Scientists at the US Department of Energy’s Oak Ridge National Laboratory (ORNL, Oak Ridge, Tennessee; www.ornl.gov) have reported the development of a chemical catalytic recycling process that can convert selected polymers in mixed plastics waste into individual monomers for reuse in producing commercial-grade plastics and other materials.

Tomonori Saito, a synthetic polymer chemist at the lab and author of a study published in Materials Horizons, said the process selectively and sequentially converts each polymer at a specific temperature ranging from 130°C to 210°C into separate, individual, pristine monomers. 

The tailored catalyst developed by the researchers is a protic ionic salt-based organocatalyst formed from triazabicyclododecane (TBD) and trifluoracetic acid and named TBD:TFA. It is said to deconstruct each selected polymer into a usable monomer at high yields, which are separated using recrystallistion or extraction methods.

According to researchers, the catalyst effectively and efficiently deconstructed selected consumer products, including safety goggles made from polycarbonate, PET water bottles, a polyurethane thermoset foam, and a rope or fishing net made from polyamide. Until now, no single catalyst has been shown to be successful on all four polymers, ORNL said. The effectiveness of the catalyst with multiple materials makes it useful for deconstructing multicomponent plastics such as composites and multilayer packaging, it noted.

In the study, two hours of processing with the chemical catalyst converted the PC into bisphenol A (BPA) at 130°C, the PU into 4,4’-methylenedianiline (MDA) at 160°C, the PET into bis (2-hydroxyethyl) terephthalate (BHET) at 180°C, and the PA to caprolactam at 210°C. Deconstruction of each polymer at a specific temperature allows for sequential recovery of the individual monomer, separately, the lab explained.

The catalyst and resulting monomers are water soluble and can be transferred into water, where impurities such as pigments can be removed by filtration, researchers noted. The catalyst can be almost entirely recovered and directly reused for multiple cycles to reducing cost, increase efficiency, enhance flexibility, and lower environmental impact, ORNL said.

Use of the catalyst replaces harsh chemicals for deconstructing polymers while offering good selectivity, thermal stability nonvolatility and low flammability. Other plastics including polyolefins, additives, and associated materials such as cotton are left intact by the catalyst due to differences in reactivity and can be subsequently recovered, according to researchers. The process and catalyst present a promising chemical recycling strategy for a variety of currently unrecyclable mixed plastics, the lab said.

A Life Cycle Assessment (LCA) reportedly showed that the production of various condensation polymers from the deconstructed monomers instead of from fossil sources would generate up to 95% fewer greenhouse gases and require up to 94% less energy input. Additionally, the comparative LCA model for mixed plastics waste of PC, PU, PET, and PA shows a reduction of 51% in fossil energy consumption compared to the combined energy demand for each polymer individually, according to the research.