paper nano finishes mlg

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Nano Finishes M.L. Gulrajani Department of Textile Technology, Indian Institute of Technology New Delhi – 110016 [email protected] 1 Introduction The most significant impetuous to the development of nano finishes for textiles has been given by the dedicated R&D work of Taiwan-born Dr. David Soane. After almost 20 years at the University of California, Berkeley, Dr. Soane left academe. Using his garage as a lab, Soane began devising ways to use nanotechnology to add unusual properties to natural and synthetic textiles, without changing a fabric's look or feel. He floated the first nanotechnology based company Nano-Tex in 1998 to specifically catering to textile industry. All most at the same time the pioneering work of Professor W. Barthlott of University of the City of Bonn, Germany lead to understanding of the mechanism by which the leaves of lotus and other plants utilize super-hydrophobicity as the basis of a self-cleaning. He now owns a patent and the ‘Lotus Effect’ trademark. The ‘Lotus Effect’ has been the basis of the NanoSphere® based stain protection and oil and water-repellent textile finishes of Schoeller Textiles A. G. of Switzerland. The most recent impetus for the development of nano finishes for textiles has come from the work of Dr. Walid Daoud and Dr. John Xin of the Hong Kong Polytechnic University, Kowloon. These scientists have invented an efficient way to coat cotton cloth with tiny particles of titanium dioxide. These nanoparticles act as catalysts that help break down

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Page 1: Paper Nano Finishes MLG

Nano Finishes

M.L. GulrajaniDepartment of Textile Technology,

Indian Institute of TechnologyNew Delhi – [email protected]

1 Introduction

The most significant impetuous to the development of nano finishes for textiles has been given by the dedicated R&D work of Taiwan-born Dr. David Soane. After almost 20 years at the University of California, Berkeley, Dr. Soane left academe. Using his garage as a lab, Soane began devising ways to use nanotechnology to add unusual properties to natural and synthetic textiles, without changing a fabric's look or feel. He floated the first nanotechnology based company Nano-Tex in 1998 to specifically catering to textile industry.

All most at the same time the pioneering work of Professor W. Barthlott of University of the City of Bonn, Germany lead to understanding of the mechanism by which the leaves of lotus and other plants utilize super-hydrophobicity as the basis of a self-cleaning. He now owns a patent and the ‘Lotus Effect’ trademark. The ‘Lotus Effect’ has been the basis of the NanoSphere® based stain protection and oil and water-repellent textile finishes of Schoeller Textiles A. G. of Switzerland.

The most recent impetus for the development of nano finishes for textiles has come from the work of Dr. Walid Daoud and Dr. John Xin of the Hong Kong Polytechnic University, Kowloon. These scientists have invented an efficient way to coat cotton cloth with tiny particles of titanium dioxide. These nanoparticles act as catalysts that help break down carbon-based molecules, and require only sunlight to trigger the reaction. The inventors believe these fabrics could be made into self-cleaning clothes that tackle dirt, environmental pollutants and harmful microorganisms.

Today we have a plethora of textile finishes that are based on the basic R&D carried out by the above-mentioned pioneers.

2 Easy-Care: Hydrophobic Nano Finish

Hydrophobic surfaces can be produced mainly in two ways. One is to create a rough structure on a hydrophobic surface, and the other is to modify a rough surface by materials with low surface free energy. Both these approaches have been used to give a hydrophobic finish to textile substrates.

During last half a century many methods of imparting hydrophobic character to cotton have been developed that includes the use of hydrophobic polymer films and attachment

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of hydrophobic monomers via physical or chemical sorption processes. Monomeric hydrocarbon hydrophobes include aluminium and zirconium soaps, waxes and wax-like substances, metal complexes, pridinium compounds, methalol compounds, and other fibre reactive finishes.

Fluorocarbon finishes constitute an important class of hydrophobic finishes. These finishes first applied to textiles in the 1960s to impart water- and oil-repellency have seen considerable growth during last decade. This growth is mainly consumer-driven since consumer demands for easy-care properties such as water- and oil-repellency, stain repellency and soil- and stain-release properties.

Fluorocarbons are a class of organic chemicals that contain a perfluoroalkyl residue in which all the hydrogen atoms have been replaced by fluorine. These chemicals have very high thermal stability and low reactivity. They considerably reduce the surface tension. The critical surface tension (C) of –CF3 is 6 mN m-1.

Fluorocarbon finishes are dispersions of per fluorinated acrylates having comonomers. The structure of the fluorinated acrylates can be chemically engineered by varying the proportion of hydrophobic and hydrophilic groups in the side chains to produce specific properties. Durable fluorocarbon finishes have reactive methalol or epoxy groups that may react to form a cross-linked net work that may also get covalently bonded to the surface of the fibres. These finishes form low-energy films that protect the fibres in the treated fabrics.

The fluorinated side chains of a polyacrylate fluorocarbon finish are oriented away from the fibre surface in the air and hence these chains form low energy repellant surface as shown in Fig. 11,2.

Fig. 1 Film of fluorocarbon acrylate polymer based finish, where R,R’ and R” = functional or polar groups, responsible for film formation and hardness, crosslinking to increase fastness to washing, emulsification, and affinity for textile surfaces

In the fluorocarbon finishes the critical surface tension (C) depends on the chain length of fluorinated side chain and is minimum for chain length of n = 9. The effect of the chain length on the oil- and water-repellency is shown in Table 11,3.

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Table 1 – Oil- and water-repellency of fabrics treated with acrylic polymers

Perfluorinated group

Measurement of oil-repellency(AATCC 118)

Spray test(ISO 4920)

–CF3 0 50CF2–CF3 3-4 70(CF2)2–CF3 6-7 70(CF2)4–CF3 7-8 70(CF2)6–CF3 7-8 70(CF2)8–CF3 8 80

To develop a more durable hydrophobic and oleophobic finish that was not blocking the pores of the fabric by formation of polymer film thereby making it more breathable Soane and coworkers4-6 patented a large number of multifunctional (nano) molecules that were capable of covalently and non-covalently attached to cellulosic and protenious fibres. Some of these multifunctional molecules were block copolymers or graft copolymers having plural functional groups such as binding groups, hydrophobic groups, hydrophilic groups and oleophobic groups. These groups may be present in the form of hydrophobic and hydrophilic regions. In these multifunctional molecules the hydrophilic groups such as the carboxyl groups act as reactive groups. These may be present in the form of poly carboxylic acid or as poly anhydrides such as poly (maleic anhydride) polymer.

One such multifunctional molecule may be represented as shown in Fig. 2

Fig. 2 Multifunctional molecule of Dr. Shone, where m, n = 0 or 1, o= 0 or 2. ‘R’ is a linear, branched, or cyclic hydrocarbon or fluorocarbon having C1 to C30 hydrocarbon or fluorocarbon groups. ‘A’ is –SO2-, -CONH-, -CH2- or CF2. ‘X’ is a nucleophilic group capable of reacting with hydroxyl, amine or thiol group.

A reaction scheme of a multifunctional molecule with cotton is shown in Fig. 3. Where a hydrophilic reactive molecule of poly (maleic anhydride) first reacts with the hydro- or fluoroalkyls having preferably C8 or C9 (for maximum hydro- and oleophobicity as

R

A

(O)n

X

( )m

( )o

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discussed above) to form a multifunctional molecules having hydrophobic, oleophilic and hydrophilic groups or regions. Subsequently this multifunctional molecule reacts with the hydroxyl groups of cotton or other cellulosic fibres and amino groups of wool to form hydrophobic whiskers on the surface of the fabric without blocking its pores.

Fig. 3 Reaction schemes of a multifunctional molecule formation and attachment with cotton to form whiskers on the surface that are floating in air away from the fabric surface

It is claimed that the attached multifunctional molecule can impart wrinkle resistance by cross-linking cellulose chains via maleic anhydride residues. The molecule can also modify the surface properties of the treated fabric and impart water-repellency, grease-repellency, soil-resistance, detergent free washing, increased speed of drying, improved strength and abrasion resistance without affecting its air permeability or breathability. Due to multiplicity of bonds and ability of the molecule to easily diffuse into the fibre because of its small molecular size (nano size) the durability of the finish is much better than the conventional fluorocarbon acrylate polymer based finish.This original research formed the basis of first commercially successful nano finish originally named as ‘Nano-CareTM’ and marketed by Nano-Tex. Thus Dr. Soane

(a) (b)

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demonstrated that 10-100 nanometer ‘whiskers’ attached to cotton fibres modify the surface tension so much that almost nothing could soak into and stain the treated fabric Red wine, soy sauce, chocolate syrup.

3 Super-hydrophobicity: Biomimatic Self-Cleaning: Lotus Effect

Hydrophobic fluorocarbon finishes as discussed above lower the surface energy and can give a maximum water contact angle of roughly 1200. To get higher contact angles and have self-cleaning ability super-hydrophobic finish with a contact angle of above 1500 is required. This type of finish is obtained by increasing the surface roughness. Increase of the surface roughness provides a large geometric area for a relatively small projected area. The roughened surface generally takes the form of a substrate member with a multiplicity of microscale to nanoscale projections or cavities.

Cassie and Baxter7 were the first to observe that water-repellency of rough surfaces was due to the air enclosed between the gaps in the surface. This enlarges the water/air interface while the solid/water interface is minimized. In this situation, spreading does not occur; the water forms a spherical droplet.

The self-cleaning propensity of plant leaves’ rough surface was investigated and reported by Barthlott and Neinhuis8 in 1997. These investigators analyzed the surface characteristics by high-resolution SEM and measured the contact-angle (CA) of leaves from 340 plant species, cultivated at the Botanical Garden in Bonn. The majority of the wettable leaves (CA < 110°) investigated were more or less smooth, without any prominent surface sculpturing. In particular, epicuticular wax crystals were absent. In contrast, water-repellent leaves exhibited various surface sculptures mainly epicuticular wax crystals in combination with papillose epidermal cells Their CAs always exceeded 150°. They observed that on water-repellent surfaces, water contracted to form spherical droplets. It ran off the leaf very quickly, even at slight angles of inclination (< 5°), without leaving any residue. Particles of all kind that were adhering to the leaf surface were always removed entirely from water-repellent leaves when subjected to natural or artificial rain, as long as the surface waxes were not destroyed. The dirt particles deposited on the waxy surface of the leaves are generally larger that the microstructure of the surface of the leaf and are hence deposited on the tips as a result the interfacial area between both is minimized. In the case of a water droplet rolling over a particle, the surface area of the droplet exposed to air is reduced and energy through adsorption is gained. Since the adhesion between particle and surface is greater than the adhesion between particle and water droplet the particle is ``captured'' by the water droplet and removed from the leaf surface.

The results presented above document an almost complete self-cleaning ability by water-repellent plant surfaces. This could be demonstrated most impressively with the large peltate leaves of the sacred lotus (Nelumbo nucifera). Barthlott and Neinhuis found that according to tradition in Asian religions, the sacred lotus is a symbol for purity, ensuing from the same observations that they made. They also found that this knowledge is

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already documented in Sanskrit writings, which fact led them to call this phenomenon the ‘Lotus-Effect’.

The self-cleaning property of lotus leaf is dependent on two important factors namely the super-hydrophobicity, that is, a very high water contact angle, and very low roll off angle.

The relation between roughness of hydrophobic surfaces and contact angle was established many years ago by Wenzel9 and Cassie and Baxter7 (see Fig. 4). The Wenzel equation relates to the homogeneous wetting regime and yields the Wenzel apparent contact angle, W, in terms of the Young contact angle, Y, and the roughness ratio, r:

cos W = r cos Y

The roughness ratio is defined as the ratio of the true area of the solid surface to its nominal area. This equation shows that when the surface is hydrophobic (Y > /2), roughness increases the contact angle.

The Cassie and Baxter equation describes the heterogeneous wetting regime and gives CB, the CB apparent contact angle, as

cos CB = rf f cos Y + f -1

In this equation, f is the fraction of the projected area of the solid surface that is wet by the liquid, and rf is the roughness ratio of the wet area. When f = 1, rf = r, and the CB equation turns into the Wenzel equation.

Fig. 4 (a) Young’ wetting equation (b) Homogeneous wetting on a hydrophobic, rough surface (c) Heterogeneous wetting on a hydrophobic, rough surface.

LV, SV, and SL are the surface energies of theliquid-liquid, solid-vapour and solid-liquid.

Young’s equation: LV, SV SL = cos

LV

SV

SL

LIQUID

SOLID

SATURATED VAPOUR

cos W = r cos Ycos CB = rf f cos Y + f -1

Y is Young’s

Wenzel Equation Cassie and Baxter Equation

(a)

(b)(c)

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It has been shown10 that the heterogeneous wetting regime is practically preferred by nature as the super-hydrophobic state on Lotus leaves. Moreover the structures that trap air give low sliding angles required for self-cleaning. A relationship between sliding angles and contact angles on super-hydrophobic surfaces with roughness has been worked out10 (Fig. 5).

Fig. 5 A drop on a rough surface: (a) the contact angle, (b) the roll-off angle,

Miwa et al.11 also prepared a transparent super-hydrophobic film whose sliding angle was approximately 1° for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfied the requirements of super-hydrophobicity, transparency, and a low water sliding angle.

Recently, Hang Ji and colleagues at Peking University in China and the Ecole Normale Supérieure in Paris, France have created a super-hydrophobic polymer structure by directly replicating the surface of a lotus leaf as shown in Fig.612. Poly(dimethylsiloxane) (PDMS) was used to replicate the lotus leaf structure. The leaf is used as a template to cast a complementary PDMS layer. An anti-stick layer is added to the PDMS, which is then used as a negative template for a second PDMS casting step. The second PDMS layer is then a positive image of the lotus leaf. The complex lotus surface patterns are transferred with high fidelity. The artificial PDMS lotus leaf has the same water contact angles and very low water roll-off angle as the natural lotus.

Fig. 7 Both the lotus leaf (top) and replicated polymer structure (bottom) have the same super-hydrophobic behavior. (© 2005 American Chemical Society.)

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The lowering of wetability by topological changes and the self-cleaning ability of the plants known as the ‘Lotus Effect’ has been patented.13 In this patent it is disclosed that it is technically possible to make the surfaces of articles artificially self-cleaning, merely providing them with a surface of elevations and depressions in a range of 5 to 200 micrometers and the height of the hydrophobic elevations in the range of 5 to 100 micrometers. It also mentions that the self-cleaning surfaces can be produced either by creating surface structures from hydrophobic polymers during the manufacture or creating the surface structures subsequently by imprinting or etching or by adhesion of a hydrophobic polymer on the surface.

‘Lotus Effect’ based textile finishes have been developed, patented and commercialized by Schoeller Textil AG of Switzerland. It is claimed that Nanospheres formation on the surface of the treated fabrics makes it superhyrodrophobic and oleophobic and it acquires self-cleaning characteristic as have been reported for the Lotus leaves. In the patent filed by Klaus,   Marte,   Meyer and Waeber of Schoeller Textile AG14, it is disclosed that the finish comprises of two water- and oil-repellent components. One of the two predominantly contains the gel-forming compound, while the other one is dominated by the nonpolar water- and/or -oil-repulsive components. A crosslinking agent is used to insolubilise the finish. During drying of the finished (padded) fabric the contraction of the film formed takes place resulting in anisotropic distribution of gel-forming component of the finish and a microstructure similar to that on the Lotus leaf is created on the surface of the finished fabric. The self organization of gel-forming component and creation of the microstructure are determined by both the phase instability and by phase transitions of the components.

For example, for the finishing of a polyester fabric, the fabric is first treated with sodium hydroxide so as to create additional hydroxyl and carboxylic acid groups on the surface. The weight reduction may be restricted to 0.5%. The fabric is then padded with the finishing composition given in Table 2 to 55% wet-pick-up, dried at 800C and subsequently cured for three minutes at 1600C.

Table 2 Padding recipe for polyester fabrics

Component Description Quantity

Aerosil R812S Hexamethyldisilazane treated silica 1.5 g/l

Cerol EWL Wax Emulsion 220 g/l

Tripalmitin Glycerol tripalmitate 4 g/l

Lyofix CHN Amino-triazine-formaldehyde precondensate

9 g/l

Glycerin 3 g/l

Aluminium sulfate 0.5 g/l

Acetic acid 5 g/l

Water 757ml/l

An alternate more durable padding recipe for polyester is given Table 3. It is claimed that

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the triglyceride in the emulsion copolymer melts at 50-900C during curing and gets dynamically oriented in the film as to give it a unique structure13.

Table 3 Alternate padding recipe for polyester fabrics

Component Description Quantity

Aerosil R 812 S Hexamethyldisilazane treated silica 5 g/lAcrylatcopolymer emulsion (35%)

Copolymer of methacrylic acid and dodecyl ester of methacrylic acid having 10% stearyl triglycerides

150 g/l

Polyvinylpyrrolidone- K90 1 g/lIsopropanol 50 g/lWater 794 g/l

Similar recipes for the finishing of cotton, polyester-cotton, polyamide, polypropylene and Lycra containing fabrics, so as to get “Lotus Effect”, have also been proposed13.

Nakajima et al.15 claim to be the first to produce transparent super-hydrophobic thin films with TiO2 by utilizing a sublimation material and subsequent coating of a fluoroalkyl silane that satisfy the requirements of transparency, super-hydrophobicity, and long lifetime simultaneously. A process and composition for producing self-cleaning surfaces from aqueous systems having TiO2 has been patented by Valpey III and Jones16. The finish consists of nano particles having a particle size of less than 300 nm and a hydrophobic film forming polymer. On application to the substrate a transparent self-cleaning coating is formed. In an experiment an aqueous solution having 1% Titania (TiO2) with a mean particle size of 25-51 nm in 1% Zonyl® 9373 (fluorinated acrylic copolymer of Du Pont) were applied on cotton to create stain-resistant surface. The treated fabric was stained with ketchup, charcoal dust, vegetable oil, transmission fluid, turmeric solution, coffee, mustard, glue, used motor oil, creamed spinach and spaghetti sauce. The treated fabrics showed very good stain resistance as compared to the control sample without Titania. This invention provides a process and composition that combines surface roughness and hydrophobicity for creating self-cleaning surfaces. The created substrates have many attributes that include, water-repellency, self-cleaning with water and stain release.

Super-hydrophobic coatings with Al2O3 gel17, gel-like isostatic polypropylene18, aligned carbon nanotubes19, silica20, ZnO nanoparticles21-24, ZnO-coated CNTs25, boehmite nanoparticles26 and CaCO3-loaded hydrogel spheres27 and many such nano-particles-hyrophobic film forming compositions have been developed.

In the studies discussed above hydrophobic particles and film forming agents used to create surfaces to achieve super-hydrophobic self-cleaning properties have a drawback of poor durability on textile substrates. On a typical textile substrate such as a woven fabric, a complex surface topology already exists. For instance, millimeter scale structures are created by the weaving of yarns; 10 to 100 micrometer scale structures are created by fibres within the yarn. Moreover, the textile substrates are mechanically flexible. On such complex structured flexible textile substrates, particles alone are not sufficient to build desired rough structures which exhibit ‘Lotus Effect’ that are durable against laundering

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and abrasion for textile applications.

An alternate approach is to use combination of both the chemical and mechanical treatments to create super-hydrophobic nanostructures on the surface of textile materials28. Mechanically roughened surfaces become an integral part of the product and are more durable. Mechanical roughening of the fabric can be carried out by any of the treatments such as calendaring, embossing, etching, schreinering, sueding, sanding, abrading or emorizing. In the conventional surface-effect finishing the abrasive roller with 400 grit or coarser are used to modify the feel of the fabric. In many such cases the surface fibres are loosened or broken which in turn increase the hairiness of the fabric surface that may hinder the rolling of the water on the surface of the fabric. In order to get fine grinding of the surface fibres without breaking the fibres abrading roller with 1200 grit or above are used and only about 20% of the whole fabric that constitute the upper surface of the fabric may receive this treatment and is considered sufficient for super-hydrophobic ‘Lotus Effect’ finish. The roughness of the abraded surface can be quantified in terms of Roughness Factor by microscopically examining the roughened fibres. The ratio of the roughened profile length to the rectilinear length along the fibres is the Roughness Factor (R.F.). A Roughness Factor of 1.1 is considered sufficient but a RF of 1.2 or even 1.3 gives better results. A subsequent treatment with crosslikable fluorocarbon having nanoparticles of, for example, silica, colloidal silica, alumina, zirconia, titania, zinc oxide, precipitated calcium carbonate, PTFE of 10 to 50nm size, significantly improves the hyrophobicity thereby reducing the rolling angle of the water droplets or the Dynamic Rolling Angle.

A micro-denier polyester 2 x 2 right hand twill having 175 warp and 80 fill yarns per inch fabric made from textured polyester 1/14/200 denier warp and 1/50/100 denier weft yarns was initially abraded with diamond coated roller at a level of 1200-30-12 (1200 grit roller at 30 psi pressure 12 cycles in each direction), whereby approximately 19% of the surface areas were roughened. The fabric was then treated with a chemical solution having 1% hydrophilic silica particles (Sipemat 22LS), 4% fluorocarbon stain repellent (Repearl F-7000) and 1% crosslinking agent (Miligard MRX) with 50% wet pick up and cured.

The treated fabric was tested for water and oil repellency, spray rating and dynamic rolling angle (DRA) and after 1, 5, 10 and 20 home washes as well as after 2000, 5000, 10,000 and 20,000 cycles of Martindale abrasion. The results of spray rating and DRA are shown in Table 4.

Similar treatments were also carried out on un-abraded fabrics, as well as on those where instead of fluorocarbon stain repellent, silicone and wax and or no nano-particles were used. The results in all these cases were found to be inferior to those obtained with abraded, fluorocarbon treated fabrics28.

Unisearch Limited has applied for a patent29 for converting micro-structured surface into a super-hydrophobic surface with a contact angle of >1500 by applying 0.1 to 1.0 micron thick coating of trifunctional alkylsilanes to the micro-structured surface that on curing forms a hydrophobic coating having a nanoscale roughness on the micro-structured

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surface. The resultant surface has both the nanoscale and microscale roughness.

Table 4 Results of spray rating and DRA measurement

Sample Spray Rating DRA* (0)

After finish treatment 100 3.0After 1 wash 90 4.5After 5 wash 75 14After 10 wash 70 18.5After 20 wash 60 26.5After 2000 cycles 75 11.0After 5000 cycles 75 17After 10,000 cycles 75 21.5After 20,000 cycles 60 NA* Dynamic Rolling Angle for rolling of 3 cm

It is assumed that the textile fabrics have micro-structure and an application of the finish and its subsequent curing will give them nanoscale roughness. A typical finish may have 45% methytrimethoxysilane, 4.5% polymethylsiloxane (-OH terminated), 9% octyltriethoxysilane, 40% ethyl acetate, 0.5% dibutyltin dilaurate and 1% 3-aminopropyltriethoxysilane. It may be prepared by mixing methytrimethoxysilane, polymethylsiloxane, ethyl acetate and 0.1% dibutyltin dilaurate in a large reaction vessel in an inert atmosphere. The mixture may be stirred and heated at 600C for 3 h. Octyltriethoxysilane and 3-aminopropyltriethoxysilane may be added during stirring. Remaining 0.4% tin catalyst may be added before padding the fabric with the finish. The fabric is padded with 5 to 20% of this mixture and cured at room temperature in air for 24 h.

It is claimed that during curing hydrolytic condensation of trifunctional silanes form a network polymers or polyhedral clusters having the generic formula (RSiO1.5) n. between that of silica (SiO2) and silicone (R2SiO), more commonly known as, silsesquioxanes or polyhedral oligomeric silsesquioxane (POSS). The POSS nanoparticles are thus deposited on the surface of the fabric. It is also claimed that organically modified silicate (Ormosil) nanoscale sol-gels may also be formed, that on curing will also give nanostructures as shown in Fig.7.

Fig. 7 Structure of POSS and ormosilPlasma treatment has also been claimed to be responsible for creating roughness on

Alkyl groupSilsesquinone

(R.O)3Si

Ormosil

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cotton fabrics. In a study carried out by Zhang et al.30 it is stated that the creation of super-hydrophobicity by applying fluorocarbon chemicals to cotton fabrics in an audio frequency (AC) plasma chamber is a result of the film formation as well as roughness of the treated fabric. During the treatment a nanoparticulate hydrophobic film is deposited onto a cotton fabric surface that has a water contact angle of about 164° that is much higher that of Scotchgard-protector-coated cotton (aprox.137°).

4 Photocatalytic Self-Cleaning

During the last two decades advanced oxidation processes that are combinations of powerful oxidizing agents (catalytic initiators) with UV or near-UV light have been applied for the removal of organic pollutants and xenobiotics from textile effluents31. Among them, TiO2 has been proved to be an excellent catalyst in the photodegradation of colorants and other organic pollutants.

Photocatalytic propensity of semiconductors such as TiO2 has been attributed to the promotion of an electron from the valence band (VB) (O 2p) to the conduction band (CB) (Ti 3d) brought about by the absorption of a photon of ultra-bandgap (≈3.2 eV) light, i.e. hν ≥ EBG. Where, EBG is the energy difference between the electrons in the VB and the CB. The photogenerated electron–hole pair, e−h+ created due to the electron transfer from VB to CB determines largely the overall photoactivity of the semiconductor material31. In the presence of oxygen and/or H2O superoxide ( O2) and/or hydroxyl ( OH) radicals are formed. These radicals attack adsorbed organic species on the surface of TiO2

and decompose them.

Under these circumstances, if an electron donor, i.e. ED such as ethanol, methanol, and EDTA, is present at the surface, then the photogenerated hole can react with it to generate an oxidised product, ED+. Similarly, if there is an electron acceptor present at the surface, i.e. EA, such as oxygen or hydrogen peroxide, then the photogenerated conductance band electrons can react with it to generate a reduced product, EA−. The overall reaction can be summarised as follows.

TiO2

EA + ED EA + ED+

hν ≈3.2 eV

Many of the current commercial systems that utilize this reaction employ the semiconductor photocatalyst TiO2 to oxidise organic pollutants by oxygen, i.e.

TiO2

Organic pollutant + O2 CO2 + H2O + mineral acidhν ≈3.2 eV

A schematic representation of this process is illustrated in Fig 8.

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Valance Bond +2.7 eV

-0.5 eV

h+

e-

Conduction Bond

OxidationOH + h+ OH▪

OH

OH▪

ReductionO2 + e O2

Eg = 3.2 eV

O2

O2

h < 400 nm

Energy

Semiconductor Nano-partic les as Photocatalysts

R-COOH CO2 + H2OOrganic

Contaminant

TiO2

Valance Bond +2.7 eV

-0.5 eV

h+

e-

Conduction Bond

OxidationOH + h+ OH▪

OH

OH▪

ReductionO2 + e O2

Eg = 3.2 eV

O2

O2

h < 400 nm

Energy

Semiconductor Nano-partic les as Photocatalysts

R-COOH CO2 + H2OOrganic

Contaminant

TiO2

Fig.8 Schematic illustration of the major processes associated with TiO2 semiconductor particle

Consequently, following irradiation, the TiO2 particle can act as either an electron donor or acceptor for molecules in the surrounding media. However, the photoinduced charge separation in bare TiO2 particles has a very short lifetime because of charge recombination. Therefore, it is important to prevent hole-electron recombination before a designated chemical reaction occurs on the TiO2 surface.

Thus, titanium dioxide displays all the desired features of an ideal semiconductor photocatalyst. It has a large bandgap, EBG≈3.2–3.0 eV, therefore it is effective in only UV light that constitutes only 5% of the day light. Despite this substantial limitation, its positive features far outweigh this one negative, and so titanium dioxide has become the semiconducting material to use in the field of semiconductor photochemistry. Its dominant position extends not only to basic research but, more importantly to commercial applications32. Although titanium dioxide exists in three crystalline forms, namely: anatase, rutile, and brookite, invariably the form used in semiconductor photochemistry is anatase as this appears to be the most active and easiest to produce of the three.

Anpo et al.33 observed that the photocatalytic activity of TiO2 increased as the diameter of its particles become smaller, especially below 100 A. Nanosized TiO2 particles show high photocatalytic activities because they have a large surface area per unit mass and volume as well as diffusion of the electron/holes before recombination.

As a part of the research project funded by the Innovation and Technology Fund (ITF) of the Hong Kong government, Dr John Xin and Dr Walid Daoud of the Hong Kong Polytechnic University’s Nanotechnology Centre for Functional and Intelligent Textiles and Apparel, developed34-39 a process for the coating of titanium oxide on textile

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substrates at low temperature. They also claimed that on coating cotton with TiO2

particles that were about 20 nm apart, photocatalytic self-cleaning properties could be imparted to the coated fabric.

In the coating composition developed by Xin and Daoud34-39, a sol mixture may be prepared, at room temperature, by mixing titanium tetraisopropoxide, ethanol and acetic acid in a molar ratio of 1:100:0.05, respectively. The mixture is then stirred for a period of time prior to coating. Ten minutes of stirring time was found to be sufficient for ethanol as the suspending medium. However, if water is used, the reaction time is preferred to be between 18-22 h in order to produce a translucent sol. The following equations summarize the principal reactions involved:

Ti(OPr)4 + 4 EtOH Ti(OEt)4 + 4 PrOH

Ti(OPr)4 or Ti(OEt)4 + H2O Ti(OH)4+ 4 PrOH or 4 EtOH

Ti(OH)4 TiO2+2H2O

The fabric to be coated was dried at 100 °C for 30 min, dipped in the above mentioned nanosol for 30 s and then pressed at a nip pressure of 2.75 kg/cm2. The pressed substrates were then dried at 80 °C for 10 min in a preheated oven to drive off ethanol and finally cured at 100 °C for 5 min in a preheated curing oven.

Samples prepared using this general procedure were found to maintain their antibacterial properties after having been subjected to 55 washes through a home laundry machine and UV protection characteristics for 20 washes. This has been attributed to the formation of interfacial bonding through a dehydration reaction between the cellulosic hydroxyl groups of cotton and the hydroxyl groups of titania38.

The investigation of the microstructure of these titania films by scanning electron microscopy (SEM) show that in contrast to the fibrillous texture of a cotton fibre (Fig. 9 (a)), the surface structure of the coated cotton fibre is rather smooth indicating the formation of a uniform continuous layer (Fig. 9 (b)). The observed particles form these images have a near spherical grain morphology and are about 15–20 nm in size38. By slightly varying the sol composition where an aqueous nanosol was prepared at room temperature by mixing and stirring for 18 h titanium tetraisopropoxide (10 g) with acidic water (200 ml) containing nitric acid (2 ml) and subsequently the treated fabric was air dried for 24 h, rinsed with sodium carbonate solution (1%) and water, a titian film having uniform sized particles of 10 nm were deposited on cotton fabric that had antibacterial properties39. In an earlier study the antibacterial activity of TiO2 in presence of UV and white light that also contains a very small fraction of UV (0.47 μW/cm2) has been attributed to the photocatalytic destruction of the bacterial cells40. However, Daoud and coworkers39 have proposed that TiO2 may simply provide no sustenance for bacteria, whereas cellulose, being a hospitable medium, offers good pores for their growth and maintains good respiration for the host. In this context, the TiO2 surface may have

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prevented the formation of a protective biofilm of adsorbed bacteria rather than actively killing them via free radical formation.

Fig. 9 SEM images of (a) uncoated cotton fiber, (b) titania coated cotton fiber showing the morphological change in the surface structure, (c) higher magnification image of titania coated cotton fiber showing the shape and size of the titania particles, and (d) higher magnification image of a titania film coated on glass38.

An elaborate investigation of the self-cleaning of modified cotton textiles by TiO2 at low temperatures under daylight irradiation has been carried out by Bozzi et al.41. These investigators initially created hydrophilic groups on ammonia treated and mercerized cotton fabrics by exposing them to RF and MW-plasma and V-UV radiations. A significant number of carboxylic, percarboxylic, epoxide and peroxide groups form upon either of these treatments. These fabrics were then padded with various concentrations of Titanium tetra-isopropoxide (TTIP as colloidal TiO2 precursor), TiCl4 (as colloidal TiO2

precursor), Colloidal TiO2 and TiO2 Degussa P25 powder (30 nm particles). The treated fabrics were stained with coffee and red wine using a micro-syringe with 50 μl of solution. The irradiation of samples was carried out in the cavity of a Suntest solar simulator (Hanau, Germany) air-cooled at 45 °C and the CO2 volume produced due to oxidation of wine during the irradiation was measured in a gas chromatograph.

It was observed that the surface pretreatment of the cotton textile used in this study allows to attach TiO2 directly on the textile by functionalization of the cotton textile with a variable density of negatively charged functional groups. Moreover, ammonia treated cotton fabrics mineralize more effectively stains of pigmented compounds upon TiO2

loading under daylight than mercerized cotton fabrics.

These investigators have suggested a different mechanism for the decomposition of red wine and the tannin in coffee stain is proposed. The decomposition of the organic

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compound goes through a cation intermediate (stain+) leading ultimately to the production of CO2. The electron generated in the process is injected into the TiO2

conduction band starts the oxidative radical-chain leading to stain discoloration as shown in Fig. 10.

Fig.10 Suggested scheme for the discoloration of wine and coffee stains under visible light irradiation by TiO2 photocatalyst

Yu et al.42 have studied the efficiency of singlet oxygen production of a photosensitied fullerene derivative, hexa(sulfo-n-butyl)[60]fullerene FC4S. Photoexcitation of C60 and fullerene derivatives induces a singlet fullerenyl excited state that is transformed to the corresponding triplet excited state, via intersystem energy crossing, with nearly quantitative efficiency. Subsequent energy transfer from the triplet fullerene derivatives to molecular oxygen produces singlet molecular oxygen in aerobic media (see Fig. 11). This photocatalytic effect becomes one of the mechanisms in photodynamic treatments using fullerene derivatives as photosensitizers complementary to TiO2. They have concluded that this photocatalytic effect makes FC4S a potential alternative sensitizer to TiO2 and feasible for use in the visible region in addition to its intrinsic high UV efficiency.

Fig. 11 Schematic representation of a FC4S-derived nanosphere in aqueous solution based on the aggregation size determined by SANS and SAXS

Stain

Stain*Stain+

Decomposition to CO2 and H2O

Wine, Coffee

e e

O2 O2

h

h > 400 nm

TiO2

O2 + RH (Stain) HO2

+ RR + O2 RO2

60 A

hv{3FC4S*}

O2

[1O2]

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Very recently Torey, Japan announced that they have developed an innovative technology to improve the durability of photocatalytic coating agents using fullerene, an allotrope of carbon, in collaboration with Riken’s Discovery Research Institute43.Firstly, Riken’s Discovery Research Institute developed the derivatives of fullerene. Then, the two partners developed a method to uniformly disperse and mix the fullerene derivatives with acrylic polymers of photocatalytic coating agents. They have confirmed that the durability of photocatalytic coating agents has been doubled, compared to the existing agents, by adding the fullerene derivatives. Further, Toray has confirmed that the new method can be used in the conventional procedures for manufacturing textiles. The company expects that it will lead to a wide range of applications in textile products, including clothes, curtains and carpets.

5. Nano Antimicrobial Finishes

Among the various antimicrobial agents used for the finishing of textile substrates, silver or silver ions have long been known to have strong inhibitory and bactericidal effects as well as a broad spectrum of antimicrobial activities44.

The inhibitor effect of silver ion/silver metal on bacteria has been attributed to the interaction of silver ion with thiol groups in bacteria45 as well as to the oxidative destruction of microorganism in aqueous medium46. Silver ion based antimicrobial finishes have been developed by interaction of a silver salt such as silver nitrate with anionic co-polymer of, for instance, styrene, ethyl acrylate, acrylic acid and divinyl benzene having at least about 0.008 m eq of carboxyl groups per g of polymer and ≥ 0.0009 m mol of silver per g of the polymer. The films of such polymeric finishes release antibacterial and antifungal silver ions slowly over a very long period of time.47 In another patent48 it is disclosed that a silver-containing antimicrobial agent that has affinity for textile fibres can be produced by treating cross-linked carboxy methyl cellulose (CMC) having > 0.4 carboxy methyl groups with silver nitrate. The antimicrobial finishing agent may have 0.01 to 1.0 % silver bound to the water resistant cross-linked CMC (Ag).

Milliken & Company, has developed a silver based antimicrobial agent, Alphasan by forming a complex between silver with zirconium phosphate. Other silver-containing antimicrobials are silver-substituted zeolite available from Sinanen under the trade name Zeomic AJ, and silver-substituted glass available from Ishizuka Glass under the trade name Ionpure. These compounds can be applied on the fabrics by exhaustion with a dye solution. The antimicrobial fabric thus produced on finishing with an acrylic copolymer makes the antimicrobial finish more durable49,50.

Another method of producing durable silver containing antimicrobial finish is to encapsulate a silver compound or nanoparticle with a fibre-reactive polymer. The resulting micro and nanocapsules when applied to a fabric react with it and thus provide durable silver based antimicrobial finish. The microencapsulation of the nanoparticles may be carried out in different ways. For instance for producing microcapsules of water soluble products, the product may first be dissolved in water and subsequently an

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emulsifier be added and an oil soluble encapsulating monomers or oligomer or polymers be added and emulsified. On polymerization and cross-linking the resulting shell encapsulates the water soluble product. One of the fibre-reactive polymers used for this purpose is poly (styrene co-maleic anhydride).51,52

Yang53 has patented a process for preparing a silver nanoparticle containing functional microcapsule having the intrinsic antimicrobial and therapeutic functions of silver, as well as additional functions of the products contained in the inner core of the capsule. For instance by emulsifying a perfume in water with a surfactant and adding an outer shell forming substance such as melanin precondensate to the resulting emulsion to form microcapsule and treating the microcapsule before hardening of the outer shell with a silver nanoparticle dispersed in water soluble styrene maleic anhydride polymer solution produces a microcapsule with the duel function. In these microcapsules the silver nanoparticles are on the surface of the capsule as shown in Fig. 12. Instead of a perfume one can have a thermosensitive pigment, thermal storage material or pharmaceutical preparation in the inner core.

Fig. 12 Structural view of a silver nanoparticle-containing functional microcapsule (a) microcapsule (b) inner core contains a functional substance such as perfume, a thermosensitive pigment, thermal storage

material or pharmaceutical preparation (c) outer shell

Even though metallic silver has adequate antimicrobial properties it is expected that conversion of silver to nanoparticles will have high specific area that may lead to high antimicrobial activity compared to bulk Ag metal. Several methods have been used to prepare silver nanoparticles by reducing silver that include chemical reduction 54, chemical and photoreduction in reverse micelles created in microemulsions55-57  and radiation induced chemical reduction.58 Besides these nano silver of 50-100nm particles

(a)

(b)

(c)

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size can be prepared by a  mechanochemical process by inducing a solid-state displacement reaction between AgCl and Na in a ball mill were elemental Ag and NaCl are formed. NaCl may be dissolved in water and removed resulting in pure nano silver59.

In a chemical reduction method of producing highly concentrated stable dispersions of nanosized silver particles silver nitrate is reduced with Ascorbic acid to precipitate metallic silver in acidic solutions according to following reaction59.

2Ag++ C6H8O6 2Ag0 + C6H6O6 + 2H+.

According to Sondi, Goia and Matijevi 60 to produce a concentrated stable silver nanosols add 16.7 cm3 of a 1.5 mol dm−3 ascorbic acid at a controlled flow rate of 2.5 cm3 min−1 to 83.3 cm3 of an aqueous silver nitrate solution, containing a dispersing agent, 5 wt% Daxad 19 (sodium salts of naphthalene sulfonate formaldehyde condensate).After completion of the precipitation process, the silver precipitates is washed with deionized water to near-neutral pH, and redispersed in water. Alternatively, the nanoparticles could be obtained as dry powder after the solids are separated by centrifugation, washed with acetone, and subsequently dried in vacuo at low temperature. The dry silver particles could be redispersed in deionized water in an ultrasonic bath to obtain concentrated dispersions. The nanosilver produced by this method yields modal diameters of 15 to26 nm. In a subsequent study61 the antimicrobial activity of silver nanoparticles produced by this method were tested against E. coli as a model for Gram-negative bacteria. These particles were shown to be an effective bactericide. Scanning and transmission electron microscopy (SEM and TEM) were used to study the biocidal action of this nanoscale material. The results confirmed that the treated E. coli cells were damaged, showing formation of “pits” in the cell wall of the bacteria, while the silver nanoparticles were found to accumulate in the bacterial membrane. A membrane with such a morphology exhibits a significant increase in permeability, resulting in death of the cell. These nontoxic nanomaterials, which can be prepared in a simple and cost-effective manner, may be suitable for the formulation of new types of bactericidal materials. Chemical reduction of silver nitrate with hydrazine in presence of a dispersing agent to produce 8 nm nano silver particles has been reported by Kim, Han, & Kim62.

Various methods of producing nano silver particles in water-in-oil microemulsions have been reviewed by Capek.63 In many of these processes the silver nanoparticles are coated or encapsulated in the chemicals used. For instance for the preparation of Dodecanethiol-capped silver ‘quantum dot’ particles the microemulsion consists of diethyl ether/AOT/water along with dodecanethiol, where dispersed microdroplets of water domains in organic bulk phase are in equilibrium with excess water. AOT ((bis(2-ethylhexyl)sulfosuccinate) as the anionic surfactant due to its higher solubility in organic phase helps to extract metal cations from the aqueous to reverse micellar phase. The metal ions concentrated in the dynamic reverse microdroplets are reduced with sodium borohydride and consequently capped by dodecanethiol particles. FT-IR investigations and elemental analyses support the encapsulation of silver particles by dodecanethiol (DT) while the transmission electron micrograph reveals an average size of 11 nm63.

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In a study Aymonier et al. 64 found that hybrids of silver particles of 1 to 2 nm in size with highly branched amphiphilically modified polyethyleneimines adhere effectively to polar substrates providing environmentally friendly antimicrobial coatings.

In a very recent study Cho et al.65 have investigated the antimicrobial activity and protection of nanosilver particles (Ag-NPs). For this study stablised Ag-NPs were prepared by sonication of a mixture of colloidal Ag-NPs (0.054%, average diameter: 10 nm) solution containing poly-(N-vinyl-2-pyrrolidone) (PVP) and sodium dodecylsulfate (SDS). Antimicrobial effect of Ag-NPs for S. aureus and E. coli was investigated using cup diffusion method. It was observed that the growth of Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria were inhibited by Ag-NPs. The minimum inhibitory concentration (MIC) of Ag-NPs for S. aureus and E. coli were 5 and 10 ppm, respectively. The main reason of PVP protecting silver nanoparticles is N in PVP coordinates with silver and forms the protection layer66.

It is well known that discoloration of silver and its compounds takes place on exposure to light. It is therefore essential to stabilize silver ions and nanoparticles. Silver ions have been stabilised by reaction with ionic polymers as described above. The stabilization of silver nanoparticles has been been achieved either by coating, encapsulation or complex formation between the lone pair of electrons on N and Ag. It is claimed that all amines that have free pair of electrons can stabilize Ag67. One such example of PVP is already discussed above.

It is claimed68 that antimicrobial yarns made from cotton, linen, silk, wool, polyester, nylon or their blends having nanosilver particles can be produced by immersing them in nanosilver particle-containing solution produced by reducing silver nitrate with glucose and drying at 120-1600C for about 40-60 min. The treated yarns were yellow-orange in colour. Electron microscopic studies of the antimicrobial yarns indicated that the yarn samples contained nanosilver particles which were evenly distributed and contained particles that were mostly below or about 10 nm size. Chemical testing indicated that the silver content in the yarns was about 0.4-0.9% by weight. The treated yarns showed effective antimicrobial activity against various bacteria, fungi, and Chlamydia that included Escherichia coli, Methicillin resistant Staphylococcus aureus, Chlamydia trachomatis, Providencia stuartii, Vibrio vulnificus, Pneumobacillus, Nitrate-negative bacillus, Staphylococcus aureus, Candida albicans, Bacillus cloacae, Bacillus allantoides, Morgan's bacillus (Salmonella morgani), Pseudomonas maltophila, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Bacillus subtilis, Bacillus foecalis alkaligenes, Streptococcus hemolyticus B, Citrobacter, and Salmonella paratyphi C. Moreover the antimicrobial activity remained intact on dyeing as well as even after 100 washes with neutral soap.

Silver-containing antimicrobials have been incorporated into wound care devices and are rapidly gaining acceptance in the medical industry as a safe and effective means of controlling microbial growth in the wound bed, often resulting in improved healing. It is known that placing surface-available silver in contact with a wound allows the silver to enter the wound and become absorbed by undesirable bacteria and fungi that grow and

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prosper in the warm, moist environment of the wound site. Once absorbed, the silver ions kill microbes, resulting in treatment of infected wounds or the prevention of infection in at-risk wounds. Some of the commercial silver antimicrobial would care products are, Acticoat, Actisorb and Silverlon.

Acticoat, a multi-layered wound care device of Smith and Nephew comprises of three layers a layer of polyethylene film, a middle layer of rayon/polyester blend nonwoven fabric, and a second layer of film. Nano-crystalline silver particles are deposited onto the film layers to provide an antimicrobial wound care device. Another product available to consumers, provided by Johnson & Johnson under the trademark Actisorb, is a highly porous, silver-impregnated charcoal cloth, sandwiched between two nylon nonwoven layers. In Silverlon , manufactured by Argentum, nanosilver produced by the reduction of silver nitrate is deposited on sensitized nylon fibres. The silver-laden polyamide fibres are then attached to a fabric. Some of these commercial products are prone to darkening on exposure to light hence coatings for fabrics used for the production of wound care devices with polyurethane binder having nanosilver particles that do not darken on exposure to light has been developed and patented69.

In 2000 and 2002, the Royal Perth Hospital (RPH) Burn Unit, Western Australia, conducted two 'before and after' patient care audits comparing the effectiveness and cost of Silvazine™ (silver sulphadiazine and chlorhexidine digluconate cream) and Acticoat™. The main findings were: when using Acticoat™ the incidence of infection and antibiotic use fell from 55% and 57% in 2000 to 10.5% and 5.2% in 2002. The total costs (excluding antibiotics, staffing and surgery) for those treated with Silvazine™ were US$ 109,357 and those treated with Acticoat™ were US$ 78,907, demonstrating a saving of US$ 30,450 with the new treatment. The average length of stay (LOS) in hospital was 17.25 days for the Silvazine™ group and 12.5 days for the Acticoat™ group - a difference of 4.75 days. These audits demonstrate that Acticoat™ results in a reduced incidence of burn wound cellulitis, antibiotic use and overall cost compared to Silvazine™ in the treatment of early burn wounds70.

In a recent study on the bacteriostasis and skin innoxiousness of nanosize silver colloids on textile fabrics, Lee and Jeong71 have observed that colloidal silver measuring 2-3 nm in diameter had a notable antibacterial efficacy at a concentration of 3 ppm; however, silver colloids measuring 30 nm in diameter had an inferior bacteriostasis at the same concentration level. According to these investigators smaller-sized silver particles in colloidal solution have a better antibacterial efficacy than larger-sized particles. The bacteriostasis of the nonwoven polyester fabric samples and a woven cotton fabric that were treated with 2-3 nm diameter silver particles was 99.99% against S. aureus and K. pneumoniae at a concentration of 10, 20, and 30 ppm for polyester and 20 ppm for cotton. Moreover, nanosize silver colloidal solution was skin-innoxious when the size of the particles was 2-3 nm and the silver concentration of colloidal solution was 100 ppm. The colloidal silver measuring 30 nm in diameter was not innoxious at the same concentration level. It is speculated that smaller-sized silver particles are less toxic to the skin than larger particles and that silver colloids measuring 2-3 nm in diameter can be used as antibacterial agents on fabrics that come into contact with human skin.

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Conclusion

Nano finishes being developed for textile substrates are at their infantile stage. The basic mechanisms and the logic of some of these finishes has been explained by the inventors. Some nano finishes such as Nano-Care, Lotus Effect finish, Nanosphere based finish and Ag Fresh have been commercialized. The commercial viability of these finishes will be customer driven and the value-addition imparted by these finishes. However, the new concepts exploited for the development of nano finishes have opened up exciting opportunities for further R&D.

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