Once a cake is baked, its shape is set for good. Thermosetting plastics, known as thermosets, behave much the same way. When heated and moulded, they undergo a chemical transformation that locks them into a permanent form. This gives thermosets exceptional strength, stability and heat resistance, making them essential for products that need to be long-lasting: thermosets form parts of our cars, computers and even kitchen countertops.
However, this strength comes with a sustainability drawback. Unlike ‘simple’ plastics, like PET in water bottles, thermosets can’t be melted down and reshaped, making them extremely difficult to recycle.
“The primary disposal methods for thermosets are mainly incineration and landfill, which add to environmental pollution and amount to a gross mismanagement of resources,” said Sheng Wang, a Research Scientist at A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).
Seeking alternatives, Wang and researchers from ISCE2 and A*STAR’s Institute of Materials Research and Engineering (IMRE) teamed up with collaborators from the King Abdullah University of Science and Technology, Saudi Arabia, and the Chinese Academy of Sciences, China, to develop a new generation of recyclable thermosets.
“Our team sought to create a material that combined the rigidity of traditional thermosets with the reprocessability of vitrimers, a new type of plastic,” said Wang. “They’re like clay: they harden to a glass-like rigidity, but can be softened, reshaped and repaired, which makes them more sustainable and versatile.”
Most crosslinked polymers that have been discovered are often too brittle to use. Wang and colleagues explored a solution hinging on an innovative use of the Knoevenagel reaction: one that used the reaction’s carbon-carbon double bond (C=C) to directly coax vitrimer molecules to trade parts with each other. With this, the team aimed to construct glassy vitrimers with networks of well-conjugated, dynamic links between molecules.
The team experimented with small molecule models to test the novel reaction’s feasibility, eventually creating a new class of polymer networks known as poly(α-cyanocinnamate)s (PCCs). Put to the test, PCCs showed a remarkable blend of rigid and tough properties: they boasted a Young’s modulus of up to 1,590 MPa, and could be stretched up to 79 percent of their original dimensions before breaking.
“One of the most surprising findings was this balance between rigidity and toughness,” said Wang. “These materials achieved high toughness without sacrificing rigidity, which is often a trade-off in materials science.”
Wang added that PCCs have the potential to transform industries and consumer goods that rely on thermosets, such as electronics, automotives and aerospace applications, by offering materials that are not only high-performing but also recyclable. The team’s novel approach to the Knoevenagel reaction may also pave new manufacturing inroads for coatings, adhesives and composites with similar properties.
“Our ultimate goal is to replace traditional thermoset resins with these advanced materials, thereby promoting a circular economy,” said Wang.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2) and the Institute of Materials Research and Engineering (IMRE).
