Why Might a Very Pure (Low-Ash) PFA Grade Heater Show Earlier Dielectric Breakdown in High-Humidity Environments?

For essential applications requiring low extractables and ionic pollutants, very pure PFA grades (low-ash, semiconductor-grade) are specified. These grades have ash content <0.01% (versus 0.1-0.5% for normal PFA). Paradoxically, very pure PFA heaters can show sooner dielectric breakdown than conventional PFA heaters in high humidity conditions (≥80% RH). That's because conventional PFA contains trace metal oxides (calcium oxide, magnesium oxide, aluminium oxide) that are water-absorbing fillers or stabilisers. These oxides react with moisture to generate hydroxides which devour water molecules and maintain high electrical resistance. These moisture scavenging organisms are absent in very pure PFA. The absorbed water molecules stay free in the polymer matrix, enhancing the ionic conductivity and reducing the dielectric strength. In high humidity environments, the electrical dependability of normal PFA can be better than ultra-pure PFA.

Mechanism of Moisture Absorption and Dielectric Breakdown
PFA takes up 0.03-0.10% water by weight at 100% RH depending on temperature and crystallinity. The absorbed water molecules are mobile, and can conduct ionic current in an electric field. If the electric field strength is higher than the material can withstand, usually 15–25 kV/mm for dry PFA, dielectric breakdown will occur. Absorbed moisture reduces the breakdown strength to 10-15 kV/mm at 80% RH and 5-10 kV/mm at 100% RH.

Standard PFA usually comprises 0.1 to 0.5 percent inorganic fillers, often calcium oxide, magnesium oxide, or aluminium silicate, which are used as acid acceptors to stabilise the polymer during processing. These oxides are hygroscopic : CaO + H2O --> Ca(OH)2 This reaction uses up free water and chemically combines it . Bound water is substantially less mobile and conductive than free water. The filler particles also serve as scattering points for electrons, increasing the breakdown path length. Very pure PFA contains no such fillers (ash <0.01%). The water that is absorbed is left as free water and creates conduction paths. Less filler content also means less physical impediments to electron avalanche.

Standard PFA vs Utra-Pure PFA Dielectric Strength
PFA Grade Ash Content (%)Water Absorption at 60°C, 80% RH ( %)Dielectric strength (kV/mm) at 80% RH Insulation resistance drop time (<100 MΩ) at 85°C, 85% RHTypical Application Standard (industrial) 0.15 – 0.30 0.05 – 0.07 12 – 15 8,000 – 12,000 hours Industrial general, high humidity
High-purity 0.05-0.10 0.06-0.08 11-13 5000-8000 hoursFood, Pharmaceuticals
Ultra-pure (low-ash) 0.005–0.01 0.08–0.10 8–10 2,000–4,000 hours Semiconductor, critical laboratories
Ultra-pure with desiccant filler (special) 0.01–0.02 (filler added) 0.04–0.06 11–14 6,000–10,000 hoursHigh Humidity + High Purity
Standard with moisture scavenger 0.20–0.40 0.04–0.06 13–16 10,000–15,000 hTropical, outdoor settings
Field Evidence from Semiconductor Manufacturing
The semiconductor fab installed ultra-pure PFA heaters (ash <0.01%) in a wet bench with 85% RH, 85°C process temperature. Heaters lasted an average of 2,500 hours before the insulation resistance dropped below 100 MΩ and triggered ground fault alarms. For the same application the fab converted to normal PFA heaters (ash 0.2%) with an expectation of reduced life due to greater extractables. All the conventional heaters surprisingly survived 9000 hours. Analysis indicated that conventional heaters had magnesium oxide filler that absorbed moisture and so maintained high resistance. Ultra-pure heaters had no filler and free water built up at the interface to the PFA-metal resulting in leakage. The fab now uses normal PFA for high humidity procedures and reserves ultra-pure PFA for dry processes (<30% RH) when extractables are the main focus.

Mitigation of Ultra-Pure PFA in Humid Service
For high humidity situations (e.g. semiconductor wet benches with process produced humidity) where ultra-pure PFA heaters are required, three mitigations improve dielectric reliability:

Nitrogen purge: Apply a constant stream of dry nitrogen to the cold side of the heater. The positive pressure keeps humid air from touching the PFA-metal interface. Nitrogen Purging increases Insulation Resistance Life 5-10X.

Lower watt density: Reduce the electric field stress by running at a lower voltage (208V vs. 480V) and/or utilising a larger heater (lower W/cm²). The lower field strength lessens the driving power for leakage current.

Moisture getter: Install a molecular sieve pack (zeolite or silica gel) in the cold region. The getter takes up any moisture that leaks through the PFA. Change the getter every 6 to 12 months.

For applications that need high purity and high humidity (e.g. pharmaceutical humid reactors), use ultra-pure PFA with additional moisture-stabilizing filler. Various manufacturers make "high-humidity grade" ultra-pure PFA with 0.01-0.02% calcium oxide or synthetic hydrotalcite. These grades have dielectric strength of 12-14 kV/mm at 85% RH, similar to normal PFA, while maintaining low extractables.

Conclusion: Standard PFA is superior to Ultra-Pure under high humidity
Very pure (low ash) Where moisture absorbers are not added to the filler, PFA heaters tend to break down earlier than normal PFA in high humidity settings (≥80% RH). The ionic conductivity is increased by the free water in the polymer matrix and the dielectric strength decreases from 12–15 kV/mm to 8–10 kV/mm. Standard PFA with 0.1–0.5 % ash will survive 2–4× longer than ultra-pure PFA in humid service. Engineers should not assume "purer is better" across all situations. For dry applications, ultra-pure PFA offers the highest degree of cleanliness. In humid applications, normal PFA or ultra-pure with moisture-stabilizing filler is preferable. If both low extractables and high humidity are desirable for application, provide nitrogen purge or moisture getters. Moisture-induced dielectric breakdown is a limitation of the design, not of the material. Choose the PFA grade according to the surroundings, not a theoretical purity ideal. A little ash is a nice thing in humidity.