The headline is simple: virgin PTFE resists almost every acid, base, solvent and fuel. The exceptions are few, and the filler is often the weak link.
What attacks PTFE itself
- Molten alkali metals (sodium, potassium) and their solutions in ammonia.
- Elemental fluorine and some fluorinating agents such as chlorine trifluoride at elevated temperature.
- Very high temperature above its rating, where it decomposes.
- High-energy radiation. PTFE is one of the least radiation-resistant polymers. Gamma doses of tens of kGy cause chain breaks and loss of strength.
- Some perfluorinated liquids at high temperature can swell it slightly.
What PTFE absorbs
PTFE does not absorb water. It can absorb small amounts of some halogenated solvents and fluorocarbon oils, causing slight swelling that reverses on drying.
Filler risks
| Filler | Avoid | Notes |
|---|---|---|
| Glass | Hydrofluoric acid, hot concentrated caustic, fluorine-releasing agents | Silane-treated glass improves but does not remove the limit |
| Bronze | Mineral acids, ammonia, steam, seawater, oxidisers | Copper alloy corrodes; discolouration |
| Carbon | Strong oxidisers at very high temperature (hot nitric, chlorine) | Otherwise inert |
| Graphite | Hot oxidising acids | Otherwise inert |
| MoS2 | Strong oxidising acids and alkalis | Check at temperature |
| Stainless steel powder | Chlorides at temperature, certain acids | Magnetic and conductive |
Temperature changes everything
Compatibility charts usually state 20 degrees C. At 150 to 250 degrees C, permeation increases, the filler corrodes faster and fluids can penetrate through the polymer and attack metal behind it (a lined pipe may corrode on the outside of the liner). Always give the supplier the real operating temperature.
Permeation is not attack
Gases and small molecules (HCl, water vapour, chlorine, hydrocarbons) diffuse slowly through PTFE without damaging it. In lined equipment this can corrode the substrate behind the liner, so vent paths or thicker or modified-PTFE liners are used.
Method
- List every fluid, including cleaning and upset media.
- Give concentration and temperature.
- Check the filler against each fluid, not only the base polymer.
- Run a soak test when the combination is unusual, then measure weight, size and hardness and look for filler staining or loss.
