Nanoparticle Networks Reduce the Flammability of Polymer Nanocomposites

06 January 2006

Nanoparticle networks may greatly improve flammability resistance in polymers, according to research from the National Institute of Standards and Technology and the University of Pennsylvania.

Results achieved with two types of carbon nanotubes (single- and multi-walled) and with carbon nanofibres could help to 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. The work appeared in the December issue of Nature Materials.

Nanoparticle fillers—especially clays—have been shown to reduce the flammability of plastics and other polymers. 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 nanofibres, 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 fibres might lend itself to forming a “continuous, network-structured protective layer” that is free of cracks.

When the researchers heated polymethyl methacrylate (PMMA) dispersed with carbon nanotubes or nanofibres, 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 networks formed as the nanocomposites underwent a change in identity, a transition from liquid to solid.

The shift occurred at an optimal composition. For single-walled carbon nanotubes, 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, 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.

The image from NIST shows the char layer formed when PMMA was filled with multi-walled carbon nanotubes. Unmodified PMMA behaves like a liquid, bubbling vigorously and leaving almost no residue.

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