A fortuitous discovery at IBM has paved the way for the development of a range of recyclable thermoset resins
The search for ways of recycling today’s widely used thermoset polymers has been a priority for some time. However, the crosslinking of monomers to form these useful but rigid plastics creates a barrier to re-use through remoulding or chemical recycling. To date, recycling by mechanical grinding and incorporation in virgin resin has been the best option.
Microelectronic wastes could be reduced
Copyright: Rex Features
Details of the advance were published in May in the journal Science. At the heart of the innovation is the use of a simple one-pot, low-temperature polycondensation reaction between paraformaldehyde and 4,4’-oxydianiline (ODA). This forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclise at high temperatures, producing polyhexahydrotriazines (PHTs).
Both materials are strong thermosetting polymers. The PHTs exhibit very high Young’s moduli, excellent solvent resistance and resistance to environmental stress cracking. However, both HDCNs and PHTs can be digested at low pH (<2) to recover the bisaniline monomers.
By simply using different diamine monomers, says IBM, the HDCN- and PHT-forming reactions lead to extremely versatile materials platforms. For example, when polyethylene glycol (PEG) diamine monomers are used to form HDCNs, elastic organogels can be formed that exhibit self-healing properties.
After the initial “accidental” discovery of the polymerisation reaction, scientists from IBM Research, in collaboration with researchers at University of California Berkeley, Eindhoven University of Technology and King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia, combined high-performance computing with synthetic polymer chemistry to hone the materials’ properties.
“These new materials,” says IBM, “are the first to demonstrate resistance to cracking, strength higher than bone, the ability to reform to their original shape (self-heal), all while being completely recyclable back to their starting material. Also, these materials can be transformed into new polymer structures to further bolster their strength by 50%, making them ultra strong and lightweight.”
The ability to selectively recycle a structural component, it adds, “would have significant impact in the semiconductor industry, advanced manufacturing or advanced composites for transportation, as one would be able to rework high-value but defective manufactured parts or chips instead of throwing them away. This could bolster fabrication yields, save money and significantly decrease microelectronic waste.”
James Hedrick, advanced organic materials scientist at IBM Research, comments “We’re now able to predict how molecules will respond to chemical reactions and build new polymer structures with significant guidance from computation that facilitates accelerated materials discovery.”
BREAKING BONDS WITH HEAT ALLOWS RESHAPING
The IBM research is not the only work that has been done in this area in recent years. Several projects have looked at thermoset materials where the cross-linking bonds can be broken or partially broken, to allow remoulding to take place, or to enable self-healing properties.
At the University of Groningen in the Netherlands, researchers have created thermoset polymers using the Diels-Alder and Retro-Diels-Alder reactions between thermosetting polyketones and bismaleimide, allowing the strong covalent bonds of the thermoset materials to be broken and reformed.
In the Groningen composition, the polyketone and bismaleimide’s links break when heated in a mould and rejoin when cooled, allowing it to be reformed. In tests, the group has remelted its polymer seven times, but theorise that it could be recycled many more times over without decomposing. The broad range of polyketone precursors also makes the system cost-effective to replicate, the researchers add. In France, scientists at the Industrial Physics and Chemistry Higher Educational Institution in Paris have created a lightweight thermoset that can be reworked and reshaped when heated. The team suggests the material could be used in many applications, from aviation to electronics, while being recyclable and repairable.
The material is made like a conventional epoxy resin by mixing a liquid resin, hardener and catalyst and then heating it to cure. After curing, the material can be reshaped and remoulded by applying sufficient heat. The team also demonstrated that it can be ground into a powder and then remoulded or injected.
It works because the material is able to flow when heated thanks to reversible exchange reactions by transesterification. These allow some of the crosslinks in the molecular network to change the topology of the material without breaking bonds in the molecular network.