Pressure pyrolysis, PTFE

The first step is set up to produce hydrogen fluoride and the second yields trichlo-romethane (chloroform). Chloroform is then partially fluorinated with hydrogen fluoride to chlorodifluoromethane using antimony fluoride as catalyst in the third step. Finally, in the fourth step, chlorodifluoromethane is subjected to pyrolysis in which it is converted to tetrafluoroethylene. The pyrolysis is a noncatalytic gas-phase process carried out in a flow reactor at atmospheric or subatmospheric pressure and at temperatures 590 to 900°C (1094 to 1652°F) with yields as high as 95%. This last step is often conducted at the manufacturing site for PTFE because of the difficulty of handling the monomer.9... 

Lewis and Naylor reported the first results on the pyrolysis of PTFE in 1947 [26], At a temperature of 600°C yields of up to 91% TFE were obtained under vacuum (7 x 10 Pa). Under an atmospheric pressure of 1.103 x 10 Pa this yield dropped to only 16%, whereas the yields of HEP and C-C4F8 were increased. 

The hrst rehable report of the preparation of tetrafluoroethene (TFE) was given in 1933 by Rulf and Bretschneider [643], who decomposed tetrafluoromethane in an electric arc. Other syntheses are based on the dechlorination of yn-chlorodifluoromethane [644], the pyrolysis of chlorodifluoromethane [645], and the decarboxylation of sodium perfluoroproprionate [646]. Since then, a number of synthetic routes have been developed [644-646]. On a laboratory scale, depolymerization of PTFE by heating at about 600 °C is probably the preferred method to obtain a smah amount of pure monomer (97%) [647], along with the highly toxic perfluoroisobutene as well as octafluorocyclobutane as a side product formed by the thermal (2ji -I- 2n) cyclodimerization of TFE [648,649]. The most common commercial approach for the preparation of TFE is the pyrolysis of chlorodifluoromethane [645,650]. The noncatalytic gas-phase reaction is carried out in a flow reactor at atmospheric pressure, yields over 95% TFE at 590 to 900 °C. The synthesis of TFE involves the following steps ... 

For continuous pyrolysis, only metal reactors (steel or nickel alloys) are used and PTFE is fed into the reactor with a screw feeder. The gaseous products are either collected using a scaled up cold trap, or fed directly into other processes [5,13], Continuous systems suffer from a drawback in that it is very difficult to maintain a pressure seal at the PTFE inlet, resulting in the inclusion of air in the reactor atmosphere, which induces PTFE combustion rather than pyrolysis and produces a mixture of mostly undesired products, such as CF4, C02 and CF2O. 

TABLE 5.1 Distribution of Products Obtained from the Pyrolysis of PTFE as Function of Temperature and Pressure... 

His work fails to take into account that OFCB, which is less stable than HFP, is preferentially formed over HFP at lower temperatures and higher pressures during pyrolysis of PTFE, in an environment where high concentrations of both TFE and CF2 are present, which would favor HFP formation according to his postulate. Until this discrepancy is explained, the mechanism, as given in Scheme 5.5, will be accepted as valid. 

HFP monomer is a gas at normal temperature and pressure (m.p. -153 C b.p. -28°C) and it does not homopolymerise easily [94]. However, it forms several industrially useful eopolymers with other fluoromonomers. Oxidation of HFP gives industrially important intermediate hexafluoropropylene oxide, which is for preparation of number of perfluoroalkyl perfluorovinyl ethers [95]. Several methods were shown in literature for preparation of HFP. For example, HFP ean be prepared in high yields by thermal cracking of TFE at 700-800°C under redueed pressure [96]. Another method for preparation of HFP good yields [97] is pyrolysis of PTFE under vaeuum and at 860°C. 

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