By Jan H. Schut
Solid State Shear Pulverization, which chills a co-rotating twin-screw extruder below the softening temperature of plastic rather than allowing it to melt, is finally commercial after more than 20 years of R&D. Zzyzx Polymers LLC, Allentown, PA (www.zpolymers.com), started up in January 2015 reportedly using the first commercial SSSP technology, according to a paper originally submitted to the Society of Plastics Engineers ANTEC 2016 conference (www.4spe.org), but not actually given. The ANTEC paper on “Commercialization of Solid-State Shear Pulverization: a Novel Polymer Processing Technology,” by Philip Brunner (co-founder of Zzyzx), Mark Tapsak, and Mike Janse, says the process has been successfully scaled up to pilot plant size for the first time.
In the SSSP process the twin-screw barrel is chilled so that tri-lobe compounding screw elements crush the polymer in its rigid state, breaking molecular bonds instead of melting the polymer. Repeated crushing turns mixed, unwashed plastics like multilayer and metalized film waste, for example, into a pinkish powder, which can be remelted and have significantly higher properties than its component polymers.
SSSP research began in the former Soviet Union in the 1980s and was continued in the 1990s by Klementina Khait, and later John Torkelson, both professors at Northwestern University, Evanston, IL (www.northwestern.edu), and by professor Katsuyuki Wakabayashi at Bucknell University, Lewisburg, PA (www.bucknell.edu). Both university labs use 25-mm twin-screw extruders from Berstorff (www.kraussmaffeiberstorff.com) with barrels modified for cooling medium (anti-freeze) to roughly -7 °C at Northwestern and -12 °C at Bucknell. But high energy cost and a two-extruder process kept SSSP from being commercialized.
Brunner’s PhD dissertation and several subsequent patent applications (U.S. Pat. Applic. # 20150131399) under Torkelson were the basis for spinning the company from Northwestern in 2013 under the name NuGen Polymers LLC. Brunner’s dissertation on “Overcoming Sustainability and Energy Challenges in Polymer Science via Solid State Shear Pulverization” models energy and cost saving from combining SSSP twin-screw extrusion directly with pelletizing in one extruder. It reportedly would use a tenth as much energy as having two extruders: one to pulverize, the other to pelletize. On a lab scale SSSP used 1-10 kw·hrs of energy per kg of material, while the scaled up, in-line extruder uses 0.25-0.40 kw·hrs/kg, Brunner’s ANTEC paper says.
The ANTEC paper describes screw speeds of 200 rpm for a mixture of PE and HMWPE with throughputs over 180 kg/hr at an energy cost of 5-7 cents/kg on a 70-mm SSSP machine. The paper exaggerates that this cost is in line with conventional melt twin-screw extrusion technologies and the SSSP process is now as energy efficient as twin-screw melt compounding.. A 70-mm co-rotating twin-screw extruder for melt compounding would produce 300 kg/hr of masterbatch and 1500 kg/hr or more of engineering plastics. So even if energy cost is comparable, output of 180 kg/hour is low for the capital cost of the machine. The ANTEC paper also describes throughput of only 145 kg/hour for PCR laminated film and 100 kg/hour for waste film containing PVC.
Recent patent applications from Zzyzx (U.S. # 20150065616 and especially # 20150131399) describe cooling channels in the extruder shafts and elements as well as in the barrel. Patent application # 20150131399 says that “systems and methods for controlling the temperature of a solid-state screw extruder may include providing an extrusion screw that incorporates one or more screw shaft channels. The shaft channels may be configured to conduct a flow of a heat conducting medium along a length of the shaft.” If channels are drilled in the shafts, that would weaken them and hurt throughput. But the patent application notes that “shaft channels may be incorporated into an exterior surface” of the shaft, which wouldn’t weaken anything.
NuGen Polymers got $150,000 National Science Foundation grant in 2013, through a Small Business Innovation Research Phase I grant (Award # 1248336). In 2014, under the Zzyzx name, the company was approved for a $150,000 loan from Ben Franklin Technology Partners of Northeastern Pennsylvania, but didn’t accept it. Instead, in 2015, Zzyzx was funded by another larger $737,640 National Science Foundation grant over two years through a Small Business Innovation Research Phase II grant (Award #1434826), plus another $203,000 from the NSF in supplements. That $940,640 of NSF support allowed Zzyzx to set up a 70-mm diameter SSSP twin-screw extruder for the first time.
This pilot scale machine became “a nine thousand lb calorimeter,” Brunner’s ANTEC paper explains, allowing them to measure and control key processing parameters including coolant flow rate, coolant temperature, and heat removal for the first time. The big surprise was that the scale-up was more successful than expected, says Zzyzx CEO Michael Janse. Unexpected efficiency meant that Zzyzx didn’t need to operate cold extrusion in the same extruder with melt extrusion, which involved an extremely steep thermal gradient for a single machine and limited throughput, Janse explains. The patent application for the one-extruder process (U.S. Pat. Applic. # 20150051339) was also somewhat awkwardly shared by Northwestern and Bucknell.
Another $150,000 grant in 2016 came from the Closed Loop Foundation, part of the Closed Loop Fund (www.closedloopfund.com) set up by a group of large consumer products companies to support plastic recycling. “It will help us purchase the equipment needed to scale up our technology for testing in a plastic processing facility,” Janse said in a statement. The Closed Loop grant was primarily sponsored by S.C. Johnson & Son Inc., Racine, WI (www.scjohnson.com), which was looking for a way to support postconsumer recycling of film including its Ziploc bags from municipal collection. The Allentown Economic Development Corp., Allentown, PA (www.allentownedc.com) also incubates Zzyzx in its Bridgeworks Enterprise Center.
Zzyzx is definitely the first commercialization of SSSP technology, but it’s not the first time that SSSP has been scaled up. Material Sciences Corp., Canton, MI (www.materialsciencescorp.com), supported R&D on an SSSP-modified, 65-mm Berstorff twin-screw extruder in 2003, according to Material Sciences’ 10-K filing Feb. 28, 2003. Reportedly the work was successfully completed at Northwestern and available for license with a patent assigned to Material Sciences Corp., which was interested to apply the powdered plastics to metal coating, but the technology wasn’t used at the time.
Currently Zzyzx is running trials for customers and trying to determine how its technology can best be used for closed loop recycling. Since the twin-screw machine is modular, Zzyzx could either license the technology to customers with a difficult waste stream, or toll recycle difficult waste for customers.
In 2016, Brunner left the company. This leaves no one at Zzyzx with PhD level experience with SSSP, which may hamper development but for now it’s too soon to tell.