Sustainable Solutions for Plastics: The Future Role of Lignins

By February 20, 2020

Plastics – Friend and Foe

If you’re like me, you’ve probably seen images of all the floating plastic in the Great Pacific garbage patch or perhaps you’ve seen the images of wildlife rescuers trying to remove a plastic straw from a sea turtle’s nose. These images can be quite disturbing and destructive to the reputation of plastic. And yet, plastic is also quite amazing and productive. Plastic helps keep our food sanitary in the form of packaging, it facilitates our mobility by being an important component of aircraft, automobiles and buses, it is used in all types of medical products from syringes to heart stents to the bottles of ibuprofen in your medicine cabinet. There’s no doubt that plastic has become an essential part of our daily lives with an ironic contradictory nature that pollutes our environment and may negatively impact our health by finding its way into our food and water supply. But I guess this also begs the question: Is there a way to minimize the bad impacts of plastic while maximizing all the good they offer? In other words, could we somehow use plastic in a more sustainable fashion?

The Challenges – From Clean-up to Renewable Sources

At first, the issue of using plastic in a sustainable fashion simply feels to be on a scale that is frankly overwhelming. For example, as an individual, I do my part in recycling and yet, the Great Pacific garbage patch exists and continues to grow. So, I must assume that some system has broken down somewhere. Maybe the origin of the plastic in the garbage patch comes from a time before contemporary environmental laws and regulations or perhaps the plastic is escaping recycling streams. Perhaps we have two challenges in using plastics in a more sustainable manner. The first includes the cleaning up of the plastic that is already in the environment. The second, and perhaps the most effective solution includes designing new plastics that are more benign to the environment so that if they do happen to escape the recycling streams, their impact on the environment is minimized or altogether mitigated.

In the first challenge, cleaning up the existing plastic in the environment is daunting and quite complex. First, there are the logistics of reclaiming the plastic and then there is the mechanical sorting of the plastic into the various types, e.g., polyethylene terephthalate (PET), high-density polyethylene plastics (HDPE), polypropylene (PP), etc. Next, there is the issue of trying to unzip the polymer back into its monomers so that it can be reconstituted into a new pristine polymer. Chemically, this is where things start to get both interesting and complicated. Industrially, there are methods such as pyrolysis, chemical, and biological depolymerization that are being used to either turn plastics into chemical feedstocks or into fuels. This area of recycling seems to be gaining interest from an industrial perspective. However, it also has several sustainability challenges to overcome such as logistics of reclaiming used plastics, energy consumption during the depolymerization processes, and evaluating whether the depolymerized polymers are free of degradation by-products and impurities before they are reused in new products.

The second challenge area in using plastics in a more sustainable fashion centers on designing plastics to be more environmentally benign in the first place. There are two facets to this challenge. First, designing the plastic so that it can be easily degraded. And second, the building of the polymer from bio-derived feedstocks. Bio-derived feedstocks have many sustainability benefits such as being renewable and can also require less energy to develop into polymers. For example, petroleum feedstocks often require an energy intensive and highly polluting oxygenation step. In contrast, the major sources of bio-derived feedstocks, i.e., cellulose, hemi-cellulose, and lignin, are inherently highly oxygenated. Hence, potentially avoiding the energy consuming and pollution generating step entirely. So, we ask ourselves another question: Could more polymers be sourced from bio-derived feedstocks and designed to degrade?

Lignins – A More Sustainable Alternative to Petroleum?

Lignins (Latin lignum = wood) gives plants their shape and sturdiness. Functionally, lignocellulose biopolymers strengthen the cell wall of plants and are made of three main components, in which cellulose and hemicellulose form a framework where a third component, lignin, acts as an adhesive that leads to the solidifying (lignification) of the cell wall. Lignification reinforces the plants cell walls of xylem tissues and provides plants protection against wind, pests, and offers resilience from other external factors. And all these structural and functional benefits that terrestrial plant life derives from lignins are also useful industrially – you can think of lignins as kind of like a sustainable warehouse of fine chemicals.

The magic of lignins doesn’t stop there because we actually have the keys to that warehouse. And, that key is depolymerization. Once the lignin is depolymerized, its polyaromatic feedstocks can often stand in for the essential feedstocks of petroleum-derived materials. These lignin-based chemicals can be used to produce the same plastics, drugs, paints, and electronic products made from traditional petroleum– onlyin a more sustainable nature and with a more biodegradable outcome. Unlike petroleum derived polymers, lignins can be derived from renewable resources including wood, straw, and even Miscanthus, a giant, hardy, and fast-growing grass that flourishes in nutrient-poor soils – which can minimize the competition of using food crops as sources of lignin. Even industrial waste products – such as paper pulp– can be used as a source for lignins.

I think it goes without saying that lignins have a huge market potential as a sustainable feedstock, but we also have to acknowledge that unlocking the full potential of lignins is an area of active research. The future of how lignins will be used in the chemical industry of the future depends on many factors – including the advancement of scientific research that enables the responsible and cost-effective scaling of lignin processing into useful chemical feedstocks. And, I’m happy to say that Analytical Chemistry is going to play an important part in fully characterizing lignins for their many applications and industrial uses.

Want to learn more about the future role of lignins?

We invite you to join our sponsored webinar with Chemistry World to learn about the potential of lignins, how to better analyze them, and the critical role they play in supporting tomorrow’s circular economy.