New method holds potential to efficiently recycle widely used polyester plastics

A new catalytic process was shown to break down polyester plastic waste into its basic, useful ingredients in an hour at moderate temperatures. If scaled up, the advance could someday allow for easier plastics recycling and help address concerns over plastics pollution.

A Washington State University-led team developed an inexpensive catalytic sorting method that can deconstruct polyester, a large class of plastics that are widely used in textiles, bottles, mixed plastics and packaging materials. Using their sequential chemical sorting method, the researchers were able to separate up to 100% of the basic ingredients of polyester even from mixes of plastics. Those ingredients, or monomers, could be used to produce new, high-quality plastics that are as good as first-generation plastics. Their work is published in the journal, Cell Reports Physical Science.

"The catalyst works very well and is very efficient, so we got very high yields, and the operating conditions are mild," said Hongfei Lin, corresponding author and a professor in WSU's Gene and Linda Voiland School of Chemical Engineering and Bioengineering.

Recycling polyester plastics can be challenging or costly, depending on the method used. Recyclers most commonly have to sort them physically first and then melt them at high temperatures and re-mold them, most often resulting in poor-quality plastics with limited marketability. Chemical recycling can produce higher quality products, but it has required high reaction temperatures and a long processing time, making it too expensive and cumbersome for many industries to adopt. Because of these challenges, only about 9% of plastic in the U.S. is recycled every year. The rest accumulates in the environment, where it can take hundreds of years to degrade.

In the study, the researchers used an inexpensive catalyst in methanol to break down one type of very common polyester, polyethylene terephthalate or PET, to its monomers, dimethyl terephthalate and ethylene glycol. They were able to completely break down the PET plastic in an hour at 160 degrees Celsius (320 degrees Fahrenheit) to its monomers. With their sequential catalytic process, they were also able to pull out the monomers from the PET plastic even when the plastic was mixed in with other materials, such as nylon, polyethylene, and polypropylene. The researchers also had success with breaking down the PET in difficult-to-recycle multi-layer plastic films like vacuum-sealed storage bags.

"If you want to deconstruct multi-layer plastic films, the first step is to deconstruct the PET," said Lin. "The catalyst used in the reaction was very selective, so they can pull out and break down the PET in the mixed plastics."

The method also worked with other types of polyesters as well, such as polylactic acid (PLA) that is used in disposable cutlery.

"The concept is to convert the polyesters to monomers. Those monomers can be re-synthesized to the polymers, and the new polymer is like brand new polymer," said Lin. "The quality is going to be the same as the polymers when they are first made - and that's the uniqueness about chemical recycling."

In addition to WSU, the project included researchers from Pacific Northwest National Laboratory and the University of Washington. The work is supported by the National Science Foundation.