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Effective Water Vapour Permeability of Wet Wool Fabric and Blended Fabrics

2018-11-14 14:26
The water vapour permeability of fabrics is one of the most important factors determining wearer comfort. Contrary to commonly accepted theories, outerwear is often used in a wet state, which has an influence on their properties. However, common standard measuring instruments mostly do not enable reliable measurement of wet fabrics due to the long time of measurement, during which the fabrics get dry. This paper presents the fast instrument- Permetest, which provides reliable measurement of the water vapour permeability of fab- rics in a dry and wet state. By means of the instrument, the relative and effective water vapour permeability of different wool fabrics in a dry and wet state were determined and the results discussed. The main contribution of the measurement was the determination of the exact ratio between the level of heat flux density of the heat flow penetrating the wet fabric, having a cooling effect, and that of the heat flux density of the heat flow caused by moisture evaporation from the fabric surface, also having a cooling effect.
Thermal comfort implies the maintenance of body temperature within relatively narrow limits. Under conditions where thermal comfort cannot be achieved by the human body’s own ability(i.e. body temperature regulation),such as very cold or hot weather, clothing must be worn to support its temperature regulation by resisting or facilitating heat exchange between the human body and the environment. Together with good insulation, the garment should allow adequate transport of water vapour from the body to the external environment. Thus the final thermo-physiological comfort is given by two principal components: thermal resistance in a wet state and active cooling resulting from moisture evaporation from the skin and passing through the garment and from direct evaporation of sweat from the fabric surface [1, 2].Fundamental papers on fabric thermal resistance and water vapour permeability have been published [3 - 5], but they did not take into consideration the aspect of changes in these parameters due to the moisture content of fabrics. In a dry state,most fabrics deliver satisfactorily permeability to water vapour (WVP), but in a wet state, a water film on the outer fabric surface is formed, which may reduce theeffective permeability of the fabric [6, 7].Meanwhile wet skin can greatly increase the cooling effect of the body, and hence, for example, a moisture content of 10 - 20% could cause a drop in thermal insulation of up to 50% compared to the dry fabric. Sweat production by the human body depends on the type and intensity of physical activity, and changes from 40 – 100 g/h (standard perspiration) to even up to 1 litre of sweat in an hour (very heavy physical work). Therefore it seems very important to investigate the water vapour permeability of fabric not only in a dry state but also in a wet state,which applies in particular to fabrics with high hygroscopicity, which wool fabrics have. Unfortunately, the permeability cannot be detected by common WVP testers because the samples during this measurement get dry as a result of too long time of measurement. The only instruments that are suitable for WVP evaluation of wet fabrics are instruments of the Skin Model Type (f.e. PERMETESTSensora Skin Model), which allow to perform tests quickly and reliably. 
Theoretical assumptions:
The heat flux density generated due to sweat evaporation determines heat lost by the body and has a cooling effect on it. The heat flux density also has an effect on cooling due to the moisture which evaporates from the surface of the fabric(see Figure 1, page 68). However, this cooling effect may not cool the body sufficiently because the heat flux density caused by the temperature drop at the fabric surface is reduced by the effect of thermal resistance of the fabric and by that of the air gap between the fabric and skin [9, 10]. In this study the effect of the contact thermal resistance is neglected. Figure 2 shows all the evaporation resistances Ret in Pa. m2/W encountered during the passage of heat flux density caused by the evaporation of sweat to the environment.
At first, the effect of skin cooling caused by the evaporation of moisture from the fabric surface was analysed. Despite the assumption of isothermal conditions, the wet fabric becomes cooler than the surrounding air because the fabric surface,due to the effect of a certain fabric thermal resistance, is not kept at the temperature of the instrument’s (Skin Model) measuring surface. The heat flux density, caused by convection mass transfer from the fabric surface (qfabw), can be described by Equation 1, on condition that the fabric surface is covered by a continuous water film.
Research program:
The researches were conducted with PERMETEST apparatus, which measured the amount of heat passing through the thermal model of human skin (Figure 3).The fabric sample was placed on a measuring head over a semi-permeable foil and exposed to parallel air flow at a velocity of 1 m/s. The measurements were carried out under isothermal conditions(23.0 ± 0.5 °C). A computer connected to the apparatus determined the evaporative resistance, Ret, and thermal resistance, Rct and RWVP, of the textile fabrics according to the standard ISO 11092, which did not refer to the fabric surface temperature when there was an air gap between the skin model surface and test fabric (Equation 8). These values served to reflect the thermophysiological properties of the textile fabrics and garments. The higher the RWVP, the lower the Ret,and the better the thermal comfort of the garment.
The samples were first dried in an air conditioner at 105.0 ± 0.5 °C in order to get rid of all moisture. The samples were subsequently soaked with water to their full volume to increase their humidity.The water used for soaking contained a surfactant to lower the surface tension.During the measurement procedure, each sample was stepwise mechanically dried and weighed. When the results of the measurement should be expressed in terms of water vapour resistance Ret in Pa.m2/W according to the ISO 11092 standard [11.
Here, qs and qo mean the heat flux density lost by the moist measuring head and pwo in Equation 11 represents the water vapour saturate partial pressure valid for the temperature of air in the measuring laboratory (21.0 ± 0.5 °C), and the partial water vapour pressure in the laboratory air. Constant C was determined by the calibration procedure. For this purpose special hydrophobic polypropylene reference fabric was delivered with the instrument.
All fabrics were tested in various states of moisture content: 1 – normal state,2 – ‘ultra-dry’ and 3 – various wet states.The experiment consisted of measuring the RWVP and Ret of dry and wet fabrics without an air layer (h = 0 mm).


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