Chapter 1: Improved textile functionality through surface modifications – Functional Textiles for Improved Performance, Protection and Health

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Improved textile functionality through surface modifications

R. Morent and N. De Geyter,     Ghent University, Belgium

Abstract:

This chapter discusses some important surface modification techniques employed for improved functional behavior of textiles. Through technological innovations, the textile industry has experienced rapid development with miscellaneous new applications. Some of these techniques are briefly introduced. The behavior of these functional textiles is strongly governed by the chemical and physical characteristics of their surface. Progress in understanding this functional behavior is only possible when these characteristics are known and therefore surface analysis is a crucial and necessary step. As a result, advanced surface characterization techniques are used and developed. The most important techniques are described in this chapter. To conclude this chapter, some applications including biomedical applications, antistatic behavior, and water and oil repellency are discussed.

Key words

surface modification

PVD

CVD

plasma

electroless deposition

grafting

sol-gel

XPS

FT-IR

contact angle

SEM

AFM

1.1 Introduction

This chapter will provide an introductory review of improved functionality of textiles through surface modifications. The focus will be on the surface modification techniques and the appropriate surface characterization techniques.

Section 1.2 will give a brief overview of the most commonly used surface modification techniques for functionalization of textiles. Taking into account the scope of this chapter, this overview is restricted to the techniques most commonly described in the literature, and moreover it is not our goal to discuss these techniques in great detail. A more detailed overview of surface modification techniques in relation to textiles can be found in Wei (2009). Section 1.3 presents the most widely used characterization techniques. Section 1.4 gives different examples of functionalized textiles achieved with the described surface modification techniques.

1.2 Types of surface modification

1.2.1 Vapor deposition

Physical vapor deposition

Physical vapor deposition (PVD) is a coating process and is an umbrella term used to describe a variety of methods to deposit thin films by the condensation of a vaporized form of a solid material onto various substrates (Mattox, 2010). A typical PVD process is carried out in vacuum. The most important steps involved in a PVD process are presented in Fig. 1.1.

1.1 Schematic representation of a PVD process.

• Evaporation: a target, typically a metal, consisting of the material to be deposited is bombarded by a high-energy source (beam of electrons or ions). Atoms are dislodged from the target surface, so they are vaporized.

• Transport: the vaporized atoms travel from the target to the substrate to be coated. In some applications, coatings will consist of, for example, metal oxides, nitrides or carbides. In such cases, the metal atoms evaporated from the target will then react with the appropriate gas during the transport stage. For the above examples, the reactive gases may be oxygen, nitrogen and methane. However, when the coating only consists of the target material, these reaction processes during transport will not occur.

• Condensation: the coating builds up at the surface of the substrate. This is the deposition of the coating. Depending on the actual process, some reactions between the target material and the reactive gases may also take place at the substrate surface simultaneously with the deposition process.

Chemical vapor deposition

Chemical vapor deposition (CVD) is a generic name for a group of processes that involve reactions which transform gaseous molecules, called precursors, into a solid material in the form of a thin film or powder on the surface of a substrate. It is similar in some respects to PVD. The main difference with CVD is that the precursors are solid in PVD, with the material to be deposited being vaporized from a solid target and deposited onto the substrate. Figure 1.2 gives a schematic representation of the CVD process. In a typical CVD process, the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Different types of CVD reactors have already been developed and are in wide use (Hitchman and Jensen, 1993). One important difference between them is the way in which energy is delivered to the reactor: thermal energy, photo energy, and so on. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber. In most CVD techniques, the temperature of the substrate is a critical issue. Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which is deposited onto the substrate. Therefore, the substrate temperature is critical and can influence what reactions will take place.

1.2 Schematic representation of the CVD process.

The process steps involved in CVD can be summarized as follows:

• Transport of precursor molecules into the reactor.

• Diffusion of precursor molecules to the surface.

• Adsorption of precursor molecules to the surface.

• Reactions at the surface: decomposition of precursor molecules on the surface and incorporation into a solid coating.

• Recombination of molecular by-products and desorption into the gas phase.

1.2.2 Wet surface modification techniques

Many surface modification techniques are applied in a wet environment. Despite the possible ecological drawbacks (e.g. water contamination), these wet-chemical techniques are still frequently used in industry for surface modification of textiles. Moreover, the majority of textile surface modifications are achieved using aqueous solutions. There are two types of wet surface modifications:

• Physical interaction with the fibers: typically fiber impurities are removed that resulted from the production process of the fiber or fabric. Desizing of cotton is a common example of such a physical interaction (Cai et al., 2003). Such processes will not be discussed in this chapter.

• Chemical interaction with the fibers: new functional groups will be introduced on the surface of the fibers to change the properties of the fiber or fabric.

Although some authors (Wang and Liu, 2009) also consider the deposition of a material as a physical interaction, in most cases the deposited layer will chemically interact with the substrate. If not, the deposited layer will not adhere well to the substrate.

In the following paragraphs, some of the most common wet-chemical surface modification techniques are discussed.

Grafting

There are two possible methods for the creation of grafted surfaces (Uyama et al., 1998):

• Direct coupling reaction of polymer molecules to the surface: this is only possible if the polymer surface to be modified possesses reactive groups capable of bonding with other components.

• Graft polymerization of monomers to the surface: an effective grafting of molecules onto the surface of textiles needs the creation of free radical sites within the macromolecules of the substrate. These free radical sites are used as initiators for copolymerization reactions with monomers from a grafting solution. Possible techniques to introduce free radicals are chemical graft polymerization, oxidizing agents, high-energy radiation or plasma-induced grafting (Abidi, 2009 ).

Electroless deposition

Textiles are usually non-conductive and therefore cannot be immersed in a plating solution to be coated in the way that metallic materials can. Electroless deposition or electroless plating of copper and nickel (Mallory and Hajdu, 1990) is a chemical reduction process which depends upon the catalytic reduction process of metal ions in an aqueous solution and the subsequent deposition of metal without the use of electrical energy. The aqueous solution contains a chemical reducing agent that is the driving force for the reduction of metal ions and their deposition. This driving potential is essentially constant at all points of the surface of the component. Therefore, electroless deposited coatings are very uniform in thickness and are interesting when used for objects with irregular shapes.

Sol-gel

A sol is a colloidal suspension of solid particles in a liquid medium. This liquid is typically an alcohol. Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloidal suspension. In the sol-gel process, the sol gradually evolves towards the formation of a wet gel-like network containing both a liquid phase and a solid phase. Curing or drying of the gel leads to a so-called xerogel after the solvent is completely removed (Brinker and Sherer, 1990 ).

1.2.3 Plasma technology: a versatile technique

The pre-treatment and finishing of textiles by non-thermal plasma technologies has become more and more popular as a surface modification technique (Morent et al., 2008; Shishoo, 2007). It offers numerous advantages over conventional processes. Plasma surface modification does not require the use of water, resulting in a more economical and ecologically friendly process. The enormous advantage of plasma processes is the drastic reduction in pollutants and a corresponding cost reduction for effluent treatment. Therefore, it can be considered as an environmentally benign technology. A non-thermal (or cold or low temperature) plasma is a partially ionized gas with electron temperatures much higher than ion temperatures. The high-energy electrons and low-energy molecular species can initiate reactions in the plasma volume without excessive heat causing substrate degradation. Non-thermal plasmas are particularly suited to apply to textile processing because most textile materials are heat sensitive polymers. In addition, it is a versatile technique, where a large variety of chemically active functional groups can be incorporated into the textile surface. Moreover, the versatility of plasma technology is shown by the fact that it is used in combination with or as part of other techniques such as magnetron sputtering PVD, plasma enhanced CVD and plasma-induced grafting.

1.3 Physical and chemical characterization of surface modifications

1.3.1 Microscopic techniques

Resolution

The resolving power or resolution of a microscope is determined by the smallest distance between two points on a surface that can be detected separately. At the lower limit of resolution, images formed by electromagnetic radiation or electrons are limited by diffraction. The resolution of two points is rigorously determined by the Rayleigh criterion and the resolution distance l (m) is given by Roth (2001) as:

[1.1]

where λ is the wavelength of the probing radiation, n is the refractive index of the medium in which the lens operates and φ is the angle subtended at the surface under observation by the edges of the last lens. The quantity n sin(φ/2) is the numerical aperture and may be as much as 1.60 for optical oil-immersion microscopes. In material science, measurements are mostly performed in air and, in that case n ≈ 1.00 ⇒ nsin(φ/2) < 1, the resolution distance l (m) is never less than:

[1.2]

An image of a non-woven PET obtained with an optical microscope can be seen in Fig. 1.3.

1.3 Microscopic image of a polyester non-woven.

Scanning electron microscopy

Scanning electron microscopy (SEM) was developed in the 1930s (Knoll, 1935) and brought a different way of imaging solids. As stated in the previous section, a modern optical microscope has a magnification of about 1000 × and enables the eye to resolve objects separated by 0.275 μm. In the continuous struggle for better resolution, it was found that the resolution of the optical microscope was not only limited by the number and the quality of the lenses, but also by the wavelength of the light used for illumination. In the 1920s, it was discovered that accelerated electrons in vacuum behave like light. Furthermore, it was found that electric and magnetic fields have the same effect on electrons as glass lenses and mirrors have on visible light. These characteristics resulted in the first electron microscope, which images the sample surface by scanning it with a high-energy beam of electrons. When electrons strike the specimen, several phenomena occur:

• emission of secondary electrons by inelastic scattering

• reflection of high-energy electrons by elastic scattering (backscattered electrons)

• absorption of electrons by the sample

• emission of X-rays by the sample

• emission of photons by the sample

All these phenomena are interrelated and all of them depend to some extent on the topography, the atomic number and the chemical state of the specimen. The number of backscattered electrons, secondary electrons and absorbed electrons at each point of the specimen depends on the specimen’s topography to a much greater extent than the other phenomena mentioned. Therefore, in SEM, the backscattered, secondary and absorbed electrons will be detected to produce the image.

Many specimens can be examined using SEM without preparation of any kind. Non-conducting specimens (such as polymers) will charge up under electron bombardment and need to be coated with a conducting layer. Because a heavy element like gold also gives a good yield of secondary electrons and thereby a good quality image, this is the favorite element for coating. In addition, it gives a fine grain coating and is easily applied in a sputter coater. The layer required to ensure a conducting layer is quite thin (about 10 nm).

The resolution of SEM can be calculated as follows: the electrons behave as a wave phenomenon with the corresponding energy-dependent de Broglie wavelength λdB (m) given by:

[1.3]

with h (J.s) the Plank constant and p the linear momentum of the electron (kg.m.s−1). If E′ is the kinetic electron energy determined by the acceleration voltage of the SEM, then equation [1.3] becomes

[1.4]

with me (kg) the electron mass. Substituting equation [1.4] into equation [1.1] yields the resolution distance l (m):

[1.5]

Assuming a SEM typical electron energy E′ of 200 keV (1 eV = 1.602 × 10−19 J) and if the numerical aperture n sin(φ/2) ≈ 1, then equation [1.5] suggests that l ≈ 1.67 pm = 0.00167 nm. This is one hundred times smaller than atomic dimensions. In practice, it will not be possible to form and focus electron beams to such small dimensions, since in real experiments the numerical aperture is smaller than 1. Depending on the instrument, the resolution can fall somewhere between less than 1 and 20 nm. Figure 1.4 shows a typical SEM picture of a polypropylene (PP) non-woven.

1.4 SEM image of a PP non-woven.

Atomic force microscopy

The previously discussed microscopes create a magnified image of an object by focusing electromagnetic radiation, such as photons or electrons, on its surface. Optical and electron microscopes can easily generate two-dimensional magnified images of an object’s surface, with a magnification as great as 1000 × for an optical microscope, and as large as 100 000 × for an electron microscope. Although these microscopes are powerful tools, the obtained images are typically in the plane horizontal to the surface of the object. Such microscopes do not readily supply the vertical dimensions of an object’s surface, the height and depth of the surface features.

Unlike traditional microscopes, atomic force microscopy (AFM) does not rely on electromagnetic radiation such as photon or electron beams to create an image. An atomic force microscope is a mechanical imaging instrument that measures the three-dimensional topography as well as physical properties of a surface with a sharpened probe. This probe is typically less than 50 nm in diameter and is placed at the end of a microscale cantilever, typically made out of Si3N4. The probe is positioned close enough to the surface such that it can interact with the force fields associated with the surface. Then the probe is scanned across the surface such that the forces between the probe remain constant. An image of the surface is then reconstructed by monitoring the precise motion of the probe as it is scanned over the surface. Typically the probe is scanned in a raster-like pattern.

AFM has several advantages compared to SEM: unlike the scanning electron microscope, which provides a two-dimensional image of a sample, AFM provides a three-dimensional surface profile. Moreover, an electron microscope requires a vacuum environment for proper operation, while an atomic force microscope can work perfectly in ambient air. In addition, AFM can provide a higher resolution compared to SEM, since typical AFM resolutions are much lower than 1 nm. Imaging of individual atoms has been possible, although in most applications this level of resolution is not required.

The development of the scanning tunneling microscope in 1981 earned Binnig and Rohrer (1986) the Nobel prize for physics in 1986.

1.3.2 Spectroscopic techniques

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a very powerful surface analysis technique in which chemical states can be determined in near surface regions (Flewitt and Wild, 1994). It is also known as electron spectroscopy for chemical analysis (ESCA). This technique provides a quantitative analysis of the atomic surface composition and can detect different types of chemical bonds at the surface. Photons are used to ionize surface atoms and the energy of the ejected photoelectrons is detected and measured. Figure 1.5 shows the experimental set-up for the XPS measurements. XPS requires an ultra-high vacuum (< 10−6 Pa) to prevent contamination of the surface of the specimen and to avoid as much as possible collisions of the emitted electrons with the background gas. Therefore the instrument normally consists of a preparation chamber to carry out initial manipulations and an analytical chamber at ultra-high vacuum with a photon source, an electron analyzer and a detector. When placed in the analytical chamber, a monochromatic source of radiation is focused on the specimen. The most commonly employed X-ray sources are MgKα and AlKα radiation with corresponding energies of 1253.6 and 1486.6 eV respectively. An X-ray photon with fixed energy (hv) is absorbed by an atom at the surface of the sample and an electron is ejected from either a valence electron shell or an inner core electron shell (see Fig. 1.6). The ejected photoelectrons are focused onto the entrance slit of an electrostatic analyzer by an electromagnetic lens system. The electrons then pass through the hemispherical electron energy analyzer which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy. These electrons are detected using an electron multiplier, which is essentially a tube with the internal surface coated with a material which produces a large number of electrons when an electron is incident upon it.

1.5 Schematic set-up for XPS measurements (UHV = ultra high vacuum).

1.6 General energy diagram of the photoionization process.

The kinetic energy Ekin of the emitted electron is given by the following relation:

[1.6]

where v is the frequency of the incident monochromatic X-ray photons, Eb the binding energy of the electron to the atom and ϕ the work function of the specimen. The work function is the minimum energy that must be given to an electron to liberate it from the surface of a particular material. In the photoelectric effect if a photon with an energy greater than the work function is incident on a metal photoelectric emission occurs. Any excess energy is given to the electron as kinetic energy (see also Fig. 1.6). By measuring the energy of the ejected electron and knowing the energy of the incident photons, the electron binding energy can be determined. It is this binding energy that permits the atom to be identified and that provides information concerning its chemical state.

For each atomic element, there will be a characteristic binding energy associated with each core atomic orbital, i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study. The exact binding energy of an electron depends not only on the electron shell from which photoemission occurs, but also on the local chemical and physical environment. A major strength of XPS is the ability to identify chemical state changes that occur at a surface when two or more atoms combine. When two atoms combine to form a compound, electron transfer occurs between the atoms, one becoming more positive, the other more negative. This results in a shift of the electron binding energies of the electrons by a small amount, usually between a fraction of an eV and a few eV. These small shifts in the peak position are responsible for peak broadening. In this way, XPS can also provide information on the type of bonds formed at the surface.

The intensity of the peaks is expressed by the peak area and is proportional to the relative abundance of the element within the analyzed region of the sample surface. A quantitative analysis of the atomic surface composition can be determined through a so-called survey scan, i.e. an XPS spectrum with a wide energy frame and a relatively low resolution. When doing XPS experiments this is the first spectrum recorded. For an accurate determination of the binding energy of a specific electron level, a narrow energy frame is placed around a specific XPS peak, where the spectrum is recorded with a high resolution and a small energy step. In this high-resolution spectrum, deconvolution of overlapping peaks is performed to resolve the different types of chemical bonds.

The intensity of the XPS peak is only determined by the photoelectrons that reach the detector without energy loss. The photoelectrons that lose energy through elastic or inelastic collisions within the sample surface contribute to the background or deviate from the detection line. Consequently, an XPS spectrum only provides information on the top atomic layers (up to 5–10 nm, or roughly 20–40 atomic layers). The spectra can be strongly influenced by the relative orientation of the source, sample and detector. The reason for this effect is simply demonstrated by referring to Fig. 1.7. If λA is the attenuation length of the emerging electron, then 95% of the signal intensity is derived from a distance 3λA within the solid (Briggs, 1990). However, the vertical depth is clearly given by

1.7 The effective depth d is determined by the attenuation length λA and the take-off angle α between the sample and the detection line.

[1.7]

with α the take-off angle between the photoelectron emission direction and the plane of the sample. This depth is maximum when α = 90° according to equation [1.7]. The attenuation length depends on the kinetic energy of the photoelectron.

Fourier-transform infrared spectroscopy

In Fourier-transform infrared spectroscopy (FT-IR), IR radiation is passed through a sample. Part of the infrared radiation is absorbed by the sample and part of it passes through (is transmitted through) the sample. The resulting spectrum represents the molecular absorption and transmission and creates a molecular fingerprint of the sample. Like a fingerprint, no two unique molecular structures produce the same infrared spectrum. Therefore, the presence or absence of a large number of functional groups can be identified at the sample surface with FT-IR and makes this technique a useful tool to be used in combination with XPS. The sampling depth of FT-IR is typically around 1 μm.

In conjunction with FT-IR, attenuated total reflectance (ATR) is often used as a sampling technique. This technique enables samples to be examined directly in the solid, liquid or gas state without special preparation. The sample is brought into contact with a crystal with high refractive index. ATR is based on the principle of total internal reflection. An infrared light beam passes through the ATR crystal in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which extends into the sample. The penetration depth into the sample is determined by the wavelength of the infrared beam, the angle of incidence and the refraction index of the ATR crystal together with the characteristics of the sample to be measured. This penetration depth typically ranges between 500 nm and 2 mm. The number of reflections may be controlled by varying the angle of incidence. As the beam exits the crystal, it is then collected by a detector. To achieve this evanescent effect, the crystal is made of an optical material with a higher refractive index than the sample being studied. Otherwise light is lost to the sample. Typical materials for ATR crystals include diamond, germanium and zinc selenide. Diamond is often used due to its excellent mechanical properties and its chemical inertness.

1.3.3 Determining the wetting and wicking of modified textiles

The transport of perspiration is an important contributor to the thermal comfort of fabrics worn next to the skin (Harnett and Mehta, 1984; Korner, 1979; Leach, 1957; Piller, 1979). The wearer perspires due to bodily activity and material worn next to the skin will get wet. Body heat is reduced by these moistened fabrics. Hence, material worn next to the skin should be capable of quick moisture release to the atmosphere. Fabric should therefore be able to:

• evaporate the perspiration from the skin surface

• transfer the moisture to the atmosphere (wicking) (Ramachandran and Kesavaraja, 2004 )

Therefore, the measurement of wetting and wicking in fabrics is of topical interest.

A clear distinction between wetting and wicking was made by Kissa (1996). Fiber wettability (Wong et al., 2001) is a prerequisite for wicking, since a liquid that does not wet fibers cannot wick into a fabric. Wicking can only occur when fibers with capillary spaces in between them are wetted by a liquid. The resultant capillary forces drive the liquid into the capillary spaces. The surface properties of the fibers and the wetting liquid completely determine wetting, whereas wicking is also affected by the way the fibers or yarns are arranged into fabrics (Van der Meeren et al, 2002).

A lot of test methods, like contact angle measurements (Le et al., 1996) or the use of the Wilhelmy technique (Hsieh and Yu, 1992) on individual fibers only provide information on the wetting behavior and do not allow evaluations of fabric wicking characteristics.

Below, the most common techniques to measure wetting or wicking behavior of textiles will be discussed briefly.

Contact angle measurement

For surfaces with a closed structure (e.g. polymer foils), the surface energy can be derived from the measurement of the static contact angle of small droplets of distilled water or other liquids on the polymer surface. For surfaces with a more open structure, such as woven and non-woven fabrics, the changes in wettability are measured by means of liquid absorptive capacity (see Section 1.4).

A simple and widely used test to evaluate the wettability of polymer films is the measurement of the static contact angle of small droplets of distilled water or other liquids on the polymer surface. Figure 1.8 shows how the contact angle is determined: the contact angle θ is geometrically defined as the angle on the liquid side of the tangential line drawn through the three-phase boundary, where a liquid, gas and solid intersect. Figure 1.8 (a) and (b) show the contact angle for an unwettable and wettable liquid–surface combination respectively. As an example, Fig. 1.9 shows contact angle images of an untreated (θ = 108°) and a plasma-treated (θ = 17°) silicone surface. The contact angle is a measure of the relative amounts of adhesive (liquid-to-solid) and cohesive (liquid-to-liquid) forces acting on a liquid and varies over the range 0° ≤ θ ≤ 180°.

1.8 Principle of contact angle measurement: (a) hydrophobic surface and (b) hydrophilic surface.

1.9 Water droplet on (a) an untreated and (b) a plasma-treated silicone surface.

Wettability and repellency of polymers against water are basic properties of the polymers. A surface can be hydrophilic or hydrophobic. Hydrophilic and hydrophobic surfaces are results of interactions at an interface between the polymer and water layers and are closely related to the surface energy of polymers. On a hydrophilic surface, polar groups exist and strong interactions between water and these polar groups occur. As a result, the water contact angle of the polymer is small. If the surface energy of the polymer is more than the surface tension of water (72.8 mN/m), the surface of the polymer will completely contact with water, and the contact angle will be zero (Inagaki, 1996). A value of the polymer surface energy lower than the surface tension of water gives rise to a non-zero contact angle. A hydrophobic surface means weak interactions with water at the polymer surface, since the surface mainly consists of non-polar groups. The contact angle of the polymer against water is as large as 90°, in some cases more than 100°.

Liquid absorptive capacity

As the contact angle can only be measured on smooth surfaces, the induced changes in wettability of fabrics are quantified by measuring the liquid absorptive capacity WA (Zamfir et al., 2002), according to DIN 53 923 (EDANA 10.1–72). WA is defined as the amount of water that a fabric has absorbed after one minute of immersion in water, relative to its own weight.

First, a dry sample is weighed (Mk). This sample is then immersed in distilled water of a constant temperature for 60 s, 20 mm below the liquid surface. After the wet sample is suspended to vertically drain for 120 s, it is weighed again (Mn). The liquid absorptive capacity WA is given by:

[1.8]

In the determination of WA the temperature of the distilled water strongly influences the value of the measured liquid absorptive capacity (Temmerman et al., 2005). Therefore, the water temperature needs to be kept constant.

This technique is rather time-consuming and depends on a highly accurate weight balance. A droplet that remains on the surface can significantly alter the sample weight, making this procedure very sensitive to misinterpretations.

Capillary rise method

The measurement of wicking in fabrics is mostly done using the capillary rise method (Ferrero, 2003). In this technique, a fabric strip is kept vertically with the lower end of the strip immersed in a water-dye liquor. A spontaneous wicking occurs due to capillary forces (Rice, 1989). The absorption height is recorded as a function of time and the absorption rate is calculated (Ferrero, 2003; Minor et al., 1960). However, during vertical upward wicking, the gravity influences the flow of liquid (Wong et al., 2001). Moreover, wicking goes on in a single dimension, causing differences in wicking behavior in warp and weft direction. To avoid these drawbacks, a horizontal wicking technique was developed by Morent et al. (2006). By continuous registration of the wicking area, the technique is able to quantify the wicking rate and it measures wicking in all dimensions. Moreover, this method is a horizontal wicking experiment and, therefore, the influence of gravity can be neglected. The developed image processing algorithms succeed in calculating the wicking area semi-automatically.

1.4 Applications for functional textiles

1.4.1 Biomedical applications

The most imaginative applications of functional textiles are those in the field of biomedical engineering. A typical application is the enhancing of the antibacterial activity of textiles. In most cases, fibers are coated with a metal containing layer. The antibacterial mechanism of the metal is a result of the effect of metallic particles on the external membrane of the bacteria; i.e. the metallic particles block their respiratory capacities, thereby preventing them from breeding (Jiang and Guo, 2009). Wang et al. (2008) deposited nanostructured silver films at room temperature on polypropyl ene (PP) non-wovens by a PVD process to obtain a fabric with antibacterial properties. They investigated the relationship between sputter parameters and antibacterial properties. The effects of a silver film deposition on surface morphology were characterized using SEM and AFM. The test bacteria were Staphylococcus aureus and Escherichia coli. All silver coated PP non-wovens were very effective against both test bacteria. The results also revealed that the silver coated samples were significantly more effective against Staphylococcus aureus than against Escherichia coli. Increased film thickness significantly improved the antibacterial performance. It is believed that increasing the coating thickness leads to the release of a larger amount of silver ions, which contributes to the antibacterial performance. However, because silver is quite expensive and because when the silver coating thickness exceeds 28 nm, it may be toxic to certain human cells (Yuranova et al., 2003), Wang et al. (2008) proposed an optimal coating thickness of approximately 3 nm to achieve excellent antibacterial properties and to reduce the cost. Similar results were obtained by Ford (1998) for electroless copper- and nickel-plated polyester fabrics. For Staphylococcus aureus, the bacteria reduction for copper- and nickel-plated polyester fabrics reached 99.6% and over 99.9% respectively; for Escherichia coli, the bacteria reduction was 99.9% and 94.4%, respectively. So, the copper-plated polyester fabric had a high antibacterial effect for both types of bacteria, whereas the nickel-plated polyester fabric showed a better antibacterial activity against Staphylococcus aureus than against Escherichia coli. Another research group (Scholz et al., 2005) investigated the PVD deposition of different metals (silver, copper, gold, platinum and platinum/rhodium) on SiO2 fibers for their performance against bacteria and fungi. It was found that copper was most effective against bacteria and fungi. Silver was also effective against bacteria, but against fungi its effectiveness proved to be limited. The other tested metals did not achieve this effectiveness. The bonding strength between the coatings and the substrates was enhanced by an air plasma treatment before the metal deposition.

1.4.2 Increasing the hydrophilic character

The application of surface modification techniques for improved wettability has been done on all possible fiber types, with varying success. The treatment aims at the introduction of water compatible functional groups such as –COOH, –OH and –NH2. As mentioned in Section 1.3, when evaluating the wettability of a treated fiber surface, it is in most cases not possible to determine the surface energy directly from contact angle measurements. The irregular surface of a textile reduces the accuracy of (large) contact angle values, while for lower contact angles their porous structure immediately absorbs the liquid drop. That is why wettability of textile (fiber) surfaces is usually monitored by indirect methods such as absorption time and wicking.

De Geyter et al. (2006; Morent et al., 2007) modified polyethylene terephthalate (PET) and PP non-wovens by a 10 kHz dielectric barrier discharge (DBD) in air, helium and argon at medium pressure (5.0 kPa). The helium and argon discharges contained a fraction of air smaller than 0.1%. Surface analysis and characterization were performed using XPS, liquid absorptive capacity measurements and SEM. The non-woven, modified in air, helium and argon, showed a significant increase in liquid absorptive capacity due to the incorporation of oxygen-containing groups, such as C–O, O–C=O and C=O. It was shown that an air plasma was more efficient in incorporating oxygen functionalities than an argon plasma, which was more efficient than a helium plasma. An air plasma treatment was the most efficient in incorporating oxygen on the surface due to the fast reaction between the radicals on the textile surface and the oxygen species present in the discharge. In an argon and helium plasma containing oxygen traces, plasma treatment led to an oxidized cross-linked structure on the textile surfaces. Since the cross-linking reaction inhibits the incorporation of oxygen and since only traces of oxygen were present, a helium and argon plasma were less efficient in oxidation of the textile surfaces. The used argon plasma contained more ions than the helium plasma resulting in a higher degree of cross-linking and a faster incorporation of oxygen-containing groups. SEM pictures of the plasma-treated non-wovens showed that the hydrophilicity of the non-wovens could be increased to a saturation value without causing physical degradation of the surface. The ageing behavior of the plasma-treated textiles after storage in air was also studied in detail. The ageing effect was the smallest for the argon plasma-treated fabrics, followed by the helium plasma-treated fabrics, while the air plasma-treated fabrics showed the largest ageing effect. During the ageing process, the induced oxygen-containing groups reorientated from the surface into the bulk of the material. A restriction of the polar group motion and therefore of the ageing process can be obtained by cross-linking of the polymer chains during plasma treatment. An air-plasma-treated non-woven was not cross-linked and therefore showed a large ageing effect. A helium plasma treatment of the non-wovens led to cross-linked textile surfaces resulting in a less pronounced ageing effect. Argon plasma-treated non-wovens had a higher degree of cross-linking resulting in an even smaller ageing effect. Borcia et al. (2003, 2006) studied a 80 kHz DBD in air for the treatment of PET and nylon fabrics and recently, a DBD in air, nitrogen and argon for the treatment of natural, synthetic and mixed fabrics. The wettability and wickability appeared to be strongly increased within the first 0.1–0.2 s of treatment. Any subsequent surface modification following longer treatment (> 1.0 s) was less important. The increased wettability could be attributed to the increased level of oxidation where supplementary polar functionalities are created on the fabric fiber surface, as observed by XPS.

1.4.3 Water and oil repellency

Next to enhancing the hydrophilic character of textiles, the opposite goal to increase water repellency also has important applications. Water repellency is important, for example, in outdoor applications of textiles: clothing, car tops, umbrellas, and so forth. Moreover, imparting water, oil and stain repellency to a textile is already a well-established application for napkins, curtains, and so forth. Because the water or oil does not diffuse into the fabric, the cost of washing these textiles is reduced. Such a treatment introduces certain functional groups via a coating or a graft co-polymer, removes hydrophilic functional groups or changes hydrophilic groups into non-hydrophilic ones. The most commonly used hydrophobic groups contain fluorine. Huang et al. (2007) reported on the functionalization of woven silk fabrics in a PVD process with a polytetrafluorethylene (PTFE) coating to improve the hydrophobic property of the silk fabric. The wettability of the fabric was characterized through measuring the surface contact angle and the contact angle of the PTFE coated fabric showed a significant increase from 68° to about 138°. In order to impart water and oil repellency, Louatie et al. (1999) chemically grafted polyester fibers with perfluorooctyl-2 ethanol acrylic monomer.

Superhydrophobicity is an effect induced on a surface so that the water contact angle becomes greater than 150°. It depends on reducing the surface energy, as for regular hydrophobic surfaces, but with the addition of rough texture on a nanoscale to trap air and prevent wetting. Superhydrophobic surfaces that are self-cleaning were achieved by a CVD process by Li et al. (2007). A nanoscale silicone layer was deposited on cotton fabric resulting in contact angles of 157°.

1.4.4 Conductivity and antistatic properties

Preventing the build-up of static charge on a fabric requires increased conductivity or decreased resistivity. Oh et al. (2001) reported on the effect of plasma treatment gas on the electrical conductivity of polyaniline-nylon-6 fabrics. Plasma surface modifications with oxygen, ammonia and argon were performed at low pressure on the nylon-6 fabrics to improve the adhesion and the rate of polymerization. Afterwards the plasma-treated fabrics were immersed in an aqueous hydrochloride solution of aniline. Successive polymerization was initiated with a mixture of an oxidant/dopant solution containing ammonium peroxydisulfate and HCl. The fabrics treated with an oxygen plasma showed the highest conductivity. The conductivity of the oxygen plasma-treated polyaniline-nylon-6 fabric was more stable with repeated washing and abrasion cycles than polyaniline-nylon-6 fabric without plasma pre-treatment.

Hautojärvi and Laaksonen (2000) studied on-line corona treatment of PP fibers during melt-spinning. Corona treatment resulted in a substantial decrease in contact angles. Treated fibers had considerably better antistatic properties than untreated fibers. Treated fibers had an electrical resistance an order of magnitude lower and about 50% less static charge built up during carding than untreated fibers.

Recently, Liu et al. (2006) modified the morphology and composition of acrylic fibers with a low-pressure nitrogen glow plasma. The overall surface roughness increased with increasing treatment time. XPS analysis showed that hydrophilic groups were inserted at the surface. Next to an increase in hydrophilicity, a drastic increase in antistatic ability was reported due to changes in surface morphology and due to the introduction of polar groups at the surface.

1.4.5 UV protection

Prolonged and repeated exposure to UV radiation from sunlight has been identified as the cause of an increase in the incidence of skin cancer in humans. Limiting the skin’s exposure to sunlight is not possible for everyone, especially for people working outdoors. Therefore, textiles blocking UV-light are the most suitable alternative. Deng et al. (2007) prepared PET non-woven in a PVD process with a ZnO coating. This coating was able to significantly reduce the transmittance in the UV-region, while the transmittance in the visible light range was only slightly reduced.

1.4.6 Photocatalytic coatings

TiO2 thin films have been deposited on textiles and offer great potential for applications involving the decomposition of various environmental pollutants in both gaseous and liquid phases. Daoud and Xin (2004) reported on the creation of titania coatings via a sol-gel process on cotton fabrics. These fabrics could be used as a self-cleaning antibacterial photocatalyst for the decomposition of organic dirt, environmental pollutants and harmful microorganisms. Degradation of red wine and coffee stains by titania coated fabrics was reported by Bozzi et al. (2005a, 2005b) and by Qi et al. (2006).

1.5 Future trends

There is a great deal of research to improve the functional properties of textiles, and new technologies have been adopted for the modification of textiles in order to achieve new properties. The newly developed functional textiles find application in fields such as environmental protection and biomedicine. The ideal surface modification techniques introduce a thin uniform layer of a desired functional group without damaging the fiber surface. Moreover, these techniques should also be environmentally friendly. Therefore, some older established surface modification techniques that are not environmentally benign will need to be replaced by sustainable techniques with good results and environmental benefits. For example, the consumption of water and energy of conventional wet treatments of textiles should be drastically reduced. In addition, progress in surface analysis is essential to develop new functionalities for textiles. This progress will lead to a better understanding of the behavior of textiles, necessary for further improvement.

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