John M. Torkelson, a Walter P. Murphy Professor in the Department of Chemical and Biological Engineering and the Department of Materials Science and Engineering at Northwestern University, has found a solution to a common problem with rubber: it can’t be recycled.
Separating recyclables from the waste stream has become routine behavior. Paper, many plastics, glass, and metals are collected and converted into reusable (economically valuable) products, reducing landfill diversion and minimizing ecological footprint.
The ability of a non-paper material to be recycled – heated at high temperatures and recast for reuse – depends largely on the way in which its polymers are linked. Polymers, or chains of molecules arranged to give structure to larger molecules, are usually configured in either linear or cross-linked chains. Polymers that we typically recycle, namely thermoplastics, are made of linear chains, which can be heated to high temperatures, remolded in their melted state, and effectively reformed when cooled without losing their original properties.
When rubber – which is made of permanent cross-linked chains – is heated, it strengthens and can’t be remolded or reheated into a usable product that retains its original durability and elasticity.
Torkelson and two of his Ph.D. students, Kailong Jin (Chem. Eng. ’17) and Lingqiao Li (Chem. Eng. ’19) have developed a simple, one-step strategy to solve this problem by modifying the way in which the polymers in rubber are linked. The research, supported by the McCormick Research Catalyst Awards Fund at Northwestern University and by discretionary funds associated with Torkelson’s Walter P. Murphy Professorship, will be detailed in an upcoming issue of Advanced Materials, and is currently available online as an early view article.
“Our approach can be used for any rubber applications that require elasticity and durability—from common products like shoes, to aerospace materials,” said Torkelson.
Torkelson sees particular impact for the tire industry. The Rubber Manufacturers Association estimates that the US disposed of 244 million scrap tires in 2013. Only about 30 percent of those disposals were down-cycled for scrap tire markets like ground rubber applications or civil engineering, while more than 50 percent were simply burned for fuel. Nearly 10 percent – 20 million tires – were landfilled.
The team’s approach would allow tires to be made using conventional polymers, maintaining the properties that make tires effective, but with modifications to the way the polymers are cross-linked. Currently, the resulting material has shown full retention of properties after two cycles of the process, with further trials to come.
The one-step synthesis approach, based largely on a process called nitroxide-mediated polymerization, separates the cross-links in rubber at high temperatures, making it possible for them to reform and retain their properties in a cooled state.
To successfully reform, the individual electrons that remain when a cross-link comes apart at high temperatures, called radicals, must find each other upon cooling. To be effective, the team needed to find a molecule that had both a stable (reforming) radical, as well as a carbon–carbon double bond, which they found in a molecule called TEMPO methacrylate.
The research builds on more than 15 years of work in the area by various researchers. Limitations of prior approaches included processes that were costly, chemically-intensive, and required many reaction steps under specific conditions. These processes were also characterized by low recovery of cross-links and material properties, according to Torkelson.
“Our approach requires only one round of chemical synthesis, uses all commercially-available components, which are also inexpensive and easy to synthesize, and is adaptable to a large fraction of cross-linked polymers out there,” said Torkelson.
The research is patent pending and will be presented at the 2016 National American Institute of Chemical Engineers Meeting in August, as well as the National American Chemical Society Meeting in November. Lingqiao Li will build on this work as a 2016-2017 ISEN Cluster Fellow.
“My group will dedicate the coming decade to this new line of research,” said Torkelson. “It has that much potential.”