10.6.3.1 State of R&D
10.6.3.1 State of R&D
10.6.3.1.1 Properties and applications of nanotechnology in the textiles sector
In last year reports  some of the most important functionalities required of textiles and their areas of application according to strategic research agenda of the “European technology platform (ETP) for the future of textiles and clothing” were presented, indicating how nanotechnology is helping to achieve these desired properties.
Table 1 lists some of the nanomaterials being utilised to improve the performance of textiles. These nanoparticles may be used to develop composite fibres, as nanoscale fillers, or through a foam-forming process and may also be applied as finishing to the textile, for example spray-coating TiO2 for biological protective materials. A more detailed analysis of nanoparticles and nanotechnology applications in the textile sector can be found on [4-7].
10.6.3.1.2 Technologies available for the production of nanotextiles
Textiles' performance improvements, by means of nanotechnologies, has been obtained utilizing the following approaches:
• Fibres containing nanomaterials - Nanometric materials can be dispersed into the polymeric precursor matrix of the fibres or deposited on their surface to give new nanocomposites with improved performances and characteristics.
• Finishing treatments - Surface treatments at nanoscale, using both wet and gas phase processes, can bring about important advantages in the finishing step. The principal technologies adopted are dip coating, spray coating, sol-gel coating, low pressure and atmospheric plasma coating, and chemical vapour deposition.
• Electrospinning - Various polymers have been successfully electrospun into ultraﬁne ﬁbres mostly in solvent solution and some in melt form. When the diametres of polymer ﬁbre materials are shrunk from micrometers to nanometres there appear several amazing characteristics such as very large surface area to volume, ﬂexibility in surface functionalities, and superior mechanical performance (such as stiffness and tensile strength), which can lead to non-woven fabrics with improved or new characteristics having multiple applications. A particular type of electrospinning is called Nanospider™, invented by the Czech Technical University of Liberc and developed by Elmarco. This technology allows for the production of nanofibres on an industrial scale, avoiding the use of capillaries and nozzles.
Most commercially available nano-enhanced textile products (>95%), incorporate nanotechnology through finishing treatments; for example coatings containing nanoparticles, or plasma treatment of finishing goods.
10.6.3.1.3 New nano-enabled functionalities in textiles
The introduction of nanotechnologies in the textile world has opened the path to a huge number of applications, some resulting in a significant improvement on current functionalities but also the introduction of new features. Here follows a list of the principal new nano-enabled functionalities acquired by textiles [4-6].
10.6.3.1.3.1 Antibacterial textiles
Textiles are carriers of bacteria and fungi. Bacterial or fungal growth on fabric can be controlled by (a) finishing, using resins to fix the bacterial agents to the textile surface or (b) grafting antimicrobials/antifungal agents to the fibre chain (cellulosic, viscose, etc). Antibacterial activity is closely related to soil-repellent and soil-release qualities of textiles.
Textiles containing a biocide - The antibacterial compound, generally silver nanoparticles, but also copper or triclosan, are applied on the textile surface by dip coating with a nanosol followed by heating to remove the solvent. Alternatively, Ag nanoparticles can be applied by plasma polymerisation co-sputtering on fabrics [5-9] or they can be mixed with the textile precursor followed by melt spinning.
Textiles with photoactive properties – A fully alternative approach is based on photocatalysis. The fabric is coated with a thin layer of nanocrystalline titanium dioxide (TiO2) particles. TiO2 is a photocatalyst that when illuminated by light of energy higher than its band-gap, electrons in TiO2 jump from the valence band to the conduction band, forming an electron and an electric hole on the photocatalyst surface. Both these species react, respectively, with oxygen and water, with the formation of two unstable species, namely O2 radicals and OH radicals, which are very reactive and react with dirt and microorganisms degrading them to CO2 and water. TiO2 is mostly applied to the textile by sol-gel processing [10-14], or alternatively by chemical vapour deposition .
Ag, TiO2 and their combination are the most commonly used nanochemicals in this field [5,9,16-21], but the use of ZnO [22-24] and chitosan  has also been reported. In addition, the cosmetic, bacteriostatic, antifungal, and wound repair properties of chitin nanofibrils have also been studied [26-28].
10.6.3.1.3.2 Self-cleaning textiles (including water repellent)
Textiles with photoactive properties - As reported for the antibacterial textiles, the photocatalytic activity of TiO2 has been applied to fabrics also with the purpose of stain degradation. TiO2 nanoparticles are applied to a textile as dip coating, by sol-gel technique [11-13,16]. The high energetic species produced by the catalyst when irradiated by light react with stains oxidising them to CO2 and H2O.
Textiles with super-hydrophobic properties - It is known that the wettability is strongly linked to two properties, the surface free energy and the surface roughness. Surface free energy is an intrinsic property of the material that can be controlled by chemical modification, such as fluorination or other hydrophobic coating [29,30]. Fluorochemicals are chemicals capable of repelling water, oil and liquids that cause stains. Ultrahydrophobic textiles have been prepared by modifying the surfaces with various fluorinated polymers, like PFTE, fluoroalkylsilanes, and pefluorinated polymer monolayers.
A second approach, derived from nature, is the so-called lotus effect [31-33] based on the observation that lotus leaves are characterised by exceptional water repellence properties because of their rough surface; a rough surface reduces the ability of water to spread out. Tiny crevices in the leaf's surface trap air, preventing the water droplets from adhering and facilitating cleaning by the water droplets rolling off the surface picking up dirt particles.
Water repellent properties have also been obtained by means of nano-whiskers , by the so-called NanoSphere(R) technique , and by plasma coating . Superhydrophobic properties have been realised on the surface of textiles by coating them with carbon nanotubes (CNT), or with silica, silver, and calcium carbonate nanoparticles in association with non-fluorinated hydrophobic polymers .
Textiles with anti-adhesive properties - A different approach to avoid the sticking of specific compounds, like proteins, to textiles, has been the one to make their surface anti-adhesive. Designed to the production of antihadesive wound dressing, good results have been obtained by sol-gel coating the textile with a SiO2 nanoparticle derivative .
10.6.3.1.3.3 Moisture absorbing textiles
With the aim of obtaining textiles with moisture wicking and transpiration absorbing features, fabrics have been coated with TiO2 nanoparticles . An alternate deposition onto the textile surface of TiO2 and poly(dimethyl diallylammonium chloride has been reported . Plasma techniques are also commercially used to achieve the desired functionality.
10.6.3.1.3.4 UV-blocking textiles
For many applications, mainly in the case of sport/outdoor textiles, UV-blocking properties are highly desirable. Materials reported to obtain textiles with this feature are metal oxide nanoparticles, mostly ZnO [22, 39-41] and also TiO2 or lutein [42-44] by coating the fabric by means of sol-gel processing [14, 41, 42].
10.6.3.1.3.5 Tear/wear resistant textiles
The mechanical properties of textiles can be improved by the help of nanotechnologies, allowing the production of fibres and fabrics with increased strength, elasticity or tear and wear resistance [45-49]. The most widely used nanomaterials for this application are carbon nanotubes (CNT). They can be mixed to many fibre precursor polymers, like polystyrene, polypropilene, polyvinyl alcohol, followed by spinning, or alternatively, applied to fabrics by spay coating or dip coating [50, 51].
In addition, the textile performance can be improved using metal oxide nanoparticles, like ZnO (increased stiffness) , Al2O3 (increased fracture toughness , or SiO2 (increased abrasion resistance) .
10.6.3.1.3.6 Insulating textiles
With the purpose of obtaining textiles with improved insulating properties for the production of exceptional environment garments, nanoporous structures that minimise the mechanisms of thermal transport have been utilised. Particularly suitable to this purpose are aerogels, which are synthetically produced amorphous silica gels impregnated into a non-woven flexible fabric substrate, offering both the benefits of exceptional thermal performance and a flexible blanket form.
In this case, the external size of the aerogel is not necessarily at the nanoscale but the voids incorporated in the amorphous silica gel matrix are. It is the nanosized void space which gives rise to the exceptional insulation performance of these materials. The voids are filled with air, lowering the density of the material; the more air enclosed within the aerogel, the more efficient the insulation. [53, 54].
10.6.3.1.3.7 Conductive textiles
Traditional textiles, both natural and synthetic, are almost always insulators. The interest in transforming them into conductors arises from the need to obtaining antistatic [55-57] or electromagnetic shielding garments, or for the production of the revolutionary electronic “smart” textiles.
The technologies utilized to make a textile conductive are based on the introduction into the fabric of conductive agents, like metal nanoparticles, carbon nanotubes (CNT), carbon black (CB), or conductive polymers, like polypyrrole, polyaniline, and polythiophen [58-59].
These conductive agents are introduced into fibres and fabrics using processes such as physical vapor deposition (Cu , dispersion into polymer followed by spinning(CNT) [61, 62] or electrospinning (CB) , dip coating , vapour or solution polymerisation of conductive polymers [64-66].
The “electronic/smart textiles” are attracting increasing attention; they contain sensors, actuators and control units but still retaining the features necessary for comfortable clothing. They may be either passive, i.e. capable of sensing the surrounding conditions, and active, i.e. containing both sensors and actuators to respond/adapt to specific inputs.
Textiles involved are natural materials, like cotton, wool and flax, and synthetic fibers, like Lycra and Kevlar, with the addition of ICPs (inherently conducting polymers), like PPY (polypyrrole) or PANi (polyaniline), and CNT (carbon nanotubes). Such fabrics are able to absorb substances from the skin or can release therapeutic or cosmetic compounds to the skin [5,7].
10.6.3.1.3.8 Textiles with controlled release agents
This type of textiles find application in many fields such as drug releasing wound dressings, insect repelling outdoors clothing, fragrance emitting clothing, moisturizers or skin care cosmetics [67-69]. The nanomaterials employed in this purpose are nanoclays, like montmorillonite, which is impregnated with the active agent, or chitin nanofibrils which are complexed with active ingredient, melt compounded with a polymer followed by spinning. As an alternative, SiO2 nanosols have been used, by addition of the active species and coating of the fabric by sol-gel-processing [10, 67].
10.6.3.1.4 Medical Textiles
Nanotechnology-related textiles can play an important role in the medical sector. Currently, woven and non-woven anti-bacterial fabrics are the most used applications of nanotechnology in the medical textiles segment, being used to prevent infection or deodorise medical clothing, wound dressing, and bedding.
The number of fields where nanotechnology-related textiles are finding applications are growing and include:
• Surgical, with surgical drapes for the aseptic techniques used in every day wound dressing, catheter changing and the like, to reduce the chances of contamination and cross-infection.
• Medical, three-dimensional textiles to prevent and reduce contact irritations and wound infections
• Prostheses, with fibres that are able to facilitate the bonding of the implant to the living bone, or with resorbable guidance devices for the regeneration of peripheral nerves.
• Dental, with textile that release medical active gases for therapeutic applications, or with multi-component nanofilament for dental care applications.
• Garments, with lightweight, flexible, lead-free X-ray shielding aprons, or clothing incorporating electronic functions to monitor biological parameters or improve the quality of life.
• Drug delivery, with drug-loaded fibres for the delivering and the controlled release of therapeutic agents.
• Fabrics surface-functionalized and utilised for tissue engineering.
• Non-woven nanofibre filters used in a variety of medical equipment, such as respiratory equipment and transfusion/dialysis machines.
• Hygiene, with composite non-wovens with improved liquid absorbing features for nappies, sanitary napkins, adult incontinence pads, panty liners, etc.
10.6.3.1.5 Sport/Outdoor Textiles
Sports textiles are one of the textile segments where performance is very important. These high performance characteristics are achieved making use of innovative technologies (e.g Gore-Tex nanotechnology). The use of these technologies aims to improve different product properties as well as incorporating new functionalities, some of the properties with higher importance for costumers include:
• Improvement in comfort - The physiological comfort of sportswear can affect not only a wearer’s wellbeing but also his performance
• Increase of protection - Protection of the wearer is of fundamental importance, dealing with: impact protection; protection against the cold; water resistance; and water vapour transfer.
• Performance enhancement –Improvement of athlete’s performances; for example, the hydrodynamic characteristics of swimmers.
• Addition of monitoring and training features in the textiles - The combination of sport textiles with sensor devices allows the monitoring of the athlete's physiological conditions leading improvement in physical abilities, training status, athletic potential, and responses to various training regimens.
Combining clothing functions with wearer comfort is a growing market trend, and for all active athletes this constitutes a vital factor for achieving a high performance level . The sports industry has driven a lot of research within the textile industry towards improving athletic performance, personal comfort, and protection from the elements . To reach the above goals, innovative fibres and fabrics, including high performance and high functional fibres, smart and intelligent textiles and coated and laminated textiles have been utilised. Synthetics that were once thought to be inferior to natural fabrics now boast high performance characteristics.
Additionally, the application of nanotechnology and combination of electronics and textiles make the manufacturing of clothing, with integrated heating elements or fabrics for example, which stimulate muscles.
The technical developments in the sports clothing industry have resulted in the use of engineered textiles for highly specialised performances in different sports. With highly functional and smart materials providing such a strong focus in the textile industry generally, companies are increasingly looking for ‘value added’ textiles and functional design in sportswear as well as intelligent textiles, which monitor performance with in-built sensors.
10.6.3.1.5.1 Nanotechnology used to improve textile performance on sport & outdoor applications
Following are some of the principal “nano-tools” applied to improve textile performance in the sport/outdoor applications.
• Carbon nanofibres are used to increase the tensile strength, improve the chemical resistance, and to introduce electrical conductivity.
• Carbon black nanoparticles are used to improve the abrasion resistance and toughness, to increase chemical resistance and to introduce electrical conductivity.
• Carbon nanotubes are used to impart outstanding increase of tensile strength, electric conductivity similar to copper and good thermal conductivity.
• Clay nanoparticles are used to increase electrical, heat and chemical resistance, as flame retardant and anticorrosive, and as blocker of UV light.
• Metal and metal oxide nanoparticles (Ag, Au, Cu, TiO2, Al2O3, ZnO, MgO) are used to impart photocatalytic activity, electrical conductivity, UV absorption, and antimicrobial properties to textiles.
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