When it comes to plastics and polymers, not all materials can be easily recycled and made into new products. Researchers are trying to create new materials with similar properties that can be reprocessed sustainably.
Professor John Torkelson is a Walter P. Murphy professor of chemical and biological engineering and materials science and engineering at Northwestern University. He presented his research on May 26 in a virtual seminar with the Program on Plastics, Ecosystems, and Public Health (PEPH) at the Institute for Sustainability and Energy at Northwestern (ISEN).
“We have set out to address a particular type of polymer material, permanently cross-linked polymers, that are known as thermosets that traditionally have just been considered to be non-recyclable, and to find a way to actually recycle them in the way that many linear polymers or thermoplastics are recycled through melt-state processing,” said Torkelson.
Materials such as rubber tires and polyurethane foam are made of permanently cross-linked polymers, which degrade before melting and therefore cannot be physically recycled. Torkelson said that about 15% to 20% of discarded rubber tires in the United States are found in landfills or become lost to the environment, while the rest are either reduced to rubber crumb or burned for energy. These are not high-value uses and can cause harm to the environment.
Torkelson’s research group uses dynamic covalent crosslinks in its polymers so that more of the material can be reprocessed. These dynamic crosslinks will come apart at high temperatures and break the polymer into branched or linear chains, allowing the material to be further processed.
“This means that at use conditions the crosslink density is reproducible before and after reprocessing within experimental error, and there are other characteristics of this particular material that are also reproducible with an experimental error,” he said.
However, Torkelson said that there is a well-acknowledged limitation to this general method with regards to creep. Creep is strain as a function of time when a material is placed under stress, like a weight stretching out a fine wire. In contrast, a rubber band with permanent crosslinks will exhibit an instantaneous strain when placed under stress but essentially no further strain as a function of time. The origin of this time-dependent strain in dynamically crosslinked polymers is the fact that at sufficiently high temperature but still within use conditions the crosslinks are dynamic and accommodate mobility associated with creep.
“You could imagine that if you had a rubber tire that had dynamic covalent crosslinks in it and you had it outside in the heat in Arizona, you don't want that rubber tire to creep under the load that it would be presented by the car sitting on top of it,” Torkelson explained. “That would be a very bad thing to have this creep take place while the automobile is actually in use.”
Torkelson’s group has found that adding a certain amount of permanently cross-linked polymers can suppress the effects of creep in the material while retaining melt-state reprocessability. They have also discovered that several types of dynamic crosslinks allow for melt-state reprocessing at temperatures close to that of tire molding (about 130 to 140 °C) but exhibit essentially no dynamic character at about 80 °C or 175 °F meaning that there the issue of creep is eliminated up to 175 °F.
Torkelson ended his presentation saying that this process could potentially be scalable to industry standards. His research team used similar materials and processing temperatures as the tire industry, but a main issue for widespread application is in reaching a faster processing time.
With this research, Torkelson hopes to continue to advance this dynamic chemistry to create better materials that can be recycled. He also hopes to continue to work with industry to make this science more accessible.
“We want to do this in a way where the chemistry is simple enough that it encourages the possibility for actual commercial application,” he said.