New foam extrusion technology is 'environmentally friendly.'

The first commercial application of a new inert-gas foam extrusion technology is expected to hit the market later this year. Last September, Plastic Extruded Products Co. (PEPCO) in Hillside, N.J., began commercial scale-up of a process that was developed over the last two years by the Polymer Processing Institute (PPI) at Stevens Institute of Technology in Hoboken, N.J. PEPCO is using the process to develop profiles of rigid PVC for wood replacement and is considering polycarbonate for this purpose as well. However, PEPCO president Edward Larsen expects lower-density foams of polyolefins or thermoplastic elastomers to be commercialized first as replacements for polyurethane or thermoset rubber in unspecified mechanical-goods applications.

The key to the technology is use of inert gases like carbon dioxide or nitrogen that are more "environmentally friendly" than ozone-depleting fluorocarbons and smog-causing hydrocarbons like pentane or isobutane. Foams made with inert gas may also be more recyclable than foams made with chemical blowing agents, which may leave reactive residues.

PEPCO president Edward G. Larsen says his firm is focusing in part on wood replacement in structural building materials such as frames and panels. Larsen says he's looking for low-density foams that offer improved compression-set properties. "We believe the driving force behind this technology will be its ability to produce foam profiles and structures with a lower density than can be obtained by traditional CFC chemical agents," he says.

For wood replacement, the inert-gas process has achieved density reductions of 20-30%. In elastomeric applications, density reductions can be as high as 60%.

REQUIRES CAREFUL CONTROL

PPI's research on inert-gas extrusion began seven years ago with a contract from a major U.S. foam manufacturer. Work on PVC started in 1993 with a $150,000 grant from the New Jersey Commission on Science & Technology, part of the state's Technology Transfer Merit Program, which is designed to foster economic growth through development and application of new manufacturing technologies. In this case, the challenge was to devise a non-polluting and cost-effective process technology that involved an affordable capital investment for small to medium-size processors.

Along with development of the fundamental process technology, PPI's research encompassed methods of achieving careful temperature control and development of a new on-line data-acquisition system to monitor processing conditions. PEPCO's Larsen notes that the inert-gas technique is more complex to monitor, compared with using chemical blowing agents. Resin pellets are metered into the extruder via a twin-screw volumetric feeder. Besides the resin feed rate, crucial processing variables include precise control of gas-injection pressure and gas-flow rate. Also essential are accurate barrel-temperature control and precise monitoring of melt temperature and pressure at the die.

PPI officials Joseph A. Biesenberger and Marino Xanthos say the research so far has focused on foaming PVC with carbon dioxide, though some work has been done with nitrogen. Gas is injected into the barrel at a pressure well below 1000 psi, which is low enough not to require special processing equipment.

The process requires a relatively long barrel residence time in order for the gas to dissolve into the melt. PPI director of research Xanthos explains that C[O.sub.2] isn't as soluble as CFC. "There is a narrow processing window, as the carbon dioxide wants to nucleate prematurely or come out of the melt solution," he says. According to Xanthos, C[O.sub.2] also doesn't plasticize the melt as well as CFC and thus generally requires a higher operating temperature.

PPI's R&D apparatus for extruding higher-density, wood-replacement foams of 0.5-0.7 g/cc is a Killion single-screw extruder of 40:1 L/D with 11 barrel-temperature zones. PEPCO's Larsen says his firm developed its own unique screw design for this process that "mimics the effect of such a long barrel without actually having to be that long."

PPI is also experimenting with tandem extrusion (using a twin-screw and a single-screw unit linked in series) of lower-density polyolefin foams (less than 0.1 g/cc) for sheet, rods, and tubes. PEPCO has not found it necessary to go to tandem extrusion for its lower-density flexible foams.

Along with a few challenges, inert-gas foam extrusion reportedly provides some processing advantages. Xanthos says the technique gives processors the ability to make on-line adjustments of foam density, which is harder to do with chemical blowing agents. The inert-gas process also can be fine-tuned to control foam surface quality, cell-size distribution, and mechanical properties.

PPI is embarking on a new development project in cooperation with the New Jersey Institute of Technology (NJIT) in Newark to utilize post-consumer recycled PET as the middle layer of coextruded foam sheet for thermo-forming. It is hoped that such a sandwich structure could be used for food packaging.