The ConversationMar 31, 2021 11:21:57 IST
It’s impossible to imagine everyday life without plastics. Lightweight, durable and cheap, these materials outperform many others in a diverse range of applications
Plastics have brought about positive change in ways we often overlook. For example, the development of plastic components in electronic devices, such as the one you’re using to read this article, means we’ve never been more connected to the world around us.
But our love of plastics has come at an environmental cost. It’s been estimated that of the 8.3 billion tonnes of plastic made between 1950 to 2015, over 75 percent is now waste, with 79 percent accumulating in either landfill or the natural environment.
For scale, that’s more than all living things on Earth, and our oceans are drowning in plastic. Because of this, recent research efforts have focused on addressing these mounting environmental concerns. One of these is chemical recycling.
The value of plastic
To overcome the huge environmental concerns created by plastic we need to start valuing plastic waste as a resource. After all, the plastic waste contains a value in the form of stable chemical bonds, so at the very least we should try to recover that energy. In fact, the stability of these bonds is why plastics linger for so long in the environment.
Beyond burning plastic to recover this energy, we can also recycle plastic. The world currently relies on mechanical recycling, where plastics are sorted, melted and remoulded to create mainly lower-grade plastic products. But this process is limited. The harsh conditions involved mean each time a piece of plastic is recycled, its performance properties are negatively affected. This limits the number of times a piece of plastic can be recycled.
To make sure plastic keeps its value in the long term, we need alternative recycling strategies. Chemical recycling provides the potential for infinite recyclability. But the challenge lies in achieving it in a sustainable and economic way at scale. Traditional methods are usually costly and energy or resource-intensive, which has limited their widespread use.
Plastics are made up of long-chain molecules known as polymers, which consist of smaller repeating building blocks called monomers. These monomers come in different shapes and sizes, and the bonding between them determines the plastic’s material properties – such as melting temperature and toughness – which affects the way it is used.
While mechanical recycling involves melting, chemical recycling relies on a chemical transformation and thus breaking the links between monomers. Chemical recycling breaks the plastic down at a molecular level. This means the monomer can be recovered in what’s called closed-loop recycling or the plastic waste can be transformed into other higher-value chemicals in open-loop recycling. For many types of plastic, it’s possible to recover monomers or other useful materials.
Some plastics, such as polyolefins – the material in a polyethene plastic bag – don’t have weak monomer links, making it harder to chemically recycle them. In such cases, a process called pyrolysis is used, a different process to burning, which relies on high reaction temperatures to typically produce fuels and waxes.
Catalysts are used in around 90 percent of industrial chemical processes. They make the process more efficient by providing the reaction with an alternative route, much like the way Google maps optimises your journey. They can also allow us to be selective about what product is created and reduce waste. Such benefits are central to ensuring chemical recycling can be performed both sustainably and economically at an industrial scale.
The enzymes that were working tirelessly during your last meal are naturally occurring catalysts that play an important role in digestion. Enzymes that can even break down plastics have been reported.
However, these processes are limited by their productivity and require specific process conditions – such as the right temperature and pH – to keep the enzyme active. But given how rapidly the field is advancing, using naturally occurring catalysts may be commercially viable in the future.
We’ve developed highly efficient metal-based catalysts for the chemical recycling of polylactic acid (PLA), a plastic made from plant starch. This work used cheap and abundant metals – such as zinc or magnesium – targeting chemicals called lactate esters, which are a potential green alternative to petroleum-based solvents.
This area remains in its infancy, but we expect significant developments, particularly in process optimisation, to be made as the field gathers momentum. This is in fact a general endeavour of the field because traditional methods typically use harsh chemicals, and can be resource and energy-intensive.
Beyond PLA, there is the potential to “up-cycle” other plastics, such as polyethylene terephthalate (PET), which is used for plastic bottles. Recent examples include building blocks for high-performance materials and antibiotics and corrosion inhibitors from PET waste.
Our recent work has also investigated the chemical recycling of PET, which is used far more extensively. PET is used more widely in plastic bottles and food containers, while PLA takes up a much smaller share of the market, used mostly for 3D printing, biomedical devices and certain packaging applications.
Given societies diverse plastic use, a one-solution-fits-all approach is not feasible. Diverse and tailored recycling strategies are needed for both existing and new emerging plastics. However, commercial-scale chemical recycling operations are underway.
In the future, we expect chemical recycling to complement its mechanical counterpart, especially for difficult to recycle materials such as thin-films. One thing is for certain, plastics are here to stay. With production expected to exceed one billion tonnes by 2050, chemical recycling promises to be an exciting space to watch.