Lego-like recycling will transform how we use these future plastics

We only have to be reminded of the Great Pacific Garbage Patch to know that plastic waste contributes to hundreds of millions of tonnes of debris that wind up not only in landfills but oceans too. And that's not all. The manufacture of plastics, which are made from fossil fuels, are set to drive nearly half of the growth in demand for oil by 2050. We also know that tiny particles of plastics, in the form of nano and microplastics, are moving freely in the human body and almost everywhere throughout the environment.

Recycling is, therefore, critical for reducing fossil fuel emissions and plastic pollution right now.

A breakthrough recycling process using a class of durable plastics extensively employed in aerospace industries

Still, traditional recycling processes use bacterial enzymes to burn or dissolve polymers after manually breaking them down into powders. The objective is to reduce larger pieces into smaller components or powders that can then be used to make other materials and objects. For instance, clothing sewn from recycled plastic water bottles or shoes made from rubber tires.

But this process does not work with all plastics. In particular, some high-performance plastics are much harder to recycle in the way, making it ineffective, in terms of cost, to recycle them. Now, researchers have developed a method to recycle some of these plastics, allowing them to be reused.

IE interviews Wei Zhang at the Zhang Lab, University of Colorado, who, along with colleagues, succeeded in achieving just that using a class of durable plastics extensively employed in the microelectronics and aerospace industries.

'Our findings reveal that [a particular high-performance polymer] can actually be dynamic'

Professor Wei Zhang tells IE that in a new study his team achieved the 'closed-loop' chemical recycling of polycyanurates (PCNs), a class of high-performance engineering plastics, for the first time. Essentially, they demonstrated that these plastics could be broken down chemically into their most fundamental building blocks and then re-formed into new materials.

Conventionally PCNs are made when three molecules of cyanate esters (high-temperature polymers) react and produce a single product. This process is known as trimerization. Before the new study, material scientists believed this process was irreversible and limited to a specific class of molecules known as 'aromatic monomers'.

However, Zhang explains, "our findings reveal that PCNs can actually be dynamic (with reversible C-O bond breaking and reformation) under certain conditions, which enable polymer sustainability and mild conditions for the production of PCNs."

Polycyanurates (PCNs) as high-performance structural composites in industry

Due to their exceptional mechanical qualities and high working temperature, PCNs are often employed as binders in high-performance structural composites, primarily in the aerospace industry.

Important products made using PCNs include fiber-reinforced composites, syntactic foams, and filament windings. In the electronic industry, PCN resins are also used for thermal interface materials, encapsulants (sealants for lead-based paint), and adhesives for electronic chips.

They offer better thermal stability, superior dielectric characteristics, and far reduced moisture absorption than other chemical compounds, such as epoxy resins, making them favored in demanding applications. However, they are often less frequently employed due to their significantly high price.

The recycling of PCNs with no compromise to physical properties can be seen as a breakthrough

Demonstrating that PCNs can be fully recyclable and malleable is a huge step in the field. This is because polymers are tough to recycle. After all, they are made to retain their integrity and shape under high heat and other adverse conditions.

The study also demonstrated that recycling did not lead to any compromise in required physical properties, which Zhang explains to IE as a breakthrough in the field.

'A nice surprise: The polymers are very stable in ethanol, even in the presence of corrosive chemical conditions

"The high resistance of [the] PCNs to various corrosive chemical conditions came as a nice surprise," Dr. Zhang tells IE. Zhang explains that while PCNs synthesis and recycling take place in basic conditions, the polymers are very stable in ethanol, even in the presence of sodium hydroxide (NaOH).

The transparent nature of the recyclable PCNs, and their light transmittance similar to that of polycarbonate, was also a surprise for the team. Polycarbonates are a group of thermoplastic (becomes plastic on heating and hardens on cooling) polymers containing carbonate molecules. They are known for their high impact resistance and are commonly used in prescription and protective eyewear.

The researchers have yet to explore applications for the new sustainable PCNs

According to a press release, the novel chemical techniques can be used as a "plug and play" with current industrial production and is ready for commercialization. Still, Zhang tells IE, "we have not systematically explored the structure-property relationship and applications of this class of novel PCNs yet."

Zhang reminds IE that PCNs are well known to be expensive, and monomer manufacturing involves hazardous conditions that limits the practical applications to some extent.

Now, with the simple and sustainable PCNs synthetic methods the team developed, the next steps will include exploring those new possibilities for their property study and application development.

Reconsidering the chemical structures of other plastic materials could result in similar (recyclable) findings

The novelty of this most recent technology resides in the fact that it applies to existing types of difficult-to-recycle polymers, as well as creating a new class of polymer materials that are simple to manufacture, recycle, and reuse — a bit like Lego.

The team prompts the reconsideration of the chemical structures of other plastic materials, as this could result in similar findings. I.e., how to completely break down and recreate their chemical bonds, which could enable the circular production of more types of plastic materials in everyday life.

"Making new materials is exciting, but reinventing new methods that can address those challenges faced by existing materials is also very important," Zhang tells IE.

A 'closed-loop' recycling process inspired by the natural world

In a press release, Zhang describes this 'closed-loop' recycling as one inspired by the natural world, which is based on the view that "animals, plants, and human beings are now a part of a planetary-level, circular system of recycling similarly.

"Why can't we make our materials the same way?" concludes Zhang.