Solid-state extrusion technique tackles commingled PCR.

New research from the BIRL Industrial Research Laboratory at Northwestern University in Evanston, Ill., suggests a novel way to turn low-performance, commingled, post-consumer recycled (PCR) plastics into useful blends with the help of a corotating twin-screw extruder.

Solid-state shear extrusion (SSSE), also known as solid-state elastic-deformation grinding, subjects polymers to high shear and pressure while rapidly removing heat from the process. No melting takes place; instead, the process grinds the commingled resins into a relatively uniform powder (see PT, Sept. '93, p. 75). "It's the only process I'm aware of that can take all six of the most common resins in the waste stream and process them together," says Dr. Klementina Khait, the scientist who conducted the BIRL research. Her study examined both PCR materials that had been sorted and separated and commingled feedstocks that included combinations of HDPE, LDPE, PP, PET, PS, and even PVC. The SSSE powders produced from these materials all were in the 50-500 micron range, depending on the composition of the feedstock and the process parameters.

Though SSSE has been around for nearly 10 years, BIRL's work has started moving the process down the road to commercialization by identifying some of the key mechanical properties for commingled-PCR powders. When injection molded into ASTM specimens, the PCR powders overcame known incompatibilities--for instance, the less-than-harmonious relationship between HDPE, LDPE, and PP. And the samples exhibited "an unusually uniform, pastel color with shiny and smooth surfaces" and without any signs of delamination, says Khalt. Most important, it looks like the commingled blends may outperform single-resin PCR in some cases--Khait cites a comparison of HDPE/PET versus HDPE as the best example.

According to Khait, SSSE produces powders suitable for blending with virgin TABULAR DATA OMITTED TABULAR DATA OMITTED resins in a variety of applications, including pipe extrusion, rotational molding, powder coating, and compounding. The appropriate ratio of PCR powder to virgin has not been determined yet, but Khait predicts it will be at least 25% PCR. "It is our firm belief that we will be able to incorporate even higher percentages of some PCR powders because of their high tensile and impact strength," she says.

HOW IT'S DONE

To transform both the sorted and commingled PCR into powders, Khait used a dieless ZE-40 corotating twin-screw extruder from Berstorff Corp. of Charlotte, N.C. Continuous pulverization occurred when pressures reached 36-44 psi and temperatures were kept in the 60-140 F range. "The cool temperatures must be coupled with high shear rates," Khait notes. Just how much shear, however, is a question Khait will not answer until she completes more research. While SSSE does require some machinery modifications, the good news is that the changes seem to be minor. In fact, Khait's research took place in Berstorff's laboratory on a machine not designed specifically for SSSE. Still, more development work must be finished before the process can produce consistent powders at anything approaching commercial throughput rates. "Making coarse powders at low rates is not difficult; making fine ones at commercial rates is," Khait points out.

Screw design is the first area that needs fine-tuning, since each resin and blend will likely require a tailor-made configuration. Khait predicts that SSSE will require more kneading blocks than conventional extrusion, and the blocks will have to be placed at strategic locations. "We still need to learn how many kneading blocks are required and where they will go," she says.

For the necessary low-temperature processing, Berstorff has come up with a proprietary water-cooled barrel design to provide additional cooling. On pilot-sized extruders--40-mm or bigger--screws with an interior cooling channel can also be used, reports Berstorff v.p. of research and development Martin Mack. (Berstorff, by the way, holds a 1986 patent on SSSE technology.)

APPLICATION RESEARCH

So far, most of the BIRL work has been an effort to determine processing parameters and examine the possibility of pulverizing unsorted, commingled, post-consumer plastics, but Khait reports that the next phase is to commercialize the SSSE technology. "In the past, we limited our experiments to the process itself, but from this point on we're focusing mostly on application development."

To that end, BIRL has ordered its own 25-mm Berstorff twin-screw extruder, the first machine in the U.S. specifically designed for SSSE. It's scheduled to start up within five months, and BIRL will make it available for industry trials.

As for the cost of SSSE relative to conventional grinding, it's too early to say--especially since no good throughput numbers exist. "We don't know the exact cost yet, but we know it will be less than conventional grinding," Khait says. She expects favorable economics because one of SSSE's biggest advantages is eliminating the expense--and heat history--of compounding reclaimed and virgin materials.

(Editor's Note: AT NPE '94, Khait will present her research on Wed., June 8 in a paper called "Non-Conventional Technology to Recover Post-Consumer Plastic Waste." She's the first speaker, so be there at 8 a.m.)