Micro-cells may be big in foam extrusion.

A proprietary process for extruding microcellular foamed polystyrene sheet with significantly improved mechanical properties is now available for commercial licensing from Trexel, Inc., Woburn, Mass. The technology, geared initially to packaging applications, has been in development for about a decade (see PT, June '88, p. 83; May '91, p. 15; Feb. '93, p. 63). The R&D involved the Massachusetts Institute of Technology (M.I.T), DuPont Co., Boeing, Eastman Kodak, Lockheed, and Kraft, Inc. Today, Trexel (formerly Axiomatics Corp.) holds the exclusive license for the material, the original batch manufacturing process, and a newer continuous process that it developed. Trexel is now licensing its continuous process, which it has dubbed MuCell, and has shelved the batch method.

In the MuCell process, a nonflammable gas such as carbon dioxide or nitrogen is injected into the melt in an extruder. The gas is pressurized to a supercritical state, in which it has some of the solvent properties of a liquid. Extremely fine nucleation is said to result when the melt is allowed to expand. Little modification of standard foam extrusion equipment is required, according to Trexel officials. "Most of our work goes into trying to use what the customer already has," says president and CEO David Bernstein.

The result is PS sheet with densities of 0.035-0.1 g/cc (2-6 pcf) and uniform closed cells measuring less than 50 microns. Cell sizes of 150-250 microns are typical for conventionally foamed PS sheet, says Matt Pallaver, executive v.p. Trexel reports output rates of 1000 lb/hr on a tandem extrusion line and somewhat lower rates with a single extruder.

BENEFITS FROM TINY CELLS

The small and uniform cell size enables MuCell sheet to show much better properties than hydrocarbon-blown foams of similar densities, according to Trexel's data (see graphs).

Trexel has recently demonstrated continuous extrusion of MuCell foams with cell sizes below 10 microns. "If the commercial market requires smaller and smaller cells, we know we can do it, but much of the current work is in the 10-30 micron range, where we can get the properties desired," says Bernstein.

And besides the work on PS, Trexel is in advanced development of a MuCell foam process for PET and polyolefins. Foams as thin as 0.005 in. have been achieved. Pallaver says MuCell technology presents an opportunity for processors of thin-wall solid PET or polyolefin sheet to reduce weight while maintaining properties. Attempts to accomplish this with conventional foams reportedly failed because resulting products were too brittle.

So far, Trexel has licensed MuCell technology to nearly 20 companies, half in the U.S. and the rest in Europe, Japan, and Korea. Over $10 million has been invested on commercial-development projects by these firms. Most projects involve thin-wall (less than 3-mm) foams. The only customer that would identify itself - but not the application - publicly is Alusuisse-Lonza Group of Zurich, Switzerland.

In its lab, Trexel recently started up an injection molding machine for developing molded MuCell foams.