Carbon dioxide foaming is explored for thin extrusions

The use of carbon dioxide as a foaming agent in extruded products made with commodity resins is finding its market niche. Early successes are in medium-to high-density, thin-section profiles, pipe, and sheet, where typically 20-40% weight reductions, as well as performance benefits, are realized by tapping CO₂. The gas is typically injected into melt in the supercritical fluid (SCF) state.

"CO₂ in foam extrusion so far has mainly found its way into what had previously been solid profiles," states David Bernstein, president of Trexel, in Woburn, MA, holder of far-reaching patents on SCF-type use if CO₂ in foam. Trexel has issued about two dozen licenses for its process in extrusion, and others in molding (Jun MP, 50: MPI, 59). At present, PVC, PS, PP, and PE are mostly foamed using physical (e.g., hydrocarbon and HCFC) or chemical blowing agents, although some PS food packaging in the U.S. is extruded using the combinations of CO₂ and CBS's (used as nucleators).

Trexel has recently publicized some commercial successes in extrusion, including vertical blind vanes, interior wall moldings, and foam-core laminated graphic board products. Trexel's MuCell process injects CO₂ in SCF form into melt under high pressure. When CO₂ reverts to gas upon exiting the die, countless voids of less than 30 micrometer dia result, creating "microcellular foam." Trexel's extrusion method requires dedicated equipment, and all end-users operate under license by paying an annual fee.

However, Rapra Technology Ltd., Shrewsbury, England, has announced another method to exploit CO₂ in SCF state. The process developer, working on behalf of several industry sponsors, says its alternative version makes fine (i.e., 100-to 200-micrometer dia) cells.

Rapra characterizes its approach as being highly flexible, since it can be retrofitted to existing extrusion lines. The equipment can also be used in a "swing" mode, i.e., for foam or standard extrusion. Rapra reports using its technology is close to being used commercially.

Martin Gale, principal consultant for processing at Rapra, says the case for CO₂ is strong. The cost of inert atmospheric gases is low relative to chemical agents, some of which run as high as $4/lb (e.g., endothermics). And CO₂ is benign in the workplace, whereas most currently used physical blowing agents face regulatory pressure.

Nevertheless, CO₂ foam extrusion is challenging , mostly because the gas has proven difficult to dissolve and fully disperse in the melt. The problem has been overcome by Trexel and Rapra by use of CO₂ in supercritical form.

New Option

To mix CO₂ in the SCF state in polymer in a standard extrusion system, Rapra uses a Cavity Transfer Mixture. At 50 bar pressure, the liquid CO₂ is transferred from the cylinder through a steel connector to a dosing pump. The CTM stator is clamped to the extruder flange and the rotor is turned within the stator by attachment to the screw tip. The CTM disperses CO₂ uniformly and keeps it dissolved.

Another innovation is cooling by means of a heat exchanger-static mixer system. Finally, small amounts of nucleator (talc) are used to thwart the natural tendency for CO₂ to diffuse out of smaller bubbles to form larger ones.

Rapra has been able to foam LDPE, HDPE, LLDPE, and PS, as well as select engineering resins, including PEEK, PPS, and SMA by using additional cooling. Rapra reports the technology is best suited for foams in the 0.4 to 0.7 g/cc density range (i.e., typical for major wood products). The process creates cells in the "fine" (100 to 200 micrometer dia) category, and also creates low-density foams.

Gale says densities as low as 0.4 g/cc (4 kilos per cubic-meter) have been achieved. But there are "grave limits" to CO₂ use in lower density foams: the gas is viable only in thin sections; suffers from surface (skin shriveling) defects; and gives poor insulation performance (CO₂ diffuses out). Hence, the current emphasis is on wood-replacement in medium - and high - density construction and other end-uses. The process is effective in thicknesses up to 10mm.

According to Gale, two sponsors of Rapra's work will commercialize CO₂-foamed products in the next year. One makes a wood-like boardstock in the U.K.; the other produces construction profiles in the U.S. A benefit is that CO₂ foaming is non-staining.

Considerable cost savings can be wrung from CO₂ use, Rapra's analysis shows. Gale estimates the modest initial investment for adapting equipment is recouped with a year due to the lower cost of foaming agents (15% of CBA cost).

Trexel makes inroads

The MuCell system is termed viable in PP, LDPE, HDPE, PS, PVC, and thermoplastic elastomer foaming. Conversion of equipment is done directly by Trexel, without involving equipment suppliers. It calls for a special screw that optimizes CO₂ mixing; modification (in some cases lengthening) of barrels to increase cooling; pressure transducer use; and drilling of injection points in the extruder barrel.

Bernstein estimated conversion costs for a medium-sized extrusion system at $40,000, with a special SCF unit for injecting the CO₂ (supplied by Trexel itself) adding $65,000 more. The payback for most foam extrusion systems is estimated at under a year.

One benefit is at least a 20-25% weight reduction in the microcellular structure, Bernstein says. The current extrusion rate is maintained, so the number of parts produced typically would increase to the extent weight is reduced. Surface properties of parts are equal to those using CBA's though in the case of PS, a lower matte effect results. Tests done for compression set values with a top thermoplastic vulcanizate TPE remained at close to that for a solid product despite high foaming levels.

"We went way out on a limb with this technology, but it has worked," says James Watson, president of Eclipse Blind Systems, in Ravenna, OH. The company, a top producer of blind systems, recently launched the Microtex line of vertical blinds based on Trexel's technology. "It outperforms its solid PVC predecessor," Watson states. Vanes of vertical blinds are typically 25-mil thick, but the new design takes them to 35 mil, giving the profile's textile-like embossment better definition.

In terms of performance, vertical blind vanes are lighter, softer, quieter and "self-healing" if damaged, according to Watson. To expedite the technology's quick adoption, Eclipse purchased a new Milacron extruder. Extensive reformulation of PVC compound was also necessary.

Eclipse manufactures horizontal blinds also, but Watson doubt these thicker (90-120 mil), more rigid PVC profiles lend themselves to CO₂ foaming.

A second licensee, Aluisuisse Composites in St. Louis, MO, recently announced use of CO₂ in its Foam-X line of laminated foam-centered boards. The company makes board for graphic arts mounting and modeling. Previously, it made sheet using hydrocarbon and HFC physical blowing agents. CO₂ foam is less prone to emissions, and reduces the time required for gas to dissipate out of the finished product prior to shipment.

Dumaplast, in Maldegem, Belgium, uses its MuCell license to foam interior cladding and trim profiles that had long been solid products. The thin (0.5 mil), tongue-in-groove profiles are 8 ft long and 10 to 25 cm wide, with integral ribbing used to elevate strength. CO₂ foaming resists warpage, offers good surface properties, and realizes 15-30% density reduction. Matt Drozdoff, Dumaplast's CEO, says the process retains virtually all of the stiffness-toughness balance required despite the density reduction.

A different perspective is offered by Dennis Keane, president of Bergen International, in Rochelle Park, NJ, a top supplier of chemical blowing agents. He concedes CO₂ foaming may be viable in some applications, but also emphasizes the limits, including restriction to higher densities an to thin cross sections only. In his view, CO₂ foaming is positioned best to replace hydrocarbon blowing agents rather than CBA's.

Keane also says comparative cost data is sometimes misleading. The endothermics, for example, since they cost as much as $4/lb, tend to be used sparingly (e.g., more as nucleators) in some end-uses. In cost-competitive markets like building profiles, however, workhorses like the azocarbonamides are used, and they offer much more favorable economics. Industry sources say there are other cases (e.g. siding) where foaming is done with unmodified sodium bicarbonate, which costs only around $0.16/lb.

More research

Other variants of CO₂ foaming, meanwhile, are being pursued by researchers in Canada at the University of Toronto, and by a non-profit group, the Polymer Processing Institute, based in Newark, NJ.

Dr. Subir Dey, processing manager at PPI, says its laboratory extrusion line can use CO₂, nitrogen, or argon gas to create extrusions with conventionally-seized cells (i.e., 250 to 750 micrometer range) but able to go down well into other densities. The major interest is in construction and building products. Design changes that can compensate for losses of mechanical properties with foams are being explored. Benefits of CO₂ foaming included non-staining, and absence of residuals that can impact negatively on using recycle-content material.

Work led by Prof. Chul Park, of the Dept. of Mechanical and Industrial Engineering at the University of Toronto, focuses on foams with density as low as 0.02 g/cc, including microcellular, fine, and conventional cell size foams. Target areas include insulation, wire and cable, and protective packaging foams.

© 2001 Trexel, Inc. · 45 Sixth Rd., Woburn, MA 01801 USA · 781-932-0202 · Fax: 781-932-3324
MuCell and Trexel are registered trademarks of Trexel, Inc.

By using this web site, you indicate your acceptance of the Terms of Use and Privacy Statement