New recycling method may reduce tons of plastic waste

A new recycling method may reduce enormous amounts of waste from the annual 100 million tons of multilayer plastic produced worldwide. 

A team of Wisconsin-based scientists devised a strategy called "solvent-targeted recovery and precipitation" (STRAP) that can separate multilayered plastic into its constituent materials at almost 100% efficiency. 

A research article on their proof-of-concept method appears in Scientific Advances on Nov. 20.

A big advantage of the new strategy is its cost competitiveness. The deconstructed materials can be made ready for new use at a cost similar to that of virgin materials.

Multilayer polymer films are used for many applications, including food containers, water bottles and medical products. These films are multilayered because different polymers provide different properties, such as heat resistance, an oxygen barrier and moisture control.

The problem addressed by researchers is not after-use waste, which may be contaminated by food or other impurities, but the waste in manufacturing: industrial processes that fabricate multilayer films to the necessary forms waste up to 40% of the new materials.

The authors point out that existing plastic waste recycling technologies cannot work with multilayer plastic materials because the layers are chemically incompatible. Their strategy deconstructs the multilayers into their constituent resins to be reused. 

The STRAP model separated out common resins that produce one type of multilayered plastic material containing polyethylene (PE), ethylene vinyl alcohol (EVOH) and polyethylene terephthalate (PET) — a particular multilayered material manufactured by Amcor Flexibles. Other multilayered plastic materials can have 10 or more layers.

The researchers applied a computational approach (the Conductor-like Screening Model for Realistic Solvents or COSMO-RS) to calculate thermodynamically which type of solvent at what temperature works best to dissolve each polymer. 

They dissolved a small sample of multilayered film in an indicated solvent, then filtered that solution and added an antisolvent to make the solution less soluble. Once cooled to room temperature, the polymer was filtered out from the mixture as a precipitated solid. The process used different solvents and temperatures to produce each component as a solid resin.

When researchers tested the end products, they found that "the PET, EVOH, and PE recovered from the physical mixture are indistinguishable from the same, corresponding virgin resins."

How economical is STRAP? The researchers note that "the viability of the STRAP process ultimately hinges on the ability to produce fit-for-use recycled resins while efficiently recovering and reusing the solvents so that production of the recycled polymer resins is cost-competitive with the production of the virgin materials."

Their analysis indicates that "the STRAP process could be deployed at scale to fully recycle realistic waste streams originating from multilayer plastic film plants."

They plan to continue STRAP research on recycling more complex polymer materials. 

The researchers are affiliated with the Department of Chemical and Biological Engineering at the University of Wisconsin (Madison), the Department of Energy Great Lakes Bioenergy Research Center at the University and the Neenah Innovation Center.

A university press release notes: "The project stems from UW-Madison's expertise in catalysis. For decades, the university's chemical and biological engineering researchers have pioneered solvent-based reactions to convert biomass--like wood or agricultural waste--into useful chemicals or fuel precursors. Much of that expertise translates into solvent-based polymer recycling as well."