Self-healing polymer

Sustainable polymers: plastics with potential

Image credit: Science Photo Library

With the ever-increasing problem of plastic waste showing no sign of relenting, can a new breed of sustainable polymer end the nightmare?

In 2016, a 17-strong team of executives and analysts working for the Ellen MacArthur Foundation claimed that by 2050 the world’s oceans are expected to contain, by weight, more plastics than fish. For three years they had pored over hundreds of pieces of literature while consulting more than 180 researchers, engineers and policy-makers from universities, industry and government.

Their report, ‘The New Plastics Economy – Rethinking the Future of Plastics’, painted a bleak scene. At least eight million tonnes of plastics leak into the ocean every year; on par with dumping the contents of a refuse truck every minute.

Meanwhile, 95 per cent of the value of plastic packaging material, worth $80-120bn annually, is lost to the economy. If plastic use grows as expected, the plastics sector will account for 20 per cent of total oil consumption and 15 per cent of the global annual carbon budget by 2050.

Since the report, David Attenborough has delivered ‘Blue Planet 2’ with its rallying call to protect the oceans, Greenpeace’s petition for a UK-wide plastic microbead ban has become the largest-ever environmental petition to reach government, and the UK single-use plastic bag charge is set to double.

Yet be it a polyethylene bag, a polyethylene terephthalate bottle, an acrylic cardigan or even the polypropylene adhesive used to seal your teabag, the world’s toxic relationship with plastic continues.

“These plastics have great properties; they really are good materials,” points out Professor Andrew Dove from the School of Chemistry at the University of Birmingham, UK. “Look at polypropylene and polyethylene; it’s a huge challenge to replace them, as – being a by-product of the oil industry – they are just so cheap. This, combined with their excellent properties, is why they have been so successful.”

Professor Marc Hillmyer, from the Department of Chemistry at the University of Minnesota and director of the Center for Sustainable Polymers, which has its headquarters at the university, echoes Dove’s comments: “I’ve got to tell you that the main polymers we have right now are pretty awesome.

“They’re lightweight, strong, have great barrier properties and can be made cheaply in so many ways,” he adds. “That’s why they are so prevalent.”

However, despite their enthusiasm for today’s plastic fantastic, both Dove and Hillmyer are part of a growing band of researchers around the world devoted to developing more environment-friendly polymers as well as sustainable processes to recycle and re-use existing plastics.

Last year, Dove and colleagues revealed a simple method to break down, or depolymerise, polyethylene terephthalate (PET) bottles in less than two hours. The shredded polymer is immersed in ethylene glycol, which when warmed to 180ºC with a thermally stable protic salt catalyst, depolymerises at break-neck speed.

Crucially, the resulting monomer building blocks are so pure, they can be re-synthesised into high-quality PET for further use, opening the door to infinite recycling. Unlike conventional organometallic catalysts, the researchers’ organic catalyst can be recovered and re-used at least five times to depolymerise plastic bottles again and again. Dove is excited.

As he points out, researchers have already engineered enzymes to effectively break down PET, but he reckons his depolymerisation method could be “one of the fastest out there”. The researchers are now working on scaling up their lab-based process, and Dove is hopeful that a spin-out company based on the technology will ensue.

“This is the first time we have presented an industrially-relevant organocatalyst for high-temperature polymer degradation and recycling,” he says. “This has high potential for commercial application.”

Yet beating the economics of uber-cheap plastics such as polyethylene and polypropylene wasn’t ever going to be easy. For years, heavyweight developers of petrochemical alternatives, such as NatureWorks and Dupont, have been producing biopolymers using plant sugars. Yet only now are prices becoming competitive with oil-based counterparts.

While developing new processes to infinitely recycle plastics could bring an end to spent plastic waste, costs still count, a fact that the University of Minnesota’s Hillmyer is only too aware of.

As he points out: “To compete in the plastics marketplace really does takes a lot of low-​cost effort, but we can compete on a sustainability and biodegradation play.”

Several years ago Hillmyer and colleagues developed a biomass-derived polymer based on a novel monomer called beta methyl valerolactone that can be quickly synthesised and just as quickly recycled. Steering clear of your cheap-as-chips polyethylene and polyethylene terephthalate plastics, they hope their biopolymer will soon be used in higher-value products such as the polyurethane foams found in mattresses.

To obtain beta methyl valerolactone, Hillmyer and colleagues simply ferment glucose, extracted from plants. This monomer is then combined with a highly active organocatalyst to swiftly produce the rubbery polymer, poly beta-methyl-delta-valerolactone.

To make a more useful plastic, the rubbery polymer is then reacted with polylactide, an established, rigid, biodegradable polymer derived from renewable sources.

The result is a bio-based polymer that can be ‘tuned’ to have a range of mechanical properties akin to those of your stretchable rubbery elastomers, polyurethane foams and other useful polymeric materials. These polymers can then be chemically recycled to yield high-quality monomer.

“We can make our starting molecule from sugar quite inexpensively and use this to get elastomers, polyurethane foams and even tough plastics,” explains Hillmyer. “Importantly, we can recover the monomer and re-use it.”

Following initial development, Hillmyer and colleagues set up ‘Valerian Materials’ in 2016 to commercialise their technology, and right now are focusing on cost-effectively scaling up processes.

“There are a lot of polymers out there that do great things, but many can’t be produced inexpensively,” says Hillmyer. “But here we have an inexpensive sugar-based polymer that is chemically recyclable and, by all our initial indicators, compostable too.  I think this is very exciting.”

Clearly recyclability is critical to any plastic of the future, but a group of chemists and materials scientists from the US-based University of Illinois at Urbana-Champaign have taken this to the extreme. Professor Jeffrey Moore and colleagues have been honing a particular polymer – cyclic poly(phthalaldehyde) (cPPA) – to, quite literally, disintegrate on demand.

Interest in cPPA first emerged as early as 1969, as it was easy to synthesise and could depolymerise at an astonishingly fast rate in response to stimuli such as acid and heat.

The trick to the polymer’s drastic degradation stems from its thermodynamical instability at ambient conditions; break its bonds and it swiftly unzips into its monomer. On the downside, cPPA’s knack for self-destruction also means it is often unstable during use and can degrade too early.

However, by adding stabilisers and a plasticiser, Moore and colleagues have raised the material’s thermal tolerance and can also mould the material into complex, solid shapes that still depolymerise on demand.

Importantly, their cPPA also has mechanical properties akin to polystyrene and polyethylene terephthalate. After depolymerisation, the monomer can be re-polymerised to produce new cPPA with identical properties to the original material.

“We’ve found that cPPA is sensitive to a number of different stimuli; you can heat it, apply mechanical force, and it’s also sensitive to sunlight,” explains Evan Lloyd from the group. “Depending on conditions, the polymer will destabilise within minutes or hours.”

A clear application for cPPA is packaging, and Lloyd reckons that a waste-collection company could easily recycle the end-of-life product by applying the necessary stimulus, so it rapidly depolymerises ready for re-use, rather than reaching landfill.

What’s more, he and colleagues are also adding carbon nanofibres to cPPA to create lightweight and high-performance composites for eventual applications in, say, sports, aerospace and automotive industries. As Lloyd points out, such cPPA-based composites could be recycled, and the high-value carbon fibres recovered, more easily than today’s materials.

“The stability of traditional composites is really exceptional by design, but if you want to recover the carbon fibres you need to treat the material at more than 400ºC for many hours to burn off the polymer,” he says. “But since we are converting our polymer to a gaseous monomer at temperatures near 100°C, we can easily recover the fibres in a pristine state, recycle the polymer and then use these two components to make a new composite.”

Yet not everyone is trying to push the self-destruct button on polymers. Polymer and materials scientist Professor Marek Urban from Clemson University, South Carolina, US, wants to make durable polymers that last longer and contribute less to the growing, and floating, mountains of plastic waste.

To this end, he and his research group have combined two common monomers found in many acrylic-based plastics – methyl methylacrylate and n-butyl acrylate – to produce a copolymer that repairs itself independently after being scratched. This spectacular act of self-healing takes place thanks to the short-range van der Waals forces that exist between the macromolecules.

“We [combined] the monomers so they would be oriented to interact with neighbouring polymer chains,” he says. “This directionality enhances the van der Waals forces so that if these forces are broken [when the polymer is scratched], they have an affinity to return to their original [orientation], and so ‘self-heal’ the material.”

According to Urban, scratches 30 microns or larger in width can re-seal within some 14 hours, while a severed cut will close in around 80 hours. What’s more, he is adamant that his simple but precisely controlled synthesis process can be easily scaled for commercial production, with the resulting self-healing polymer used to protect any surface, expand the shelf life of, say, electronics devices and batteries, or serve as a sustainable structural material.

“I have a film on my cellphone that’s like a protective screen, and I can peel it off and then put it back on,” he says. “I can also stretch it and it will still come back... I am doing this right now as we talk, so you see, this all takes place under ambient conditions.

“This technology is available to anyone that is interested,” he adds. “We are just waiting for someone who’s willing to pick it up and run with it.”

Urban isn’t alone in his quest to deliver a self-healing plastic. While his latest polymer heals entirely on its own, other researchers have devised materials to seal larger cracks and holes in response to stimuli such as light and heat. Indeed, as well as mastering the art of self-destruction, the Moore Group at the University of Illinois has achieved just this.

As early as 2001, and alongside the late Professor Scott White, Moore and colleagues were devising self-healing polymer composites that contained microcapsules filled with liquid monomer. If a crack was to run through the polymer and break open the capsule, the monomer would be released, react with an embedded catalyst, and polymerise to bind the crack.

Over the years, the researchers pioneered more sophisticated polymer systems that could heal ever-larger cracks. And then, in 2014, they revealed a regeneration strategy that could replace lost material and recover its strength after large-scale damage, such as a crack in a water pipe or a bullet-hole in an aircraft wing.

Inspired by the human blood-clotting system, this polymer contains a network of capillaries to deliver ‘healing’ chemicals to damaged areas. Upon damage, fluid streams are pumped into the crack or hole which then mix, start to gel, and harden over time into a structural polymer.

Importantly, the researchers can tune the kinetics of this reaction so that, say, gelation takes place more quickly or hardening is slowed down to fit the type of damage. By doing this, the system can effectively repair a gaping-wide bullet hole as well as any tiny microcracks emanating from it.

Development continues, and the researchers eventually hope to create a polymer that continually regenerates itself, opening the door to plastic structures that last a very long time. As Lloyd points out, one day these self-healing aspects of a polymer could even be combined with the ability to self-destruct.

“The goal of our research is to develop a [polymeric material] that actually has life-cycle control,” he says. “We could design a material that can repeatedly heal itself if subjected to damage while in service, but once damage is irrecoverable, we trigger depolymerisation, get the materials back and manufacture it again.”

Lloyd also envisages having a polymer that could self-heal via depolymerisation and repolymerisation. In this way, the plastic would revert to the monomer at the damage site and then rebuild itself locally.

“You could even imagine designing a polymer with alternative chemistries that react with the monomer once available, after damage,” he adds.

“With this, the polymer could be rebuilt with different mechanical properties than the original material, so it doesn’t [succumb] to that particular damage again.”

Lloyd’s vision of a new generation of plastics may seem incredible, but the underlying sentiment is echoed by many in the field. Urban, for one, believes that tailoring a polymer to a particular purpose is going to be crucial.

“If I want a durable architectural coating, then I want to be able to make this self-healing so I don’t have to spend money and energy remaking it five years down the road,” he says. “But if I want a plastic bag just to carry my groceries home, then I want to make it so it disintegrates on its own, within a day or two.

“We need to get smarter about designing polymers, so they can be tuned to have desirable properties for specific applications,” he adds. “We don’t do this right now; all we have is inexpensive processes that aren’t thought through, and that’s why we have tonnes of plastic in landfills.”

Still, as the University of Birmingham’s Professor Dove says, any change in the set-up of existing polymer manufacturing processes won’t be easy or cheap. He believes that right now, our biggest chance for change comes from public demand and government legislation, especially given today’s “high level of consciousness” of the plastics problem.

“This is a complex problem with no single solution,” he points out. “We need new polymers, sustainable sources, and we need to look at end-of-life plastics separation and collection as well as recycling to retain value and usability, and reduce waste.

“All of this needs to be considered and that’s what’s hitting me the most about our field. Many people are focused on a single aspect and the solution is so much broader than that.”

Plastic population

Recyclable or not?

Several biodegradable plastics are on the market today, but are they truly recyclable? Not necessarily, as once made they can’t be completely reconstituted into their original monomeric states without forming, often, unwanted by-products.

In the true chemical sense, recyclability means monomer molecules can be synthesised into a useful material and then completely converted back to the same molecules. This is what researchers worldwide are striving to achieve.

One frontrunner, Professor Eugene Y-X Chen from Colorado State University, hit the headlines in 2015 when he unveiled a polyester that could be returned to its original monomer state simply by heating it for an hour. His starting building block was a biorenewable monomer – y-butyrolactone – that, at the time, textbooks had declared non-polymerisable.

Importantly, Chen and colleagues had devised precise, low-temperature reaction conditions, using both metal-based and metal-free catalysts, to polymerise the stubborn monomer. They could then heat it between 220ºC and 300ºC to convert the polymer back to its original monomer. The polymer – poly-y-butyrolactone – was the chemical equivalent to commercial poly(4-hydroxybutyrate), or P4HB, used in medical applications.

Fast-forward to today, and Chen has revamped his recyclable polymer and can now produce a more rigid version with a higher molecular weight. Crucially, the monomer can also be polymerised under more industrially realistic conditions.

So, like many of his peers, he is now devising more cost-effective ways to make his polymers at scale. “These polymers can be chemically recycled and reused, in principle, infinitely,” Chen says. “It would be our dream to see this chemically recyclable polymer technology materialise in the marketplace.”

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