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NIST, UPenn Find Carbon Nanoparticle Fibers Reduce Flammability

by Editor1 last modified November 19, 2007 - 10:27

The National Institute of Standards and Technology at the University of Pennsylvania reports that carbon nanotubes or nanofibers may greatly improve flammability resistance

NIST, UPenn Find Carbon Nanoparticle Fibers Reduce Flammability

Video sequences reveal how different additives affect the behavior of a plastic material (PMMA) when heated under fire-like conditions.

Nanoparticle fillers--especially clays--have been shown to reduce the flammability of plastics and other polymers.

Both single- and multi-walled carbon nanotubes achieved results and paired with nanofibers, they could help eliminate trial and error in designing and producing nanocomposite materials with flame-retarding and other desired properties optimized for applications in areas ranging from packaging and electronics to construction and aerospace.

Previous work on these nanoclay flame retardants, says NIST fire researcher Takashi Kashiwagi, indicates that the additives are most effective when they migrate to form a continuous surface layer, creating a "heat shield" on top of the more flammable polymer matrix. The shield, he explains, suppresses the "vigorous bubbling" that can occur as the matrix breaks down.

However, if the plate-like nanoclay particles cluster into islands, heat escapes through cracks between them, compromising their performance as flame retardants.

To get around this problem, Kashiwagi and colleagues chose to investigate carbon nanotubes and nanofibers, which tend to be narrower and longer than nanoclays. These structures also have been shown to enhance strength, electrical conductivity and other material properties. The researchers reasoned that the extended, sinuous geometry of the tiny tubes and fibers might lend itself to forming a "continuous, network-structured protective layer" that is free of cracks.

When the researchers heated polymethyl methacrylate (PMMA)--a clear plastic--dispersed with carbon nanotubes or nanofibers, the material behaved like a gel. In a process dictated by their type, concentration, and other factors, the nano additives dispersed throughout the PMMA matrix and eventually achieved a "mechanically stable network structure." The researchers say the "jammed networks" formed as the nanocomposites underwent a change in identity, a transition from liquid to solid. The shift occurred at an optimal composition that the team called the "gel concentration."

For single-walled carbon nanotubes--sheets of carbon atoms rolled into cylinders--top fire retardant performance was achieved when the fillers made up only 0.5 percent of the total mass of the material. For multi-walled carbon nanotubes, which are nested sets of carbon cylinders, the gel concentration was 1 percent. Both types of nanotubes have the potential to surpass nanoclays as effective fire retardants, says NIST materials scientist Jack Douglas.

Results suggest that the gel concentration also may mark the point at which other nanotube-enabled improvements in material properties are maximized, Douglas adds.