Thermal stability polytetrafluoroethylene

The high thermal stability of the carbon-fluorine bond has led to considerable interest in fluorine-containing polymers as heat-resistant plastics and rubbers. The first patents, taken out by IG Farben in 1934, related to polychlorotri-fluoroethylene (PCTFE) (Figure 13.1 (a)), these materials being subsequently manufactured in Germany and the United States. PCTFE has been of limited application and it was the discovery of polytetrafluoroethylene (PTFE) (Figure... 

Fluorocarbon elastomers represent the largest group of fluoroelastomers. They have carbon-to-carbon linkages in the polymer backbone and a varied amount of fluorine in the molecule. In general, they may consist of several types of monomers poly-vinylidene fluoride (VDF), hexafluoropropylene (HFP), trifluorochloroethylene (CTFE), polytetrafluoroethylene (TFE), perfluoromethylvinyl ether (PMVE), ethylene or propylene.212 Other types may contain other comonomers, e.g., 1,2,3,3,3-pentafluropropylene instead of HFP.213 Fluorocarbon elastomers exhibit good chemical and thermal stability and good resistance to oxidation. 

The high-molecular-weight perfluoropolyethers obtained in the first step have excellent thermal stability (TGA initial decomposition >370°C) and chemical resistance and are true fluorocarbon elastomers that are, in contrast to polytetrafluoroethylene, flexible. Other methods for the synthesis of perfluoropolyethers and functionalized perfluoropolyethers are under development in our laboratories. 

All polyolefins have low dielectric constants and can be used as insulators, ill particular, PMP has llie lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and a low dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon) These properties remain nearly constant over a wide temperature range. The dielectric characteristics of polyfvinylcyclohexane) are especially attractive its dielectric loss remains constant between-180 and 160 C, which makes it a prospective high frequency dielectric material of high thermal stability. 

Perfluoroelastomers represent a special subgroup of fluorocarbon elastomers. They are essentially rubbery derivatives of polytetrafluoroethylene (PTFE) and exhibit exceptional properties, such as unequaled chemical inertness and thermal stability. Currently, there are two types of known commercial perfluoroelastomers Kalrez and Perlast. These have ASTM designation FFKM. 

Commonly used high speed substrates such as polyethylene and polytetrafluoroethylene meet the low dielectric constant and low moisture requirements, but lack the thermal stability required... 

Fluorine substitution on carbon confers particular reactivity because of its particular properties Fluorine as the most electronegative element forms very strong CF bonds which are sized to "cover" perfluorocarbons tightly. Polytetrafluoroethylene (Teflon) is one practical example illustrating the exceptional chemical and thermal stability of saturated perfluorocarbons in a sharp contrast to explosive tetrafluoroethylene or to fluoroacetylenes. 

The thermal stability of polytetrafluoroethylene oxide and PTFE have been compared under the same conditions by Donato et al. [263] between 450 and 600°C. The decomposition rate has a maximum at 628°C for the oxide and at 568°C for PTFE. The activation energy for the first-order degradations are 98 kcal mole"1 between 8.5 and 85% for the oxide polymer and 85 kcal mole-1 between 523 and 571°C for PTFE. The rate of weight loss is less than 1.2% per min for both polymers below T = 550°C for the oxide and T = 590°C for PTFE. The oxide, however, loses weight below 390° C whereas PTFE does not. The main components of the volatile material are trifluoroacetyl fluoride, carbonyl fluoride and tetrafluoroethylene. An end-initiated thermal degradation with small zip length is proposed. 

In this section, heat and temperature related or dependent properties of polytetrafluoroethylene resins are discussed. These include thermal stability, thermal expansion, thermal conductivity, and specific heat (heat capacity). These characteristics are important to both design and use of PTFE parts. 

Ryton Polyphenylene Sulfide is a new commercial plastic which is characterized by good thermal stability, retention of mechanical properties at elevated temperatures, excellent chemical resistance, a high level of mechanical properties, and an affinity for a variety of fillers. It is produced from sodium sulfide and dichlorobenzene. Its unusual combination of properties suggests applications in a variety of molded parts such as non-lubricated bearings, seals, pistons, impellers, pump vanes, and electronic components. Tough coatings of polyphenylene sulfide can be applied to metals or ceramics by a variety of techniques and are used as protective, corrosion-resistant coatings in the chemical and petroleum industries. Incorporation of small amounts of polytetrafluoroethylene provides excellent non-stick properties in both cookware and industrial applications. 

In another TG study, Chiu (45) compared the relative thermal stabilities of five polymers, as shown in Figure 4.40. These polymers, poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), high-pressure polyethylene (HPPE), polytetrafluoroethylene (PTFE), and an aromatic poly-pyromellitimide (PI), were all heated under identical conditions in the... 

In the cardiovascular area, PET has a dominant position with heart valves. Expanded polytetrafluoroethylene, known for its chemical inertia and its high-thermal stability, possesses other advantages compared with PET, especially its biostability and the possibility of sterilization by heat. Finally, substitutes based on synthetic textiles now open up an alternative therapy for the replacement of injured... 

Polytetrafluoroethylene, PTFE, is the polymerization product of tetraflu-oroethylene discovered in 1938 by R.J. Plunkett of Du Pont. The polymer is linear and free from any significant amount of branching. The highly compact structure leads to a molecule of great stiffness and results in a high crystalline melting point and thermal stability of the polymer. 

---