Plastic. This simple yet ingenious material literally changed our lives. Have a look around you, and you’ll notice that most things contain plastics. The computer, which I’m using to write this blog, the clothes that I’m currently wearing, and pretty much everything I can lay my eyes on in my room contains plastic. We’ve been so used to plastic, it’s hard, if not impossible, to imagine life without it — and for good reasons too. They prevent food loss, keep our medicine safe and are great as a single-use material to prevent contamination of deadly microbes or chemicals.
Plastic is amazing for us humans; it’s what enables our society and our lifestyles.
I’m not a chemist, but I’d imagine that it’s a synthetic chemist’s dream. Imagine being able to create something that can withstand nature. Water and electricity can’t pass through plastic, microbes and insects can’t degrade it, and it’s mostly unaffected by seasons. Plastic is virtually indestructible. And therein lays the problem.
Modern life wouldn’t be possible without plastic, but we’re drowning in plastic waste because of its durability. They’re buried everywhere in our land, forests, mountains and cities. They float around in our oceans, rivers and lakes. They’re in the air that we breathe finding their ways into our lungs as well as in the water that we drink. Plastic is everywhere, and we can’t get away from it.
Plastic is terrible for our environment; it’s useless for animals, plants, microbes, insects and fish.
OK — so there’s a clear contradiction here. The plastic itself is turning out to be an oxymoron: plastic is great for our society; it’s also an environmental nightmare. What we need to remember is that there’s no society without the environment. We co-exist with other organisms (big and small), who maintain the ecosystems that enable our existence. It’s these beautiful creatures that create the perfect, pleasant conditions in which we live. Plastic waste threatens that harmony, and we must do something immediately to tackle the plastic waste crisis before it’s too late.
Before I start talking about what we can and must do, let’s start by asking ourselves: “how did we get to this point?”
No exit strategy
There are different types of plastics whose chemical composition dictate their properties and appearance. All plastics are polymers, which is a chemical chain structure made up of repeating units of molecules. “Poly” means many and “mono” means one — so a polymer contains many repeats of a single building block subunit molecule called a “monomer”. The monomer that makes up plastic is usually hydrocarbons (chemicals made of hydrogen and carbon) from crude oil, coal or natural gas.
Making polymers from crude oils, which contain thousands of chemicals, is a long process. First, crude oils undergo an extensive refining and distillation process to separate the crude oil into different petroleum (hydrocarbon) products. Of the several hydrocarbon layers that form during crude oil distillation, one of them is “naphtha”, which contains four to eleven carbon atoms. Naphtha is further processed to produce several different monomer molecules for a range of plastic production. These monomers undergo a process called “polymerisation”, which is when monomers are chemically stitched together by a bond to form chemical chains or polymers. Finally, polymers are moulded into different forms of plastics with distinct properties.
The most common polymer materials used to make plastics are “polystyrene” (PS), “polypropylene” (PP), “polyethylene” (PE), “polyvinyl chloride” (PVC) and “polyethylene terephthalate” (PET). Plastic production is dominated by the products made from “polyolefin” polymers, which includes PE and PP plastic derivatives.
All plastics are polymers, and the plastic that I described above is a synthetic polymer. Still, all polymers aren’t plastic — and bioplastics also exist (but the focus here is mostly on synthetic plastic). Anyway, almost all other polymers (natural or synthetic) can breakdown into simpler molecules.
For example, proteins or carbohydrates are also polymers, but unlike plastics, they can undergo degradation. And when these polymers are broken down, they’re consumed by other organisms (microbes, insects or plants). When we eat food, our bodies break down the protein and carbohydrate into simpler molecules that give us energy and maintain the balance of molecules in our cells that keep us alive. Therefore, the molecules that make up these proteins or carbohydrates are constantly recycled in a “circular ecosystem”.
Our handling of plastics is a classic example of “linear economy”, where we make things, use and dispose of them. Waste is a massive problem in the way we live, but in most cases, nature takes its course and breaks down our waste materials. Not in case of plastic — there seems to be no after-life for plastics. When chemists first made plastic, they designed it without an exit strategy. They wouldn’t have known that the polymer they made would be undegradable. Despite all of our scientific technology and expertise, we still don’t have a clear way out of the plastic waste crisis — and neither does nature.
Problems with recycling plastic
Ideally, we’d make plastic, use it and recycle it to use again in a “circular economy” for plastic. Unfortunately, our record of recycling plastic is terrible. By 2015, we’ve produced 8,300 million tonnes of plastics but only recycled 500 million tonnes. That’s only 6 percent — and it gets worse if we further breakdown to what happens to the recycled plastic.
Only 100 million tonnes of recycled plastic is still in use, while 300 million tonnes are dumped away, and 100 million tonnes are burnt. In summary, just over 1 percent of the total plastic we’ve made is recycled and still used (see the figure below).
Yes, plastics’ properties provide easy and cheap utilities, but there’s a significant untapped economic potential in reusing plastic. 95 percent of the value of plastic is lost after a single use — this clearly needs to change. In economic terms, that’s a loss of 80–120 billion US dollars every year.
Wider adoption of plastic recycling is vital for the environmental crisis, and it’s also good for the economy. So why aren’t we recycling plastics? To answer this question, we need to understand what happens to plastics after they’re used.
Plastic waste usually undergoes one of the following three routes:
- Dumped in landfills. Most of the plastic end up wasted in landfills, causing pollution and harm to the environment. As landfill waste, they lose all their economic value. We can all agree that this is a terrible option.
- Burnt for energy production (energy recovery process). As shown in the figure above, about 12 percent of all plastic waste are burnt. The idea is that energy stored in plastic can be recovered in the form of heat. While the idea is well-intended, the benefit of energy produced from burning plastic is significantly offset by the environmental damage it causes. Burning plastic releases greenhouses gases and other toxic chemicals into the atmosphere that cause global heating and air pollution. Additionally, the amount of energy we generate from burning plastic is significantly less than the energy saved by recycling.
- Recycled for reuse. With the two options above being useless, recycling is the only viable and sustainable option to manage post-use plastics’ fate. There are several forms of plastic recycling (see the figure below). “Closed-loop” recycling of plastics involves reprocessing used plastic to return them into their original product of the same or similar quality. Also termed as primary recycling, this form of recycling only works on clean plastic waste. However, almost all plastic waste is contaminated or mixed with other materials, making it extremely challenging to recycle plastics to the same quality as the plastic made from virgin feedstock material. That’s why most of the plastic collected for recycling undergo “mechanical recycling”. This involves sorting, washing and drying plastics before melting to produce new plastic materials. The processing in mechanical recycling, which is a significant challenge in itself, causes degradation of the plastic by changing the polymer’s chemical structure. Mechanical recycling compromises the desirable properties of plastic such as its toughness or elongation at break. It results in lower quality and lower value plastic than the original material. Eventually, the plastics that undergo mechanical recycling also end up dumped in landfill or are burnt.
If we look at the three routes that the post usage plastics undergo, we’ll all agree that they should be recycled. But, the current recycling methods themselves aren’t good enough to produce sufficiently high-quality material from recycling — unless the plastic waste is pristine and uncontaminated, which rarely happens.
That’s why we end up dumping or burning most of the plastic rather than recycling them to make useful materials again.
So how do we solve the plastic waste problem? We must start tackling this problem with two approaches: 1) design next-generation plastics using reusable materials for easy post-use recycling, and 2) design new catalysts and enzymes to breakdown existing plastic into starting materials to make virgin-quality plastic, repurpose plastics into other value-added materials, or decompose plastics for biological recycling.
New plastics, new design principles
Future design and development of new plastics must consider plastics’ fate at the end of its life. After using plastic, we must recover its economic value and materials to improve its environmental impact. To resolve the plastic waste problem, we need to design our plastic to enable breaking them down into simpler molecules that could be reused to make virgin-grade polymer again, repurpose plastic into other value-added products or undergo biological recycling. Ideally, we’d also like to retain plastic’s current properties to be strong, elastic, clean, etc. These design principles must be key considerations for any future plastic development.
As I explained earlier, polymers (which includes plastics) are chains of monomers linked together formed by a polymerisation reaction. New plastics’ design could include “depolymerisation”, where we break down polymers into their single units (starting material) and feed them back into plastic production to make virgin quality polymers.
Polymers are abundant at our disposal, and there are opportunities in polymer chemistry to identify polymers that could undergo depolymerisation (to release monomers) followed by repolymerisation. Chemists are designing built-in mechanisms for polymer degradation under controlled conditions. This approach could lead to plastics development for targeted function, whose recovery strategy is embedded in its design.
One challenge in recovering monomers from polymers is that the process is expensive and energy-intensive. It’s made more difficult as plastic waste is often mixed with other complex contaminants, so the plastic monomer isn’t always pure. To overcome these problems, scientists have created a next-generation polymer (“polydiketoenamines” or PDK), using special chemical linkage that makes it easy to separate the plastic monomers from other waste contaminations.
Adding a strong acid to PDK is enough to release its constituent monomers. This process doesn’t need a special catalyst and works at room temperature, requiring less energy. Moreover, chemists showed that the monomers could be recovered from a complex coloured plastic waste mix. The recycled polymers can form new virgin plastic without the colour. Researchers say that “the ease with which PDK can be manufactured, used, recycled and reused — without losing value — points to new directions in designing sustainable polymers with minimal environmental impact.”
Scientists are developing several new polymers with the design principles to recover their monomers. For example, a polymer that breaks down into its monomer simply by heating at 260-degree Celsius for an hour; or a plastic that only takes a few minutes to polymerise at room temperature and breakdown into monomers in the presence of a catalyst. The science is promising, but the challenge is to make these processes competitive and implement them at a bigger scale.
The strong acid that breaks down PDK — and 260-degree Celsius — aren’t easily available, so they’re unlikely to fall apart before their time. The durability of plastic allows its wide range of functionality, but the polymer that breaks down easily loses one of its most desirable properties. We want polymers that are easy to recycle, but they must also be robust enough to fulfil their functions.
One category of plastic, which has a good exit strategy (in theory) are biodegradable plastics or “bioplastics”. These polymers can be degraded by enzymes or hydrolysis (reaction with water) into natural molecules to be part of a circular ecosystem. However, bioplastics have their own associated problems. Their starting materials usually come from plants, which has associated environmental impacts, including deforestation for agricultural land and high water use. Bioplastic may also threaten food security as the agricultural products are used to make the biopolymer rather than feeding human or livestock.
You may think that biodegradable plastics break down quickly to enter the circular ecosystem, where plants and other microbes take up the carbon from the bioplastic. However, some of the most common biodegradable plastics aren’t degraded by microbes, which is why they’re slow to breakdown. That’s why experts say that bioplastics should be composted at industrial facilities rather than buried in landfills. As most countries don’t have the infrastructure required to support biowaste treatment, bioplastics also end up in landfills. With a high volume of plastic use, bioplastics in landfills can also cause excess waste contributing to environmental pollution just like synthetic plastics.
Biodegradable plastics are also much more expensive to make in comparison to synthetic plastics. Economically, we’d want to lower the price of bioplastic and recover their value after use. Several innovators are developing a new generation of efficient bioplastics bringing down the cost. Cheaper bioplastic is vital for it to be economically competitive against synthetic plastic, whose price is dependent on the oil price that continually fluctuates. But the fact is that bioplastic is economically inefficient as most of its end of life value of bioplastic isn’t recovered. The degradation of biopolymers results in carbon dioxide, water and other molecules — which may form part of a circular ecosystem — but it’s still doesn’t close the loop on the economy.
In terms of profit, the future design of polymers should adopt the principles of the circular economy. Developing infrastructures to enable bioplastic composting, which can be converted into biogas or plant fertiliser can close the economic loop by recovering some of the value of bioplastic, and the environmental loop through biological recycling.
When designing new plastics, it’s also important to remember why we use them. Plastics made from “polyolefin” — for example, polyethylene and polypropylene — make up more than half of total polymer production and waste. These plastics are so popular because they’re the most useful for us.
Next-generation polymers with similar properties to polyolefins will make the new material more competitive against common plastics, and deliver the biggest environmental and economic impact. In summary, next-generation polymers must be designed to give high performance and recyclable by break down under right conditions at the end of their life. New polymers’ production must meet specific needs.
Existing plastic waste
Yes, we must design and make new polymers that are easily degradable, but that doesn’t get rid of the current plastic waste. Two options for the current plastic waste include degrading them or recycling them. Both of these options require breaking down the plastics into simpler molecules. We must prioritise identifying new approaches to break down existing plastics. New catalysts and enzymes that break down plastics will help the recycling by:
- degrading plastics to allow biological recycling so that the molecules that make up the plastic enters a circular ecosystem, or produce byproducts with practical use;
- breaking plastic polymers into their starting subunits to enable the development of virgin quality polymers; or
- breaking down polymers into simpler molecules to be repurposed into high-value materials, like other plastic forms, fuels or waxes.
Degrading current plastic waste
Scientists have found dozens of organisms that breakdown plastic, including bacteria, fungi and insects. As a result, we’ve seen some sensational headlines that suggest these organisms to be saviours of environmental pollution. While it’s positive that several species can break down plastics, it’s not practical (or desirable) to expect them to eat all plastics around.
Most organisms that breakdown plastics, degrade them extremely slowly or degrade them partially; some shred the plastic rather than degrade it. We need to make the “plastic-eating” organisms better at breaking down plastics — and that’s a big challenge.
If a magical bug eats all plastics in a short time, it would be a massive problem in itself. Such is our reliance on plastics that our society would collapse without them. We won’t find anything that will eat all plastic anytime soon, so we can’t rely on nature to solve this problem in the timeframe that we want. However, the ideas to resolve the plastic waste crisis could come from nature.
Scientists found that worms can consume plastic, and have used the frass (faeces) that the worms produce as a fertiliser to support plant growth. Turning plastic waste into fertilisers for food production with zero waste is still an early idea, but it has the potential to reduce plastic waste. This is similar to turning biodegradable plastic into compost with many logistic challenges to turn this idea into a scalable practice. To make the biggest impact in resolving the plastic waste problem, we must use science and seek inspiration from nature.
One tactic is to extract the enzymes from organisms that breakdown plastic and use biotechnology to create a better version of these enzymes that degrade plastics faster. There is so much plastic waste around that biocatalysts won’t solve the plastic waste crisis alone. But if the cost of the enzyme-based degradation of plastic is competitive with other recycling approaches, it could provide a partial solution to the problem.
In 2016, scientists identified a bacterium (Ideonella sakaiensis)that breaks down the plastic (“PET”, commonly used to make plastic) and use its basic building blocks for growth and energy. Scientists have subsequently found that the bacterium breaks down PET in a two-step reaction catalysed by two enzymes: “PETase” and “MHETase”.
First, PETase breaks down PET into an intermediate chemical (“MHET”), which is then broken down to basic subunits used to make the plastic, PET. Researchers are using biotechnology to enhance these enzymes’ performance to make them more efficient at breaking down PET. As these enzymes breakdown PET into their monomer subunits, they can undergo polymerisation again to make virgin-quality PET again.
Of course, not all biological degradation of plastic results in the polymer breaking down into its building block subunits. For example, scientists have found that waxworms also degrade polyethylene into “glycol” (a type of alcohol). The waxworms contain gut bacteria that degrade polyethylene. However, both the bacteria and the waxworms degrade the plastic independently, but they’re much more effective when working together.
Biotechnology will play a vital role in developing this promising area to reduce plastic waste and recoup polymers’ value. Engineering better enzymes to breakdown plastics, or designing multi-enzyme systems to breakdown mixed plastic wastes could help us design better plastic biodegradation systems. However, such is the magnitude of waste, it would be unreasonable to expect enzymes and plastic-eating organisms to solve the plastic waste crisis.
Recycling current plastic waste
Clearly, the current approach to recycling doesn’t work well enough to solve plastic waste problems. Closed-loop recycling of plastics does result in virgin quality plastic — but it requires uncontaminated pristine waste. Most plastics are contaminated with several materials, so it’s practically impossible to scale up closed-loop recycling. Therefore, we need to identify ways to recycle plastic by breakdown polymers to their basic subunits so that we’re able to make virgin quality material from the existing plastic waste.
As I mentioned above, some enzymes can breakdown plastic polymers into their basic monomer form. We need to be actively finding these enzymes and make these biocatalysts more efficient at breaking down plastics. And scientists are already using the biological tools at our disposal to engineer better enzymes to depolymerise plastics.
A study reported a bioengineered enzyme that breaks down 90 percent of the PET plastic in just 10 hours. This enzyme variant outperforms all other PETase enzymes that have attracted recent interest. Innovators the enhanced enzyme to breakdown PET plastic waste into its monomer form, and subsequently purified the resulting monomer to polymerise PET. They closed the circular economy’s loop by using the biologically recycled PET to make new plastic bottles.
The biologically recycled PET produced virgin quality material with the same properties as PET made from crude oil. Carbios, the biotech company behind this technology, has also made textile fibre from enzyme-recycled plastic waste. More recently, the company also announced that it successfully made plastic from waste clothes using its biological recycling technology. Carbios is now partnering with the leading biotech company (Novozymes) to scale up its enzyme production at an industrial scale.
Industrial-scale implementation of the technology may provide a sustainable solution for the infinite recycling of PET-based bottles, packaging and textiles. “Carbios has launched the construction of an industrial demonstration plant that will use this enzymatic depolymerisation process from the beginning of 2021 to recycle post-consumer PET plastics and polyester fibres,” says Carbios’s CEO.
Optimists may claim that biological recycling of PET may be looking promising, but it’s certainly not the same for other types of plastics. So far, no enzymes that breakdown “high molecular weight” plastics are known. These polymers makeup 80 percent of annual plastic production, and include polystyrene, polypropylene, PVC and polyethylene. We should continue looking for new enzymes and organisms that breakdown these plastics. We must also use biological tools to improve the performance of these enzymes and scale up the application of these enzymes so that they’re able to degrade plastic waste at industrial levels.
Yes, enzymes that breakdown plastics show promise, but we can’t only rely on enzyme recycling to close the loop on plastic waste. Another way of recycling plastic to make virgin polymers is through “chemical recycling”. Two forms of chemical recycling exist: the first method depolymerises polymers to release monomers that can be used to create virgin plastic (chemical recycling to monomers); the second method repurposes plastic waste into high-value products (chemical recycling to products). In my view, the principles of chemical recycling to monomers show more promise as it enables infinite recycling of polymers to recycle virgin quality materials.
However, in reality, chemical recycling to monomers isn’t always feasible for all plastics. A combination of both forms of chemical recycling will add value to our aim to reuse and reduce plastic waste. Polyvinyl chloride (PVC), polyethylene (PE) and polypropylene (PP) are the most abundant plastics around. Unfortunately, it’s extremely challenging to recycle these polymers by breaking them into monomers and repolymerising them.
Depolymerisation usually occurs by applying high temperatures to the plastic waste in an oxygen-depleted reactor. In theory, the energy input breaks the polymer chains into smaller fragments, eventually releasing monomers in the polymer chain. Polyolefin plastics (PE and PP) are held together by single bonds. These carbon-carbon and hydrocarbon bonds are strong and saturated, so they require high energy to degrade.
Depolymerisation of PP by breaking its carbon-carbon bonds through thermal energy (a process called “cracking”) requires temperature above 500 degrees Celcius. Depolymerisation of other plastics also requires large energy input at high temperatures, which isn’t sustainable itself. Therefore, researchers have developed several catalysts to crack plastics at lower temperatures (see this review).
When polypropylene undergoes thermal degradation in a plasma reactor, it depolymerises into its basic subunit and converts to a gas that contains 94 percent propylene. Scientists have also developed a method to degrade PE plastics at 175 degrees Celcius. This system uses a highly efficient catalyst to degrade PEs into oils and waxes in just a day. Of course, the challenge is to turn these proof-of-concept laboratory experiments into a full-scale industrial process to recycle and repurpose plastic waste.
PVC starts breaking down at 200 degrees Celcius, at which point it starts releasing chlorine and strong acid (hydrochloric acid). These by-products can be dangerous and cause equipment corrosion. Moreover, PVC materials are often mixed with fillers, dyes and plasticisers (to make plastics more flexible), which means that PVCs can contaminate an entire batch of polymers in recycling plants. To overcome the cross-contamination problems, plastic mixtures undergo an energy-intensive pretreatment process. Heating the mixture at 300 degrees Celsius for an hour reduces the chlorine content by about 75 percent.
Implementing new recycling approaches
Removing the chlorine from PVC has a high energy cost, which must come down. Scientists are looking for alternative methods or improve current processes to overcome the problems associated with recycling PVC plastics. Efficient PVC recycling also requires finding better catalysts to break down PVC into its monomers in the presence of chlorine, hydrochloric acid and other plastics. We should also use chemicals inhibitors and identify new approaches to prevent acid by-product formation in PVC recycling.
Mechanical recycling of plastics into other products significantly reduces the quality of polymers (downcycling). However, chemical recycling to products can generate value-added materials (upcycling). For example, researchers have shown that “polycarbonate” (polymers used to make milk bottles, etc.) can be repurposed into “polyaryl ether sulphones” (polymers used to make wastewater purification membranes). Scientists have also managed to degrade “polyethlyene” plastic waste into valuable transportation fuel (diesel) and waxes. Of course, there are environmental consequences to converting plastics into hydrocarbon fuels, which contribute to global heating, climate change and cause air pollution.
Earlier in the blog, I mentioned some of the desirable principles for recycling polymers to monomers. Chemical recycling to monomers should be easy to recycle polymers to monomer with the ability to have an infinite polymerisation — depolymerisation cycles or turn the monomers into other useful products. The process should use one of the Earth-abundant catalysts to enable the depolymerisation reaction. Easy and accessible depolymerisation with an abundant catalyst will bring down the cost, too, which is also important.
Countries have also begun targeting plastics to reduce waste and mandating more recycling. The European Union adopted its first-ever Plastic Strategy in earlier 2018. Large companies like Coca-Cola and Unilever have also made ambitious commitments to use recycled plastics in their packaging. Under increasing pressure to keep tabs on plastic waste, several companies are now using the principle of chemical recycling to breakdown plastics (see this post). For example, a recycling company Agilyx is working with 30 other companies to depolymerise plastics. Agilyx’s depolymerisation plants can breakdown about 10 metric tons of polystyrene each day.
“Plastic Energy, which uses pyrolysis to transform mixed plastics into diesel and naphtha, plans to build ten plants in both Asia and Europe by 2023, including one at Sabic’s chemical complex in the Netherlands. Loop Industries is building a commercial plant to break down PET into its raw materials in Spartanburg, South Carolina, as part of a joint venture with the big polyester maker Indorama. And Loop aims to build three more plants by 2023,” as reported here.
Successful chemical recycling to monomer must strike a delicate balance between polymerisation and depolymerisation. Several factors affect this balance, including temperature, solvents used, and catalysts. The success of chemical recycling to monomer is very much dependent on the development of catalysts. We’ve invested significant intellectual and financial efforts into identifying and improving the catalysts for polymerisation. If we’re to achieve our goal of a circular plastic economy, then we must make similar investments into depolymerisation catalysis research and development.
In summary, plastics are an integral part of our lives and we still want them around. However, we must find ways to deal with plastic after we use them. We need to break down plastic waste to recycle and reuse them or repurpose them with new properties for different uses. In parallel, we must also identify next-generation plastics that are easier to recycle and breakdown, which will eventually replace our unsustainable dependencies of the current plastics.
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