PTFE is very resistant to chemical attack and thermally stable, which is mainly due to the many C-F bonds present within the polymer. The table below shows the bond strenghs of several C-X bonds:
| Bond | Bond enthalpy/kJ mol-1 |
|---|---|
| C-F | 484 |
| C-H | 416 |
| C-Cl | 318 |
| C-Br | 276 |
| C-I | 238 |
The very strong C-F bonds means polymers containing a -CF2- chain are very stable from both an energetic and kinetic viewpoint. Fluorine atoms form a more effective shield around the C-C backbone of the polymer because they are slightly larger and so obstruct the approach/attack of other reagents. Fluorine atoms are also not very polarizable therefore there are weaker intermolecular forces between PTFE chains than in hydrocarbons which results in higher volatilities and solubilities for fluorocarbons.
The melting point of PTFE is around 330°C (compared to that of polythene - a hydrocarbon which is only around 120°C) and it is so thermally stable that its electrical and mechanical properties don't change at temperatures of about 250°C for long intervals (months). Also, PTFE doesn't show conventional melting behaviour because it doesn't liquify on heatng. Instead it forms a high viscocity, transparent material that fractures upon applied force.
Surface hydrophobicity is estimated by the contact angle formed when a drop of water is placed on the surface of the polymer. PTFE has a contact angle of 107° which is high and shows there is little interaction bewteen the polymer and the water droplet making PTFE a waterproof (hydrophobic) material.
The surface tension of PTFE is lower than hydrocarbons at both 20°C and 40°C:
| Material | Surface tension at 20°C (dynes/cm) | Surface tension at 40°C (dynes/cm) |
|---|---|---|
| PTFE | 23.9 | 16.9 |
| Polythene | 35.7 | 28.8 |
This means PTFE has very low values of coefficient of friction - in fact, the lowest of any solid and so PTFE has been listed in the the Guiness book of World Records as the Slipperiest material in the world!
PTFE can withstand attack from concentrated acids/alkalis, reducing/oxidising agents and most other chemicals. There is also no know solvent that can dissolve it unless at temperatures of around 300°C. This excellent chemical inertness, accompanied with its heat resistance, non-stick, slippery, hydrophobic surface, non-melting and good electrical insulation properties means PTFE is a first choice material for many applications.