Whether as soft bags or sturdy furniture, plastics make much of our world. A class of durable plastics, known as thermosets, are widely used in hard-wearing places like car engines, plane bodies and buildings. However, that durability has a downside: once worn down, thermoset products often have nowhere else to go but landfills.
To make these plastics more sustainable, researchers at the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2), are developing new thermosets based on dynamic covalent adaptable networks (CANs). These unusual chemical bonds can be as tough as those in typical thermosets, but with one key difference: like door hinges, they can open and close repeatedly without degrading or breaking.
“Traditional thermosets are valued for their robust strength, heat resistance and stability, but can’t be reprocessed or recycled, which adds to substantial plastic waste,” said Zibiao Li, A*STAR ISCE2 Division Director (Resource Circularity). “However, thermosets designed with dynamic CANs can be recycled repeatedly without sacrificing thermal or mechanical performance.”
Currently, chemical reactions used to make CANs can be costly and complicated as they need catalysts or cause unwanted side reactions. Li and A*STAR ISCE2 Research Scientists Jie Zheng and Sheng Wang recently set out to explore if a novel reaction—a type of C═C/C═N metathesis—could circumvent these hurdles.
“Much like switching dance partners, metathesis reactions let molecules trade functional groups in a seamless, orderly fashion,” explained Zheng. “This avoids unwanted side reactions, but most known metathesis reactions still need catalysts, which can deactivate under harsh conditions and make processing more complex.”
With other A*STAR ISCE2 colleagues and those from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and King Abdullah University of Science and Technology, Saudi Arabia, Li, Zheng and Wang designed, synthesised and observed a series of compounds with flexible C═C (carbon-carbon) and C═N (carbon-nitrogen) double bonds. Built from α-cyanocinnamates and aldemines, these compounds rapidly formed plastic-like materials without catalysts at room temperature.
Encouraged by the results, the team scaled up, adding these linkages into a larger polymer network called V1. Using dihydroxyl and isocyanate groups as cross-linkers, they created a polyurethane thermoset that withstood heat up to 329 °C without breaking down. The V1 thermoset also proved highly recyclable: sliced into pieces and hot-pressed at 180 °C, its mechanical properties, such as tensile strength, were largely restored.
“Our new direct C═C/C═N metathesis reaction addresses the limitations of most metathesis reactions by enabling efficient, catalyst-free bond exchange,” said Wang. “Our innovative approach not only simplifies thermoset recycling, but also enhances their durability and environmental compatibility in advanced materials.”
The team plans to expand their reaction’s applications to common industrial materials, develop its scalability and cost-effectiveness, and closely collaborate with manufacturers to integrate it into existing recycling systems.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2) and the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
