Image courtesy of Kat Moon.
Avocado seeds, lobster shells, fish scales, red algae, cactus leaves… and now, wood. What do these items have in common? As it turns out, researchers around the world have turned to all these unexpected materials to develop biodegradable alternatives to plastic.
Petrochemical plastics are plastics derived from crude oil and natural gas. Such nonrenewable resource-based plastics have pervaded modern life—they can be found everywhere, from fertilizers to packaging to clothing, and are cemented as an integral part of society. According to the International Energy Agency, petrochemical feedstock now accounts for twelve percent of oil demand around the world. But it is not the quantity alone that renders plastic a global issue—instead, it is the lack of biodegradability.
A potential solution may lie in bioplastics, alternative materials that use different biomass feedstock to create bio-based plastics that are biodegradable. A breakthrough study on the creation of bioplastic from natural wood was recently published in Nature Sustainability, co-authored by Yuan Yao, assistant professor of industrial ecology and sustainable systems at the Yale School of the Environment (YSE), and Liangbing Hu, a professor at the Center for Materials Innovation at the University of Maryland.
The quest for a biodegradable plastic to combat the billions of metric tons of plastics accumulating in the environment has led to the creation of these bioplastics. Currently, petrochemical plastics last for hundreds to thousands of years due to the stable long polymer chains they contain, such as those found in polyethylene, polystyrene, and polyvinyl chloride-based plastics. Because previous efforts to produce bioplastics have been associated with the use of toxic chemicals, weak mechanical strength, and poor water stability, researchers started to wonder: how could a balance between degradability and durability be achieved?
In this study, the research team reports a method of fabricating lignocellulosic bioplastic that not only demonstrated recyclability and biodegradability, but also dramatically improved durability. “There are many people who have tried to develop these kinds of polymers in plastic, but the mechanical strands are not good enough to replace the plastics we currently use, which are made mostly from fossil fuels,” Yao said in a Yale School of the Environment News article. Overall, their bioplastic showcases high mechanical strength as well as improved water and thermal stability.
Building Better Bioplastic
To create bioplastics, researchers typically extract lignin and cellulose, two organic polymers responsible for plant structure, from wood. The team performed a process called in situ lignin regeneration, whereby instead of isolating lignin and cellulose, they homogenized wood powder, or made it uniform, to form a high-density, viscous slurry.
Next, deep eutectic solvent (DES)—a group of biodegradable and recyclable substances—was used to dissolve lignin and break apart the hydrogen bonds between cellulose fibers. Water was then added to the slurry for lignin regeneration from the DES. Finally, the DES was removed from the mixture through a filtration and washing process, leaving behind a stable cellulose-lignin material. Through a simple casting process, the team fabricated bioplastic films.
In sharp contrast to cellulose film, lignocellulosic bioplastic was found to be considerably more resistant to water damage. The researchers demonstrated that it absorbed water at a much slower rate than standard cellulose film. As a result, lignocellulosic bioplastic samples maintained their shape well past thirty days of being submerged in water, far beyond the point at which cellulose film degraded.
While the durability of lignocellulosic bioplastic is critical for its large-scale use, the most important characteristic of this new bioplastic was its ability to break down under the right conditions. When the researchers subjected it to an outdoor environment, they found that exposure to sun, wind, and rain was enough to completely break it down within five months. For comparison, PVC—a commonly-used synthetic plastic—subjected to identical conditions remained unchanged. “That [characteristic] is what really makes this plastic good: it can all be recycled or biodegraded,” Yao told the Yale School of the Environment News. “We’ve minimized all of the materials and the waste going into nature.”
The more sustainable production process of lignocellulosic bioplastic, in addition to the material’s impressive physical properties, makes it a promising replacement for less environmentally friendly petrochemical plastics. This bioplastic follows a closed-loop cycle in which microorganisms in soil can naturally degrade the material. Typical plastics take hundreds of years to decompose in nature, filling up landfills and potentially leaching toxic chemicals into groundwater; in contrast, this type of bioplastic breaks down far more quickly and poses less risk to communities and nature in the process. The lignocellulosic bioplastic can also be recycled.
Barbara Reck, a senior research scientist at the Yale School of the Environment, characterized the project’s findings as an impressive innovation. “A bioplastic is fully degradable under regular outdoor conditions, offering an opportunity to reduce putting a much-needed end to the accumulation of at least some plastics in the natural environment,” she said.
The Future of Bioplastics
The environmentally friendly, closed-loop cycle used to create this bioplastic signals hope for a future where strong, biodegradable bioplastics can be produced from resource-abundant, sustainable, and renewable biomass.
Hu told the Yale School of the Environment News that the malleability of this bioplastic will allow for several applications. From being molded into a film for use in plastic bags to being shaped for use in automobile manufacturing, this bioplastic may help solve our society’s current dependence on plastic.
Because this lignocellulosic bioplastic is made of biomass feedstock, its production would entail the use of local materials rather than nonrenewable fossil fuels, which would further mitigate environmental damage. “It is very promising to see the flexibility in the biomass feedstocks used for this process,” Reck said. “One can imagine a decentralized production network that uses predominantly local materials, which in turn would keep the overall environmental impacts [of the lignocellulosic bioplastics] rather low.”
Yao’s team’s work on using wood as a substitute for plastic presents significant substitution potential for petroleum derived plastics. Mark Ashton, Morris K. Jesup Professor of Silviculture and Forest Ecology and the Director of the Yale Forests, further emphasized the benefits of using bioplastic over petrochemical plastics. “[Bioplastic usage can] mov[e] a product that has a large environmental impact because of its persistence to one that might potentially be relatively benign,” Ashton said.
In determining a sustainable production network, the team has continued to research how scaling up manufacturing for this bioplastic could impact forests. The issue with using wood byproducts is that large-scale production may require massive amounts of wood, which could negatively impact forests and local ecosystems.
According to Yao, the research team is responding to this potential issue by working with a forest ecologist to build forest simulation models, which could elucidate the connection between the growth cycle of forests to the manufacturing process of this bioplastic. Such advances in the processability and functionality of wood could motivate better forest management practices in addition to realizing the lower environmental impact of using wood as a sustainable material.
Considering our far-reaching reliance on plastic in society, the discovery of a biodegradable, durable bio-based plastic is worth its weight in gold. In this way, the researchers are like Rumpelstiltskin—spinning something valuable out of a natural material in ways we never thought possible.
Further reading:
Anusewicz, J. (25 March 2021). Turning Wood Into Plastic. Yale School of the Environment News. https://environment.yale.edu/news/article/turning-wood-into-plastic
Xia, Q., Chen, C., Yao, Y., Li, J., He, S., Zhou, Y., … & Hu, L. (2021). A strong, biodegradable and recyclable lignocellulosic bioplastic. Nature Sustainability, 1-9.
Xiao, S., Chen, C., Xia, Q., Liu, Y., Yao, Y., Chen, Q., Hartsfield, M., Brozena, A., Tu, K., Eichhorn, S. J., Yao, Y., Li, J., Gan, W., Shi, S. Q., Yang, V. W., Lo Ricco, M., Zhu, J. Y., Burgert, I., Luo, A., … Hu, L. (2021). Lightweight, strong, moldable wood via cell wall engineering as a sustainable structural material. Science, 374(6566), 465–471.