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Description
Water vapour permeability, often expressed as evaporation resistance Ret, belongs to the most important thermal comfort parameters of protective, sport and other functional clothing (garments) and related textile fabrics. Water vapour permeability of all textile products should be as high as possible, in order to prevent the accumulation of sweat in the garment system and simulatenously enable efficient cooling of a body by the sweat evaporation. For outdoor or industrial applications, these garments should also exhibit the so-called semipermeability: the textile laminates creating the outside garment fabrics contain micro- or nanoporous membranes, which prevent the penetration of outside liquid water into the laminates, but simultaneously allow the passage of water vapour from the body through the whole fabric system.
In microporus membranes, this semi-permeability is given by differences between the dimension of minimal water drops – compact lusters of water molecules (hundreds of micrometers) and size of pure water (water vapour) molecules (about 0,4 nm). In nanoporous membranes, when entering the laminate, water vapor molecules first condensate into single water molecules, which then travel through the macromolecular structure of the polymer fibres. The more water rmolecules are present in the amorphous phase of the polymer, the higher is the flux of moisture. The average moisture level inside the fibre structure is then proportional to the moisture level on both sides (surfaces) of the nanoporous membranes. The moisture level is in this case expressed (proportional) to the level of water vapour partial pressure on both surfaces of the nanoporous membrane.
In the study, a new observation based on the requirement of the highest level of water vapour partial pressure on both membrane surfaces is presented.
When testing the evaporation resistance Ret of proper commercial laminates with micro and nanoporous membranes, the next to skin membrane surface is (in Skin models or gravimetric testers) exposed to highest level of moisture, presented by the saturated water vapour partial pressure (WVPP). The other surface faces lower WVPP simulating the environmental conditions. Thanks to this high WVPP on one of the membrane surfaces, the experimentally determined evaporation resistance Ret levels of the tested nanoporous laminates are low, thus indicating protective clothing with excellent WV permeabiliy. The manufacturers and vendors of jackets with nanoporous laminates are promising excellent wearing comfort of these goods. As follows from the recent research results presented in this study, their optimistic marketing declarations may not be justified, due to the negative effect of underwear and other fabric layers, which are worn (within the garment structure) under the outside semipermeable laminate. All evaporative resistances of these textile interlayers are linked in series and reduce significantly the water vapor flow, before this gazeous form of moisture reaches the next to skin surface of the nanoporous laminate.Thus, the average moisture level inside the nanoporous membrane can be significantly lower, then in case of testing the nanoporous laminate by means of testing instruments. This low moisture inside these membranes will significantly, in non-linear way, reduce WV permeability of the laminates, resulting in very high evaporation resistance linked in series with evaporation resistances of other textile layers. That is why the effective water vapour permeability of e. g. outdoor jackets with nanoporous membranes, when worn over several fabric layers, can be much lower then marketed levels of these outdoor garments. Clients wearing these clothing may suffer from sweat accumulation and overheating caused by reduced transfer of the evaporated sweat.
In this study, theoretical analysis of the above problem is presented, along with experimental determination of evaporation resistance of 10 nanoporous and one microporous laminate, tested in the quick PERMETEST Skin model. In the first step, evaporation resistances of the single laminates were detemined, and in the second step, additional fabric, simulating the evaporation resistance of the underwear, created a sublayer of the tested laminates. The measurements confirmed, that in case of the included evaporation resistance of the simulated underwear, evaporation resistance levels of the proper laminates with nanoporous membranes were significantly, in non-linear way, higher then in case without the simulated interlayers.
The achived results were expressed in the form of a simple mathematical model of the VW transfer in the analyzed garments. It was also confirmed, that contrary to the nanoporous laminates, evaporation resistances of laminates with microporous membranes keep constant, their level is not influenced by the presence of evaporation resistances of other fabrics in the garment system.
