following shrinkage processes a rough surface is obtained(‘micro-folding’) [65]. Typical examples of the surface topographies achieved bythis curing process are shown in Fig. 15.

The experimental lacquers (smooth surfaces) had water contact angles on theorder of 100 to 115◦ and rather good scratch resistance. In none of the studiedsystems, however, did the micro-structuring of the surfaces by UV induced micro-folding lead to the envisaged increase in hydrophobicity. The reported SEM analy-ses reveal that the surfaces of samples pre-cured with 126 or 172 nm typically havetopographies with peak-to-peak distances Sy of 10 to 20 µm, but rather small aspectratio. Accordingly, a Cassie–Baxter state of a roughness-dependent contact anglewas not achieved in any case. It is worth noting that Prager et al. [64] extensivelystudied the dirt take-up behavior of the experimental topcoats, also. Assuming typ-ical fields of applications of outdoor products — e.g. roofs, tarpaulins, blinds, etc.—, the measurements concentrated on dirt typical for industrial and urban areas.In comparison to conventional systems — e.g. fluorine containing lacquers —, allexperimental topcoats had a reduced take-up of oil- and pigment-containing dirt.No correlation was found between the dirt take-up and the water contact angle onthe samples, however. It can be assumed from these findings that the mechanismsresponsible for the self-cleaning of certain plant and animal surfaces is not neces-sarily valid for dirt as found in industrial areas with potentially different adhesionto surfaces, take-up by a water droplet, etc.

7. Summary

Technical textiles are a growing and important market for high performance prod-ucts. Depending on their application in fields as diverse as medical engineering,automotive industry, or filtration, a super-hydrophobic character is required to at-tain effective liquid repellence, self-cleaning, uni-directional liquid transport, orto create barrier coatings on fiber surfaces. Numerous approaches to create super-hydrophobic textile surfaces, which make use of developments in modern materialsscience, which either have been investigated in fundamental research or are alreadyapplied in various industrial branches, have been published in the last years from

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Figure 15. SEM micrographs of UV cured surfaces which have been produced from a lacquer coatingof given chemistry (mixture of acrylated mono- and bi-functional monomers and aliphatic urethaneacrylates, filled with nano-sized Aerosil particles) with one-step or two-step curing procedures [65].

a number of research groups. The scope of this paper was to give an introductionto these approaches and discuss their potential using examples from various recentstudies.

Developments in the field of wet-chemical finishing processes, predominantlyby suppliers of auxiliaries for the textile industry, have concentrated on innov-ative short-chain fluorocarbon products. Also silicone-based finishes have beenincreasingly discussed in the last years as an interesting alternative to fluorocar-bons. Inorganic–organic hybrid polymers deposited in very thin layers on the fibersurface through a sol–gel process combine qualities of ceramics and synthetic poly-mers and offer an immense potential for creative modifications of surface properties

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with a low technical effort at moderate temperatures. The actual coating process canbe carried out with a comparatively low technical effort, i.e. by simple dipping orpadding processes which are common techniques in the textile industry. Mostly,sol–gel coatings are derived from alkoxysilanes modified with additional organicgroups. Water contact angles up to 145◦ are reported using this technique. Highlyhydrophobic coatings can be created by adding a fluorinated alkoxysilane in con-centrations as small as 1 Vol%.

An interesting perspective, with regard to consumption of auxiliaries, waste wa-ter or gases, as well as durability, is given by using physical processes, such asplasma-polymerization or photo-chemical treatments, to deposit functional thin lay-ers. Plasma polymerization processes — low-pressure as well as atmospheric pres-sure — using hydrocarbon, organosilicon, fluorocarbon process gases have beeninvestigated widely as a means to achieve hydrophobic surface properties of tex-tiles. In general, it can be stated that novel technical (and scientific) approachesfor the treatment of textiles favor more and more the atmospheric pressure plasma.High water and oil repellence has been reported by a number of researchers, butthe often low resistance of the deposited thin layers, e.g. against washing, remainsa general problem. It has been shown, however, that thermal post-treatments couldbe employed to enhance durability. Similar to plasma-based processes, a photo-chemical treatment performed in the presence of reactive atmospheres other thanair, such as 1,5-hexadiene, 1,7-octadiene, or diallylphthalate, can lead to radicalsurface modifications, especially of hydrophobic character. With most reported re-active substances, the hydrophobic modification was extremely resistant againstmechanical strain, e.g. abrasion, as well as against chemical attacks.

While strong water repellence — characterized by water contact angles or droppenetration — was achieved in these chemical or physical modification techniques,but no definite statement concerning super-hydrophobic properties can be made.Most papers did not state roll-off angles or contact angle hysteresis.

Several research groups have reported modifications that aim at the generationof multi-dimensional fractal surfaces, comparable to well-reported plant surfaces.Summarizing, it can be stated that a Cassie–Baxter wetting behavior was achievedonly by deposition of nano-sized particles on the textured surfaces of fibers, yarnsand fabrics. Water contact angles of the order of 170◦ were reported in examples. Ithas to be said however that — in absence of self-healing mechanisms as in plant andanimal surfaces — the permanence of the particles on the textile surface is a criticalfactor with regard to durability. Alternative approaches to generate structured, i.e.fractal, surfaces using physical treatments have not led to Cassie–Baxter wettingbehavior up to now.

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