Multilayer plastic materials are ubiquitous in food and medical supply packaging, particularly since the specific properties of layered polymers can keep moisture from fouling sterile syringes, or light and oxygen from making potato chips stale. But despite their utility, those ever-present plastics are impossible to recycle using conventional methods.
About 100 million tons of multilayer thermoplastics — each composed of as many as 12 layers of varying polymers — are produced globally every year. Forty percent of that total is waste from the manufacturing process itself and because there has been no way to separate the polymers, almost all of that plastic ends up in landfills or incinerators.
A team of engineers has pioneered a method for reclaiming the polymers in these materials using solvents — a technique called Solvent-Targeted Recovery and Precipitation (STRAP) processing. By using a series of solvent washes guided by thermodynamic calculations of polymer solubility, the team 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 separated polymers appear chemically similar to those used to make the original film.
The team now hopes to use the recovered polymers to create new plastic materials, demonstrating that the process can help close the recycling loop. In particular, it could allow multilayer-plastic manufacturers to recover the 40 percent of plastic waste produced during the production and packaging processes. The process was demonstrated with one multilayer plastic; the team will try other multilayer plastics and scale the technology.
As the complexity of the multilayer plastics increases, so does the difficulty of identifying solvents that can dissolve each polymer. That’s why STRAP relies on a computational approach called the Conductorlike Screening Model for Realistic Solvents (COSMO-RS) to guide the 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 team can then experimentally explore the candidate solvents. The goal is to eventually develop a computational system that will allow researchers to find solvent combinations to recycle all sorts of multilayer plastics. The team also hopes to look at the environmental impact of the solvents it uses and establish a database of green solvents that will allow them to better balance the efficacy, cost and environmental impact of various solvent systems.