Polymers and supercritical carbon dioxide

We have learned so far that the reaction that joins small molecules to form big macromolecules is called polymerization. This page is about the polymerization used to make fluoropolymers. So what is a fluoropolymer? It is just a polymer that contains fluorine atoms. That isn't so hard to understand, now is it? A fluoropolymer you're probably familiar with is polytetrafluoroethylene (PTFE), also known as Teflon^¨. PTFE is the material that keeps food from sticking to frying pans and is also a major component of the Gore-Tex^¨ that keeps you warm and dry on cold, rainy days.

PTFE is made by polymerizing tetrafluoroethylene (TFE). During the polymerization, TFE is dissolved in chlorofluorocarbons (CFCs). But CFCs are not the best materials for the envirnoment. These compounds have been shown to cause depletion of the ozone layer. So lately, DuPont, a big maker of fluoropolymers, has been developing a new type of solvent for the polymerization of TFE, with help from a chemistry professor at the University of North Carolina named Joseph DeSimone. Instead of CFCs, DuPont dissolves the TFE in supercritical carbon dioxide.
No, no, supercritical carbon dioxide isn't CO₂ that goes around criticizing everything and everybody. Let's explain. At atmospheric pressure, many materials can be either solids, liquids, or gases. Take water, for example. You know that if the temperature is cold enough, less than 0ĄC, water freezes and becomes solid ice. Warm the ice above 0ĄC and it will melt to become liquid water. If you keep heating the water to above 100ĄC, it will boil and become gaseous water vapor.
But above a certain pressure, called the critical pressure, the boundary between the liquid phase and the gas phase can become blurred. At these pressures, we don't talk about the material as being either a liquid or a gas. Instead, we say the material is in the supercritical phase. Supercritical materials are in some ways like gases and in some ways like liquids. Supercritical CO₂ is a very powerful solvent, capable of dissolving many substances, including tetrafluoroethylene.
Unlike CFCs, supercritical CO₂ does not destroy ozone. In fact, it is obtained by extracting it from air. So if supercritical CO₂ escapes into the atmosphere, we've only put back into the atmosphere something we've taken from it. A test factory is being made by DuPont in Fayetteville, North Carolina. If this technology works, full-scale use of environmentally friendly supercritical CO₂ to make PTFE could begin by 2006.
Would you like to know more about how we learned about CFCs and how they can destroy ozone? Read the story of Mario Molina in the Faces in the Molecular Sciences Environment module.

For more information, at other Web sites...

Curiosity Motivates SFU Chemist in Study of 'Supercritical' Water — read about supercritical water and waste disposal, from Simon Fraser University.

References

McCoy, Michael. "DuPont, UNC Partnership Yields Teflon Investment," Chemical and Engineering News, 26 April 1999, 77, 10.

Copyright ©2001 The Chemical Heritage Foundation