Polytetrafluoroethylene stability

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... 

Figure 1. Implantation of electrons in polytetrafluoroethylene films to form electric transducer of high stability and sensitivity.

Fluorocarbon Resins. This term includes polytetrafluoroethylene, polymers of chloro-trifluoroethylene (fluorothene), vinylidene fluoride (H2C CF2)j hexafluoropropylene (C3Ffl) and similar compds. These polymers are thermoplastic, inert to chemicals and oxidation. They have high heat stability, retain their useful props at both extremely low and high temps, have high electrical resistance to moisture. The materials are available as re sins, powders, and dispersions, and as films, sheets, tubes, rods and tapes. Some of them are rubber-like. Commercially available varieties are Kel-F , Teflon , Fluorel , Aclar and "Halon ... 

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. 

C is clearly an important quantity for a latex dispersion since it essentially represents the electrolyte concentration at which complete loss of stability occurs. It may be obtained experimentally by a variety of different methods (14,17, 18,19). It should be noted, however, that since coagulation is a kinetic phenomena time enters as a variable and consequently the various methods may yield somewhat different numerical results. This effect is illustrated by results obtained for the coagulation of polytetrafluoroethylene (PTFE) latices with sodium chloride as a function of pH (19). From Figure 4 it can be seen that different results are obtained according to whether the system was examined after 2 h or 24 h. As expected the results indicate that the state of aggregation is more advanced after 24 h and consequently systems at a lower electrolyte concentration have coagulated. Care must therefore be taken when comparing values... 

The stability of methylmercury contained in samples of fish, fish muscle and dried shellfish was investigated by storing samples in a deep-freeze and subjecting them to thermal cycling over a period of 32 months (Horvat and Byrne, 1992). Losses of up to 30% of methylmercury contents of shellfish were recorded, but in the other samples contents were stable. Aqueous samples containing methylmercury stored in polytetrafluoroethylene containers were stable for up to 20 days, and acidified solutions (nitric acid) were stable in glass containers stored in a refrigerator. Seawater samples were acidified with 2% hydrochloric acid for better stability (Leermakers et al., 1990). 

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. 

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