Could next-gen biodegradable polymers help solve the plastic problem?
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Biodegradable plastics might seem an obvious solution to the plastic waste problem, but their current form lags behind fossil-based polymers in required properties. Sometimes they don’t even biodegrade that well. New research is trying to find a solution.
“Polyethylene terephthalate, or PET, which plastic water bottles are made of, is a remarkable polymer,” says Professor Alex Conradie, who leads the Sustainable Process Technologies research group at the University of Nottingham. “It has very useful properties and given decades of process optimisation invested into PET manufacturing, it’s also very cost effective. It is a tall order to match the properties of such a versatile material.”
PET is made from oil; the material which scientists hope will one day replace it in its multitude of applications should be made from something else. If thrown into the ocean, PET will stay floating around almost forever – its replacement should decompose into innocuous chemical compounds such as water and carbon dioxide within a reasonable period so that whales, dolphins and other sea creatures don’t ingest it.
Professor Conradie and his colleagues are part of a research project, supported by the UK government, that is aimed at bringing a fully renewable and compostable plastic packaging material closer to reality.
Single-use plastics, such as those used for food packaging, are the biggest contributor to the plastic waste problem. Unlike some more challenging applications, such as in the automotive industry, it appears that food packaging could quite easily be served by less versatile biodegradable materials.
The challenges, however, are plenty. Since the 1970s, researchers have been trying to create all sorts of biodegradable plastics, mostly from renewable materials such as potato starch, sugar cane or cellulose. Despite these efforts, biodegradable products have so far taken off only marginally and currently account for less than 5 per cent of all plastics in the market.
Their comparatively higher cost is putting off many manufacturers who might doubt whether their products could absorb the two to three times higher price one has to pay for a renewable food wrapper.
As Conradie admits, many ecologically minded consumers would also be disappointed by the actual biodegradation performance of some biodegradable plastics currently in the market.
“Many biodegradable polymers need compostable conditions to actually degrade at an appreciable rate,” he says. “If some biodegradable polymers are discarded into the environment, such polymers can persist for quite some time. Not as long as PET, but much longer than just a few months.”
A study by researchers from University College Dublin, published in the journal Environmental Science & Technology in August 2018, looked at the actual biodegradability of the most commonly used biodegradable plastics. The researchers found that in a natural marine environment, only two types would fully biodegrade. The rest, if thrown into the ocean, would likely join one of the plastic garbage islands or end up in a whale’s stomach.
According to the study, the two types of biodegradable polymers that promptly dissolve in the ocean are thermoplastic starch and polyhydroxybutyrate (PHB).
Polylactic acid (PLA) is one of the most commonly used biodegradable polymers and can be used to make plastic food packaging. The study found it showed no signs of biodegradation in the ocean-like environment after 56 days of testing. In a soil environment, PLA and its blends showed no significant biodegradation after 141 days of testing.
“Many types of biodegradable plastics don’t degrade in a natural environment,” says Ramesh Babu Padamati, senior research fellow in the polymer materials research unit at Trinity College Dublin and one of the authors of the paper. “You need to have certain conditions – certain temperature, certain microbial cultures – for the degradation to occur.”
Most of the tested plastics degraded completely under industrial composting conditions and many decomposed well in a home composting heap. The popular PLA cannot be home-composted, the researchers advise. Blending it with other polymers, however, such as polycaprolactone (PCL), makes it home compostable.
An obvious solution to the plastic pollution and waste problem would be to use more of the easily biodegradable plastics. Yet it’s not that straightforward. Plastics that fall apart too easily when exposed to humidity, for example, might start decomposing before their duty ends and that’s obviously not what users want.
Babu Padamati says chemical engineers must look for the right balance between the material’s ability to biodegrade and the required mechanical, chemical and thermal properties.
“Starch, for example, is a water-soluble polymer,” he says. “You can’t use it for food packaging. You need to add some material into it that will give it the properties that you need. You can’t just add materials that would increase the degradation behaviour but would not help with the application.”
Krisztina Kovacs-Schreiner, project and business development manager at Biome Bioplastics, who collaborates with the team from the University of Nottingham on a project developing next-generation bioplastics, agrees that most biodegradable polymers are deliberately designed with industrial composting in mind.
“In a controlled environment, in an industrial composting facility, where you have the right temperature, the right moisture and the right type of microbes, you can achieve complete degradation within three months,” says Kovacs-Schreiner.
“Industrial composting is our preferred method of disposal. If you just leave it somewhere in nature, it would degrade eventually, unlike fossil-based plastics, but it might take years.”
Kovacs-Schreiner admits that even if consumers were ready to pay the higher price for biodegradable plastics, the current waste management system is nowhere near ready for the green materials’ mass arrival. Industrial composting facilities are few and consumers are, in most cases, not aware of the differences between traditional fossil-based and renewable bioplastics. Throwing a biodegradable plastic waste item into a plastic recycling bin is, in fact, a mistake.
“Introducing biodegradable polymers into the current recycling system can have detrimental effects on the ability to recycle the more traditional PET and other polymers,” says the Univeristy of Nottingham’s Conradie.
“It’s a contaminant. If they are not separated, if they are blended together, it’s going to impact on the properties of the conventional polymer and the ability to reuse it in certain applications.”
Recycling facilities, Conradie says, use different methods to separate various types of polymers. However, at this stage, they are not likely to be equipped with spectroscopic technology to distinguish bioplastics.
Kovacs-Schreiner says some of Biome Bioplastics’ biodegradable materials could be handled with food waste or green waste rather than with conventional plastics.
Biome Bioplastics and the University of Nottingham researchers hope to address shortcomings of existing biodegradable polymers and develop new materials that could, at least in some applications, replace PET. The new polymers must, of course, be cheaper than the existing biodegradable polymers, have better properties and completely biodegrade when needed.
One biodegradable polymer the researchers are hoping to improve is polybutylene adipate terephthalate (PBAT), which can be used to make cling wrap, plastic bags or cups. Although the polymer visibly biodegrades, it leaves behind invisible residues, its monomers, which could contaminate the natural environment.
“PBAT breaks down over time into its monomeric constituents,” says Conradie. “The problem is the terephthalate part, which is essentially the monomer terephthalic acid (TPA). It is a human-made compound and there are only a few types of microorganisms that can actually degrade it.”
Moreover, PBAT is only typically made with up to 30 per cent of renewable content, according to Conradie. Although visibly biodegradable, it is not all that environmentally friendly since most of the material originates from fossil reserves. Its biodegradation, therefore, adds net CO2 into the atmosphere.
“It’s infeasible at this stage to produce terephthalic acid from renewable feedstocks,” says Conradie. “Thus, there is a constant need to draw on fossil reserves and therefore you are ultimately not only polluting, but you would be exacerbating climate change if you were to biodegrade PBAT.”
The researchers want to fix the PBAT problem by creating a new polymer building block that would replace the TPA with pyridinedicarboxylic acid (PDCA). This new component would not only fully biodegrade, but would also be made from renewable resources instead of oil.
“PBAT is otherwise a versatile polymer and has many useful applications,” says Conradie. “If we can create new biodegradable polymers, replacing the TPA as a building block with PDCA, we would have something that under industrial composting conditions breaks down fully in 60 to 90 days, breaks down eventually even if it bleeds to the natural environment, and doesn’t release more CO2 into the atmosphere.”
The University of Nottingham team and Biome Bioplastics has recently received £800,000 for a two-year project that will use microorganisms to produce the PDCA monomer at large laboratory scale. The team hopes to develop a product that could be used to make biodegradable plastic food packaging and for the material to be tested by a major UK food manufacturer.
Biome Bioplastics’ Kovacs-Schreiner calls the team’s novel polymers – which are under development – ‘second-generation bioplastics’. Compared to earlier types of bioplastics, the second generation is made from building blocks (monomers), which are structurally and functionally similar to oil-based polymers, but are made from fully renewable resources with the help of bacteria.
“First-generation bioplastics may contain starch-based materials such as corn starch and potato starch, various fillers and various additives,” Kovacs-Schreiner says. “These materials are dry-blended and then compounded. You melt them and then create the resins from which you manufacture the final products.
“Second-generation bioplastics are about creating entirely new materials.”
The chemical engineers work with a wide range of renewable materials, which would lead to a lower price of next-generation materials compared to first-generation starch-based bioplastics.
“The ultimate goal is to use material from any biomass-producing facility,” says Kovacs-Schreiner. “For example, agricultural waste or waste from the paper and pulp industry could potentially all be used as a raw material in our fermenters.”
Further cost reduction will come with increased adoption, Kovacs-Schreiner believes, as production moves towards larger scale.
“Customers also need to understand why the end product has a different price tag and be aware that this is a different packaging so the manufacturers can recover the cost of the packaging material itself,” she says.
During the production process, the raw material is broken down by microbes, which synthesise the monomeric building blocks. The monomers are purified, polymerised and mixed with additives to obtain the required properties.
The University of Nottingham researchers are experimenting with various engineered microbial strains that can convert biomass into monomers based on requirements for different applications.
“We are essentially modifying the microbes, or microbial cell factories, as we call them, to use the carbon feedstock to overproduce the product of our interest instead of producing biomass,” says Samantha Bryan, assistant professor in white [industrial] biotechnology at the University of Nottingham.
“We are using various metabolic engineering techniques, knocking out genes and improving biochemical pathways to make these monomers. There are always a number of biochemical pathways, which can be used once the carbon feedstock crosses the cell wall of the bacteria and we can promote carbon flux down optimal biochemical chemicals pathways leading to the monomer product.”
Conradie believes biodegradable plastics are the ultimate solution to the plastic waste problem. There is a reason why industries rely on plastic materials and would certainly find it difficult to do away with them completely.
“Plastics are an intrinsic solution to preventing food spoilage,” says Conradie. “The ideal situation is that we have the best of both worlds; we can still use single-use plastics but in the full knowledge of what occurs should such plastics be inadvertently introduced into the natural environment.”
For the next two years, researchers at the University of Nottingham will be developing their process with the aim to gradually scale up to a pilot project.
“We will make polymers and gradually introduce them to our customers,” says Kovacs-Schreiner. “These polymers can be potentially very exciting, but they need to go through rigorous testing as well because we want to make sure they are food-contact approved. There is a lot more development work that needs to be done.”
Biome Bioplastics hopes its materials will eventually provide a viable alternative to PET. The company is also designing biodegradable polymers for use in agriculture, such as support structures for plants that disintegrate after a couple of years, and is experimenting with high-performance materials that could withstand temperatures of up to 120°C.
While some uses will probably always require fossil-fuel-based polymers, some rather interesting biodegradable applications have already been introduced. For example, car manufacturer Ford is reportedly using soy-based foam to make car seats and is experimenting with making bioplastics from tequila production leftovers.
Biodegrading the non-biodegradable
Fossil fuel-based polymers such as PET, when exposed to UV light and natural forces, break down in nature into tiny fragments called micro-plastics. While those fragments might be invisible to the human eye, they are extremely harmful to the environment. Scientists are therefore trying to devise ways to get rid of the plastic pollution completely.
In 2016, researchers from Japan discovered a type of bacteria called Ideonella sakaiensis breaks down PET into its basic building blocks. The bacteria, found accidentally outside a bottle-recycling facility, produces an enzyme, which is now called PETase. This enzyme can be extracted from the bacteria and used to break down PET directly.
Two years after the original discovery, researchers from the University of Portsmouth created a mutant version of this enzyme, which breaks down PET 20 per cent more efficiently than the natural variant. The scientists believe they might be able to create even more potent PET eaters in the future by further mutating the PETase enzyme.
The scientists said the enzyme causes degradation to a piece of PET that could be observed with electron microscopy in just a matter of days. The researchers believe the enzyme could in the future help clean up the world’s seas and land that are contaminated with plastics. It could also be used in recycling facilities to digest PET and produce its building blocks for manufacturing of a fresh new polymer. Current methods of plastic recycling are quite limited since the polymers degrade with every use and can only be reused for a limited number of times. Breaking the waste down back into the basic building blocks would allow reusing the material almost indefinitely.
In 2017, Spanish researchers found a caterpillar of the wax moth can digest polyethylene, the most popular plastic used to manufacture shopping bags, children’s toys and shampoo bottles. However, a German team later questioned whether the larvae actually break down polyethylene into simpler components, or just shred and excrete it as broken down little pieces, which would further add to the invisible contamination problem.
Some have also cautioned that mass breeding of wax moth for an ocean clean up could backfire as the insects could cause an unexpected disruption in the ecosystem by, for example, attacking bee colonies.
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