Siwen Bi, UMass Lowell
In March 2018, a Nature paper published by Lebreton et al. revealed that the floating plastics island (more than 99.9% plastics) between Hawaii and California has grown to more than 1.6 million square kilometers (about twice of Texas). More and more microplastics are found in ocean ecosystems, even in the deep ocean. As the pollution from non-degradable plastics has been attracting increasing attention, the demand for the management of plastics pollutants and development of sustainable (biobased and biodegradable) plastics has become pressing for academia and industry. To achieve sustainable use of plastics, academia and industries have been devoted to making the following come true: involution of alternative renewable sources, recycling and reuse of plastics, transitioning from plastics to valuable chemicals (or energy recovery) and last but not least, control of plastics degradation. We will take a quick look at each of these strategies in this article.
Size of the Problem
Packaging is the largest market of plastics production and consumption and estimated to be more 35.0% of the overall revenue by 2025. (1 )Worldwide, 1 million plastic bottles are consumed per minute, projected to increase by another 20% by 2021, while about 2 million plastic bags are used per minute and the average lifetime of a plastic bag is only 12 min. (2)
How can WE get to a sustainable packaging industry?
First, we can transition from fossil fuel-based plastics to bio-based plastics such as bio-polyethylene and bio-poly(ethylene terephthalate) (PET). Provided we can transition economically, this would be a simple solution to deploy in industry since PE and PET are already used in packaging. Drop-in replacements for some petroleum-derived monomers such as ethylene glycol (made from sugarcane ethanol) are already available in the marketplace.
Taking PET as an example, the rigid packaging industry (e.g., Coca-Cola Co) pushed the development of bio-PET starting with the ethylene glycol fraction for 30% bio-based PET dating back to 2009, with plans to demonstrate 100% bio content by 2015.3 In 2018, bio-PET had taken up 26.6% of the bioplastics market as shown in the following graph. It is expected that the bio-PET market will grow more than 42% annually till 2024. (4)
Although it is produced from renewable resources, bio-PET is not considered biodegradable due to its low degradation rate (around 16-48 years to powders and even more than 1000 years at low RH). In human and animal bodies, the lifetime of PET is estimated over 30 years.(5) On the other hand, compared with the fossil-based PET, the production of bio-PET could have a similar environmental effect due to the agricultural production stream (especially the influences of intensive agriculture). Therefore, more EFFICIENT acts should be taken.
Recycling – a practical solution
Recycling is a more conventional solution to the plastics waste problem, and it could be the best option according to the results of life cycle assessment on fossil-based/bio-based PE and PET. Recycling provides the greatest benefits at the plastics end of life compared with biodegradation or composting. What’s more, the production of recycled plastics only needs about 1/8 of the energy compared to producing virgin plastics. Packaging is also the largest application for recycled plastics with a market share of 45.98% in 2016. Rigid PET bottles (e.g., water and soda bottles) achieved a recycling rate of 20% in 2017. However, the overall U.S. plastics recycling rate is estimated to drop from 9.1% in 2015 to 2.9% in 2019 with the influence of China’s waste import ban in Table 1. The end life of most plastics wastes even those that are tossed into recycle bins is still landfilling. Therefore, it’s time to implement a practical solution, particularly to “short-life” plastics (most plastics packaging).
Table 1 Summary of U.S. plastic waste generation and rates of waste management (7)
Last but not least, controlling even durable plastics degradation according to the lifetime in various applications CAN be the best path to keep the plastics out of landfill. Researchers are working on genetically modified bacteria that can accelerate PET degradation to the raw materials (E - ethylene glycol and T - terephthalic acid), which provides the potential for repeated chemical recycling .(9)
Biodegradable plastics provide a real practical solution for plastic wastes management – composting instead of landfilling. If PET is hard to biodegrade in a real environment (such as ocean or soil), why not design a new plastic having similar structure/properties and FASTER degradation? The idea of adding a biodegradable unit has been realized by scientists and engineers for example in “PBAT” (polybutylene adipate terephthalate) which shows 90% degradation in 90 days.8 Recently, another promising PET alternative has been attracting increasing attention in academia, furan-based polyester. Polyfuranoates can have better mechanical properties (than aliphatic polyesters) and barrier properties (than PBAT). In our lab, we are working to create biobased copolyesters from a variety of aliphatic and aromatic monomers to tune the properties of strength and degradation. The concept of an ideal biodegradable plastic that could replace the functions of PET and PE is not so far out of reach.
1. Lebreton, L.; Slat, B.; Ferrari, F.; Sainte-Rose, B.; Aitken, J.; Marthouse, R.; Hajbane, S.; Cunsolo, S.; Schwarz, A.; Levivier, A.; et al. Evidence That the Great Pacific Garbage Patch Is Rapidly Accumulating Plastic. Scientific Reports 2018, 8 (1), 4666. https://doi.org/10.1038/s41598-018-22939-w.
6. Müller, R. J., Kleeberg, I., & Deckwer, W. D. (2001). Biodegradation of polyesters containing aromatic constituents. Journal of biotechnology, 86(2), 87-95.
7. Hottle, Troy A., Melissa M. Bilec, and Amy E. Landis. "Biopolymer production and end of life comparisons using life cycle assessment." Resources, Conservation and Recycling 122 (2017): 295-306.