MONTAJE EN LA PARED

The above section briefly explained moisture absorption in a textile fabric and different measurement techniques. The important definition of textile comfortability is to keep the wearer with a moisture-free, dry environment and also to ensure that moisture in terms

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of sweat is moved away from the skin to the outer-most layer of a garment for evaporation (Watt, Sampath et al., 2012). The human body is a self-regulatory system in which the core body temperatures has to be maintained at an optimum level for the proper function of vital organs and systems. This is achieved easily during normal activities. However, an imbalance is caused when performing activities such as sports in which the body core temperature is increased due to muscle movements. In order to remove this excess heat, sweat is produced via the skin which comes into contact with the inner layers of the clothing; often called the second skin. As such this layer must possess high absorption capabilities and also the ability to transfer the absorbed moisture to the outer garment layers for evaporation.

Literature (Yi Li and Qingyong Zhu, 2003, Srikiatden and Roberts, 2007, Gibson and Charmchi, 1997) shows that during sweating, moisture vapour can be created on the human skin during exercise, and this moisture vapour has to be transmitted to outer layers of the garment in order to avoid moisture condensation on the skin, i.e. within the microclimate. The microclimate is the mini space formed between the human skin and the textile, comprising its own temperature, pressure and humidity. As a result the relative humidity in the microclimate region could increase due to the accumulation of moisture which may cause condensation and bring about a clammy feeling for the wearer.

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Figure 2.5.1: Humidity variation in microclimate(Nahla Abd El - Mohsen Hassan Ahmed, 2012)

Therefore, textile fabrics should be designed to facilitate efficient moisture vapour removal. As the mechanism of vapour transmission is by diffusion, absorption and desorption, different techniques for measuring moisture vapour transmission have been developed by several researchers (Nahla Abd El - Mohsen Hassan Ahmed, 2012). These have been summarised in Table 2.5.1 below.

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Table 2.5.1: Test methods to quantify moisture vapour transmission

Literature (Kissa, 1996, Watt) also reports that the main mechanism for transmitting moisture in a textile fabric is the capillary effect. This effect causes water to be

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transmitted through narrow tubes. The main factors which influence the capillary system of a textile structure are the types of fibres used, the length of capillary and the hydrostatic pressure difference between the two sides of the capillary. Generally the type of yarns used to manufacture textile fabric with good moisture-transfer characteristics should possess high capillary effect which is created due to the formation of micro channels between the fibres of the yarn; this would allow micro tubes to be formed in the textile structure to allow moisture to transfer in the textile fabric. Fibres with different cross- sections have been developed by synthetic fibre manufacturers for the sportswear industry who have designed fabrics with enhanced moisture management (Drirelease, Gore-tex, DuPont). The length of the capillary in the textile structure also plays an important role as it influences the amount of moisture that can be transmitted to the outer environment.

Lucas Washburn kinetics can be used to explain the dynamics of the capillary effect. This shows that the longer the capillaries are, the less time is required to move moisture within the fabric. Another factor affecting moisture transmission is the hydrostatic pressure difference between the two ends of the capillary. The capillary ends next to the skin, in the region that is sometimes known as the microclimate, possess conditions which create an inside pressure (Buck, 1981). Similarly the other end, which will be on the surface of the fabric, and exposed to outside environmental conditions, will be at atmospheric pressure. This difference in pressure plays a major role in the transmission of moisture across the fabric. As such, in order to achieve effective moisture flow within fabric, the pressure on one side of the fabric has to be higher than the pressure on the other side of the fabric. A higher pressure difference, known as hydrostatic pressure difference, will enhance moisture transmission at a high rate.

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Figure 2.5.2: Cloth-body system

The moisture transmission across a textile structure can be studied by subjecting the textile structure to real environmental conditions and then by monitoring its performance. This means that two different environmental chambers have to be created, i.e. hot and cold chambers to mimic actual environments, in which the fabric can be tested. To continuation in ensuring realistic test conditions, actual fabrics made into garments are used on a human manikin. Currently, fabrics are tested on a manikin that is created to mimic a human, in order to create repeatable data. Among the pioneers of this technology are the Hohenstein Institute in Germany (Hohenstein). Three different manikins named ‘Charlie’, ‘Charlene’ and ‘Sherlock’ have been designed to test fabrics at different environment conditions. Sensors, sophisticated sweating mechanisms and weighing systems are integrated within the manikins to facilitate moisture transmission and to determine moisture transmission characteristics precisely. This technology is used by researchers and manufacturers to study the performance of garments under different conditions. Clothing layers Atmosphere Microclimate Skin Moisture

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Figure 2.5.3: Hohenstein Testing manikin(Hohenstein)

Recently, a new technology was developed (GHANMI H.) which utilise electrical capacitance measurement to monitor moisture transmission across fabrics (Figure 2.3.4). This involves placing electrode plates on both sides of the textiles fabric to measure the capacitance. The research has shown that the measurements can be influenced by the moisture passing through the fabric. Experimental results have demonstrated similar moisture transmission behaviour of fabrics tested with Hohenstein and capacitance measurement technologies.

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Figure 2.5.4: Capacitance method schematic diagram(GHANMI H.)