John Torkelson Seminar
Professor of Chemical and Biological Engineering at Northwestern University, John Torkelson, will deliver a department seminar titled, “Upcycling Virgin and Waste Thermoplastic Polyethylene, Polypropylene, and Related Copolymers into Covalent Adaptable Networks that Recover Cross-link Density upon Recycling,” Tuesday, October 14th at 1:25 p.m. in room B75 Amundson Hall.
Abstract
Nine hundred billion pounds of synthetic polymers/plastics are produced annually on a global basis. Unfortunately, less than 10% of spent polymers and plastics are effectively recycled. Polyethylene (PE) and polypropylene (PP) constitute more than one-half of worldwide polymer production; thus, any approach contributing significantly to overcoming this recycling crisis should consider PE, PP, and their related copolymers. We will describe how thermoplastic PE, PP, and associated copolymers, constituted of linear or branched chains, can be upcycled into covalent adaptable networks (CANs) using a one-step, radical-based reactive processing method that is a simple “drop-in” modification of the commercial process used to make non-recyclable, permanently cross-linked PE (XLPE) networks or thermosets from thermoplastic PE. The commercial process to make XLPE thermosets, which have enhanced properties relative to thermoplastic PE and are produced at levels equivalent to thermoplastic polystyrene or poly(vinyl chloride), two of the six most widely produced polymers, involves melt processing PE with a low level of radical initiator, resulting in the transfer of a hydrogen atom from PE to the initiated radical. This transfer leaves a radical on the PE backbone that can react with another PE backbone radical, leading to a permanent cross-link. We have developed several dynamic covalent cross-linkers with two carbon-carbon double bonds that can be “dropped into” the reactive process at several weight percent relative to PE or ethylene-based copolymer, leaving other conditions unchanged and resulting in CANs with cross-links that are robust at use conditions but dynamic at melt reprocessing conditions, leading to recyclability. Our dynamic covalent cross-linkers result in PE CANs that quantitatively recover the original cross-link density and associated properties after multiple recycling steps and lead to massive reductions in elevated-temperature creep relative to thermoplastic PE. Our approach has also led to overcoming the challenge of making networks by reactive melt-state processing of PP and propylene-based copolymers. No commercial cross-linked PP thermoset is made by radical-based reactive processing because of accompanying chain scission. We have overcome this problem by developing approaches involving resonance stabilization of the radicals, leading to recyclable PP CANs that recover cross-link density upon recycling. Finally, we will also show that fully recyclable PE CANs can be produced by direct free-radical copolymerization of ethylene with very low levels of a dynamic covalent cross-linker.
John Torkelson Biography
John Torkelson is a Walter P. Murphy Professor in the Department of Chemical and Biological Engineering and the Department of Materials Science and Engineering at Northwestern University. John received his B.S. in Chemical Engineering from the University of Wisconsin-Madison in 1978 and his Ph.D. in Chemical Engineering from the University of Minnesota in 1983. He has been a faculty member at Northwestern since 1983, where he also served as Assistant Department Chair in Chemical Engineering from 1992 to 1995 and Associate Dean for Graduate Studies and Research in the McCormick School of Engineering and Applied Science from 1997 to 2002. During 2003 - 2006, John was the Director of the Materials Research Center at Northwestern University, which receives its main funding from the NSF-Materials Research Science and Engineering Center (MRSEC) Program.
