Process for forming an expanded porous tetrafluoroethylene polymer

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.