Which substances is PTFE not chemically resistive to? Key Limitations for Safe Material Selection

PTFE (Polytetrafluoroethylene) is renowned for its exceptional chemical resistance, making it a preferred material in industries like chemical processing, pharmaceuticals, and food handling. However, it does have limitations when exposed to certain highly reactive substances, particularly alkali metals and fluorine compounds under specific conditions. Understanding these limitations is crucial for selecting the right material for applications involving aggressive chemicals, especially when considering custom PTFE parts tailored for specific environments.

Key Points Explained:

1. Alkali Metals

   • PTFE is not resistant to molten or dissolved alkali metals, including:
     • Sodium (Na)
     • Potassium (K)
     • Rubidium (Rb)
     • Cesium (Cs)
     • Francium (Fr)
   • These metals can aggressively attack PTFE, especially at elevated temperatures or in molten states, leading to material degradation.

2. Fluorine and Fluorine Compounds

   • PTFE reacts with:
     • Elemental fluorine gas (F₂)
     • Fluorine-based compounds like chlorine trifluoride (ClF₃) and oxygen difluoride (OF₂)
   • The reactivity increases under high pressure, high temperature, or concentrated exposure, making PTFE unsuitable for such environments.

3. High-Temperature Limitations

   • While PTFE is stable up to 260°C, it decomposes at ~400°C, releasing toxic gases.
   • Prolonged exposure to temperatures near its upper limit can reduce its chemical resistance, even to otherwise compatible substances.

4. Exceptions in Food and Common Chemicals

   • PTFE excels in resisting food products (e.g., oils, dairy, acids like vinegar) and most industrial chemicals (acids, bases, solvents).
   • This makes it ideal for non-stick coatings, lab equipment, and custom PTFE parts in food processing or pharmaceutical applications.

5. Comparative Advantage Over Other Plastics

   • PTFE outperforms most polymers in chemical inertness, with no solubility in solvents and negligible water absorption.
   • Its non-flammability and broad compatibility justify its use despite the few reactive exceptions.

Practical Considerations

When designing custom PTFE parts, always evaluate the operating environment for potential exposure to alkali metals or fluorine compounds. For instance, in semiconductor manufacturing (where fluorine plasmas are used) or nuclear applications (involving molten metals), alternative materials like PEEK or ceramics may be necessary. Conversely, for handling corrosive acids or food-grade applications, PTFE remains unmatched. Always cross-check chemical compatibility charts for specific combinations of temperature, concentration, and duration of exposure.

Summary Table:

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