Explosion-free process for the production of polytetrafluoroethylene

#The invention pertains to a safe technology for the production of polytetrafluoroethylene (PTFE) through aqueous emulsion polymerization of tetrafluoroethylene (TFE), whereby a significant cause of explosions is avoided.

#The radical polymerization of TEE to PTFE in the aqueous medium is carried out according to two different processes, namely, suspension polymerization (S-process) and so-called emulsion polymerization (E-process). The E-process is more susceptible to explosions than the S-process. With the S-process, little or no emulsifier with a non-telogenic effect is used, but with the E-process, a relatively large amount of this emulsifier is used. The products differ accordingly:

#The S-process leads to a relatively coarse-particle suspension of PTFE that is essentially processed using the sintering process, whereas the E-process leads to a colloidal PTFE dispersion with latex particles having a particle size of up to about 250 nm that can either be used—directly or after being concentrated—for coatings, for example, or else following isolation of the solids. In the latter case, the “fine resin powder” is then mixed into a paste with organic liquids such as benzene, and extruded into formed objects such as strands, tubes, hoses, or strips, which—following removal of the liquid—are sintered or stretched into flat sheets. For that reason, this form of PTFE is also known as “pasteware”.

#The polymerization, its preparation and the measurements of the SSG per ASTM D 1457-66 are carried out as described in (US-A-4 391 940. The polymerization takes place in a 50-liter vessel boiler equipped with an impeller agitator and a baffle. The agitator speed is 200 rpm. The reactor is filled with 22 g of anunonium perfluoro octanoate dissolved in 31 liters of deionized water. The air is removed by alternately evacuating and pressing TFE, as is known in the state of the art. The vessel is pressurized to 13 bar with TFE. The temperature is set to 35° C. Pumped into the reactor are 15 mg CuSO4.12 H2O dissolved in 20 cm3 concentrated ammonia solution (28 wt-%), and a solution of 1.0 g (NH4)2S2O8 (ALPS). The polymerization is started by pumping in an aqueous solution of 0.5 g Na2S2O5. Pressure and temperature are kept constant during the polymerization. The TFE consumption is measured with a flow meter. After 8.5 kg of TFE has been consumed, the charging of TFE is ended, the pressure is reduced through ventilation, and the vessel is evacuated and refilled with nitrogen. The evacuation and filling with nitrogen is repeated twice in order to remove remaining TFE. The resulting dispersion has a solids content of 22 wt-%. The ploymer resin that has formed is separated and dried, and its SSG determined.

#In a series of tests, the partial pressure of R134a is varied per Table 1. The R134a is pressurized before the starter is added.

#In a further series of tests, the initiator KMnO4 is used in place of APS, while the rest of the conditions remain the same. The aqueous phase contains 22 g ammonium perfluoro-octanoate, 1.5 g oxalic acid and 10 mg K2S2O5. The polymerization is sustained through the continuous charging of 30 mg KMnO4 per liter with a charging ratio of 0.3 liters/h, Pressure and temperature (35° C.) remain constant during the polymerization. Once a solids content of 22 wt-% has been reached, the polymerization is ended, the PTFE resin isolated, and the SSG measured.