Where Does Our Plastic Go?

Where Does Our Plastic Go?

by Amy Lai and Zach Hern

What are plastics?

Plastics are polymers, long repeating chains of small molecules called monomers, and are categorized by the monomers they are made of. For the consumer to know what type of plastic they using, companies have to include the Resin Identification Code, a small recycling symbol containing a number from 1 to 7. This code provides information on what they are made out of (Table 1). We use plastics for a myriad of applications, such as packaging, medicine, clothing, electronics, cars, and construction materials. Over the last 70 years, plastics have become such a ubiquitous material due to their versatility and relatively inexpensive production cost. In fact, the global production of plastics in 1950 was merely 2 metric tonnes (4409 pounds) and as of 2015, this has increased to 381 metric tonnes (839,961 pounds). ^[2][6]

Resin Identification Code	Description	Molecular Structure	Applications
#1 PET	Polyethylene terephthalate		Water and soft drink bottles, peanut butter, vegetable oil containers, and textile.
#2 HDPE	High-density polyethylene		Milk cartons, shampoo and detergent bottles, spray bottles, and plastic bags (not recyclable).
#3 PVC	Polyvinyl chloride		Loose leaf binder, pipes, electrical wires, window frames, and car parts.
#4 LDPE	Low-density polyethylene		Tote bags, furniture, carpet, and containers.
#5 PP	Polypropylene		Yogurt containers, medicine bottles, clear Starbucks cup, and automotive parts.
#6 PS	Polystyrene		Take-out containers, disposable plates and cups, and packing peanuts.

What happens to our recyclables after they are collected?

There are three traditional routes for dealing with plastic waste. The most optimistic route is that the consumer cleans and recycles their post-consumer waste (PCW); this PCW is collected, processed, and sorted by material type at a Materials Recovery Facility (MRF), combined into bales, and sold to companies to perform the actual recycling. For example, PET water bottles (plastic # 1) are highly recyclable and have been recycled into “new” T-shirts, socks, and backpacks. Unfortunately, not all plastics make it through this route. Out of the 381 metric tons of plastic that is produced annually, it is estimated that only 34 metric tons (75,600 pounds) or 9% is actually recycled. In the other two routes, 12% of plastics are incinerated and the rest of the 79% ends up in a landfill or the environment. ^[2]

Why are most plastics not recycled?

One of the limiting factors for the low recycling rate is the recycling process itself. The most widely used method for recycling is thermomechanical recycling. This is where “recyclables” are collected, sorted, washed, shredded, melted, and re-molded into new materials. The nature of this method limits what can be recycled and only compatible materials can undergo this process together. Any mixing with other plastic types or contaminants will yield a lower grade recycled plastic, which limits its application and recyclability. Generally speaking, virgin materials, on average, can only be recycled a couple of times before they completely lose their value (too much is lost in mechanical properties) and eventually end up in landfills or the natural environment. Additionally, the low cost of producing virgin material makes the recycling process less economically feasible. ^[5]

Which plastics actually get recycled?

#1 (polyethylene terephthalate), #2 (high-density polyethylene), and #5 (polypropylene) plastics are highly recyclable.^[1][2] For the other plastic types, the demand and interest in recycling them are too low and, therefore, no market exists for recycling the rest of the plastic types. The fate of these plastics is, unfortunately, residing in landfills or incinerators. ^[6]

Environmental consequences of non-recycled plastic

Petroleum-based plastics (plastics #1-6) can inherently degrade into smaller plastics under ultraviolet (UV) radiation, like that from sunlight, but this process can take up to hundreds of years to complete. For example, it can take up to 30 years for coffee cups to degrade, 450 years for water bottles, 500 years for coffee pods and plastic toothbrushes.^[4] One reason why PET water bottles have expiration dates is that the water may not go bad, but the plastic sure can especially under direct sunlight for long periods of time.

Additionally, this method of degradation is prolonged for plastics that are in the ocean due to a combination of lower temperatures and lower direct sunlight exposure. Over the years, plastic and trash have been swept by ocean currents, trapped by a gyre, and accumulated in large patches big enough to be seen from space. The largest patch is known as the Great Pacific garbage patch, which stretches over 1.6 million square kilometers (more than 2x the size of Texas) and weighs about 80,000 tonnes.^[3] The plastics that have degraded into 0.05 – 0.5 cm pieces are known as secondary microplastics; these plastics are difficult to remove, remain in the ocean, and consequently block off sunlight from reaching planktons and algae, and may be further consumed by marine animals, and eventually humans.^[7] It is estimated that 8 million metric tons of macroplastic (5-50 cm) and 1.5 metric tons of primary microplastic enter the ocean every year.^[4]

Bioplastics vs. Biodegradable and Biocompostable Plastics

Bio-based plastics (most of which are plastic #7) have been recently developed as a more sustainable alternatives to petroleum-derived plastics. Bio-based plastics are polymers made from biological resources, meaning they directly or indirectly come from CO₂. Some examples include bio-polyethylene, poly(lactic acid) (PLA), polyhydoxybutryate (PHB), and bio-PET. Bio-polyethylene is identical to it’s oil-derived counterpart so it has the same slow degradation rate. Other bio-based plastics are either biodegradable (PHB) or biocompostable (PLA), meaning they can naturally, or with the help of an industrial composting facility (not a backyard compost bin), be converted back to CO₂ or other biologically relevant small molecules in a short period of time.^[8][10] This plastic life-cycle offers a more circular route and bypasses the wasteful and contaminating linear life-cycle of traditionally used plastics. Currently, the physical and mechanical properties are not as desirable as the majority of plastics being manufactured for packaging, but further research and development may lead to a greener and more sustainable plastic economy.

Perspective

Plastics are cheap to produce but are costly to get rid of. Our current plastic economy follows a mostly linear lifecycle, where plastics are produced from raw materials and end up as waste at the end of their use. A recent model showed that even with tremendous efforts in plastic waste reduction, waste management, and environmental recovery strategies, it is still not enough to eliminate plastic pollution. The model predicted that the annual plastic waste emission into our aquatic systems could reach 40-90 metric tonnes by 2030 following current trajectories.^[9] With global commitment and strategies in place, this could be reduced to 20-54 metric tonnes, which remains far too high. As a society, we need to evaluate and redesign the life cycle of plastics and transition into a circular economy, where new plastic production is reduced and all plastic “waste” is a valuable feedstock. One avenue to achieve this is by designing new polymeric materials that can be chemically degraded into their basic monomeric form. The advantage of this recovery strategy is that the recovered monomers can be used as feedstock, polymerize once more, and produce a similar polymer without any significant loss in properties. ^[10]

While many of the solutions towards a circular plastic economy fall on companies and governments to incentivize or develop new materials, there still lies some burden on us as consumers. Firstly, we must rethink how we contribute to the growing post-consumer plastic waste problem. Refusing to buy products with unnecessary plastic packaging – such as pre-cut pineapples and strawberries – is a simple yet effective lifestyle change that can help mitigate the issue. Actively and purposefully seeking products that use post-consumer materials will help elevate the market for recycled plastics and push companies to implement them more into product packaging.^[1] We also need partnerships with local and national governments to pass and enforce laws put pressure on plastic producers and polluters. This will drive the implementation of environmentally friendly alternatives just as environmental regulations have lead to cleaner water and air. As always, we should constantly strive to (in order of greatest impact) reduce, reuse, and/or recycle the plastics we cannot avoid using.

References

[1] Billiet, S.; Trenor, S. R. 100th Anniversary of Macromolecular Science Viewpoint: Needs for Plastics Packaging Circularity. ACS Macro Lett. 2020 9 (9), 1376–1390; doi.org/10.1021/acsmacrolett.0c00437

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[2] World Economic Forum, Ellen MacArthur Foundation and McKinsey and Company; The New Plastics Economy – Rethinking the future of plastics 2016;  www.ellenmacarthurfoundation.org/publications

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[3] The Great Pacific Garbage Patch. The Ocean Cleanup; www.theoceancleanup.com/great-pacific-garbage-patch

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[4] The Lifecycle of Plastics. World Wide Fund for Nature, 19 June 2018, www.wwf.org.au/news/blogs/the-lifecycle-of-plastics

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[5] Garcia, J. M.; Robertson, M. L. The Future of Plastics Recycling. Science 2017, 358 (6365), 870-872; doi.org/10.1126/science.aaq0324

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[6] Geyer, R., Jambeck, J. R., Law, K. L. Production, use, and fate of all plastics ever made. Science Advances. 2017, 3 (7), e1700782; doi.org/10.1126/sciadv.1700782

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[7] Revel, M., Châtel, A., Mouneyrac, C. Micro (nano) plastics: A threat to human health? Current Opinion in Environmental Science & Health. 2018, 5,17-23; doi.org/10.1016/j.coesh.2017.10.003

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[8] Wei, R., Tiso, T., Bertling, J. et al. Possibilities and limitations of biotechnological plastic degradation and recycling. Nat Catal 3, 867–871 (2020); doi.org/10.1038/s41929-020-00521-w

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[9] Borrelle, Stephanie B., et al. Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science 2020, 369 (6510), 1515–1518; doi.org/10.1126/science.aba3656

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[10] Coates, G.W., Getzler, Y.D.Y.L. Chemical recycling to monomer for an ideal, circular polymer economy. Nat Rev Mater 5, 501–516 (2020); doi.org/10.1038/s41578-020-0190-4

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— Amy Lai and Zach Hern are both graduate student researchers at the University of California, Los Angeles (UCLA).