Plastic recycling studies need reliable polymer data. This database is ready to inform them

Researchers have created a polymer characterization database to serve as a basis for plastic recycling studies.¹ In doing so, they discovered that many commercially available polymers had physical and chemical properties that differed from manufacturers’ specifications.

The polymer industry uses over 10,000 unique chemicals.² Polymer additives can be classified into four classes: functional additives (including flame retardants and plasticizers), colorants, fillers and reinforcements. However, polymer additives are known to thwart the chemical recycling processes of polymers.³ For example, a 2022 study found that amine, phenolic, phosphite, and metal stearate additives all slowed polyethylene decomposition compared to zeolite catalysts.⁴ Similarly, separate research has shown that phenolic additives alter the product distribution of polyethylene hydrocracking.⁵

Physical properties of polymers, such as molecular weight and branching density, can also affect their recycling chemistry. For example, isotactic polypropylene forms much smaller hydrogenolysis products than similar syndiotactic and atactic feedstocks.⁶

Inspired by comparative studies in the field of biomass valorization, a team led by Gregg Beckham and Nicholas Rorrer of the US National Renewable Energy Laboratory determined the chemical and physical properties of 59 commercially available polymers and compiled their data in an online database. It includes information on molecular composition, polymer morphology, molecular weight distributions, thermal properties, elemental compositions and the presence of additives.

This work will open the eyes of many people.

The 59 polymers in the database represent about 95% of the polymers manufactured worldwide in 2018 and include polyolefins, condensation polymers and copolymers. “We selected these polymers because they are the ones that are manufactured at more than a million tons per year,” Beckham notes.

“Our main motivations in doing this work were to provide a resource to the community using these particular plastics and to provide a description of the complete characterization methods,” Beckham explains.

Analytical Arsenal

The team characterized the polymers using an arsenal of analytical tools including Fourier transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry, and inductively coupled plasma mass spectrometry. They found that not all properties were as advertised. For example, six polymers had bimodal molecular weight distributions, ten polymers had unexpected thermal properties, and five polymers had mass distributions that differed from supplier specifications.

The polymers also contained additives such as titanium dioxide and antimony, as well as several others that the team could not identify. “I had no idea,” said Anne McNeil, a polymer sustainability expert at the University of Michigan in the US, when she learned that antimony is used as a catalyst in the production of polyethylene terephthalate. “This work will open a lot of people’s eyes to how impurities and additives can influence their chemical composition.”

“Polymer blends are even more challenging than pure materials,” McNeil continues. “An intermediate step between using complex materials and what they’ve done here would be to go into the grocery store and pull out real products and see how different they are from what (the researchers) studied here. I think the real products are going to be even more contaminated with more additives,” McNeil concludes. “I think that’s the next most important step.”

The references

1 AA Cuthbertson et al, Green chemistry2024, 267067 (DOI: 10.1039/d4gc00659c)

2 H Wiesinger, Z Wang and S Hellweg, Environ. Sci. Technol.2021, 559339 (DOI: 10.1021/acs.est.1c00976)

3 LD Ellis et al, Catal. nat.2011, 4539 (DOI: 10.1038/s41929-021-00648-4)

4 AC Jerdy et alAppl. Catal., B2023, 325122348 (DOI: 10.1016/j.apcatb.2022.122348)

5 ZR Hinton et al, Green chemistry2022, 247332 (DOI: 10.1039/d2gc02503e)

6 RA Hacker et al, Macromolecules2022, 556801 (DOI: 10.1021/acs.macromol.2c00805)