Method of making polytetrafluoroethylene composite sheet

A method of making high tensile strength polytetrafluoroethylene composite sheet material comprising:

• (1) dry blending one or more types of particulate material;
• (2) blending said particulate material with polytetrafluoroethylene particles in a weight ratio of about 3:1 to 50:1 particulate material to polytetrafluoroethylene particles to form a putty-like mass, wherein said polytetrafluoroethylene particles have a particle size in the range of 0.05 to 0.5 microns and are added in the form of an aqueous dispersion comprised of about 30% to about 70% by weight solids;
• (3) adding sufficient lubricant water to said mass to exceed the absorptive capacity of the solids yet maintain a putty-like consistency;
• (4) mixing said putty-like mass in an intensive mixer at a temperature between 50.degree. C. and 100.degree. C. for a time sufficient to cause initial fibrillation of said polytetrafluoroethylene particles;
• (5) biaxially calendering said putty-like mass between calendering rolls maintained at about 50.degree. C. to about 100.degree. C. to cause additional fibrillation of said polytetrafluoroethylene particles to form a self-supporting sheet, while maintaining the water content of said mass at least at a level above the absorptive capacity of the solids and also closing the gap between the calendering rolls with each successive calendering operation, for a time sufficient to produce a high tensile strength sheet having a tensile strength of at least one megapascal;
• and
• (6) drying the resultant sheet to remove water.