http://www.sciam.com/article.cfm?chanID=sa006&colID=17&articleID=0003DDF5-2417-13A8-9E4D83414B7F0101
January 16, 2006
Scientific American
Clean and Green
Shorter carbon chains make stain beaters safer
By Rebecca Renner
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.
