Production of string-like polytetrafluoroethylene

Claim 1

A process for producing a string-like porous material of a polytetrafluoroethylene, comprising the steps of: molding polytetrafluoroethylene into a string-like shape, the polytetrafluoroethylene containing a liquid lubricant, the molding being performed by a paste process: removing the liquid lubricant; providing a low speed set of heating rolls and a high speed set of heating rolls, wherein the heating rolls are each provided with a plurality of grooves, and the temperature of the low speed heating rolls is higher than the temperature of the high speed heating rolls; placing the molded polytetrafluoroethylene on the low speed pair of heating rolls by threading it in a FIG. 8 configuration around the low speed rolls and within the grooves thereof; placing the molded polytetrafluoroethylene on the high speed pair of heating rolls by threading it in a FIG. 8 configuration around the high speed rolls and within the grooves thereof; rotating each of the high speed pair of rolls at the same speed and each of the low speed pair of rolls at the same speed, the high speed pair of rolls being rotated faster than the low speed pair of rolls to create a uniform compression force during the stretching, the force being applied in the peripheral direction of the string-like material; sintering the stretched string-like material at a temperature of 327° C. or more.