The search for plastics capable of decomposing naturally in the environment has been ongoing since the 1970s. So far, cost and undesirable mechanical properties have stymied progress. But recent advances mean that’s set to change, reports Jasmin Fox-Skelly
Since the 1950s, we’ve thrown away 6.3bn t of plastic waste. Of this, 600m t has been recycled; 4.9bn t has ended up in landfill, where it can take between 500 and 1000 years to completely degrade; and the rest has ended up in rivers, lakes, beaches and oceans, posing unknown risks to wildlife.1 By 2050, the World Economic Forum predicts there will be more plastic in the ocean than fish by weight.
Biodegradable plastics can be decomposed by bacteria, and broken down into water, carbon dioxide and biomass. They are usually made from renewable materials such as potato starch, sugar cane or cellulose, and unlike their fossil-fuel based counterparts, which contain long hydrocarbon chains with few or no functional groups, they tend to consist of smaller chains with oxygen and nitrogen groups that bacterial enzymes can latch onto.
However, despite their initial promise, the current crop of biodegradables, including polylactic acid (PLA), polyhydroxyalkanoates (PHAs) and thermoplastic starches (TPS), currently account for less than 5% of all plastics on the market; although there is evidence that this is changing, with the global biodegradables market doubling in the last 12-18 months.
According to Paul Mines, CEO of Biome Technologies, a UK manufacturer of biodegradable natural polymers, one of the main reasons for this lag in the market is cost. ‘Compostable plastics don’t have the scale of oil based persistent plastics, and their starting raw materials are often significantly more expensive than oil/gas,’ says Mines.
This problem is especially pronounced for single-use plastics, such as those used for food packaging, which are the biggest contributor to the plastic waste problem. Attempts to replace them with biodegradables haven’t got off the ground because their higher cost has deterred manufacturers who worry that consumers won’t pay extra for a renewable food wrapper.
A second reason for lack of investment so far is performance. Until recently, the current crop of biodegradable plastics lacked some of the desired properties of fossil-fuel based polymers. After all, conventional plastics are widely used for a reason: they are light, mouldable, and you can make them as tough or flexible as you want. You can colour them and make them transparent or water resistant. Biodegradable plastics are not yet able to match all of these properties. For example, currently no biodegradable plastic has the barrier properties expected of a fizzy drinks bottle, although that may be set to change as Netherlands-based renewable chemicals company Avantium recently won the support of Carlsberg, Coca-Cola and Danone to develop plastics made from plant sugars. The bioplastics are designed to contain carbonated drinks and should decompose in one year in a composter.
‘The performance of compostable and bio-based plastics is improving, but we have a lot of catching up to do on the oil industry,’ says Mines.
To add insult to injury, often ‘biodegradable’ plastics don’t even degrade that well in the environment, with most designed to breakdown under industrial composting conditions including temperatures of between 50 and 60°C. A 2018 study by researchers from University College Dublin found that, of the most commonly used biodegradable plastics, only two types – thermoplastic starch and polyhydroxybutyrate (PHB) – fully degrade in an ocean environment.2 The rest showed no signs of biodegradation in an ocean or soil environment after 141 days of testing.
An obvious solution would be to use more of the easily biodegradable plastics. It’s not that simple, however – plastics that fall apart too easily when exposed to humidity, for example, might start decomposing too early.
The current crop of biodegradables account for less than 5% of all plastics on the market, although there is evidence that this is changing, with the global biodegradables market doubling in the last 12-18 months.
As single-use plastics, especially those used for food packaging, are the biggest contributor to the plastic waste problem, many researchers are focusing on biodegradable alternatives to plastics like PET.
UK firm Biome Bioplastics, for example, is collaborating with researchers at the University of Nottingham to create a fully renewable and compostable plastic packaging based on polybutylene adipate terephthalate (PBAT), used to make cling film, plastic carrier bags and cups. However, the problem is that although the polymer partially biodegrades, it leaves behind terephthalic acid (TA) monomers, which few microbes are capable of digesting. In addition, only about 30% of PBAT comes from renewable sources; as most of the material is fossil fuel-based, its biodegradation releases CO2 into the atmosphere, potentially exacerbating climate change.
However, Mines believes that PBAT can be modified to get around these difficulties by substituting TA with another monomer building block called pyridinedicarboxylic acid (PDCA).
‘What we have found is that the nitrogen in the PDCA ring results in both faster biodegradation and enhanced mechanical and chemical performance, compared with just the carbon atoms in TA,’ says Mines.
Tests show that replacing TA with PDCA results in a material able to degrade fully under industrial composting conditions in 60 to 90 days, which, according to Mines, is ‘a significant enhancement in biodegradation rate, compared with traditional PBAT and other bioplastics in the market’.
Another advantage of using PDCA is that it can be produced from renewable resources.
‘We have a good chance of sourcing the PDCA from lignin, which is found in straw [and] wood, so we have the opportunity to use a low cost starting material and convert it into a useful biodegradable plastic using engineered microbes,’ says Mines.
In 2018, the team received an £800,000 grant to use microorganisms to produce the PDCA monomer at a larger laboratory scale, with Mines ultimately hoping to develop a product that could be used to make biodegradable plastic food packaging.
‘Scale is the next challenge. We can make single kgs at present and expect to be 10-20kg during 2020, circumstances permitting,’ says Mines.
‘We are currently planning our next phase that should see us get to 100s of kgs in the next couple of years. Our plan involves a staged scale-up that allows us to take limited investment risks as we prove the material performance, refine the production process and carry out enough market testing to ensure that we have a winner as a product.’
Another new class of materials generating excitement is polyhydroxyalkanoates (PHAs), chemicals naturally produced by bacteria as an energy reserve when under stress. The kind of bacteria used, and their diet – be it sugars, starches, glycerin, triglycerides, or methane – determines the PHA produced, with more than 150 PHAs currently in existence.
The physical properties of PHAs make them ideal candidates for single-use products like food packaging, straws and cutlery, allowing them to replace plastics like polyethylene and polypropylene. They could also be used in applications where biodegradation is a must, such as products that inevitably end up in the environment, eg slow-release fertiliser coatings.
Theoretically, the raw materials for PHA bioplastics could come from any biomass-producing facility, be it agricultural waste, or waste from the paper and pulp industry. This waste could then be broken down by microbes in fermenters, and converted into the desired monomeric building blocks, which could then be purified, polymerised and mixed with additives to obtain the required properties.
‘As more and more people see the need to reduce the use of traditional plastics throughout society, we are seeing overwhelming demand for PHA,’ says Molly Morse, CEO of Mango Materials, a California-based start-up company producing biodegradable plastics from waste biogas.
‘PHAs can be formulated to have a wide range of mechanical, processing, and end-of-life properties. Wherever there are plastics that threaten the environment, the PHA family can act as a substitute.’
Competing with large-scale, low-cost, polluting plastics presents a challenge to companies specialising in PHA and other biopolymers. Molly Morse CEO of Mango Materials, California, US
One of the first to invest in PHAs was US firm Metabolix, which in 2010 built a commercial plant capable of producing 50,000t of plastics. However, after ploughing hundreds of millions of dollars into development, the company was forced to abandon its plans in 2016 due to lack of demand.
Nevertheless, a number of start-ups seem undeterred. Mango Materials has a pilot facility converting raw biogas from a nearby water treatment plant into the biopolymer poly(3-hydroxybutyrate), or PHB. Bacteria in vats feed on methane, carbon dioxide and hydrogen sulphide. When they are ready, scientists break open their cells to retrieve the PHB polymer. Mango extracts about 250kg/year and hopes that the product could eventually be used as packaging for beauty care products. The company is now focused on scaling up its technology, however, this is no easy task.
‘Competing with large-scale, low-cost, polluting plastics presents a challenge to companies specialising in PHA and other biopolymers,’ says Morse.
‘Significant production infrastructure for traditional plastics is already in place, resulting in billions of dollars of steel in the ground. Mango Materials and other biopolymer companies must scale up to increase supply and ultimately compete on price. However, funding infrastructure for first-of-its kind plants can be a challenge.’
Another company focusing on PHB, Italian firm Bio-On, recently launched a line of sun creams called MyKAI with British-Dutch consumer goods company Unilever. The sun creams incorporate micropowders made with PHB, produced from the fermentation of molasses and by-products of sugar beet production at the company’s demonstration plant near Bologna. The plant also produces PHB microbeads for use in personal care applications, and recently announced a joint venture with the Italian fruit distributor Rivoira to produce PHB packaging for fresh fruit and vegetables. However, Bio-On hit financial trouble in late 2019, and was declared bankrupt in January 2020. Since then the firm has received an ‘extraordinary wages guarantee fund’ from the Italian government to enable the company to continue operating for another 12 months.
‘PHAs are among the best candidates for traditional oil-based plastics replacement, thanks to their excellent balance of physical, thermal and mechanical properties,’ says Diego Torresan, Bio-On research advisor and Business Development Manager. ‘They also have properties that allow them to adapt to existing plastic manufacturing processes, such as injection moulding, extrusion, blow and cast filming and thermoforming’.
Plant based plastics
Meanwhile, other scientists are focusing on producing bioplastics from cellulose or lignin, polymers that give plants strength and rigidity. Manufacturers must first break them into their building blocks, or monomers – something easier said than done, as both are insoluble in water, and lignin does not dissolve in most solvents.
However, researchers have recently found ways to degrade both substances. For example, in 2016, Washington State University researchers used an ionic liquid solvent to selectively separate lignin from wood and woody plants.3 Genetically engineered enzymes similar to those in fungi and bacteria can then break down the dissolved lignin into its components. This is promising, as lignin’s monomers contain aromatic rings – the chemical structures that give some plastics their mechanical strength and other desirable features.
Companies capitalising on these findings include Chrysalix Technologies, now known as Lixea, which has developed a way of using low-cost ionic liquids to separate cellulose and lignin from plants; MetGen, a Finnish biotech company using genetically engineered bacteria to cleave lignins into bioplastics for a range of applications; and US firm Mobius, which is developing lignin-based plastic pellets for use in biodegradable flower pots, agricultural mulches and other products.
However, even if biodegradable plastics take off, the UK’s waste system isn’t currently set up to handle compostables. Ideally such items should go to the UK’s few industrial composting facilities, however, councils don’t collect biodegradable plastics from households, so consumers are left with no option but to put them in the general waste where they’ll head to landfill or incineration.
Consumers are also not aware of the difference between fossil fuel and bio-derived plastics, so there is a danger that people could inadvertently throw biodegradable plastics into the plastic recycling bin. As recycling facilities currently have no way of distinguishing bioplastics from fossil fuel-derived polymers, this would inevitably lead to contamination, making it harder to recycle PET and other plastics.
According to Mines, a better solution would be for biodegradable materials to be handled with food waste or green waste rather than with conventional plastics.
‘In reality, compostables will piggyback on food-waste management,’ says Mines. ‘We have a bit of time to get ready, as optimistically it will probably take us at least three to five years until the UK is generating 100kt of compostable plastic.
‘We also need to ensure proper labelling and segregation. It will never be perfect, but neither is the segregation of existing plastics. As you might expect there are lots of technical solutions to this as well, and we are ensuring that materials that we intend to put on the market can be identified quickly in industrial recovery processes.’
In 2016, researchers discovered a type of bacteria called Ideonella sakaiensis living outside a bottle-recycling facility in Japan. Amazingly, the bacteria has evolved the ability to break down PET into its basic building blocks.
Two years after the original discovery, researchers from the University of Portsmouth created a mutant version of the enzyme responsible, PETase, which is 20% more efficient at breaking down PET.4 Scientists believe they might be able to create even more potent versions of the enzyme in the future.
Not only could the enzyme help clean up the world’s plastic-contaminated seas and lands directly, an engineered version of PETase could also be used in recycling facilities to breakdown PET and produce building blocks that could then go towards manufacturing a new polymer.
1 R. Geyer, Science Advances, doi: 10.1126/sciadv.1700782
2 K.E O’Connor, Environ. Sci. Technol, doi: 10.1021/acs.est.8b02963
3 X. Zhang, Green Chemistry, doi: 10.1039/C6GC01007E
4 H.P Austin, Proc. Nat. Acad. Sci., doi: 10.1073/pnas.1718804115