Researchers have built a database of polymer characterisation data to serve as a baseline for plastic recycling studies.1 In doing so they found that many commercially available polymers had physical and chemical properties that differ from the manufacturers’ specifications. 

The polymer industry uses more than 10,000 unique chemicals.2 Polymer additives can be categorised into four classes – functional additives (including flame retardants and plasticisers), colourants, fillers and reinforcements. However, polymer additives are known to frustrate chemical polymer recycling processes.3 For example, a 2022 study found that aminic, phenolic, phosphite and metallic stearate additives all slowed the decomposition of polyethylene over zeolite catalysts.4 Similarly, separate research has shown that phenolic additives alter the product distribution of polyethylene hydrocracking.5 

The physical properties of polymers, such as molecular weight and branch density, can also affect their recycling chemistry. For example, isotactic polypropylene forms much smaller hydrogenolysis products than similar syndiotactic and atactic starting materials.6 

Now, inspired by benchmarking studies in the biomass valorisation community, a team led by Gregg Beckham and Nicholas Rorrer from the US National Renewable Energy Laboratory has determined the chemical and physical properties of 59 commercially available polymers and compiled their data into an online database. It includes information on molecular composition, polymer morphology, molecular mass distributions, thermal properties, elemental compositions and the presence of additives.

This work is going to open a lot of people’s eyes

The 59 polymers in the database represent approximately 95% of polymers manufactured globally in 2018 and included polyolefins, condensation polymers and co-polymers. ‘We selected these polymers as they are the ones made at over 1 million metric tons per year,’ notes Beckham.

‘Our primary motivations in doing this work were to provide a resource for the community using these particular plastics and to provide a description of comprehensive characterisation methods,’ says Beckham. 

Analytical arsenal 

The team characterised the polymers with an arsenal of analytical tools that included Fourier-transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry and inductively coupled plasma-mass spectrometry. They found that not all of the properties were as they were reported to be. For example, six polymers had bimodal molecular mass distributions, 10 polymers displayed unexpected thermal properties, and five polymers had mass distributions that differed from the vendors’ specifications.

The polymers also contained additives such as titanium dioxide and antimony as well as several the team couldn’t identify. ‘I had no idea,’ comments Anne McNeil, an expert in polymer sustainability from the University of Michigan, US, on finding out that antimony is used as a catalyst in the production of polyethylene terephthalate. ‘This work is going to open a lot of people’s eyes to thinking more about how impurities and additives might influence their chemistry.’ 

‘[Polymer mixtures] present even bigger challenges than just the pure materials,’ continues McNeil. ‘A step in-between going to complex materials and what they did here would be to go to the grocery store and pull-out real products to see how different they are from what [the researchers] studied here. My guess is that real products are going to be even more contaminated with more additives,’ concludes McNeil. ‘I feel like that’s the most important next step.’