Abstract: A porous polytetrafluoroethylene material produced by forming a mixture of an adequate amount of a liquid lubricant and polytetrafluoroethylene fine powder having an average molecular weight of at least 6,000,000, an amorphous index larger than 0.1 and a number average primary particle size of 0.1 to 0.8 microns, and a clear endothermic peak at a temperature of 347.+-.3.degree. C. and an another endothermic peak or shoulder at a temperature between 330.degree. C. and the temperature of said clear endothermic peak on a crystalline melting chart recorded by a differential scanning calorimeter, removing the lubricant to obtain an unsintered PTFE material and expanding the unsintered polytetrafluoroethylene material at least monoaxially at a temperature higher than the melting point of polytetrafluoroethylene.

Abstract: A thermopervaporation apparatus comprising a microporous membrane which is impervious to liquids while readily allowing the vapor to pass therethrough, the passage of a hot feed solution situated on one side of said microporous membrane, a porous spacer disposed on at least part of the other side of said microporous membrane, and a heat-transmission wall disposed on said spacer, in such a manner that the vapor of the component to be separated from the feed solution permeates said membrane, diffuses to said heat-transmission wall through said spacer and is cooled on said heat-transmission wall to form a condensate which is withdrawn through said spacer, thereby improving a yield of the condensate.