Plastics are among the most widely used materials worldwide but are also a major pollutant. The accumulation of plastics on land and in marine environments severely affects the atmosphere and poses harm to humans and aquatic life.1
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Plastic pollution is a major threat to human survival, with more than six hundred thousand tons of plastics on Earth.2 In an effort to reduce the concentration of conventional plastics, scientists are exploring more sustainable options, among which biodegradable plastics emerge as the most promising. These innovative materials are synthesized using abundant natural, sustainable raw materials.
Biodegradable Plastics: A “Green” Material with Unique Properties
Traditional plastic materials utilized in houses and for commercial uses are not biodegradable; microorganisms cannot decompose them. In contrast, biodegradable plastics can be decomposed into water and other harmless compounds by microbes found in the environment and, therefore, do not contribute to land pollution.3
Bio-polymers, produced from renewable resources like plants instead of oil, represent a sustainable alternative. However, it is important to note that not all bioplastics are biodegradable. Bio-based plastics are made from natural raw materials, but this does not mean they will decompose naturally.4 Both biomass-based and fossil-based polymers can be either biodegradable or non-biodegradable.5
Artificially produced biodegradable polymers are modified during production to attain the desired properties. These synthetic polymers are utilized in various fields, such as cosmetics, coatings, wound dressings, enzyme immobilization, gene delivery, and scaffolds for tissue engineering.
Structurally modified biopolymers are also recommended for use in nanotechnology, augmentation, cryopreservation, prosthetics, medical implants, devices, and sanitation products, including surgical sutures.6
How are Biodegradable Plastics Made?
The manufacturing process for biodegradable plastics is not standardized, as their properties vary depending on the application. Therefore, experts design the synthesis process with the final product and its uses in mind. However, several general steps are consistently followed in the production of biodegradable plastics.
First, the raw materials required for plastic production, such as starch, polymer pellets, and additives, are unloaded and stored.7 These materials are often transported from storage to processing equipment using pneumatic conveying systems.7
The next step involves the mixing and thorough blending of materials, followed by extrusion and cooling to the desired shape using a pelletizer. The final product can be stored in bulk in storage silos before final packaging. Packaging is accomplished using an automatic or semi-automatic machine to pack the product into small or jumbo bags for dispatch.
One innovative approach involved using potato peels to obtain biodegradable plastics for everyday use.8 In this approach, starch is leached from the potatoes. The extracted starch is then heated to 100 degrees Celsius and mixed with acetic acid and glycerol.
Further heating of the solution results in a gel-like liquid that is dried for four to five days to form the biodegradable plastic. This study found that lower reaction and drying temperatures benefit the resulting plastic, improving its mechanical properties and hardness.
In another development, researchers from the University of Washington created a biodegradable bioplastic using powdered blue-green cyanobacteria cells, also known as spirulina.9 Through hot-pressing, the plentiful and photosynthetic algae spirulina creates cohesive bioplastics with a flexural modulus ranging from 3 to 5 GPa and strength between 25.5 and 57 MPa.
This rapid, straightforward, and scalable method offers a means to convert raw microalgae into a self-bonded, recyclable, and backyard-compostable bioplastic. These bioplastics exhibit appealing mechanical properties that outperform other biobased plastics like thermoplastic starch.9,10
Challenges Associated with Biodegradable Plastics
Owing to their environment-friendly nature and viable attributes, biodegradable plastics may appear to be an ideal focus for industry expansion. However, this sector faces significant challenges.
The high production cost of biodegradable plastics stands as a major concern. Many of the plants used in bioplastic production are genetically modified. The process of developing polylactic acid (PLA) is costly, involving high-temperature and vacuum conditions to convert lactic acid into a pre-plastic form, which is then further processed into PLA building blocks.
Despite PLA being considered an eco-friendly plastic, this process involves multiple intermediate steps that use metals and generate waste.11
Achieving the optimal mechanical properties for specific purposes, such as food packaging, PET bottles, and general-use plastics, poses a challenge. Most polymers lack sufficient mechanical strength and functionality for specialized applications requiring electric and thermal conductivity.11 As a result, 95 % of polymers are combined with inorganic or organic additives to meet the necessary properties for certain uses.
Thus, while biodegradable plastics are not harmful, their economic impacts and emissions during the synthesis process raise significant sustainability concerns.
Using Carbon Dioxide to Enhance Biodegradable Plastics
In recent years, the rise in emissions from automobiles and industries has significantly increased carbon dioxide (CO2) levels. Scientists are exploring using CO2 as a raw material in chemical reactions to utilize it for advantageous purposes.
Polyhydroxybutyrate (PHB), a popular biodegradable plastic, can be produced using CO2 as a raw material. This approach could reduce both CO2 levels and plastic pollution.
Researchers have developed a hybrid system that combines a chemical process for CO2 reduction with hydrogen and a biological process for synthesizing PHB synthesis.12 This system efficiently converts CO2 into bioplastics by optimizing the synthetic pathway through the improved catalytic efficiency of key enzymes.
The cost of raw materials for producing 1 kg of PHB using this hybrid approach is approximately US$0.70, significantly lower than current market prices for raw materials. The operation yielded 5.96 g L−1 of PHB with a productivity rate of 1.19 g L−1 per hour and a molar CO2 utilization efficiency of 71.8 %.
The high efficiency of PHB production and the extremely low cost of raw materials make this system a preferred choice for commercial-scale applications.
Recent Breakthrough: Crosslinkable CO2-Based Biodegradable Plastic
The fixation of CO2 into biodegradable polymers offers a promising approach for carbon neutralization, addressing plastic pollution, and mitigating energy depletion.13
Poly(propylene carbonate) (PPC), an alternating copolymer of propylene oxide (PO) and CO2, represents a commercially available biodegradable plastic known for its excellent transparency, biocompatibility, and oxygen/water barrier properties. Despite these advantages, its applications are restricted due to a low glass transition temperature (T g) and inadequate tensile strength.14
Researchers have developed cross-linkable PPC-P by incorporating functional anhydride monomers with double bonds into the copolymerization of PO/PA/CO2, catalyzed by the TEB/PPNCl pair.15 The integration of nitrile anhydride (NA) into the PO/PA/CO2 terpolymer, without significantly impacting its reactivity, resulted in the synthesis of a cross-linkable biodegradable plastic.
The crosslinked PPC-P with double bonds could be constructed in situ at elevated temperatures using the thermal initiator DCP (0.3 wt %). Optimal crosslinking conditions were determined to be 170 ℃ for 13 minutes. The crosslinked polymers exhibited mostly unchanged thermal properties, improved tensile performance, and satisfactory transparency. By using CO2 in its production, researchers have demonstrated the potential for PPC-P as a foaming material due to the improvement in tensile attributes, facilitating its path toward commercialization.
Future Outlooks
The technology used to synthesize biodegradable plastics requires optimization in terms of power, energy, and financial considerations. Governments worldwide should establish uniform standards and implement policies that promote the development of biodegradable plastics.
Extensive research studies are also required to completely understand the synthesis process of biodegradable plastics and identify the critical parameters affecting their production. Such steps are essential to facilitate significant advancements in the field, ensuring that traditional, environmentally harmful plastic products are substituted with sustainable, biodegradable alternatives.
More from AZoM: Sustainability in the Plastics Industry
References and Further Reading
1. C. Moore. (2024). Plastic Pollution. [Online]. Britannica. Available at: https://www.britannica.com/science/plastic-pollution. [Accessed 9 March 2024].
2. Walker, T. et al. (2022). Micro (nano) plastic toxicity and health effects: Special issue guest editorial. Environment International. doi.org/10.1016/j.envint.2022.107626.
3. Rosenboom, J. et al. (2022). Bioplastics for a circular economy. Nature Reviews Materials. doi.org/10.1038/s41578-021-00407-8.
4. E. Anderson., Zagorski, J. (2024). Real-time Science – Biodegradable Plastics and Polymers. [Online]. Michigan State University. Available at: https://www.canr.msu.edu/news/real-time-science-biodegradable-plastics-and-polymers. [Accessed 7 March 2024].
5. European Commission, Directorate-General for Research and Innovation (2021). Biodegradability of plastics in the open environment. [Online] European Union. Available at: https://data.europa.eu/doi/10.2777/690248.
6. Mukherjee, C., et al. (2023). Recent advances in biodegradable polymers–Properties, applications and future prospects. European Polymer Journal. doi.org/10.1016/j.eurpolymj.2023.112068.
7. IndPro, (2022). Towards a Sustainable Future: Exploring Biodegradable Plastic Manufacturing. [Online] Indpro Engineering Systems Pvt. Ltd. Available at: https://indpro.com/blog/biodegradable-plastic-manufacturing-process/#:~:text=Certain%20raw%20materials%20are%20used,the%20manufacturing%20of%20biodegradable%20plastics. [Accessed 08 March 2024].
8. Zidan, N., et al. (2023). Production of Biodegradable Plastic from Biomass. [Online]. Food Technology Research Journal. Available at: https://ftrj.journals.ekb.eg/article_289773_f3c5b7b7019f93a1ff4098cf3942f7bd.pdf.
9. McQuate, S. (2023). New biodegradable plastics are compostable in your backyard. [Online] University of Washington. Available at: https://www.washington.edu/news/2023/07/10/new-biodegradable-plastics-compostable-in-your-backyard/. [Accessed 09 March 2024].
10. Iyer, H., et al. (2023). Fabricating strong and stiff bioplastics from whole spirulina cells. Advanced Functional Materials. doi.org/10.1002/adfm.202302067.
11. Moshood, T., et al. (2021). Expanding Policy for Biodegradable Plastic Products and Market Dynamics of Bio-Based Plastics: Challenges and Opportunities. Sustainability. doi.org/10.3390/su13116170.
12. Zhang, J., et al. (2023). Hybrid synthesis of polyhydroxybutyrate bioplastics from carbon dioxide. Green Chemistry. doi.org/10.1039/D3GC00387F.
13. Ravanchi, M., et al. (2021). Catalytic conversions of CO2 to help mitigate climate change: Recent process developments. Process Safety and Environmental Protection. doi.org/10.1016/j.psep.2020.08.003.
14. Scharfenberg, M., et al. (2018). Functional polycarbonates from carbon dioxide and tailored epoxide monomers: degradable materials and their application potential. Advanced Functional Materials. doi.org/10.1002/adfm.201704302.
15. Liang J., et al. (2023). A new biodegradable CO2-based poly(ester-co-carbonate): Molecular chain building up with crosslinkable domain. Journal of CO2 Utilization. doi.org/10.1016/j.jcou.2023.10240
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