The uses of poly(tetrafluoroethene) (PTFE) are a function of its resistance to chemical attack, unreactivity (even above 500 K), low friction, non-stick properties and high electrical resistance.
Uses of poly(tetrafluoroethene) (polytetrafluoroethylene)
In many applications, tetrafluoroethene (TFE) is co-polymerized with other fluorinated monomers, such as hexafluoropropene and perfluoropropyl vinyl ether, and also with ethene.
Poly(tetrafluoroethene) (PTFE) and its co-polymers are used in:
Annual production of poly(tetrafluoroethene) (polytetrafluoroethylene)
PTFE is made from methane in a series of reactions:
(a) Production of trichloromethane (chloroform)
Trichloromethane is one of the products formed by the reaction of methane and a mixture of chlorine and hydrogen chloride. This can be performed in the liquid phase at 370-420 K using a zinc chloride catalyst. Alternatively, the reaction is carried out in the vapour phase, using alumina gel or zinc oxide on silica as a catalyst at 620-720 K.
(b) Production of chlorodifluoromethane
Trichloromethane is reacted with anhydrous hydrogen fluoride in the presence of antimony(III) and antimony(V) chlorofluoride to give chlorodifluoromethane:
(c) Production of tetrafluoroethene (tetrafluoroethylene, TFE))
Since TFE is an explosive gas (bp 197 K), it is usually made when and where required for polymerization so that there is minimum storage time of the monomer between its production and its polymerization.
Chlorodifluoromethane is heated in the absence of air, a process known as pyrolysis:
Low pressures (atmospheric) and high temperatures (940-1070 K) favour the reaction.
Steam, preheated to 1220 K, and chlorodifluoromethane, at 670 K, are fed into a reactor. Steam is used to dilute the reaction mixture and hence reduce the reactant partial pressure, and thus the formation of carbon and toxic by-products. The steam also supplies all the heat required by this endothermic reaction. Very little hydrolysis of reactant and product occurs.
Once formed, the product must be rapidly cooled to 770 K to prevent the reverse reaction occurring and the explosive decomposition of TFE:
The cooling is done by passing the vapour through a water-cooled heat exchanger, made of graphite to resist chemical attack and thermal shock. Reactor residence time is 1 second.
The monomer is transformed into the polymer, PTFE, by radical polymerization. The reaction is carried out by passing TFE into water containing a radical initiator, e.g. ammonium persulfate, (NH4)2S2O8, at 310-350 K and a pressure of 10-20 atm.
Two different procedures are used:
There is a group of co-polymers which are formed by the co-polymerization of tetrafluoroethene and other unsaturated organic compounds such as ethene, hexafluoropropene and perfluoropropylvinyl ether. As described above, these co-polymers are used in many of the examples given for PTFE.
The co-polymer produced from ethene and tetrafluroethene is an alternating co-polymer usually known by its trivial name, ethylene tetrafluoroethylene (ETFE):
It is used, in particular as a lining for containers as it is stable to attack by concentrated solutions of acids and alkalis, and because of its good electrical properties of insulation and its strength, it is used as a coating for wires and cables. Its most spectacular use is as a roofing material in buildings such as the O2 Arena in London, the Eden Project in Cornwall and the Birds Nest Olympic Stadium in Beijing. The roofs are made up of 2 - 5 layers of large cushions of ETFE. It is also used as an outer skin of large buildings (Figure 2).
Date last amended: 7th October 2013