Pyrolysis, polytetrafluoroethylene

The first report of the cyclodimerization of fluorinated olefins was provided by Lewis and Naylor,3 working at E.I. DuPont de Nemours Co., in 1947. While studying the pyrolysis of polytetrafluoroethylene (PTFE), the compound octa-fluorocyclobutane was isolated from the pyrolysis off-gas stream. The researchers identified the product and speculated that it was formed by the cyclodimerization... 

Polyvinyl fluoride and PVDF are more stable to elevated temperatures than the corresponding chloride polymers. The decomposition temperatures of polytrifluoroethylene and polytetrafluoroethylene (ptfe) are progressively higher than polymers of vinyl fluoride or vinylidene fluoride. The pyrolysis of PAN and polymethacrylonitrile yields polycyclic ladder polymers. 

Early study of the pyrolysis of polytetrafluoroethylene was reported by Lewis and Naylor. A series of subsequent investigations have been described by Siegle, Muus, and Lin and by workers at the National Bureau of Standards (Florin, Wall, Brown, Hymo, and Michaelsen Madorsky, Hart, Strauss, and Sedlak Michaelsen and Wall Wall and Michaelsen). 

The vacuum pyrolysis of thin samples of polytetrafluoroethylene follows first-order kinetics with monomer as the major decomposition product in the temperature range from 360—510° C (Siegle, Muus, and Lin). The rate constant does not depend on either the molecular weight or the type of polymer and is characterized by an activation enthalpy of 83.0 kcal/mole and a frequency factor of 3 x 1019 sec-1. The melt viscosity decreases during pyrolysis. 

Michaelsen, J. D., and L. A. Wall Further studies on the pyrolysis of polytetrafluoroethylene in the presence of various gases. J. Research Natl. Bur. Standards 58, 327-331 (1957). 

Siegle, J. C., L. T. Muus and T. P. Lin Pyrolysis of polytetrafluoroethylene. Meeting of the American Chemical Society, Atlantic City, September 1956. 

Siegle JC, Muus LT, Lin T-P, Larsen HA (1964) The molecular structure of per-fluorocarbon polymers II. Pyrolysis of polytetrafluoroethylene. J Polym Sci Part A 2 391... 

There are several methods to produce HEP Eor example, thermal cracking of TFE at reduced pressure and temperatures 700°C to 800°C (1292°F to 1472°F) produces HFP in high yield [38,39]. Another process is pyrolysis of polytetrafluoroethylene under vacuum at 860°C (1580°F) with a 58% yield [40]. More recently, a technique involves the pyrolysis of a mixture of tetrafluoroethylene and carbon dioxide at atmospheric pressure and temperatures 700°C to 900°C (1292°F to 1652°F) [41]. Additional routes to HFP are described in [42,43]. 

C. M. Simon and W. Kaminsky, Chemical recycling of polytetrafluoroethylene by pyrolysis, Polymer Degradation and Stability, 62, 1-7 (1998). 

ANALYSIS OF VOLATILE PRODUCTS FROM PYROLYSIS OF POLYTETRAFLUOROETHYLENE AT ELEVATED TEMPERATURES IN A VACUUM [255]... 

Active carbons are obtained by partial combustion and foermal decomposition of various raw materials followed by carbonization and activation. A detailed look at the process of pyrolysis and carbonization of a parent feedstock and the processes of generation of an isotropic porous carbon and its activation illustrates the complexity of reactions involved in its manufacturing. Active carbons are manufactured from a wide variety of materials wood [1, 2], coal [1, 2], bituminous coal [29], rubber [30, 31], ahnond shells [32], oil-palm stones [33], polymers (e.g., vinylopyridine resin [34] and polytetrafluoroethylene [35]), phenolic resins [36], rice husk [37], etc. Very interesting active carbon honeycomb structures were fabricated from combination of synthetic precursors, i.e., phenolic resins, along with several organic and inorganic additives [38]. 

Waritz RS, Kwon BK. The inhalation toxicity of pyrolysis products of polytetrafluoroethylene heated below 500 degrees Centigrade. Am Indust Hygiene Assoc J. 1968 19-26. 

Polytetrafluoroethylene decomposition products Overexposures result in polymer fume fever, a disease with fiu-iike symptoms inciuding chills, fever, and cough. See also p 259. Produced by pyrolysis of Teflon and related materials. Perlsofluorobutylene and carbonyl fluoride are among the pyrolysis products. 

Arenes from Aliphatic and Allcyclic Precursors. —The formation of perfiuoromesityl-ene and perfluoropseudocumene by pyrolysis of polytetrafluoroethylene at 530— 560 °C over metal catalysts (Vol. 2, p. 359) has been patented,and improved yields of cyclic aromatic trimers have been obtained by coupling reactions of perfluoro-(l,2-di-iodocyclo-butene and -pentene) (see p. 74). i The formation of polyfluoronaphthalene derivatives from one of the bicyclic compounds (24) obtained during the reaction of carbon tetrachloride with 3,3,4,4-tetrafluorohexa-... 

Although polytetrafluoroethylene has the reputation for being quite stable thermally, it does degrade at elevated temperatures. The polymer upon pyrolysis yields almost 100% of monomer. The mechanism is believed to be free-radical unzipping of the chains until the entire chain is consumed. This can be illustrated as follows [457]. Initially, the chain ruptures ... 

Pentafluoropyridine, although reported to be stable when pure at 550—600 °C, has been found to react at these t nperatures with potassium fluoride and with polytetrafluoroethylene to afford a moderate yield of perfluoropicolines (see Vol. 1, p. 256), typically a mbcture of the 3-isomer (88%) and the 2-and 4-isomers (6%). The reaction is thought to involve a difluorocarbene intermediate (Vol. 1, pp. 187, 256) formed by the decomposition of the <7-complex of the pyridine and potassium fluoride, or from the pyrolysis of polytetrafluoroethylene. Both 4-methoxy- and 4-nitro-tetrafluoropyridines... 

---