Why does Nafion® membrane tubing turn brown over time?
Reactive organic gas components within the surrounding air may undergo
chemical changes when exposed to the super-acid catalytic activity of Nafion,
which may combine to form larger compounds that are liquid or solid in
nature. Over time, these organic residues will build up a
deposit on the Nafion causing the original color to gradually change from
translucent to yellow, then brown, then even black.
Under typical storage or operating conditions, Nafion will turn somewhat
yellow within a year and brown within three to five years. Even after turning
brown, the Nafion is still fully functional, and no loss of performance
efficiency is likely to be observed.
These unwanted chemical reactions in the air surrounding Nafion are
stimulated by exposure to light and to elevated temperatures. Storage in sealed
bags in the dark will extend the pristine original appearance of Nafion.
The original clear appearance of Nafion can be restored by cleaning. Various
cleaning solvents may be used if desired, usually non-polar solvents such as
hexane. The most effective cleaning method is to boil it in strong acid. Nafion
is not damaged by these procedures, but the discolored surface residues are
eliminated.
More information can be found on this site: http://www.permapure.com/TechNotes/Nafion%20physical%20&%20chemical.htm