Bioplastics: Polylactic Acid (PLA) and Starch

The second article in a series about the science behind plastic waste. Find the first here– you’ll probably want to read that first.


Plastics are unavoidable in modern life. Your grocery shop is full of it, your takeaway is wrapped in it and even the phone in your pocket depends upon it. The ocean is also a plastic soup, and sustainability activists are calling for something to be done.

But can your lunch be wrapped in a material made from its contents?

Made of natural materials sourced from renewable resources, bioplastics are usually biodegradable in nature and are a hot topic in the plastics industry. As always, there are many many different types- so what options do we have for our coffee cups?


POLYLactic ACID- PLASTIC FROM FERMENTED SUGAR CANE

Polylactic acid (PLA) is the bioplastics industry leader, with its production costs dipping as low as polystyrene in recent years. This renewable plastic is a chain of fermented sugars from sugar cane, whose individual chain links can be found in yoghurt or in your muscles when you exercise.

PLA is seen by many as the ‘gold standard’ of biodegradability. In a field study in Costa Rica, PLA banana ropes buried underground (at 27°C and 80% moisture content) deteriorated in only a few weeks. PLA has also been observed being degraded by enzymes in your digestive system, as well as breaking down into its chain links in acid, implying we may be able to recycle it back into virgin plastic.

The only problem is applying the label of ‘biodegradable’ to this plastic ignores mountains of evidence to the contrary. Whilst this bioplastic is easily observed degrading in acid and base, it is very hard to determine at the neutral conditions found in nature.

PLA is also almost completely non-biodegradable in water, only breaking down a minuscule amount in 30°C seawater- a temperature far hotter than the ocean in California, where the study took place.

With an estimated 4.8 to 12.7 million metric tons of plastic entering our oceans in 2010 alone, biodegradability in marine systems is vitally important. 

PLA is not a perfect ‘plastic’ by modern expectations, either. When used as a thin film like the bag that wraps your bananas at the supermarket, PLA is fragile and is oxygen-permeable. This means it won’t even help prevent your food going bad- which is the main reason we wrap them in the first place!

All is not lost; we may be able to combine PLA with another biodegradable plastic to improve both of their qualities.


STARCH- SUGAR, BUT HEATED UP

Starch itself, the infamous sugar in potatoes, is itself a polymer. Carbohydrates are long chains of sugars joined together, much like the sugars in PLA joining end-to-end to form plastic.

Used as a filler for conventional plastics since the 1960s, starch was never investigated further as a polymer because of its extreme water-loving properties, making it a pain to dry out and study.

Granules of starch do seem to enhance biodegradability though- with Ecostar’s 6% starch plastic bag claiming to biodegrade in three to six years, in comparison to the hundreds of years for normal plastic.

This sugar is critical in the production of bioplastics like PLA since the lactic acid monomer is often produced by starch fermentation. This usually has 45% efficiency, compared to near-100% when starch is used directly as a polymer- a far more efficient process, putting far less strain on farmers. 

Granular starch doesn’t need any complex chemistry to become a plastic; when the crystalline starch is heated above a certain temperature with lots of water and a few additives, the crystal structure is destroyed, and the thermoplastic is formed.

This thermoplastic starch (TPS) can be melted and moulded the same as normal plastic without needing expensive new machinery, making this polymer very interesting to industry. 

This polymer is rarely studied alone; instead, TPS is usually used when mixed with another polymer, such as polyesters. These composites are ideally suited to plastic bags, which can decompose in a variety of situations- including home composting.

When mixed with polyethylene, which thin plastic bags are made of, the composite becomes completely biodegradable without changing its practical qualities.

With each material complimenting the other’s weaknesses, the blend of starch and PLA seems almost too good to be true- which is inevitably the case. 

Starch’s extreme hydrophillicity clashes with PLA’s hydrophobicity, meaning the two don’t mix well when melted together. As a result, more additives are needed, usually in the form of potentially hazardous chemicals which would be released into the environment when it breaks down.


The moral of the story? If a promise seems too good to be true, it probably is. Plastics that just break down are exceedingly complicated to engineer- especially since current one-use plastics are perfect for their applications. If we make them truly biodegradable, they lose the qualities that we wanted them for in the first place.

However, we do have other hopes- perhaps Nestle’s funding into another polymer will prove successful? Read about Nodax here in two weeks.


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