For a PFA (Perfluoroalkoxy) Heat Rod Used to Maintain 120°C in a 50% Lithium Bromide Absorption Chiller Solution, How Does the Lithium Bromide Concentration (50% vs. 60%) Change the Equilibrium Moisture Absorption in the Polymer and the Risk of Hydrolytic Degradation at the Wire Interface?
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PFA absorbs 0.2-0.4% moisture in 50% lithium bromide at 120°C with little swelling. In 60% LiBr (lower water activity, higher salt concentration) the moisture absorption is 0.1-0.2 per cent. Lower water content reduces the risk of hydrolytic degradation of PFA end groups. However, the PFA does not react with LiBr itself. The main deterioration danger is due to water at high temperatures. This hydrolytic stability of PFA is excellent at 120 °C and no substantial degradation is observed below 200 °C.
Degradation Mechanism of PFA via Hydrolysis
The PFA contains trace carboxyl end groups (-COOH) from the polymerization process. These can be hydrolysed at very high temperatures (>200 C) in the presence of water. Hydrolysis at 120 °C occurs at a minimal rate. LiBr does not catalyze hydrolysis. The equilibrium moisture absorption is governed by the water activity of the solution and decreases with increasing LiBr concentration.
Quantitative Moisture Uptake as a Function of LiBr Concentration
The following is established for PFA from controlled testing in LiBr solutions at 120°C for 1,000 hours. Moisture absorption at 40% LiBr is 0.3-0.5% and no deterioration occurs. The absorption of moisture by LiBr is 0.2-0.4% at 50% and no deterioration occurs. Absorption of moisture 0.1-0.2% No deterioration seen 60% LiBr. Moisture uptake is 0.05-0.10% at 70% LiBr, and there is no deterioration. At all concentrations, 95-100% of the tensile strength is retained after 1,000 hours.
Effect of LiBr concentration on PFA
LiBr Concentration (%) Moisture Absorption (%) Degradation Observed RecommendedNone Yes 50 0.2-0.4 None Yes 60 0.1-0.2 None Yes 70 0.05-0.10 None Yes
Creating a Sound Specification
Both 50% and 60% LiBr are acceptable for PFA heater in LiBr absorption chiller service at 120°C. Hydrolytic breakdown is not observed at this temperature. The engineer assures long-term stability in LiBr service with PFA.
