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Technology Solutions
The long and the short of perfluorinated replacements
An accidental discovery at 3M in the 1950s catalyzed the development of the first fluorochemical cleaning products—amazing chemicals that make life easier. But emerging data indicate that these chemicals have an environmental dark side. Now scientists are beginning to use their knowledge of chemistry and fluorochemical behavior in the environment to design products that do the job without harming the environment.
The most famous fluorochemical stain repellent, 3M’s Scotchgard, was discovered by accident in 1953 when an experimental compound spilled on a lab assistant’s tennis shoe. The spill kept the spot clean, and a new market was born. Scotchgard spawned a whole family of immensely successful chemicals. The Scotchgard family of products was worth $300 million annually in 2000 when 3M discovered that some of these chemicals were persistent, bioaccumulative, ubiquitous in the environment, and linked with adverse effects in laboratory tests. As regulators began to investigate the compounds, the company voluntarily withdrew those surfactants and began the search for environmentally benign alternatives.
All surfactants have a hydrocarbon-loving head and a tail that repels water. Surfactants, or surface-active chemicals, migrate to surfaces or interfaces and lower surface tension. In fluorosurfactants, the tail contains fluorine bonded to carbon. The incorporation of fluorine, the most electronegative atom, makes fluorosurfactants repel fats like oil and grease, in addition to water. Fluorosurfactants are also more effective at reducing surface tension than other surfactants. In general, the characteristics of these chemicals form a continuum based on the number of fluorine and carbon atoms they contain. The greater the number of carbon–fluorine (C–F) bonds and the longer the C–F chains, the better the surfactant.
Before environmental concerns appeared, fluorosurfactants relied on perfluorinated C–F chains that consist of 8 carbon atoms and 17 fluorine atoms, because this C8–F17 chain length was found to be optimal, according to industry scientists.
Although 3M abandoned C8–F17 chemistry, other manufacturers still make PFCAs (perfluorocarboxylates) by using C8–F17 molecules via a telomerization process. These companies include DuPont (U.S.), Atofina (France), Clariant (Germany), Asahi Glass (Japan), and Daikin (Japan).
The U.S. EPA is currently investigating the best-known of these PFCAs: PFOA (perfluorooctanoic acid). The agency’s Science Advisory Board issued a draft PFOA risk assessment in July recommending that EPA classify the chemical as a “likely” carcinogen in humans. The recommendation is a significant step that could eventually lead to regulation, according to observers.
To develop environmentally friendly replacements, the chief strategy is to shorten the C8–F17 chain, because chain length has a big impact on bioaccumulation and toxicity. The C–F bond is so strong that all C–F chains are likely to persist in the environment, but studies suggest that chain lengths of C8 and longer are more likely to bioaccumulate and to have more potent toxicity. Chains containing four carbon atoms or fewer do not appear to bioaccumulate.
Although the fluorochemicals used in cleaning products all function as surfactants, there are two main markets for these products. Stain repellents are the biggest market, worth close to $1 billion annually. But fluorosurfactants are also incorporated into polishes, paints, and coatings to make them easier to apply. This market is worth about $100 million per year, according to University of Akron fellow Barry Rosenbaum, who was previously research director at OMNOVA Solutions, a U.S.-based specialty chemicals company.
Stain repellents
At the tip of any fully fluorinated C–F chain, one carbon atom is surrounded by three fluorine atoms. Stain repellents work because upright C–F chains link together to form an immense forest, with the tips of the molecules extending above the surface of the fabric, carpet, or paper to create an invisible protective armor.
C8–F17 chains do such a great job repelling oils and water because they are strong and stiff. When packed closely, these chains form physical links and create an impenetrable barrier. In the past, using a C4–F9 group to replace a C8–F17 group didn’t work well, because the C4–F9 chains were not “stiff enough” to get to the surface, says Ji Guo, a Ph.D. student at the University of North Carolina (UNC) who is working on a project to design new stain repellents under the leadership of Joseph DeSimone. Instead of packing tightly together, the chains were floppy.
Guo, DeSimone, and Paul Resnick, an ex-DuPont chemist, added extra hydrocarbon groups to prop up the chains, so that the C–F3 tips could get to the surface. The hydrocarbon groups also promote the formation of physical links between the chains, Guo says. The UNC scientists introduced this new stain repellent at the American Chemical Society’s national meeting in Washington, D.C., in August.
Since June 2003, 3M has been using a C4-based compound, perfluorobutane sulfonate, in place of the C8-based PFOS (perfluorooctanyl sulfonate) in its Scotchgard stain repellents. 3M is also marketing two new fluorosurfactants based on perfluorobutane sulfonate. A battery of environmental and safety tests indicates that these C4-based chemicals are neither bioaccumulative nor toxic. The company considered even shorter-chained fluorochemicals but couldn’t obtain competitive performance, according to published reports. 3M declined to cooperate with ES&T for this article.
However, DeSimone claims that the product they are developing performs better than the reformulated Scotchgard. “These new compounds can go a long way toward reducing PFOA in the environment while still providing the convenience of stain-repellent coatings,” he says. The scientists have filed a patent for these new materials. Several textile companies have already expressed an interest, he adds.
Coatings
Polishes and coatings all work because they form a thin film that thoroughly covers and wets the surfaces of floors, walls, and furniture. To effectively wet these surfaces, the polish or paint must have surface energy lower than that of the item being covered. Fluorosurfactants are the most effective surfactants for lowering surface energy, so they work well at very low concentrations. They are also very stable.
In general, these fluorosurfactants consist of much smaller molecules than the large polymers used in stain repellents, according to Rosenbaum.
Because such short molecules might easily enter the environment, OMNOVA has created a range of larger fluorosurfactants (molecular weight greater than 1000 daltons) that consist of a long, flexible hydrocarbon backbone that bristles with small C–F chains. These chains can be in either the C–3F or C2–5F configuration, depending on the application.
Currently, OMNOVA’s fluorosurfactants are used in a variety of products, including anti-graffiti paint, whiteboard wallpaper, no-polish floor waxes, and automotive clear coats.
At the Fluoros meeting in Toronto in August, OMNOVA presented test results showing that its PolyFox fluorosurfactants perform as well as or better than traditional C8-based surfactants in polishes, because they create less foaming and promote a smoother, glossier finish.
PolyFox global marketing manager Joe Twichell hypothesizes that OMNOVA’s fluorosurfactants work well because the molecule is designed to get as many C–F3 groups to the surface of the polish, paint, or coating as possible. OMNOVA attaches as many as 20 of these short C–F chains to each flexible polyether backbone, he says.
OMNOVA has won new chemical regulatory approval in the U.S. and Europe, with partial approval in Japan. The company is pursuing approval in China, Korea, and Australia.
With products by several companies already on the market and more in the pipeline, the use of short-chain fluorosurfactants as an environmentally friendly solution to our collective need to be clean and green seems to be well under way. But, when they look at all of the potential applications for short-chain perfluorinated surfactants, the scientists involved say that they have not even scratched the surface.
