According to Textile Intelligence estimates, worldwide sales of activewear and sportswear increased by 23% between 1997 and 2001. In the European Union, the market for sports apparel and equipment is now worth over Euro 37 bn (US$ 43 bn) and in the USA sales are estimated at around US$ 46 bn.
The dramatic growth in activewear and sportswear market has significant implications for the textile industry. Spending in the UK alone exceeded £4 billion in 2002 and is predicted to reach £5 billion in 2007. The sector ranges from specialist apparel for specific sports to sportswear worn for its fashion value. Different sports require garments to fulfil different functions. For example wind proofing and high thermal insulation are required for skiwear, whereas efficient thermoregulation and moisture management are required in football shirts.
The paper discusses the textile materials and products specifically designed for activewear and sportswear. It discusses fibres, yarns, fabric structures and fabric finishes currently used to enhance comfort in the wearer during strenuous exercise. The testing, analysis and benchmarking of sportswear structures will be discussed for specific test methods used for a wide range of sportswear and activewear garments developed at the University of Bolton.
The dramatic growth in the activewear and sportswear sector has significant implications for the textile industry worldwide. According to Pierre Duffar, DuPont’s European active sportswear manager, worldwide sales in this sector increased by 75% between 1987 and 1998. He anticipated growth of 23% between 1997 and 2001. Sales within the European EU15 market are currently worth at least £16 billion with £11 billion being spent on sports clothing. A recent Key Note report estimated that, in 2002, UK consumers spent £4.05 billion on sportswear. The report estimated that £2.9 billion was spent on clothing and £1.15 billion on footwear. This represents 10.3% of the total clothing and footwear market. Table 1 illustrates the total UK sportswear market since 1998.The sector includes specialist apparel for specific sports each with its own particular functions. The performance fibres, yarns, fabrics and finishes developed for this specialist sector are increasingly transferring to the mass market in the high street. The increasing cultural importance of sportswear in fashion meant that only 25% of sportswear was used for active sports or during exercise. In the report, Key Note forecast a 17.6% growth in the sportswear market over the next five years resulting in UK sales of £5 billion in 2007.
Consumers demand high levels of comfort, design and easy care in all types of clothing. However, in sportswear, where thermophysiological comfort can significantly enhance the performance of the wearer, the use of innovative textile products and materials is increasingly common.
Table 1: Total UK Sports Clothing and Footwear Market at Current Prices (Source: Key Note)
|1998||1999||2000||2001||2002 (est)||% change 1998 – 2002||% change 1999 – 2002|
|Sports clothing (£ billion)||2.63||2.55||2.65||2.75||2.90||10.5||13.7|
|Sports footwear (£ billion)||1.20||1.15||1.20||1.20||1.15||-4.2||0.0|
|Total (£ billion)||3.83||3.70||3.85||3.95||4.05||5.9||9.5|
The above total is predicted to reach £5 billion in 2007 with growth rate of 17.5% from 2002 to 2007.
2. What is Comfort?
For the consumer the comfort of any garment stems from a combination of its sensorial properties, its psychological properties and its thermophysiological properties. Comfort is determined by the interaction of the body, its microclimate and its clothing. Where garments are worn as layers, it is a combination of the properties of the individual garments that determine the comfort of the whole clothing system.
Whilst undergoing strenuous activity a body generates additional metabolic heat. Sweat is produced as part of the natural mechanism for the dispersion of that heat. A naked man can control his heat loss almost instantly as sweat is evaporated very quickly during the period of activity leaving no accumulated sweat when activity stops. Clothing can act as a barrier to heat and moisture loss. If over-heating is to be avoided, thermoregulation and moisture management are key functions of clothing designed for use as sportswear or activewear. For sportswear that has transferred to the mass market and may be worn on a daily basis, such as football and rugby shirts, the psychological and sensorial functions are as important as the thermophysiological properties.
Psychological comfort consists of a combination of consumer prejudice and prevalent fashion trends. Where garments are worn during strenuous activity, psychological comfort also occurs when the garment is extensible and does not restrict mobility. A garment with low intrinsic weight can significantly aid sporting performance. Colour and shape retention over multiple wash/wear cycles are a further prerequisite for success in the mass market.
Sensorial comfort is focussed on the tactile sensation of a garment on the human body. Garments should be soft and pliable during wear and, especially when damp, should not prickle / irritate or cling to the body. To a lesser extent, sensorial comfort can be improved by the control of odour and by use of UV resistant materials. Waterproofing can improve sensorial comfort but may impair thermophysiological comfort.
Thermophysiological comfort entails both thermoregulation and moisture management. Garments should be designed to maintain the human body temperature and moisture output close to its normal levels under diverse conditions. The thicker the layer of air trapped inside the clothing system the greater its thermal resistance and its resistance to moisture transmission. If perspiration is trapped next to the skin during exercise it can lead to an increase in body temperature; this will cause dehydration, fatigue and decreased performance. The thermal insulation properties of a fabric usually decline when the fabric is wet resulting in rapid heat loss from the wearer. This wetting can occur both from outside a garment (rain) and from inside a garment (perspiration). During strenuous activity wet fabric can aid in the cooling of the hot skin surface. However, once the activity and the excessive heat production stop, this heat loss must be restricted. A wet body cools very quickly leading to post-exercise ‘chill’ or, in extreme cases, hypothermia.
Garments that are designed for sportswear and activewear should be dynamic or responsive. Through effective thermoregulation and moisture management a clothing system can maximise heat loss when the wearer is hot then increase thermal insulation when perspiration stops. In a sports arena dynamic or responsive garments can enhance performance, control weight build up in clothing and reduce the potential for skin damage.
3. Test Methods Used to Measure Thermophysiological Comfort
Various test methods exist for the measurement of comfort properties. BS 4745 uses either a two plate togmeter or a single plate togmeter to measure thermal conductivity. ASTM D1518 uses a guarded hot plate method to measure thermal transmittance. Water vapour permeability can be measured using the cup method as defined in BS 7209 and in ASTM E96-80. Alternatively moisture vapour permeability can be determined using a sweating guarded hotplate method in which a plate is heated to skin temperature and supplied with water in order to simulate sweating. Absorption and wicking are typically measured separately. Absorption is measured by a static immersion test as defined in BS 3449. The wicking test suspends a strip of fabric vertically with its lower edge in a reservoir of distilled water. The rate of rise of the leading edge of the water is monitored. Transverse wicking can be determined using a plate test. The test methods currently used at the University of Bolton, UK, to determine comfort are described below.
Alambeta instrument, developed by Sensora, Czech Republic, determines a number of thermal transmission parameters. The instrument, shown in Figure 1, measures the behaviour of the heat that flows through the test material due to the different temperatures of the lower measuring body and the heated measuring head. Measurements do not include the layer of air associated with fabric during actual use. Alambeta is used to measure fabric thickness, thermal conductivity, thermal diffusivity and thermal resistance. It also measures thermal absorptivity (W m-2 s1/2 K-1), which Sensora define as the warm – cool feeling at the first contact of the human skin with a textile fabric. All five above – mentioned properties can be determined in dry and wet state fabrics.
Figure 1: Alambeta Instrument by Sensora, Czech Republic (1 – Control and evaluation computer unit; 2 – Control panel with display; 3 – Frame and housing of unit; 4 – Measuring plate; 5 – Measuring head)
Permetest instrument, also developed by Sensora, determines the relative water vapour permeability, that is the percentage of water vapour transmitted through the fabric sample compared with that through the equivalent thickness of air. It also measures the resistance to evaporative heat loss of a fabric with its associated layer of air. The apparatus consists of a heated porous membrane, which is used to simulate sweating skin. A current of air passing over the plate can be controlled to simulate the effect of different wind speeds. The heat required to evaporate the water from the membrane with and without a fabric covering is measured. The results are automatically recorded on an ink recorder attached to the instrument.
Combined Wicking and Absorption Apparatus was developed at the University of Bolton, UK, to measure concurrent absorption and wicking. Traditional methods measure absorption horizontally and wicking vertically. This new method, illustrated in Figure 2, determines the mode of water transport through the fabrics that is not evidenced by separate tests. The method simulates the absorption of sweat by a garment from a profusely sweating body and its transport across the fabric surface. Initial uptake of water is through absorption in a direction perpendicular to the fabric plane over an area of 23.75 cm2. Wicking starts as soon as the area under the plate is saturated. This computerised instrument is highly reproducible and determines the absorption and wicking capacity and rate. The results are graphically illustrated by the computer.
4. Properties Relevant to the Measurement of Comfort
For sportswear and activewear the properties most relevant to the measurement of comfort are those related to thermoregulation and moisture management. These are listed first followed by those properties relevant to sensorial comfort.
Intrinsic thermal insulation (units = m2 ºC W-1, or m2 K W-1, or Tog = 0.1 m2 °C W-1, or Clo = 0.155 m2 °C W-1) measures the resistance of a fabric to dry or conductive heat loss. Intrinsic thermal resistance can be measured in a dry or damp fabric. It is generally proportional to fabric thickness. Intrinsic thermal insulation does not include the effect of the layer of air associated with a fabric during actual use.
Thermal insulation (units = m2 °C W-1, or m2 K W-1, or Tog = 0.1 m2 ºC W-1, or Clo = 0.155 m2 ºC W-1) measures the resistance of a fabric and its associated layer of air to dry or conductive heat loss. Thermal insulation, unlike intrinsic thermal insulation, will vary with wind speed. Increasing wind speeds decrease the thermal insulation afforded by the layer of air.
Resistance to evaporative heat loss of a fabric with its associated layer of air (units = =m2 mm Hg W-1 or m2 Pa W-1) determines the resistance of fabric to the cooling of the body through evaporation of heat generated during activity. This can also be measured on dry or damp fabrics.
Thermal conductivity of fabric determines the rate of transmission of heat through a fabric. Thermal conductivity is the reciprocal of thermal insulation or resistance.
Moisture vapour permeability (units = g m-2 hr-1) determines the resistance of a fabric to the transfer of water vapour or insensible perspiration emanating from the body. Relative moisture vapour permeability (%) determines the percentage of water vapour transmitted through the fabric sample compared with that through the equivalent thickness of air. An increase in fabric thickness tends to lead to a decrease in the rate of water vapour transmission through the fabric. Low moisture vapour permeability prevents perspiration from passing through the fabric leading to a precipitation and accumulation of sweat in the clothing.
Water (sweat) absorption determines the capacity (units = g m-2 or g g-1) and rate (units = g m-2 s-1) of a fabric to mop up the liquid sweat generated by the body. Ideally the absorption capacity should be low at the surface of the fabric in contact with the skin to prevent wet clinging.
Wicking determines the capacity (units = g m-2 or g cm) and rate (units = g m-2 s-1) of a fabric to transport absorbed sweat away from the point of absorption, that is away from the skin.
Air permeability (units = cm3 cm-2 s-1 @ 10 mm Hg) determines how well air can flow through a fabric. It can be measured in dry or damp fabrics and will be expected to reduce in fabrics where absorption of water leads to fibre and yarn swelling. Air resistance is the reciprocal of air permeability. Air permeability doesn’t always equate to good moisture vapour permeability.
Rate of drying (units = g m-2 s-1) from the outer surface of a fabric must be optimal for continuous wicking and hence prevention of saturation of the fabric with sweat.
Wind proofing a garment provides a mechanism by which heat loss by convection is reduced thereby improving the thermal insulation properties of the clothing system.
Surface coefficient of friction of a fabric contributes to the sensorial comfort of a fabric. The coefficient can increase significantly in a wet fabric leading to rubbing or chafing of the skin. Low surface coefficient of friction is essential where one layer of fabric must move freely against another layer of fabric.
Handle of a garment describes its tactile qualities and includes softness, compressibility, pliability, drape etc. These properties, though less important in specialized sportswear than in clothing worn on a daily basis, should not impair the performance of the wearer.
UV resistance is important in clothing worn under high levels of exposure to the sun. It is particularly important in skiwear where the wearer is less aware of exposure.
Anti-microbial / anti-bacteria / anti-odour properties are particularly important in items such as sports socks, underwear etc. which are worn, and are in contact with sweat, for long periods of time.
5. Heat Loss from the Clothing of a Person Carrying Out Strenuous Activity
The optimal conditions for comfort as measured at the skin’s surface are 31.5 – 32.5 ºC and 60% relative humidity in a static environment and 33.5 – 34.5 ºC and 70% relative humidity in a dynamic environment. When a person is involved in strenuous activity the additional metabolic heat produced can be dissipated by conduction, convection and radiation, as well as by evaporation.
Conduction of heat from the body occurs through direct contact with clothing and is determined by the difference in temperature of the two substances and by their thermal conductivities.
Convection occurs when air, in contact with the body, is heated by conduction and then carried away from the body by convection. Convection losses can be reduced by restricting the movement of air close to the body.
CText FTI Professor of Technical Textiles,
The University of Bolton, Bolton, UK