In existing mechanical recycling processes, plastic items are chopped into fine pieces, melted and extruded into new plastic feedstock, which causes them to degrade in quality. Recycled virgin plastic is often used for lower grade applications, including park benches, water pipes, and traffic cones. For that reason, scientists from the Universities of Bath and Birmingham are researching a method of chemical recycling that breaks polymers like PLA and PETG into their base molecules.
The paper, published in ChemSusChem, describes how McKeown et al., sought about developing a viable method of chemically recycling PLA plastic into its constituent chemical components. Using a complex made up of zinc as a catalyst, the team successfully performed the break up of PLA and the formation of lactic acid, the basis for PLA plastic. The researchers employed several zinc complexes for dissolving PLA samples, with the most active complex consuming the PLA completely within 30 minutes.
The research team was able to derive different constituent materials depending on the exact nature of the zinc complex. When combined with ethanol as a solvent, for instance, the zinc complex was able to form alkyl lactates, which are considered “green” solvents and, therefore, a useful and potentially sustainable result of PLA recycling. In addition to PLA, the researchers were able to run small tests on the recycling of PET, the plastic used to make water bottles, using the same process.
While chemical recycling has been demonstrated in the past, the researchers consider the zinc complexes used in this study to be more environmentally friendly than catalysts used in previous methods of chemical recycling. The team was also able to perform the recycling at lower temperatures, which means less energy was used. The scale at which the recycling was carried out was very small, but the University of Birmingham team is exploring ways to scale the system up to produce larger quantities of starting chemicals.
Compared to fossil fuel used for transportation, electricity and industrial applications, plastics represent a small, but still significant portion of industrialized society’s carbon footprint. Finding the global numbers is a bit more difficult, but the production of plastics makes up 1 percent of U.S. greenhouse gas (GHG) emissions and 3 percent of the country’s primary energy use. Moreover, the Center for International Environmental Law points out that, as polymers are often made using the byproducts of fossil fuel extraction, “the two product chains [plastics and fossil fuels] are intimately linked.”
If society wants to maintain any semblance of its current industrial and consumer form, it will be necessary to ditch the petro-plastics, leaving industrialized society in need of alternatives. This means recycling the plastics we do have and finding other sources for our polymers. For the moment, the dominant form of polymer not derived from fossil fuels is polylactic acid (PLA), primarily derived from the sugar in corn starch and sugarcane, as well as cassava roots, chips or starch.
A 2017 study suggested that by using PLA as a replacement for plastics derived from fossil fuels, GHG emissions could be cut by 25 percent. Using renewable energy sources to power PLA production could reduce emissions even further. However, switching to PLA alone is not enough to completely drop its ecological impact. The material must be disposed of and recycled properly.
PLA releases GHGs as it degrades, though fewer than its petro-based counterparts, and, according to a 2010 study, it may release more pollutants due to the fertilizers and pesticides used to grow the crops that serve as the basis for PLA. NatureWorks, the largest supplier of PLA, allows partners to purchase Ingeo made without genetically modified (GM) crops. However, the use of GM plants is the default practice and, because GM plants have high pesticide resistance, they are typically linked to increased pesticide usage.
The actual growing of the feedstock for PLA also results in its own significant GHG emissions with the nitrous oxide used in low-cost fertilizers 310 times as powerful in terms of its effect on the atmosphere than CO2 and 15 times more powerful than methane. According to one calculation, the use of these fertilizers in NatureWorks products results in the emission of “56 Tg of carbon dioxide equivalent, more than all of the landfills combined in the United States according to the US Gas and Sinks.”
In other words, the sort of circular economy envisioned by the Bath and Birmingham scientists is needed more than one might even imagine. Of course, it’s important to remember that, even in such a scenario, this process is not a zero-sum game. It still requires extra material and energy inputs to recycle the plastic, which have to come from somewhere.
In this case, zinc is the major material input, though at very small quantities. Though the U.S. has its own share of zinc reserves, it imported nearly as much refined zinc as the amount of raw zinc that it produced in 2018. Other industrialized countries, like every country in Europe except for Sweden, have no zinc reserves of their own and must rely on imports entirely, which means shipping, which means more emissions.
This may sound like needless nitpicking, but given the fact that our ecosystem is collapsing due in no small part to global society’s near-total reliance on fossil fuels, it is completely necessary to consider every material input and output when embarking on new technological endeavors. Such an accounting probably should have been employed form the beginning, but you live and you learn, right?
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