Bubbles by the Billions

If the classic 1960s film The Graduate were remade today, Benjamin might receive two words of advice instead of one: “microcellular plastics.” These polymer materials, permeated by bubbles less than 50 microns in diameter, are tough, light, and can be made remarkably thin-features that suggest all kinds of uses. Now people just have to figure out which of them to realize first.

Microcellular plastics (MCPs) are the brainchild of Nam Suh, head of MIT’s Mechanical Engineering Department, who invented the process for making them in 1979. The idea was triggered by a question posed during lunch by an Eastman Kodak executive who was trying to devise a way for his company to “reduce the amount of plastics used in their products, and thus reduce manufacturing costs, without sacrificing physical properties,” Suh recalls. The savings could be substantial, given that foamed plastics are widely used for packaging, thermal insulation, cushioning, and disposable dishes, among other items. Suh quickly suggested introducing microscopic bubbles into the polymer in order to cut down on the amount of material used.

“Coming up with the idea was easy,” he recalls. “The hard part was figuring out how to make it.”

Assisted by several MIT students, Suh hit upon a technique that involved mixing gas with liquid plastic under high pressure and then suddenly lowering the pressure to form bubbles (also called voids or cells) that are distributed uniformly throughout the material. Suh’s approach created much smaller, more densely packed bubbles than the conventional method of adding “impurity particles” to the polymer melt-particles that create discontinuities in the polymer, which in turn give rise to voids. “The number of voids you can make that way is limited by the fact that you cannot add too many impurities without ruining the original material,” Suh explains. Furthermore, the voids in conventional foamed plastics are typically at least 200 microns in diameter.

Suh patented his process in 1984, but suspended work on MCPs until 1988, when he returned to MIT after serving for four years as an assistant director at the National Science Foundation.

Suh argues that manufacturers of many, if not most, plastic products could save materials and money by using MCPs. Because the tiny voids are so densely packed (on the order of 10 billion cells per cubic centimeter), MCPs use far less material than conventional foamed plastics. Simply put, they contain a greater proportion of empty spaces. “We can cut material costs by 10 to 95 percent, depending on the application,” Suh notes. “And by using less material overall, less energy is needed for manufacturing and there will be less waste to be disposed of, or recycled, later.” In addition, MCPs are better able than conventional plastics to resist breakage, shattering, fatigue, and cold temperatures, according to Matt Pallaver, executive vice president of Trexel, Inc., a company formed in 1994 to develop commercial uses for the new materials. (Trexel is the successor to Axiomatics, a company founded by Suh in 1983 to make instrumentation for the plastics-processing industry.)

“A big area for us is developing new thin products-plastic foams that are as thin as paper,” he says. These could be used, for example, as coatings for data communications wires. “You cannot use conventional foams for this because the cells themselves are bigger than the thing you’re trying to make,” he explains.

Trexel is now working with major plastics manufacturers to develop at least 18 different applications for the material. Trexel representatives are unable to discuss the details of these projects for proprietary reasons, but say the applications they are exploring include automotive parts, pipes, insulated housing for cables and wires, construction materials, containers (such as bottles and cans), and-at the low end-disposable plates and cups. “We’re still trying to figure out new applications,” Pallaver says.

“With new uses for plastics being discovered every day,” adds Chul Park, a mechanical engineer at the University of Toronto who worked with Suh at MIT, “MCPs have more market potential than people can imagine.”