Work in the laboratories 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 deg C(73°F ± 2 deg F). permeability of fabricated PTFE items has shown that the permeability is very dependent on crystallinity (as indicated by relative density).
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°(680 to 716°F) and at shear stresses of about 106 dynes/cm2
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. 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.
The effect of radiation dose on mechanical properties, as it is found that as little as 0.01 to 0.1 Megarad dose can affect mechanical properties. 1 Megarad has a measurable effect and 2—3 Megarads in air reduce strength by 40—75%. 4 Megarads reduce tensile strength to 2% of the original.
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 or even swollen 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.
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.
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