Multilayer plastic substances are ubiquitous in food and medical supply packaging, especially since layering polymers can give those films specific properties, like heat resistance or oxygen and moisture control. But despite their utility, these ever-present plastics are impossible to recycle using traditional procedures.
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 the total is waste in the production process itself, and because there has not been any way to separate the polymers, almost all of the plastic ends up in landfills or incinerators.
Now, University of Wisconsin-Madison engineers have pioneered a method for reclaiming the polymers in these materials using solvents, a method they have called Solvent-Targeted Recovery and Precipitation (STRAP) processing. Their proof-of-concept is detailed in the journal Science Advances.
With a series of solvent packs directed by thermodynamic calculations of polymer solubility, UW-Madison professors of biological and chemical engineering George Huber and Reid Van Lehn and their students used the STRAP procedure to divide the polymers at a commercial plastic composed of common layering materials polyethylene, ethylene vinyl alcohol, and polyethylene terephthalate.
The result? The separated polymers appear chemically like those used to generate the originals.
The group now hopes to utilize the recovered polymers to create new plastic materials, demonstrating the procedure can help close the recycling loop. In particular, it might allow multilayer-plastic manufacturers to regain the 40 percent of plastic waste generated throughout the production and packaging procedures.
“We have demonstrated this with one multilayer plastic,” says Huber. “We need to try other multilayer plastics and we need to scale this technology.”
Since the complexity of the multilayer plastics increases, so does the difficulty of identifying solvents that could dissolve each polymer. That is why STRAP relies on a computational strategy utilized by Van Lehn known as the Conductor-like Screening Model for Realistic Solvents (COSMO-RS) to direct the process.
COSMO-RS is able to calculate the solubility of target polymers and the staff can then explore the candidate solvents.
“This allows us to tackle these much more complex systems, which is necessary if you’re actually going to make a dent in the recycling world,” says Van Lehn.
The objective is to eventually develop a computational system that will allow researchers to find solvent combinations to recycle all kinds of multilayer plastics. The team also hopes to look at the environmental impact of the solvents it utilizes and establish a database of green solvents that will enable them to better balance the efficiency, cost and environmental impact of various solvent systems.
For decades, the university’s chemical and biological engineering researchers have pioneered solvent-based responses to convert biomass — like wood or agricultural waste — into useful chemicals or gas precursors. Much of this expertise translates into solvent-based plastic recycling too.
The group is continuing its research on STRAP processing via the recently established Multi-University Center on Chemical Upcycling of Waste Plastics, directed by Huber. Researchers at the $12.5 million U.S. Department of Energy-funded center are exploring several chemical Pathways for recycling and recovering polymers.