Of all the major types of materials used today, plastics are one of the fastest-growing categories and offer the greatest opportunity for innovation. Decades of development towards improving the properties of plastics, making them easier to process and lowering the cost of production has led to their broad use in a variety of different applications. However, advancements in end-of-life solutions have not kept up. Earlier this year, Amazon joined the United States Department of Energy’s BOTTLE Consortium to help develop foundational technologies that will address some of the end-of-life issues we see with today’s plastics, while also looking towards new types of plastics, some of which are highlighted in this collection.
The impressive physical properties of the most common plastics used today are attributable to the long carbon–carbon chains that make up their molecular backbone (Fig. 1). However, the strong carbon–carbon bonds are not easy to break apart, which means that the molecular structure that confers diverse properties also makes plastics persistent in the environment. To address the end-of-life challenges with plastics, we should be thinking beyond the common plastics that exist today towards new types of materials and recycling technologies that are developed in concert.
The repeating units of polyethylene polymer chains are characterized by strong carbon–carbon bonds.
Today, when considering end-of-life solutions for plastics, there is generally an underlying dichotomy between recyclability and biodegradability, or, more specifically, compostability. Solutions to improve the recycling of existing plastics often cite compostable or other biodegradable plastics as contaminates. Conversely, plastics that can biodegrade in specific environments, such as an industrial composting facility, are posed by some as a simpler end-of-life solution that avoids landfilling, incineration, or the recycling of traditional plastics into lesser value applications (also known as downcycling). However, there is no fundamental reason why this dichotomy has to exist for plastics in general. Paper is a good example of a useful material that has high recycling rates in many regions around the world, yet is also biodegradable.
Biodegradable and recyclable
Polyesters are a class of polymer that have the potential to overcome this dichotomy and create a new value chain of plastics that can be efficiently recycled back into equal or higher-value applications, while also being compostable and, in some cases, biodegradable in natural environments. The chemical bonds that make a polymer susceptible to biodegradation, such as the carbon–oxygen ester linkages within the molecular backbone of polyesters, also enable low-energy deconstruction mechanisms that can be used to breakdown these types of polymers in controlled environments where the remaining molecules can then be recycled (Fig. 2). Solvolysis techniques, such as methanolysis and glycolysis, are being developed for polyethylene terephthalate, but these deconstruction processes could also be extended to other polyesters, such as polylactic acid (PLA) or polyhydroxyalkanoates (PHAs), that are more readily biodegradable.
The natural environment should never be considered a viable end-of-life solution for plastics. Nonetheless, if the chemical bonds of a plastic enable more efficient ways to keep the molecules in use through new recycling techniques, while also enabling the material to safely degrade in natural environments, then innovation in that direction should be encouraged.
New recycling methods can be used to deconstruct a mixed waste stream of polyester plastics. The recycled molecules can then be used to make new materials without degraded properties.
Bio-based feedstock
Another major challenge with the common plastics used today is that they are derived from non-renewable fossil fuels. When considering an individual application, plastics often have the lowest carbon footprint when compared to alternative materials1. This goes back to the impressive properties of plastics, which enables different types of products and packaging to meet the necessary functional requirements with very little material. However, collectively, the production of all plastics is a major contributor to global carbon emissions2. One pathway to reduce the carbon emissions associated with the full life cycle of plastics is to utilize bio-based feedstock instead of fossil fuels for the production of these materials3,4.
The various types of fossil fuel feedstock used to make most plastics are rich in carbon and hydrogen, which is why the molecular structure of most plastics today are comprised mainly of these two elements. In fact, polypropylene, polystyrene, and the different types of polyethylene (high density polyethylene, low density polyethylene and linear low-density polyethylene), which make up a large share of all plastics produced, contain no heteroatoms at all.
Today, it is possible to produce plastics made solely of carbon and hydrogen from bio-based feedstock rather than fossil fuels. Indeed, some bio-based feedstock options, such as sugarcane, are starting to scale, which can be used as feedstock for producing polyethylene. The use of bio-based feedstock for producing today’s plastics has inherent disadvantages because the molecular structure of bio-based feedstocks is generally rich in oxygen. This means the oxygen molecules in the feedstock are lost as waste during production, making bio-based feedstocks less competitive when compared to fossil fuels5.
Whereas the common plastics mentioned above are made of only carbon and hydrogen, polyester plastics contain oxygen within their molecular structure. That is, the carbon–oxygen bonds that enable these materials to be selectively deconstructed using new solvolysis techniques, and a pathway for potential biodegradation, also offer the opportunity for more efficient production methods from biobased feedstock.
As a company that offers a wide range of products and services to our customers, we recognize the need for many different types of materials to meet the functional requirements for a diverse set of applications. It is highly unlikely that one specific type of polyester plastic would be sufficient to meet the needs of all the different types of applications where conventional plastics are used today. Nevertheless, considering the entire family of emerging polyesters that already exist at moderate scales, including PLA, various types of PHAs, polybutylene succinate, and polybutylene adipate terephthalate, as well as new polyesters that are being developed, such as polyalkylene xylosediglyoxylates6 and poly(δ-valerolactones)7, this category of plastics offers the opportunity to shift away from fossil fuels as feedstock, while also enabling new recycling pathways and materials that are less persistent in the environment. In some cases, new bio-based polyester plastics have been shown to also have better physical properties than traditional fossil-derived plastics, so we see these new materials as an opportunity for an entirely new and better value chain for plastics5.
A future vision for plastics recycling
When considering a diverse set of polyesters that could potentially be used for different applications, developing separate recycling streams for each individual type of material (as we commonly do with today’s plastics) is likely to be impossible or, at the very least, highly unlikely to ever be economical. Working backwards from this future longer-term vision for polyester-based plastics, there will be a need to be able to efficiently and economically recycle a mixed waste stream of these materials, which is not possible today.
This is why we joined the BOTTLE Consortium with the goal of developing an energy-efficient chemical processing technology that can selectively deconstruct a mixed waste stream of polyester plastics and separate the remaining molecules into valuable feedstock that can be used to make new materials. We are at the early stages of this work, but we are excited about the progress being made. We are also working with the consortium members to help develop new types of polyester plastics that meet the functional requirements of our applications, while also ensuring these new materials can be easily recycled and are biodegradable.
We know that creating a new, circular value chain for bio-based, biodegradable polyester plastics is not something one company or organization can do alone. Amazon is sponsoring this collection to help catalyze others around this concept as one approach to help mitigate the issues with today’s plastics.