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: Polytetrafluoroethylne fine powder is disclosed 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 micron, and further having a clear endothermic peak at a temperature of 347.degree..+-.3.degree. C. and 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. An unsintered article is made which is sintered and expanded at a temperature higher than the melting point of the polytetrafluoroethylene to give a porous article having improved properties.