Microcellular foams: lots of R&D, but little commercial action yet.

Active research on these novel foams continues to develop more exotic versions with unusual properties. So far, most commercial activity is in Japan.

Although microcellular foams have yet to make much impact on commercial processing operations, new and ongoing research activity suggests interesting market potential in several applications. Microcellular foams, which are said to offer a new range of insulating and mechanical properties while reducing material costs, are characterized by extremely small bubble size and very low density. A second claimed advantage of the foams is that they may be produced using "environmentally friendly" gases, such as C|O.sub.2~ and nitrogen, rather than ozone-damaging CFCs or HCFCs, which makes them a viable alternative to current foam processes.

The process for producing microcellular foam was initially developed by a consortium of the Massachusetts Institute of Technology (MIT) and several U.S. companies, including Eastman Kodak Co. of Rochester, N.Y. A patent on the material was issued in 1984. Today, at least two companies in the U.S. are actively involved in sublicensing this technology for commercial production. Microcellular Plastics Technology (MPT) claims it has an exclusive license under Kodak's patents to make, use, and sell microcellular materials and products, and to sublicense the technology to other manufacturers. Kodak had developed and patented (in 1988) a continuous process for producing microcellular webs having a smooth integral skin, as well as a related process for producing clear "windows" in thermoformed microcellular sheet. Meanwhile, Axiomatics Corp. claims an exclusive license to the original MIT patent, which is based on a batch process.

WHY ALL THE INTEREST?

One gauge of its commercial potential is the healthy amount of research activity currently taking place. During the SPE ANTEC meeting last May in Detroit, seven papers were presented during a special session on microcellular foams. According to Nam Suh, head of MIT's department of mechanical engineering, microcellular foams are characterized by cell sizes between 1 and 10 microns, cell densities from |10.sup.9~ to |10.sup.12~ cells/cc, and specific-gravity reductions exceeding 95%. Material properties include high impact strength (up to five times that of unfoamed plastic), high stiffness-to-weight ratio, low dielectric constant, and low thermal conductivity. These properties reportedly make microcellular foams suitable for food packaging, refrigerator linings, circuit-board insulators, and molecular-grade filters, among other applications.

In August 1991, MPT licensed its extrusion processes for integral-skin foams to Sekisui Plastics of Tokyo, Japan, which reportedly is using it to make disposable food packaging. Since then, MPT claims to have licensed its processes to a second, unnamed, client, which v.p. Mark Wentworth describes as a large Japanese oil company. Wentworth says MPT is working on a proposal to build a pilot plant somewhere in the southern half of the U.S. in an effort to expose more domestic companies to its technology. Axiomatics recently completed a small project with the U.S. Navy to produce buoyant cable of microcellular foam. It is working on other development projects, although the company would not elaborate on details.

SUPER-FINE FOAMS

Much of the activity on the research side has taken place at MIT, through its Microcellular Plastics Consortium, established in 1988. A second research consortium, known as the Cellular Composites Consortium, has recently been established at the University of Washington in Seattle. It consists of three member companies representing the aerospace, packaging, and telecommunications industries, according to Vipin Kumar, assistant professor of mechanical engineering at the University of Washington.

Among the most recent developments are so-called "supermicrocellular" plastics, recently patented by MIT. Supermicrocellular plastics have cell diameters on the order of 0.1 to one micron (an order of magnitude smaller than those of microcellular plastics), and cell densities of around |10.sup.12~ to |10.sup.15~ cells/cc (three orders of magnitude greater than microcellular plastics).

Supermicrocellular foams are formed using a supercritical fluid--a gas in its supercritical state--as the foaming agent. A supercritical fluid has properties that cause it to act like both a gas and a liquid--for example, exhibiting the solvent characteristics of a liquid but diffusing readily into a polymer like a gas. At some point after the introduction of the supercritical fluid into the polymer, a saturated solution of the fluid and the polymer is produced. When the solution contains a sufficient amount of supercritical fluid at a suitable temperature and pressure, the temperature and/or pressure is rapidly changed, inducing thermodynamic instability and foaming the polymer. In some cases, foaming can be achieved at room temperature.

The manufacturing process recently developed at MIT has used supercritical C|O.sub.2~ as a foaming agent. The use of C|O.sub.2~ as a supercritical fluid instead of in its gaseous state is said to reduce saturation time, increase cell nucleation density, and improve control of foam cell size.

Foaming with supercritical fluids reportedly makes possible the production of increasingly smaller cell sizes and higher cell densities. These are of interest because they open the way to new types of products, according to Daniel F. Baldwin, a researcher at MIT's Laboratory for Manufacturing and Productivity. Some of these include impact-resistant transparent foams, dyeable plastics, and microscale insulators in applications such as computer chips. Baldwin adds that working with the small cell sizes also expands the understanding of gas/polymer systems, and could open the way for new types of processes for manufacturing polymers.

The next stage is development of "ultra-microcellular" foams, characterized by cell sizes down to 0.01-0.1 micron and cell densities in the range of |10.sup.15~ to |10.sup.18~ cells/cc. According to Suh, ultra-microcellular foams have implications in medical, industrial, and consumer applications. Attractive properties could include enhanced electrical and thermal insulation, high impact strength, and optically transparent foams. Ultra-microcellular foams reportedly have unique potential as transparent materials. Because the cell size is smaller than the wavelength of visible light, the majority of light may be transmitted through "ultra-foamed" polymer without being deflected.

Another ongoing project at MIT is directed at foams with open cell structures, which may have applications as filters and insulators. Initially the research will focus on microcellular foam and will progress toward smaller cell sizes. Baldwin sees possible applications in filters, insulation, and synthetic papers.

Axiomatics reportedly has used the supermicrocellular process to produce foams with very low specific gravities to be used as in insulation for refrigerator doors. The so-called ultra-low-density microcellular foam (U-LDMCF) is said to combine densities below 0.1 g/cc with improved impact resistance, insulation, and buoyancy in the thermoplastics tested. Ultra-low specific gravities to 0.02 g/cc have been demonstrated in PS, with tensile strength improvements of 60% over non-microcellular PS foams of similar density. Characterization of U-LDMCF PC is now under way. Because of difficulties in converting to a continuous process, U-LDMCF foams have so far not proved to be commercially viable, although research continues.

CONTINUOUS PROCESSES

What may be an important step toward commercializing supermicrocellular foams is MIT's recent development of an extrusion process for the continuous production of microcellular thermoplastic filaments. MIT also recently patented a method for producing continuous microcellular foamed sheet. In one version, supercritical fluid is supplied to the barrel of a twin-screw extruder with a sheet die. Downstream of the die is a pressurized chamber with a dynamic seal at its exit end. Cell nucleation takes place at the exit of the die and foaming takes place in the pressurized chamber, which controls bubble growth.

MIT's Suh also sees potential for applying the microcellular foam process to blow molding and injection molding as well.