26 February 2021

Half of the plastic products labelled as “bio” were found to contain varying amounts of fossil fuel-based plastics when they were analysed using a new method developed by GNS Science, Scion, and others.

Plastic bags, cutlery and disposable plates were most likely to be mislabelled, while coffee cups and lids were most accurately labelled.

Plastic polymers are primarily made up of the elements carbon and hydrogen. The new method uses stable carbon and hydrogen isotope ratios and the plastic carbon content to distinguish plant-based plastics from oil-based plastics.

The researchers believe their new method is a robust way to identify biobased materials. This approach is affordable and can be used to screen or check bioplastic claims where manufacturers use a “bio” label to influence consumer perceptions. Unfortunately, the results of this work show this is common.

Aside from “greenwashing”, mislabelling can lead to contaminated bioplastic recycling, increased pollution, including more microplastics in the environment, and more waste.

How it works

Elements like carbon and hydrogen can exist in forms (isotopes) that have different weights. For example, the most common form (or isotpe) of carbon has a “mass” of 12 (¹²C). However, other heavier forms, carbon 13 (¹³C) and carbon 14 (¹⁴C), exist in tiny but measurable amounts. The isotopes of hydrogen (¹H) include deuterium (²H) and tritium (³H).

Carbon 14 is radioactive, and the amount present decays over time. As plants grow, they absorb ¹⁴C from the air and it is found in any bioplastics made from plant matter. In contrast, fossil fuels, which were once plant matter millions of years ago and which have been out of contact with the air, contain practically no ¹⁴C. Measuring the amount of ¹⁴C in plastic is an accurate method of distinguishing between petroleum and biobased plastics. However, the cost and time the test takes mean it is not practical for large scale bioplastics screening.Carbon-13 and ²H are stable isotopes, and their levels do not change with time. We know the ratio of carbon to ¹³C in plants and petrochemicals. This information, combined with the ²H ratios and the percentage of carbon in the final product, the method’s developers could identify plastics made from either plants or petroleum products and analyse plastics of mixed origins. The findings were also confirmed by measuring ¹⁴C levels.

Rogers, K. M., Turnbull, J. C., Dahl, J., Phillips, A., Bridson, J., Raymond, L. G., ... & Hill, S. J. (2021). Authenticating bioplastics using carbon and hydrogen stable isotopes–An alternative analytical approach. Rapid Communications in Mass Spectrometry, e9051.