Polytetrafluoroethylene: Synthesis and Characterization of the Original Extreme Polymer Gerard J. Putsa, Philip Crousea and Bruno M. Amedurib* aDepartment of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa. b Ingenierie et Architectures Macromoléculaires, Institut Charles Gerhardt, UMR 5253 CNRS, UM, ENSCM, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France. Correspondence to: Bruno Ameduri (E-mail: [email protected]) 1 ABSTRACT This review aims to be a comprehensive, authoritative, and critical review of general interest to the chemistry community (both academia and industry) as it contains an extensive overview of all published data on the homopolymerization of tetrafluoroethylene (TFE), detailing the TFE homopolymerization process and the resulting chemical and physical properties. Several reviews and encyclopedia chapters on the properties and applications of fluoropolymers in general have been published, including various reviews that extensively report copolymers of TFE (listed below). Despite this, a thorough review of the specific methods of synthesis of the homopolymer, and the relationships between synthesis conditions and the physico-chemical properties of the material prepared, has not been available. This review intends to fill that gap. As known, PTFE and its marginally modified derivatives comprise some 6065 % of the total international fluoropolymer market with a global increase of ca. 7 % per annum of its production. Numerous companies, such as Asahi Glass, Solvay Specialty Polymers, Daikin, DuPont/Chemours, Juhua, 3F and 3M/Dyneon, etc., produce TFE homopolymers. Such polymers, both high molecular-mass materials and waxes, are chemically inert, hydrophobic, and exhibit an excellent thermal stability as well as an exceptionally low co-efficient of friction. These polymers find use in applications ranging from coatings and lubrication to pyrotechnics, and an extensive industry (electronic, aerospace, wires and cables, as well as textiles) has been built around them. South Africa, being the third largest producer of fluorspar (CaF2), the precursor to hydrogen fluoride and fluorine, has embarked on an industrial initiative to locally beneficiate its fluorspar reserves, with the local production of fluoropolymers being one projected outcome. As our manuscript focuses specifically on the homopolymerization of TFE (the starting point for all fluoropolymer industries), it will be of considerable use to start-up companies and other commercial entities looking to enter the fluoropolymer market, as well as end-user companies. After a short discussion on the synthesis and production of TFE (both at industrial and laboratory scales), including the safety aspects surrounding handling (since that monomer is regarded as explosive if brought into contact with oxygen due to the formation of peroxides), transport and storage, and then expands into detailed discussions dealing with aspects such as the various additives used (buffers, chain transfer agents, surfactants etc.), the solvent environment, and the reaction conditions. A further section reports the properties of PTFE with respect to the polymerization conditions as well as an overview on the specialized techniques used to characterize PTFE. Finally, the applications of PTFE into various topics, ranging from electrical insulation, tribological to medical applications, as well as chemically resistant coatings and pyrotechnics are discussed. 2 TABLE OF CONTENTS ABSTRACT ................................................................................................................................................. 2 1 INTRODUCTION ........................................................................................................................... 7 2 SYNTHESIS OF TFE AND CONCOMITANT SAFETY ASPECTS .................................. 9 2.1 Ultra-fast pyrolysis of CF2ClH ............................................................................................... 11 2.2 Dehalogenation of halofluorocarbons .................................................................................. 12 2.3 Pyrolysis of perfluoropropanoic acid salts ............................................................................ 12 2.4 Pyrolysis of PTFE under vacuum .......................................................................................... 12 2.5 TFE process safety ................................................................................................................... 14 2.5.1 Autogenic decomposition of TFE ................................................................................ 15 2.5.2 Storage of TFE ................................................................................................................. 17 2.5.3 Heat removal during polymerization ............................................................................ 18 2.6 Conclusions ............................................................................................................................... 18 3 POLYMERIZATION PROCESS AND EQUIPMENT ........................................................ 18 3.1 Polymerization procedures ..................................................................................................... 19 3.1.1 Batch and semi-batch polymerization ........................................................................... 20 3.1.2 Post-polymerization processing ..................................................................................... 21 3.2 Polymerization equipment ...................................................................................................... 21 3.3 Reaction mechanisms and kinetics ........................................................................................ 22 3.3.1 Kinetics of suspension free-radical polymerization .................................................... 23 3.3.2 Kinetics of dispersion free-radical polymerization ..................................................... 24 3.3.3 Kinetics of supercritical CO2 mediated free-radical polymerization ........................ 25 3.3.4 Gamma radiation induced free-radical polymerization .............................................. 25 3.4 Conclusion................................................................................................................................. 25 4 POLYMERIZATION INITIATORS ......................................................................................... 26 4.1 Inorganic free-radical generating initiators ........................................................................... 26 4.2 Organic free-radical generating initiators .............................................................................. 27 4.3 Free-radical generating redox initiators ................................................................................. 27 3 4.4 Fluorinated free-radical generating initiators ....................................................................... 28 4.5 Miscellaneous free-radical generating initiators ................................................................... 29 4.6 Photoinitiators .......................................................................................................................... 29 4.7 Other initiators ......................................................................................................................... 30 4.8 Ziegler-Natta catalysts ............................................................................................................. 31 4.9 Conclusions ............................................................................................................................... 31 5 REACTION CONDITIONS ....................................................................................................... 33 5.1 Monomer purity ....................................................................................................................... 33 5.2 Solvent environment ................................................................................................................ 34 5.2.1 Solvents for free-radical polymerization of TFE ........................................................ 35 5.2.2 Solvents for coordination polymerization of TFE ..................................................... 37 5.2.3 Solvents for electrochemical polymerization of TFE ................................................. 37 5.2.4 Solvent pre-treatment ...................................................................................................... 37 5.3 Pressure and temperature ........................................................................................................ 37 5.3.1 Pressure ............................................................................................................................. 39 5.3.2 Temperature...................................................................................................................... 39 5.4 Agitation .................................................................................................................................... 40 5.5 Polymerization additives ......................................................................................................... 40 5.5.1 Buffering agents ............................................................................................................... 40 5.5.2 Dispersants for emulsion polymerization .................................................................... 40 5.5.3