Polymer Engineering & Chemistry: Molecular Structures Enable Circularity

Circularity begins at the roots of a material: it is feedstock. On the other hand, it is only possible to think about circularity if the material is known down to the molecular level. These two aspects bring with them a rough distinction between the polymer material classes of polyolefins, polyesters, and polyamides. Concerning the end-of-life options (EOL) for these materials, the recycling options are significantly different but can overlap, as well.

It is no big news saying that to date polymer materials are almost exclusively produced from fossil fuels, starting from naphtha and via multi-step synthetic routes. However, following the concept of a “circular carbon economy” the only sustainable carbon source on a planetary level is CO2 that is captured from air. The most common route of carbon sequestration is to collect it in biomass: meaning everything between cyanobacteria to giant redwoods. Photosynthesis is the superpower of nature and we are learning from it.

Bio-based Polyolefins

Polyolefins, bio-based or not, have the same basic structure: a continuous carbon backbone. This carbon backbone may or may not have different side groups or side chains attached leading to polymers like polyethylene (LDPE, HDPE, LLDPE, etc.), polypropylene (PP), copolymers of them (e.g. PP-Copo) and polystyrene (PS) are the most common ones. Depending on the chemistry behind the production polymers like “metallocene PE” can be produced, which again have some specific material properties.

Bio-based PE can be produced from two feedstocks. Oily feedstocks, like plant oils from rape seeds, sunflower, canola, or used cooking oils, or from starch or sugar feedstocks. The former feedstock is converted after pre-treatment via steam cracking to olefins as ethylene or propylene. For these existing facilities are used. The resulting materials come in a physical mixture with fossil-based materials but are 100% traceable via ISCC+ certification. This material range is provided by Plastrans Technologies. The other possibility is to produce ethylene from sugar or starch by fermentation using specially engineered bacteria. The ethylene can further be polymerized to PE.

Bio-based Polyesters

Although significantly lower in volume compared to polyolefins, polyesters are a very important material class that almost every human on Earth uses every day. Most people would describe this whole material class as one single material, which is often perceived as “the” polyester: Polyethylene terephthalate (PET). However, polyesters offer even more variability than polyolefins. Depending on the used monomers to produce the respective polyester it may be either well mechanically recyclable or easily biodegradable at low temperatures.

This makes “the” polyester look from a biopolymer perspective to be rather PLA. The production of biobased polyesters (not only PLA) has in common that it always involves a fermentation process using an energy source. This energy source is not necessarily sugar or starch, but it is the case in the most prominent example of PLA. The fermentation of sugar with specified bacteria produces lactic acid. This is the only fermentation step required in this process. The next step to obtain PLA is basic chemistry to first produce lactide (a lactic acid dimer) with subsequent polymerization of the lactide to PLA.

Bio-based Polyamides

Bio-based polyamides are, yet, relatively rare on the market, and generally, polyamides can be produced in various ways. It is either produced from one monomer like PA6 or PA10 or several like PA5.6 or PA6.6. Without going too deep into the chemistry background the bottom line is that not every monomer is available from bio-biobased sources. The two main bio-based sources are sugars and castor oil to produce monomers for PA5, PA10, PA5.10, and PA11.

Irrespective of the feedstock polyamides are generally very stable against elevated temperatures and need high temperatures to be molten. Additionally, many polyamides are reinforced with e.g. glass fibers to unfold their full potential as high-performance materials.

Circularity/Structure Relationship

Finally, seeing the different structures and having an idea of the material properties that arise from them, sheds some light on the complexity that is to tackle to provide circular solutions for the respective materials. All the mentioned materials require different EOL treatments but also have different EOL options. Polyolefins are great thermoplastics and can easily be reworked by mechanical recycling methods. Additionally, chemical gives further options for contaminated packaging. Different recycling techniques will be covered later this year. Polyesters allow mechanical recycling as polyolefins, as well as, low-temperature chemical recycling with enzymes or chemicals exploiting their susceptibility to undergo hydrolysis reactions. Therefore, depending on the exact polymer, polyesters can either be mechanically, chemically, or organically recycled. The highly stable polyamides are hardly (mechanically) recycled at all. Nevertheless, after breaking up their crystalline structure the polymerization reaction can be reversed, which means it can be chemically recycled, as well. Because of the underlying chemistry polyamides can also biodegrade under special conditions, which was scientifically proven.[Kyulavska, M., Toncheva-Moncheva, N., Rydz, J. (2019). Biobased Polyamide Ecomaterials and Their Susceptibility to Biodegradation. In: Martínez, L., Kharissova, O., Kharisov, B. (eds) Handbook of Ecomaterials. Springer, Cham. https://doi.org/10.1007/978-3-319-68255-6_126]

Overall, it is evident that there might not be a “one size fits all” circular solution. But it is also evident that there is at least one circular solution for almost every material class. The foundation to be able to be circular is a regenerative production. The basis of being fully circular is to be biobased.

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?Dr. Rudi Eswein

Director of Sustainability