8 Self Cleaning Finishes for Textiles

Self cleaning fi nishes for textiles D. Gupta , M. L. Gulrajani Indian Institute of Technology Delhi , New Delhi, India

8

8.1 Introduction

The original idea of self cleaning textiles envisioned a scenario where tablecloths and men ’ s suits shrug off coffee, tea, wine and other stains; or where large awnings, tents and other architectural structures stay spotlessly clean without requiring any washing or cleaning. Due to the remarkable developments made in this fi eld during the last few decades, the concept of self cleaning widened to include apparel that cleanses itself of body odour, curtains that rid themselves of tobacco odours to stay ‘ever fresh’, and hospital sheets that disinfect themselves to reduce the incidence of cross-infections. As research continues and knowledge matures in the area, more exciting developments are expected that will allow self cleaning to expand its frontiers into many as yet unknown and unexplored domains.

The self cleaning property was for many years associated with superhydrophobic surfaces – a phenomenon fi rst observed in the lotus leaf and so named the lotus effect . However, subsequent research established that a surface can also be made self cleaning by photocatalytic breakdown of organic compounds such as adsorbed dirt, contaminants, pollutants or microorganisms into carbon dioxide and water, by metals having semiconductor properties. More recently, it has been shown that even the phenomenon of superhydrophilicity can impart effective self cleaning ability to specifi c surfaces.

Superhydrophobicity, or the lotus effect , is achieved by chemical and geometrical modifi cation of a surface. By creating micro and nano scale roughness accompanied by a hydrophobic coating, dirt on the surface can be removed by water droplets as they bead up and roll off the hydrophobic surface, thus keeping the surface clean. Initial research studies in the fi eld were specifi cally targeted towards understanding the self cleaning power of superhydrophobic structures that mimicked the lotus effect. Synthetic structures with a variety of nano scale geometries have since been produced and tested (Tung and Daoud, 2011 ). Photocatalytic self cleaning, on the other hand, is based on chemical breakdown of dirt through photo oxidation and photo reduction reactions in the presence of light (Fujishima et al ., 2006 ). Titanium dioxide or titania is the most frequently used substance in this fi eld. Photocatalytic activity can break down the organic compounds present in cell walls of bacteria, as well as organic odour molecules present in the atmosphere, thus inducing a self cleaning function as well as conferring antimicrobial and deodorization properties to treated surfaces. As TiO 2 treated surfaces absorb light they provide protection to surfaces against Functional Finishes for Textiles. http://dx.doi.org/10.1533/9780857098450.1.257Copyright © 2015 Elsevier Ltd. All rights reserved.

258 Functional Finishes for Textiles

photoyellowing and photodegradation. Because of such multifunctional properties imparted by TiO 2 coatings, this technology is receiving increasing attention in research as well as commercial processes in the last few years (Fujishima et al ., 2000 ).

This chapter discusses the two major approaches, namely superhydrophobicity and photocatalysis using TiO 2 , that can be used to impart self cleaning properties to textile surfaces. The chapter begins by defi ning the phenomenon of superhydrophobicity and its measurement. It then discusses the nature of superhydrophobic (SH) structures found in nature which have served as the inspiration for most developments in the fi eld, and follows with a discussion on the various techniques used to create SH structures on textiles, mainly cotton. Challenges faced in the creation of SH surfaces on textiles and their applications are also discussed. The second section deals with the photocatalytic activity of TiO 2 and development of self cleaning treatments for textiles based on such activity. The chapter ends by discussing the convergence of the two technologies, the challenges facing the commercialization of self cleaning technologies for textiles and the future scope of research in the fi eld.

8.2 Superhydrophobicity and self cleaning

8.2.1 Measurement of surface wettability

Wettability of a surface is usually determined by measuring the static contact angle (CA) of a water droplet contacting the surface. The angle between the surface and the water meniscus near the line of contact, measured through the droplet, gives an indication of the wettability of the surface (Roach et al ., 2008 ) (Figure 8.1 ). Surfaces are called hydrophobic when CA > 90° and hydrophilic when CA < 30°. Superhydrophilic surfaces show CA < 5°. Materials containing low surface energy groups like –CF 3 typically give contact angles of 120°. These surfaces are hydrophobic and easy to clean but cannot be categorized as self cleaning (X.M. Li et al ., 2007 ).

While the water CA has commonly been used as a criterion for the evaluation of hydrophobicity of a surface, it alone is not suffi cient to evaluate the sliding properties of water that characterize a superhydrophobic surface. On such surfaces water beads up and roll off the surface, removing all dirt and dust with them. The sliding properties

Figure 8.1 Static contact angle measurement for a hydrophobic and a hydrophilic surface.

qq

Hydrophobicsurface

Hydrophilicsurface

Self cleaning fi nishes for textiles 259

of water are affected by values of CA hysteresis and the sliding angle. Surfaces exhibit superhydrophobicity when the CA is high (150–180°) and CA hysteresis and sliding angle are low (Zhang et al ., 2008 ).

For CA measurement, a drop is deposited on the surface and the angle is measured by a goniometer, while for calculating the CA hysteresis the contact angle of a water drop is measured in static as well as dynamic mode. Dynamic CA is measured during the growth (advancing CA, θ a ) as well as the receding stage (receding CA, θ r ) of a water drop, and the difference between θ a and θ r is taken as the CA hysteresis. CA hysteresis is an important characteristic of a solid–liquid interface as it indicates the ‘stickiness’ of the surface or the amount of energy dissipated when a drop fl ows along a solid surface. CA hysteresis is infl uenced by the roughness and heterogeneity of a surface (Gao and McCarthy, 2006a ).

The sliding angle, also known as the tilt angle, is defi ned as the critical angle to which a surface must be tilted for the water drop to roll off smoothly (Nosonovsky and Bhushan, 2007 ). The tilt angle does not equal, but refl ects, the difference between the advancing and receding contact angles. Surfaces with low CA hysteresis have a very low sliding angle. A low water roll-off angle of ∼ 3° is desirable in self cleaning applications.

8.2.2 Two states of superhydrophobicity

The wettability of a surface expressed by the CA of a water droplet is given by Young ’ s equation:

cosθ γ γγ

= −SV SL

LV

(8.1)

where γ SV , γ SL and γ LV refer to the interfacial surface tensions between solid (S), liquid (L) and vapour (V) phase respectively. Young ’ s angle is a result of thermodynamic equilibrium of the free energy at the solid–liquid–vapour interphase.

On the basis of experimental data obtained in various studies, empirical models have been proposed to explain the surface wetting properties (Callies and Quéré, 2005 ). These include the well known Wenzel and Cassie–Baxter models which describe the relation between surface roughness and water repellency. These models explain the two possible states that can cause superhydrophobicity – in one case the liquid enters the texture and follows the solid surface, while in the other case it is suspended above the rough surface, leaving air inside the texture (Figure 8.2 ). The former state, called the Wenzel effect, causes an increase in the surface area due to the textured effect which amplifi es the hydrophobicity of the material. The key parameter controlling the apparent contact angle θ * on this surface is the solid roughness r , defi ned as the ratio between the true surface area and the apparent one. It is called the apparent CA as it does not represent the real CA of the corresponding fl at surface:

cos * cosθ θ= r (8.2)


where θ is the Young ’ s angle on a fl at surface, determined by the chemical nature of the solid, liquid and vapour phase. The surface roughness r can enhance the hydrophilicity as well as the hydrophobicity of the surfaces (Sun et al ., 2005 ). According to the Wenzel model, increasing the roughness of a hydrophilic surface increases its hydrophilicity, and increasing the roughness of a hydrophobic surface increases its hydrophobicity (Rosario et al ., 2004 ). In other words, the CA as well as its hysteresis increases with increase in roughness of the surface. In experimental studies, this increase was observed until the roughness factor reached 1.7, beyond which the CA hysteresis started to decrease. This decrease was ascribed to the shift from the Wenzel state to the Cassie–Baxter state as the increased fraction of air trapped in the surface leads to the suspension of water droplets on top of the surface. The Cassie state is given by the following equation:

cos * cosθ θ= − −( )f f1 (8.3)

where f is the fraction of the solid–water interface, while ( 1 − f ) is that of the air–water interface. This indicates that when a rough surface comes in contact with water, air may be entrapped in the trough area, leading to hydrophobicity. A hierarchical structure with convex shaped nanostructures resists the destabilization of the liquid–air interface by pinning the triple line (line of contact between solid, liquid and air), thus leading to stable equilibrium (Bhushan et al ., 2009 ).

The Wenzel and Cassie–Baxter models give a fundamental explanation of how the CA can be increased with the help of surface roughness. But these models do not

Figure 8.2 Models of superhydrophobicity: (a) liquid penetrates into spikes (Wenzel state); (b) liquid suspends on the spikes (Cassie–Baxter state).

(a)

(b)


consider the dynamics of the water drop as it moves on a surface. Oner and McCarthy ( 2000 ) showed that the structure of the three phase contact line defi nes the dynamic behaviour of a water droplet on a solid surface. Surfaces where the contact line can be easily destabilized have lower hysteresis and a droplet on such surfaces moves better (Sas et al ., 2012 ).

8.3 Superhydrophobic structures in nature

Research on superhydrophobic surfaces has largely been based on mimicking nature, where over 200 species have been found to have the proper combination of surface chemistry and morphology that allows them to stay clean (Parkin and Palgrave, 2005 ). Hence a lot of effort has been devoted to understanding the surface structures of different plants and animals that have self cleaning properties (Zhang et al ., 2006 ). This section summarizes some important studies which have helped decode the superhydrophobic phenomenon witnessed in natural surfaces.

The concept of self cleaning materials originated with the lotus ( Nelumbo nucifera) , which is venerated and considered to be pure and sacred in several cultures, because it grows in muddy water yet its leaves never get dirty (Figure 8.3 (a)). This self cleaning effect was fi rst noted by botanists Neinhuis and Barthlott of the University of Bonn in Germany in 1997, who subsequently patented the idea as the ‘Lotus effect’ (Forbes, 2008 ). According to them, this effect is caused by a combination of two properties – the presence of micrometre scale papillae structures that trap a large amount of air when contacted by water, and the low surface energy epicuticular wax crystalloids that coat the leaf surface. However, numerical calculations showed that this model could, at best, yield a contact angle of about 147°, which was much lower than the experimental value of 160° (Sun et al ., 2005 ). Feng et al . ( 2002 ) showed that in addition to microscale papillae of 5–9 micrometres, the lotus leaf has further branchlike nanostructures of diameter 124.3 ± 3.2 nm (Figure 8.3 (b)). When these hierarchical structures were taken into account, the theoretical model predicted a contact angle that was much closer to the experimental value of 160°.

Several other examples of SH surfaces have been observed in nature. Rice leaves show a hierarchical structure similar to that of the lotus leaf (Feng et al ., 2002 ). However, unlike the lotus leaf, where the papillae are distributed uniformly on the surface, the papillae on the rice leaf are arranged in one direction only, that is parallel to the edge of the leaf. Because of this arrangement, the water drop can roll off freely at a sliding angle of 4° in the direction of the papillae, but moves with diffi culty at sliding angle of 12°, along the perpendicular direction, hence giving these leaves a directional or anisotropic SH effect. In comparison, a lotus leaf has a sliding angle of < 2° in all directions, due to the homogeneous arrangement of surface protrusions.

Other examples of SH surfaces in nature include taro ( Colocasia esculenta ) leaves, having a microstructure comprised of elliptic protrusions of about 10 μ m diameter along with the nanoscaled pins that form the hierarchical structure which is similar


Figure 8.3 Superhydrophobic structures found in nature: (a) lotus leaf; (b) SEM of lotus leaf showing micro and nano level structures (Ensikat et al ., 2011 ); (c) water strider (Xu et al ., 2012 with permission of Elsevier); (d) SEM of the aligned setae present on leg of water strider; the arrow denotes the orientation of the aligned setae (i.e., AS direction) and (inset) the aligned setae with an inclination angle of θ to leg surface (Xu et al ., 2012 with permission of Elsevier); (e) Stenocara beetle (Amato, C&EN, 2006 ); (f) schematic of the beetle back showing the movement of water (a) through alternating hydrophilic (b) and hydrophobic (c) zones.

a b

c

(a) (b)

(c) (d)

(e) (f)

10 μm

10 μm

50 μm

AS

θ


to the lotus leaf. Indian canna ( Canna generalis bailey ) leaves are also reported to be superhydrophobic due to the binary structures (micro and nanostructures) on the surface. The Colocasia ( Colocasia esculenta ) leaf, which is superhydophobic, has bumps similar to those in the lotus leaf, but these bumps are separated by surrounding ridges. The bumps and ridges create air pockets between the water droplet and the surface, thus making the surface hydrophobic in nature (Yan et al ., 2011 ).

Water repellent structures have been observed in animals as well. Gao and Jiang ( 2004 ) discussed the amazing non-wetting properties of the legs of the water strider ( Gerris remigis ) that show a water contact angle of 167.6 ± 4.4° and allow it to stand and move effortlessly on water. The legs are covered with oriented, needle shaped setae about 50 μ m in length and 3 microns to several hundred nanometres in diameter (Figure 8.3 (c) and (d)). Many nanoscale grooves are present on each microseta to form a unique hierarchical structure that gives a strong repellent force to the legs and helps the insect stride across water (Yan et al ., 2011 ). Wings of some butterfl ies have also been reported to be superhydrophobic. They have scales which overlap like roof tiles on the wing surface. The length and width of the scales range between 50–150 μ m and 35–70 μ m, respectively. Superhydrophobicity has also been observed in ducks ’ feathers and cuticles of other insects such as the cicada ( Cicada orni ) (Zhang et al ., 2006 ).

The most interesting fi nding, however, relates to the discovery of surfaces which have both superhydrophobic and superhydrophilic properties. This unique combination has been found to exist in nature in a small beetle belonging to Stenocara sp., found in the Namib Desert in Southern Africa (Forbes, 2008 ). The back of the beetle is made up of hydrophilic hills and waxy superhydrophobic channels (Figure 8.3 (e) and (f)). The hydrophilic tops capture water from the fog which grows into little droplets and trickles slowly through the hydrophobic channels into its mouth. This technique is being used to develop fog harvesting systems for arid regions.

SH surfaces are a complex combination of hierarchical or fractal surfaces made up of two or more layers of different sizes, each made up of convex cells which are covered with a layer of three-dimensional hydrophobic wax tubules. This waxy coating creates a surface having low surface energy. However, it is worth noting that lotus leaves achieve a wet contact angle of > 160° with these paraffi nic wax crystals predominantly made up of –CH 2 – groups, thus indicating that nature does not depend on lower surface energy groups like –CH 3 or fl uorocarbons to achieve SH effects (Ma and Hill, 2006 ). It has been proposed that wetting of such surfaces is minimized, because air is enclosed in the cavities of the convex cell sculptures. It therefore follows that the key to superhydrophobicity lies not in the surface coating but in the pattern of structures that are hierarchically organized at micro and nanoscale on the surface (Yan et al, 2011 ). Feng et al . ( 2002 ) carried out a mathematical analysis to prove that nanostructures are essential for fabricating SH surfaces with high CA and that multiscale structures can reduce the CA hysteresis of water. They proposed a mathematical model to explain the relationship between CA on a rough surface ( θ f ) and that on a smooth surface of the same solid ( θ ):

cos cosθ θf s v/= ( ) −−f L I fD 2 (8.4)


where ( L / I ) D-2 is the surface roughness, L and I denote the upper and lower limit scales of the fractal surface, D is the fractal dimension and f s and f v correspond to the fraction of the surface under the water droplet occupied by solid material and air, respectively ( f s + f v = 1). In the lotus leaf, L and I correspond to the diameters of the papillae and the branched nanostructures respectively. The value of CA for the lotus leaf obtained with this model was 160°, which corresponded to the diameter of 128 nm for the nanostructure. These values were very close to the experimental values obtained for the lotus leaf, thus indicating that this model can be used successfully to engineer surfaces that yield the required high contact angle and low CA hysteresis. This realization led to a plethora of research wherein micro, nano and hierarchical structures of variable geometries and dimensions were created on various surfaces to impart the SH effect.

8.4 Creating superhydrophobic surfaces on textiles

While superhydrophobicity is a relatively recent term, coined around the 1990s, reports on the study of water repellency and hydrophobicity in the textile industry preceded the term by almost 50 years. The fi rst patent for making textile surfaces hydrophobic was fi led in 1945, although the term ‘superhydrophobic’ did not appear anywhere in the patent (Roach et al ., 2008 ). This patent described the use of an alkyl silane to produce hydrophobic paper or fabric. The silane reacts with moisture in the fi brous material and hydrolyses and condenses to form a hydrophobic layer on the surface. Gao and McCarthy ( 2006b ) used this patent to reproduce an ‘artifi cial lotus leaf’ surface on polyester fabric that had a CA higher than and a hysteresis value lower than that of the lotus leaf. Both Cassie and Wenzel, who fi rst put forth the theories of surface wetting, were originally involved in work on waterproofi ng textiles (Ma and Hill, 2006 ). This indicates that, though a lot of work was done by the textile industry on hydrophobic fabrics during the 1940s, this work is not being recognized or cited in the current literature published on the subject by physicists, materials scientists and nano technologists (Gao and McCarthy, 2006b ).

Textile surfaces are different from other solid substrates like metal sheets and glass surfaces in that they are fl exible and have inherent micrometre-scale roughness coming from fi bres and the woven structure (Zhao et al ., 2010 ). They can easily be rendered SH by adding secondary nanoscale roughness, followed by surface hydrophobization. Researchers have primarily followed two techniques to fabricate SH surfaces: making a rough surface from a low surface energy material, or modifying a rough surface using a material of low surface energy (Ma and Hill, 2006 ).

A variety of methods, which vary from the conventional padding and coating to highly precise layer by layer (LbL) assembly of composite layers, have been used to deposit nano coatings on textiles (Han et al ., 2010 ; Ji et al ., 2006 ). A plethora of techniques have similarly been employed to produce shaped features to introduce nanoscale roughness on textile surfaces. These include particles, rod arrays or pores (Roach et al ., 2008 ), carbon nanotubes (Hsieh et al ., 2008 ), silica particles, ZnO


nanorods (Xu and Cai, 2008 ) and silver nanoparticles (Xue et al ., 2012 ). Although it is relatively easy to create superhydrophobic/self cleaning surfaces on textiles, the challenge is to retain these properties during use. Pretreatments like laser and plasma etching have been used for this purpose (X.M. Li et al ., 2007 ; Balu et al ., 2008 ; Zheng et al ., 2010 ).

A simple technique to produce SH fabrics is to use low surface energy fabrics having micro/nanostructures. Gao and McCarthy ( 2006b ) used this technique to produce SH polyester by grafting a silicone coating on to a microfi bre polyester fabric. Polyester provided the required hydrophobicity, while the tightly woven ∼ 2 μ m long microfi bres provided the desired roughness. Electrospinning can also be used to produce fi ne hydrophobic webs. In an exhaustive review, Sas et al . ( 2012 ) have shown that electrospinning is a promising technique that can be used to fabricate fi brous substrates having self cleaning and SH properties. Coating of electrospun webs with some additional materials to increase roughness or reduce surface energy allows creation of webs with high CA and low roll-off angles in synthetic fi bres. A great advantage with these structures is that wetting properties of the surface can be tuned simply by changing the fi bre diameter. Electrospinning allows creation of such high roughness that electrospun webs made of hydrophilic polymers can also be tuned to be hydrophobic without additional coating. Despite these advantages, electrospinning is not a mainstream technology for production of SH fabrics, since it is an expensive technique. The mechanical stability of the webs is poor and the robustness of the self cleaning properties needs to be improved.

Flock coating of nylon 6,6 fi bres on polyester fabric has been used by Lee and Michielsen ( 2007 ) to create a SH surface. They grafted poly(acrylic acid) chains onto nylon-6,6 surfaces followed by grafting of 1H,1H-perfl uorooctylamine to create a SH surface having a CA of 178°. The method relies on the use of an expensive coupling agent, 4- (4,6-dimethoxy-1,3,5-triazin-2yl)-4-methylmorpholinium chloride, which makes it unsuitable for commercial exploitation.

The above mentioned approaches are suitable for fi bres which are inherently hydrophobic in nature and cannot be used for hydrophilic fi bres like cotton. Discussed below are some of the techniques which have been used to create SH cotton textiles.

8.4.1 Metal salts

Researchers have used metals to impart nanoscale roughness to cotton and followed it up with a hydrophobic coating. Use of metals often imparts additional properties such as antimicrobial and conductivity to fabrics, thus making it a multifunctional treatment. Xu and Cai ( 2008 ) employed a technique based on a hydrothermal method to apply ZnO nanocrystals to cotton. This was followed by fabrication of oriented ZnO nanorod arrays to develop nanoscale roughness. Finally, the fabrics were coated with dodecyltrimethoxysilane to impart surface hydrophobicity. Khalil-Abad and Yazdanshenas ( 2010 ) created SH surfaces by treating cotton with aqueous KOH and AgNO 3 , followed by surface hydrophobization with octyltriethoxysilane. Contact angles as high as 151° were achieved and the fabric showed antimicrobial properties due to the presence of silver. Xue et al . ( 2012 ) used in situ production of silver


nanoparticles on cotton fi bres to impart a dual-size roughness to the fabric surface. Silver coated fi bres were made hydrophobic with hexadecyltrimethoxysilane to give a CA of 157°. Treated SH fabric showed conducting and antimicrobial properties because of the treatment.

Most methods reported for making cotton SH typically involve two to three steps. To address this problem, Zhang et al . ( 2013 ) propose a single step drop coating method to fabricate a composite thin fi lm comprising modifi ed ZnO nanoparticles and polystyrene (PS) on cotton, which gave a static water contact angle of 155°. This fabric has the unique property of separating water from oil (or oil from water). Such processes offer the opportunity to develop methods for large scale production of superhydrophobic textiles for a variety of industrial applications.

Other than metals, carbon nanotubes (CNTs) have been used to create nanoscale roughness on textiles. Liu et al . ( 2008 ) reported developing lotus leaf like structures on cotton textiles which had been dipped into a CNT suspension. Roughness could be modifi ed by use of poly(butyl acrylate) modifi ed CNTs.

8.4.2 Silane treatments

Silane based sol gel treatments have been used widely to create SH surfaces on textiles. Hoefnagels et al . ( 2007 ) fabricated biomimetic superhydrophobic and highly oleophobic cotton textiles. Silica particles coated with amine groups were generated in situ and covalently bonded to cotton by a one or two step reaction. The amine groups were reacted with mono-epoxy functionalized polydimethylsiloxane to hydrophobize the surface. This process yielded large particles of about 1 μ m, which may affect the softness and fl exibility of cotton. Zhou et al . ( 2012 ) used polydimeth-ylsiloxane fi lled with fl uorinated alkylsilane functionalized silica nanoparticles and fl uorinated alkylsilane to produce a superhydrophobic coating on fabrics. They found that when a long chain alkyl (C16), phenyl or fl uorinated alkyl (C8) was the functional group, the treated surfaces were rendered superhydrophobic with a CA greater than 170° irrespective of the nature of the substrate fi bres. Introducing a third silane containing an epoxide group during synthesis improved the washing durability of the fi nish. Wang et al . ( 2011 ) demonstrated that fabrics coated with hydrolysed fl uorinated alkyl silane have a remarkable self healing, super liquid repellent surface. Gao et al . ( 2011 ) coated cotton by a series of polyhedral oligomeric silsesquioxanes to develop excellent water and oil repellency.

8.4.3 Gas coating methods

Zimmermann et al . ( 2008 ) reported a simple, one step gas phase coating method for producing fabrics with long term water resistance, by growing a layer of polymethylsilsesquioxane nanofi laments onto individual textile fi bres. The plasma enhanced chemical vapour deposition (PECVD) technique is capable of depositing nanometric coatings on textiles for developing SH surfaces. S. Li et al . ( 2007 ) used chemical vapour deposition followed by hydrolysation and polymerization to deposit a nanoscale coating of silicone to make cotton SH. Cotton fabric was plasma coated


with a perfl uorocarbon layer to make it superhydrophobic (Zhang et al ., 2003 ). Pulsed laser deposition of thin Tefl on fi lms has been shown to convey additional nanometre sized granular roughness to cellulose fabric (Daoud et al ., 2006 ).

8.4.4 Layer by layer assembly

Polyelectrolytes, inorganic nanoparticles and macromolecules have been used to build up multilayered composite fi lms on charged fi bres in a controlled manner, by electrostatic layer by layer (LbL) assembly. The technique is based on alternate adsorption of oppositely charged particles in a sequential manner. Compared with traditional methods like dip coating and padding, the LbL assembly technique is versatile in that it allows tailoring of surface nanostructures by controlling the assembly cycles. Because of electrostatic interactions between negatively charged silica nanoparticles and the polycations, the assemblies tend to be durable. Zhao et al . ( 2010 ) carried out electrostatic LbL assembly of polyelectrolyte/silica nanoparticle multilayers on cotton fi bres, followed by a fl uoroalkylsilane treatment. When one to three layers were deposited, cotton showed a CA of > 150° and a CA hysteresis of > 45°. However, when the number of layers increased to fi ve or more, a CA hysteresis lower than 10° was achieved. The effect was found to be durable for up to 30 machine cycles.

Zhang et al . ( 2012 ) used a facile method to produce SH cotton by the LbL deposition of cationic poly(dimethyldiallylammonium chloride) and negatively charged silica particles, followed by modifi cation of (heptadecafl uoro-1,1,2,2-tetradecyl) trimethoxysilane. Xue et al . ( 2008 ) used the covalent LbL assembly process to prepare superhydrophobic surfaces on cotton. A dual-size hierarchical structure was created based on a complex coating of silica nanoparticles with functional groups on epoxy functionalized microscale cotton fi bres, followed by a hydrophobization with stearic acid, 1H,1H,2H,2H-perfl uoro decyl trichloro silane (PFTDS), or their combination. They reported a static CA of 168° for treated cotton.

8.5 Challenges in the development of superhydrophobic surfaces

Although a large body of research is available and a lot of work is going in the fi eld of creating superhydrophobic surfaces to impart self cleaning properties to textiles, several challenges remain to be addressed. These pertain mostly to the ageing and durability of the special surface effects. The complex hierarchical textures are fragile and are easily destroyed by impact or even gentle rubbing. There is a need to build structures which are strong enough to resist the forces acting on them during use. These surfaces also suffer from the problem of contamination (Callies and Quéré, 2005 ). They have a tendency to absorb oily substances which percolate deep into the structure and are extremely diffi cult to remove. Such problems limit the use of these surfaces in long term use applications such as textiles. Research is going on to fi nd solutions to many of these issues.


8.6 Non-textile applications of superhydrophobic surfaces

The interest in self cleaning surfaces is being driven by the desire to fabricate such surfaces for satellite dishes, solar energy panels, photovoltaics, exterior architectural glass and greenhouses, and heat transfer surfaces in air conditioning equipment (Ma and Hill, 2006 ). Self cleaning SH surfaces are also much in demand for paints used in building exteriors, window and windshield glasses, boat and sail coatings, etc.

8.7 Photocatalysis using TiO 2 for self cleaning

Biomimetic approaches based on superhydrophobicity, such as those discussed above, were the only known methods for production of self cleaning surfaces for several decades. Although research on TiO 2 started in the late 1960s, it was limited at the time to study of photo electrochemical solar energy conversion. In 1990, Japanese scientists led by A. Fujishima discovered that nanothin fi lms of titania had a photocatalytic effect in the presence of UV light (Fujishima et al ., 2000 ). The next major breakthrough came in the mid-1990s, when Japanese researchers annealed a fi lm of titania particles at 500°C and found that it had become superhydrophilic. This superhydrophilic coating could be wetted completely by both oil and water and had good self cleaning properties. Water would spread across the superhydrophilic surface and form a continuous sheet that washed away the dirt as it fl owed. This was the fi rst incidence of superhydrophilicity being associated with the self cleaning effect. Three years later, the antimicrobial property of titania coated materials was discovered. Since then, extensive work on development of self cleaning surfaces based on TiO 2 has been carried out by several research groups across the world (Fujishima et al ., 2006 ). The following sections briefl y discuss the principle and applications of TiO 2 photocatalysis in self cleaning textiles.

8.7.1 Principle of TiO 2 photocatalysis

A photocatalyst is a substance which is photosensitive in nature and exhibits a strong oxidation effect in the presence of light. A self cleaning property can be imparted to a surface by coating it with a photocatalytic oxide of a transition metal. The mineral titanium dioxide, also known as titania, is close to being an ideal photocatalyst in several respects. It is relatively cheap, chemically stable, non-toxic and biocompatible, besides having exceptional photocatalytic activity (Fujishima et al ., 2000 ).

TiO 2 produces self cleaning by two routes – photocatalytic oxidation and superhydrophilicity. Titania is photocatalytic because it is a semiconductor, meaning that a moderate amount of energy is required to lift an electron from its valence band to the conduction band. In the non-excited state, the photocatalyst is at the ground state with the electrons being localized in the valence band (VB). When the surface


is illuminated, electrons in TiO 2 absorb energy and move from the band gap to the conduction band (CB), provided the photon energy is equal to or greater than the band gap energy ( ≥ 3.2 eV) of the photocatalyst (Figure 8.4 ). In the activated state, the photocatalyst generates electron hole pairs as highly active electrons are formed in the conduction band while positive holes are created in the valence band (Tung and Daoud, 2011 ). The generation of these electron hole pairs is the cause of the light induced semiconductor properties of TiO 2 (Veronovski et al ., 2009 ).

The redox potential for photogenerated electron hole pairs is + 2.53 V and they can oxidize the water molecules or adsorbed hydroxide ions to form highly oxidizing hydroxyl radicals (˙OH). The redox potential for conduction band electrons is − 0.52 V, which is negative enough to react with the adsorbed oxygen molecules to produce highly reactive superoxide radical ions ( O2

− ). The excited radicals generated from these reactions are responsible for powerful photo-electrochemical reactions which lead to the decomposition of organic compounds like dirt, stains, microbes and other contaminants into carbon dioxide and water without the application of any external voltage (Figure 8.5 ). The electrons may have the tendency to recombine on the surface or in the bulk of the photocatalyst, thus dissipating the energy. This recombination reaction must be suppressed to favour the photocatalytic effect (Tung and Daoud, 2011 ; Fujishima et al ., 2006 ).

Figure 8.4 Schematic showing the photocatalytic phenomenon of TiO 2 .

Ground state Excitation state

Conduction

band

Conduction

band

Valence

band

Valence

band

Light

(hv < ΔE)

Light(hv < ΔE)

e–

e– h+

Figure 8.5 Decomposition of contaminants by excited radicals.

Contamination

O2–

CO2H2OOH·

+


Reactions involved in the process are shown below (Tung and Daoud, 2011 )

TiO h e h2 + ⎯ →⎯ +− +ν (8.5)

e h TiO− ++ ⎯ →⎯ 2 (8.6)

e O O− −+ ⎯ →⎯2 2 (8.7)

O H HO2 2− ++ ⎯ →⎯ i (8.8)

h H O HO H+ ++ ⎯ →⎯ +2i (8.9)

h O+ −+ ⎯ →⎯2 20i (8.10)

The second effect of TiO 2 was discovered accidentally in 1995 when it was found that TiO 2 , combined with a certain percentage of SiO 2 , became superhydrophilic in the presence of UV radiation. In this case, the electrons reduce the Ti(IV) cations to the Ti(III) state and the holes oxidize the O 2 − anions. Due to this, some oxygen atoms are ejected from the surface, thus creating oxygen vacancies. Water molecules occupy these vacancies, resulting in a patchwork of nanoscale domains where hydroxyl groups get adsorbed and produce the superhydrophilic effect (Fujishima et al ., 2000 ). TiO 2 fi lms form a very low contact angle ( < 1°) with water. As a result, water does not bead up but covers the coated surface completely with a continuous fi lm that washes away dirt and cleans the surface. The effect can stay for several days after UV exposure but slowly reverts to the original state after the surface is kept in the dark.

The most fascinating aspect of TiO 2 is that while the chemistry responsible for photocatalysis and superhydrophilicity is totally different, both effects can occur on the same surface simultaneously. In fact, photocatalytic activity is closely related to hydrophilicity and both activities reinforce each other to maintain the self cleaning effect (Guan, 2005 ). The self cleaning effect obtained due to superhydrophilicity has been put to use in the creation of antifogging and self cleaning mirrors for cars. Not much work has been done on textiles in this area.

8.7.2 Factors affecting the photocatalytic activity of TiO 2

The intensity of photocatalytic activity of TiO 2 is determined by several factors such as the crystalline form in which it is deposited on the fabric, the particle size and the specifi c surface area, as well as the porosity and structure of the layers deposited (Yuranova et al ., 2006 ). Titanium dioxide is found in three common crystalline forms – anatase, rutile and brookite (Tung and Daoud, 2011 ). Out of these, the brookite form has not been used much, while the anatase form has been found to have the maximum photocatalytic activity (Oh et al ., 1997 ). Mixtures of anatase and rutile have been reported to be more effi cient than anatase alone. A commercial product, Degussa P-25, composed of 80% anatase and 20% rutile along with some amorphous


TiO 2 , has been used extensively by scientists due to its advanced photocatalytic ability which is attributed to a more effi cient charge separation arising out of the presence of multiphase particles (Carp et al ., 2004 ).

The photocatalytic activity of anatase TiO 2 is affected by particle morphology. When TiO 2 is converted to the nanoscale, photocatalytic activity is found to be higher because of the diffusion of the electron holes before recombination and an increase in the surface area per unit mass and volume (Mills et al ., 1993 ). Wahi et al . ( 2005 ) found that an anatase particle size of ∼ 10 nm shows the maximum photocatalytic activity, because it has the best balance between charge separation and surface area. Activity was lower in particles that were smaller or larger than the optimum size. Hebeish et al . ( 2013 ) found TiO 2 nanowires to be more effi cient in photodegradation than TiO 2 nanoparticles due to the relatively large surface area of the former.

8.7.3 Applications of TiO 2 coatings

TiO 2 treatments impart several functional properties to textiles besides self cleaning. The photocatalytic action of TiO 2 adds deodorizing and antimicrobial properties to the coated items by breaking down organic molecules responsible for causing odours and by killing bacteria. A further property is protection from UV light due to its absorption by the coating, thus offering UV protection to the fabric as well as the user (Forbes, 2008 ). In a recent review, Radeti ć ( 2013 ) has summarized the latest advances in fi nishing of textile materials with TiO 2 nanoparticles.

With such multifunctional properties, innovative commercial applications of self cleaning surfaces on various solid materials like glass, tiles, silica, ceramics and quartz have already been achieved. TiO 2 -containing wallpaper can clean the indoor air with illumination of fl uorescent lamps in the room (Fujishima et al ., 2006 ). Bacterial counts were found to decrease to negligible levels within a period of 1 h after application of antibacterial tiles on the fl oors and walls of hospital operating rooms in Japan. TiO 2 based products such as tiles, fi bres and sprays having antimicrobial properties are commercially available.

8.8 TiO 2 based self cleaning treatments for textiles

Self cleaning fabrics have several useful applications such as industrial and military uniforms, outdoor and indoor upholstery and carpets, window blinds, textiles for industrial use such as tents and awnings, fi lter fabrics, agricultural textiles and so on. Self cleaning coatings must have certain specifi c properties for them to be suitable for use in apparel and home textile applications. For example, the fi lms must adhere strongly to the substrate for durability and should be thin and fl exible. They should be optically transparent and colourless so that the colour or appearance of the treated fabric remains unaffected (Qi et al ., 2011 ). Other requirements pertain to comfort in that the fi lm should not have any adverse effect on the breathability or handle characteristics of the substrate. TiO 2 is suitable for textile applications since it is


capable of forming thin and transparent fi lms which meet most of these requirements (Fujishima et al ., 2000, 2006 ).

Most of the current work in the fi eld of TiO 2 based coatings is directed towards improving adhesion between the substrate and the photocatalyst for enhanced durability. This has been accomplished by the use of physical methods such as irradiation with plasma or vacuum UV light to generate functional groups on the surface. Chemical approaches include use of spacers or crosslinking agents based on polycarboxylic acids. Processes have been developed to suppress the recombination of electron hole pairs by enhancing charge generation, or by modifying the quantum effi ciency of heterogeneous photosensitive catalysts (Tung and Daoud, 2011 ). Doping of TiO 2 coatings with metals or their oxides such as Ag/TiO 2 , Au/TiO 2 or SiO 2 /TiO 2 improves the uniformity and reproducibility of coatings and boosts the photocatalytic effi ciency of TiO 2 . Attempts are being made to expand the photocatalytic ability of TiO 2 fi lms on textiles, by extending the absorption range of TiO 2 from UV to the visible light region. The following sections discuss some of the processes used in the development of TiO 2 based self cleaning textiles.

8.8.1 Cotton

8.8.1.1 Sol–gel application

Among all the preparation techniques used for producing TiO 2 coatings, sol–gel is the most popular as it allows nucleation of anatase TiO 2 at relatively low temperatures, making it suitable for application to materials like textiles and polymers which have low thermal resistance. However, it is not possible to directly prepare anatase/polymer nanocomposite fi lms from sol–gel titania on polymer fi lms. Post treatment with boiling water or water vapour is required to crystallize TiO 2 in most cases. TiO 2 combined with SiO 2 has a synergistic effect on the self cleaning ability of the treated surface. The presence of silica increases the surface area in the vicinity of titanium dioxide as well as the surface acidity of the photocatalyst (Kim et al ., 2005 , Anderson and Bard, 1995 ). In addition, the presence of silica has a protective effect on the substrate and protects it from degradation during photocatalysis.

Yuranova et al . ( 2006 ) developed a 20–30 nm thick TiO 2 –SiO 2 layer on cotton at 100°C. Since TiO 2 was deposited in the amorphous form, its photocatalytic effi ciency was poor. Qi et al . ( 2011 ) prepared anatase nanocrystalline TiO 2 by a sol–gel process in an acidic aqueous solution, using a simple dip pad–dry cure process at 60°C. The sol was used to fabricate thin fi lms having high photocatalytic activity and high resistance to laundering on cotton. Fabric strength, handle and breathability were not affected during treatment or use.

8.8.1.2 Pretreatment by surface irradiation

One approach that has been used to improve the adhesion between textile and substrate is pretreatment with various types of plasma and UV light to produce negatively charged functional groups such as carboxylic, percarboxylic, epoxide and


peroxide on the surface (Fei et al ., 2007 ). Positively charged Ti 4 + of TiO 2 is deposited on the negatively charged surface by electrostatic bonds. The surplus charge is 3 + , which is quite considerable in electrostatic terms and enhances the adhesion between TiO 2 and the substrate. Pretreatment by UV radiation or plasma is environmentally friendly as the treatment time is short and no solvents are used. However, the loading of TiO 2 has to be done immediately after pretreatment as the surface generated reactive species have a very short lifespan.

Bozzi et al . ( 2005a ) pretreated cotton fabric with RF plasma, MW plasma and UV irradiation. TiO 2 fi lms developed on pretreated cotton were found to be durable to 20 washings. Bleached and mercerized fabric surfaces were activated differently depending on the type of system used for pretreatment. UV pretreatment was found to be most effective in enhancing the photocatalytic activity. Kiwi and Pulgarin ( 2010 ) loaded nanocrystalline anatase TiO 2 on RF plasma and vacuum UV treated cotton and found good discoloration of wine stains.

8.8.1.3 Doping of TiO 2 with metals

Doping of TiO 2 with metals such as Ag and Au offers several advantages (R.H. Wang et al ., 2010 ; Kubacka et al ., 2008 ; Hebeish et al ., 2013 ):

• enhancement of the photocatalytic effect due to improvement in the rate of electron transfer to oxygen and decrease in the rate of recombination between excited electron hole pairs

• self cleaning in the visible light range due to strengthened adsorption in the near UV-A region and the visible light region, improvement of UV-vis vacancies, quantum effi ciencies, and a red shift in the adsorption edge

• durability of the self cleaning effect to multiple washes due to the low polycondensation levels of metal/TiO 2 /SiO 2 nanosols, low calcination temperatures, and the bonding between the hydroxyl groups of cotton and the nanosols during curing.

R.H. Wang et al . ( 2010 ) used Au/TiO 2 /SiO 2 nanocomposites to improve the visible light self cleaning performance of cotton. Due to the high cost of gold, a more viable and frequently used option is silver because of its high stability and excellent electrical and thermal conductivity. Multifunctional textiles having antimicrobial, self cleaning and UV protective properties have been reported on doping of TiO 2 with silver. Hebeish et al . ( 2013 ) fabricated nanocomposite fi lms based on TiO 2 nanowires doped with Ag and TiO 2 nanoparticles doped with Ag-PVP on cotton. Addition of PVP enhanced the effi ciency of stain photodegradation by increasing the transportation of photoinduced electrons and holes in the photocatalyst (Wang et al . 2003). Good antimicrobial activity as well as a high degree of stain degradation under normal laboratory conditions and sunlight was obtained. The process, however, is quite tedious and complex and involves several steps, thus making it unsuitable for commercial applications.

Onar et al . ( 2011 ) developed fi lms of TiO 2 doped with AgNO 3 , tetraethyl orthosilicate (TEOS) or a quaternary ammonium compound on cotton. They report that a high temperature, high pressure sol–gel process gives better deposition of TiO 2 as compared to dip coating. Best UV protection and antimicrobial property with better durability to laundering was obtained when silver and titanium were used in


combination. Best self cleaning was obtained with undoped titania sample. In their study, doping with silver enhanced the antimicrobial activity and worsened the self cleaning effect.

8.8.1.4 Chemical crosslinking

Polycarboxylic acids have been used to crosslink TiO 2 durably to cotton (Meilert et al ., 2005 ; Mirjalili and Karimi, 2011 ). One carboxylic group of the spacer is esterifi ed by a hydroxyl group of cellulose and the second anchors TiO 2 by electrostatic interaction. Transparent but non-homogeneous layers of TiO 2 are produced on cotton, with good self cleaning ability under daylight. The high temperature of about 210°C needed for crosslinking causes yellowing and tendering of cotton. Up to 47% reduction in strength was observed in crosslinked samples due to acid hydrolysis of cellulose.

8.8.2 Wool

In addition to a self cleaning property, TiO 2 based treatments have been reported to impart several other functionalities to wool. These include enhanced hydrophilicity, antimicrobial and antiphotoyellowing properties. Due to the low chemical and thermal resistance of protein fi bres, special processes are needed to apply TiO 2 on them. Montazer and Seifollahzadeh ( 2011 ) treated wool fabric with titanium dioxide nanoparticles to improve its hydrophilicity with UV illumination. Pakdel et al . ( 2012 ) developed a uniform coating of TiO 2 /SiO 2 (50:50 and 30:70) nanocomposites on wool and obtained a self cleaning function as well as superhydrophilicity in wool. A higher concentration of silica gave a higher self cleaning ability. FTIR spectra showed the presence of Ti–O–Si and Si–O–Si linkages. Enhanced hydrophilicity of wool was attributed to a higher surface acidity caused by the presence of Ti–O–Si linkages which provide a higher concentration of hydroxyl groups. M. Zhang et al . ( 2009 ) found that nano TiO 2 reduces the rate of photoyellowing of wool, thus indicating that crystalline titanium dioxide acts primarily as a UV absorber on wool in dry conditions and not as a photocatalyst. Tang et al . ( 2012 ) used a mixture of silica and silver nanoparticles to impart antimicrobial and superhydrophilic properties to wool.

Nitric acid and hydrochloric acid were used as acid catalysts for the hydrolysis and condensation reactions of titanium dioxide precursor for wool (Tung and Daoud, 2009 ). Although a self cleaning property was imparted, signifi cant losses were observed in tearing strength. HCl was a better acid, since it prevented photoyellowing and photodegradation of treated samples. Chemical pretreatments have been used to improve the adhesion between wool and TiO 2 . Daoud et al . ( 2008 ) carried out acylation of wool surfaces by succinic anhydride to impart enhanced functionality and reactivity towards titanium dioxide. Montazer et al . ( 2011 ) oxidized wool surfaces with potassium permanganate and hydrolysed polyester with lipases before treating the PET/wool blend with TiO 2 . Butane tetra carboxylic acid was used as a crosslinking agent to stabilize the nanoparticles and increase their adsorption. A good self cleaning property was observed under daylight as well as UV radiation. Treated fabrics showed


improved resistance to photoyellowing, indicating that TiO 2 had a powerful UV absorbing action that could protect fabrics from adverse effects of UV rays (Montazer and Pakdel, 2010 ).

8.8.3 Polyester

Mihailovic et al . ( 2010 ) treated polyester with RF oxygen and argon plasma to functionalize the surface before application of colloidal TiO 2 nanoparticles. Although both treatments were effective, better results were obtained with oxygen plasma. Treated fabrics showed good reduction of E. coli and UV blocking properties besides excellent degradation of juice stains.

Bozzi et al . ( 2005b ) exposed wool–polyamide and polyester textiles to RF plasma, MW plasma and vacuum UV irradiation. TiO 2 in suspension and colloidal form was attached to the modifi ed textile surface using post treatment temperatures of about 100°C. Colloidal TiO 2 as a basic layer with a second coating of TiO 2 Degussa P-25 having bigger crystallite size showed best discoloring performance. This has been attributed to the deposition of small TiO 2 colloidal particles on the textile which provide effective anchoring sites for the bigger Degussa P-25 TiO 2 crystallites. The nanoparticles of TiO 2 remain fairly stable on the textile surface after photochemical discoloring of stains.

8.9 TiO 2 photocatalysis versus superhydrophobicity

The TiO 2 induced photocatalytic self cleaning effect has several advantages over self cleaning superhydrophobic surfaces. For one, it is relatively easy to develop self cleaning surfaces with TiO 2 as compared to fabrication of SH surfaces. Surfaces treated with TiO 2 are more robust and durable than the fragile hierarchical SH structures which have limited durability. TiO 2 surfaces exhibit multifunctional properties of being self cleaning, antimicrobial, deodorant and UV absorbing. Titania based coatings can be made suitably durable to washing, rubbing and environmental conditions using various methods. It is because of these properties that TiO 2 fi nishing is emerging as the preferred mode of imparting self cleaning effects to large area surfaces for commercial use. This is apparent from the number of research publications appearing in the two related areas.

8.9.1 Convergence of superhydrophobic and photocatalytic surfaces

The two apparently contrasting approaches to self cleaning, where one is based on the superhydrophobic effect and the other on the photocatalytic effect, followed their own separate paths for several years, without showing any signs of convergence. Towards the turn of the century, the fi rst signs of convergence appeared when Fujishima et al . ( 2000 ) found that the lotus effect could be prolonged signifi cantly


by adding a very small quantity of titania to the superhydrophobic coating. Following this discovery, several attempts have been made to combine the two effects and produce them using very similar materials (Forbes, 2008 ). Surfaces having a controlled combination of hydrophobic and hydrophilic arrays provide a unique opportunity to control the geometry and shape of liquids and thus have many emerging applications (Ueda and Levkin, 2013 ).

8.10 Future trends

As the quality of coatings improves and prices fall, the demand for and production of self cleaning surfaces is likely to increase in the global markets. Based on the wealth of knowledge available in the fi eld, many of the drawbacks faced till recently can be minimized by selection of right materials and processes.

Although superhydrophilic coatings have been commercialized for antifogging glasses and mirrors, not much work has been done in this area on textiles. Although TiO 2 has been the most studied photocatalyst so far, there are issues related to its performance in visible light. Research is being conducted to design and develop undoped, single phase photocatalysts to extend the adsorption wavelength range into the visible light region. Two such photocatalysts which have been studied, though not for application on textiles, are benzophenone and bismuth vanadate (BiVO 4 ). In future there may be possibilities for their use in textile applications.

In the near future, self cleaning treatments will focus on combining the two approaches of superhydrophobicity and photocatalysis to create smart and responsive textile surfaces with durable self cleaning properties for a wide range of applications. Surfaces which have alternating hydrophilic and hydrophobic domains for separation of oil from water and other applications will be a focus area of research. The ultimate objective is to engineer textile surfaces that switch from superhydrophilic to superhydrophobic and vice versa by changing their chemical make-up in the presence of a trigger such as temperature, pH, solvent or electric potential. Hence the fi eld of self cleaning textiles continues to provide opportunities for further research and development to develop processes and techniques which will fulfi l the many current as well as future requirements of this sector.

8.11 Conclusion

Development of self cleaning surfaces is an active fi eld of research with several publications directed towards improvement of processes or effi ciency of treatments on various substrates. Starting from the concept of superhydrophobicity, the scope of self cleaning surfaces has expanded to encompass photocatalytic as well as superhydrophilic phenomena exhibited by TiO 2 coated surfaces. The science of self cleaning now relates to the study of wettability of surfaces and interfaces with surface chemistry and physics, nanotechnology and mechanics.


A plethora of research is being conducted in the area of self cleaning textiles. Methodologies for nano roughening followed by hydrophobization of textile surfaces have been developed. Coatings based on TiO 2 in combination with a host of materials have been fabricated. Several different routes have been explored for production of these surfaces, using many different types of materials as substrates or as surface modifi ers. Theoretical understanding of the relationship between surface morphology, surface chemistry and wettability of textile surfaces is maturing. However, commercial processes in self cleaning textiles are as yet limited since most of the experimental techniques involve tedious and multiple step procedures, thus making them impractical for large scale production. There is a need to develop methods which are based on a simple one step preparation and coating process that can be carried out at low temperatures. A major challenge is posed by the low surface energy of textile surfaces which makes it diffi cult to achieve suffi cient interfacial adhesion for the coating to adhere to the substrate. Development of coatings that are homogeneous and do not affect the feel, handle, appearance or comfort of textiles is another challenge. Other unsolved issues in the area relate to the physiological burden, biocompatibility and biodegradability as well as long term stability, property durability and effi ciency of self cleaning effects.

References and further reading

Amato I ( 2006 ), ‘ Desert beetle wings inspire high-tech, liquid-controlling surfaces ’, C&EN , available from https://pubs.acs.org/cen/news/84/i20/8420notw8.html ( accessed on 20 July 2013 ).

Anderson C and Bard AJ ( 1995 ), ‘ An improved photocatalyst of TiO 2 /SiO 2 prepared by a sol–gel synthesis ’, J. Phys. Chem. , 99 ( 24 ), 9882 – 9885 .

Balu B , Breedveld V and Hess DW ( 2008 ), ‘ Fabrication of rolloff and sticky superhydrophobic cellulose surfaces via plasma processing ’, Langmuir , 24 , 4785 – 4790 .

Bhushan B , Koch K and Jung YC ( 2009 ), ‘ Fabrication and characterization of the hierarchical structure for superhydrophobicity and self-cleaning ’, Ultramicroscopy , 109 , 1029 – 1034 .

Bozzi A , Yuranova T , Guasaquillo I , Laub D and Kiwi J ( 2005a ), ‘ Self-cleaning of modifi ed cotton textiles by TiO 2 at low temperatures under daylight irradiation ’, J. Photochem. Photobiol. A , 174 , 156 – 164 .

Bozzi A , Yuranova T and Kiwi J ( 2005b ), ‘ Self-cleaning of wool–polyamide and polyester textiles by TiO 2 –rutile modifi cation under daylight irradiation at ambient temperature ’, J. Photochem. Photobiol. A , 172 , 27 – 34 .

Callies M and Quéré D ( 2005 ), ‘ On water repellency ’, Soft Matter , 1 , 55 – 61 . Carp O , Huisman CL and Reller CL ( 2004 ), ‘ Photoinduced reactivity of titanium dioxide ’,

Prog. Solid State Chem. , 32 , 33 – 177 . Daoud WA and Xin JH ( 2004 ), ‘ Nucleation and growth of anatase nanocrystallites on cotton

fabrics at low temperatures ’, J. Am. Ceram. Soc. , 87 , 953 – 955 . Daoud WA , Xin JH , Zhang YH and Mak CL ( 2006 ), ‘ Pulsed layer deposition of superhydrophobic

thin tefl on fi lms on cellulosic fi bres ’, Thin Solid Films , 515 , 835 – 837 . Daoud WA , Leung SK , Tung WS , Xin JH , Cheuk KKL and Qi K ( 2008 ), ‘ Self cleaning

keratins ’, J. Chem. Mater. , 20 , 1242 – 1244 .


Ensikat HJ , Ditsche-Kuru P , Neinhuis C and Barthlott W ( 2011 ), ‘ Superhydrophobicity in perfection: the outstanding properties of the lotus leaf ’, Beilstein J. Nanotechnol. , 2 , 152 – 161 .

Fei B , Zhang Y and Xin JH ( 2007 ), ‘ Titania nano crystals mixtures for cloth fi nishing ’, Solid State Phenom. , 121–123 , 1217 – 1220 .

Feng L , Li S , Li Y , Li H , Zhang L , Zhai J , Song Y , Liu B , Jiang L and Zhu D ( 2002 ), ‘ Super hydrophobic surfaces: From natural to artifi cial ’, Adv. Mater. , 14 , 1857 – 1860 .

Forbes P ( 2008 ), ‘ Self-cleaning materials: Lotus leaf-inspired nanotechnology ’, Scientifi c American Magazine , 30 July 2008 .

Fujishima A , Rao TN and Tryk DA ( 2000 ), ‘ Titanium dioxide photocatalysis ’, J. Photochem. Photobiol. C: Photochem. , 1 ( 1 ), 1 – 21 .

Fujishima A , Zhang X and Chimie CR ( 2006 ), ‘ Titanium dioxide photocatalysis: Present situation and future ’, Comptes Rendus Chimie , 9 ( 5–6 ), 750 – 760 .

Gao LC and McCarthy TJ ( 2006a ), ‘ Contact angle hysteresis explained ’, Langmuir , 22 ( 14 ), 6234 – 6237 .

Gao L and McCarthy TJ ( 2006b ), ‘ Artifi cial lotus leaf prepared using a 1945 patent and a commercial textile ’, Langmuir , 22 ( 14 ), 5969 – 5973 .

Gao XF and Jiang L ( 2004 ), ‘ Biophysics: water-repellent legs of water striders ’, Nature , 432 , 36 .

Gao Y , He C and Qing FL ( 2011 ), ‘ Polyhedral oligomeric silsesquioxane-based fl uoroether-containing terpolymers: Synthesis, characterization and their water and oil repellency evaluation for cotton fabric ’, J. Polym. Sci., Part A: Polym. Chem. , 49 ( 24 ), 5152 – 5161 .

Guan K ( 2005 ), ‘ Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO 2 /SiO 2 fi lms ’, Surf. Coat. Technol. , 191 , 155 – 160 .

Han JT , Zheng Y , Cho JH , Xu X and Cho KJ ( 2010 ), ‘ Stable superhydrophobic organic–inorganic hybrid fi lms by electrostatic self-assembly ’, J. Phys. Chem. B , 109 , 20773 – 20778 .

Hebeish AA , Abdelhady MM and Youssef AM ( 2013 ), ‘ TiO 2 nanowire and TiO 2 nanowire doped Ag-PVP nanocomposite for antimicrobial and self-cleaning cotton textile ’, Carbohydr. Polym. , 91 , 549 – 559 .

Hoefnagels HF , Wu D , de With G and Ming W ( 2007 ), ‘ Biomimetic superhydrophobic and highly oleophobic cotton textiles ’, Langmuir , 23 ( 26 ), 13158 – 13163 .

Hsieh CT , Chen WY and Wu FL ( 2008 ), ‘ Fabrication and superhydrophobicity of fl uorinated carbon fabrics with micro/nanoscaled two-tier roughness ’, Carbon , 46 ( 9 ), 1218 – 1224 .

Ji J , Fu J and Shen J ( 2006 ), ‘ Fabrication of a superhydrophobic surface from the amplifi ed exponential growth of a multilayer ’, Adv. Mater. , 18 , 1441 – 1444 .

Khalil-Abad MS and Yazdanshenas ME ( 2010 ), ‘ Superhydrophobic antibacterial cotton textiles ’, J. Colloid Interface Sci. , 351 , 293 – 298 .

Kim HJ , Shul YG and Han H ( 2005 ), ‘ Photocatalytic properties of silica-supported TiO 2 ’, Top. Catal. , 35 ( 3–4 ), 287 – 293 .

Kiwi J and Pulgarin C ( 2010 ), ‘ Innovative self-cleaning and bactericide textiles ’, Catalysis Today , 151 , 2 – 7 .

Kubacka A , Ferrer M , Arias MA and Garcia MF ( 2008 ), ‘ Ag promotion of TiO 2 -anatase disinfection capability: Study of Escherichia coli inactivation ’, Appl. Catal. B , 84 , 87 – 93 .

Lee HJ and Michielsen S ( 2007 ), ‘ Preparation of a superhydrophobic rough surface ’, Journal of Polymer Science: Part B: Polymer Physics , 45 , 253 – 261 .

Li S , Xie H , Zhang S and Wang X ( 2007 ), ‘ Facile transformation of hydrophilic cellulose into super hydrophobic cellulose ’, Chem. Commun. , 46 , 4857 – 4859 .


Li XM , Reinhoudt D and Calama MC ( 2007 ), ‘ What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces ’, Chem. Soc. Rev. , 36 , 1350 – 1368 .

Liu Y , Wang X , Qi K and Xin JH ( 2008 ), ‘ Functionalization of cotton with carbon nanotubes ’, J. Mater. Chem. , 18 , 3454 – 3460 .

Ma M and Hill RM ( 2006 ), ‘ Superhydrophobic surfaces ’, Curr. Opin. Colloid Interface Sci. , 11 ( 4 ), 193 – 202 .

Ma M , Hill RM , Lowery JL , Fridrikh SV and Rutledge GC ( 2005a ), ‘ Electrospun poly(styrene co -dimethyl) block co-polymer fi bres exhibiting microphase separation and superhydrophobicity ’, Langmuir , 21 ( 12 ), 5549 – 5554 .

Ma M , Mao Y , Gupta M , Gleason KK and Rutledge GC ( 2005b ), ‘ Superhydrophobic fabrics produced by electrospinning and chemical vapour deposition ’, Macromolecules , 38 , 9742 – 9748 .

Ma M , Hill RM and Rutledge GC ( 2008 ), ‘ A review of recent results on superhydrophobic materials based on micro and nano fi bres ’, J. Adhes. Sci. Technol. , 22 , 1799 – 1817 .

Meilert KT , Laub D and Kiwi J ( 2005 ), ‘ Photocatalytic self-cleaning of modifi ed cotton textiles by TiO 2 clusters attached by chemical spacers ’, J. Mol. Catal. A: Chem. , 237 , 101 – 108 .

Mihailovic D , Saponjic Z , Molina R , Puac N , Jovancic P , Nedeljkovic J and Radetic M ( 2010 ), ‘ Improved properties of oxygen and argon RF plasma-activated polyester fabrics loaded with TiO 2 nanoparticles ’, Appl. Mater. Interf. , 2 ( 6 ), 1700 – 1706 .

Mills A , Davies RH and Worsley D ( 1993 ), ‘ Water purifi cation by semiconductor photocatalysis ’, Chem. Soc. Rev. , 22 , 417 – 425 .

Mirjalili M and Karimi L ( 2011 ), ‘ Photocatalytic degradation of synthesized colorant stains on cotton fabric coated with nano TiO 2 ’, JFBI , 3 ( 4 ), 208 – 215 .

Montazer M and Pakdel E ( 2010 ), ‘ Reducing photo-yellowing of wool using nano TiO 2 ’, Photochem. Photobiol. Sci. , 86 ( 2 ), 255 – 260 .

Montazer M and Seifollahzadeh S ( 2011 ), ‘ Pretreatment of wool/polyester blended fabrics to enhance titanium dioxide nanoparticle adsorption and self-cleaning properties ’, Color. Technol. , 127 ( 5 ), 322 – 327 .

Montazer M , Pakdel E and Moghadam MB ( 2011 ), ‘ The role of nano colloid of TiO 2 and butane tetracarboxylic acid on the alkali solubility and hydrophilicity of proteinous fi bers ’, Colloid Surface A , 375 ( 1–3 ), 1 – 11 .

Nosonovsky M and Bhushan B ( 2007 ), ‘ Multiscale friction mechanisms and hierarchical surfaces in nano- and bio-tribology ’, Mater. Sci. Eng. R , 58 ( 3–5 ), 162 – 193 .

Oh WS , Xu C , Kim DY and Goodman DW ( 1997 ), ‘ Preparation and characterization of epitaxial titanium oxide fi lms on Mo(100) ’, J. Vac. Sci. Technol. A , 15 , 1710 – 1716 .

Onar N , Aksit AC , Sen Y and Mutlu M ( 2011 ), ‘ Antimicrobial, UV-protective and self-cleaning properties of cotton fabrics coated by dip-coating and solvothermal coating methods ’, Fiber Polym. , 12 ( 4 ), 461 – 470 .

Oner D and McCarthy TJ ( 2000 ), ‘ Ultrahydrophobic surfaces: effects of topography length scales on wettability ’, Langmuir , 16 ( 20 ), 7777 – 7782 .

Pakdel E , Daoud WA and Wang X ( 2012 ), ‘ Self-cleaning and superhydrophilic wool by TiO 2 /SiO 2 nanocomposite ’, Appl. Surf. Sci. , 275 , 397 – 402 .

Parkin IP and Palgrave RG ( 2005 ), ‘ Self cleaning coatings ’, J. Mater. Chem. , 15 , 1689 – 1695 .

Qi K , Wang X and Xin JH ( 2011 ), ‘ Photocatalytic self-cleaning textiles based on nanocrystalline titanium dioxide ’, Text. Res. J. , 81 ( 1 ), 101 – 110 .

Radeti ć M ( 2013 ), ‘ Functionalization of textile materials with TiO 2 nanoparticles ’, J. Photochem. and Photobiol. C: Photochemistry Reviews , 16 , 62 – 76 .


Roach P , Shirtcliffe NJ and Newton MI ( 2008 ), ‘ Progess in superhydrophobic surface development ’, Soft Matter , 4 , 224 – 240 .

Rosario R , Gust D , Garcia AA , Hayes M , Taraci JL , Clement T , Dailey JW and Picraux ST ( 2004 ), ‘ Lotus effect amplifi es light induced contact angle switching ’, J. Phys. Chem. B , 108 ( 34 ), 12640 – 12642 .

Sas I , Gorga RE , Joines JA and Thoney KA ( 2012 ), ‘ Literature review on superhydrophobic self-cleaning surfaces produced by electrospinning ’, J. Polym. Sci. Part B: Polym. Phys. , 50 , 824 – 845 .

Sun T , Feng L , Gao X and Jiang L ( 2005 ), ‘ Bioinspired surfaces with special wettability ’, Acc. Chem. Res. , 38 ( 8 ), 644 – 652 .

Tang B , Wang J , Xu S , Afrin T , Tao J , Xu W , Sun L and Wang X ( 2012 ), ‘ Function improvement of wool fabric based on surface assembly of silica and silver nanoparticles ’, Chem. Eng. J. , 185–186 , 366 – 373 .

Tung WS and Daoud WA ( 2009 ), ‘ Photocatalytic self-cleaning keratins: A feasibility study ’, Acta Biomater. , 5 ( 1 ), 50 – 56 .

Tung WS and Daoud WA ( 2011 ), ‘ Self-cleaning fi bers via nanotechnology: A virtual reality ’, J. Mater. Chem. , 21 , 7858 – 7869 .

Ueda E and Levkin PA ( 2013 ), ‘ Emerging applications of superhydrophilic-superhydrophobic micropatterns ’, Adv. Mater. , 25 ( 9 ), 1234 – 1247 .

Veronovski N , Rudolf A , Smole SM , Kreže T and Geršak J ( 2009 ), ‘ Self-cleaning and handle properties of TiO 2 -modifi ed textiles ’, Fibers and Polymers , 10 ( 4 ), 551 – 556 .

Wahi RK , Yu WW , Liu Y , Mejia ML , Falkner JC , Nolte W and Colvin VL ( 2005 ), ‘ Photodegradation of Congo Red catalyzed by nanosized TiO 2 ’, J. Mol. Catal. A: Chem. , 242 , 48 – 56 .

Wang W , Gu M and Jin Y ( 2003 ) ‘ Effect of PVP on the photocatalytic behaviour of TiO 2 under sunlight ’, Materials Letters , 57 ( 21 ), 3276 – 3281 .

Wang H , Ding J , Xue Y , Wang X and Lina T ( 2010 ), ‘ Superhydrophobic fabrics from hybrid silica sol–gel coatings: Structural effect of precursors on wettability and washing durability ’, J. Mater. Res. , 25 ( 7 ), 1336 – 1343 .

Wang H , Xue Y , Ding J , Feng L , Wang X and Lin T ( 2011 ), ‘ Durable, self-healing superhydrophobic and superoleophobic surfaces from fl uorinated-decyl polyhedral oligomeric silsesquioxane and hydrolyzed fl uorinated alkyl silane ’, Angew. Chem. Int. Edit. , 50 ( 48 ), 11433 – 11436 .

Wang RH , Wang XW and Xin JH ( 2010 ), ‘ Advanced visible-light-driven self-cleaning cotton by Au/TiO 2 /SiO 2 photocatalysts ’, Appl. Mater. Inter. , 2 ( 1 ), 82 – 85 .

Wang T , Hu X and Dong S ( 2007 ), ‘ A general route to transform normal hydrophilic cloths into superhydrophobic surfaces ’, Chem. Commun. , 18 , 1849 – 1851 .

Xu B and Cai Z ( 2008 ), ‘ Fabrication of a superhydrophobic ZnO nanorod array fi lm on cotton fabrics via a wet chemical route and hydrophobic modifi cation ’, Appl. Surf. Sci. , 254 ( 18 ), 5899 – 5904 .

Xu L , Yao Xi and Zheng Y ( 2012 ), ‘ Direct-dependent adhesion of water strider ’ s legs for water-walking ’, Solid State Sciences , 14 ( 8 ), 1146 – 1151 .

Xue CH , Jia ST , Zhang J , Tian LQ , Chen HZ and Wang M ( 2008 ), ‘ Preparation of superhydrophobic surfaces on cotton textiles ’, Sci. Technol. Adv. Mater. , 9 , 1 – 7 .

Xue CH , Jia ST , Zhang J and Tian LQ ( 2009 ), ‘ Superhydrophobic surfaces on cotton textiles by complex coating of silica nanoparticles and hydrophobization ’, Thin Solid Films , 517 , 4593 – 4598 .

Xue CH , Chen J , Yin W , Jia ST and Ma JZ ( 2012 ), ‘ Superhydrophobic conductive textiles with antibacterial property by coating fi bers with silver nanoparticles ’, Appl. Surf. Sci. , 258 , 2468 – 2472 .


Yan YY , Gao N and Barthlott W ( 2011 ), ‘ Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces ’, Adv. Colloid Interface Sci. , 169 , 80 – 105 .

Yuranova T , Mosteo R , Bandara J , Laub D and Kiwi J ( 2006 ), ‘ Self-cleaning cotton textiles surfaces modifi ed by photoactive SiO 2 /TiO 2 coating ’, J. Mol. Catal. A: Chem. , 244 , 160 – 167 .

Zhang GM , Zhang J , Xie GY , Liu ZF and Shao HB ( 2006 ), ‘ Cicada wings: A stamp from nature for nanoimprint lithography ’, Small , 2 , 1440 – 1443 .

Zhang H , Millington KR and Wang X ( 2009 ), ‘ The photostability of wool doped with photocatalytic titanium dioxide nano particles ’, Polym. Degrad. Stab. , 94 ( 2 ), 278 – 283 .

Zhang J , France P , Radomyselskiy A , Datta S , Zhao J and van Ooij W ( 2003 ), ‘ Hydrophobic cotton fabric coated by a thin nanoparticulate plasma fi lm ’, J. Appl. Polym. Sci. , 88 ( 6 ), 1473 – 1481 .

Zhang M , Shi L , Yuan S , Zhao Y and Fang J ( 2009 ), ‘ Synthesis and photocatalytic properties of highly stable and neutral TiO 2 /SiO 2 hydrosol ’, J. Colloid. Interf. Sci. , 330 ( 1 ), 113 – 118 .

Zhang M , Wang S , Wang C and Li J ( 2012 ), ‘ A facile method to fabricate superhydrophobic cotton fabrics ’, Appl. Surf. Sci. , 261 , 561 – 566 .

Zhang M , Wang C , Wang S and Li J ( 2013 ), ‘ Fabrication of superhydrophobic cotton textiles for water–oil separation based on drop-coating route ’, Carbohydr. Polym. , 97 ( 1 ) 59 – 64 .

Zhang X , Shi F , Niu J , Jiang Y and Wang Z ( 2008 ), ‘ Superhydrophobic surfaces: From structural control to functional application ’, J. Mater. Chem. , 18 , 621 – 633 .

Zhao Y , Tang Y , Wang X and Lin T ( 2010 ), ‘ Superhydrophobic cotton fabric fabricated by electrostatic assembly of silica nanoparticles and its remarkable buoyancy ’, Appl. Surf. Sci. , 256 , 6736 – 6742 .

Zheng JY , Feng J and Zhong MQ ( 2010 ), ‘ Fabricating polymer superhydrophilic/superhydrophobic surfaces by replica/molding method using CaCO 3 particles as template ’, Acta Polym. Sin. , 10 , 1186 – 1192 .

Zhou H , Wang H , Niu H , Gestos A , Wang X and Lin T ( 2012 ), ‘ Fluoroalkyl silane modifi ed silicone rubber/nanoparticle composite: A super durable, robust superhydrophobic fabric coating ’, Adv. Mater. , 24 ( 18 ), 2409 – 2412 .

Zimmermann J , Reifl er FA , Fortunato G , Gerhardt LC and Seeger S ( 2008 ), ‘ A simple, one-step approach to durable and robust superhydrophobic textiles ’, Adv. Funct. Mater. , 18 ( 22 ), 3662 – 3669 .

8 Self Cleaning Finishes for Textiles

Documents

Transcript of 8 Self Cleaning Finishes for Textiles