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Recycling the ‘unrecyclable’: a new class of thermoset plastics

Plastics comprise around 10% of solid waste in Australia. And while we can recycle certain types, there is a group of particularly stable plastics called thermosets, common in electronic devices, which…

Electronic thermoset components, such as those found in mobile phones, are destined for landfill – but new research points to a way to make them recyclable. David Goehring/Flickr, CC BY

Plastics comprise around 10% of solid waste in Australia. And while we can recycle certain types, there is a group of particularly stable plastics called thermosets, common in electronic devices, which can’t be broken down and recycled – until now.

In a paper published in Science today, researchers describe a way to manufacture recyclable thermosets, thus allowing the plastics to be broken down and reformed.

So what is a thermoset, and what makes it so difficult to recycle? First, let’s examine the properties of traditionally recyclable plastics – thermoplastics – and compare them to thermosets.

Walter Benson/Flickr, CC BY-SA

Thermoplastics – such as plastic milk bottles, Lego bricks and guitar picks – can be melted and reshaped repeatedly. If you hold a flame to a plastic milk bottle, it will warp and melt (although I recommend against doing so as the fumes aren’t very good for you).

But thermosets – found in kitchen utensils (think of a plastic spatula) and in electronic components such as circuit boards – retain their strength and shape even when heated, making them ideal for the production of permanent components with large, solid shapes.

This ability to withstand heat is due to the “curing” step in manufacturing – such as heating, pressure and adding catalysts – which changes the state of the material at the molecular level.

As a result of curing, molecular chains in a thermoset are chemically bonded into exceptionally strong cross-linked networks. These prevent a thermoset object from melting as a thermoplastic would; when thermosets are heated, they crack or become charred.

Most common types of thermosets include epoxies, phenolics, polyesters and silicones. In general, thermosets are known for their good adhesion, high chemical and heat resistance and excellent mechanical and electrical insulating properties.

Because of these superior properties, thermosets are used in a broad range of applications in the microelectronics, transportation and aerospace industries, as well as in varnishes and glues.

Thermosets resist heat much better than thermoplastics.

Breaking down the unbreakable

The authors of today’s Science paper created recyclable thermosets called poly(hexahydrotriazine)s (PHTs) which break down in strong acid. The pure monomers (single small molecules of the plastic material) were recovered and reused (polymerised) to make new products.

The properties of PHTs were similar to those of the traditional, unrecyclable thermosets: rigid, heat-resistant and chemically stable and displaying excellent resistance to solvents and environmental stress, especially when reinforced with carbon nanotubes.

Moreover, by varying the combinations and compositions of the monomers used in their reactions, the researchers were able to produce elastic gels with self-healing properties. These gels can also be broken down in strong acid.

This work is an important advance as thermosets have long been considered impossible to recycle.

Depending on the manufacturing process, substances can yield both thermoset and thermoplastic products:

arbyreed/Flickr, CC BY-NC-SA

  • paraformaldehyde is commonly used for manufacturing of thermosets such as phenolics, and thermoplastic polyoxymethylene (POM). POM is used in precision parts and high performance engineering components such as small gear wheels and ball bearings
  • aromatic and aliphatic amines are used for production of thermoset benzoxazine resins, which are then used for manufacturing products such as high-temperature composites and electronic components. These amines are also raw materials used for producing thermoplastics including aromatic nylons for manufacturing high–strength Kevlar fibres (such as those found in some bullet-proof vests).

Simply using different combinations of monomers provides an extremely versatile materials platform. The polymerisation process reported in Science today is based on the well-understood reaction of amines with paraformaldehyde to prepare a new class of thermosets.

This method can be easily used on a wider scale with the existing facility and equipment in the resin industry. It is both technically feasible and inexpensive to produce these recyclable thermosets at the commercial scale.

These novel thermosets would replace existing thermosets in many applications such as in modern electronic devices, as well as applications in the automotive and aerospace industries.

But most importantly, the majority of products made with traditional thermosets go to landfill. If this research is implemented widely, we’ll see fewer thermosets in landfill and more being recycled, just like thermoplastics.

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8 Comments sorted by

  1. Victor Jones

    Freelance

    Excuse my ignorance, but why aren't non-recyclable materials burned and the heat used as energy for grid power, as is done in other countries?

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    1. Peter Cunningham

      MD

      In reply to Victor Jones

      I'm just guessing here, but I would have thought the answer to be the unwillingness by communities - particularly 'green' types - to have high temperature incinerators built. It's easier to whinge and complain than do anything constructive - so the problem(s) worsen.
      Such facilities couldn't survive on plastics alone, which means other really nasty industrial wastes need also be handled - but "NIMBY" (not in my back yard).
      This then gives rise to transport objections and the everpresent "safety…

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    2. Andrew Fisher

      Worker

      In reply to Peter Cunningham

      High temperature incineration is very expensive to build and maintain and is not without its own problems. given the limited resources we are willing to devote waste management/polution reduction, incineration may not be the best solution.

      For a start, an incinerator would require a constant supply of the fuel/waste in order to be cost effective. This could produce commercial/political pressure against continued efforts to reduce the use of these hazardous materials. Besides, High temperature…

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  2. ian cheong

    logged in via email @acm.org

    The mot commonly recognised thermosets are probably old brown Bakelite electrical switches, domestic light and power switches, melamine plates and bowls. Most plastic kitchen utensils are thermoplastic. A smaller number are melamine or silicone. Most polyester people are familiar with is the thermoplastic type as used in (flammable) clothing.

    Thermoset plastics in electrical devices are mostly in switch mechanisms and high voltage insulators. Integrated circuit packages are ceramic. Printed circuit boards are fibreglass resin.

    So how much thermoset plastic is there really in a phone (as implied by the photo in the article???

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    1. Andrew Fisher

      Worker

      In reply to ian cheong

      fiberglass uses either polyester resin or epoxy resin, some IC packages are ceramic, but many are epoxy.

      A phone may not have that much of these materials and besides, separating the different elements from each other is another challenge.

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  3. Steve Hindle

    logged in via email @bigpond.com

    Thanks Qipeng, This could be an excellent development. In a world of finite resources it is essential we get better at recycling plastics (and the worlds oceans need a break)
    I am guessing that there would be a higher cost involved in manufacturing. As it is normal for competitive pressures to work against any inputs that are more costly, if successful, these plastics would need to be pushed into use through Govt mandates.
    Perhaps it is early days yet, but I hope the chemistry problems can be solved at somewhere near realistic prices.

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  4. George Michaelson

    Person

    Many plastics include complexes of soft and hard parts. I agree its a shame the hard part thermosets are challenging for recycle/reuse, but what annoys me is the tendency of the soft-plastics to degrade. I cannot count the number of times I've gone to a device with soft touch feet, to find they have converted back to a sticky black liquid treacle mess.

    The thermosets can go brittle. I have 1920s bakelite-style formed goods and some of them have gone like a toffee converting to fudge.

    Lastly, softeners include the class of materials which are bioactive and a problem. I don't believe the phenolics are, but the formaldehyde can't be good for us. Whats going to be potential for leachout in the new stuff? If not easy to recycle, landfill is the destination. If landfill is acidic damp, what happens?

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  5. Alex Cannara

    logged in via LinkedIn

    Every hydrocarbon is recyclable. The types we don't recycle are simply unprofitable in present markets, which is our typical short-sighted dumb-as-a-hoe thinking.

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