Why Does Polymer Type Matter in Microplastics Research?

This week I thought I would explain the theory and reasoning behind the polymer analysis step that I do with the laser spectroscope to make sense of why microplastic polymer type matters. Plastics are complex structures of carbon, hydrogen, oxygen, often nitrogen, chlorine, and even sulfur. Each polymer, also known as the specific type of plastic, has its own structure and varies in the organization of the atoms and the amounts and types of atoms used to construct the skeleton, so to say. The differences in the atomic skeleton of the plastics are what the laser spectroscopy analysis notices and helps identify the types of materials used to make the particles.

Polymer types are exceptionally important indicators of pollution sources and they also provide some insight into future pollution concerns. The most abundantly produced polymers are polyethylene (PE), polypropylene (PP), and polystyrene (PS), and are joined by polyester (PES), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) as the most abundant polymers in marine plastics pollution. While there are many uses for each of these polymers, a combination of physical characteristics of microplastic particles, input sources, and economic data can help scientists understand the likely cause of the marine plastics pollution we find in our samples. For example, fibrous polyester and polyethylene microparticles often originate from textiles, which is why they are so abundant near wastewater treatment plant sites. Alternatively, fragments of polystyrene, regardless of color, often originate from disposable food and beverage containers. While we cannot link microplastics to the their origin with certainty, we can use the information about polymer and particle type as well as input pathway to drive policy.

Another subset of microplastics science is research regarding how plastics fragment in the environment and become microparticles over time. When disposable food containers collide with rocks, scrape against sandy beaches, and embrittle under UV rays, the containers fragment in predictable ways and in variable amounts depending on polymer type. In a fragmentation study by Chubarenko et al. (2020), single-use plastic items were exposed to different sediment grain sizes in an oversized tumbler and ranked according to the percent mass of microplastics produced compared to the original plastic item. The research found that solid PS, then low density (LD) PE released the highest amounts of microplastics during fragmentation, while foamed PS and PP released the least. Although the work assumed that differences in polymer densities do not affect fragmentation in nature, which is not realistic, it does provide strong evidence for leading sources of secondary microplastics in coastal areas, where single-use plastic waste is abundant. As I noted before, while this evidence is not concrete, a combination of the particle types, polymers, and input sources can provide support for regulations in coastal areas. 

To end on a slightly lighter note, I did get to analyze more of my suspected microparticles this week at the laser lab. I have completed work on 110 particles, with only 400+ to go. I know that sounds like a lot, but now that I have a reliable method (see last week's blog), I feel comfortable working through the samples at a speedy pace.

​*Interested in the Chubarenko et al. (2020) paper? Click here.