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Flame retardants that fight fire and pollution

By Martin Leggett - 31 Aug 2011 16:31:0 GMT
Flame retardants that fight fire and pollution

Flame retardants may save lives and prevent traumatic injuries - but the chemicals used are some of the most persistent chemicals clogging up the natural world. Health worries have dogged some long-banned retardants, such as polychlorinated biphenyls (PCBs), as well as their replacements, brominated flame retardants (such as PBDE).

These chemicals are fat-soluble, and so build up the food chain, working their way into breast milk, for example - or devastating killer whale numbers, as PCBs continue to do, even three decades after they were banned.

The ideal solution would be low-toxicity water-soluble retardants, ones that disperse in the environment, rather than building up. And just such compounds are being explored by chemists at Texas A&M University.

They have turned to the nano-scale to find chemicals that stop fires - without dangerously polluting the planet. "People are concerned about the potential toxicity of flame retardants that are currently used on a variety of products, especially children's pajamas and the foam in children's car seats," said team lead, Jaime Grunlan.

Building inspiration

The flame retardants that Grunlan and his team have been working on are for clothing - specifically cotton wear. Cotton is both easy to set light, and burns fast and hot, so the aim of fire retardants is to slow down that burn, and to reduce the heat. This gives the wearer a chance to get the burning clothes off before serious damage is done. The new compounds that have been developed borrow from building fire retardants, which are used on steel girders. These are painted with tumescent retardant, which swells like foam, and so smothers flames.

It is that property of swelling and smothering which the Texas team were seeking to reproduce - but at a minute scale. Alternating layers of polymers, of opposite charges, are built up layer-by-tiny-layer, soaking and coating each and every cotton fiber. The way this new retardant penetrates those fibers is a real advantage over existing approaches. Current retardants just coat the outside of the cotton layer, and can't stop the cotton from burning - only slow it down.

'Much less toxic'

This new retardant technology - which is being called nano-intumescence - can actually stop flames in their tracks. It expands, under heat, around each fiber, choking off oxygen supply. Put that together with its relatively low toxicity and persistence, and the Texan team may have a winning combination. "The water-based ingredients in this new coating are much less toxic to humans than the so-called 'halogenated' or 'brominated' flame retardants used in the past, and they are more environmentally friendly," said Grunlan.

Softening and launder-proofing still needed

There are still hurdles to overcome, however. The layered retardant currently used can make the fabric harsh - but Grunlan expects that this can be dealt with. "The look and texture of the fabric would depend on the thickness of the coating and also on the specific polymer we use," he said. A different polymer could be used, to make things softer and more flexible.

More seriously, 'water-soluble' means the retardant could be lost after repeated washing. Grunlan concedes that more research will be needed to make it launder-proof. "We haven't done anything yet to protect the coating, but we believe that with further research, we could make this an almost permanent addition to the fabric," he conjectured.

Top Image Credit: © RmWood