Q&A: A breakthrough in plastics recycling

Recyclable thermoset plastics are one of 10 emerging technologies of 2015 highlighted by the World Economic Forum’s Meta-Council on Emerging Technologies.

There are multiple types of plastic, and historically only some of them, including thermoplastics, could be recycled. A prominent type, thermoset polymers – which are used in everything from the sheathing of electrical cables to toys, Tupperware and electronic circuit boards – instead ends up in landfill at the end of its useful life.

However, a breakthrough discovery at IBM Almaden Research Center now looks set to make this type of plastic widely recyclable, too. We spoke to Julia R. Greer, Professor of Materials Science and Mechanics at the California Institute of Technology, about what has changed and why it matters. This is an edited transcript of the interview.

Q: Why couldn’t we recycle thermoset polymers until now?

These polymers are highly cross-linked, which means the molecules have extremely strong chemical bonds. Once you’ve initially heated them up to set them into the shape you want – hence the name “thermoset” – they’re irreversibly bound. That means they’re much stronger than thermoplastics, but also that heating them up again won’t break down the bonds in a way that enables you to usefully reshape them, as with thermoplastics. It’ll just char them and crack them instead.

What the IBM researchers realized was that they could make a kind of thermoset polymer that could be broken down again, not by heating it up but by dissolving it in strong acid and then reforming it into a different shape. The researchers used a chemical mechanism called the Diels-Alder reaction, which has been known about since 1928. The breakthrough was in realizing that it could be applied to thermoset polymers, to unlink the polymers into monomers that can then be reshaped into a new functionality.

Q: If the reaction is so well established, why had nobody thought of it before?

Sometimes it takes a fresh perspective combining different disciplines to make this kind of breakthrough. People have been using the Diels-Alder reaction for decades, and the thermoset polymers have been used for decades, but these areas of expertise have largely resided in different scientific communities with different mindsets. Quite simply, nobody had thought of putting them together until now.

From my experience, also, we shouldn’t underestimate that it’s a real challenge to discover reversible reactions in chemistry – you have to find the right catalyst, the right conditions; the reaction has to be thermodynamically favourable, and so on. There are lots of constraints. This is an impressive discovery because it’s so simple and elegant – but it’s only in retrospect that it seems obvious.

Q: It was only last year that this discovery was published. What are the next steps?

There’s nothing overly complicated about the chemistry involved, we already have large-scale facilities and equipment that can do it; and the Diels-Alder reaction itself is not subject to any kind of patent, although specific polymers might be. Given this, we can expect it should be a relatively straightforward process to get from demonstrating feasibility in the lab to reaching commercial scale. I would imagine there will be quick progress this year.

What we will need to do, however, is test the properties of these recycled polymers over multiple life cycles. This is not a completely loss-less process – there may be some degradation. And some applications of thermoset polymers depend on the presence and types of defects, as well as on mechanical properties like stiffness and strength, to assure structural integrity. We’ll need to be confident that the thermoset polymer still has the same attributes after being recycled multiple times. Because this process is so new, we have no idea yet whether problems such as increasing brittleness may start to develop after a certain number of cycles.

Q: Assuming the testing goes well, is recyclability likely to lead to any new applications for thermoset polymers?

Not necessarily, but that’s only because they are already so ubiquitous – thermoset polymers are in everything from insulation and tubing to storage boxes and glue, and in every industry from micro-electronics to automotive and aerospace.

One potential interesting consumer application for the longer term is that recyclable thermoset polymers could expand the possibilities of 3D printing. You can imagine we might one day develop home units for recycling – so if you print a piece from the thermoset polymer and realize its design was wrong, you could recycle and reprint it rather than having to throw it away. That’s just a small part, however, of the wider environmental benefits we can anticipate.

Q: What will those environmental implications be?

For a start, we should be putting a lot less in landfill. And thermoset polymers are particularly problematic in landfill because all the attributes that make us value them during their useful lifespan – that is, they’re highly resistant to sunlight, heat, moisture, other chemicals and environmental agents – also mean that they take hundreds of years to degrade, and take up lots of space.

We can also expect to be releasing a lot less carbon, as plants that currently synthesize new thermoset polymers from raw materials are repurposed to recycling them – although here, again, we’ll need more testing on the possible environmental implications of the by-products created by the recycling process. Recycling is certainly likely to be much less environmentally harmful than the initial synthesis, but given the newness of the discovery it will take time to figure out all the implications.

Something like a third of all the polymers currently produced in the world are thermoset, so you can see how enormous the scope is for reducing waste and environmental damage.

Reporting by Andrew Wright for the World Economic Forum.

Image: Crushed plastic bottles are seen at a recycling plant near Laval, western France October 20, 2011. REUTERS/Charles Platiau