Method of making a controlled pore composite polytetrafluoroethylene

A method for controlling porosity and mean pore size in a composite sheet comprising the steps of:

• a) admixing lubricant with a blend comprising insoluble, non-swellable, sorptive particles and polytetrafluoroethylene to form a soft dough-like mass, the lubricant being present in an amount to exceed the sorptive capacity of the particles by at least 3 weight percent, said mass having a cohesive consistency, and the ratio of insoluble particles to PTFE being in the range of 40:1 to 1:4;
• b) intensively mixing said mass at a temperature and for a time sufficient to cause initial fibrillation of said PTFE particles;
• c) biaxially calendering said mass between gaps in calendering rolls maintained at a temperature and for a time, while closing the gap between the calendering rolls with each successive calendering operation, to cause additional fibrillation of said PTFE particles to form a self-supporting tear-resistant sheet having a void volume in the range of 30 to 80 percent and a mean pore size in the range 0.3 to 5.0 micrometers, wherein said void volume and mean pore size vary directly with and are controlled by the amount of lubricant present during processing.