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post #7 of Old 03-28-2006
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ummmmm the intrusion pressure of liquid/free water of typical gortex material is in the range of 75 pounds per square inch of surface area when loaded with free water - and that pressure to allow free water to pass would crush a puny human body into 'paste'. The way a hydrophobic membranes works with respect to vapor transmission is that the amount of relative humidity from one side to the other 'equilibrates' from the higher humidity to the zone of lower humidity (partial pressures of the mixture differences) ... no little 'flaps' or valves involved at all just a structure that microscopically looks like an open celled foam. The moisture/vapor *diffuses* through the fixed pore structure membrane.
The 'competitors' and cheap equivalent suppliers of coated hydrophobics simply coat each microfiber with a hydrophobic substance .... and the permeability of the open structure allows the diffusion and equilibration of humidity to the lower side. Such 'coatings' are very vulnerable to coming off the microfiber .... but are a lot cheaper than the PTFE expanded Goretex, etc.
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