RJTA Vol. 16 No. 3 2012
Antimicrobial and Self-cleaning Textiles using Nanotechnology E.M. El-Khatib* National Research Centre, Textile Research Division, El Buhouth St., Dokki, Cairo, Egypt
[email protected]
ABSTRACT The present article review is pertaining to recent developments in antimicrobial and self-cleaning textiles using nanotechnology. Nanosized metal and metal oxide particles in single use or aided by other means are emphasized as versatile tools to achieve such properties. Mechanisms involved in effecting self-cleaning as well as antimicrobial functional characteristics to different textile material are reported. Also reported are the impacts of these characteristics on the performance of the textile products. Moreover, future research and developmental works are envisioned. This concept means we can be protecting ourselves from diseases by preventing pollution and microbial effect to our bodies. So, we not need to any drugs. Keywords: Antimicrobial, Self-cleaning, Nanotechnology, Nanosized Metal, Nanosized Metal Oxide and Textiles
1.
Introduction
Nanotechnologies refer to design, characterization, production and application of structures, devices and systems by controlling shape and size at the nanometer scale. Nanomaterials are fundamentally different from normal materials for two reasons:
Nanotechnology has real commercial potential for the textile industry. This is mainly due to the fact that conventional methods used to impart different properties to fabrics often don't lead to permanent effects, and will lose their function after laundering or wearing (Brust & kiely, 2002).
• •
Nanotechnology has a great role in textile industry. Current applications in textile industry take place in fibers, yarns, fabrics, nonwovens and polymeric materials. The purpose of using nanotechnology in textile and apparel applications are low chemical usage, low energy costs and less change in physical and maechanical properties such as handle, strength and air permeability (Bahadani et al., 1996) .
The surface area of nanoparticles increases dramatically with decreasing particle size. With decreasing size of the material, the behavior of matter becomes more reliant upon quantum effects. As a result the optical, electrical and magnetic behavior is changed from that of macro-scale.
One consequence of the increasing surface area is for example a strongly increasing reactivity. An important implication of the change of optical properties of nanoscale materials, especially for textile applications, is the non absorbance of visible light (400-800 nm) by particles of below 100 nm diameter. Any textiles treated with nanoscale materials do not absorb visible light, they are transparent (Qian & Hinestroza, 2004; Qian, 2004).
The term "Nano" in nanotechnology is derived from a Greek world "nanos" means dwarf. One nanometer is one billionth of a meter or 10-9 meter. The nanoscale is defined for products with a controlled geometry size of at least one functional component below 100 nanometers in one or more dimensions that makes physical, chemical or biological effects available which cannot be achieved above this critical dimension.
Nanotechnology can provide high durability for fabrics, because nanoparticles has a large surface area-to-volume ratio and large surface energy, thus presenting better affinity for fabrics and leading to an increase in durability of the function. Coating is
* Corresponding author. Tel.: (+202) 33371362; Fax: (+202) 33370931 E-mail address:
[email protected] (E. M. El-Khatib)
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a common technique used to apply nanoparticles onto textile.
mechanisms that act around the cell wall of the micro-organism (Scott, 2005; Royal Academy of Engineering, Royal Society (Great Britain) 2004).
A coating of nanoparticless on fabrics will not affect their breathability or handle feel. Therefore, the interest in using nanotechnologies in the textile industry is increasing (Anonymous, 2003).
Thus cell wall damage, alteration of cytoplasm membrane permeability, alteration of the physical or chemical state of proteins and nucleic acids, inhibition of enzyme action, or inhibition of protein or nucleic acid synthesis are all chemical approaches that can be utilized by antimicrobial finishes to inhibit or kill the micro-organism.
The properties imparted to textiles using nanotechnology include water repellence, self-cleaning, antimicrobial, UV-protection, wrinkle resistance and anti-static. There are various potential applications of nanotechnology in the textile industry. 1.1
1.2
Self-Cleaning Textiles
Water and soil repellency has been one of the major targets for fiber and textile scientists and manufacturers for centuries. Combinations of new materials for fiber production with a variety of surface treatments have been developed to reach the condition of limited wettability (Kair, 2007).
Antimicrobial Finishes
As some lifestyles have become more active, sportswear, active wear, and casual wear may become more easily contaminated by perspiration leading to bacterial growth and body odors (King, & Jones, 2006; Holme, 2007; Dring, 2003; Schindler & Hauser, 2004).
Nature has already developed an elegant approach that combines chemistry and physics to create super repellant surfaces as well as self cleaning surfaces. “Lotus leaves” is the best example of self cleaning surfaces. The concept of self cleaning textiles is based on the lotus plant whose leaves are well-known for their ability to ‘selfclean’ by repelling water and dirt. More recently, botany and nanotechnology have united to explore not only the beauty and cleanliness of the leaf, but also its lack of contamination and bacteria, despite its dwelling in dirty ponds (Lotus-Effect®, 2007). Basically, the lotus leaf has two levels of structure affecting this behavior – micro-scale bumps and nanoscale hair-like structures – coupled with the leaf’s waxy chemical composition. On the basis of lotus leaf concept scientist developed a new concept “Self cleaning textile” the textile surface which can be cleaned by itself without using any laundering action.
Microbes are the tiniest creatures not seen by the naked eye. They include a variety of microorganisms like bacteria, fungi, algae and viruses. Bacteria are uni-cellular organisms which grow very rapidly under warmth and moisture. Further, sub divisions in the bacteria family are gram positive (Staphylococcus aureus), gram negative (E.coli). Some specific types of bacteria are pathogenic and cause cross infection. Fungi, molds or mildew are complex organisms with slow growth rate. They stain the fabric and deteriorate the performance properties of the fabrics (Ramachandran et al., 2004). Antimicrobial finishing of textiles for preventing bacterial and fungal infection which causes skin irritation and deteriorate the performance properties of the fabrics.
Nanotechnology provides a new concept self cleaning textiles which gives self cleaning as well as fresh cloths every day, this not only technically benefited but also techno economically benefited. There are basically two types of self-cleaning surfaces involving nanotechnology.
No one antimicrobial finish, as yet, fulfils all the necessary criteria for all textile end uses but some very effective and durable antimicrobial finishes have been developed by nanotechnology, and their utilization in textiles, clothing and footwear is increasing, as their undoubted benefits take hold in the conscious minds of consumers worldwide.
In the first place extremely water repellent, microscopically rough surfaces: dirt particles can hardly get a hold on them and are, therefore, removed by rain or by a simple rinse in water. Self
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cleaning surface having a water contact angle greater than 150° and a very low roll off angle. Water through these surfaces easily rolls off and completely cleans the surface in the process. Self cleaning fabrics not only repellant to water but are also resist stains, dirt, odor and are antimicrobial as well. The second example is given by photo catalytic layers: due to a layer of nanocrystalline titanium dioxide, organic material (i.e. dirt, pollutants, and micro organisms) is destroyed by solar irradiation.
2.
Fig. 1. Photocatalytic Self-Cleaning of titanium dioxide
Manufacturing Method
2.1. The Manufacturing of Self-Cleaning Textiles Using Nanotechnology • • • •
Using photo catalysts Using microwaves Using carbon nanotubes Using silver nanoparticles
2.1.1 Using Photo Catalysts Starting with photo catalysts, it is catalytic selfcleaning process. In this process Nanosized titanium dioxide and zinc oxide are used for imparting self cleaning and anti-bacterial properties .The fabric is coated with a thin layer of titanium dioxide particles heaving 20 nanometers diameter. Titanium dioxide is a photo catalyst as illustrated in figure 1, when it is illuminated by light of energy higher than its band gap, electrons in TiO2 will jump from the valence band to the conduction band, and the electron (e )־and electric hole (h+) pairs will form on the surface of the photo catalyst. The negative electrons and oxygen will combine to form O2־, radical ions, whereas the positive electric holes and water will generate hydroxyl radicals OH ־. Since both products are unstable chemical entities, when the organic compound (i.e. dirt, pollutants, and micro organisms) falls on the surface of the photo catalyst it will combine with O2 ־and OH ־and turn into carbon dioxide (CO2) and water (H2O). Since the titanium dioxide acts as a catalyst, so it is never used up. This is how the coating continues breaking down stains over and over. Zinc oxide is also a photo catalyst, and the photo catalysis mechanism is similar to that of titanium dioxide.
Fig. 2. Fabric coated with a thin layer of titanium dioxide particles The fabric is coated with a thin layer of titanium dioxide particles as shown in figure 2, which measure only 20 nanometers in diameter. When this semi-conductive layer is exposed to light, photons with energy equal to or greater than the band gap of the titanium dioxide excite electrons up to the conduction band. The excited electrons within the crystal structure react with oxygen atoms in the air, creating free-radical oxygen. These oxygen atoms are powerful oxidizing agents, which can break down most carbon-based compounds through oxidation-reduction reactions. In these reactions, the organic compounds (i.e. dirt, pollutants, and micro organisms) are broken down into substances such as carbon dioxide and water. Since the titanium dioxide only acts as a catalyst to the reactions, it is never used up. This allows the coating to continue breaking down stains over and over. The self-cleaning fabrics work using the photo catalytic properties of titanium dioxide as illustrated in figure 3.
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provides a bionic route to create hydrophobic textiles. Furthermore, considering the novel mechanical and electric properties of carbon nanotubes, these carbon nanotubes coated cotton fabrics as shown in figure 5, will find potential application in sensing, conducting and special textiles (Paula et al., 2010). 2.1.4 Using Silver Nanoparticles Fig. 3. Working of self cleaning textiles A highly water-repellant coating made of silver nanoparticles that can be used to produce suits and other clothing items that offer superior resistance to dirt as well as water and require much less cleaning than conventional fabrics (Wange et al., 2007).
2.1.2 Using Microwaves New technology attaches nanoparticles to clothing fibers using microwaves as shown in figure 4. Then, chemicals that can repel water, oil and bacteria are directly bound to the nanoparticles.These two elements combine to create a protective coating on the fibers of the material. This coating both kills bacteria, and forces liquids to bead and run off. The same technology, created by scientists working for the U.S. Air Force, has already been used to create tshirts and underwears that can be worn hygienically for weeks without washing.
Fig. 5. Treated cotton fabrics
Fig.
4.
Self-cleaning clothing fibers microwaves
using
2.1.3 Using Carbon Nanotubes (a) Fig. 6. (a) Untreated textile surface (b) Treated textile surface
Artificial lotus leaf structures were fabricated on textiles via the controlled assembly of carbon nanotubes. Carbon nanotubes (CNTs) and surface modified carbon nanotubes are used as building blocks to biomimic the surface microstructures of lotus leaves at the nanoscale. Cotton fabrics, which otherwise have perfect water absorbabilities, have been endowed with super-hydrophobic properties water contact angles greater than 150° were measured. The method
(b)
Figure 6 shows that (a) untreated textile surface, (b) Treated textile surface which is treated with silver nano particles The untreated surface having dust particles, when water droplets rolls over it do not get washed off because dust particles are adhere by textile 159
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surface. While treated textile surface do not adheres the dust particles hence when water particles rolls over it dust get washed off.
deodorant, antibacterial, and anti-soiling properties as well as excellent functional stability, as compared to fabrics using a conventional photo catalytic reaction. “SELFCLEAR” yarn has a multi-layer structure with micro voids tens of nanometers in diameter on the surface of the acrylic fiber, and that makes it a nanovoid structure creating a greater surface area as shown in figure 7. Because special Nanosized photo catalytic titanium dioxide, one-tenth the size of conventional titanium dioxide used for fibers, is used for “SELFCLEAR” yarn. It has a ten times greater surface. With this synergistic function of double nano-technology, it can produce excellent effects on higher self-cleaning functions with deodorant, antibacterial and anti-soiling properties compared to fibers using conventional photo catalytic reactions (Japan Exlan develops, 2008).
In figure 6: (b) Silver nano particles treated surface shows self cleaning property. 2.2
Photo Catalytic Degradation of Odors Compound
Japan develops a photo catalytic acrylic fiber “SELFCLEAR” yarn with higher-dimensional self-cleaning properties which had been exclusive with conventional photo catalytic fibers, and started recently sending their product for manufacture of clothing, sportswear, uniforms, bedding, carpets, and daily goods. Because titanium dioxide is applied into “SELFCLEAR” yarn, it has a higher self-cleaning activity with
Fig. 7. Photocatalytic degradation of odorous component
3.
3.1
Innovation in Self-Cleaning Finishes
Researchers at Clemson University are developing a highly water-repellant coating made of silver nanoparticles. This can be used to produce suits and other clothing items that offer superior resistance to dirt as well as water and require much less cleaning than conventional fabrics.
Self-cleaning Wool and Silk Developed Using Nanotechnology
Wool and silk, which are composed of natural proteins, are among the most prized, luxurious, delicate and widely used fabrics in the clothing industry. However, they are difficult to keep clean and are easily damaged by conventional cleaning agents. Self cleaning forms of wool and silk have been developed with the help of nanotechnology. It was suggested that, wool socks, skirts and silk ties may soon clean themselves of smells and stains in the sunshine.
The patented coating "a polymer film (polyglycidyl methacrylate) mixed with silver nanoparticles" can be permanently integrated into any common fabric, including silk, polyester and cotton, the researchers say. In the long run, it can save time and money by reducing expensive dry cleaning bills. It is also environmentally friendly (Sampson, 2006).
The secret is a nano particle coating, one already used to keep windows clear, that could lead to "self-cleaning" versions of wool and silk fabrics. 160
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Thus, wool fabrics were prepared with and without a nanoparticle coating, particles around five nanometres across (five billionths of a meter) composed of anatase titanium dioxide. A substance already used that is known to break down and destroy contaminants upon exposure to sunlight.
Keratinous protein fibers such as wool, find numerous applications in addition to textiles. Although, protein fibers offer excellent physical and processing properties, as biological materials, they lack the minimally required thermal and chemical resistance and intrinsic reactivity of their constituting polypeptide chains to enable modification or fine-tuning of their properties. They are also subject to photodegradation in presence of UV containing light such as solar and indoor lighting. These special characteristics have hindered further functionalization of proteinic materials and limited their full utilization despite their abundance, practicability and biodegradability.
The self-cleaning technology in this work uses titanium dioxide photo catalyst that, when triggered by light, it decomposes dirt, stains, and harmful microorganisms and so on. Fabric samples were stained with red wine. After 20 hours of exposure to simulated sunlight, the coated fabric showed almost no signs of the red stain, whereas the untreated fabric remained deeply stained as shown in figure 8.
Proteinic fibers, capable of converting incident light to self-cleaning power to decompose its stains, dirt, and harmful microorganisms in a process of photo catalytic purification, are very interesting materials for various applications.
It was noted that, the coating, which is non-toxic, can be permanently bonded to the fibre and does not alter its texture and feel. So a silk tie would still feel silky (Highfield, 2008).
In this contribution, self-cleaning keratin fibers have been realized following a bottom-up nanotechnology approach in which anatase nanocrystals of titanium dioxide are formulated and carefully applied to the fibers via a near room temperature sol–gel process in order to maintain their intrinsic properties while conferring self-cleaning properties and self-protection against UV degradation. This may enable wider utilization of these natural fibers. Colloids of anatase titanium dioxide, a highly efficient photo catalyst, have recently been prepared via a near room temperature process (Bozzi et al., 2005; Bozzi et al., 2005). Fig. 8. Red wine stained wool with no treatment (top), a stain-treating agent (middle) and nano particle coating (bottom) 3.2
This synthetic innovation implies that the application of anatase nanocrystals can be extended to low thermal resistance materials such as biomaterials (Meilert et al., 2005; Yuranova et al., 2006; Qi et al., 2006).
Self-Cleaning Keratins
Keratins, a class of biologically fibrous proteins, are tough and insoluble due to the formation of adjacent peptide bond that allow close alignment of the sulfur-containing amino acid constituent, cysteine, enabling the formation of disulfide bridges, cross-links, that confer rigidity and thermal stability to keratinous materials, which make them an important class of fibrous materials. Keratins are a type of natural protein and the main structural constituents of animal tissues.
However, due to additional low chemical and thermal resistance and liability to photo degradation of protein fibers, a tailor-made anatase colloid should be devised that suits the target application. It was found that a synthetic formulation of an acidic aqueous colloid of titanium dioxide using hydrochloric acid produces single-phase anatase nanocrystals (4-5 nm), which is applicable to a keratin fiber representative, wool, without affecting its intrinsic properties. 161
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Although these functionalized fibers were found to possess excellent self-cleaning and UV-protection properties, these properties may not be stable.
before recombination (Anpo et al., 1987), high oxidizing ability, high stability, non-toxicity, and low-cost, TiO2 has been regarded as an ideal photo catalyst. Among all preparation techniques for producing TiO2 (Bickmore et al., 1998; Sandell et al., 2003), the sol–gel method is the most widely used due to its ability to nucleate anatase at relatively low temperatures (Daoud & Xin, 2004; Daoud & Xin 2005), which makes it suitable for application in materials with low thermal resistance such as plastics and biomaterials.
TiO2 has high affinity toward hydroxyl and carboxylic groups (Brinker et al., 1990; Dhananjeyan et al., 2001; Yu et al., 2002; Campostrini et al., 2004) especially the latter. However, keratin fibers contain less than 50% of those functional groups. Therefore, it is necessary for these fibers to undergo a chemical modification to their backbone chains to enable stable bonding with the anatase nanocrystals. Acylation of fibers by acid anhydride allows for enrichment of carboxylic groups. Other functional groups on the amino acid residues such as amino, hydroxyl, phenol and thiol groups are also potent reactive sites. Following the introduction of additional carboxylic groups into the fiber, the change of the binding ability toward metal ions has been studied (Fried & Li, 1990; Tsukada et al., 1992; Freddi et al., 1999; Arai et al., 2001 Taddei et al., 2003).
Self-cleaning treatment technology of fibers by incorporation of titanium dioxide nanoparticles is a new concept that has been introduced in recent years (Daoud & Xin, 2004; Peplow, 2004). With the fast-growing demand towards functional fibers, where fibers not only have the basic characteristics such as maintaining thermal insulation, air permeability and elasticity, but also possessing extra functionality such as self-cleaning, anti-bacterial, environmental friendly, and anti-pollution (Daoud et al., 2005).
Succinic anhydride is a nontoxic and mild acylating agent. On the basis of the observed properties of the modified fibers, it was found that the acylation reaction allows for increasing functionality and reactivity toward anatase nanocrystals which in turn resulted in enhanced self-cleaning functionality.
It is anticipated that self-cleaning fibrous materials would have significant potential in the global commercial market. Therefore, this novel concept continues to open up exciting opportunities for further research and development.
Since the discovery of photo catalytic water splitting on TiO2 electrodes in the late 1960 (Hashimoto et al., 2005; Fujishima & Zhang, 2006), great scientific attention has been given to TiO2, particularly, in exploring its functions of purification, sterilization and deodorization (Parkin & Palgrave, 2005; Kikuchi et al., 1997; Sopyan et al., 1996; Dong et al., 2006; Yu et al., 2006; Jia et al., 2007; Wang et al., 2007).
Protein keratin fibers such as silk and wool are luxurious, delicate, have low thermal and chemical resistance with poor photo-stability in presence of UV-containing light sources, difficult to maintain and care for, and more liable to attack by microorganisms as compared to cellulose fibers such as cotton and flax. Therefore, it is essentially significant for protein fibers to possess self-cleaning property. Although much work has been conducted on cellulose fibers such as cotton, due to the low chemical and thermal resistance and liability to photo degradation of protein materials ( Xin et al., 2004; Qi et al., 2007; Ichiura et al., 2003 Zhang et al., 2007; Uddin et al., 2007; Yuranova et al., 2007), a tailor-made anatase colloid should be devised wherein the effect of the acid catalyst is taken into account.
Owing to its impressive physical and chemical properties such as efficient photo catalytic activities, facilitated by its particle size to diffuse the excited electrons and holes towards the surface
It is believed that the development of self-cleaning protein fibers may lead to new possibilities of self-cleaning materials. In this contribution, a systematic investigation was performed in order to
This finding may open up new applications and extendibility to other proteinic materials (Daoud et al., 2008). 3.2.1 Photocatalytic Self-Cleaning Keratins: A Feasibility Study
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study the feasibility of the application process, the impact on wool’s intrinsic characteristics, and the efficiency and stability of the introduced self-cleaning properties (Tung & Daoud, 2009). 3.3
TiO2 and polyester as it is known that the adhesion between TiO2 and polyester is not good because of the lack of chemical bonding. To improve the adhesion, surface treatments for altering the chemical and physical properties of the polyester surface may be needed. Low temperature plasma (LTP) pretreatment of polyester surface is probably the most versatile technique to improve the bondability of TiO2 on polyester fibers. The surface pretreatment only modifies the outermost surface layers of the polymer without affecting the bulk properties (Kinloch, 1987) and the modifications are dependent on the composition of the gaseous medium and the processing parameters. These two factors are important with regard to the increase of the adhesion to the titania.
Self-Cleaning Polyester Fibers
Recently, several studies have reported nanocrystalline TiO2 layers on textiles that were prepared from sol–gel TiO2 at relatively low temperatures. In one of previous studies, nanocrystal anatase on cotton fabrics was obtained by boiling the fabric in water for 3 h after a fabric treatment using ethanol-based sol-gel titania (Daoud & Xin 2004). Kiwi and colleagues 2005, reported the preparation of self-cleaning wool-polyamide, polyester, and cotton textiles coated with TiO2. In their work, the TiO2 colloidal solution was prepared using an isopropanol-based sol-gel process, followed by hydrothermal treatment at100°C for 16 h and finally it was used to coat textiles.
The use of direct current glow discharge plasma was the most successful approach for an improvement in polymer surface reactivity (Friedrich et al., 1993) compared with radio frequency plasma and microwave plasma. Oxygen gas is the most currently used gas in plasma treatment (Petasch et al., 1995; Carlotti & Mas, 1998; Chen et al., 1999; Inagaki et al., 2004) leading to the introduction of negative groups COO-, –O–O- onto the polyester surface and it appears that oxygen plasma gives the most oxygenated polyester surface, which results in better adhesion. The TiO2 can be attached to the modified polyester surface through ionic attraction with the positively coated Ti4+ of TiO2.
However, it was the rutile phase that was attained on the textiles, which is less photoactive than the anatase (Fujishima et al., 2002). Furthermore, the long treatment time is not feasible in the industrial textile application and the use of organic solvents is not desired because it makes the whole chemical approach not environmental friendly. More recently, Daoud, W.A., et. al., investigated self-cleaning cotton by coating with single-phase anatase sols (Daoud et al., 2008).
Single-phase anatase-coated polyester fibers pretreated with low-temperature oxygen plasma showed significant improvement in self-cleaning performance as demonstrated by their bactericidal activities, colorant decomposition, and degradation of red wine and coffee stains under simulated day light irradiation. The adhesion between TiO2 layers and polyester substrates was also improved after plasma treatment. The UV absorption of the titania-coated polyester was significant enough to promote excellent UV protection to polyester (Daoud, 2007).
These sols were synthesized using a sol–gel process at low temperatures. The fibers coated with the anatase sol prepared at 60°C showed the highest photo catalytic activities. In this work, this anatase sol is applied to functionalize polyester fibers owing to its high photo catalytic activities (Qi et al., 2007). Polyesters have found widespread applications in textiles, bottles, films, filters, and so on. Improving their properties, especially functional properties, is important for their added value. When applying TiO2 to polyester textile products that may likely be subject to frequent washing, it is necessary to improve the adhesion between the
No photo degradation of the molecular chains of polyester by the titania layers could be observed as demonstrated by a comparison study of the tearing strength of coated polyester fabrics before and after light irradiation (Inagaki et al., 2001; Krump et al., 2005; Wang et al., 1999). 163
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3.4
composites thereof. This study was to obtain self-cleaning properties for regenerate cellulose surfaces (Lyocell fabric) by nano-modification, using TiO2 nano-coating and to define the impact of the modification on fabrics end-use properties. Two different modified fabrics with self-cleaning effect were prepared (method 1): TiO2/SiO2 composite nano-coating, (method 2): TiO2 nano-coating and analyzed, i.e. the modification efficiency was determined. In addition, the influence of fibre modification on several textile properties was determined. However, a soft handle, good appearance and some other surface properties accompanied by appropriate mechanical properties represent the basis for a high quality fabric therefore the influence of the modification procedure on textiles handle was improved (Veronovski et al., 2009).
Self-Cleaning Blend Fabrics
The bondability of TiO2 on wool-polyamide and polyester textiles increases by surface textile modifications induced by radio frequency plasma (RF-plasma), microwave plasma (MW-plasma), and vacuum-UV light irradiation. These pretreatments allowing the loading of TiO2 by wet chemical techniques in the form of transparent coatings constituted of nanoparticles of diverse sizes. These loaded textiles show a significant photo-oxidative activity under visible light in air under mild conditions, which discolors and mineralizes persistent pigment stains contained in wine and coffee. The mineralization of stains on the textile loaded with TiO2 was monitored quantitatively to assess the appropriate surface pretreatment in conjunction with the most suitable deposition method of TiO2 colloids, powders, or combination of both. Their photocatalytic activity allowed, in kinetically acceptable times, the almost complete discoloration of coffee and wine stains. The observed discoloration of colored stains seems to involve visible light sensitization of the stain pigment on the TiO2-loaded textile. The size of the particles obtained from colloidal precursors of TiO2 varied between 5 and 25 nm.
3.6
Nanocrystalline anatase coatings were prepared on cotton fabrics by a near room temperature sol-gel process. The anatase nanocrystallite coated fabrics showed significant self-cleaning performance as demonstrated by their bactericidal activities, colorant decomposition and degradation of red wine and coffee stains under UV irradiation. The UV absorption by the titania coating was quite substantial, promoting excellent UV protection to the cotton. No photo degradation of the cellulosic chains of cotton by the titania coating could be observed as demonstrated by a comparison study of the tearing strength of coated cotton fabrics before and after prolonged solar-simulated light irradiation.
These different pretreatment methods of textile surfaces have been studied that allow the TiO2 coatings to discolor wine and coffee stains under visible light in reasonable time. Post treatment temperatures of 100°C or less were shown to be sufficient to attach TiO2 to the textile surfaces. The nanoparticless of TiO2 remain fairly stable on the textile surface after the photochemical discoloring of stains. A combination of TiO2 powder and titanium tetraisopropoxide (TTIP) colloids deposited on wool-polyamide or polyester textiles showed to be kinetically suitable for the self-cleaning of wine and coffee stains using a solar simulated light. Under neon light the self-cleaning effect needed much longer times. 3.5
Self-Cleaning Cotton
The photo catalytic activity of TiO2–SiO2-coated cotton textiles was investigated through the self-cleaning of red wine stains. It was shown that a TiO2–SiO2 species could be produced at temperatures of 100°C with acceptable photo-activity on non-heat resistant materials. The most suitable Ti-content of the coating was found to be 5.8% and for SiO2, the content was 3.9% (w/w). The discoloration of red wine led to CO2 evolution that was more efficient for TiO2-SiO2-coated cotton samples than of TiO2-coated ones. The reasons for these results are discussed.
Self-Cleaning Modified Cellulose
Self-cleaning surfaces based on photo catalysis are an extremely promising nano-technological field of extensive research and development. Recently comprehensive research work has been performed to evaluate the optical, photo catalytic and antimicrobial properties of TiO2 nanoparticless and
The textile surface did not show any change after several consecutive light-induced discoloration 164
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acid enhanced it's reactively towards silver nanoparticles by nano layer condensation. Treatment of wool fabrics with silver nanoparticles was found to be effective in antimicrobial activity.
cycles of a red wine stain. By high-resolution transmission electron microscopy (HRTEM), the TiO2–SiO2 layer thickness on the cotton fibers was detected to 20–30 nm. The TiO2 and SiO2 were both observed to have particle sizes between 4 and 8 nm. Further electron microscopy work coupled with energy dispersive spectroscopy (EDS) showed that the Ti-particles were always surrounded by amorphous SiO2 and never alone by themselves. Infrared spectroscopy revealed that no modification of the cotton could be detected after photo-discoloration processes with TiO2–SiO2, taking a wine stain as model compound.
This treatment was durable to repeated washing. UV-protection properties were improved. Treated wool fabrics with silver nano particles have a positive impact on UV-absorption. This indicate that silver nano particles renders treated samples less permeable to solar UV-rays whereas untreated samples has the less blocking effect. Furthermore electrical conductivity and smoothness of the wool fabrics surface were improved by this treatment. This treatment does not involve hazardous chemicals and thus this treatment is environmentally friendly (Abdel-Fattah et al., 2010).
The mixed TiO2 and SiO2 colloids lead during the dip-coating and subsequent thermal treatment on cotton to an organized structure of highly dispersed TiO2 particles always surrounded by amorphous silica.
4.
Excellent antibacterial properties could be achieved by treating cotton fabrics with Nanosized silver nanoparticles at a concentration of 50 ppm (prepared by chemical method) in presence of 1% binder (Hebeish et al., 2009), and prepared by biological method (El-Rafie et al., 2009).
Properties Imparted to Textiles by Nanotechnology
In addition to self-cleaning, other properties imparted to textiles by nanotechnology include antimicrobial, water repellence, UV-protection, anti-static and wrinkle resistance. 4.1
Beside the high efficiency of such finishing treatments, the finish is durable to repeated washing.
Antimicrobial
For imparting anti-bacterial properties, Nanosized silver (Russell, 2002; & Xin et al., 2004; Yeo, et al., 2003), titanium dioxide (Burniston et al., 2004; Sherman & Jonathan, 2003) and zinc oxide (Saito, 1993) are used. Metallic ions and metallic compounds display a certain degree of sterilizing effect. It is considered that part of the oxygen in the air or water is turned into active oxygen by means of catalysis with the metallic ion, thereby destroying the organic substance to create a sterilizing effect (Anonymous, 2003; Lee et al., 2003).
Wool and wool garments provide an excellent environment for a microorganism to grow because of their ability to retain moisture which can deteriorate these materials. These effects include generation of unpleasant odor, stains and discoloration in the fabric and/or a reduction in fabric mechanical strength (Purwar & Joshi, 2004; Povlidou, 2005). Already silver nanopartcles have found applications like socks impregenated with silver nanpparticles of size 25nm (Parthasarathi & Borkar, 2007).
With the use of Nanosized particles, the number of particles per unit area is increased, and thus anti-bacterial effects can be maximized.
Metalized textile is a kind of new composite that has relatively new applications which attracted the attention due to its special properties such as antistatic, electrical conductivity as well as antiodor performance (Freddiet et al., 2001).
Nano-silver particles have an extremely large relative surface area, thus increasing their contact with bacteria or fungi, and vastly improving their bactericidal and fungicidal effectiveness. Nano-silver is very reactive with proteins. When
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combine with O2- and OH- respectively, and turn into carbon dioxide (CO2) and water (H2O). This cascade reaction is called ‘oxidation-reduction’ (Yang et al., 2003) and the mechanism is shown in Figure 9. Through the reaction, the photo catalyst is able to decompose common organic matters in the air such as odor molecules, bacteria, fungus and viruses.
contacting bacteria and fungus, it will adversely affect cellular metabolism and inhibit cell growth. It also suppresses respiration, the basal metabolism of the electron transfer system, and the transport of the substrate into the microbial cell membrane. Furthermore, it inhibits the multiplication and growth of those bacteria and fungi which cause infection, odor, itchiness and sores. Hence, nano-silver particles are widely applied to socks in order to prohibit the growth of fungus and bacteria. In addition, nano-silver can be applied to a range of other healthcare products such as dressings for burns, scald, skin donor and recipient sites (Athinson, 2003).
Several papers have discussed the use of the photo catalytic property of TiO2 in the field of textiles (Cui et al., 2003). It was determined that a fabric treated with nanoTiO2 could provide effective protection against bacteria and the discoloration of stains, due to the photo catalytic activity of nanoTiO2. On the other hand, zinc oxide is also a photo catalyst, and the photo catalysis mechanism is similar to that of titanium dioxide; only the band gap (ZnO: 3.37eV, TiO2: 3.2eV) is different from titanium dioxide. NanoZnO provides effective photo catalytic properties once it is illuminated by light, and so it is employed to impart anti-bacterial properties to textiles (Chen, 2002; Wang et al., 2004; Yasuhide et al., 1997).
Titanium dioxide is a photo catalyst; once it is illuminated by light with energy higher than its band gaps, the electrons in TiO2 will jump from the valence band to the conduction band, and the electron (e-) and electric hole (h+) pairs will form on the surface of the photo catalyst. The negative electrons and oxygen will combine into O2-, the positive electric holes and water will generate hydroxyl radicals. Since both are unstable chemical substances, when the organic compound falls on the surface of the photocatalyst it will
Fig. 9. Photo catalysis mechanism of titanium dioxide 4.2
Water Repellence
remains on the top of the whiskers and above the surface of the fabric (Russell, 2002; Draper, 2003; Kathiervelu, 2003). However, liquid can still pass through the fabric, if pressure is applied. The performance is permanent while maintaining breathability.
Nano-Tex improves the water-repellent property of fabric by creating nano-whiskers, which are hydrocarbons and 1/1000 of the size of a typical cotton fibre, that are added to the fabric to create a peach fuzz effect without lowering the strength of cotton. The spaces between the whiskers on the fabric are smaller than the typical drop of water, but still larger than water molecules; water thus
On the other hand, a hydrophobic property can be imparted to a cotton fabric by coating it with a thin nanoparticulate plasma film. The audio frequency 166
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plasma of some kinds of fluorocarbon chemical was applied to deposit a nanoparticulate hydrophobic film onto a cotton fabric surface to improve its water repellent property. Superhydrophobicity was obtained due the roughness of the fabric surface, without affecting the softness and abrasion resistance of cotton fabric (Burniston et al., 2004). 4.3
Nanosized titanium dioxide and zinc oxide were more efficient at absorbing and scattering UV-radiation than the conventional size, and were thus better able to block UV. This is due to the fact that nanoparticless have a larger surface area per unit mass and volume than the conventional materials, leading to the increase of the effectiveness of blocking UV radiation. For small particles, light scattering predominates at approximately one-tenth of the wavelength of the scattered light. Rayleigh’s scattering theory stated that the scattering was strongly dependent upon the wavelength, where the scattering was inversely proportional to the wavelength to the fourth power. This theory predicts that in order to scatter UV radiation between 200 and 400 nm, the optimum particle size will be between 20 and 40 nm.
UV-Protection
The most important functions performed by the garment are to protect the wearer from the weather. However it is also to protect the wearer from harmful rays of the sun. The UV-blocking property of a fabric is enhanced when a dye, pigment, delustres or ultraviolet absorber finish is present that absorbs ultraviolet radiation and blocks its transmission through a fabric to the skin. Metal oxides like ZnO as UV-blocker are more stable when compared to organic UV-blocking agents. Hence, nano ZnO will really enhance the UV-blocking property due to their increase surface area and intense absorption in the UV region as shown in figure 10.
Various research works on the application of UV-blocking treatment to fabric using nanotechnology were conducted. UV-blocking treatment for cotton fabrics was developed using the sol-gel method. A thin layer of titanium dioxide is formed on the surface of the treated cotton fabric which provides excellent UV-protection; the effect can be maintained after 50 home launderings. Apart from titanium dioxide, zinc oxide nanorods of 10 to 50 nm in length were applied to cotton fabric to provide UV protection. According to the study of the UV-blocking effect, the fabric treated with zinc oxide nanorods demonstrated an excellent UV protective factor (UPF) rating.
Inorganic UV blockers are more preferable to organic UV blockers as they are non-toxic and chemically stable under exposure to both high temperatures and UV. Inorganic UV blockers are usually certain semiconductor oxides such as TiO2, ZnO, SiO2 and Al2O3. Among these semiconductor oxides, titanium dioxide (TiO2) and zinc oxide (ZnO) are commonly used. It was determined that
Fig. 10. Photo catalytic mechanism of ZnO
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4.4
Anti-Static
• • •
Static charge usually builds up in synthetic fibres such as nylon and polyester because they absorb little water. Cellulosic fibres have higher moisture content to carry away static charges, so that no static charge will accumulate. As synthetic fibres provide poor anti-static properties, research work concerning the improvement of the anti-static properties of textiles by using nanotechnology were conducted. It was determined that Nanosized titanium dioxide (Dong & Huang, 2002), zinc oxide whiskers (Zhou et al., 2003), nano antimony-doped tin oxide (ATO) (Wu et al., 2002) and silane nanosol (Xu et al., 2005) could impart anti-static properties to synthetic fibres. TiO2, ZnO and ATO provide anti-static effects because they are electrically conductive materials. Such material helps to effectively dissipate the static charge which is accumulated on the fabric. On the other hand, silane nanosol improves anti-static properties, as the silane gel particles on fibre absorb water and moisture in the air by amino and hydroxyl groups and bound water (Anonymous, 2002). 4.5
6.
• • •
Limitation of Self-Cleaning Fabrics
Breakthroughs in nanotechnology have made self-cleaning fabrics both practical and economical. With commercial production making the technology readily available to the masses, will washing machines and laundry detergent become obsolete. There are several factors limiting how quickly current self cleaning fabric would be able to break down organic compounds. Sunlight is the best source of light for activating the self-cleaning process. A ketchup-stained shirt would have to be left outside in the sun for at least a day in order to remove the stain. However, for military persons or hikers, who are outside in the sun for long periods of time without the time or means to clean their clothes, self-cleaning fabric would be ideal. Further research would be required to test ways of applying titanium dioxide nanofilms to textiles.
Wrinkle Resistance
7.
To impart wrinkle resistance to fabric, resin is commonly used in conventional methods. However, there are limitations to applying resin, including a decrease in the tensile strength of fibre, abrasion resistance, water absorbency and dyeability, as well as breathability. To overcome the limitations of using resin, some researchers employed nano-titanium dioxide (Song et al., 2001; Wang & Chen, 2005) and nano-silica to improve the wrinkle resistance of cotton and silk respectively. Nano-titanium dioxide was employed with carboxylic acid as a catalyst under UV irradiation to catalyze the cross-linking reaction between the cellulose molecule and the acid. On the other hand, nano-silica was applied with maleic anhydride as a catalyst; the results showed that the application of nano-silica with maleic anhydride could successfully improve the wrinkle resistance of silk.
5.
Smart textiles Upholstery Under garments
Problems of Self-Cleaning Fabrics
The main reasons that self-cleaning fabrics require a lot of time to break down stains is because titanium dioxide is very inefficient at using energy from sunlight. The titanium dioxide serves as a catalyst for the break down of dirt molecules by providing electrons that oxidize oxygen molecules in the surrounding air. The electrons are freed from the titanium dioxide via the photoelectric effect. But because of titanium dioxide's high band gap energy, only high energy blue and UV light photons have enough energy to excite electrons to the conduction band. High energy blue and UV light only make up 3% of the solar spectrum, so titanium dioxide can only use a very small portion of the sun's energy to break down stains. Excitation of electrons to the conduction band is only the beginning of the cleaning process. These electrons must then react with oxygen atoms, which then react with the dirt particles. All of these reactions are limited by access to and the amount of freed electrons in the titanium dioxide. So for a large stain, a lot of light energy is needed before the fabric can fully break it down.
Areas of Applications Hospital garments Sports wear Military uniform 168
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8.
Conclusion •
Nanotechnology is an essential cornerstone in development of self-cleaning textiles. Nanosized metal and metal oxide particles such as Silver, TiO2, ZnO, etc. have been synthesized and successfully applied to different textiles to impart self-cleaning and antimicrobial properties. Besides, the application of nanotechnology induces water repellency, UV-protection, anti static and wrinkle resistance.
•
•
Mechanisms of attachment of nanoparticles to the textiles as well as their mechanisms to inhibit and/or kill the microbes are emphasized.
•
Self cleaning effect on textile materials lead to an efficient use of materials and are therefore in agreement with the principles of sustainable development.
property will become a standard feature of future textile. Self-cleaning technology is commonly used materials to maintain hygiene and prevent the spreading of pathogenic infection. Self-cleaning technology can also help in reducing the consumption of chemicals, such as detergents and dry-cleaning solvents, water, and energy. The use of a self-cleaning coating is attractive as they are labor saving and effectively improve the appearance of the environment. It can save time and money by reducing expensive dry cleaning bills and cleaning efforts. The coating could be applied to suits, hospital garments, sportswear, military uniforms and outdoor fabrics. It could appear in consumer products within few years.
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•
•
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Ease of maintenance and environmental protection. Time, material, energy reduction and consequently cost-efficiency during production Makes textiles longer-lasting People need not to suffer from heavy laundry bills and cleaning efforts Improved ageing behavior by extended surface purity effect.
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