Bioplastics & Biopolymers; Are They Really Biodegradable?
by Andrew Dent on 10/17/2018 | 3 Minute Read
Bioplastics and biopolymers—are they really biodegradable?
To some that might seem a tautology, because they have to be, right?
The answer is no, though it’s a bit more complicated than that. Not all biodegradable polymers are bio-based, and just because a brand might say their packaging is biodegradable doesn’t always mean that it's compostable. Compostable packaging materials are exceptional, but if they end up in your recycling bin, it can potentially contaminate the recycling stream. Ultimately, those rejected compostables will end up in a landfill, and then we're right back where we started.
So let's break down these materials, no pun intended.
In the same way that plastics are a subsection of polymers, plastics is a shortened version of ‘thermoplastics’ such as polystyrene, distinct from ‘thermosets’ such as epoxies, both of which are also polymers.
In the same way, bioplastics are thermoplastics derived from a bio-based source, such as sugar, seaweed or starch. Biopolymers are the broad class of materials that include bioplastics, but that also includes natural polymers such as silk, chitosan and wool. PLA is a bioplastic and a biopolymer while silk can be classed as a biopolymer but NOT a bioplastic.
Many bioplastics biodegrade quickly. PLA needs some help in an industrial compost, but PHA will compost either in your backyard or the ocean. Tenite, a bioplastic from wood chips and cotton waste, is a durable material that will take many years to break down. You'll find this material in Craftsman orange-handled screwdrivers, but also your tortoiseshell eye-glasses.
Some nylon parts are made from castor oil (a vegetable oil pressed from castor beans) and thus are a bioplastic, but they’re also intended for durable applications. There are also some quick biodegrading plastics that come from oil like Ecoflex. Typically you'll find this material widely used as a biodegradable compost bag.
Did you notice that I added ‘quickly’ to biodegradable up there? That's because all organic things will eventually biodegrade given the right combination of water, light, air and micro-organisms. All plastics will degrade to a certain degree, though it may still take hundreds of years.
Biodegradability only matters when it's quick enough to provide resources for future plant growth and, typically, the word ‘Compostable’ is used for plastics and bioplastics when the material biodegrades at a rate that has been certified according to a recognized test, such as the ASTM D6400 in the US, the EN 13432 in Europe or JIS K693 in Japan. The material itself needs to break down within a few months.
There are various ways in which different bioplastics are made, depending upon the raw plant source and the type of plastic created. Here I’ve highlighted four types that are currently in use: PLAs from corn, thermoplastic starches from potatoes, nylon from castor seeds (which is also used to create industrial and motor oil), and PHA plastics that are manufactured in the cells of bacteria as they feed on sugar-based sources such as sugarcane.
Knowing about what bioplastics are and how to assess their biodegradation, however, does not solve our current problem. Even with certified compostability, many of these bioplastics won't degrade quickly in our natural environment. There's not enough humidity in Arizona, not enough heat in northern climates, and often, there isn't the right bacterial combination that will help break these materials down. They're not degrading in our oceans, and landfills aren't much better.
Solutions that offer fast-acting bio breakdown in salt and fresh water (natural polymers, starch and PHA-based plastics as well as some cellulose-based films and PBS resins) or materials that offer recyclability multiple times will likely prove to be better options in the short term.