Stain repellents confer easy-to-clean convenience to carpets and clothing thanks to substances called fluorosurfactants. Yet this benefit comes at a price: the processes used to make these surfactants--which are also used to improve paints and polishes--generate chemicals that have become pervasive in the environment. Of particular concern is perfluoro¿octanoic acid (PFOA), one of the most common fluorosurfactant breakdown products. Last July the science advisory board of the Environmental Protection Agency recommended that the EPA classify it as a "likely" human carcinogen. Canada has already banned some compounds that have the potential to break down to PFOA in the environment. Chemists, however, are now changing the structure of fluorosurfactants so that they do the job but are safer and do not accumulate in the environment.
Fluorosurfactants basically consist of chains of carbon atoms surrounded by fluorine. Long rigid chains work best because they get many carbon-fluorine molecules to the surface, where they can do their jobs, and keep a large part of the chain embedded in a substrate. In stain repellents (the more demanding application), they pack tightly together so that their tips form an invisible protective armor. But chain length is also at the root of the environmental problem. Long-chain fluorosurfactants, based on a lineup of eight carbon atoms (C8), enter the body more readily than shorter analogues, and they can break down to form PFOA. PFOA and other related long-chain fluorochemicals stick to blood proteins and masquerade as digestive acids. Consequently, they are difficult to eliminate, says environmental toxicologist Jonathan Martin of the University of Alberta in Edmonton.
Scott Mabury, a University of Toronto chemist, has done the most to link fluorosurfactants and their environmental consequences. Among the options to solve the problem, he suggests, are shortening the carbon-fluorine chains to make the chemicals less bioavailable. Indeed, 3M Company took this approach when it reformulated Scotchgard in 2001. Its change from C8 to C4, however, also caused some loss of performance, so 3M and other firms are continuing to look for more effective solutions.
To replace long-chain fluorosurfactants in polishes and paints, Omnova Solutions in Fairlawn, Ohio, has patented chemicals that consist of a long, flexible polymer backbone that bristles with small (C1 and C2) carbon-fluorine chains. The flexibility enables the short chains to reach the surface and to perform as well as or better than traditional C8 polishes and waxes. Tests show that these compounds neither accumulate in fish nor degrade during wastewater treatment, a process that appears to cause conventional fluorosurfactants to release long-chain chemicals. Omnova fluorosurfactants are now being used in industrial polishes and paints, which makes them easier to apply, and in antigraffiti coatings, which cause paint to ball up.
Chemists have also achieved some success with the tougher task of making short-chain stain repellents. University of North Carolina at Chapel Hill researchers hope to patent a new antistain chemical based on short chains they unveiled at the American Chemical Society meeting last August. They found a way to stiffen the C4 chains by propping them up with extra hydrocarbon groups. These achievements may only be scratching the surface of short-chain fluorosurfactants' potential.