PTFE is a crystalline polymer with the crystallite melting point of 327°С and the noncrystalline vitrification temperature of -100 to -120°С. Even at a temperature higher than the decomposition point (415°С), PTFE does not go into the plastic state (at 370°С, the viscosity of molten PTFE ≈1011P, i.e. 1,000,000 greater than viscosity required for die-casting). Therefore, it can only be processed by sintering pressed preforms.
Depending on the cooling rate (to a temperature below 250°С), the sintering process can produce hardened items with the crystallinity of ≈50% and specific gravity of ≈2.15 g/cm3, or non-hardened items with the crystallinity above 65% and specific gravity above 2.20 g/cm3.
Within the operating temperature range of -269°С to +260°С, the crystallinity achieved during this cooling process does not change, while at a temperature higher than 260°С crystallinity gradually increases. Crystallinity increases sharply at a temperature of 310 - 315°С.

   The absence of porosity is evidenced by full transparency of a specimen at 370-390°С. Even insignificant porosity makes the specimen opaque. Porosity of about 0.1-0.2% significantly affects the accuracy of determination of specific gravity.
   Data on correlation between specific volume and specific gravity and the temperature for a specimen with the crystallinity of 68% (specific gravity of a slowly cooled item) is provided below:

* Heating from 19.6 to 22°C increases specific volume by 0.74%
** At 327°С, specific volume increases by 20%.

PHYSICAL AND MECHANICAL PROPERTIES OF POLYTETRAFLUOROETHYLENE (PTFE)

Main physical and mechanical properties of polytetrafluoroethylene (PTFE) are specified below:

   Data on correlation between physical & mechanical properties of PTFE and the temperature is provided below.

Physical & mechanical properties of PTFE at low temperatures

    One of the key strength parameters is tensile yield stress, i.e. a stress leading to residual deformation. This is dependent on the crystallinity, stretching speed and temperature. At the crystallinity of 65% and stretching speed of 100 mm/min., the correlation between the yield stress and absolute temperature Т (in К) is described by the following empirical function (valid to the temperature range of 20 to 300°С):
                                          lgσT= 0,53166+483,64/Т
   Yield stress values calculated for some temperatures using this formula are provided below:
Temperature, °С ........................ 25.........50.........75......100.......150......200......250
Yield stress, kgf/cm2 ....... 42,4.....106,9.....83,5.....67,2.....46,6.....35,5.....28,6

   Under long-term exposure to loads, residual deformations occur at lower stresses (40-50% of those calculated using the formula).
   PTFE items should be designed taking account of creeping. Creeping (deformation under long-term exposure to loads) is calculated using the following formula:
                                        lgγt=lgγ1+a·lgt
where γt - deformation for t days; γ1 - deformation for 1 day; а - factor which is mainly dependent on the temperature and, to a lesser degree, on the load if it does not exceed 40-50% of the yield stress.
   Values of the a-factor and some data on creeping for specimens with the crystallinity of 50% are provided in the table below. Deformation during 1 day (γ1) under other loads and temperatures should be determined empirically. Where the crystallinity is 65-68%, the creep is lower.

   PTFE creep

   
                                           PTFE ANTIFRICTION PROPERTIES

Data on correlation between the coefficient of friction and loads [static and dynamic (at a low speed); coefficients of friction of PTFE on steel without lubricants are the same] are provided below:

Where lubricants are used, it is about 2 times lower.
   The dynamic coefficient of friction of PTFE on steel without lubricants under a load of ~ 20 kgf/cm2 depends on the slip velocity:

    Where a filler is used, the coefficient of friction somewhat increases at low slip velocities, а while at high slip velocities it is lower than coefficient of friction of virgin PTFE on steel.
   At 327°С (on the friction surface), the coefficient of friction of PTFE on steel significantly increases (a few times) which results in disastrous wear and fast destruction of the bearing.

                                  THERMOPHYSICAL PROPERTIES OF PTFE

Unsintered PTFE (powder form) has the crystallinity of 95 - 98%, while sintered PTFE has the crystallinity of 50% (hardened) to 68 - 70% (non-hardened). At a temperature below 19.6°С, unit cell of PTFE consists of 13 groups of CF2, while at temperatures higher than 19.6°С it consists of 15 groups of CF2. At the temperature of 19,6°С, the triclinic packing turns into a less ordered hexagonal lattice, and the volume of crystallites increases by 0,0058 cm3/g (1.2 vol. %), or the volume of the specimen with the crystallinity of 68% increases by 0.74%. Where external pressure is applied, the transition point decreases by 0.013°С per each atm. At the temperature of 30°С, the second transition of the crystalline structure occurs, but the volume changes by only 1/10 of the volume change at 19.6°С. Under high pressure (4,500 kgf/cm2 at 70°С), the third transition of the crystalline structure occurs.
The glass transition temperature of non-crystalline sections as determined based on the brittleness temperature varies from -97 to -100°С, while the glass transition temperature of non-crystalline sections as determined based on the inflection point of the modulus of elasticity is -120°С. The temperature of transition from a non-crystalline solid state to the supercooled liquid state is 127°С.
At the temperature of 327°С, PTFE crystallites melt, and PTFE becomes fully amorphous (non-crystalline), totally transparent (if there is no porosity) and highly elastic but does not flow (viscosity is higher than 1011 P). The volume increases by 20%.
The melting point depends on external pressure and increases by 0.154 °С per each atm. When molten PTFE cools off to a temperature below 327 °С, the specimen becomes opaque, non-transparent and milk-white. The crystallization rate depends on the temperature (maximum rate is achieved at 310-315 °С), exposure of PTFE in a molten state to a temperature of 370-390 °С (the longer the sintering period, the faster the crystallization of the specimen) and average molecular weight of polymer (the lower the molecular weight of polymer, the faster the crystallization). The indirect PTFE molecular weight estimation method is based on these factors: a PTFE plate specimen with a thickness of 2 mm2 is sintered at 370 °С during 13 hours and then cooled off from 370 to 250 °С during 5 hours. The molecular weight can be estimated based on the specific gravity of the specimen at 23 °С: 2.16-2.19 g/cm3 for a high-molecular polymer and 2.20-2.22 g/cm3 for a low-molecular polymer.

Some of the thermophysical properties of PTFE are summarised below:

The thermal coefficient of linear expansion depends on the temperature:

   In practice, it is more convenient to use average values of the thermal coefficient of linear expansion for certain temperature ranges. It should also be taken into account that internal stresses frequently occur in heated PTFE items, leading to irreversible size changes. In some cases, the specimen may contract instead of expected thermal elongation.   The values shown below apply to specimens with zero internal stresses:

     * As compared to the size at 25°С.

ELECTRICAL PROPERTIES OF PTFE

Electrical properties of PTFE are summarised below:

Data on the correlation between the dissipation factor and frequency is provided below:

PTFE dielectric conductivity at frequencies of up to 1,010 Hz depends on specific gravity rather than frequency: є= 1+0,238*d
                         2-0,119*d 
where d - specific gravity at the given crystallinity and temperature.
The dissipation factor remains constant at a temperature of -60 to 250^оС.
Heating PTFE at 300^оС during 6 months does not affect dielectric properties of PTFE.

CHEMICAL AND OTHER PROPERTIES OF PTFE

PTFE has the highest resistance of all known materials such as plastics, metals, glasses, enamels, alloys, etc. It is absolutely inert to acids, oxidants, alkalis and solvents. PTFE only reacts with molten alkali metals and their ammonia, naphthalene and pyridine compounds, as well as trifluorochlorine and elementary fluorine at high temperatures. At a temperature above 327^оС, PTFE swells in liquid fluorocarbons, e.g. perfluorokerosene. At the temperature of 20^оС, PTFE swells a little (3 - 9%) in fluorinated and chlorinated gases (freons).
At a temperature above 350^оС, PTFE reacts with alkali earth metals and their compounds (oxides and carbonates), and also with oxides of some other metals (lead, cadmium, copper).
PTFE is not wetted during a short dip time (wetting angle 126^о) but can be wetted by a long-term exposure to distilled water (15 - 20 days). In brines (e.g. sea water), a film of salt is formed on the surface of PTFE after 15 - 20 days. This film can be washed off by distilled water.
Water absorption during 24 hours (and longer periods) is lower than the weighing error (0.00%).
PTFE is totally resistant in tropical conditions and is not affected by fungi (however, it does not inhibit their growth, either).
PTFE moisture permeability at 20^оС is 3·10-9 - 6·10-9 g/(cm·h·mm of mercury column);
PTFE vapour permeability at 20^оС is 0.6·10-9 - 1.2·10-9 g/( cm·h·mm of mercury column).
Data of gas permeability of PTFE films (non-porous) with the thickness of 0.1 mm at 20^оС [in cm3/(cm·sec· mm of mercury column)] is provided below:

Permeability may increase by up to 1,000 times in porous PTFE.
Only thin PTFE films can be used for visible light applications: