Virgin PTFE has outstanding Permeability Resistance, Remarkable High Energy Radiation resistance & excellent chemical resistance even at elevated temperature mainly due to strong Carbon Flurome bond.
Work using a sample of cast film produced from Fluon® GP1 led to the following test results for permeability:
Oxygen 10.5 x 10-10
Nitrogen 4.0 x 10-10
Air 5.3 x 10-10
The units are cm3 of gas at NTP x cm (thickness) / cm2 (area). s. cm Hg measured at 23°C ± 1°C (73°F ± 2°F)
Work published by Barton, who uses the same units, can be summarised as follows:
Hydrogen 2.4 x 10-9
Helium 7.0 x 10-8
Nitrogen 3.1 x 10-10
Oxygen 1.0 x 10-9
Argon 5.8 x 10-10
Yasuda and Stone obtained a substantially higher figure, 23.7 x 10-10 (units as above), for gaseous oxygen, and an even greater value, 91.0 x 10-10, for dissolved oxygen, while Pasternak et al. obtained a much lower value, 4.2 x 10-10, in experiments with membranes in the thickness range 0.081-0.145mm (0.0032-0.0057 inch). The latter authors also give values for hydrogen (9.8 x 10-10), nitrogen (1.4 x 10-10), and carbon dioxide (11.7 x 10-10). Casper and Henley, using PTFE film 0.094mm (0.0037 inch) thick found a value of 11.6 x 10-10 for hydrogen, and 0.65 x 10-10 for ethane. Work on the helium permeability of fabricated PTFE items has shown that the permeability is very dependent on crystallinity (as indicated by relative density). For isostatically-moulded granular PTFE discs the helium permeability at 25°C (77°F) varied from about 30-40 x 1015 mol m s-1 N-1 at a relative density of 2.08 to about 5-15 x 10-15 moI m s-1 N-1 at a relative density of 2.15: the effect of crystallinity was much greater than that of varying the moulding pressure, or the type of PTFE polymer used. For tubing extruded from coagulated dispersion (CD) polymers, a similar effect of crystallinity on permeability was observed. At a relative density of 2.15 the permeability was about 15-25 x 10-15 mol m s-1 N-1 and this fell to about 5-10 x 1015 mol m s-1 N-1 at a relative density of 2.21. Again, no effect of CD polymer type could be detected, even though a considerable number of both homopolymers and copolymers were examined. Gerritse has measured the permeability of PTFE to oxygen and nitrogen as a function of temperature in the range 50-125°C (122-257°F); for both gases the permeation rate at 125°C was about 5-6 times greater than at 50°C. Felder, Spence and Ferrell investigated the permeation of sulphur dioxide through a range of polymers including PTFE. The permeability of PTFE to water vapour has been studied by Konovalov and by Korte-Falinski who both found that PTFE has a lower permeability to water vapour than almost any other plastics material examined. For PTFE films in the thickness range 0.050.20mm (0.002-0.008 inch), values were found equivalent to about 0.9-1.8 m2 per 24 hours, per 0.025mm (0.001 inch), at 20°C (68°F). Toren, using a special electrolytic measuring technique, obtained a value equivalent to 2.7 g/m2 per 24 hours per 0.025mm (0.001 inch), for a PTFE film 0.08mm (0.003 inch) thick. A value of 5.4 g/m2 per 24 hours per 0.025mm (0.001 inch) at 30°C (86°F) has also been quoted. These somewhat variable results for water vapour permeability of PTFE may, most probably, be explained by differences in the film fabrication techniques used, as well as by different methods of measurement.
Figure 23 shows the infra-red transmission spectrum for PTFE.
Billmeyer, using sodium yellow light and a sample of PTFE of density 2.12 reported a refractive index of:
nD= 1.376
Using a far infra-red maser and monochromatic radiation of wave-length 337 µ m Chamberlain and Gebbie report a figure of 1.391 ± 0.017.
The refractive index of PTFE would be expected to vary with density, or more strictly with crystallinity, in accordance with the equation:
(n2 - 1) / (n2 + 2) x 1 / d = k
where
n = refractive index
d = density
K = constant
The melt viscosity of PTFE is extremely high by comparison with other polymers. The observed value will depend somewhat on the experimental method used, of which the parallel plate plastometer, the capillary rheometer and creep methods are the most important. The melt viscosity of PTFE varies with the shear stress applied to the polymer and with the temperature of the polymer but, in general, commercial samples of granular polymer display viscosities of about 1011 poise in the temperature range 360 to 380°C (680 to 716°F) and at shear stresses of about 106 dynes / cm2.
![Figure 23. Infra-red transmission spectrum (specimen thickness 0.05mm [0.002 inch])](http://www.hindustan-nylons.com/images//products/advanced-info-of-ptfe/Infra-red-transmission.png)
The effect of high energy radiation on PTFE was first noted by Liversage who found that the electrical resistance of the polymer fell on exposure to X-rays. Harrington and Giberson, in a study of the decline in the tensile strength and elongation of PTFE when exposed to gamma radiation, showed that irradiation in a vacuum was less damaging than irradiation in air. This point was confirmed by Wall and Florin and a summary of their results is given below:
| Radiation dose eV / g x 10-20 | % retention of original tensile strength | |
|---|---|---|
| Irradiation in air | 2.4 4.1 |
2 0 |
| Irradiation in a vacuum | 0.7 4.1 32.0 |
73 51 43 |
As might be expected of a saturated aliphatic fluorocarbon PTFE is almost completely inert chemically. Molten or dissolved alkali metals degrade PTFE by abstracting fluorine from the molecule, while at elevated temperatures fluorine and compounds capable of releasing fluorine can break the carbon skeleton and form low molecular weight fluorocarbons. Apart from these not very important exceptions, PTFE resists attack by all the acids, bases and solvents that might be encountered in industrial practice. In addition to its remarkable chemical inertness, PTFE is not dissolved by any solvent within its normal range of working temperatures. Small quantities of solvents may be absorbed by PTFE on prolonged exposure especially at elevated temperatures but this in no way impairs the usefulness of the polymer. Rossa has given details of the effect of 79 chemicals on PTFE with, in many cases, data on weight gain
The velocity of sound in PTFE and the way in which the velocity changes with changes in temperature has been studied by Kravtsov. He showed the velocity to pass through a maximum at 20°C (68°F) in the region of the first-order transition.