Let’s start with bioplastics. Bioplastics are a diverse group of materials, and the term has several meanings. They can be made using polymers derived from plant-based sources such as starch, oils, cellulose etc. Bio-based polymers can also be used to create non-biodegradable plastics that behave in exactly the same way as conventional plastics, so just because ‘bio’ is in the name does not mean it is biodegradable. Coca Cola’s PlantBottle, for example, though partly derived from sugarcane is chemically identical to hard-to-breakdown polyethylene terephthalate (PET) bottles. So, it can be recycled many times but it won’t break down for centuries. Equally, the term bioplastic can refer to plastics created from fossil fuels which are made to be biodegradable (as neatly explained in the diagram below from WRAP). The term is used for a wide range of materials that do not necessarily all behave in the same way.
Then we have compostable plastics. There is an industry standard that plastics must meet to be deemed ‘compostable,’ which ensures they can be decomposed/biodegraded in industrial composting conditions. Not all biodegradable plastics are compostable, but all compostable plastics are biodegradable, in the right conditions. What this does not mean is that they can be placed in household composting – these products must be specifically labelled as such.