In the 2010s, covalent adaptable networks (CANs) emerged as a new class of plastic materials designed to combine recyclability with high mechanical performance. Researchers from the P3R team at Softmat have recently developed a new CAN based on an innovative chemistry capable of thermally activating or deactivating the material’s recyclability. These results have been published in the journal Polymer Chemistry.
Thermosetting plastics are cross-linked polymeric materials in which the polymer chains are chemically bonded together. They play a vital role in high-tech sectors such as aerospace, automotive and electronics, thanks to their high-performance properties, such as exceptional resistance to heat and chemicals. But this robustness has a downside: once moulded and cured, they cannot be remelted or recycled. Over the past decade, covalent adaptable networks (CANs) have attracted considerable interest in this field. Indeed, these next-generation plastics combine the robustness of thermosetting materials with the ability to be reshaped and/or recycled. This property is based on so-called “dynamic” chemical bonds, capable of breaking and reforming in response to a stimulus, such as temperature.
Until now, these rearrangements required an excess of highly reactive chemical groups, particularly sulphur-containing groups known as thiols. However, this excess makes the material more susceptible to “creep”, a phenomenon of slow and irreversible deformation under prolonged stress which, over time, compromises the material’s durability and mechanical performance.
To overcome this challenge, scientists led by Marc Guerre have developed an innovative strategy akin to a molecular “padlock”. Their idea was to temporarily mask the excess thiol groups responsible for creep, thereby preventing their reactivity at operating temperatures, whilst retaining the ability to reactivate them when necessary.
They have thus succeeded in protecting the sulphur atoms of the thiol groups within stable chemical structures that act as locked padlocks. At room temperature, these padlocks prevent any undesirable interaction between the sulphur atoms and their environment. The material remains stable, strong and resistant to creep. Once heated, these locks gradually open, releasing the thiol groups, which can then participate in exchange reactions that allow the material to be reshaped or recycled. This strategy thus enables the material’s recyclability to be thermally activated or deactivated, whilst limiting creep at operating temperatures.
This breakthrough opens up new possibilities for the development of more sustainable plastics that have a longer lifespan whilst remaining recyclable. Furthermore, this approach could be extended to the design of materials capable of adapting their properties according to the conditions in which they are used.
Image caption: reversible protection of sulphur-containing chemical groups, which give the material its dynamic properties © Marc Guerre & Alexis Millan / Softmat