October 1, 2005
A bulletproof nano solution
Adding a miniscule amount of nano particles to polymers may someday stop a bullet.
"You can improve the barrier to gases, such as hydrogen, carbon dioxide, and oxygen. You can increase material strength with little increase in weight," said Don Baird, professor of chemical engineering at Virginia Tech.
Adding the right amount of particles is the issue. "While 1% by weight of nano particles will change a material's properties dramatically, 2 or 3% provides hardly any additional enhancement," he said. "The particles just clump together, and thereby reduce the advantages associated with the surface area of single particles."
Another problem: The incorporation of nano particles changes a polymer's flow properties leading to potential processing problems. Baird's team developed a new method for improving the dispersion, or exfoliation, of individual nano particles into polymers.
Using supercritical carbon dioxide, the researchers are able to exfoliate nano particles at higher concentrations, leading to further enhancement of mechanical properties than possible using just mechanical mixing. "Carbon dioxide is soluble in a lot of polymers. It attaches to the particles, so they don't attach to each other, and helps disperse the particles throughout the polymer. It is a benign, natural substance," Baird said.
The rheological properties including the normal stresses (elastic properties) and the stress relaxation response monitor particle dispersion.
Researchers also discovered the changed flow behavior is good news, an indication the material will exhibit improved mechanical properties.
Baird's team observed that nano clay particles well dispersed in polypropylene and polycarbonate plastics tend to promote polymer chain orientation, or alignment, and then retard relaxation or loss of orientation. "The result is they make the polymer chains act like longer or higher molecular weight chains. The material is stronger than one would expect given the size of a polymer chain."
Pointing to a bobbin of fiber, Baird said, "If that contained nano particles and was stretched, it is possible that the fiber could be woven into a vest that would stop a bullet? An ordinary polymer material with well dispersed high levels (8 wt %) of nano particles could have exceptional mechanical properties."