Researchers at Case Western Reserve University are working on a simple way of turning scrap thermoset plastics into new thermosets with similar properties.
Their process involves mixing scrap thermoset with a catalyst and then pulverizing the mixture. In the process, the catalyst bonds at the cross-linking sites where it can break the cross-link bonds and then form new ones into a new thermoset. They call the technology vitrimization.
Researcher Liang Yue said in a phone interview the research stems from the desire to recycle tough thermoset scrap rather than landfill it.
“These are strong materials and resistant to impact, but until now they were single use. We’re trying to change that,” Yue said.
Wind turbine blades, auto parts and a host of other industrial and commercial products are made of tough thermosets such as epoxies, polyurethanes and reinforced unsaturated polyesters. These products can end up in landfills or be incinerated, which are undesirable outcomes at the products’ end of life.
Much work has been done on grinding up thermoset scrap and adding the resultant powder as a filler in virgin thermoset resin, but this approach is frustrated by poor bonding between the filler and virgin resin.
Yue and others began work on the concept at Cleveland-based Case Western’s School of Engineering laboratory two years ago when he joined with lead researcher Ica Manas-Zloczower, professor of advanced materials and energy. Their team, which included several other Case Western researchers, first announced its discovery recently in journal ACS Macro Letters.
The team reported on recent work with thermoset epoxy materials and a zinc-based catalyst. It created a material with so-called dynamic cross-links. The “permanent” cross-links in the scrap bond with zinc ions, which under heat and pressure catalyse the formation of new cross-links.
The powder they made can be compression molded into a new thermoset part. The process does not rely on solvents and in theory such scrap could be infinitely recycled. These benefits call to mind the ability to recycle thermoplastics through many cycles.
Yue said the researchers want to expand on their work with more thermoset polymers. He said it should be applicable to elastomeric thermosets like rubber, which then could be injection molded. Down the road, even polyolefins might be applicable to vitrimization-based recycling.
Various approaches to vitrimization have been studied by several research groups for more than a decade. The Case Western work seems more promising and economically feasible than other approaches tried so far, Yue said.
The team’s paper sparked interest in industry, according to Yue. He and Manas-Zloczower are negotiating with potential industry partners to scale up their process via mechanical-chemical ball milling of scrap to make thousands of pounds of reusable powder resin.
“We can recycle tons of epoxy waste in a matter of hours,” he said.