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Application of woven fabrics
23
Application of woven fabrics
Abstract: This chapter deals with the application of modeling in the area of technical textiles. The chapter reviews such examples as sportswear, medical applications, textiles for electronics and airbag construction in automotive engineering. Key words: textile modeling, technical textiles.
23.1
Introduction
The formal structure of a woven fabric is defined by weave, thread density, crimp and yarn count. The design of a fabric to meet the requirements of a particular end use is a complicated engineering problem. Theoretically, it is possible to design a fabric structure to achieve any desired characteristic, but in actual practice it is not so easy because of inherent nonlinearity and complex relationship between structure and properties of the textile materials along with their visco-elastic behavior. The factors associated with fabric design include fiber type, yarn geometry, fabric structure and finishing method. The use of different weaves alters the ability of the component threads to move relative to one another, and as a result mechanical properties such as shear characteristics and drapeability of fabric change significantly. The strength of the woven fabric is highest in the warp and weft direction, while in bias, the fabrics show lower mechanical properties, higher elasticity and lower shear resistance. The utility performance properties of speciality woven fabrics depend on the combined effect of the properties of the constituent fibers, yarn and the fabric structure. Thus, designing such a fabric consists of identifying the best combination of those variables that renders the fabric, able to meet the performance requirements. In order to enhance mechanical properties, a triaxial woven structure that consists of three systems of threads (one system for weft and two systems for warp) can be constructed. Warp threads in a basic triaxial fabric are interlaced at 60° and the structure is fairly open with a diamond-shaped center. A modification of basic triaxial fabric is basket weave that forms a closer structure with different characteristics. Woven fabrics have broad application in all segments of technical textile production. Conventional and triaxial fabrics fall into the group of 2-D fabrics. The application of woven fabrics in production of technical textile (as reinforcement in composite 413
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Woven textile structure
production), has inflicted the need for production of 3-D fabrics, which have high mechanical properties in x, y and z directions [1]. The uniqueness and challenge of technical textiles lie in the need to understand and apply the principles of textile science and engineering to provide right solutions to the growing and widely varying demands of their applications in areas such as protective clothing, sportswear, automotive textiles, geotextiles, agricultural textiles, medical textiles and textiles for building and construction and specialty textiles for defence and military applications. Thus it is very important to have an in-depth knowledge of the principles involved in the selection of raw materials and their conversion to desired yarns and fabric structures followed by various treatments such as coating and finishing of fabrics to introduce special technical and commercial features of a wide range of specific areas of applications. High performance technical textiles should not only possess general fabric characteristics but should also satisfy the critical performance requisites. Such fabrics should be engineered with utmost precision for its performance requirements.
23.2
Fundamental aspects of woven textile structure and function
23.2.1 Configurational functions of fiber The reason why textiles are selectively used for a certain specified end use instead of the other kinds of materials is that the textile products in which some of the configurational functions of fiber are effectively utilized can become most valuable among several forms of materials such as powder and solid sheet. The configurational functions of fiber consist of the following four elements: ∑ ∑ ∑ ∑
flexibility (pliable); high ability in axial transmission of such properties as mechanical; high specific surface area; technological easiness in transformability into textile structure; materials such as woven and nonwovens.
23.2.2 Structural system of fiber assembly Fiber assembly structural systems can be summarized based on constitutional element, orientation of element, bonding manner of element and macroscopic form. ∑ ∑ ∑
uniaxial bundle of membrane hollow fiber; nonwovens; glass matt thermoplastic sheet called stampable sheet;
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∑ ∑ ∑
415
woven fabrics; knitted fabrics; braids.
23.2.3 Function system of fiber assembly structures The functions of fiber assembly structures for technical textiles are classified as mechanical bearing, shielding, removal or separation, medium transportation, axial conduction and reinforcement.
23.2.4 The maximum fiber volume fraction There is a geometrical upper limit in the fiber volume fraction which is dependent on fiber orientation and aspect ratio. It must be considered as an inevitable restrictive condition for designing assembly structure.
23.2.5 Pore size and its distribution within fiber assembly structures Pore size is an important parameter for considering such functional properties as shielding, medium transportation and removal or separation. Total fiber lengths by unit volume, inter-fiber coherence are main factors to determine pore size and its distribution.
23.2.6 Directive fiber assembly structures for required functions Directive fiber assembly structures can be roughly assigned to meet required functions using following consideration. ∑ ∑ ∑ ∑ ∑ ∑
For the mechanical functions of tensile loading, impact loading, tear resistance, ballistic resistance, oriented filament is used as constitutional element. For shielding, fiber or yarn is used as constitutional element formed into sheet. For removal or separation, there are typically two kinds of fiber assembly forms; randomly oriented fibers in sheet and uniaxially oriented bundle of membrane hollow fiber. For medium transportation the constitutional element is usually fiber, to obtain a high contact area of assembling struct with medium. For axial conduction, fiber or filament is usually uniaxially oriented. For reinforcement, there are typically two types: fiber randomly distributed within the matrix and yarn unidirectionally oriented within the matrix.
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Woven textile structure
These kinds of analysis can provide a directional systematic knowledge for designing optimum assembly structure for developing different textile products suitable for various end-use applications.
23.3
Medical textiles
Medical textiles are the products and constructions used for medical and biological applications for clinical and hygienic purposes, scaffolds for tissue culturing and a large variety of prostheses for permanent body implants. They consist of all those textile materials used in health and hygienic applications in both consumer and medical markets. A broad classification of medical textiles can be as under: ∑ ∑ ∑ ∑ ∑
Protective and healthcare textiles – surgical wear, operation dresses, staff uniforms, etc. External devices – wound dressings, bandages, pressure gauze, prosthetic aids, etc. Implantable materials – sutures, vascular grafts and artificial limbs are the products where textiles are used. Hygiene products – incontinence pads, nappies, tampons, sanitary towels, etc. Extracorporeal devices – artificial liver, artificial kidneys and artificial lung are the recent advances in medical textiles.
23.3.1 Woven medical textiles Woven medical textiles are typically used for products requiring extreme stability and; high durability over a significant number of loading cycles; or to precisely control porosity for air or fluid flow. For example, a good quality surgical gown must be made of light and comfortable, breathable fabric material, yet be tough and durable enough to withstand abrasion, ripping and puncture. Surgical gowns must act as a barrier between the sources of infection (micro-organisms such as bacteria, and viruses of different size and geometry) and the user (healthy person), and must also demonstrate good wearing comfort. The latter is important for the surgeon who often has to wear the surgical gown for several hours while doing hard work. Hydrophobic wovens from polyester are used for shorter surgical operations with a small amount of liquid, and have so far been the only reusable surgical gowns which are currently able to fulfill both these contrary demands of barrier effect and wearing comfort [2]. The barrier function of such fabrics depends on the surface structure, and also on the number and size of the continuous pores running through the fabric; the pores which run both between the filaments in the filament yarn
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and also between the filament yarns. When the ‘barrier’ is the feature to be evaluated, there are three measurable characteristics: ∑ ∑ ∑
resistance to dry microbial penetration; resistance to wet microbial penetration; resistance to liquid penetration.
When the mechanical resistance is the feature to be evaluated, there are four measurable characteristics: ∑ ∑ ∑ ∑
dry bursting strength (combination of air movement and mechanical action); wet bursting strength (combination of wetness, pressure and rubbing); dry tensile strength; wet tensile strength.
When the freedom from contamination or unwanted foreign matter is the feature to be evaluated, there are two measurable characteristics: ∑ ∑
microbial (not viable micro-organisms); particulate matter (freedom from particles not generated by mechanical impact).
When the release of fiber fragments or other particles, originally from the fabric itself, is the feature to be evaluated, there is one very important measurable characteristic: linting. The only obvious characteristic that is exclusively requested for the surgical drapes (but not for the surgical gowns) is adhesion for fixation for the purpose of wound isolation.
23.4
Automotive textiles
Automotive textiles are finding extensive use in the product categories of interior trims, safety devices such as seatbelts and airbags, carpets, filters, battery separators, hood liners, hoses and belt reinforcement. Textiles, which constitute approximately 20–25 kg in a car, are not only used for enhanced aesthetics of automotives but also for sensory comfort and safety. Additionally, few textile products found their application as design solutions to engineering problems in the form of composites, tyre reinforcement, sound insulation and vibration control. Apart from woven and knitted constructions, nonwovens also find applications in automotive textiles due to certain advantages served by them. The fabrics used worldwide as surface material for car interior can be woven, knitted or nonwovens. Woven fabrics represent the dominant application areas in seat covers, headrests and door panels. Fabrics are characterized by a large variety in design, stable shape retention and high mechanical resistance.
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Woven textile structure
23.4.1 3-D woven fabrics for automotive textiles 3-D sandwich woven fabrics are made of 100% E-glass yarns, as well as carbon fiber, basalt fiber or other high performance fibers. Two deck layers are bonded together by Z-piles to the specified height of 2–25 mm. When the fabric is impregnated with a thermoset resin, it immediately absorbs the resin and, due to the capillary forces of the Z-piles, the fabric rises to the preset height. Because the composite material is a kind of hollow integrated core sandwich structure that offers excellent mechanical properties and design versatility. The composite material is widely used for many applications in the core composites industry, such as ships, railways, automotive, aviation, wind energy, double wall tanks and construction, while offering multiple advantages (delamination, impact, etc.) against traditional sandwiches such as honeycombs, foams, balsa and more.
23.4.2 Airbags Airbags are a type of automobile safety restraint like seatbelts. They are gas-inflated cushions built into the steering wheel, dashboard, door, roof or seat of the car that use a crash sensor to trigger a rapid expansion to protect a person from the impact of an accident. The working of airbag is a precision application. Airbags should begin to inflate 0.03 seconds after a crash and should be fully inflated after 0.06 seconds. Airbags may be built into steering or in some other strategic location. Nitrogen gas is commonly used in airbags. Fabrics used for airbags must be able to withstand the force of hot gases and they must not penetrate through the fabric. Airbags are typically woven from high tenacity (HT) multifilament nylon 6,6. Nowadays, a one-piece weaving system produces an airbag directly on loom, but formerly two pieces were sewn together with suitable threads. Typically, front airbags are uncoated. The size of airbags may vary according to their position. The concept of the airbag, a soft pillow to land against in a crash, has been around for many years [3]. Airbag working Airbags inflate, or deploy quickly, faster than the blink of an eye. Imagine taking one second and splitting it into one thousand parts. In the first 15 to 20 milliseconds, airbag sensors detect the crash and then send an electrical signal to fire the airbags. Typically a squib, which is a small explosive device, ignites a propellant, usually sodium azide (NaN3). The azide reacts with potassium nitrate (KNO3) and burns with tremendous speed, generating nitrogen, which inflates the airbags. Within 45 to 55 milliseconds the airbag
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is supposed to be fully inflated. Within 75 to 80 milliseconds, the airbag is deflated and the event is over [4]. When airbags work properly, they dramatically reduce the chance of death or serious injury. However, the speed with which airbags inflate generates tremendous forces. Passengers in the way of an improperly designed airbag can be killed or significantly injured. Unnecessary injuries also occur when airbags inflate in relatively minor crashes when they are not needed. Airbag inflation The goal of an airbag is to slow the passenger’s forward motion as evenly as possible in a fraction of a second. There are three parts to an airbag that help to accomplish this feat: ∑ ∑
∑
The bag itself is made of a thin nylon fabric, which is folded into the steering wheel or dashboard or, more recently, the seat or door. The sensor is the device that tells the bag to inflate. Inflation happens when there is a collision force equal to running into a brick wall at 10 to 15 miles per hour (16 to 24 km per hour). A mechanical switch is flipped when there is a mass shift that closes an electrical contact, telling the sensors that a crash has occurred. The sensors receive information from an accelerometer built into a microchip. The inflation system which inflates the airbag through the production of nitrogen.
Airbag manufacture Airbag fabrics are made of nylon 6,6 multifilament yarns with counts from 235 to 940 tex. Airbag fabrics are generally dense which presents a challenging task to weave. Tensile strength, elongation, tear proportion resistance and weight requirement of airbag fabric are critical [5]. Air permeability of airbag fabric should be uniform across the whole width of the fabric. There are currently two principal material types which are used in the manufacture of airbags. They are uncoated nylon (polyamide 66) and coated nylon. Two types of commonly used coatings are silicone and neoprene. In general, coated materials are used for driver’s side airbags and side impact bags, while passenger side airbags are made from uncoated nylon materials [6,7]. All airbag cushions manufactured worldwide are constructed with fabric made from nylon 6,6 yarn. The key initial drivers were performance, cost and benefit. As one can imagine, the key function of the airbag cushion is to absorb the impact. Nylon 6,6 has superior capability in energy absorption. The balance between the strength and elongation gives it unmatched suitability for airbag cushion materials. The performance attributes that have
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Woven textile structure
led to industry standardization are its breaking strength or tenacity; energy absorption capability or toughness; heat resistance; and stability over time as measured by accelerated aging tests [6]. More recently, a third selection criterion has started to come to the fore, that of proven performance or confidence. Since the airbag system forms part of the automotive supply chain there is continuous and necessary consideration of other materials such as polyester, for use in the airbag cushion. It is possible to demonstrate that all these alternatives, including polyester are significantly inferior to nylon 6,6 when measured against the key performance attributes listed above and that their use would pose a risk of failure in airbag applications. That lower performances of polyester coupled with increasing legislative and litigation activity around automotive safety and the lack of a track record in the global market for polyester have led to the conclusion that polyester is not appropriate for airbag cushion use. For comparison purpose, the key physical properties of nylon 6,6 and polyester are shown in Table 23.1. The key differences between the two polymers are density and specific heat capacity. Comparison of key properties of nylon 6.6 and polyester Although nylon 6,6 and polyester have similar melting points, the large difference in the specific heat capacity causes the amount of energy required to melt polyester to be about 30% less than that required to melt nylon 6,6. Hence in any inflation event that uses a pyrotechnic or pyrotechnic-containing inflator, cushions made from polyester yarn are far more susceptible to burn or melt through in the body of the cushion or at the seam. The second advantage of nylon 6,6 is its lower density. Lower mass has key advantages; reducing the mass of the cushion lowers the kinetic energy of impact on the occupant in out-of-position situations thus enhancing safety, while allowing the overall weight of the vehicle to be reduced. The difference in density between the two polymers leads to polyester yarns usually being of higher denier or decitex (weight per unit length) than nylon 6,6, to generate the same filament diameter. These results in reduced
Table 23.1 Key physical properties of nylon 6,6 and polyester 3
Density (kg/m ) Specific heat capacity (kJ/kg/K) Melting point (°C) Softening point (°C) Energy to melt (kJ/kg)
Nylon 6,6
Polyester
1140 1.67 260 220 589
1390 1.3 258 220 427
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fabric coverage. Using polyester yarn, the cushion fabric is more open for gas permeation. This reduces thermal protection for the vehicle occupants, and makes it more difficult for the cushion designer to control the bag deployment dynamics. In addition, since seam strength is strongly dependent on cover factor, seam performance is negatively impacted. This is particularly important since seam leakage of hot gas is one of the principal concerns of the engineer and the potential for an increase in leakage combined with a reduction in thermal resistance is critical [8,9]. Coated base fabric for airbags A coated airbag base fabric made of a textile fabric that has an excellent air barrier property, high heat resistance, improved mountability, and compactness and excellent adhesion to a resin film is characterized in that at least one side of the textile fabric is coated with resin, at least part of the single yarns of the fabric are surrounded by the resin, and at least part of the single yarns of the fabric are not surrounded by the resin. An airbag is characterized by using such a coated airbag base fabric [10]. A method for manufacturing the coated airbag base fabric is characterized by applying a resin solution having a viscosity of 5–20 Pa s (5000–20 000 cP) to the textile fabric using a knife coater with a sharp-edged coating knife at the contact pressure between the coating knife and the fabric of 1–15 N/cm [11]. Laminated material for airbag A polymer film, preferably polyamide polymer film, which comprises at least one first layer and a second layer, is laminated onto a fabric. The material of the first layer has a glass transition temperature of less than 10 oC, while the material of the second layer has a glass transition temperature of less than 20 oC. Preferably, the polymeric materials contain portions of polyamide blocks. The fabric–polymer film laminate is suitable as a laminated material especially for an airbag [11].
23.5
Filter fabrics
The surface quality of a woven fabric has become an increasingly important factor which influences the property of the filter in many different ways. Surface treatments modify the functionality of the woven fabric according to the different requirements and thereby significantly increase the effectiveness of the resulting filter in a specific environment [12]. The following surface treatment technologies improve the surface property:
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∑ ∑ ∑
Woven textile structure
wet chemical surface modification (mainly hydrophilic); low temperature plasma treatment (mainly activation, fictionalization and plasma polymer deposition); metal coating (aluminum, copper and nickel).
The selection of the appropriate fabric type and coating depends on the functionality needed for the test. The phenomenon of wetting or non-wetting of a solid by a liquid is better understood by studying contact angle of a liquid drop on a condensed surface as demonstrated in Fig. 23.1. The lower the contact angle between liquid drop and fabric surface, the better the hydrophilic action of the fabric. Woven filters are used in many acoustic devices, loudspeakers and microphones. Filters not only improve the speech and sound quality in mobile phones by adsorbing unwanted frequency peaks; they also protect the sensitive electronic equipment from moisture, dust and dirt. Hydrophobic surface treatments offer an excellent protection against moisture uptake and contamination. Acoustic fabrics are hydrophobically treated to provide optimal protection against moisture uptake and to repel dust and dirt. Hydrophobic woven fabrics protect loudspeakers and microphones from moisture, dust and dirt. In automobiles, a series of filter systems are used to protect sensitive components. High precision woven fabrics are mounted in the fuel, injection and hydraulic filter systems. Since the fuel can be contaminated with particles and dirt originating from refining, transport and storage, from the filling process or from simply the automobile tank itself, a series of security filters are installed along the fuel transport path. These filters, from the tank to the injector system, protect the mechanical parts from damage and prevent the injection nozzles from blocking. In addition, high water content is usually found in fuels, especially in diesel, which has to be removed as it may lead to corrosion. In order to enhance water removal, the fabrics used in fuel systems should be hydrophobic. Fabrics remove water from kerosene and other fuels. An airplane carries thousands of liters of fuel which must be free of particles and water to guarantee trouble-free operation of the jet engines. Jet fuel contains a certain amount of dissolved particles (approx. 60 parts per
23.1 High-contact angle, hydrophobic fabric and low-contact angle, hydrophilic fabric.
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million [ppm], ca. 60 micrograms per kilogram of kerosene); free water must be removed to prevent freezing and blocking of the fuel transport system. Coalescer systems are used in the removal of water from jet fuel and others (e.g. diesel). Water droplets combine at the coalescer surface (filter medium) to form drops which by flow and or gravity move to a separator. The fuel then flows through the separator while the water drops are retained by the hydrophobic filter [13]. The surface of the separator consists of a hydrophobic woven filter medium supported by a stainless steel element.
23.6
Textiles for electronics
E-textile technology holds out the promise of truly wearable computers as well as inexpensive large-scale computational devices. To achieve these goals, e-textiles combine high volume, low cost textile manufacturing capability with discrete electronics and novel fiber technologies. Industrial weaving machines capable of inserting thousands of meters per minute of weft yarn can efficiently produce large volumes of complex woven textiles while individually controlling the position of every fiber in the design. New fibers are being created for inclusion in e-textiles, including battery fibers, conductive fibers and mechanically active fibers. Methods are being developed for attaching discrete components to e-textiles, including processors, microphones and speakers. Two broad categories of e-textile applications are envisioned, wearable and large-scale non-wearable [14]. Many specific applications in the field of wearable computing have been envisioned and realized, though most suffer bulky form factors. In the new field of large-scale non-wearable e-textiles, applications include large-scale acoustic beam-forming arrays (STRETCH), self-steering parafoils (Draper) and intelligent, inflatable decoys (DARPA). Both categories of e-textile applications share three common design goals: low cost, durability and long running. ∑
∑
∑
Low cost dictates the use of inexpensive, commercial, off-the-shelf (COTS) electronic components and yarns as well as the design of weaves and architectures that are manufacturable in current or slightly modified textile production systems. Durability dictates that the system must tolerate faults, both permanent and transient, that are inherent in the manufacture and use of the device. In addition, there is an expectation that individual components may not be repairable and that system functionality should gracefully decline as components fail. Long running dictates that the system must manage power consumption in an application-aware fashion to minimize the need for bulky batteries and/or external power recharge. Power scavenging and distributed power management are essential.
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Woven textile structure
In the category of wearable computing, a garment that provides the user with precise location information within a building is analyzed. A large-scale, non-wearable acoustic beam-forming array is analyzed. Textiles are woven as opposed to alternative textile manufacturing techniques such as knitting, embroidering, or non-woven technologies. Woven textiles allow for stable fabrics with a high degree of precision, but impose directional limitations on the fabric, which have implications for communication within the textile. Virtually all woven e-textiles are expected to use the new fiber batteries and solar cells under development. This will lead to highly distributed, redundant power supplies for e-textiles in which some parts of the textile have more remaining power than others [15].
23.6.1 Mapper garment The mapper garment tracks the motion of the user through a structure by monitoring the user’s body position, the user’s movement, and the distance of the user from surrounding obstacles. Such a garment would allow users to be given directions in a building, maintenance workers to be automatically shown blueprints for their current room, or users to automatically map existing structures. The user’s body position is measured by a set of piezoelectric strips woven into the clothing; by measuring the deformation along tens of strips, the physical configuration of the user’s body can be detected [16]. The user’s activity, such as walking up stairs, climbing a ladder and walking on a flat surface, can be detected. The user’s movement rate can be measured by a small set of discrete accelerometers as well as a digital compass attached to the garment. The distance from obstacles is measured using ultrasonic signals. An array of approximately ten ultrasonic transmitters, also piezoelectric strips, are distributed around the garment to periodically send signals in each direction; a similar number of receiver piezoelectric strips are used to detect the reflected signals, allowing time-of-flight to be measured. The primary challenge in this application is interfacing to a large number of sensors and actuators in a reliable fashion. Simply attaching the leads of every sensor or actuator to a single processing unit and power supply would not meet the design goal of a durable e-textile. In the event of a tear in the fabric, single leads running to one collection point could lead to significant rather than graceful degradation in performance; in addition, the potentially long leads required would cause degradation in analog signal quality unless significant amplification is applied, leading to larger power consumption. The garment needs multiple points at which analog data is converted to digital data; these conversion units, likely in the microcontroller or digital signal processor (DSP) class, would need to communicate within a fault tolerant network. The sample rate at each conversion point is very low (10–100 per second) for the body position sensors and moderate (100 000 per second)
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for the occasional ultrasonic reception. Once the data has been converted to a digital format, low power data transmission and coding techniques can be applied. By carefully managing the active sensors and processing units the power requirements of the system can be reduced. When determining the location of a wall, the garment must activate a transmitter in the direction desired and then sample a receiver for the return signal. This will accurately compute the distance, but gives no information on the direction in which it is located. To compute direction, the location of the transmitter on the body, the position of that part of the body relative to the torso, and the direction in which the user’s torso is pointing must be known. The location of the transmitter should be known from the manufacturing process and the digital compass can provide the torso direction. A number of techniques are available for determining the location of one part of the e-textile with respect to other parts of the e-textile; in the garment, the body position is available and acoustic beam forming could be used to determine the location of all of the ultrasonic transceiver with respect to one another. To accurately compute the body position requires the analysis of samples from a large set of sensors that have been collected at several processing elements. Due to its nature, the analysis is best done at a single processing element. To initialize the collection of the data, the processing element would send out a query to the selected elements, such as ‘give me your current reading’ or ‘give me your reading every 100 ms’. In both cases, some time stamp should be applied to both the query and the replies, though high accuracy is not required in this particular application [17].
23.6.2 Beam-forming array The beam-forming array textile gathers data from a large array of acoustic sensors and analyzes this data to determine the direction of an acoustic emitter (e.g., a moving vehicle or a human voice). Through the use of acoustic beam-forming algorithms, a set of three acoustic sensors can identify the direction of a single emitter if the sensors and the emitter are all in the same plane. Identifying the direction of multiple emitters or working in three dimensions requires data from more acoustic sensors. Further, given noise and potential miscalibrations in the acoustic data, the use of redundant acoustic sensors is advisable. If the fabric is large enough, then not only the direction, but also the location of an emitter can be found. Like the mapper garment, for reasons of robustness, the large number of sensors is not all handled by one conversion or processing node; the fabric is sprinkled with many communicating processing nodes. It is important to note that each acoustic sensor must know its location precisely with respect to the other sensors; small errors in sensor location result in increasingly larger errors as the distance to the emitter increases. Although this is not a wearable
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Woven textile structure
textile, it is flexible and thus subject to movement; at a minimum, the initial position of each acoustic sensor must be computed. In contrast to the mapper garment, the positions of each sensor must be known quite accurately. To accomplish this, the textile is augmented with speakers that are physically co-located with a subset of the acoustic sensors; by systematically activating the speakers, the distances between the microphones can be determined. Once enough distances are known, the relative positions of all of the microphones can be computed. The frequency, direction and distance of a potential target all affect the optimal selection of a subset of sensors on which to perform beam forming. Because beam forming is fairly computationally demanding, it would be wasteful of resources, including power, to collect and analyze the data from the entire set of acoustic sensors. An efficient strategy, therefore, is to have a small active set of sensors look for emitters while the rest of the sensors sleep to conserve power. Upon the tentative identification of an emitter’s characteristics along with an assessment of remaining power at processing nodes, an optimal set of sensors can be activated. Once a set of sensors is selected, the time series data from those sensors must be collected at a single processor where the beam-forming algorithm is to be run. If multiple beams are formed, then the direction and intensity data must be combined at a single node; this is much less demanding of communication resources than the exchange of time series data. It is expected that some nodes along potential routes may be asleep, others out of power, and some broken; routes must be found in spite of these drawbacks. In addition, time series data from different nodes must be synchronized; small errors in time can lead to large errors in the computed results [17].
23.6.3 Nanotube-based e-textile Conductive woven fabrics are made by utilization of metallic conducting wires which are flexible wearable electrical circuits; the properties of nanotube-based yarns could be leveraged to produce more robust systems far lighter and more flexible than metallic wires. The applications and methods of integration of conductive yarns into clothing have been dealt with the integration of conventional integrated circuits and the utilization of fabricbased components as part of wearable computing devices. As a step in the direction of creation of nanotube-based electronic components for fabric applications, super-capacitors woven into a textile fabric as shown in Fig. 23.2 has been reported. The capacitors were shown to have capacitances and life cycles comparable to commercially available super-capacitors. Since nanotubes are also known to have effects similar to piezoelectric materials (when exposed to strains), such materials could
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be integrated into structures to produce sensors that monitor the stresses in structures [18].
23.6.4 Organic field effect transistors (OFETs) By coating fibers with conducting polymer, they can join them with solid electrolyte at cross-points to form micrometer-sized organic electrochemical transistors (OECTs). They then use the OECTs as components for logic circuits, offering an alternative approach to organic field effect transistors (OFETs) commonly used in flexible electronics. The wire OECTs provide an easier route to weaving electronics directly into fabrics Fig. 23.3 [19]. Common OFETs operate like electrical valves, with the flow of current between the sources and drain electrodes controlled by the voltage of the gate electrode. Increasing the gate voltage injects charges into the conducting polymer next to the gate electrode, creating a conduction channel, through which the source-to-drain current flows. The magnitude of the voltage applied by the gate controls the size of this conduction channel. OFETs can be flexible, but printing them onto fibers is a complex process. Their successful operation depends on the accurate control of the electric field applied to the polymer by the gate electrode, which in turn requires a very thin and uniform insulating layer to be deposited between the gate and active polymer. Precise micropatterning of source drain and gate electrodes on a fiber is therefore needed. These requirements make the implementation of OFET-based knitted or woven structures very impractical. ∑
OFET on fibers; switching from the Off to the On state operates through charge accumulation in the polymer channel under the action of the electric field from the gate electrode, which increases the polymer conductivity in the conduction channel.
23.2 Woven textile super-capacitor.
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Woven textile structure Source
Drain
Gate
Drain
Gate Insulator
Off
Off
Source
Drain
Drain
Gate
Gate
On (a)
Insulator
(b)
On
Source
Conductor Semiconductor in the low-conduction state Semiconductor in the high-conduction state Ion reservoir
23.3 Working principles of (a) organic field effect transistors (OFET) and (b) wire electrochemical transistors (WECT).
∑
∑
Wire electrochemical transistor (WECT) switching from the On to the Off state occurs when ions are depleted from the polymer semiconductor channel by electrodiffusion through the solid electrolyte under the action of the gate voltage. The WECT structure is symmetrical; the source–drain fiber and gate fiber can be interchanged, which adds flexibility to circuit design. In the OECTs the conducting channel is made by ions being injected from or removed to a solid electrolyte reservoir, which is in turn controlled by the gate electrode. This means very large conductivity changes can be easily induced, and these conductivity changes control the sourceto-drain current in the same way as the injected charges from the gate electrode in the OFET. Because of the differences in the way the devices work, the need for accurate dimensions and precise positioning of the transistor subparts is greatly relaxed in OECTs compared with OFETs. OECTs also benefit from needing very low operating voltages of the order of a couple of volts [20].
Wires are coated with conducting polymer to create cylindrical electrodes. At junctions where the wires cross each other, a drop of solid electrolyte is used both as a physical joining method and as the ion reservoir to create a transistor (WECTs). If a polymer with lower conductivity is used to
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join the wires instead, a resistor is made. The logic circuits can be made using arrays of wires crossed and connected in these ways as shown in Fig. 23.4 [21]. ∑ ∑
Classical circuit diagram of an inverter, consisting of a transistor and three resistors. Illustration of how this circuit can be realized using WECT technology.
Despite all the positive features outlined, there is a major intrinsic constraint on the performance of OECTs that is highly relevant to their implementation on fibers. The rate-limiting process of OECT operation is electrodiffusion of ions within the solid electrolyte; ionic charge carriers have very low mobility, which does not allow speeds of operation comparable to either inorganic or conventional organic transistors. The long response time and the consequent low switching frequency of WECT-based logic circuits will certainly limit the dynamic characteristics of any application. Dimensional scaling to speed up operation is an option because the time characteristics of electrodiffusion
+Vdd
–Vdd
R1 Vout
Vin
R2
Vin
R3
Vout
–Vdd
Vout
Vin +Vdd (a)
(b) Coated fiber
Wect
Resistor Insulating
Low resistance High resistance
23.4 Logic circuits constructed from WECTs (a) classical circuit diagram of an inverter, consisting of a transistor and three resistors (b) illustration of how the circuit in (a) can be realized using WECT technology.
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processes scale with the inverse square of characteristic lengths. However, other rate phenomena might be dominant, limiting operations to very low frequencies and hence confining WECT technology to quasi-static applications. This aspect needs investigation to assess the full potential of WECT woven logic in the field of e-textiles and wearable technology. Body signals with slow dynamics, such as sweat rate and composition changes, body surface temperature mapping and surface-strain field mapping for posture recognition and respiration monitoring, are therefore likely to be ideal targets in the area of wearable monitoring systems for health, sport and ergonomics using this technology [22]. Chemo-, piezo- and thermo-resistive fibers are now available in textilecompatible form, and their integration into fabrics and garments is being actively pursued [17,18]. Using simple and elegant solutions, the inganas team [23] has interwoven WECTs with passive electronic components on fibers for local signal acquisition and early conditioning in e-textiles, paving the way for a revolution in existing wearable technology [21].
23.7
Sports textiles
Sport textiles are textiles used in sports. These are sports goods and sportswear. Sportswear is clothing, including footwear, worn for sport or exercise. Typical sport-specific garments include short pants, tracksuits, T-shirts, polo shirts and trainers. Specialized garments include wet suits and salopettes. It also includes some underwear, such as the jockstrap. Sportswear is also often worn as casual fashion clothing. For most sports the athletes wear a combination of different items of clothing, e.g. sports shoes, pants and shirts. Some athletes wear personal armour such as helmets or American footbal body armour.
23.7.1 Functional considerations Almost every piece of sports clothing is designed to be lightweight so the athlete is not encumbered by its weight. The best athletic wear for some forms of exercise, for example cycling, should not create drag or be too bulky. On the other hand it should be loose enough so as not to restrict movement. Clothing worn for some other forms of exercise should not unduly restrict movement and may also have specific requirements, for example the Keikogi used in karate. It should allow freedom of movement in competition. Various physically dangerous sports require protective gear, e.g. for fencing, American football or ice-hockey. Standardized sportswear may also have the function of a uniform. In team sports the opposing sides are usually made identifiable by the colours of their clothing, while individual team members can be made recognizable by a back number on a shirt.
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Garments should allow the wearer to stay cool in hot weather and warm in cold weather. In cold climates the best athletic wear should not only provide warmth but also transfer sweat away from the skin. For activities such as skiing and mountain climbing this is achieved by using layering; moisture transferring materials must be worn next to the skin, followed by an insulating layer, and wind and water resistant shell garments.
23.7.2 Moisture-transferring fabric Waterproof, breathable fabrics are designed for use in garments that provide protection from the environmental factors such as wind, rain and loss of body heat. Waterproof fabric prevents the penetration and absorption of liquid water. The term ‘breathable’ implies that the fabric is actively ventilated. Breathable fabrics passively allow water vapour to diffuse through them yet prevent the penetration of liquid water. High functional fabrics support active sportswear with importance placed on functions as well as comfort. Finally, materials with heating and or cooling property have newly attracted the interest of the market. All these materials do not pursue a single function, but different functional properties combined on a higher level. Fabrics that can convey water vapor from body perspiration out through the material while remaining impervious to external liquids such as rainwater are widely used in sportswear and similar applications. Water-resistant and moisture-permeable materials may be divided into three main categories, high density fabrics, resin-coated materials and film-laminated materials. These are selected by the manufacturers according to the finished garment requirements in casual, athletics, ski or outdoor apparel.
23.7.3 Densely woven water breathable fabrics The densely woven waterproof breathable fabrics consist of cotton or synthetic micro-filament yarns with compacted weave structure. One of the famous waterproof breathable fabrics known as Ventile was manufactured by using long staple cotton with minimal space between the fibers. Usually combed yarns are woven parallel to each other with no pores for water to penetrate. Usually oxford weave is used. When the fabric surface is wetted by water the cotton fibers swell transversely reducing the size of pores in the fabric and require very high pressure to cause penetration. Therefore waterproof is provided without the application of any water repellent finishing treatment. Densely woven fabrics can also be produced from microdenier synthetic filament yarns. The individual filaments in these yarns are of less than 10 mm in diameter, so that fabrics with very small pores can be engineered [24].
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Woven textile structure
23.7.4 Laminated waterproof breathable fabrics Laminated waterproof breathable fabrics are made by application of membranes into the textile product. These are thin membrane made from polymeric materials. They offer high resistance to water penetration but allow water vapor at the same time. The maximum thickness of the membrane is 10 mm. They are microporous and hydrophilic membranes. Microporous membranes have tiny holes on their surface, smaller than a raindrop but larger than water vapour molecule. Some of the membranes are made from polytetrafluoroethylene (PTFE) polymer, polyvinylidene fluoride (PVDF) [3,4]. The hydrophilic membranes are thin films of chemically modified polyester or polyurethane. These polymers are modified by the incorporation of polyethylene oxide [6], which constitutes the hydrophilic part of the membrane by forming amorphous regions in the main polymer system. This amorphous region acts as intermolecular pores allowing water vapour molecules to pass through but preventing the penetration of liquid water due to the solid nature of the membrane [24].
23.7.5 Coated waterproof breathable fabrics Coated fabrics with waterproof breathable fabrics consist of polymeric material applied to one surface of fabric [6,7]. Polyurethane is used as the coating material. The coatings are microporous and hydrophilic membranes. In microporous membrane the coating contains very fine interconnected channels much smaller than the finest raindrop but larger than water vapor molecules. Hydrophilic coatings are similar to hydrophilic membranes; the microporous material allows water vapour through the permanent airpermeable structure whereas the hydrophilic material transmits vapour through adsorption-diffusion and de-sorption mechanism. The desirable attributes of functional sportswear and leisurewear are: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑
optimum heat and moisture regulation; good air and water vapour permeability; rapid moisture absorption and conveyance capacity; absence of dampness; rapid drying to prevent catching cold; low water absorption of the layer of clothing just positioned to the skin; dimensionally stable even when wet; durable; easy care; lightweight; soft and pleasant touch.
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It is not possible to achieve all of these properties in a simple structure of any single fiber or their blend. The two layer structure has a layer close to skin of the wicking type from synthetic fibers, e.g. microdenier polyester, and the outer layer is usually cotton or rayon that absorbs and evaporates. Microdenier polyester is ideal for wicking perspiration away from the skin. The use of superfine or microfiber yarn enables production of dense fabrics leading to capillary action that gives the best wicking properties. No single fiber or blend of different fibers can give ideal sportswear. The right type of fiber should be in the right place. Blending of fibers does not give the same effect as that of multi-layer fabric. The wicking behavior of the fabric is mainly dependent on its base fiber’s moisture properties [24].
23.7.6 Moisture transport mechanism The mechanism by which moisture is transported in textiles is similar to the wicking of a liquid in capillaries. Capillary action is determined by diameter and surface energy of its inside face. The smaller diameter or greater surface energy gives greater tendency of a liquid to move up the capillary. In textile structures, the spaces between the fibers effectively form capillaries. Hence, narrow spaces between the fibers enable the textile to wick moisture. Fabric constructions, forming narrow capillaries pick up moisture easily. Such constructions include fabrics made from microfibers, which are packed closely together. However, capillary action ceases when all parts of a garment are equally wet. The surface energy in a textile structure is determined largely by the chemical structure of the exposed surface of the fiber. ∑ ∑
Hydrophilic fibers have a high surface energy. Consequently, they pick up moisture more readily than hydrophobic fibers. Hydrophobic fibers, by contrast, have low surface energy and repel moisture.
Special finishing processes can be used to increase the difference in surface energy between the face of a fabric and the back of the fabric to enhance its ability to wick [24].
23.7.7 Factors affecting moisture transport There are several factors which affect moisture transport in a fabric. The most important are: fiber type, cloth construction or weave, weight or thickness of the material and presence of chemical treatments. Synthetic fibers can have hydrophilic (wetting) or hydrophobic (nonwetting) surfaces. They also have a range of bulk absorbencies, usually reported by suppliers and testing
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organizations as the percentage moisture regain [1] by weight. Synthetic fabrics are generally considered to be the best choice for garments worn as a base layer. This is due to the ability to provide a good combination of moisture management, softness and insulation. While most fabrics, both natural and synthetic, have the ability to wick moisture away from the skin, not all of these are fast-drying and air permeable; two factors which have a direct influence on cooling and perceived comfort. High-tech synthetic fabrics are lightweight, are capable of transporting moisture efficiently, and dry relatively quickly. It is generally agreed that fabrics with moisture wicking properties can regulate body temperature, improve muscle performance and delay exhaustion. While natural fibers such as cotton may be suitable for clothing worn for low levels of activity, synthetic fabrics made of nylon or polyester are better suited for high levels of activity. They absorb much less water than cotton, but can still wick moisture rapidly through the fabric. The main parameters for comfort and functionality are: ∑ ∑ ∑ ∑ ∑
water- and windproof, breathability and comfort; moisture and sweat management; warmth and temperature control; easy-care performance; smart and functional design [24].
23.7.8 Characteristics of sports clothing Sports clothing should have following characteristics: ∑ ∑
∑
Protective properties against variable atmospheric conditions existing during the clothes use as well as protection against physical damage. A high resistance to external influences, including tear strength, resistance to abrasion, shape stability, colour fastness, making-up quality, constancy of protective functions, and other features contributing to the service life of such materials. Comfort-providing properties, generally described as wellness, including first of all physiological comfort. This includes protection against overwarming or cooling, owing to high water vapour permeability, i.e. carrying off perspiration, good warmth retention and adequate air permeability. Moreover, the person wearing the garment will be positively affected by soft handle and good shape assumption by the fabric and cloth cut that does not limit their ease of movement, as well as the cloth’s aesthetic appeal and practical constancy of protective and aesthetic functions throughout the period of use.
Application of woven fabrics
23.8
435
References
1. Demboski G and Bogeva-Gaceva G (2005), ‘Textile Structures for Technical Textiles Part II: Types and Features of Textile Assemblies,’ Bulletin of the Chemists and Technologists of Macedonia, 24, 1, 77–86. 2. Textiles Intelligence Limited (2007), ‘Developments In Medical Textiles,’ Technical Textiles Market, Issue 70. 3. www.scienceservingsociety.com 4. www.howstuffworks.com/airbag 5. Siejak V (1997), ‘New yarn for light weight airbag fabrics’, Technical Textiles, 40, E54–E56. 6. Sato H (1997), ‘The other fabric for air bag,’ Technical Usage Textiles, Trimestre, 49–50 7. Siejak V (1997), ‘Now yarns for the light-weight air bag fabrics’, Technical Textile, 40, E54–E56 8. Bandyopadhayay B N (2000), ‘Flame retardant automotive Textile’, Manmade Textile in India, 113–118 9. Menzol (1992), ‘Coated fabric structure for air bag applications’, US Patent No. 5,110,666. 10. US Patent No. 5,110,666, May 1992, ‘Coated fabric structure for air bag applications’. 11. jit.sagepub.com/cgi/reprint/37/1/5/fabric 12. www.sefar.us/cms/medien.nsf/img 13. www.usfabricsinc.com 14. Amit S (2007), ‘Design and development of a textile patch antenna using slow wave propagation technique’, M.Tech. Thesis, IIT, Delhi. 15. Marculescu D et al. (December 2003), ‘Electronic textile: a platform for pervasive computing’, Proceeding for the IEEE, 91, 12. 16. Stone R (2003), ‘Electronic textile charge ahead’, Science 301, 15 August. 17. Edmison J, Jones M, Martin T and Nakad J (2002) ‘Using pizzoelectric materials for wearable electronic textiles’, Proceedings of the sixth international symposium on wearable computers, Oct 2002. 18. Laxminarayana K, Ramaratnam A, Rajoria H, Cherian V (2003), Electrical and Computer Eng.: Lonkar, K. and Zhang, X., Nader Jalili, ‘Functional Fabric with Embedded Nanotube Actuators/Sensors,’ National Textile Center Annual Report. 19. Bar-Cohen Y (2001), ‘Electroactive polymers as artificial muscles – reality and challenges’, AIAA Review, (JPL), California Institute of Technology. 20. De Paoli M A and Gazoti W A (2002), ‘Electrochemistry, polymers and optoelectronic devices: a combination with a future’, J. Braz. Chem. Soc., 13(4), 410–424. 21. Rossi D de (2007), ‘Electronic textiles: A logical step’, Nature Materials, 6, 328–329. 22. Mattila H R (2006), Intelligent Textiles and Clothing, Woodhead Publishing, 399–419. 23. Graham-Rowe D (2007), New Scientist, of 07 April 2007, Magazine issue 2598. 24. Chaudhari S S, Chitnis R S and Ramkrishnan R, Waterproof Breathable Active Sports Wear Fabrics, The Synthetic and Art Silk Mills Research Association, Mumbai.
Application of woven fabrics
Abstract: This chapter deals with the application of modeling in the area of technical textiles. The chapter reviews such examples as sportswear, medical applications, textiles for electronics and airbag construction in automotive engineering. Key words: textile modeling, technical textiles.
23.1
Introduction
The formal structure of a woven fabric is defined by weave, thread density, crimp and yarn count. The design of a fabric to meet the requirements of a particular end use is a complicated engineering problem. Theoretically, it is possible to design a fabric structure to achieve any desired characteristic, but in actual practice it is not so easy because of inherent nonlinearity and complex relationship between structure and properties of the textile materials along with their visco-elastic behavior. The factors associated with fabric design include fiber type, yarn geometry, fabric structure and finishing method. The use of different weaves alters the ability of the component threads to move relative to one another, and as a result mechanical properties such as shear characteristics and drapeability of fabric change significantly. The strength of the woven fabric is highest in the warp and weft direction, while in bias, the fabrics show lower mechanical properties, higher elasticity and lower shear resistance. The utility performance properties of speciality woven fabrics depend on the combined effect of the properties of the constituent fibers, yarn and the fabric structure. Thus, designing such a fabric consists of identifying the best combination of those variables that renders the fabric, able to meet the performance requirements. In order to enhance mechanical properties, a triaxial woven structure that consists of three systems of threads (one system for weft and two systems for warp) can be constructed. Warp threads in a basic triaxial fabric are interlaced at 60° and the structure is fairly open with a diamond-shaped center. A modification of basic triaxial fabric is basket weave that forms a closer structure with different characteristics. Woven fabrics have broad application in all segments of technical textile production. Conventional and triaxial fabrics fall into the group of 2-D fabrics. The application of woven fabrics in production of technical textile (as reinforcement in composite 413
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Woven textile structure
production), has inflicted the need for production of 3-D fabrics, which have high mechanical properties in x, y and z directions [1]. The uniqueness and challenge of technical textiles lie in the need to understand and apply the principles of textile science and engineering to provide right solutions to the growing and widely varying demands of their applications in areas such as protective clothing, sportswear, automotive textiles, geotextiles, agricultural textiles, medical textiles and textiles for building and construction and specialty textiles for defence and military applications. Thus it is very important to have an in-depth knowledge of the principles involved in the selection of raw materials and their conversion to desired yarns and fabric structures followed by various treatments such as coating and finishing of fabrics to introduce special technical and commercial features of a wide range of specific areas of applications. High performance technical textiles should not only possess general fabric characteristics but should also satisfy the critical performance requisites. Such fabrics should be engineered with utmost precision for its performance requirements.
23.2
Fundamental aspects of woven textile structure and function
23.2.1 Configurational functions of fiber The reason why textiles are selectively used for a certain specified end use instead of the other kinds of materials is that the textile products in which some of the configurational functions of fiber are effectively utilized can become most valuable among several forms of materials such as powder and solid sheet. The configurational functions of fiber consist of the following four elements: ∑ ∑ ∑ ∑
flexibility (pliable); high ability in axial transmission of such properties as mechanical; high specific surface area; technological easiness in transformability into textile structure; materials such as woven and nonwovens.
23.2.2 Structural system of fiber assembly Fiber assembly structural systems can be summarized based on constitutional element, orientation of element, bonding manner of element and macroscopic form. ∑ ∑ ∑
uniaxial bundle of membrane hollow fiber; nonwovens; glass matt thermoplastic sheet called stampable sheet;
Application of woven fabrics
∑ ∑ ∑
415
woven fabrics; knitted fabrics; braids.
23.2.3 Function system of fiber assembly structures The functions of fiber assembly structures for technical textiles are classified as mechanical bearing, shielding, removal or separation, medium transportation, axial conduction and reinforcement.
23.2.4 The maximum fiber volume fraction There is a geometrical upper limit in the fiber volume fraction which is dependent on fiber orientation and aspect ratio. It must be considered as an inevitable restrictive condition for designing assembly structure.
23.2.5 Pore size and its distribution within fiber assembly structures Pore size is an important parameter for considering such functional properties as shielding, medium transportation and removal or separation. Total fiber lengths by unit volume, inter-fiber coherence are main factors to determine pore size and its distribution.
23.2.6 Directive fiber assembly structures for required functions Directive fiber assembly structures can be roughly assigned to meet required functions using following consideration. ∑ ∑ ∑ ∑ ∑ ∑
For the mechanical functions of tensile loading, impact loading, tear resistance, ballistic resistance, oriented filament is used as constitutional element. For shielding, fiber or yarn is used as constitutional element formed into sheet. For removal or separation, there are typically two kinds of fiber assembly forms; randomly oriented fibers in sheet and uniaxially oriented bundle of membrane hollow fiber. For medium transportation the constitutional element is usually fiber, to obtain a high contact area of assembling struct with medium. For axial conduction, fiber or filament is usually uniaxially oriented. For reinforcement, there are typically two types: fiber randomly distributed within the matrix and yarn unidirectionally oriented within the matrix.
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These kinds of analysis can provide a directional systematic knowledge for designing optimum assembly structure for developing different textile products suitable for various end-use applications.
23.3
Medical textiles
Medical textiles are the products and constructions used for medical and biological applications for clinical and hygienic purposes, scaffolds for tissue culturing and a large variety of prostheses for permanent body implants. They consist of all those textile materials used in health and hygienic applications in both consumer and medical markets. A broad classification of medical textiles can be as under: ∑ ∑ ∑ ∑ ∑
Protective and healthcare textiles – surgical wear, operation dresses, staff uniforms, etc. External devices – wound dressings, bandages, pressure gauze, prosthetic aids, etc. Implantable materials – sutures, vascular grafts and artificial limbs are the products where textiles are used. Hygiene products – incontinence pads, nappies, tampons, sanitary towels, etc. Extracorporeal devices – artificial liver, artificial kidneys and artificial lung are the recent advances in medical textiles.
23.3.1 Woven medical textiles Woven medical textiles are typically used for products requiring extreme stability and; high durability over a significant number of loading cycles; or to precisely control porosity for air or fluid flow. For example, a good quality surgical gown must be made of light and comfortable, breathable fabric material, yet be tough and durable enough to withstand abrasion, ripping and puncture. Surgical gowns must act as a barrier between the sources of infection (micro-organisms such as bacteria, and viruses of different size and geometry) and the user (healthy person), and must also demonstrate good wearing comfort. The latter is important for the surgeon who often has to wear the surgical gown for several hours while doing hard work. Hydrophobic wovens from polyester are used for shorter surgical operations with a small amount of liquid, and have so far been the only reusable surgical gowns which are currently able to fulfill both these contrary demands of barrier effect and wearing comfort [2]. The barrier function of such fabrics depends on the surface structure, and also on the number and size of the continuous pores running through the fabric; the pores which run both between the filaments in the filament yarn
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and also between the filament yarns. When the ‘barrier’ is the feature to be evaluated, there are three measurable characteristics: ∑ ∑ ∑
resistance to dry microbial penetration; resistance to wet microbial penetration; resistance to liquid penetration.
When the mechanical resistance is the feature to be evaluated, there are four measurable characteristics: ∑ ∑ ∑ ∑
dry bursting strength (combination of air movement and mechanical action); wet bursting strength (combination of wetness, pressure and rubbing); dry tensile strength; wet tensile strength.
When the freedom from contamination or unwanted foreign matter is the feature to be evaluated, there are two measurable characteristics: ∑ ∑
microbial (not viable micro-organisms); particulate matter (freedom from particles not generated by mechanical impact).
When the release of fiber fragments or other particles, originally from the fabric itself, is the feature to be evaluated, there is one very important measurable characteristic: linting. The only obvious characteristic that is exclusively requested for the surgical drapes (but not for the surgical gowns) is adhesion for fixation for the purpose of wound isolation.
23.4
Automotive textiles
Automotive textiles are finding extensive use in the product categories of interior trims, safety devices such as seatbelts and airbags, carpets, filters, battery separators, hood liners, hoses and belt reinforcement. Textiles, which constitute approximately 20–25 kg in a car, are not only used for enhanced aesthetics of automotives but also for sensory comfort and safety. Additionally, few textile products found their application as design solutions to engineering problems in the form of composites, tyre reinforcement, sound insulation and vibration control. Apart from woven and knitted constructions, nonwovens also find applications in automotive textiles due to certain advantages served by them. The fabrics used worldwide as surface material for car interior can be woven, knitted or nonwovens. Woven fabrics represent the dominant application areas in seat covers, headrests and door panels. Fabrics are characterized by a large variety in design, stable shape retention and high mechanical resistance.
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Woven textile structure
23.4.1 3-D woven fabrics for automotive textiles 3-D sandwich woven fabrics are made of 100% E-glass yarns, as well as carbon fiber, basalt fiber or other high performance fibers. Two deck layers are bonded together by Z-piles to the specified height of 2–25 mm. When the fabric is impregnated with a thermoset resin, it immediately absorbs the resin and, due to the capillary forces of the Z-piles, the fabric rises to the preset height. Because the composite material is a kind of hollow integrated core sandwich structure that offers excellent mechanical properties and design versatility. The composite material is widely used for many applications in the core composites industry, such as ships, railways, automotive, aviation, wind energy, double wall tanks and construction, while offering multiple advantages (delamination, impact, etc.) against traditional sandwiches such as honeycombs, foams, balsa and more.
23.4.2 Airbags Airbags are a type of automobile safety restraint like seatbelts. They are gas-inflated cushions built into the steering wheel, dashboard, door, roof or seat of the car that use a crash sensor to trigger a rapid expansion to protect a person from the impact of an accident. The working of airbag is a precision application. Airbags should begin to inflate 0.03 seconds after a crash and should be fully inflated after 0.06 seconds. Airbags may be built into steering or in some other strategic location. Nitrogen gas is commonly used in airbags. Fabrics used for airbags must be able to withstand the force of hot gases and they must not penetrate through the fabric. Airbags are typically woven from high tenacity (HT) multifilament nylon 6,6. Nowadays, a one-piece weaving system produces an airbag directly on loom, but formerly two pieces were sewn together with suitable threads. Typically, front airbags are uncoated. The size of airbags may vary according to their position. The concept of the airbag, a soft pillow to land against in a crash, has been around for many years [3]. Airbag working Airbags inflate, or deploy quickly, faster than the blink of an eye. Imagine taking one second and splitting it into one thousand parts. In the first 15 to 20 milliseconds, airbag sensors detect the crash and then send an electrical signal to fire the airbags. Typically a squib, which is a small explosive device, ignites a propellant, usually sodium azide (NaN3). The azide reacts with potassium nitrate (KNO3) and burns with tremendous speed, generating nitrogen, which inflates the airbags. Within 45 to 55 milliseconds the airbag
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is supposed to be fully inflated. Within 75 to 80 milliseconds, the airbag is deflated and the event is over [4]. When airbags work properly, they dramatically reduce the chance of death or serious injury. However, the speed with which airbags inflate generates tremendous forces. Passengers in the way of an improperly designed airbag can be killed or significantly injured. Unnecessary injuries also occur when airbags inflate in relatively minor crashes when they are not needed. Airbag inflation The goal of an airbag is to slow the passenger’s forward motion as evenly as possible in a fraction of a second. There are three parts to an airbag that help to accomplish this feat: ∑ ∑
∑
The bag itself is made of a thin nylon fabric, which is folded into the steering wheel or dashboard or, more recently, the seat or door. The sensor is the device that tells the bag to inflate. Inflation happens when there is a collision force equal to running into a brick wall at 10 to 15 miles per hour (16 to 24 km per hour). A mechanical switch is flipped when there is a mass shift that closes an electrical contact, telling the sensors that a crash has occurred. The sensors receive information from an accelerometer built into a microchip. The inflation system which inflates the airbag through the production of nitrogen.
Airbag manufacture Airbag fabrics are made of nylon 6,6 multifilament yarns with counts from 235 to 940 tex. Airbag fabrics are generally dense which presents a challenging task to weave. Tensile strength, elongation, tear proportion resistance and weight requirement of airbag fabric are critical [5]. Air permeability of airbag fabric should be uniform across the whole width of the fabric. There are currently two principal material types which are used in the manufacture of airbags. They are uncoated nylon (polyamide 66) and coated nylon. Two types of commonly used coatings are silicone and neoprene. In general, coated materials are used for driver’s side airbags and side impact bags, while passenger side airbags are made from uncoated nylon materials [6,7]. All airbag cushions manufactured worldwide are constructed with fabric made from nylon 6,6 yarn. The key initial drivers were performance, cost and benefit. As one can imagine, the key function of the airbag cushion is to absorb the impact. Nylon 6,6 has superior capability in energy absorption. The balance between the strength and elongation gives it unmatched suitability for airbag cushion materials. The performance attributes that have
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led to industry standardization are its breaking strength or tenacity; energy absorption capability or toughness; heat resistance; and stability over time as measured by accelerated aging tests [6]. More recently, a third selection criterion has started to come to the fore, that of proven performance or confidence. Since the airbag system forms part of the automotive supply chain there is continuous and necessary consideration of other materials such as polyester, for use in the airbag cushion. It is possible to demonstrate that all these alternatives, including polyester are significantly inferior to nylon 6,6 when measured against the key performance attributes listed above and that their use would pose a risk of failure in airbag applications. That lower performances of polyester coupled with increasing legislative and litigation activity around automotive safety and the lack of a track record in the global market for polyester have led to the conclusion that polyester is not appropriate for airbag cushion use. For comparison purpose, the key physical properties of nylon 6,6 and polyester are shown in Table 23.1. The key differences between the two polymers are density and specific heat capacity. Comparison of key properties of nylon 6.6 and polyester Although nylon 6,6 and polyester have similar melting points, the large difference in the specific heat capacity causes the amount of energy required to melt polyester to be about 30% less than that required to melt nylon 6,6. Hence in any inflation event that uses a pyrotechnic or pyrotechnic-containing inflator, cushions made from polyester yarn are far more susceptible to burn or melt through in the body of the cushion or at the seam. The second advantage of nylon 6,6 is its lower density. Lower mass has key advantages; reducing the mass of the cushion lowers the kinetic energy of impact on the occupant in out-of-position situations thus enhancing safety, while allowing the overall weight of the vehicle to be reduced. The difference in density between the two polymers leads to polyester yarns usually being of higher denier or decitex (weight per unit length) than nylon 6,6, to generate the same filament diameter. These results in reduced
Table 23.1 Key physical properties of nylon 6,6 and polyester 3
Density (kg/m ) Specific heat capacity (kJ/kg/K) Melting point (°C) Softening point (°C) Energy to melt (kJ/kg)
Nylon 6,6
Polyester
1140 1.67 260 220 589
1390 1.3 258 220 427
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fabric coverage. Using polyester yarn, the cushion fabric is more open for gas permeation. This reduces thermal protection for the vehicle occupants, and makes it more difficult for the cushion designer to control the bag deployment dynamics. In addition, since seam strength is strongly dependent on cover factor, seam performance is negatively impacted. This is particularly important since seam leakage of hot gas is one of the principal concerns of the engineer and the potential for an increase in leakage combined with a reduction in thermal resistance is critical [8,9]. Coated base fabric for airbags A coated airbag base fabric made of a textile fabric that has an excellent air barrier property, high heat resistance, improved mountability, and compactness and excellent adhesion to a resin film is characterized in that at least one side of the textile fabric is coated with resin, at least part of the single yarns of the fabric are surrounded by the resin, and at least part of the single yarns of the fabric are not surrounded by the resin. An airbag is characterized by using such a coated airbag base fabric [10]. A method for manufacturing the coated airbag base fabric is characterized by applying a resin solution having a viscosity of 5–20 Pa s (5000–20 000 cP) to the textile fabric using a knife coater with a sharp-edged coating knife at the contact pressure between the coating knife and the fabric of 1–15 N/cm [11]. Laminated material for airbag A polymer film, preferably polyamide polymer film, which comprises at least one first layer and a second layer, is laminated onto a fabric. The material of the first layer has a glass transition temperature of less than 10 oC, while the material of the second layer has a glass transition temperature of less than 20 oC. Preferably, the polymeric materials contain portions of polyamide blocks. The fabric–polymer film laminate is suitable as a laminated material especially for an airbag [11].
23.5
Filter fabrics
The surface quality of a woven fabric has become an increasingly important factor which influences the property of the filter in many different ways. Surface treatments modify the functionality of the woven fabric according to the different requirements and thereby significantly increase the effectiveness of the resulting filter in a specific environment [12]. The following surface treatment technologies improve the surface property:
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∑ ∑ ∑
Woven textile structure
wet chemical surface modification (mainly hydrophilic); low temperature plasma treatment (mainly activation, fictionalization and plasma polymer deposition); metal coating (aluminum, copper and nickel).
The selection of the appropriate fabric type and coating depends on the functionality needed for the test. The phenomenon of wetting or non-wetting of a solid by a liquid is better understood by studying contact angle of a liquid drop on a condensed surface as demonstrated in Fig. 23.1. The lower the contact angle between liquid drop and fabric surface, the better the hydrophilic action of the fabric. Woven filters are used in many acoustic devices, loudspeakers and microphones. Filters not only improve the speech and sound quality in mobile phones by adsorbing unwanted frequency peaks; they also protect the sensitive electronic equipment from moisture, dust and dirt. Hydrophobic surface treatments offer an excellent protection against moisture uptake and contamination. Acoustic fabrics are hydrophobically treated to provide optimal protection against moisture uptake and to repel dust and dirt. Hydrophobic woven fabrics protect loudspeakers and microphones from moisture, dust and dirt. In automobiles, a series of filter systems are used to protect sensitive components. High precision woven fabrics are mounted in the fuel, injection and hydraulic filter systems. Since the fuel can be contaminated with particles and dirt originating from refining, transport and storage, from the filling process or from simply the automobile tank itself, a series of security filters are installed along the fuel transport path. These filters, from the tank to the injector system, protect the mechanical parts from damage and prevent the injection nozzles from blocking. In addition, high water content is usually found in fuels, especially in diesel, which has to be removed as it may lead to corrosion. In order to enhance water removal, the fabrics used in fuel systems should be hydrophobic. Fabrics remove water from kerosene and other fuels. An airplane carries thousands of liters of fuel which must be free of particles and water to guarantee trouble-free operation of the jet engines. Jet fuel contains a certain amount of dissolved particles (approx. 60 parts per
23.1 High-contact angle, hydrophobic fabric and low-contact angle, hydrophilic fabric.
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million [ppm], ca. 60 micrograms per kilogram of kerosene); free water must be removed to prevent freezing and blocking of the fuel transport system. Coalescer systems are used in the removal of water from jet fuel and others (e.g. diesel). Water droplets combine at the coalescer surface (filter medium) to form drops which by flow and or gravity move to a separator. The fuel then flows through the separator while the water drops are retained by the hydrophobic filter [13]. The surface of the separator consists of a hydrophobic woven filter medium supported by a stainless steel element.
23.6
Textiles for electronics
E-textile technology holds out the promise of truly wearable computers as well as inexpensive large-scale computational devices. To achieve these goals, e-textiles combine high volume, low cost textile manufacturing capability with discrete electronics and novel fiber technologies. Industrial weaving machines capable of inserting thousands of meters per minute of weft yarn can efficiently produce large volumes of complex woven textiles while individually controlling the position of every fiber in the design. New fibers are being created for inclusion in e-textiles, including battery fibers, conductive fibers and mechanically active fibers. Methods are being developed for attaching discrete components to e-textiles, including processors, microphones and speakers. Two broad categories of e-textile applications are envisioned, wearable and large-scale non-wearable [14]. Many specific applications in the field of wearable computing have been envisioned and realized, though most suffer bulky form factors. In the new field of large-scale non-wearable e-textiles, applications include large-scale acoustic beam-forming arrays (STRETCH), self-steering parafoils (Draper) and intelligent, inflatable decoys (DARPA). Both categories of e-textile applications share three common design goals: low cost, durability and long running. ∑
∑
∑
Low cost dictates the use of inexpensive, commercial, off-the-shelf (COTS) electronic components and yarns as well as the design of weaves and architectures that are manufacturable in current or slightly modified textile production systems. Durability dictates that the system must tolerate faults, both permanent and transient, that are inherent in the manufacture and use of the device. In addition, there is an expectation that individual components may not be repairable and that system functionality should gracefully decline as components fail. Long running dictates that the system must manage power consumption in an application-aware fashion to minimize the need for bulky batteries and/or external power recharge. Power scavenging and distributed power management are essential.
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Woven textile structure
In the category of wearable computing, a garment that provides the user with precise location information within a building is analyzed. A large-scale, non-wearable acoustic beam-forming array is analyzed. Textiles are woven as opposed to alternative textile manufacturing techniques such as knitting, embroidering, or non-woven technologies. Woven textiles allow for stable fabrics with a high degree of precision, but impose directional limitations on the fabric, which have implications for communication within the textile. Virtually all woven e-textiles are expected to use the new fiber batteries and solar cells under development. This will lead to highly distributed, redundant power supplies for e-textiles in which some parts of the textile have more remaining power than others [15].
23.6.1 Mapper garment The mapper garment tracks the motion of the user through a structure by monitoring the user’s body position, the user’s movement, and the distance of the user from surrounding obstacles. Such a garment would allow users to be given directions in a building, maintenance workers to be automatically shown blueprints for their current room, or users to automatically map existing structures. The user’s body position is measured by a set of piezoelectric strips woven into the clothing; by measuring the deformation along tens of strips, the physical configuration of the user’s body can be detected [16]. The user’s activity, such as walking up stairs, climbing a ladder and walking on a flat surface, can be detected. The user’s movement rate can be measured by a small set of discrete accelerometers as well as a digital compass attached to the garment. The distance from obstacles is measured using ultrasonic signals. An array of approximately ten ultrasonic transmitters, also piezoelectric strips, are distributed around the garment to periodically send signals in each direction; a similar number of receiver piezoelectric strips are used to detect the reflected signals, allowing time-of-flight to be measured. The primary challenge in this application is interfacing to a large number of sensors and actuators in a reliable fashion. Simply attaching the leads of every sensor or actuator to a single processing unit and power supply would not meet the design goal of a durable e-textile. In the event of a tear in the fabric, single leads running to one collection point could lead to significant rather than graceful degradation in performance; in addition, the potentially long leads required would cause degradation in analog signal quality unless significant amplification is applied, leading to larger power consumption. The garment needs multiple points at which analog data is converted to digital data; these conversion units, likely in the microcontroller or digital signal processor (DSP) class, would need to communicate within a fault tolerant network. The sample rate at each conversion point is very low (10–100 per second) for the body position sensors and moderate (100 000 per second)
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for the occasional ultrasonic reception. Once the data has been converted to a digital format, low power data transmission and coding techniques can be applied. By carefully managing the active sensors and processing units the power requirements of the system can be reduced. When determining the location of a wall, the garment must activate a transmitter in the direction desired and then sample a receiver for the return signal. This will accurately compute the distance, but gives no information on the direction in which it is located. To compute direction, the location of the transmitter on the body, the position of that part of the body relative to the torso, and the direction in which the user’s torso is pointing must be known. The location of the transmitter should be known from the manufacturing process and the digital compass can provide the torso direction. A number of techniques are available for determining the location of one part of the e-textile with respect to other parts of the e-textile; in the garment, the body position is available and acoustic beam forming could be used to determine the location of all of the ultrasonic transceiver with respect to one another. To accurately compute the body position requires the analysis of samples from a large set of sensors that have been collected at several processing elements. Due to its nature, the analysis is best done at a single processing element. To initialize the collection of the data, the processing element would send out a query to the selected elements, such as ‘give me your current reading’ or ‘give me your reading every 100 ms’. In both cases, some time stamp should be applied to both the query and the replies, though high accuracy is not required in this particular application [17].
23.6.2 Beam-forming array The beam-forming array textile gathers data from a large array of acoustic sensors and analyzes this data to determine the direction of an acoustic emitter (e.g., a moving vehicle or a human voice). Through the use of acoustic beam-forming algorithms, a set of three acoustic sensors can identify the direction of a single emitter if the sensors and the emitter are all in the same plane. Identifying the direction of multiple emitters or working in three dimensions requires data from more acoustic sensors. Further, given noise and potential miscalibrations in the acoustic data, the use of redundant acoustic sensors is advisable. If the fabric is large enough, then not only the direction, but also the location of an emitter can be found. Like the mapper garment, for reasons of robustness, the large number of sensors is not all handled by one conversion or processing node; the fabric is sprinkled with many communicating processing nodes. It is important to note that each acoustic sensor must know its location precisely with respect to the other sensors; small errors in sensor location result in increasingly larger errors as the distance to the emitter increases. Although this is not a wearable
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Woven textile structure
textile, it is flexible and thus subject to movement; at a minimum, the initial position of each acoustic sensor must be computed. In contrast to the mapper garment, the positions of each sensor must be known quite accurately. To accomplish this, the textile is augmented with speakers that are physically co-located with a subset of the acoustic sensors; by systematically activating the speakers, the distances between the microphones can be determined. Once enough distances are known, the relative positions of all of the microphones can be computed. The frequency, direction and distance of a potential target all affect the optimal selection of a subset of sensors on which to perform beam forming. Because beam forming is fairly computationally demanding, it would be wasteful of resources, including power, to collect and analyze the data from the entire set of acoustic sensors. An efficient strategy, therefore, is to have a small active set of sensors look for emitters while the rest of the sensors sleep to conserve power. Upon the tentative identification of an emitter’s characteristics along with an assessment of remaining power at processing nodes, an optimal set of sensors can be activated. Once a set of sensors is selected, the time series data from those sensors must be collected at a single processor where the beam-forming algorithm is to be run. If multiple beams are formed, then the direction and intensity data must be combined at a single node; this is much less demanding of communication resources than the exchange of time series data. It is expected that some nodes along potential routes may be asleep, others out of power, and some broken; routes must be found in spite of these drawbacks. In addition, time series data from different nodes must be synchronized; small errors in time can lead to large errors in the computed results [17].
23.6.3 Nanotube-based e-textile Conductive woven fabrics are made by utilization of metallic conducting wires which are flexible wearable electrical circuits; the properties of nanotube-based yarns could be leveraged to produce more robust systems far lighter and more flexible than metallic wires. The applications and methods of integration of conductive yarns into clothing have been dealt with the integration of conventional integrated circuits and the utilization of fabricbased components as part of wearable computing devices. As a step in the direction of creation of nanotube-based electronic components for fabric applications, super-capacitors woven into a textile fabric as shown in Fig. 23.2 has been reported. The capacitors were shown to have capacitances and life cycles comparable to commercially available super-capacitors. Since nanotubes are also known to have effects similar to piezoelectric materials (when exposed to strains), such materials could
Application of woven fabrics
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be integrated into structures to produce sensors that monitor the stresses in structures [18].
23.6.4 Organic field effect transistors (OFETs) By coating fibers with conducting polymer, they can join them with solid electrolyte at cross-points to form micrometer-sized organic electrochemical transistors (OECTs). They then use the OECTs as components for logic circuits, offering an alternative approach to organic field effect transistors (OFETs) commonly used in flexible electronics. The wire OECTs provide an easier route to weaving electronics directly into fabrics Fig. 23.3 [19]. Common OFETs operate like electrical valves, with the flow of current between the sources and drain electrodes controlled by the voltage of the gate electrode. Increasing the gate voltage injects charges into the conducting polymer next to the gate electrode, creating a conduction channel, through which the source-to-drain current flows. The magnitude of the voltage applied by the gate controls the size of this conduction channel. OFETs can be flexible, but printing them onto fibers is a complex process. Their successful operation depends on the accurate control of the electric field applied to the polymer by the gate electrode, which in turn requires a very thin and uniform insulating layer to be deposited between the gate and active polymer. Precise micropatterning of source drain and gate electrodes on a fiber is therefore needed. These requirements make the implementation of OFET-based knitted or woven structures very impractical. ∑
OFET on fibers; switching from the Off to the On state operates through charge accumulation in the polymer channel under the action of the electric field from the gate electrode, which increases the polymer conductivity in the conduction channel.
23.2 Woven textile super-capacitor.
428
Woven textile structure Source
Drain
Gate
Drain
Gate Insulator
Off
Off
Source
Drain
Drain
Gate
Gate
On (a)
Insulator
(b)
On
Source
Conductor Semiconductor in the low-conduction state Semiconductor in the high-conduction state Ion reservoir
23.3 Working principles of (a) organic field effect transistors (OFET) and (b) wire electrochemical transistors (WECT).
∑
∑
Wire electrochemical transistor (WECT) switching from the On to the Off state occurs when ions are depleted from the polymer semiconductor channel by electrodiffusion through the solid electrolyte under the action of the gate voltage. The WECT structure is symmetrical; the source–drain fiber and gate fiber can be interchanged, which adds flexibility to circuit design. In the OECTs the conducting channel is made by ions being injected from or removed to a solid electrolyte reservoir, which is in turn controlled by the gate electrode. This means very large conductivity changes can be easily induced, and these conductivity changes control the sourceto-drain current in the same way as the injected charges from the gate electrode in the OFET. Because of the differences in the way the devices work, the need for accurate dimensions and precise positioning of the transistor subparts is greatly relaxed in OECTs compared with OFETs. OECTs also benefit from needing very low operating voltages of the order of a couple of volts [20].
Wires are coated with conducting polymer to create cylindrical electrodes. At junctions where the wires cross each other, a drop of solid electrolyte is used both as a physical joining method and as the ion reservoir to create a transistor (WECTs). If a polymer with lower conductivity is used to
Application of woven fabrics
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join the wires instead, a resistor is made. The logic circuits can be made using arrays of wires crossed and connected in these ways as shown in Fig. 23.4 [21]. ∑ ∑
Classical circuit diagram of an inverter, consisting of a transistor and three resistors. Illustration of how this circuit can be realized using WECT technology.
Despite all the positive features outlined, there is a major intrinsic constraint on the performance of OECTs that is highly relevant to their implementation on fibers. The rate-limiting process of OECT operation is electrodiffusion of ions within the solid electrolyte; ionic charge carriers have very low mobility, which does not allow speeds of operation comparable to either inorganic or conventional organic transistors. The long response time and the consequent low switching frequency of WECT-based logic circuits will certainly limit the dynamic characteristics of any application. Dimensional scaling to speed up operation is an option because the time characteristics of electrodiffusion
+Vdd
–Vdd
R1 Vout
Vin
R2
Vin
R3
Vout
–Vdd
Vout
Vin +Vdd (a)
(b) Coated fiber
Wect
Resistor Insulating
Low resistance High resistance
23.4 Logic circuits constructed from WECTs (a) classical circuit diagram of an inverter, consisting of a transistor and three resistors (b) illustration of how the circuit in (a) can be realized using WECT technology.
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Woven textile structure
processes scale with the inverse square of characteristic lengths. However, other rate phenomena might be dominant, limiting operations to very low frequencies and hence confining WECT technology to quasi-static applications. This aspect needs investigation to assess the full potential of WECT woven logic in the field of e-textiles and wearable technology. Body signals with slow dynamics, such as sweat rate and composition changes, body surface temperature mapping and surface-strain field mapping for posture recognition and respiration monitoring, are therefore likely to be ideal targets in the area of wearable monitoring systems for health, sport and ergonomics using this technology [22]. Chemo-, piezo- and thermo-resistive fibers are now available in textilecompatible form, and their integration into fabrics and garments is being actively pursued [17,18]. Using simple and elegant solutions, the inganas team [23] has interwoven WECTs with passive electronic components on fibers for local signal acquisition and early conditioning in e-textiles, paving the way for a revolution in existing wearable technology [21].
23.7
Sports textiles
Sport textiles are textiles used in sports. These are sports goods and sportswear. Sportswear is clothing, including footwear, worn for sport or exercise. Typical sport-specific garments include short pants, tracksuits, T-shirts, polo shirts and trainers. Specialized garments include wet suits and salopettes. It also includes some underwear, such as the jockstrap. Sportswear is also often worn as casual fashion clothing. For most sports the athletes wear a combination of different items of clothing, e.g. sports shoes, pants and shirts. Some athletes wear personal armour such as helmets or American footbal body armour.
23.7.1 Functional considerations Almost every piece of sports clothing is designed to be lightweight so the athlete is not encumbered by its weight. The best athletic wear for some forms of exercise, for example cycling, should not create drag or be too bulky. On the other hand it should be loose enough so as not to restrict movement. Clothing worn for some other forms of exercise should not unduly restrict movement and may also have specific requirements, for example the Keikogi used in karate. It should allow freedom of movement in competition. Various physically dangerous sports require protective gear, e.g. for fencing, American football or ice-hockey. Standardized sportswear may also have the function of a uniform. In team sports the opposing sides are usually made identifiable by the colours of their clothing, while individual team members can be made recognizable by a back number on a shirt.
Application of woven fabrics
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Garments should allow the wearer to stay cool in hot weather and warm in cold weather. In cold climates the best athletic wear should not only provide warmth but also transfer sweat away from the skin. For activities such as skiing and mountain climbing this is achieved by using layering; moisture transferring materials must be worn next to the skin, followed by an insulating layer, and wind and water resistant shell garments.
23.7.2 Moisture-transferring fabric Waterproof, breathable fabrics are designed for use in garments that provide protection from the environmental factors such as wind, rain and loss of body heat. Waterproof fabric prevents the penetration and absorption of liquid water. The term ‘breathable’ implies that the fabric is actively ventilated. Breathable fabrics passively allow water vapour to diffuse through them yet prevent the penetration of liquid water. High functional fabrics support active sportswear with importance placed on functions as well as comfort. Finally, materials with heating and or cooling property have newly attracted the interest of the market. All these materials do not pursue a single function, but different functional properties combined on a higher level. Fabrics that can convey water vapor from body perspiration out through the material while remaining impervious to external liquids such as rainwater are widely used in sportswear and similar applications. Water-resistant and moisture-permeable materials may be divided into three main categories, high density fabrics, resin-coated materials and film-laminated materials. These are selected by the manufacturers according to the finished garment requirements in casual, athletics, ski or outdoor apparel.
23.7.3 Densely woven water breathable fabrics The densely woven waterproof breathable fabrics consist of cotton or synthetic micro-filament yarns with compacted weave structure. One of the famous waterproof breathable fabrics known as Ventile was manufactured by using long staple cotton with minimal space between the fibers. Usually combed yarns are woven parallel to each other with no pores for water to penetrate. Usually oxford weave is used. When the fabric surface is wetted by water the cotton fibers swell transversely reducing the size of pores in the fabric and require very high pressure to cause penetration. Therefore waterproof is provided without the application of any water repellent finishing treatment. Densely woven fabrics can also be produced from microdenier synthetic filament yarns. The individual filaments in these yarns are of less than 10 mm in diameter, so that fabrics with very small pores can be engineered [24].
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Woven textile structure
23.7.4 Laminated waterproof breathable fabrics Laminated waterproof breathable fabrics are made by application of membranes into the textile product. These are thin membrane made from polymeric materials. They offer high resistance to water penetration but allow water vapor at the same time. The maximum thickness of the membrane is 10 mm. They are microporous and hydrophilic membranes. Microporous membranes have tiny holes on their surface, smaller than a raindrop but larger than water vapour molecule. Some of the membranes are made from polytetrafluoroethylene (PTFE) polymer, polyvinylidene fluoride (PVDF) [3,4]. The hydrophilic membranes are thin films of chemically modified polyester or polyurethane. These polymers are modified by the incorporation of polyethylene oxide [6], which constitutes the hydrophilic part of the membrane by forming amorphous regions in the main polymer system. This amorphous region acts as intermolecular pores allowing water vapour molecules to pass through but preventing the penetration of liquid water due to the solid nature of the membrane [24].
23.7.5 Coated waterproof breathable fabrics Coated fabrics with waterproof breathable fabrics consist of polymeric material applied to one surface of fabric [6,7]. Polyurethane is used as the coating material. The coatings are microporous and hydrophilic membranes. In microporous membrane the coating contains very fine interconnected channels much smaller than the finest raindrop but larger than water vapor molecules. Hydrophilic coatings are similar to hydrophilic membranes; the microporous material allows water vapour through the permanent airpermeable structure whereas the hydrophilic material transmits vapour through adsorption-diffusion and de-sorption mechanism. The desirable attributes of functional sportswear and leisurewear are: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑
optimum heat and moisture regulation; good air and water vapour permeability; rapid moisture absorption and conveyance capacity; absence of dampness; rapid drying to prevent catching cold; low water absorption of the layer of clothing just positioned to the skin; dimensionally stable even when wet; durable; easy care; lightweight; soft and pleasant touch.
Application of woven fabrics
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It is not possible to achieve all of these properties in a simple structure of any single fiber or their blend. The two layer structure has a layer close to skin of the wicking type from synthetic fibers, e.g. microdenier polyester, and the outer layer is usually cotton or rayon that absorbs and evaporates. Microdenier polyester is ideal for wicking perspiration away from the skin. The use of superfine or microfiber yarn enables production of dense fabrics leading to capillary action that gives the best wicking properties. No single fiber or blend of different fibers can give ideal sportswear. The right type of fiber should be in the right place. Blending of fibers does not give the same effect as that of multi-layer fabric. The wicking behavior of the fabric is mainly dependent on its base fiber’s moisture properties [24].
23.7.6 Moisture transport mechanism The mechanism by which moisture is transported in textiles is similar to the wicking of a liquid in capillaries. Capillary action is determined by diameter and surface energy of its inside face. The smaller diameter or greater surface energy gives greater tendency of a liquid to move up the capillary. In textile structures, the spaces between the fibers effectively form capillaries. Hence, narrow spaces between the fibers enable the textile to wick moisture. Fabric constructions, forming narrow capillaries pick up moisture easily. Such constructions include fabrics made from microfibers, which are packed closely together. However, capillary action ceases when all parts of a garment are equally wet. The surface energy in a textile structure is determined largely by the chemical structure of the exposed surface of the fiber. ∑ ∑
Hydrophilic fibers have a high surface energy. Consequently, they pick up moisture more readily than hydrophobic fibers. Hydrophobic fibers, by contrast, have low surface energy and repel moisture.
Special finishing processes can be used to increase the difference in surface energy between the face of a fabric and the back of the fabric to enhance its ability to wick [24].
23.7.7 Factors affecting moisture transport There are several factors which affect moisture transport in a fabric. The most important are: fiber type, cloth construction or weave, weight or thickness of the material and presence of chemical treatments. Synthetic fibers can have hydrophilic (wetting) or hydrophobic (nonwetting) surfaces. They also have a range of bulk absorbencies, usually reported by suppliers and testing
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Woven textile structure
organizations as the percentage moisture regain [1] by weight. Synthetic fabrics are generally considered to be the best choice for garments worn as a base layer. This is due to the ability to provide a good combination of moisture management, softness and insulation. While most fabrics, both natural and synthetic, have the ability to wick moisture away from the skin, not all of these are fast-drying and air permeable; two factors which have a direct influence on cooling and perceived comfort. High-tech synthetic fabrics are lightweight, are capable of transporting moisture efficiently, and dry relatively quickly. It is generally agreed that fabrics with moisture wicking properties can regulate body temperature, improve muscle performance and delay exhaustion. While natural fibers such as cotton may be suitable for clothing worn for low levels of activity, synthetic fabrics made of nylon or polyester are better suited for high levels of activity. They absorb much less water than cotton, but can still wick moisture rapidly through the fabric. The main parameters for comfort and functionality are: ∑ ∑ ∑ ∑ ∑
water- and windproof, breathability and comfort; moisture and sweat management; warmth and temperature control; easy-care performance; smart and functional design [24].
23.7.8 Characteristics of sports clothing Sports clothing should have following characteristics: ∑ ∑
∑
Protective properties against variable atmospheric conditions existing during the clothes use as well as protection against physical damage. A high resistance to external influences, including tear strength, resistance to abrasion, shape stability, colour fastness, making-up quality, constancy of protective functions, and other features contributing to the service life of such materials. Comfort-providing properties, generally described as wellness, including first of all physiological comfort. This includes protection against overwarming or cooling, owing to high water vapour permeability, i.e. carrying off perspiration, good warmth retention and adequate air permeability. Moreover, the person wearing the garment will be positively affected by soft handle and good shape assumption by the fabric and cloth cut that does not limit their ease of movement, as well as the cloth’s aesthetic appeal and practical constancy of protective and aesthetic functions throughout the period of use.
Application of woven fabrics
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435
References
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