Extract from ABC News
Biodegradable plastic bags, cutlery and coffee cup lids may seem like a win for the environment, but they often introduce more problems than solutions.
Despite being touted as "green", many of these plastics take just as long as their conventional counterparts to break down in home composts and landfill, leading to more pollution in soils and waterways.
Many are also not recyclable, and can only be broken down by industrial composting under high temperatures.
Now, a team of researchers at the University of California, Berkeley have finally created a biodegradable plastic that disappears almost entirely in household compost within a matter of days, just by adding heat and water.
The new material includes built-in enzymes that chew the plastic down to non-toxic molecules without leaving behind traces of harmful microplastics.
The researchers' study has been published today in Nature.
"Enzymes are really just catalysts evolved by nature to carry out reactions," materials scientist and study co-author Ting Xu said.
Biodegradable does not equal compostable
A 2015 study estimated that just 9 per cent of the world's plastics are recycled, with most of it ending up in landfill.
Australia is marginally better, with about 18 per cent of our plastic waste ending up at recycling facilities.
Biodegradable plastics — which break down into water, carbon dioxide or organic material with the help of microorganisms — have been proposed as an environmentally friendlier alternative to petroleum-based varieties.
Many of these plastics are made from polyesters, such as polylocatic acid (PLA) and polycabrolactone (PCL).
They comprise tightly packed chains of molecules, called polymers.
This makes them durable, but also difficult for water and soil microbes to penetrate enough to degrade them.
While the chemical make-up of these traditional materials is technically biodegradable, they can only be broken down in industrial composting facilities under tightly controlled temperatures and conditions, said materials scientist Hendrik Frisch at the Queensland University of Technology, who was not involved in the study.
"Under other conditions such as soil or marine environments, these materials often display a similar durability as their conventional fossil-fuel-based counterparts, causing significant environmental damage and pollution," Dr Frisch said.
In response to this problem, the federal government launched a National Plastics Plan earlier this year that aims to phase out plastics that "do not meet compostable standards".
The power of enzymes
Professor Xu has been exploring how to use enzymes to tackle pollution and make materials more biodegradable for more than a decade.
In 2018, Professor Xu and her team created fibre mats with embedded enzymes that break down toxic chemicals, found in insecticides and chemical warfare agents, in water.
In their new study, Professor Xu and colleagues dispersed billions of polyester-eating enzymes throughout PLA and PCL beads, which are used early in the manufacturing process to create plastic products.
After melting these beads down, they shaped the material into filaments and sheets for testing.
To prevent these enzymes from falling apart before they had a chance to do their job, the researchers coated them in custom-designed polymers to keep them embedded in the plastic.
Without this supportive polymer coating, the enzymes could only partially chew through the molecular chains, leaving behind polluting microplastics.
But when wrapped in the coating, the enzymes were able to chomp these large molecules down to their building blocks, similar to unthreading a pearl necklace.
"The enzyme doesn't leave the plastic [behind]," Professor Xu said.
"Even when the plastic degrades into very small pieces, the enzymes keep working."
Plastics pull a disappearing act
When the team added their enzyme-studded polyesters to household soil compost with a little tap water, 98 per cent was converted into their individual building blocks in just a few days.
The small molecules left behind were harmless, with the enzymes turning PLA into lactic acid, a food source for soil microbes.
Reassembling the remains of the degraded plastics into new products may also require a specialised recycling facility, he added.
"Implementing multiple cycles of making and unmaking will be something that has to be investigated in the future."
Professor Xu said the approach could one day be applied to make products that are more biodegradable, from polyester clothing to biodegradable glue in phones and electronics.
"We want to work with industry to really move this forward, so that it's in the grocery store and on your countertop."
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