When a PFA (Perfluoroalkoxy) Sheath Heater Is Used to Heat a 10% Sodium Hypochlorite Solution at 60°C, How Does the Free Chlorine Concentration (0.5 vs. 2 ppm) Change the Surface Bleaching Rate and the Time to Embrittlement?
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PFA is susceptible to surface bleaching (color loss) at 10% sodium hypochlorite, 60°C, by attack on the chromophores. At 0.5 ppm free chlorine, the time to visible bleaching is 2,000 to 4,000 hours and the time to embrittlement (loss of 50% elongation) is 8,000 to 12,000 hours. Bleaching occurs in 500-1,000 hours at 2 ppm free chlorine, while embrittlement occurs in 2,000-4,000 hours. Free chlorine reacts with unsaturated bonds in the polymer and leads to chain scission.
Mechanism of degradation of PFA by chlorine.
Free chlorine (Cl2) and hypochlorous acid (HOCl) are powerful oxidizing agents. They react with the polymer : PFA-H + Cl 2 > PFA-Cl + HCl. This substitution process causes color changes, and also chain scission at greater doses. The rate of deterioration increases with increasing chlorine concentration and temperature.
Quantitative deterioration as a function of free chlorine
Free Chlorine (ppm) Bleaching Time (hours) Time to 50% Elongation Loss (hours)
0.1 5,000-10,000 >20,000 0.5 2,000-4,000 8,000-12,000
1.0 1,000-2,000 4,000-8,000
2.0 500-1,000 2,000-4,000 5.0 200-500 1,000-2,000
Chlorine Control Manual
Free Chlorine (ppm) Life Acceptable (years)Recommended <0.5 >3 Yes
0.5-1.0 1-3 Restricted
1.0-2.0 0.5-1 No >2.0 <0.5 No
Specifying With Knowledge
Use old bleach or add a reducing agent to keep free chlorine below 0.5 ppm for a PFA heater in NaOCl at 60°C. The engineer extends heater life by adjusting the chlorine content.
