Indian Joual of Fibre & Textile Research Vol. 31, March 2006, pp. 1 87-20 1 Nano fin ishes M L Gulrajani' Department of Textile Technology, Indian Inst itute of Technology, New Delhi 1 10 0 16 Techno-science of some recently introduced nano finishes for textile substrates has been reviewed. The logic of using low molecular weight fibre-react ive fluorocarbons that form the basis of Nano-Care™ finish to impart durable hydrophobic- oleophobic characteristics to fabrics, as described in a patented l iterature, has been discussed. Super hydrophobicity as exhibited by lotus leaves and the finishes to get self-cleaning textile fabrics based on the Lotus Effect™ have been covered giving some typical recipes. Mechanisms proposed to explain the photo-catalytic self-cleaning effect of Ti02 have been described. Developments in the product ion and evaluation of nano s ilver and wound care devices based on ant imicrobial activity of silver have been covered in detail. Keywords: Acti coat™, Antimicrobial, Lotus EffectT�l , Nano-Care™, Nanosphere™, Photo-catalytic, Super hydrophobicity, Self-cl eaning fabric, Whiskers IPC Code: Int.CI. 8 8828, 0068 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 Unive rsity of California, Berkeley, Dr Soane left academe. Using his garage as a lab, Dr Soane began devising ways to use nanotechnology to add unusual properties to natural and synthetic texti les 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. Almost at the same time, the pioneering work of Prof W Barthlott of the University of the City of Bonn, Germany, led to understand the mechanism by which the leaves of lotus and other plants utilize super hydrophobicity as the basis of a self-c leaning. 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-repel lent textile finishes of Schoeller Textiles A. G . of Switzerland. The most recent impetus for the development of nano finishes for textiles has come f rom the work of Dr Walid Daoud and Dr John Xin of the Hong Kong Polytechnic University, Kowloon. These scientists invented an efficient way to coat cotton cloth with tiny particles of titanium dioxide. These nanopartic les act as catalysts that help break down of carbon-based "E-mail: mlg54@hotmail.com molecules and require only sunlight to t rigger the reaction. The inventors believe that these fabrics could be made into se lf-c leaning clothes that tackle dirt, environmental pollutants and harmful . . mlcroorgams ms. Today, we have a plethora of textile finishes that are based on the basic R&D carried out by the above- mentioned pioneers. The techno-science of some recently developed nano finishes for texti le substrates has been reviewed and reported in this paper. 2 Easy Care-Hydrophobic Nano Finish Hydrophobic surfaces can be produced mainly in two ways: (i) by creating a rough st ructure on a hydrophobic surface, and (ii) by modifying a rough surface using materials with low surface free ene rgy. Both these approaches have been used to give a hydrophobic fin ish to textile substrates. During last half a century, many methods of imparting hydrophobic character to cotton have been developed that inc ludes the use of hydrophobic polymer films and attachment of hydrophobic monomers via physical or chemical sorption processes. Monomeric hydrocarbon hydrophobes include aluminium and zirconium soaps, waxes and wax-like substances, metal comp lexes. pridinium compounds, methalol compounds and other fibre reactive finishes. Fluorocarbon fin ishes constitute an important class of hydrophobic finishes. These finishes first applied to textiles in the 1960s to impart water- and oil-
Transcript
Indian Journal of Fibre & Texti le Research
Vol . 3 1 , March 2006, pp. 1 87-20 1
Nano finishes
M L Gulrajani'
Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 016
Techno-science of some recently introduced nano finishes for textile substrates has been reviewed. The logic of using
low molecular weight fibre-reactive fluorocarbons that form the basis of Nano-Care™ finish to impart durable hydrophobicoleophobic characteristics to fabrics, as described in a patented l i terature, has been discussed. Super hydrophobicity as
exhibited by lotus leaves and the finishes to get self-cleaning textile fabrics based on the Lotus Effect™ have been covered giving some typical recipes. Mechanisms proposed to explain the photo-catalytic self-cleaning effect of Ti02 have been described. Developments in the production and evaluation of nano si lver and wound care devices based on antimicrobial activity of silver have been covered in detai l .
Keywords: Acticoat™, A ntimicrobial, Lotus EffectT�l , Nano-Care™, Nanosphere™ , Photo-catalytic, Super hydrophobicity, Self-cleaning fabric, Whiskers
IPC Code: Int.CI.8 8828, 0068
1 Introduction The most significant i mpetuous to the development
of nano finishes for textiles has been g iven 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, Dr 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 1 998 to specifically catering to texti le industry.
Almost at the same time, the pioneering work of Prof W Barthlott of the University of the City of Bonn, Germany, led to understand the mechanism by which the leaves of lotus and other plants uti l ize 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 oi l and water-repellent texti le finishes of Schoeller Texti les A. G . of Switzerland.
The most recent i mpetus for the development of nano finishes for textiles has come from the work of Dr Wal id Daoud and Dr John Xin of the Hong Kong Polytechnic Universi ty , Kowloon. These scientists invented an efficient way to coat cotton cloth with tiny particles of t itanium dioxide. These nanoparticles act as catalysts that help break down of carbon-based
molecules and require only sunlight to trigger the reaction . The inventors believe that these fabrics could be made i nto self-cleaning clothes that tackle dirt, environmental pollutants and harmful
. . mlcroorgamsms.
Today, we have a plethora of texti le finishes that are based on the basic R&D carried out by the abovementioned pioneers. The techno-science of some recently developed nano fin ishes for textile substrates has been reviewed and reported i n this paper.
2 Easy Care-Hydrophobic Nano Finish Hydrophobic surfaces can be produced mainly in
two ways: ( i ) by creating a rough structure on a hydrophobic surface, and ( i i ) by modifying a rough surface using 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 i mparting hydrophobic character to cotton have been developed that includes the use of hydrophobic polymer films and attachment of hydrophobic monomers via phys ical or chemical sorption processes. Monomeric hydrocarbon hydrophobes i nclude aluminium and zirconium soaps, waxes and wax-l ike substances, metal complexes. pridin ium compounds, methalol compounds and other fibre reactive finishes.
Fluorocarbon fin ishes constitute an important class of hydrophobic finishes. These fin ishes first applied to textiles i n the 1 960s to i mpart water- and oil-
1 88 INDIAN 1. FIBRE TEXT. RES. , MARCH 2006
repellency have shown considerable growth during last decade. This growth is mainly consumer-driven since consumer demands for easy care properties such as water- and oi l-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 stabi l ity and low reactivity . They considerably reduce the surface tension . The critical surface tension (Yc) of -CF3 is 6 mN m-I .
Fluorocarbon finishes are dispersions of per fluorinated acrylates having comonomers . The structure of the fluori nated acrylates can be chemically engineered by vary ing the proportion of hydrophobic and hydrophil ic 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-l inked net work that may also get covalently bonded to the surface of the fibres. These finishes form low energy fil ms 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. 1 (refs 1 ,2) . ]n the fluorocarbon finishes, the critical surface tension (Yc) depends on the chain length of fluorinated side chain and is minimum for chain length of Il = 9. The effect of the chain length on the oil- and waterrepellency is shown in Table I (refs 1 ,3).
To develop a more durable hydrophobic and oleophobic finish that does not block the pores of the fabric by the formation of polymer film thereby making it more breathable, Soane et al.4-6 patented a
CF3 C F3 CF] CF3 CF] I I I I I
( C F2 )n (CF2 )n C H 3 (C F2)n (CF2)n (U2 )n I I I I I I (CH2)2 (CH2 )2 ( CH2)n R (CH2 )2 (CH2 )2 (C H2 )2 I I I I I I I 0 0 0 0 0 0 0 I I I I I I I
c = o c = o c = o c=o c=o R' c=o c=o I I I I I I I I
/C H /C H ";: H CH C H C H CH CH -' "cH '-...CH ........ CH/ '-...CH/ ........ CH/ ........ CH/ '-...C H/ "-. I I I , I I I H ICH3 H IC� HICH3 H ICH3 H1CH3 R� H1CH3
Fig. I-Fi lm of fluorocarbon acrylate polymer based finish CR, R' and R" are the functional or polar groups responsible for fi lm formation and hardness, crosslin k ing to i ncrease fastness to washing. emulsification and affin i ty for textile surfaces )
large number of multifunctional (nano) molecules that were capable of forming covalent and non-covalent bonds with 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, hydrophi l ic groups and oleophobic groups. These groups may be present in the form of hydrophobic and hydrophi l ic regions. In these multifunctional molecules, the hydrophil ic groups such as the carboxyl groups act as reactive groups. These may be present i n the form of poly carboxylic aci d or as poly anhydrides such as poly (maleic anhydride) polymer. One such multifunctional molecule may be represe-nted as shown in Fig. 2
A reaction scheme of a multifunctional molecule with cotton is shown in Fig. 3, where a hydrophil ic reactive molecule of poly (maleic anhydride) first reacts with the hydro- or fluoro-alkyls having preferably Cs or C9 (for maximum hydro- and oleophobicity as discussed above) to form a multifunctional molecules having hydrophobic, o leophi l ic and hydrophi l ic groups or regions. Subsequently, this multifunctional molecule reacts
Table I -Oil- and water-repel lency of fabrics treated with acryl ic polymers
Perfluorinated Oil-repel lency test Spray test group (AATCC I i8) ( ISO 4920)
Fig. 2-Multifunctional reacti ve molecule as disclosed by Dr Soane, where m, 11 = 0 or I , 0= 0 or 2. 'R ' is the Ii near. branched or cycli c hydrocarbon or fluorocarbon having C , -C30 hydrocarbon or fluorocarbon groups; 'A' , the -SOr, -CONH-. -CHr or CF2: and 'X' , the nucleophi l ic group capable of reacting with hydroxyl , amine o r thiol group
GULRAJANI: NANG FINISHES 1 89
(0 )
Poly ( m oloic anhydrido) j H O - R R = hydro - a n d
Fig. 3-Multifunctional molecule formation and attachment with cotton to form whiskers on the surface that are floating in air away from the fabric surface [(a)-using poly (maleic anhydride), and (b)-starting with maleic anhydride]
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.
It is c1airped that the attached multifunctional molecule can i mpart wrinkle resistance by crosslinking cellulose chains v ia maleic anhydride residues. The molecule can also modify the surface properties of the treated fabric and i mpart waterrepellency, grease-repellency, soil-resistance, detergent free washing, increased speed of drying, improved strength and abrasion resistance without affecting i ts air permeability or breathabil ity. Due to multipl ic i ty of bonds and abil i ty of the molecule to easi ly diffuse into the fibre because of i ts small molecular size (nano size), the durabi l ity of the finish is much better than the conventional fluorocarbon acrylate polymer based finish.
Thi s orig inal research formed the basis of first commercially successfu l nano finish originally named as 'Nano-Care™,
and marketed by Nano-Tex. Thus, Dr Soane demonstrated that ' 1 0- 1 00 nm 'whiskers'
attached to cotton fibres modi fy the surface tension so much that almost nothing could soak into and stain the treated fabric, be it red w ine, soy sauce or chocolate syrup.
3 Super Hydrophobicity-Biomimatic Self-cleaning: Lotus Effect Hydrophobic fluorocarbon fin ishes as discussed
above lower the surface energy and can give a maximum water contact angle of roughly 1 200• To get higher contact angles and to have self-cleaning abi l i ty, super-hydrophobic finish with a contact angle of above 1 500 is required. This type of finish i s obtained by increasing the surface roughness. The increase in 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
1 90 INDIAN J. FIBRE TEXT. RES., MARCH 2006
enlarges the water/air i nterface while the solid/water interface is minimized. I n this s ituation, spreading does not occur; the water forms <: spherical droplet.
The self-cleaning propensity of plant leaves' rough surface was investigated and reported by Barthlott and Neinhuis8 in 1997. These i nvestigators analyzed the surface characteristics by h igh-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 < 1 10°) investigated were more or less smooth without any prominent surface sculpturing. In particular, epicuticular wax crystals were absent. I n contrast, water-repellent leaves exhibited various surface sculptures mainly epicuticular wax crystals i n combination with papi l lose epidermal cells. Their CAs always exceeded 1 50°. They observed that on water-repellent surfaces, water contracted to form spherical droplets. I t came off the leaf very quickly, even at slight angles of i ncl ination « 5°) , without leaving any residue. Particles of all kind that were adhering to the leaf surface were always removed entirely from water-repel lent 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 than the microstructure of the surface of the leaf and are hence deposited on the tips, as a result the interfacial area between both i s minimized. I n the case of a water droplet rolling over a particle, the surface area of the droplet exposed to air i s reduced and energy through adsorption is gained. S ince the adhesion between particle and surface is greater than the adhesion between parti cle and water droplet, the particle i s ' captured' by the water droplet and removed from the leaf surface .
The results presented above document an almost complete self-cleaning abi li ty by water-repellent plant surfaces. This could be demonstrated most i mpressively with the large peltate leaves of the sacred lotus (Ne:umbo nucifera). Barthlott and Neinhuis found that according to tradition in Asian rel igions, the sacred lotus i s a symbol for purity, ensuing from the same observations that they made. They also found that this knowledge is already documented in sanskrit writings, which led them to call this phenomenon the 'Lotus-Effect ' .
The self-cleaning property of lotus leaf is dependent on two i mportant factors namely the superhydrophobicity , i . e . a very high water contact angle and a very low roll off angle.
The relation between roughness of hydrophobic surfaces and contact angle was established many years ago by Wenzd and Cassie and Baxter' (Fig. 4). The Wenzel equation relates to the homogeneous wetting regi me and y ields the Wenzel apparent contact angle (8w) in terms of the Young contact angle (8y) and the roughness ratio (r), as shown below:
cos8w = r cos8y
The roughness ratio is defined as the ratio of true area of the solid surface to i ts nominal area. This equation shows that when the surface is hydrophobic (8y > nl2), roughness increases the contact angle.
The Cassie and Baxter equation descr;bes the heterogeneous wetting regime and gives 8cIl (apparent contact angle) as
cos8cIl = rrf cos8y + f -1
In this equation,fi s the fraction of projected area of the solid surface that is wet by the l iquid; and rl, the roughness ratio of the wet area. When f = 1 and rf = r, the CB equation turns i nto the Wenzel equation.
It has been shown 1 0 that the heterogeneous wetting regime is practically preferred by nature as the superhydrophobic 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-
(b)
(a)
SOLID TLV, T5v, and T5L are the surface energies of the
liquid-/iquld. soUd-vapour and solid-/iquid.
Young's equation: TLV, Tsv - T5L ' cos (J
cos 9w = ' cos (}y Wenzel Equation
cos Bee = , ''cos (}y + '-1 Cassie and Baxter Equation
(}y is Young's ()
Fig. 4-(a) Young' wetting equation, (b) homogeneous wetting on a hydrophobic rough surface, and (c) heterogeneous wetting on a hydrophobic rough �urface
GULRAJANI : NANO FINISHES 1 9 1
hydrophobic surfaces with roughness has been worked out lO (Fig. 5) .
Miwa et al. I I also prepared a transparent superhydrophobic fi lm whose sl iding angle was approximately I ° for a 7 mg water droplet. On this fi lm, there was almost no resistance to the sliding of water droplets. The flim, thus obtained, satisfied the requirements of super hydrophobici ty , transparency and a low water sl iding angle.
Recently, Hang Ji and his colleagues at Peking University of China and the Ecole Normale Superieure in Paris, France, have created a superhydrophobic polymer structure by directly replicating the surface of a lotus leaf as shown in Fig.6 (ref. 1 2) . Poly(dimethylsi loxane) (POMS) was used to repl icate the lotus leaf structure. The leaf was used as a template to cast a complementary POMS layer. An anti-stick layer was added to the POMS, which was then used as a negative template for a second POMS casting step. The second POMS layer was then a positive i mage of the lotus leaf. The complex lotus surface patterns are transferred with high fidelity. The artificial POMS lotus leaf has the same water contact angles and very low water roll-off angle as the natural lotus.
The lowering of wetabil i ty by topological changes and the self-cleaning abil i ty of the plants known as the 'Lotus Effect' has been patented l 3, where the technical possibil ity of making the surfaces of articles artificially self-cleaning has been discussed, by providing them with a surface of elevations and depressions in a range of 5-200 micrometers and the height of the hydrophobic elevations in the range of 5- 100 micrometers. It also mentions that the selfcleaning surfaces can be produced either by creating surface structures with hydrophobic polymers during the manufacture by adhesion of a hydrophobic polymer on the surface or creating the surface structures subsequently by i mprinting or etching.
'Lotus Effect' based textile fin ishes have been developed, patented and commercial ized by Schoeller Textil AG of Switzerland. It is clai med that the Nanospheres® formation on the surface of the treated fabrics makes it super-hyrodrophobic and oleophobic and hence acquires self-cleaning characteristic as reported for the lotus leaves. In the patent filed by Alfred et at. 1 4 , it is d isclosed that the finish comprises two water- and oil-repel lent components. One of the two predominantly contains the gel-forming compound, while the other one i s dominated by the
Fig. 'i-A drop on a rough surface: (a) contact angie, e and (b) roll-off angle. a
nonpolar water- and/or oil-repulsive components. A crossl inking agent i s used to i nsolubil ise the finish. Ouring drying of the fin i shed (padded) fabric , the contraction of the fi lm 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 fin ished fabric. The self organization of gel-forming component and the creation of microstructure are determined by both the phase i nstabili ty and phase transitions of the components.
For example, for the finishing of a polyester fabric, the fabric is first treated w ith 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 (Table 2) to 55% wet p ick-up, dried at 80°C and subsequently cured for 3 min at 1 60°C.
An alternate more durable padding recipe for polyester i s given in Table 3 . It i s clai med that the triglyceride i n the emulsion copolymer melts at
1 92 INDIAN J. FIBRE TEXT. RES., MARCH 2006
Table 2-Paddi ng recipe for polyester fabrics
Component Description Quantity gIL
Aerosil R8 1 2S Hexamethyldis i lazane treated 1 . 5 s i l ica
Cerol EWL Wax emulsion 220 Tripalmit in Glycerol tripalmitate 4
Glycerin 3 Aluminium 0.5 sulfate Acetic acid 5 Water 757
Table 3-Alternate padding recipe for polyester fabrics
Component
Aerosil R 8 1 2 S
Acrylatcopolymer emulsion (35%)
Polyvinylpyrrol idoneK90 Isopropanol Water
Description
Hexamethyldisi lazane treated s i l ica Copolymer of methacrylic acid and dodecyl ester of methacrylic acid havi ng 1 0% stearyl triglycerides
Quantity giL 5
1 50
50 794
50-90°C during curing and gets dynamically oriented in the film to give it a unique structure. 1 3
S imilar recipes for the finishing of cotton, polyester-cotton, polyamide, polypropylene and Iycra contain ing fabrics, so as to get 'Lotus Effect ' , have also been proposed. 1 3
Nakaj ima e t al. 1 5 claim to be the first to produce transparent super-hydrophobic thi n films with Ti02 by utilizing a sublimeable material and subsequent coating of a f1uoroalkyl s i lane that satisfy the requirements of transparency, super hydrophobicity and long l ifetime s imultaneously. A process and composition for producing self-cleaning surfaces from aqueous systems hav ing Ti02 has been patented by Valpey I I I and Jones. 1 6 The finish consists of nano particles having a particle s ize of less than 300 nm and a hydrophobic fi lm forming polymer. On application to the substrate, a transparent self-cleaning coating is formed. In an experiment, an aqueous solution having 1 % Titan ia (Ti02) with a mean particle size of 25-5 1 n m i n 1 % Zonyl® 9373 (fluorinated acryl ic copolymer of DuPont) was
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 composi tion that combines surface roughness and hydrophobicity for creating self-cleaning sUlfaces. The created substrates have many attributes that include water-repellency, selfcleaning with water and stain release.
Super-hydrophobic coatings with AI203 gel 1 7 , gellike isostatic polypropylene l 8, aligned carbon
b 1 9 ' 1 ' 20 Z 0 .
I ' 1 '4 nanotu es , Sl lca , n nanopartIc es- -" , ZnO-coated CNTs
25, boehmite nanoparticles26 and CaCOr loaded hydrogel spheres27 and many such nanoparticles hyrophobic film forming compositions have been developed.
In the above described studies, the hydrophobic particles and fi lm 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 texti le substrate such as a woven fabric, a complex surface topology already exists. For instance, mi l l imeter scale structures are created by the weaving of yarns ; 1 0- 1 00 micrometer scale structures are created by fibres within the yarn. Moreover, the texti le substrates are mechanically flexible. On such complex structured flexible textile substrates, the particles alone are not sufficient to build desired rough structures which exhibit 'Lotus Effect' that are durable against laundering 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 materials .28 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 . I n many such cases, the surface fibres are loosened or broken, which, in turn, increase the hairiness of the fabric surface that may h inder the roll ing of water on the surface of fabric. I n order to get fine grinding of the surface fibres without
GULRAJANI : NANO FINISHES 1 93
breaking, the fibres abrading roller wi th 1 200 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 quantifi ed in terms of roughness factor by microscopically examining the roughened fibres. The ratio of the roughened profi le length to the recti l inear length along the fibres is the roughness factor (RF). A Roughness Factor of 1 . 1 i s considered sufficient but a RF of 1 .2 or even 1 .3 gives better results. A subsequent treatment with crossl ikable fluorocarbon having nanoparticles of, for example, sil ica, colloidal si lica, alumina, zirconia, titania, zinc oxide, precipi tated calcium carbonate, PTFE of 1 0-50 nm size, significantly improves the hyrophobicity, thereby reducing the rol l ing angle of the water droplets or the dynamic rol l ing angle.
A microdenier polyester fabric (2x2 right hand twill, and 1 75 warp and 80 fil l yarns per inch), made from textured polyester 1 1 1 4/200 denier warp and 1 /50/ 100 denier weft yarns, was in i tially abraded with diamond coated roller at a level of 1 200-30- 1 2 ( 1 200 grit roller at 30 psi pressure and 1 2 cycles in each direction), whereby approx imately 1 9% of the surface areas were roughened. The fabric was then treated with a chemical solution having 1 % hydrophil ic si l ica particles (Sipemat 22LS), 4% fluorocarbon stain repellent (RepearJ F-7000) and 1 % crossl inking agent (Miligard MRX) with 50% wet pick-up and cured.
The treated fabric was tested for water and oil repellency, spray rating and dynamic rol l ing angle (ORA). These were also carried out after 1 , 5 , 10 and 20 home washes as well as after 2000, 5000, 1 0,000 and 20,000 cycles of Martindale abrasion. The results of spray rating and ORA are shown in Table 4. Simi lar treatments were also carried out on unabraded fabrics as well as on those where i nstead of fluorocarbon stain repellent, sil icone 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 fabrics.28
Unisearch Limited has applied for a patent29 for converting micro-structured surface into a superhydrophobic surface with a contact angle of > 1 500 by applying 0. 1 - 1 .0 micron thick coating of trifunctional alkylsilanes to the micro-structured surface that on curi ng forms a hydrophobic coating having a nanoscale roughness on the micro-structured surface.
Table 4-Spray rating and dynamic rol l ing angle (DRA)
Sample Spray rating DRA (for 3 em rol l ing)
deg After finish treatment 1 00 3 . 0
After I wash 90 4.5
After 5 wash 75 1 4
After 1 0 wash 70 1 8 .5
After 20 wash 60 26.5
After 2000 cycles 75 1 1 .0
After 5000 cycles 75 1 7
After 1 0.000 cycles 75 2 1 .5
After 20,000 cycles 60 NA
The resultant surface has both the nanoscale and microscale roughness.
It is assumed that the textile fabrics have microstructure 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% octyltriethoxysi lane, 40% ethyl acetate, 0.5% dibutyltin dilaurate and 1 % 3-aminopropyltriethoxysi lane. 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 60°C for 3 h. Octyl triethoxysilane and 3-aminopropyltriethoxysi lane 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 5-20% of this mixture and cured at room temperature in air for 24 h .
I t i s claimed that during curing the hydrolytic condensation of trifunctional si lanes form a network polymers or polyhedral clusters having the generic formula (RSiOI .5)1l between si l ica (Si02) and sil icone (R2SiO), more commonly known as, s ilsesquioxanes or polyhedral oligomeric silsesquioxane (POSS). The POSS nanoparticles are thus deposited on the surface of the fabric . I t i s also claimed that organically modified sil icate (Ormosi l ) nanoscale sol-gels may also be formed, that on curing wil l also give nanostructures as shown i n Fig. 7 .
Plasma treatment has also been c laimed to be responsible for creating roughness on cotton fabrics. In a study carried out by Zhang et al. 30, i t 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 fi lm formation as well as roughness of the treated fabric.
1 94 INDIAN 1. FIBRE TEXT. RES., MARCH 2006
Fig. 7-Structure of POSS and ormosi l
During the treatment, a nanoparticulate hydrophobic fi lm is deposited onto a cotton fabric surface that has a water contact angle of about 1 64° that i s much higher as compared to Scotchgard-protector-coated cotton (- 1 37°).
4 Photocatalytic Self-cleaning During the last two decades, advanced oxidation
processes that are combinations of powerful oxidizing agents (catalytic ini tiators) with UV or near-UV l ight have been applied for the removal of organic pollutants and xenobiotics from textile effluents] ' . Among them, Ti02 has been proved to be an excellent catalyst in the photodegradation of colorants and other organic pollutants.
Photocatalytic propensity of semiconductors such as Ti02 has been attributed to the promotion of an electron from the valence band (VB) (0 2p) to the conduction band (CB) (Ti 3d) brought about by the absorption of a photon of ultra-bandgap (:::::3 .2 eV) l ight, i .e. Izv :::: EBG (EBG i s 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 material ." In the presence of oxygen and/or H20, superoxide C02) and/or hydroxyl COH) radicals are formed. These radicals attack adsorbed organic species on the surface of Ti02 and decompose them.
Under these circumstances, if an electron donor (ED), such as ethanol, methanol and EDT A, is present at the surface then the photogenerated hole can react with it to generate an oxidised product (ED+) . S imilarly, if there i s an electron acceptor present at the surface, i .e. EA, such as oxygen or hydrogen
Semiconductor Nano-partlcles as Photocatalysts
I!nel1lY
OH-O"ldatlon
OH . ".-. Ott
"" R-COOH CO2 + H20 Organic
Contaminant Fig. g-Schematic i l lustration of the major processes associated
with TiOz semiconductor part icle
peroxide, then the photogenerated conductance band electrons can react with it to generate a reduced product (EA -). The overall reaction can be summarized as fol lows :
Ti02 EA + ED -7EA- + EO+
hv ::::3 .2 eV
Many of the current commercial systems that uti l ize this reaction employ the semiconductor photocatalyst Ti02 to oxidize organic pollutants by oxygen, i .e.
Ti02 Organic pollutant+02 -7C02+H20+mineral acid
hv ::::3 . 2 eV
A schematic representation of this process i s shown in Fig. 8 .
Consequently, fol lowing i rradiation, the Ti02 particle can act as ei ther an electron donor or acceptor for molecules in the surrounding media. However, the photo-induced charge separation in bare Ti02 particles has a very short l i fetime because of charge recombination. Therefore, i t is i mportant to prevent hole-electron recombination before a designated chemical reaction occurs on the Ti02 surface.
Thus, the t itanium dioxide displays all the desired features of an ideal semiconductor photocatalyst. It has a large bandgap, EBG:::::3 .2-3.0 eV, and therefore i s effective i n only UV light that consti tutes only 5% of the day l ight. Despite this substantial l imitation, its posit ive features far outweigh this one negative, and so titanium dioxide has become the semiconducting material to use in the field of semiconductor
GULRAJANI: NAND FINISHES 1 95
photochemistry . I ts dominant position extends not only to basic research but more i mportantly to commercial applications . :l2 Although titanium dioxide exists in three crystall i ne 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.
Anpo et al. 33 observed that the photocatalytic activity of Ti02 increased as the diameter of its particles become smaller, especial ly below 1 00 A. Nanosized Ti02 particles show high photocatalytic activities because they have a large surface area per unit mass and volume as well as diffusion of the electronlholes 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 Wal id Daoud of the Hong Kong Polytechnic Univers i ty ' s Nanotechnology Centre for Functional and I ntell igent Textiles and Apparel, developed:l4-39 a process for the coating of titanium oxide on texti le substrates at low temperature. They also claimed that on coating cotton with Ti02 particles that are about 20 nm apart, photocatalytic self-cleaning properties could be imparted .t0 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 : 1 00:0.05 respectively . The mixture i.s then stirred for a period of time prior to coating. Ten minutes of stirring t ime was found to be sufficient for ethanol as the suspending medium. However, if water i s used, the reaction time is preferred to be 1 8-22 h in order to produce a translucent sol . The fol lowing equations summarize the principal reactions i nvolved:
Ti(OPr)4+4EtOH--7Ti(OEt)4+4PrOH
The fabric to be coated was dried at 1 00°C for 30 min, dipped i n 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 1 0 min in a preheated oven to drive off ethanol and finally cured at 1 00°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 hO,:1e 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 titania.38
•
The i nvestigation of the microstructure of these titania films by scanning electron microscopy (SEM) shows that in contrast to the fibril lous texture of a cotton fibre [Fig. 9 (a)) , the surface structure of the coated cotton fibre is rather smooth indi cating 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 1 5-20 nm in size.38 By sl ightly varying the sol composition where an aqueous nanosol was prepared at room temperature by mixing and stirring (for 1 8 h) titan ium tetraisopropoxide ( 1 0 g) w ith acidic water (200 ml) containing n i tric acid (2 ml), and subsequently the treated fabric was air dried for 24 h, rinsed with sodium carbonate solution ( 1 %) and water. A titian fil m having uniform sized particles of 1 0 nm was deposited on cotton fabric that had antibacterial properties.39 I n an earlier study, the antibacterial acti v ity of Ti02 i n presence of UV and white l ight that
. also contains a very small fraction of UV (0.47 JlW/cm2) has been attributed to the photocatalytic destruction of the bacterial cells.40
However, Daoud et aL. 39 have proposed that Ti02 may s imply 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 Ti02 surface may have prevented the formation of a protective biofilm of adsorbed bacteria rather than actively k i l ling them via free radical formation.
An elaborate investigation of the self-cleaning of modified cotton textiles by Ti02 at low temperatures under daylight irradiation has been carried out by Bozzi et aL.4 1 • These i nvestigators i nitially created hydrophil ic groups on ammonia treated and mercerized cotton fabrics by exposing them to RF and MW-plasma and V isible-UV radiations. A significant number of carboxylic, percarboxylic, epoxide and peroxide groups form on either of these treatments . These fabrics were then padded with various concentrations of titanium tetraisopropoxide (TTIP as colloidal Ti02 precursor), TiCl4 (as colloidal Ti02
1 96 INDIAN I. FIBRE TEXT. RES., MARCH 2006
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 i mage of a t i tania fi lm coated on glassJ8
precursor), colloidal Ti02 and Ti02 Degussa P25 powder (30 nm particles). The treated fabrics were stained with coffee and red wine using a microsyringe with 50 III of solution. The irradiation of samples was carried out i n the cavity of a Suntest solar s imulator (Hanau, Germany) air-cooled at 45 °C and the CO2 volume produced due to oxidation of wine during the i rradiation was measured i n a gas chromatograph.
It was observed that the surface pretreatment of the cotton textile used in this study allows to attach Ti02 directly on the textile by functionalization of the cotton textile with a variable density of negat ively charged functional groups. Moreover, ammonia treated cotton fabrics mineral ize more effectively stains of pigmented compounds upon Ti02 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. The decomposition of the organic compound goes through a cation intermediate (stain+) , leading ultimately to the production of CO2. The electron generated in the process i s i njected into the Ti02 and the conduction band starts the oxidative radical chain. leading to stain d iscoloration as shown in Fig. 1 0.
Yu et al.42
have studied the efficiency of singlet oxygen production of a photosensitied ful lerene
Wine, Coffee
Sg;� 1 Stain
Decomposition to CO2 and H20
hv > 400 02-+ RH (Stain) -+ H02- + R R + 02 -+ R02
Fig. IO--Suggested scheme for the discoloration of wine and 'coffee stains under vi sible l ight i rradiation by Ti02 photocatalyst
derivative. hexa(sulfo-n-butyl ) [60]fullerene (FC4S). Photoexcitation of C60 and fullerene derivatives i nduces a s inglet fullerenyl excited state that is transformed to the corresponding triplet excited state, via i nter-system energy crossing, with nearly quantitative efficiency. Subsequent energy transfer from the triplet fullerene derivatives to molecular oxygen produces s inglet molecular oxygen in aerobic media (Fig. 1 1 ) . This photocatalytic effect becomes one of the mechanisms in photodynamic treatments using fullerene derivatives as photosensitizers complementary to Ti02 . They have concluded that this photocatalytic effect makes FC4S a potential alternative sensitizer to Ti02 and feasible for use i n the visible region in 'addition to i ts i ntrinsic high UV efficiency.
GULRAJANI: NANO FINISHES 1 97
\.. .... _-- 60 A � __ .... J
� {3FC4S*} +02 [10�
Fig. 1 1- Schematic representation of a FC4S-derived nanosphere in aqueous solution based on the aggregation size determined by SANS and SAXS
Very recently, Torey of Japan developed an innovative technology to improve the durabil i ty of photocatalytic coating agents using fullerene, an allotrope of carbon, i n collaboration with Riken' s Discovery Research Institute43 .
Firstly, Riken ' s D iscovery Research I nstitute developed the derivatives of ful lerene. Then, the two partners developed a method to uniformly disperse and mix the fullerene derivatives with acryl ic polymers of photocatalytic coating agents. They confirmed that the durabi li ty of photocatalytic coating agents has been doubled, compared to the existing agents, by adding the fullerene derivatives. Further, Toray also confirmed that the new method can be used in the conventional procedures for manufacturing texti les. The company expects that i t will lead to a wide range of applications i n texti le products, including clothes, curtains and carpets.
5 Nano Antimicrobial Finishes Among the various antimicrobial agents used for
the finishing of textile substrates, s ilver or si lver ions have long been known to have strong inhibitory and bactericidal effects as well as a broad spectrum of antimicrobial activities.44
The inhibitory effect of s i lver ion/s ilver metal on bacteria has been attributed to the interaction of silver ion with thiol groups i n bacteria45 as well as to the oxidative destruction of microorganism in aqueous medium.46 Si lver ion based ant imicrobial fin i shes have been developed by the in teraction of a silver salt such as si lver n itrate with anionic copolymer of styrene, ethyl acrylate, acrylic acid and divinyl benzene having at least about 0.008 m eq of carboxyl groups per gram of polymer and ::: 0.0009 m mol of silver per gram of the polymer. The films of such
polymeric fin ishes release antibacterial and antifungal silver ions slowly over a very long period of time.47 In another patent48, it is disclosed that a s i lver-containing antimicrobial agent that has affinity for textile fibres can be produced by treating cross-l inked carboxy methyl cellulose (CMC) having > 0.4 carboxy methyl groups with si lver n itrate. The antimicrobial finishing agent may have 0.0 1 - 1 .0% silver bound to the water resistant cross-linked CMC (Ag).
Mi l l iken & Company has developed a si lver based antimicrobial agent, AlphasanTM by forming a complex of silver with zirconium phosphate. Other silver-containing antimicrobials are s i lver-substituted zeol ite available from S inanen under the trade name Zeomic™ AJ, and s ilver-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 durable.49,5o
Another method of producing durable si lver 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 i t and thus provide durable silver-based antimi crobial finish. The microencapsulation of the nanoparticles may be carried out i n different ways. For instance, for producing microcapsules of water soluble products, the product may first be d issolved in water and subsequently emulsified after adding an emulsifier and oil-soluble encapsulating monomers or oligomer or polymers. On polymerization and cross-l inking, the resulting shell encapsulates the water soluble product. One of the fibre-reactive polymers used for this purpose is poly (styrene co-maleic anhydride).s l ,s2
Yang5J has patented a process for preparing a silver nanoparticle contammg functional microcapsule having the i ntrinsic antimicrobial and therapeutic functions of si lver as well as additional functions of the products contained in the i nner core of the capsule. Such microcapsules can be prepared by a two step process. I n the first step an emulsified solution of a perfume is encapsulated with melanin precondensate. The microcapsule so produced is treated with si lver nanoparticle d ispersed i n water soluble styrene maleic anhydride polymer solution before it fully dries. Thus microcapsules with duel function are produced. In these microcapsules, the silver
1 98 INDIAN 1. FIBRE TEXT. RES .. MARCH 2006
nanoparticles are on the surface of the capsule (Fig. 1 2) . I nstead of a perfume, one can have a thermosensitive pigment, thermal storage material or pharmaceutical preparation i n the i nner core.
Even though metal l ic si lver has adequate antimicrobial properties, it is expected that the conversion of si lver to nanoparticles wil l have h igh specific area that may lead to high antimicrobial activity compared to bulk Ag metal . Several methods have been used to prepare s ilver nanoparticles by reducing silver that include chemical reduction54, chemical and photoreduction in reverse micelles created in microemulsions55.57 and radiation induced chemical reduction.58 Besides, these nano si lver of so-100nm particles s ize can be prepared using a mechanochemical process by inducing a solid-state displacement reaction between AgCI and Na i n a ball mill where elemental Ag and NaCI are formed. NaCI may be dissolved in water and removed, resul ting in '1 59 pure nano Sl ver.
In a chemical reduction method of producing highly concentrated stable dispersions of nanosized silver particles, silver nitrate is reduced with ascorbic acid to precipitate metall ic s i lver in acidic solutions according to fol lowing reaction59:
2Ag + + Cr,HR06 <=> 2Ago + C6H606 + 2H+
According to Sondi et al.60, to produce a concentrated stable si lver nanosols add 1 6.7 cm3 of a 1 .5 mol dm -3 ascorbic acid at a controlled flow rate of 2.5 cm' min - I to 83 .3 cm' of an aqueous si lver n itrate solution contain ing a dispersing agent, S wt% Daxad 1 9 (sodium salts of naphthalene sulfonate formaldehyde condensate). After completion of the precipitation process, the silver precipitate 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 i n 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 1 5-26 nm. In a subsequent stud/ I , the antimicrobial activi ty of si lver nanoparticles produced by thi s method was 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) techniques were used to
Fig. 1 2-Structural view of a si lver nanoparticle-containing functional microcapsule (a) microcapsule, (b) inner core contains a functional substance such as perfume. a thermosensit ive pigment, thermal storage material or pharmaceutical preparation, and (c) outer shel l
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 s i lver 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 nontoxi c nanomaterials, which can be prepared in a s imple and cost-effective manner, may be sui table for the formulation of new types of bactericidal materials. Chemical reduction of si lver n itrate with hydrazine i n presence of a dispersing agent to produce 8 nm nano si lver particles has been reported by Kim et al. 62.
Various methods of producing nano si lver particles in water-in-oil microemulsions have been reviewed by Capek.63 In many of these processes, the si lver nanoparticles are coated or encapsulated in the chemicals used. For instance, for the preparation of dodecanethiol-capped si lver '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 h igher solubili ty 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-I R i nvestigations and elemental analyses support the encapsulation of s ilver
GULRAJANI : NANO FINISHES 1 99
particles by dodecanethiol (DT) while the transmission electron micrograph reveals an average size of 1 1 nm (ref. 63).
In a study, Aymonier et al.64 found that the hybrids of silver particles of 1 -2 nm in size with highly branched amphiphil ically modified polyethyleneimines adhere effectively to polar substrates providing environment-friendly antimicrobial coatings. Recently, Cho et al .65
investigated the anti microbial activity and protection of nanosilver particles (Ag-NPs) . In this study, stabilized Ag-NPs were prepared by sonication of a mixture of colloidal Ag-NPs (0.054%, average diameter 1 0 nm) solution contain ing 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. I t was observed that the growth of gram-positive (S. aureus) and gram-negative (E. coli) bacteria were inhibited by Ag-NPs. The minimum inhibi tory concentration (MIC) of Ag-NPs for S. aureus and E. coli were 5 and 1 0 ppm respectively. The main reason of PVP protecting silver nanoparticles is nitrogen in PVP coordinates wi th silver and forms the protection layer.66
It is well known that the discoloration of silver and i ts compounds takes place on exposure to l ight. It is therefore essential to stabil ize si lver ions and nanoparticles. Si lver ions have been stabil ised by reaction with ionic polymers as described above. The stabil ization of silver nanoparticles has been achieved either by coating, encapsulation or complex formation between the lone pair of electrons on N and Ag. I t is claimed that all amines that have free pair of electrons can stabilize Ag (ref.67). One such example of PVP is already discussed above.
It is claimed68 that the antimicrobial yarns made from cotton, l inen, si lk, wool, polyester, nylon or their blends having nanosilver particles can be produced by immersing them in nanosilver particle-containing solution produced by reducing si lver n itrate with glucose and drying at 1 20- 1 60°C for about 40-60 min. The treated yarns were yellow orange in colour. Electron microscopic studies of the antimicrobial yarns i ndicated that the yarn samples contained nanosilver particles which were evenly distributed and contained particles that were mostly below or about 1 0 nm size. Chemical testing indicated that the silver content in the yarns was about 0.4-0.9% by weight . The treated yarns showed effecti ve antimicrobial activity against various bacteria, fungi
and Chlamydia that included Escherichia coli, Methicillin resistant Staphylococcus aureus, Chlamydia trachomatis, Providencia stuartii, Vibrio VUlllificus, 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, Citrobacfer and Salmonella paratyphi C. Moreover, the antimicrobial activity remained intact on dyeing as well as even after 1 00 washes with neutral soap.
Si lver-containing antimicrobials have been i ncorporated 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 i mproved healing. I t i s known that placing surface-available silver in contact with a wound allows the si lver to enter the wound and become absorbed by undesirable bacteria and fungi that grow and prosper in the warm, moist environment of the wound s i te . Once absorbed, the silver ions kil l microbes, resulting in treatment of infected wounds or the prevention of i nfection in atrisk wounds. Some of the commercial si lver antimicrobial wound care products are Acticoat™, Actisorb™ and Si lverlon™.
Acticoat™, a multi-layered wound care device of Smi th and Nephew comprises three layers-a layer of polyethylene film, a middle layer of rayon/polyester blend nonwoven fabric, and a second layer of film. Nano-crystal l ine silver particles are deposited onto the fil m layers to provide an aritimicrobial wound care device. Another product available to consumers, provided by Johnson & Johnson under the trademark Actisorb™, is a highly porous, s ilver- impregnated charcoal cloth, sandwiched between two nylon nonwoven layers. In Si lverion™, manufactured by Argentum, nanosilver produced by the reduction of silver nitrate is deposited on sensitized nylon fibres. The si lver-laden polyamide fibres are then attached to a fabric. Some of these commercial products are prone to darkening on exposure to l ight, 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 l ight has been developed and patented.69
In 2000 and 2002, the Royal Perth Hospital (RPH) Burn Unit, Western Australia, conducted two 'before
200 INDIAN J. FIBRE TEXT. RES., MARCH 2006
and after' patient care audits companng the effectiveness and cost of Silvazine™ (silver su lphadiazine and chlorhexidine digluconate cream) and Acticoat™. The main fi ndings were: when using Acticoat™ the incidence of infection and antibiotic use fel l from 55% and 57% in 2000 to 1 0.5% and 5 .2% i n 2002. The total costs (excluding antibiotics, staffing and surgery) for those treated w ith Si lvazine™ were US$ 1 09,357 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) i n hospital was 1 7 .25 days for the Si lvazine™ group and 1 2 .5 days for the Acticoat™ group-a difference of 4.75 days. These audits demonstrate that Acticoat™ results in a reduced i ncidence of burn wound cellulit is, antibiotic use and overall cost compared to Si lvazi ne™ in the treatment of early burn wounds.70
In a recent study on the bacteriostasis and skin innoxiousness of nanosize si lver colloids on texti le fabrics, Lee and Jeong7 1 observed that colloidal si lver measuring 2-3 nm in diameter had a notable antibacterial efficacy at a concentration of 3 ppm; however, silver col loids measuring 30 nm in diameter had an i nferior bacteriostasis at the same concentration level . According to these investigators, smal ler-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 si lver particles was 99.99% against S. aureus and K. pneumoniae at a concentration of 1 0, 20 and 30 ppm for polyester and 20 ppm for cotton. Moreover, nanosi ze si lver colloidal solution was skin-innoxious when the s ize of the particles was 2-3 nm and the si lver concentration of colloidal solution was 1 00 ppm. The colloidal si lver measuring 30 nm in diameter was not i nnoxious at the same concentration level. It i s speculated that smaller-sized silver particles are less toxic to the skin than larger particles and that si lver colloids measuring 2-3 nm in diameter can be used as antibacterial agents on fabrics that come into contact with human skin .
6 Conclusions Nano fin ishes being developed for textile substrates
are at their infantile stage. The basic mechanisms and the logic of some of these fin ishes ha� been explained by the i nventors. Some nano fin ishes such as NanoCare™, Lotus Effect™ finish, Nanosphere™ based . finish and Ag Fresh™ have been commercialized. The
commercial v iabil ity of these finishes wil l be customer driven and the perception of value addition due to the additional functional i ty imparted by these finishes. The new concepts exploited for the development of nano fin ishes have opened up exciting opportuni ties for further R&D.
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