Polytetrafluoroethylene strength

Electrical Properties. Polytetrafluoroethylene is an excellent electrical insulator because of its mechanical strength and chemical and thermal stabihty as well as excellent electrical properties (Table 6). It does not absorb water and volume resistivity remains unchanged even after prolonged soaking. The dielectric constant remains constant at 2.1 for a temperature range of —40 to 250°C and a frequency range of 5 Hz to 10 GHz. 

Fig. 11. Effect of polyolefin primers on bond strength of ethyl cyanoacrylate to plastics. All assemblies tested in accordance with ASTM D 4501 (block shear method). ETFE = ethylene tetrafluoroethylene copolymer LDPE = low-density polyethylene PFA = polyper-fluoroalkoxycthylene PBT = polybutylene terephthalate, PMP = polymethylpentene PPS = polyphenylene sulfide PP = polypropylene PS = polystyrene PTFE = polytetrafluoroethylene PU = polyurethane. From ref. [73].

It resembles polytetrafluoroethylene and fluorinated ethylene propylene in its chemical resistance, electrical properties, and coefficient of friction. Its strength, hardness, and wear resistance are about equal to the former plastic and superior to that of the latter at temperatures above 150°C. 

It may be noted that the strength of the CF bond and the tendency of polytetrafluoroethylene to degrade rather than cross-link on irradiation suggests the breaking of a CC rather than a CF bond as a likely source of radicals in this polymer. 

The general structure of this class of materials can, therefore, be summarized as a fine dispersion of metal oxide in a polymer matrix very similar to plasma polytetrafluoroethylene and in principle any metal should be able to be incorporated. Clearly, if the films are protected from the atmosphere, for metals which form involatile fluorides having a relatively weak metal-fluorine bond strength, it should be possible to produce films having metal atoms dispersed in the matrix. It is expected that these films will have many interesting chemical, optical, electrical and magnetic properties., ... 

The characteristics of a covalent bond formed by two atoms are due mainly to the properties of the atoms themselves and vary only a little with the identities of the other atoms present in a molecule. As a result, some characteristics of a bond can be predicted with reasonable certainty once the identities of the two bonded atoms are known. For instance, the length of the bond and its strength are approximately the same regardless of the molecule in which it is found. Thus, to understand the properties of a large molecule, such as the resistance of polytetrafluoroethylene (Teflon ) to chemical attack, we can study the character of C—F bonds in a much simpler compound, such as tetrafluoromethane, CF4, and expect the C—F bonds in the polymer to be similar. 

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

In order to obtain the optimum dielectric strength, it is necessary to avoid the formation of voids. Thomas, Lontz, Sperati, and McPherson have shown that the dielectric strength may be as low as 200 volts/mil when polytetrafluoroethylene is fabricated in a way which produces several per cent voids. Sugita, Nagao, and Toriyama also showed how the elimination of voids improved the corona resistance of polytetrafluoroethylene and decreased the rate of dielectric breakdown. The dielectric breakdown under prolonged stress was studied by Brodhun. He also showed how the presence of corona causes an additional effect. Pao and Bjorklund performed quantum mechanical calculations of the energies of excited electronic states. 

The almost universal chemical inertness of polytetrafluoroethylene has been attributed to the strength of the carbon-fluorine bond and the way in which the fluorine atoms protect the carbon chain from chemical attack (Doban, Sperati, and Sandt). From the theory of solubility, it is expected that the miscibility of hydrocarbons and fluorocarbons will be low. Experimental measurements indicate that the miscibility is even less than was expected from the theory. The possible explanations for this have been discussed by Scott. 

Teflon-6a Polytetrafluoroethylene 10.5 20% White Usually 40-60 (U.S.) mesh size for relatively nonpolar liquid phases low mechanical strength high inert surface difficult to handle due to static... 

Keywords multiwall carbon nanotubes (MWNT), polytetrafluoroethylene (PTFE), surface groups, conventional yield strength, coefficient of elasticity... 

In addition to high breakdown strength, the electrical insulators for superconducting magnets must have excellent dielectric properties at cryogenic temperatures. Chant reported the results of measurements on dielectric constant and loss tangent (tan 5) for several polymers over the temperature range from 4.2 to 300 K [83], The variation of dielectric constant of samples as a function of temperature is shown in Fig. 15. The dielectric constants of nonpolar polymers, such as polyethylene, polypropylene and polytetrafluoroethylene, are substantially independent of temperature, whereas those of polar polymers except polyimide decrease by a maximum of 20% as the temperature is reduced. The values of tan 8 at the frequency of 75 cps for nonpolar polymers decreased by... 

A fluoroepoxy compound was made from a fluorodiepoxy resin cured with an amine adduct. The use of fluoroepoxies is most advantageous in bonding fluorocarbons such as polytetrafluoroethylene. Good bond strengths are achieved on fluorocarbons without the need to surface-treat the substrate because the surface tension of the fluoroepoxy adhesive is reduced from about 45 to 33 dyn/cm.25... 

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