Nafion: Physical and Chemical Properties

Nafion: Physical and Chemical Properties

Background

Nafion™, was developed by Dr. Walther Grot at DuPont in the late 1960’s by modifying Teflon®. Nafion™ was the first synthetic polymer ever developed with ionic properties, and it started an entirely new class of polymers called ionomers. These ionic properties of Nafion™ were created by adding sulfonic acid groups, a chemical with very strong ionic properties, into the bulk polymer matrix. Nafion™ combines the physical and chemical properties of its Teflon® base material with ionic characteristics that give the final material the following properties:

  1. Like Teflon®, Nafion™ is extremely resistant to chemical attack. According to DuPont, only metallic alkali metals (sodium in particular) can attack Nafion™ directly under normal conditions of temperature and pressure. This means Nafion™ does not release fragments or degradation products into the surrounding medium.
  1. Like Teflon®, Nafion™ has relatively high working temperatures compared to many polymers. Nafion™ is used in some applications at temperatures up to 190° C.
  1. Unlike Teflon®, Nafion™ is highly ion-conductive. It functions as a cation exchange polymer.
  1. Nafion™ is a super-acid catalyst. The sulfonic acid groups attached to the Teflon® backbone within Nafion™ function as an extremely strong proton donor due to the stabilizing effect of the large polymer matrix attached to the sulfonic acid.
  1. Nafion™ is very selectively and highly permeable to water. The sulfonic acid groups in Nafion™ have a very high water-of-hydration, so they very efficiently absorb water. Interconnections between the sulfonic acid groups lead to very rapid transfer of water through the Nafion.

Applications

These unusual properties are exploited in the four main applications of Nafion™ today.

  1. Nafion™ is used to fabricate ion-exchange membranes used to produce chlorine gas and lye (sodium hydroxide) by the electrolysis of salt water.
  2. Nafion™ is used to selectively dry or to humidify gases. The largest segment of this application involves drying or humidifying breath for anesthesia, respiratory care, or plethysmography, but many applications exist in the laboratory and in industrial use.
  3. Nafion™ is used as the proton exchange membranes in polymer electrode fuel cells.
  4. Nafion™ is used as a super-acid catalyst in the production of fine chemicals.

Physical Appearance

Nafion™ is initially produced in a salt form with its sulfonic acid groups neutralized. In this form, Nafion™ is thermoplastic (extrudable and formable using heat and pressure), but not chemically active. Once extruded into its final form (typically sheet or tubing), Nafion™ is chemically activated by conversion of the salt form to the acid form. At this stage, Nafion™ is a translucent plastic that is similar in appearance to Teflon, but clearer and less opalescent.

Once activated, Nafion™ immediately begins to react with its surrounding environment. Moisture is absorbed and exchanged with the surroundings. Reactive organic gas components within the surrounding air may undergo chemical changes when exposed to the super-acid catalytic activity of Nafion. These organic gases may combine to form larger compounds that are liquid or solid in nature. Over time, these organic residues build up a deposit on the Nafion. The original color of the Nafion™ changes gradually from translucent to yellow, then brown, then even black.

The chemical nature of the Nafion™ itself is not changed by this process. The Nafion™ is not directly attacked, and the Nafion™ is not degraded. Its properties of water permeability, ion-exchange, and acid catalysis are not directly affected.

For Nafion™ to transport water (dry or humidify gases), the water must be able to reach the active sites on the surface of the Nafion. In extreme cases, deposits of organic residues can build up on the Nafion™ to the point where the surface of the Nafion™ is sufficiently occluded to reduce its functionality. When this occurs, the discoloration of the Nafion™ is severe, black rather than merely yellow or brown. Even in these extreme cases, Nafion™ is still functional, it merely loses efficiency. Loss of a few percent of its total functionality due to partial occlusion means that a gradual decline in performance will be observed, not a catastrophic failure.

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 (used by Perma Pure in its dryer service procedures) is to boil it in strong acid. Nafion™ is not damaged by these procedures, but the discolored surface residues are eliminated.