Fighting flames with greener materials

New flame retardants rely on alternating nanolayers of relatively nontoxic materials

Materials scientists in Texas have developed flexible coatings mere billionths of a meter thick that keep cotton clothing from going up in flames and plastic foam from melting. Unlike the widely used but potentially toxic flame retardants they’ve been designed to replace, these nano-coatings appear relatively safe, their designers say.

FOAMING BUBBLES An electron micrograph shows cotton fabric coated with 20 paired layers of chemicals before (left column) and after (right column) burning. Bubbles in the lower right photo show the coating’s heat-triggered foaming. Y.-C. Li et al/Advanced Materials 2011/Wiley-VCH Verlag

Yu-Chin Li and Jaime Grunlan of Texas A&M University in College Station described their team’s new technologies August 31 at the American Chemical Society national meeting in Denver.

Because fabric fibers are so thin, “being able to fire-retard them is a big deal,” observes chemist Charles Wilkie of Marquette University in Milwaukee, a fire-retardant specialist not involved with the new work. “So I’m encouraged. The new work is impressive.”

Grunlan’s team has been seeking safer alternatives to brominated fire retardants, some of which have been banned over concerns about their potential toxic effects. The researchers’ initial prototypes consisted of alternating layers of garden variety clay and a commercial polymer. But the polymer, a synthetic chemical, did not rely on renewable, green constituents. So the engineers swapped it out for an inexpensive waste material: chitosan, a natural compound extracted from shrimp and lobster shells.

Like the earlier clay-based recipes the group had worked with, the new formulation proved disappointing on cotton fabric, Grunlan says. But it was a game changer for plastic foam, such as the type used as cushioning in furniture. Untreated foam held over a propane torch flame for 10 seconds quickly ignited, melted and burned up. But after applying 10 dual layers of clay and chitosan to an identical piece of foam, the 10-second flame created a thin veneer of char but left the interior unscathed.

Still intent on finding fabric protectants, the team turned to materials that intumesce — undergo a foaming chemical reaction — at high temperatures. In the construction industry, millimeter-thick intumescent coatings on steel girders protect a skyscraper’s skeleton. Grunlan’s group scaled the technology down to nanometer-thick alternating layers of the compounds, polysodium phosphate and poly-allylamine. 

When cotton fabric treated with 10 alternating layers of each chemical was exposed to a flame for 10 seconds, the fabric charred but didn’t burn up, Grunlan says. The only sign of damage was localized, minor charring where the flame touched the fabric. Li, Grunlan and colleagues published details of these experiments online July 29 in Advanced Materials.

These data “sound really good,” Wilkie says — if the concern is preventing ignition of flammable materials. But in the real world, fires last considerably longer than 10 seconds, he points out, so these materials might retard burning or limit its spread, but wouldn’t withstand an inferno.

Vince Baranauskas of NanoSonic Inc. in Pembroke, Va., a firm that engineers materials including fire retardants, cautions that nano-layered flame retardants may not be feasible on a commercial scale. “Alternating dips would cost a great deal more than the fabric itself,” he says.

But Serge Bourbigot of the University of Lille in Villeneuve d’Ascq, France, remains optimistic. The new nanolayer fire retardants — with or without intumescence — are novel and show great potential, he says, even if they might need refinements before they are marketed.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

More Stories from Science News on Chemistry

From the Nature Index

Paid Content