Plastic pollution is an incredibly common problem for us these days, especially when it comes to food and medical supply packaging. One of the major issues is that many of the plastics used in packaging are tailored to have a wide variety of applications, meaning that there are a massive host of different varieties of plastic compounds that need to be broken down—this doesn’t just need one quick-fix polymer for all plastics, but many different polymers to address each variety of plastic compound currently inhibiting our environment. For example, some plastics are engineered for package durability; others for temperature management; others for aesthetic appeal; others for traceability, etc. Each variety of these multilayer thermoplastics is a different chemical compound, and each one needs its own specific polymer to help break it down instead of just throwing it all into what amounts to the same incinerator.
Luckily, ScienceDaily (Nov. 20, 2020) reports that scientists at the University of Wisconsin-Madison have made some progress on a new method for reclaiming polymers in these materials using solvents dubbed Solvent-Targeted Recovery and Precipitation (STRAP) processing. In a research paper by Walker et al. (Nov. 20, 2020), researchers describe how they’ve discovered a means to deconstruct multilayer films into their constituent resins using a series of solvent washes that are guided by thermodynamic calculations of polymer solubility. In other words, they can take these multilayer plastics and break them down into polymers that can be broken down and recycled much more easily.
By using a series of solvent washes guided by thermodynamic calculations of polymer solubility, the team from UW–Madison used the STRAP process to separate the polymers in a commercial plastic composed of common layering materials polyethylene, ethylene vinyl alcohol, and polyethylene terephthalate. The end result produced separated polymers that appear chemically similar to those used to make the original film. These new polymers demonstrate a process that can help close the recycling loop—in other words, turn those plastics into materials that can thoroughly be recycled and used again without creating extra unusable waste material after processing (Daley, Nov. 20, 2020).
STRAP uses a computational approach called the Conductor-like Screening Model for Realistic Solvents (COSMO-RS) to guide the polymer-identification process. COSMO-RS is able to calculate the solubility of target polymers in solvent mixtures at varying temperatures, narrowing down the number of potential solvents that could dissolve a polymer. The main principle underlying the STRAP process is “to selectively dissolve a single polymer layer in a solvent system in which the targeted polymer layer is soluble, but the other polymer layers are not. The solubilized polymer layer is then separated from the multilayer film by mechanical filtration and precipitated by changing the temperature and/or adding a cosolvent (an antisolvent) that renders the dissolved polymer insoluble. The solvent and antisolvent are distilled and reused in this process, and the targeted polymer layer is recovered as a dry, pure solid” (Walker et al., Nov. 20, 2020). This process can then be repeated for each layer of polymer, resulting in creating segregated streams that can then be easily recycled.
The research team’s ultimate goal is to develop a computational system that will allow researchers to catalogue solvent combinations to help recycle all sorts of different varieties of plastics. By having a list of different varieties of plastics that can be cross-referenced with a list of breakdown solvents, researchers hope to be able in the near future to break down every listed variety of plastic available.
Consider this a master plan for breaking down every type of plastic imaginable. Having a fully developed system like this could be a key step in finding reliable methods to break down all pollutant plastics currently in production. Having that system in place could potentially revolutionize recycling processes, and that could wind up making significant progress in global efforts to combat pollution and climate change.