Unlike micro-porous membrane permeation, which transfers water through a relatively slow diffusion process, Nafion™ tubing removes water by absorption as water-of-hydration. This absorption occurs as a First Order Kinetic reaction, so equilibrium is reached very quickly (typically with in milliseconds). Because this is a specific chemical reaction with water, gases being dried or processed are usually entirely unaffected.
Nafion™ polymer is a copolymer of tetrafluoroethylene (Teflon®) and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid. Like Teflon®, Nafion™ polymer is highly resistant to chemical attack, but the presence of its exposed sulfonic acid groups confers unusual properties. Sulfonic acid has a very high water-of-hydration, absorbing 13 molecules of water for every sulfonic acid group in the polymer; consequently, Nafion™ tubing absorbs 22% by weight of water.
Nafion™ tubing is very stable at high temperatures. As noted previously, its chemical resistance is similar to Teflon®, and very few things can attack it. Even samples containing high concentrations of very corrosive gases like hydrogen fluoride or hydrogen chloride can be dried or processed. In addition to the Nafion™ tubing, most Perma Pure dryers and humidifiers include a shell to contain the purge gas as well as fittings for gas connections to the gas stream and purge lines. Shells and fittings are available in stainless steel, fluorocarbon polymer, or polypropylene. For higher gas stream flow rates, bundles of Nafion™ tubing are formed using chemically resistant thermosetting epoxy resin. The temperature resistance properties of the dryers are dependent on these other materials as well. The gas stream does not come into contact with the shell but the fittings for gas stream connection do contact the gases.