Plastics impart a lot of useful properties during a global health pandemic. They’re cheap, disposable, and easy to clean. However, plastic buildup in the environment hasn’t been affected by the pause button that has hit most of the world in 2020 and is still a pressing challenge. One of the ways that scientists are approaching this challenge is developing replacement components for plastics that are more environmentally friendly, using green chemistry.
The Large and the Small: Polymers and Plasticizers
Plastics are mostly large, long-chain molecules called polymers, which don’t readily break down in the environment. However, other smaller molecules are often blended in with the polymers to give plastics the properties and colors by which we recognize them.
For example, PVC pipes and plastic cling film are often both made out of poly(vinyl chloride) (PVC) polymer. However, plastic cling film can additionally be made up of up to 50% of smaller molecules, making it much more malleable and flexible. Because these “small” molecules can dramatically impact the properties of the plastics that contain them, they are called plasticizers. Plasticizers are used for a variety of reasons: improving flexibility and malleability, to ease processability, and to change colors. Most dyes are smaller molecules blended in with colorless (or maybe off-white) polymers to make characteristic plastics, such as purple Kangaroo shampoo bottles, green recycling bins, and black trash bags.
Since plasticizers are blended into plastics, rather than chemically attached, they can eventually escape out of their plastic housing and into the environment. The broad and ubiquitous use of plasticizers in plastics means that this escaping, or leaching, into their environment is also a concern to human health, particularly when it comes to plastics used in food packaging.
Sometimes plasticizers are not added intentionally, but are a residual product from the chemical reaction that forms polymers. However, even residual amounts can alter the properties of the plastics and leach into the environment and food they come into contact with. A familiar example of this is Bisphenol A (BPA) in polycarbonate water bottles and tupperware. It has largely been phased out due to health concerns, hence “BPA-free” labelling on many of these items.
A Green Solution
In order to improve the sustainability of plastics, we need to find ways to make both polymers and plasticizers more bio-sourced (made from materials other than fossil-fuels), biodegradable (breaks down in the environment), and environmentally-friendly. Luckily– scientists have made headway in this effort! Researchers at McGill University have recently published a study in ACS Sustainable Chemistry and Engineering in which they synthesized and tested a green plasticizer that can be used in PVC.
What does it mean to be a green plasticizer? It’s one that uses the principles of green, or sustainable, chemistry. These green chemistry principles apply to the entire lifecycle of a compound and seek to reduce the generation and use of hazardous substances in its synthesis, production, and disposal. In the case of their green plasticizer, the researchers designed a compound that they synthesize from sustainable materials, uses cleaner solvents in its manufacturing, and is biodegradable.
A fully-sustainable, non-toxic, and biodegradable plasticizer for PVC is a big deal, because 90% of worldwide plasticizer production is to plasticize PVC. This large use of plasticizers is how you can go from the strong, stiff material used in PVC pipes to the flexible material that makes up the most popular form of cling film. Historically, manufacturers used chemicals called phthalates as plasticizers. However, these compounds are now global contaminants because they break down slowly but are used, and unintentionally released, widely.
The most common historical plasticizer, di(2-ethylhexyl)phthalate (DEHP) can break down into compounds that have toxic effects. It has been restricted in toys, medical devices, and food packaging around the globe. New regulations have spurred research for safer replacement plasticizers to fill the void. Unfortunately, such replacements have had their own problems: some are persistent in the environment, some break down into harmful products, some need toxic or petroleum-derived ingredients in their synthesis and processing.
The alternative proposed by the team of researchers at McGill University, di-n-heptyl succinate (DHPS), is made from succinic acid (produced by microbes) and n=heptanol (from castor oil) with the use of minimal solvents. The researchers blended DHPS into PVC as a plasticizer and compared the blend’s properties to those of PVC and more traditional plasticizers. They found that DHPS performed equal or better in a variety of physical and mechanical tests, indicating that it is a very good replacement plasticizer. They also found that it degraded much more rapidly in soil than DEHP, showcasing excellent biodegradability. Hopefully this green plasticizer will be incorporated commercially and will help to make PVC plastics safer and more environmentally-friendly!
Green chemistry is an active area of research. For more green chemistry success stories, check out this list of winners of the EPA’s Green Chemistry Challenge. All this being said, reducing your generation of plastic waste is still a great way to lessen plastic’s grip on the environment!
Source article: B. Elsiwi, O. Garcia-Valdez, H. Erythropel, R. Leask, J. Nicell, and M. Maric ACS Sustainable Chem. Eng. 2020 https://doi.org/10.1021/acssuschemeng.0c02859
Cover Image Source: https://www.flickr.com/photos/maveric2003/2275970847