Recent estimates suggest that it will take 450 years for a plastic bottle to completely break down.However, since plastic bottles didn’t enter into widespread use until the 1960s, it’s hard to know exactly how long it will take for them to disappear.
Breakdown of plastics is caused by solar UV-radiation and is most intensive in environments like beaches and the sea surface. The rate of degradation depends on a number of factors, including composition and temperature.Most plastics “are extremely durable. This means the majority of polymers manufactured today will persist for decades and probably for centuries, if not millennia.”In addition, it is not known when, if ever, full degradation will occur. Larger plastics may fragment into many small microplastics that take longer to break down. And there is no research yet on the degradation of nanoplastics.In fact, there is actually little evidence showing that plastics will ever fully break down in the marine environment.
So, “450 years” is a useful illustration but not a verifiable figure. We don’t know exactly how long it will take. But we do know plastics take extremely long to deteriorate, highlighting the urgent need to prevent more entering the marine environment.
 Australian Department of Environment and Conservation. Litter – How long does it take to break dowmn? Fact Sheet. Available here
 Meikle, J.L., 1995. American plastic: a cultural history. Rutgers University Press.
 Andrady, A.L., 2015. Persistence of Plastic Litter in the Oceans. In: Bergmann M., Gutow L., Klages M. (eds) Marine Anthropogenic Litter. Springer, Cham.
 UNEP and GRID-Arendal, 2016. Marine Litter Vital Graphics. United Nations Environment Programme and GRID-Arendal. Nairobi and Arendal. www.unep.org, www.grida.no
 Hansson, L.A. et al., 2015. Nano-plastics in the aquatic environment. Environmental Science: Processes & Impacts, 17(10), pp.1712-1721.
 O’Brine, T. and Thompson, R.C., 2010. Degradation of plastic carrier bags in the marine environment. Marine pollution bulletin, 60(12), pp.2279-2283.