Plexiglas Fully Decomposed Into Basic Components

Today, plastics recycling is primarily limited to the collection of sorted PET or polyethylene beverage bottles. The plastic collected is of identical chemical composition, with polymer molecules of similar lengths. The additives used to enhance properties such as colour, softness or sunlight resistance are also similar.

This process enables plastic to be melted down and reformed into new bottles. In contrast, plastics of various types and qualities (known as mixed plastics) are usually incinerated to generate heat in cement plants, for example.

A team of scientists led by Athina Anastasaki from the Laboratory of Polymeric Materials at ETH Zurich has discovered a method that enables the near-complete breakdown of Plexiglas into its monomer building blocks. By using additives, these building blocks can be easily purified through distillation into virgin-grade starting products for the synthesis of new Plexiglas polymers.

The potential implications are significant: with a global annual production of around 3.9 million tonnes, Plexiglas (chemically known as PMMA or polymethyl methacrylate) is a durable and lightweight acrylic material. It is gaining in popularity in the aerospace and automotive industries, in the manufacture of screens and monitors and in the construction industry.

The process developed by the ETH researchers and presented in the journal Science is highly robust. It is also effective with very long polymer chains made up of 10,000 monomer building blocks. Additionally, the presence of additives such as copolymers, plasticisers, dyes and most other plastics have minimal impact on chain scission. Even when using multicoloured Plexiglas from the DIY market, the yield remains between 94 and 98 percent.

Surprisingly simple process

"Our process is extremely simple," stresses Anastasaki: "All we need is a chlorine-based solvent and to heat the dissolved recycling mixture to a temperature of between 90 and 150°C to start the depolymerisation reaction with the aid of UV or visible light."

The ETH professor was amazed at how straightforward the process is. Like many other important plastics, such as polyethylene or polypropylene, Plexiglas polymers consist of a polymer chain of carbon atoms with various side groups branching off, depending on the type of plastic. Until today, these uniform carbon chains have posed an insurmountable chemical challenge for targeted splitting into monomers, as they do not provide specific points of attack for splitting reactions.

The only method currently used in industry that completely breaks down homogeneous carbon chains is pyrolysis. This involves the thermal decomposition of carbon chains at around 400°C. However, these reactions are non-specific, resulting in a mixture of various cleavage products. The large amount of energy required for this process, along with the costs associated with purifying the resulting mixture, severely limits the economic efficiency of pyrolysis.

For several years, various research groups have been experimenting with modified polymers. They have introduced easily detachable molecular groups at the ends of the polymer chains, which then trigger deconstruction from the end of the chain. In this way, the researchers have achieved yields of up to over 90 percent.

However, these designer polymers have several major disadvantages. In addition to the need to be first of all integrated into established plastic production, their reactive end groups significantly limit the thermal stability of the polymers and thus their possible uses. Furthermore, many of the commonly used plastic additives reduce the yield of the reactions, with the result that depolymerisation only works to a limited extent, even in the case of the long polymer chains that often occur in commercial plastics.

The solvent determines the reaction

As is so often the case in chemistry, the new method was discovered by chance. As Anastasaki explains: "We were actually looking for specific catalysts that would promote the targeted breakdown into monomers. But a control experiment led to the surprising revelation that the catalyst was not even necessary." The chlorinated solvent in which the crushed Plexiglas sample was dissolved was enough to virtually completely split the polymer with the help of UV light.

When the researchers took a closer look at the splitting reaction, they came across a surprising mechanism. They discovered that the chemically active particle in the reaction was a chlorine radical that is split off from the chlorinated solvent when excited by UV light. What was unexpected was that the high-wavelength light can break the chlorine's bond with the solvent molecule. This happens as part of a relatively esoteric photochemical phenomenon whereby a very small fraction of the solvent molecules absorbs high-wavelength UV light.

Anastasaki was able to count on the help of specialists from other ETH research groups to investigate the mechanism behind the splitting reaction. Tae-Lim Choi from the Laboratory of Polymer Chemistry calculated the theoretical electronic states of the molecules involved, while Gunnar Jeschke from the Institute of Molecular Physical Science carried out electron paramagnetic resonance measurements, which were used to experimentally verify the theoretical predictions.

The chlorine must go

In the future, however, the ETH researcher wants to dispense with the chlorinated solvent in her recycling process: "Chlorinated chemical compounds harm the environment. Our next goal is therefore to modify the reactions to enable them to work without the chlorinated solvent."

It is still unclear how and when the ETH method will be implemented in practice. In any case, Anastasaki and her team of researchers have opened the door to new recycling methods that can be used to bring about the targeted breakdown of previously chemically inaccessible carbon chains of plastics.