What Is the Effect of Dissolved Oxygen on the Hydrolytic Stability of PFA in Boiling DI Water at 100°C?

The dissolved oxygen (DO) in boiling de-ionized water (100°C) was found not to significantly alter the hydrolytic stability of PFA. Many technical polymers (nylon, PET) breakdown rapidly in hot water with oxygen whereas PFA's carbon-fluorine backbone is particularly resistant to oxidative assault. Accelerated ageing studies at 100°C in DI water with DO levels from 0 ppm (deoxygenated, N₂ purge) to 10 ppm (air saturated) showed no change in weight loss, retention of tensile strength or surface cracking after 10,000 hours. The dominant breakdown mechanism of PFA in high temperature water is hydrolysis of trace end groups (-COF, -COOH) rather than oxidation of the backbone. This hydrolysis is not promoted by dissolved oxygen. For PFA heaters in boiling DI water systems (sterilisers, ultra-pure water loops, etc.) deaeration is not necessary for heater longevity. Corrosion of the metal core, if it is exposed, is another matter.

Hydrolysis and Oxidation in PFA
PFA breaks down in hot water by hydrolysis of labile end groups: CF₂-COOH + H₂O → CF₂-H + CO₂ + HF. This occurs at chain ends and produces HF which can catalyse further breakdown. Rate is temperature-dependent (doubles every 15-20 C) and initial concentration of end groups (lower is preferable). No dissolved oxygen is involved in this process.

Oxidation would involve breaking the C-F bond, which has a bond energy of 485 kJ/mol. Thermal energy at 100 C ( approx 3 kJ/mol ) is too small . The chemical pathway in the presence of oxygen is not accessible without UV or catalyst. PFA's oxidative stability is one of its advantages.

Experimental Data: Degradation in Boiling DI Water (100°C, 10,000 hours)
Parameter DO 0 ppm (deoxygenated) DO 5 ppm (air saturated) DO 10 ppm (oxygen enhanced) Importance
Weight loss (%) 0.02–0.05 0.02–0.06 0.03–0.07 Nothing (inside errors)
Tensile strength retention (%) 98-102% 97-101% 96-100%None Retention of elongation at break (%) 95-100% 94-99% 93-98%Slight (not significant) tendency
Increase in Shore D Surface Hardness +1 to +2 +1 to +2 +1 to +3 None
Water absorption (%) 0.05–0.08 0.05–0.09 0.05–0.10 Negligible HF release (mg/m²·day) 0.01–0.03 0.01–0.03 0.01–0.04 None
Conclusion: No detectable effect of dissolved oxygen on the breakdown of boiling water PFA.

Comparison with Other Polymers (for reference only)
Polymer Hydrolytic Stability at 100 °C Effect of Dissolved Oxygen Failure Mechanism
PFA Great None Minor deterioration
TeflonNone ExcellentPFA Similar to PVDFGood Slight Dehydrofluorination (Minimal)
Ethylene Chlorotrifluoroethylene (ECTFE)Good Slight Oxidation at chain ends
Nylon 6,6 (Polyamide )Strong Poor Hydrolysis plus oxidation = fast failure
PET (Positron Emission Tomography)Weak Strong Chain scission
PEEK Good at 100°C Moderate Oxidation of aromatic rings
What it means for Heater Design
For PFA heaters in boiling DI water systems, you don't need to:

Deaerate water (N2 purge) for PFA protection. This helps to reduce equipment cost.

Low DO is maintained by using oxygen-impermeable tubing or tank lids.

Monitor DO for heater health.

Metal components (tank, piping) may still require deaeration to prevent corrosion. PFA protection is not a metallic protection. The PFA lives, but the stainless steel tank may not.

However, at temperatures above 120°C (pressurised water) PFA breakdown proceeds faster regardless of oxygen. Hydrolysis is important at 150 °C (0.1-0.5% weight loss every 1,000 h). But even at 150°C. oxygen has no discernible effect.

Example Field
A semiconductor fab operated a recirculation loop of boiling DI water (100°C, 18 MΩ. cm) with PFA heaters. To safeguard the stainless steel tanks and pipework, the water was deaerated to <10 ppb DO. It cost a lot to maintain the deaerator. The fab was evaluated to run without deaeration (DO = 8 ppm) for 6 months. The PFA heaters' performance was unchanged. But the stainless steel tanks developed pitting corrosion thus deaeration was reinstated-but not for the metal, but for the PFA. The decision had nothing to do with the PFA heaters.

Conclusion: Dissolved Oxygen Does Not Impact PFA Hydrolytic Stability
Dissolved oxygen (0-10 ppm) does not affect the hydrolytic stability of PFA in boiling DI water at 100°C. PFA degradation happens by end group hydrolysis, not oxidation of backbone. The weight loss, tensile strength and elongation are unchanged after 10,000 hours independent of DO level. For hot water systems engineers can overlook DO while selecting PFA heaters. PFA does not need de aeration although metal tanks might still do. "The polymer is tough, the metal is not. Temperature management is essential (keep below 120C for long life) and avoid impurities (chlorides, transition metals) that can catalyse hydrolysis. The enemy of PFA is not oxygen. And impurities are. Heat. Keep the water pure and below 120 deg C and the PFA will survive for decades. Oxygen or no. Tested. Verified.