Pore size control of PTFE membranes by stretch operation with asymmetric heating system

Polytetrafluoroethylene (PTFE) is a remarkable membrane material. Due to its high melting point, PTFE fine powder cannot be processed using the conventional melting processing methods. Instead, techniques such as paste extrusion, rolling and sintering have to be employed. In this paper, a PID control PTFE paste extrusion system was used to improve the properties of PTFE paste during the extrusion process and the quality of the PTFE membrane. A range of lubricant content (18, 20 and 22 wt.%) was used to monitor the pressure drop at the same reduction ratios (RR ¼ 26.47) and different extrusion speeds (0.5, 1 and 2 mm/s). In addition, we describe a precision PTFE stretch operation with asymmetric heating system, and how it can be used to improve the properties of PTFE membrane. Different stretching ratio (none and 50%), asymmetry heating temperature (420 and 5 C), and different heating time (5 and 10 s) were used to modify the PTFE membrane pore size. It was found that a higher stretching rate tends to result in larger pore size and broader pore size distribution at the same heating time. At a shorter heating time and without stretch at asymmetric heating process, the porosity of PTFE membrane was increased from 50 to 70% and the mean pore size was decreased from 0.15 to 0.08 mm.

Introduction of the PID control into a PTFE extrusion line has improved the stability of extrusion and consequently the PTFE membrane morphology. A stable extrusion at lower speed could dramatically reduce lubricant migration, resulting in an extrudate with more uniform and lower density. The stretching operation plays an important role in the PTFE membrane formation. A higher temperature tends to produce larger pore size and broader pore size distribution at the same stretching rate. Asymmetric heating process also affects the PTFE membrane process significantly. PTFE membranes prepared by the optimized extrudate result in higher porosity and smaller mean pore size. Stretching operation and the heat treatment play important roles to control the pore size of the membrane. In the asymmetric heating process, set adequate operation conditions (temperature and time) were necessary to restrict unstable pore size distribution.