All Weather Clothing

Transcription

1 All Weather Clothing By Dr. Sanjay Gupta, Professor at The National Institute of Fashion Technology, Hauz Khas, New Delhi India. Intelligent developments are undoubtedly going to feature strongly in the textile & fashion industries over the next decade and even become a part of our everyday life. The so-called intelligence arises from the incorporation of particular components into the fabric, which may be electronic devices, specially constructed polymers or even some type of colorant. Many intelligent textiles are designed to respond to adverse conditions in their environment and there by provide enhanced protection. They can also either alter their nature in response to external factors or confer additional benefits to their users. There has been extensive innovation, for example, in clothing fabrics that can provide extra insulation in hot as well as cold conditions. Such textiles are becoming increasingly important in the fashion industry. The first question, which may be asked, is what exactly is all weather clothing? A more technical term will be temperature-regulated or thermo-regulated clothing. To consider thermo regulation only as staying warm would be incorrect, or only half correct. The term is broader than that and actually means maintaining the body temperature to the level that will maximize performance and comfort, while also protecting the user. Temperature regulation or thermo-regulation is best defined by its goal, which is to maintain both the core body temperature and the comfort of the wearer in diverse environments. The body itself regulates its temperature through a group of biological processes. Even at rest the human body is a mass of ongoing chemical reactions that regulates body heat within an optimum temperature range called the thermo-neutral zone (usually set at 37+1oC). When the body temperature extends beyond the limits of the thermo-neutral zone, bodily systems operate less efficiently, and when pushed to extremes can even result in death. Therefore, thermo-regulation is critical both from a safety and performance standpoint. Human Body regulates its own temperature. Typically, fabrics do not inherently provide thermo-regulation. Their thermo-regulation is affected by not inhibiting or rather supporting the thermo-regulation efforts of the body itself. The role of the fabric will be to allow air to circulate around the body and at the same time provide a cushion of insulation (either hot or cold) when the body needs it. The fabric must be able to adjust to the needs of the body over a wide range of external temperatures and activities. Certain combinations of fabric construction, chemical finishes and garment construction can also keep the body warmer or cooler, depending on the environmental conditions. Usually fabrics are geared for one or the other. Cold weather garments must address both radiant and convective heat loss. On the other hand, warm-weather garments must aid evaporative heat loss by increasing moisture movement, and increasing the velocity of heat conduction through the 1

2 material. Control of airspace in the microclimate between the skin and the garment, or between layers is of prime importance. There are three main characteristics that have been observed in materials that provide thermo-regulation. First is breathability/moisture management. The absorption and retention of water must be as close to zero as possible and there should be a mechanism to ensure that the moisture is moved away from the skin. The second characteristic is insulation. The fabric must have a good insulation value to supplement the boundary layer or air gap on the surface of the skin. There can be a mechanism to vary the degree of insulation. Last of all, the fabric must be lightweight with good bulk to achieve maximum comfort. Some of these characteristics and materials displaying them are discussed below: BREATHABILITY/MOISTURE MANAGEMENT A highly efficient breathable fabric material enables the user to control body temperature and experience physical comfort by controlling heat loss from the system while at the same time removing excessive sweat. The overall effect is to create a more comfortable condition on the skin s surface. One of the first types of fabrics marketed to confer improved insulation was the range of breathable Gore-Tex fabrics. These fabrics are constructed by lamination of a waterproof bi-component membrane to a range of substrates, such as expanded poly-tetrafluoroethylene impregnated with an oleo phobic polymer. The membrane is highly porous, and the width of the pores, around 100 nm, is critical to the effectiveness of the fabric: the fabric can allow perspiration to escape but still confer protection from rain. Another range of materials providing enhanced insulation includes the Stomatex fabrics. In these fabrics an elevated temperature is maintained to prevent condensation of perspiration. Vapor trapped beneath the fabric is removed by the action of tiny pumps present in the material. Each pump consists essentially of a deformable chamber and an exit pore. During the use of the material, vapor is released from each chamber by virtue of the flexing of the fabric. With higher levels of physical activity on the part of the wearer, the pumping action is correspondingly increased. The performance of the material is thus controlled to match the rate at which the wearer is perspiring. Micro Thermal Systems of England developed its award-winning technology over five years and in 1999 it received a 2

3 Department of Trade and Industry Smart prize for the successful commercialization of its innovative technology. Neoprene is an oil/heat-resistant synthetic rubber. Current specifications of Stomatex described as breathable neoprene are made from closed-cell foam neoprene. These fabrics can be applied as laminates or loose linings according to users requirements. The thickness (thermal insulation) of the insulating component varies from two millimeters (mm) to six mm, with the most frequently used being between two and five mm. In dry condition, the thousands of tiny chambers and pores formed in the body of the Stomatex garment efficiently evacuate sweat as it evaporates from the skin s surface. Used in wet conditions in thermal vests, shorts, under dry suits and wet suits, Stomatex traps air in the chambers, which, acting as an excellent insulator, makes the garment warmer than a similar garment made from ordinary neoprene, which does not allow sweat to evaporate. Because Stomatex material has the ability to remove excess heat and sweat, the suit does not allow the inside temperature to rise in or out of the water. The cast of 'Harry Potter and the Philosopher's Stone' was grateful for the warmth provided on the set by tailor-made Stomatex thermal undergarments. Stomatex is on permanent exhibition at the British Science Museum as an example of an advanced biomimetic material. Multi-layer composite yarns and textiles have other physical possibilities for achieving wear comfort in terms of absorbing sweat released from the human skin surface by an internal sweat absorbent layer. Toyobo Co. Japan developed a cool and dry three layer composite yarn, which consists of a polyester filament yarn on the surface, a staple polyester yarn in the middle and a polyester filament yarn in the core. The finest components lying in the middle, i.e. fine fibers offer greater porosity, which increases capillary action, conveying the absorbed sweat to the yarn surface. The coarse polyester filament yarn in the yarn interior has a Y-shaped cross section in order to increase moisture absorption capacity. INSULATION The required thermal insulation of clothing systems primarily depends on the physical activity and on the surrounding conditions, such as temperature and relative humidity. The quantity of heat produced by humans depends to a large extent on the physical activity and can vary from 100W while resting to over 1000W during maximum physical performance. In cooler seasons, for example when ambient temperature is approx. 0ºC, thermally insulated clothing is recommended in order to ensure that the body is sufficiently warm when resting. If, however, the body is involved in a more intensive activity (as in case of winter sports), the body temperature increases with enhanced heat production. To keep this increase within limits, the body perspires in order to withdraw energy from the body by evaporative cooling. If the thermal insulation of the clothing is reduced during physical activity, a part of the produced heat can be removed by convection and the body will not be required to perspire so much. The quality of insulation in a garment will be extensively governed by the thickness and density of its component fabrics. While high thickness improves insulation, a garment made from a thick fabric will have greater weight 3

4 impairing the freedom of movement of the wearer. Thus low density is also important for improving insulation. In many practical examples, thermal insulation is provided by air gaps between the garment layers. The external temperatures also affect the effectiveness of the insulation. The more extreme the temperature, be it very high or very low, the less effective the insulation becomes. Thus, a garment designed for its ability to protect against heat or cold is selected by its wearer on the expectation of the climate in which the garment is to be worn. Clearly there is a need for garments made from intelligent fabrics that can provide superior protection as well as wear comfort. A number of intelligent materials and textiles are available today. (a) Phase change materials When a material changes phase with increasing temperature, e.g. from solid to liquid state, a large quantity of latent heat is absorbed. This input of heat is necessary to transform the solid material to the liquid state, and the change will occur at an almost fixed temperature the melting point of the material. The heat is, in effect, stored in the material in its liquid state and is only released when the liquid is cooled back to its solid state. This behavior forms the basis of phase change materials. In normal circumstances, heat will flow through the garment to the outer environment. With the presence of PCM s within the garment, this flow is interrupted as the PCM absorbs or releases the heat preventing the heat access to the outer environment. In this way an active thermal barrier (insulation) is created that is in addition to the normal passive thermal barrier inherent in the garment s design. Burlington Worldwide (BWW), in partnership with Outlast Technologies and Ciba, has created a finish that allows fabrics to adjust to changes in temperature for more comfortable and versatile clothing. The patent-pending technology, called Smart Fabric Technology, is built around micro encapsulated phase-change materials called Thermocules. These materials absorb and release heat for increased comfort without compromising the fabric's inherent characteristics. A paraffin-pcm, for example, absorbs approximately 200 kilojoules per kilogram of heat to undergo a melting process. This high amount of heat absorbed by the paraffin in the melting process is released into the surrounding area during the cooling process starting at its crystallization temperature. During the complete melting process, the temperature of the PCM as well as its surrounding area remains constant. The excess heat generated by a body in action is absorbed by the paraffin in the PCM which melts in the process and stores the heat. Since the excess heat has been taken away from the body, the undesired temperature increase concomitant with the normal heating process does not occur. The same is true for the crystallization process. During the entire crystallization process the temperature of the PCM does not change either. The high heat transfer during the melting process as well as the crystallization process without temperature change makes PCM interesting as a source of heat storage. 4

5 The PCM is normally present in microcapsules, which can resist wear and tear during the life of the garment. Microencapsulated PCMs can either be distributed within fibers or coated onto the fabric. Whilst the concept of using PCMs is clearly a very attractive one, there are still a number of limitations. Acrylic is the only commercially available fiber that is compatible with PCMs, and there is an upper limit to the amount of PCM that can be incorporated in the fibers before tensile properties are appreciably reduced. Where PCMs have been coated onto fabrics, fabric hand may be compromised, and durability to abrasion during wear and to washing and dry-cleaning may be lowered. The selection and incorporation of PCMs in textiles requires care. The most important consideration is the actual temperature of the phase change, and in garments this should normally be 30-35º C, close to the body s temperature. Other key factors include cost, toxicity and availability. The initial applications of the finish will be in fabrics for active wear apparel. Future planned applications include men swear, uniform and barrier products. (b) Shape memory materials. These types of materials are those that can revert from the current shape to a previously held shape, usually due to the action of heat. The UK Defence Clothing and Textiles Agency have extensively pioneered this technology. When these shape memory materials are activated in garments, the air gaps between adjacent layers of clothing are increased, in order to give better insulation. The incorporation of shape memory materials into garments thus confers greater versatility in the protection that the garment provides against extremes of heat or cold. There are shape memory alloys and polymers. A shape memory alloy is usually in the shape of a spring. The spring is flat below the activation temperature but becomes extended above the activation temperature. By incorporating these alloys between the layers of a garment, the gap between the layers can be substantially increased above the activation temperature, which considerably improves protection against external heat. Shape Memory Polymers have the same effect as the alloys but, being polymers, are potentially more compatible with textiles. They could also be employed as flame retardants. The shape memory effect is observed when a plastic conforming to one shape returns, at a particular temperature, to a previously adopted shape. For clothing applications, Polyurethane films have been made which can be incorporated between adjacent layers of clothing. When the temperature of the outer layer of clothing has fallen sufficiently, the polyurethane film responds so the air gap between the layers of clothing by becoming broader. This broadening is achieved if, on cooling, the film develops an out-of-plane deformation, which must be strong enough to resist the weight of the clothing and the forces induced by the movements of the wearer. The deformation must be capable of reversal if the outer layer of clothing subsequently becomes warmer. Based on methods first developed by Mitsubishi Heavy Industries, for manufacturing polyurethane-based shape memory polymers, a unique, new, high performance material that features temperature sensitivity has been developed. Called DiAPLEX, this novel material can be used to make comfortable garments that are watertight without clamminess. To maintain a comfortable environment within garments DiAPLEX is designed to react at a transition temperature, which adapts the state of the material to variations in the internal and external environment. When, following strenuous activity or changes in the external environment, the temperature inside the garment reaches the transition temperature, the material automatically becomes either more waterproof or more permeable to water vapor. Structural representation of twin layer of Diaplex fabric 5

6 Diaplex has Self-Control through which it Memorizes Conditions of Comfort and Responds to Changes in Environmental Temperature DiAPLEX can be called a material with self-control because it senses changes in the environment and adjusts itself accordingly to maintain comfortable in-garment condition. In addition to being highly waterproof and effectively breathable, DiAPLEX also features anti-condensation characteristics, heat retention, wind proofing and water repellency required in severe weather conditions; while it also has stretchability, durability and a sensitive soft touch that make it suitable for sportswear. Corpo Nove, through their R&D spin-off Grado Zero Espace, has used Thermal Shape Memory metals as a fabric for the manufacturing of a shirt with long sleeves. The sleeves can be programmed to shorten immediately as the room temperature becomes a few degrees hotter. The fabric can be screwed up into a hard ball, pleated and creased then, just by a flux of hot air (even a hairdryer) pop back automatically to its former shape. (c ) Aerogels The aero gel insulation blanket produced by Aspen Aerogels in the U.S. is being employed as a lining for extreme winter clothing, and is said to be three times more effective than 3M's Thinsulate and 39 times more insulating than the best fiberglass insulation. Aerogels are produced through the creation of gelatinous structures and then removal of all liquid without allowing any shrinkage. It is therefore packed with microscopic insulative air pockets of silica, alumina, carbon or other such materials with diameters of less than 100 nanometers that make it impossible for most gas molecules, including air, to pass through, resulting in virtually zero heat loss. A wafer thin layer is sufficient to protect a hand from a blowtorch. A block the size of a person weighs less than a kilogram and yet can support a small car. Aerogels were used as insulation on the rover vehicle of the Mars Pathfinder. 6

7 Fig. A jacket made with Aerogel (d) Nettle fibers Textiles made of nettle fibers are naturally insulating. The fibers of the stinging nettle have a special characteristic in the fact that they are hollow which means they can accumulate air inside thus creating a natural insulation. To create a cool fabric for summer, the yarn lengths are highly twisted closing the hollow core and reducing insulation. For winter fabrics, yarns with a low twist are used. The hollow core of the fibers remains open maintaining a constant temperature. CONDUCTIVE MATERIALS Fig. A garment made of Nettle fibers. Conductive fabrics combine the breathing/ moisture managing finishes with high metallic content in textiles. With the addition of nickel, copper and silver coatings of varying thickness, these fibers provide a versatile combination of physical and electrical properties for a variety of applications. For example, the thermal conductivity of fabric is 7

8 increased a thousand-fold after the fibers are metallized. Clothing constructed with conventional polymers have minimal thermal insulation. Another type of conductive fiber is carbon. ECT (Electric Conductive Textile) is a carbonized glass fiber textile, each filament of which is coated with a few nanometers of carbon. The textile can be woven, braided or knitted with any kind of glass fiber and any kind of yarn. The carbon creates the electrical conductivity and therefore electrical resistance. This electrical resistance presents the opportunity to use the textile as an electric heating medium. Heat is distributed evenly over the entire surface of the textile with resistance varying between 10 and 3000 Ohm per square of surface area. However, whether the square is 1 cm2 or 1 m2 in dimension, the resistance per square is constant. The applied power can be up to 600 Volts (AC or DC). The combination of both parameters allows ECT to generate an energy output of between 50 and 10,000 Watt/m2. Fig. A carbonized glass fiber Robert Rix from Rotherham, South Yorkshire, has created a high conductivity carbonised-fibre material called Gorix. The material is highly conductive and is able to regulate its own temperature without a thermostat. It does this by sensing how much voltage it is taking from a power source. The material ensures this voltage level is maintained, restricting temperature variation to within 0.2 degrees Celsius of the limit. With Gorix, an entire expanse of cloth is regulated, preventing "hot spots". EXO², based in Edinburgh, UK, is now actively marketing what it claims is the first, "intrinsically safe and totally adaptable" textile and polymer-based heating system. Called Fabroc, the system is a combination of conductive yarn, braiding and carbon loaded silicon elements that can be used to apply heat to a wide range of products including textiles, footwear, dry suits, back supports, custom heated gloves etc. Fabroc employs no wires to break or overheat, has no solid panels to accommodate and uses discreet and efficient energy sources, operating at very low voltages (typically volts). 8

9 The BMW Heated Vest has been designed to plug into a BMW power socket but it is also possible to connect it to the battery of any motorcycle. Designed without a collar, the black and gray vest can be worn with virtually any style of riding jacket. Its maximum heating performance requires 3.5 amps at 12 volts meaning it needs less power than most headlight bulbs. The carbon fiber fabric, responsible for the heating performance, allows a more uniform and pleasant distribution of heat than most conventional heated garments. The vest is lined and has exceptional thermal insulation properties. This has been achieved by vapor-depositing aluminum onto the waistcoat s outer material. Concluding remarks Intelligent textiles provide rich evidence of the enormous wealth of opportunities still to be grasped by the textile industry. These opportunities appear equally abundant in the clothing and fashion sector of the industry and in the technical textiles sector. In particular, future developments will arise from active collaboration between people representing a whole variety of backgrounds and disciplines, including engineering, science, process development, design, commerce and marketing. Within the next few years, intelligent (SMART) devices will significantly influence our everyday lives, and many of these devices will be present in textile and clothing. Indeed, progress in intelligent textiles is occurring so fast that there are likely to be significant developments between the time of submission of this paper and its subsequent publication. March