Biodegradable, Bioplastics and Green Plastics, What’s the Difference?
by By Matthew Thompson, Kexing Xiao, Gretchen Brown and Christine Vo
As plastics continue to face greater public scrutiny over pollution concerns, many companies have pounced on the opportunity to gain some good favor by rolling out nice-sounding terms like bioplastics, plant-based, biodegradable, green, sustainable, etc. With all these terms being thrown around, there is significant confusion over what they actually mean. Today we would like to help out and determine what each term means and how this knowledge can help you make sustainable choices.
Bioplastics, plant-based and natural plastics.
A bioplastic is any plastic primarily sourced from natural materials. The most common sources are carbohydrate rich plants like sugar cane and corn as well as algal plastics which account for 80% of overall bioplastics market.1 Some bioplastics are directly sourced from the plant or algae like starch based plastics. Other bioplastics are made by taking those existing starches and chemically convert them into molecules like ethylene or lactide that can then be polymerized. Some of these plastics are biodegradable, like poly(lactic acid) (PLA), poly(butylene succinate) (PBS) and more, but many others are not like poly(ethylene) (HDPE, LLDPE and LDPE) or poly(ethylene terephthalate) (PET) (for more information on identifying common plastics see our earlier blog post “Where Does Our Plastic Go?”). It doesn’t matter if the PET water bottle is made from oil or plants, it still won’t be biodegradable. On the other hand some petroleum-derived plastics are biodegradable including the poly(caprolactone) (PCL) in medical sutures or the poly(propylene carbonate) (PPC) that is partially made from both oil-derived propylene oxide and carbon dioxide.
There can be benefits to deriving non-biodegradable plastics from plants like oil independence, lower carbon footprints and we already have the infrastructure to make large amounts of these plastics2 ,but these options don’t contribute to the reduction of persistent plastic waste. For biodegradable bioplastics there has to be consideration to how much land is being used for plastic and how much for food. Algal and bacteria bioplastics as well as non-food crops like switch grass are promising future sources for the world’s bioplastics, but there are still many technical hurdles to overcome to scale up these options. Bioplastics are an incredibly broad term but environmentally, it’s more important to know what your plastic is rather than what it is made from.
What Makes a Plastic Biodegradable?
The reason why some plastics degrade readily while others do not is not always immediately obvious from their chemical structures. Now nothing lasts forever and even the non-biodegradable plastics will degrade eventually, but these processes are slow enough to not reenter the biosphere anytime soon. Some plastics may even be considered degradable if they break into small fragments, but not biodegradable if these microplastics do not further breakdown in a reasonable timeframe.3 What is surprising is that non-biodegradable plastics like polyethylene look similar to natural fats and waxes while non-biodegradable poly(ethylene terephthalate) has the same ester repeat units as industrially compostable poly(lactic acid). Important factors for making plastics degradable is their hydrophilicity, or ability to absorb or dissolve in water and their crystallinity, or ability to pack with itself. Even among natural plastics hydrophilic, amorphous starch degrades much faster than hydrophobic, semicrystalline cellulose in a cotton t-shirt even though both are made from sugars.3
There are also different types of biodegradability that are very different from each other. Generally, plastics are considered biodegradable if: 1) >90 % of the mass biodegrades with six months under composting conditions (58 +/- 2° C ) 2) It is 50 % organic matter and doesn’t contain large amounts of heavy metals 3) It disintegrates into fragments < 2mm in size within 12 weeks under controlled composting conditions 4) the compost obtained at the end of the process does not cause negative side effects on the germination and growth of plants.2 These industrial composting conditions are more intense than most home composting environments which causes considerable confusion with biodegradable plastics like poly(lactic acid) degrading readily under industrial composting conditions, but much slower under either home composting or as marine waste. Researchers are looking at organisms either natural or genetically modified that can break down non-biodegradable plastics like HDPE and polystyrene (PS) using different bacteria, molds or insects, but these solutions still need extensive research before being considered as reliable options.4 Future labels are needed to improve the clarity to consumers and producers on the differences between these different disposal methods so informed choices can be made about the plastics we consume.
Green Plastics
Since “green” is a rather vague term it can be difficult to determine which plastics are green. Here we will define a green plastic as any that is sustainable in regards to both production and disposal. For a plastic to be green it must be easily produced from renewable resources and it Other plastics can have green characteristics, if they help reduce one’s carbon footprint compared to alternatives (ex. Lightweight plastic containers compared to heavy glass that requires more petroleum to transport) but ultimately have some drawback in regards to disposal. Since green is a relative term unlike bioplastic or biodegradable, new methods like chemical recycling could improve the greenness of plastics like PET by making recycling highly efficient and minimizing waste. Alternatively growing cash crops for plastic production that rely heavily on nitrogen fertilizer may not be very green at all even if the final material is biodegradable. Whether a plastic is really green will depend on where you live, how it is being used and how often it will be used.5 Companies will try to greenwash products by
Closing Thoughts
Plastic is everywhere so it is important to know what you are consuming. While Bioplastics can tell you the source of your plastic the term can describe a wide range of plastics with vastly different degradation rates. Biodegradability is far more specifically defined, but improved clarity will make it easier for consumers to dispose of biodegradable plastics responsibly. Finally, green plastics are not a specific material but a way to look at the whole lifecycle of a material and should be an important consideration. These terms are often thrown around like they are interchangeable, but there are key differences that make a big difference in terms of a plastic’s environmental impact. We hope you feel empowered with knowledge the next time you choose a takeout container or a water bottle.
Answers to first poll
Cellophane- plant based, from cellulose, biodegradable but very slow
Polyethylene- not normally plant-based, ethylene is found in natural gas or cracked from oil, but can be made from sugar. Not biodegradable
Agar- from algae, easily biodegradable
Polystyrene- from styrene which is made from oil
Poly(lactic acid)- Made from lactide derived from sugar fermentation and processing, industrially biodegradable
PET- Poly(ethylene terepthalate), copolymer of ethylene glycol and terephthalic acid. Ethylene glycol can come from oil but more commonly both it and terephatlic acid are more commonly derived from oil, very slowly degrades
References
[1] Thiruchelvi, R.; Das, A.; Sikdar, E. Bioplastics as Better Alternative to Petro Plastic. Mater. Today Proc.2020, No. xxxx. https://doi.org/10.1016/j.matpr.2020.07.176
[2] H. E. Mogens Lykketoft. The New Plastics Economy Rethinking the Future of Plastics; 2014.
[3] Song, J. H.; Murphy, R. J.; Narayan, R.; Davies, G. B. H. Biodegradable and Compostable Alternatives to Conventional Plastics. Philos. Trans. R. Soc. B Biol. Sci. 2009, 364 (1526), 2127–2139. https://doi.org/10.1098/rstb.2008.0289.
[4] Drahl, C. Plastics Recycling with Microbes and Worms Is Further Away than People Think. Current Biology. 2017, pp R292–R293.
[5] Miller, S. A. Five Misperceptions Surrounding the Environmental Impacts of Single-Use Plastic. Environ. Sci. Technol. 2020, 54 (22), 14143–14151. https://doi.org/10.1021/acs.est.0c05295.