So-called nanocomposites are one of the hottest areas in material science today, and Ford Motor Co. has developed a new process for making them that may lead to important new applications for big parts such as body panels. The process uses sound waves to improve the compatibility between the microscopic reinforcement materials and the plastic resin that are used to make nanocomposite parts.
Reinforcement materials are key because they add important engineering properties such as strength and stiffness to plastic parts, just like adding straw to bricks makes them stronger and less likely to crack. Nanocomposites get their name from the fact that their reinforcement materials are only a nanometer — one millionth of a millimeter — thick. That's about 1,000 times thinner than glass-fiber or talc fillers typically used to reinforce plastic parts.
In automotive applications, nanocomposites are created by mixing solid microscopic particles of smectite clay with plastic resin. The use of the tiny clay particles avoids some of the harmful characteristics associated with the much larger talc, mica or glass-fiber fillers, which often make surface finishes bumpy and cause parts to crack more easily in cold temperatures.
Most importantly, nanocomposites can be much stronger and lighter than conventional plastic composites. As a result, they eventually could replace steel, aluminum and conventional plastics in applications such as body panels.
General Motors Corp, which also is developing nanocomposite technology, currently uses a thermoplastic olefin nanocomposite for the step-assist on its Chevy Astro/GMC Safari minivan.
However, nanocomposite development has been slowed by the clay's less-than-optimal interaction with typical automotive plastics such as polypropylene and polyethylene. Other nanocomposite production processes add special agents to improve compatability, but they're expensive, and they make the material softer.
Ford researchers have found that bombarding clay particles with sound vibrations during mixing causes the filler particles to better disperse throughout the design matrix — improving a part's strength without using costly agents.
The American Chemical Society recently honored the Ford scientists who developed the new procedure: Ellen Lee and Deborah Mielewski.
“What we've developed is a method that enhances the ease of dispersing by taking the clay platelets and being able to disperse them as single platelets throughout the matrix material,” explains Lee, a plastics technical specialist. “Once you have a very well dispersed material, that's what really gives it reinforcement.”
Ford still uses the basic mechanical dispersion form for nanocomposite production. The resin and clay are put through an extruder, heated and then mixed as they melt. But Ford also uses its ultrasonic sound wave method during the melt state. “Clay is very hydrophilic,” explains Lee. “It likes to be with water. But if you just put them together, it (clay) wouldn't necessarily disperse well in the water, just because of the difference in density. But if you use ultrasonics, take a wand and start vibrating with ultrasonic energy at a frequency of 20,000 hertz, that helps the dispersion. It takes the clay into separate particles, and you get a very different viscosity.”
The result, Ford says, is nanocomposite parts that are cheaper and lighter. Impact characteristics should improve, too, and the coefficient of linear and thermal expansion is lowered. “So you should be able to get around things like big gaps for clearances between doors,” Lee points out.
GM's Saturn Corp. has used plastic door panels for years. But in order to allow the material to expand and contract, larger than normal gaps are placed between the door and body side panels. In a marketplace where tight gaps define quality, plastic vertical panels can look sloppy.
Lee says mass production of nanocomposite parts using Ford's proprietary technology is still several years away.