Electrical Properties of PTFE

Electrical stability of polytetrafluoroethylene is outstanding over a wide range of frequency and environmental conditions. This plastic makes an excellent electrical insulator at normal operating temperatures. Dissipation factor and dielectric constant values are virtually constant up to 10 MHz. Dielectric strength of PTFE drops off with increasing frequency slower than most other material. 

PTFE dielectric constant and dissipation factors remain constant over a broad temperature range (-40 to 240°C) as seen in Fig. 3.28.1 They are not 

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. 

Polytetrafluoroethylene resins are very stable at their normal use temperature range ( 260°C). They exhibit a small degree of degradation at higher temperatures. The rate of decomposition is a function of the specific polymer, temperature, time at temperature, and, to some extent, on the pressure and nature of decomposition environment. In actual processing, degradation is tracked by indirect measurement of mo- 

In vacuum, polytetrafluoroethylene degrades into nearly pure monomer. Products of PTFE degradation in air include carbonyl fluoride (COF2), tetra-fluoroethylene (TFE), and small amounts of perfluoroisobutylene (PFIB).l fl i PFIB and COF2 are highly toxic if they are inhaled. 

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The electrical properties of PTFE are dominated by its extremely low dielectric constant (2.1) This value is invariant over a broad range ol temperatures (—40 to 250 °C) and frequencies (5 Hz to 10 GHz). Similarly, PTFE has an unusually low dissipation factor, which is also quite independent of temperature and frequency This behavior results from the high degree of dipolar symmetry of the perfluonnated and unbranched chains The dielectric strength, resistivity, and arc resistance are very high... 

The volume resistivity of polytetrafluoroethylene remains unchanged even after a prolonged soaking in water, because it does not absorb water. The surface arc-resistance of PTFE resins is high and is not affected by heat aging. They do not track or form a carbonized path when subjected to a surface arc in air [39]. The electrical properties of PTFE are summarized in Table 3.6. 

Table 3.31. Effects of Oven-Aging at 300°C on the Electrical Properties of PTFE Resins ...

Electrical Properties of PTFE, PFEP and Plasticised PCFE... 

Table 2. Mechanical and Electrical Properties of Commercial PTFE Resins [7]...

Chemical resistance and electrical properties of PMP are similar to those of the other polyolefins, except that it retains these properties at higher temperatures than do either PE or PP. In this respect PMP tends to compare well with PTFE up to 150C (300F). Molded parts made of this plastic are hard and shiny, yet their impact strength is high at temperatures down to -29C (-20F). Their specific gravity of 0.83 is the lowest of many commercial solid plastics. 

The size of the fluorine atom allows the formation of a uniform and continuous sheath around the carbon-carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm)[ i and low coefficient of friction (0.05-0.08, static)[ i of PTFE. Another attribute of the imiform fluorine sheath is the electrical inertness (or non-polarity) of the PTFE molecule. Electrical fields impart only slight polarization to this molecule, so volume and surface resistivity are high. Table 1.1 summarizes the fundamental properties of PTFE, which represents the ultimate polymer among all fluoroplastics. 

Electrical properties. Fillers and additives significantly increase the porosity of polytetrafluoroethylene compounds. Electrical properties are affected by the void content as well as the filler characteristics. Dielectric strength drops while dielectric constant and dissipation factor rise. Metals, carbon, and graphite increase the thermal conductivity of PTFE compounds. Tables 3.19 and 3.20 present electrical properties of a few common compounds. 

Table 3.19. Electrical Properties of Filled PTFE Compounds Measured According to ASTM D149,0150 ...

Major applications of unsintered polytetrafluoroethylene are as tape in thread sealing and wrapping electrical cables, and as rod and tape in packings. Important properties of PTFE like chemical resistance, broad service temperature, low friction, flexibility, high machine direction strength, and deformability in the cross direction make unsintered fine powder PTFE ideal for these applications. 

The exceptional properties of PTFE make it highly useftil. It is selected for a wide range of apphcations that affect every person. The applications fall in the live areas that require one or more of its chemical, mechanical, electrical, thermal, and surface properties. In essentially every instance, a successful application employs at least two of the outstanding properties of this polymer. However, because of its high volume cost, PTFE is not generally used to produce large objects. 

FEP will melt and flow. The high chemical resistance, low surface energy, and good electrical insulation properties of PTFE are retained. 

Hubacek T, Lyutakov O, Rybka V, Svorcik V (2010) Electrical Properties of Flash-Evaporated Carbon Nanolayers on PTFE. J. mater, sci. 45 279-281. 

The most important property of PTFE is its extreme resistance to the attack of chenucals and solvents. Only molten or liquid NHj-dissolved alkali metals would attack PTFE through the removal of fluorine atoms. PTFE has a very low coefficient of friction (0.04-0.05) but poor abrasion resistance and is completely unaffected by water. It has very good electrical properties with its dielectric constant as low as 2.0. PTFE is a weak polymer compared with other polyolefins. Upon radiation, the material degrades rather than cross-links and its degradation occurs preferentially due to the sdssion of backbone C-C bonds before C-F bonds. The presence of C-F bonds provides an easy detection of PTFE by IR at 8.2-S.3 pm absorption bands. 

The PTFE and PFEP polymers have some of the best electrical properties of all known polymers and their more important electrical properties are shown in Table 10.13. 

Parylene films have excellent electrical properties with high volume and surface resistivitities whilst their dielectric constants and dissipation factors are quite low, although not as good as PTFE. The two main types of parylene are Parylene N and Parylene C. Parylene N is the unsubstituted polymer, whilst Parylene C is chlorine substituted. The best overall electrical properties are exhibited by Parylene N but Parylene C is superior with respect to D.C. dielectric breakdown voltage for films under 5 fxm in thickness. The principal electrical properties of parylenes are shown in Table 10.14. 

Some of the electrical properties of ECTFE are given in Table 13.39. The dielectric strength (per ASTM D149) is higher than breakdown strength of PVDF. The dielectric constants and dissipation factors of ECTFE, PVDF, and PTFE are given in Figs. 13.91 -13.96 as a function of temperature and frequency. 

Nelson et al. [16] have shown that the etching solution attacks only the surface regions of the fluorocarbon polymer. They examined a cut section of etched PTFE and, from optically microscopy, determined that the depth of the coloured surface region was about 1 /xm. However, they reported that the bulk electrical properties of the treated and untreated polymer were virtually identical and that their electron diffraction patterns were indistinguishable, indicating that no changes had been induced in the bulk polymer s crystallinity. These observations may be attributed to the thinness of the treated surface layer and thus the properties of the layer are not measured when analytical techniques are employed which monitor the bulk properties of the material. Indeed, Benderley [17] has since reported that the treated layer does have a substantially lower surface resistivity than the untreated material. 

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