In food and medical supply packaging, multilayer plastic materials are ubiquitous, particularly because layering polymers can give unique properties, such as heat resistance or oxygen and moisture control, to those films. But those ever-present plastics, considering their usefulness, are difficult to recycle using traditional methods.
Every year, approximately 100 million tonnes of multilayer thermoplastics, each consisting of as many as 12 layers of different polymers, are manufactured globally. Forty percent of the amount is waste from the processing process itself, and almost all of that plastic ends up in landfills or incinerators because there is no way to separate the polymers.
Now, engineers from the University of Wisconsin-Madison have pioneered a technique to reclaim the polymers using solvents in these products, a technique they have called Solvent-Targeted Recovery and Precipitation (STRAP) processing. In the journal Science Advances, their proof-of-concept is detailed today (November 20, 2020).
UW-Madison professors of chemical and biological engineering George Huber and Reid Van Lehn and their students used the STRAP process to separate the polymers into a commercial plastic consisting of traditional layering materials polyethylene, ethylene vinyl alcohol, and polyethylene terephthalate by using a series of solvent washes driven by thermodynamic measurements of polymer solubility.
Outcome? Chemically, the separated polymers look identical to the ones used to make the original film.
The team now hopes to produce new plastic products using the recycled polymers, showing that the process will help close the recycling loop. Multilayer plastic manufacturers will in particular, be able to recover 40 percent of the plastic waste produced during the manufacturing and packaging processes.
"We demonstrated this with one plastic multilayer," says Huber. "We need to try other plastics with multilayers and we need to scale up this technology."
The difficulty of finding solvents that can dissolve each polymer increases as the complexity of the multilayer plastics increases. That is why STRAP relies on a Van Lehn computational approach called the Conductor-like Practical Solvents Screening Model (COSMO-RS) to direct the process.
COSMO-RS is able to measure, at different temperatures, the solubility of target polymers in solvent mixtures, minimising the number of possible solvents that a polymer may dissolve. The team can then investigate the candidate solvents experimentally.
This helps us to tackle these far more complicated processes, which is important if in the field of recycling you are really going to make a dent," Van Lehn says."
Ultimately, the objective is to build a computer 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 uses and create a green solvent database that will allow them to better balance the effectiveness, cost and environmental impact of different solvent systems.
The project stems from the expertise of UW-Madison in catalysis. Researchers in chemical and biological engineering at the university have pioneered solvent-based reactions for decades to turn biomass into useful chemicals or fuel precursors, such as wood or agricultural waste. Most of the experience also translates into solvent-based recycling of polymers.
Through the newly formed Multi-University Center on Chemical Upcycling of Waste Plastics, directed by Huber, the team continues its research on STRAP processing. In the $12.5 million U.S., researchers The Energy-Funded Center Department invites