For a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 100°C in a 20% Hydrochloric Acid Solution, How Does the Polymer’s Water Absorption (0.03% vs. 0.08%) Change the Permeation Rate of HCl and the Time to Acid Attack on the Resistance Wire?

PFA absorbs 0.03-0.08% water in 20% HCl @ 100°C depending on grade and processing. The HCl permeation rate through the polymer wall increases with increasing water absorption (0.08%). The HCl permeation rate is 0.02-0.05 g/m2.day at 0.03% water absorption, and the time to acid attack on the resistance wire is >10000 hours. The water absorption is 0.08% with a permeation rate of 0.08 to 0.15 g/m2-day and a time to attack of 2,000 to 5,000 hours. The absorbed water acts as a plasticizer of the polymer, increasing the free volume and permitting a quicker diffusion of HCl.

Mechanism of HCl permeation enhancement by water

The amorphous portions of PFA can absorb water molecules which enhance the free volume. The HCl molecules are spread out over these locations. The increased water content results in a higher diffusion coefficient (D) and solubility (S) of HCl. P = D x S ( Permeation Rate )

Quantitative HCl Permeation vs Water Absorption

Water Absorption (%) HCl Permeation Rate (g/m2·day)Wire Corrosion Time (hours)
0.01 0.01-0.02 >20,000 0.03 0.02-0.05 >10,000 0.05 0.04-0.08 5,000-10,000 0.08 0.08-0.15 2,000-5,000 0.10 0.12-0.20 1,000-3,000
Water Absorption Control Manual

Water Absorption (%) Recommended Grade Expected Life (1,000 hours) <0.03 High-purity PFA >10,000 0.03-0.05 Standard PFA 5,000-10,000 0.05-0.08 Off-grade PFA 2,000-5,000 >0.08Not acceptable <2,000
Creating an Informed Specification

Use PFA with water absorption less than 0.03% (high-purity grade) as a PFA heater in 20% HCl at 100°C. The engineer reduces the permeation of HCl by regulating the absorption of water and so preserves the resistance wire.