Process for forming an expanded porous tetrafluoroethylene polymer

Claim 1

A process for preparing an expanded porous polytetrafluoroethylene product having a higher density, greater flexibility, and a lower matrix tensile strength than conventionally extruded, stretched, and sintered polytetrafluoroethylene products, and for which the resulting properties can be varied while maintaining a constant rate of expansion during the process, the method comprising: extruding a paste formed from a polytetrafluoroethylene resin and a lubricant at a reduction ratio of less than 100 to thereby produce an extruded paste at a relatively low extrusion pressure; calendering the extrudate; removing the lubricant from the calendered extrudate; concurrently heating and expanding the calendered extrudate, and wherein the heating step further comprises, heating the thus calendered extrudate to a temperature greater than the crystalline melting point of polytetrafluoroethylene but below its thermal decomposition temperature for a time period sufficient to substantially equilibrate the temperature of the extrudate while expanding the extrudate, but short enough to prevent substantial decomposition of the polytetrafluoroethylene, and wherein the expanding step further comprises, concurrently expanding the extrudate during heating at a fixed rate of expansion and to at least twice its original length; and varying the heating temperature at a constant rate of expansion to control the porosity and deformability of the resulting polytetrafluoroethylene.