Not all biobased plastics are biodegradable, nor do they need to be

If you use the word ‘bioplastic’ in Maarten van der Zee’s presence, you can expect to be politely corrected. ‘Bioplastic is not a scientific term. What is more, it is confusing.’ Van der Zee, who is a senior researcher in WUR’s Sustainable Plastics Technology group, thinks we would be better off avoiding the word altogether. ‘When you talk about bioplastics, it is easy to get two categories of plastics confused: biobased plastics on the one hand, made from biomass rather than fossil raw materials, and biodegradable plastics on the other, ones that break down in nature under certain conditions.’

Not at all biobased products are biodegradable. A bio-polyethylene carrier bag   is relatively sustainable because the production process involves lower emissions, but if you were to drop that bio-polyethylene bag in the woods, the remains would still be there some 2,000 years later. It is therefore only logical that Van der Zee wants to make clear what exactly we are talking about right at the start of the interview. That is because he and his team are developing a new method for measuring the biodegradability, which would be a significant step towards plastics that are not only more sustainable but also safer for the environment.

Why is it that not all plastics made from starch, sugar or even wood fibres break down in nature? The reason lies in the fact that ‘plastic’ is a type of material, just like metals and ceramics are. In other words, all the materials we group under the label ‘plastic’ are defined mainly by their plasticity — the fact that they can be moulded, bent or blown into all kinds of shapes. The exact properties of a plastic depend strongly on its chemical composition. A hard Lego block is made from ABS; a light, elastic sandwich bag is made from LDPE.

If you make a shopping bag from biobased polyethylene, the chemical composition is no different from fossil-fuel polyethylene: it is composed of chains of ethene molecules. The difference is in the raw materials from which those building blocks are derived. The ethene normally comes from petroleum, a fossil fuel, but it can also be derived from sugar cane. That makes producing carrier bags from it more sustainable, but it does not mean it’s OK to abandon the bag as litter. Nature can’t cope with polyethylene bags, biobased or otherwise

‘But that doesn’t mean all plastics have to be both biobased and biodegradable,’ says Van der Zee. ‘Some products, such as window frames or water butts, need to be highly durable and resistant to weathering. A plastic that is not biodegradable could be the right choice for those products.’

While Van der Zee sees benefits from plastics, he stresses that we need to be far more discerning in how we use plastics. ‘Every year, 400 million tons of plastics are produced globally. Fossil-based or not, that is a huge burden on the planet.’ Even so, it is unrealistic to aim for a world without plastics. That is why Van der Zee and his colleagues argue that all plastics that are manufactured should be biobased, that plastic should be recycled wherever possible, and that plastic products that constitute a pollution risk should be biodegradable.

Van der Zee explains what he means by ‘pollution risk’. ‘There are basically four ways in which plastics can end up in the environment. First, there is the deliberate use of plastics in the environment, such as enriching seed coatings in horticulture. Then you have losses; for example, a lot of fishing nets are lost and left behind in the ocean. A third source is wear and tear, for instance of clothes, brooms or paint. Finally, there is plastic litter, whether from an overfull waste container or because the plastic is dumped in a waste disposal site.’

Plastics that are biodegradable can help reduce the amount of pollution. Although Van der Zee hastens to add that biodegradable products are not the solution to the problem of litter. ‘Litter is a social problem and the result of human behaviour. Even when plastics are biodegradable, we can’t just dump them en masse in the streets. Not only is that another burden on the environment, it is also a public nuisance and prevents the plastic from being recycled or turned into compost.’

‘Biodegradability is not a material property in the same way the hardness or colour is,’ explains Van der Zee. ‘The breakdown of a product is an interaction between the material and the microbiology of its surroundings. If those microscopic creatures — the local fungi and bacteria — are unable to deal with the material, it will never break down completely.’

Complete biodegradation, to use the scientific term, has a very specific meaning. ‘A lot of plastics do fall apart eventually, for example under the influence of sunlight. But just because you can no longer see it, that doesn’t mean it has been broken down completely, chemically. There might still be microscopic particles in the environment — the infamous microplastics.’ A plastic object has only been broken down fully once microorganisms have consumed all the building blocks of the material and converted that into energy, for instance. The end product of the breakdown process is then carbon dioxide. ‘We say the material has been mineralised.’

The microorganisms present in the environment are not the same everywhere. So the speed at which a material is broken down depends on the environment. How do scientists make sure that plastics that are labelled biodegradable will indeed definitely degrade fully? They measure the extent to which the material mineralises. ‘The standard test makes use of a respirometer. That essentially measures the respiration of the microorganisms — the amount of oxygen they use during the breakdown process and the amount of carbon dioxide that is then released.’

In the depths of one of the WUR buildings, Van der Zee shows a typical respirometer setup, consisting of a couple of large cabinets with transparent doors. Inside these reactors are rows of one-litre bottles filled with a liquid or soil. ‘Plus plastics and bacteria.’ The bottles are connected together by tubes. ‘And all the cabinets are attached to this apparatus.’ Van der Zee points to the tubes coming out of each reactor and leading to a smaller piece of equipment. ‘This apparatus measures the amount of carbon dioxide released into the air above the samples. You can then measure how well the plastic degrades down to the last microgram.’

That precision is a major advantage of the standard method, but it is at the expense of efficiency. ‘This setup has 36 channels. If we want to measure how well a material is broken down in water or soil and we also want to test that against a control, we can measure one plastic per cabinet. The respirometer takes up a lot of room, so you can only test a few samples at any one time.’

In a lab a couple of floors higher up, Van der Zee holds up the alternative test developed by him and his team. In his hand, he has a rack the size of an A5 with 24 small test tubes. ‘We put one-and-a-half millilitres of a sample solution in each test tube.’ The new test can cope with such small quantities because it uses a new method to measure the carbon dioxide emissions. ‘Each test tube has a microsensor.’ Van der Zee shows the minuscule grey sensor. ‘The sensor measures the amount of carbon dioxide dissolved in the water. This is because the CO₂ is evenly distributed over the water and air in the tube, so you can measure it in either the water or the air.’

The small size of the test equipment makes it possible to test lots of different materials at the same time. ‘We can fit between five and ten of these racks in our lab’s reactor, so we can perform measurements in up to 240 test tubes in one go. Even if you include duplicates of every material plus a control group, that’s still 80 plastics.’ 

The new test that Van der Zee and his colleagues have developed is not as precise as the respirometer. ‘This test doesn’t let us quantify exactly how much carbon dioxide is released during biodegradation, but it does tell us whether it is a lot or not much,’ he explains. The test may not give very precise numbers, but this new method can still help in developing new, sustainable, biodegradable plastics.

Developers can use this test for screening new materials. ‘Developing plastics is a complex chemical process. You’re constantly trying out new chemical compounds in combination with substances intended to enhance the flexibility, hardness or some other property. Our test lets you assess a lot of variants on one formula at the same time, and therefore gives you an efficient way of finding out which variants are most promising in terms of their biodegradation. Manufacturers will then be able to take biodegradation into account from the start when developing new plastics.’ Preferably new biobased plastics, of course.