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Researchers
at the National Institute of Standards and Technology
(NIST) have discovered that the addition of carbon
nanotubes to a common commercial polymer, polypropylene,
leads to dramatic changes in how the molten polymer
flows. This process eliminates a widespread manufacturing
headache known as "die-swell" in which polymers
swell in undesirable directions when passing through
the exit port of an extruder (a machine for producing
more or less continuous lengths of plastic sections).
Researchers have been adding small amounts of nanotubes--tiny
tubes of carbon about 1,000 times thinner than a human
hair--to polypropylene in hopes of dramatically enhancing
the material's strength and other properties. Once
realized, this enhanced polymer could be processed
at high speed through extruders for use in manufacturing.
NIST materials scientists were concerned that because
nanotubes make the polypropylene rubbery, the material
would be difficult to process or its enhanced properties
would be lost. To their surprise, the opposite proved
true. When sheared (forced) between two plates, the
polymer normally separates the plates. However, when
nanotubes are added, the plates are pulled together.
The scientists discovered that this "pulling-together"
completely alleviated die-swell. Industry currently
uses various time-consuming trial-and-error solutions
to deal with the problem.
Eliminating die-swell should help manufacturers improve
their time-to-market by simplifying their die design
processes and enabling the controlled manufacture
of smaller components.
The NIST work appears in the August 2004 edition of
the journal Nature Materials
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