A Study On Heat And Water Vapor Transfer Through Waterproof Breathable Fabrics With Micro-pore Membrane Under Subzero Climate

Waterproof breathable fabrics (WBFs) is a kind of advanced functional textiles. Now, the markets are largely dominated by relatively small groups such as Gore-tex(?) fabrics consisting of micro-pores PTFE membrane was launched in the USA by W. Gore & Associates, Sympatex(?) fabrics with the pore-less hydrophilic polyester membrane was developed in Germany by Akzo Nobel. More recently hydrophilic polyurethane coating technology is used to produce WBFs in our enterprises. They still have no good water vapor permeability for lacking of key technique.The membrane modifies the ordinary fabrics' structure formed by interlacing of yarns. It can influence the fabrics' characteristic of heat and water vapor transport. It is also known that WFBs widely used in low temperature climate such as raining and snowing. Although much work has been dedicated to heat and water vapor transport through fabrics, there is a great lack of understanding on water vapor transport through fabrics under subzero climate.In this paper, we report on a novel apparatus, which is used to measure the moisture transport through fabrics under subzero climate.The temperature of the small climate chamber can be reduced down to subzero by semiconductor refrigerators. The sensors of temperature and relative humidity are connected to a computer through an A/D converter for data acquisition. Then this apparatus is used to measure the moisture transport through fabrics under conventional (20 ℃) and subzero (-20 ℃) climatic condition. The evaporation from the water bath, the condensation on the fabric surface, the vapor transmission through the fabric and the temperature and relative humidity of inner and outer fabrics are recorded. Experimental results show that the accumulation of condensates on fabric surfaces increase linearly with time and the rate of moisture transfer through fabrics is little affected by the accumulation of condensates on fabric surfaces under both conventional (20 ℃) and subzero (-20 ℃) climate. Nevertheless, the water evaporation from the water bath, condensation on fabric surface and vapor transmission through the fabric is greater and the moisture vapor resistance is smaller under the subzero (-20℃) climate than those under the conventional (20℃) climate.The mechanism of condensation occurred on the fabrics is also investigated based on the Kelvin theory, which describe the relationship between the water vapor pressure and water droplet radial when condensation occurs. The results obtain from an experiment, which can acquire temperature and relative humidity simultaneously, show that condensation is prone to occur on the WBFs. The p-T figure is first usedto judge condensation occurred on the fabrics' surface. The water vapor transport through the multi-layer fabrics is explored under subzero climate. The condensation is also prone to occur on the inner layer and middle layer.Finally the structure of WBFs is carried out on the Scanning Electron Microscope. The non-line equation of heat and mass transfer through WBFs with micro-pore membrane is built up on Hagen-poiseuill model. Base on this model, numerical simulation is calculated to better understand the effect of pore diameter and environmental temperature on the moisture transmission. It is found that more condensation and less water vapor transport occur in -20 ℃ climate temperature. Ice sublimation results in increasing of water vapor transport under -20 ℃ climate temperature in experiment. The presented numerical also showed that changing the pore size and porosity of membrane can increase water vapor transport through the WBFs.The results of expounding the mechanism of water vapor transport through WBFs suggest that improved the vapor permeability of WBFs can be achieved and continued development of WBFs design is recommended.