Plastic waste is a big problem, with estimates suggesting that worldwide plastic waste generation may reach up to 53 million metric tons per year by 2030.1
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There are growing fears that large amounts of intact plastic waste might not be the only problem associated with plastics in the environment. While there is still a great deal of uncertainty around the exact environmental impact and danger microplastics pose to human health, there is a clear concern that preventative measures need to be taken to reduce their spread.2
One of the key environmental issues with plastic waste is the lack of biodegradability. Many of the key properties that make plastics something of a wonder material for manufacturing, such as its excellent durability, ease of molding and manufacture, and its inertness, also make plastic an environmental nightmare. The hardness of plastic materials makes them difficult to mechanically break down, and the strength of the chemical bonds that make up the polymer chains in the plastic materials also makes many types of plastic highly chemically and biologically resistant.
Burning plastic waste releases a huge number of hazardous chemicals. Given the long breakdown times and risk of these chemicals leaching into the environment, landfill sites are not an optimal solution either.
The difficulties of dealing with plastic waste make it clear that solutions are needed to deal with existing waste. One of the tools that can help achieve this is spectroscopy.
The Use of Spectroscopy in Plastic Recycling Plants
Spectroscopy techniques use the spectral signals produced when light interacts with matter typically as a way of quantifying and identifying the molecular species in a sample.
Different spectroscopic techniques are sensitive to several aspects of a molecule’s unique properties, such as vibrational bond frequencies or the energies of transitions between different electronic states that determine the color and appearance of a sample.
With advances in laser technologies that have made even relatively power-hungry techniques such as Raman spectroscopy feasible in small portable spectrometers, spectroscopy has become a powerful tool that can be integrated as part of machine vision systems for the automated recognition and identification of samples.
For plastic waste, spectroscopy is now being used to automatically sort different kinds of plastic for recycling.3 Many plastics that are made from different polymers and require different recycling procedures are colorless and indistinguishable by eye, but techniques such as Raman and infrared spectroscopy, which are sensitive to the molecular vibrations in the sample, can be used to tell them apart.
By combining the spectral outputs of these measurements with advanced chemometrics methods for automated data analysis, it is possible to make fully automated sorting systems that can significantly improve the efficiency of plastic recycling plants.3 Combining hybrid spectroscopic systems to use the strengths of different techniques is another route to further enhancing the efficiency and accuracy of such systems.
Manufacturing Recycled Plastics with Spectroscopy
Spectroscopy methods are often used as analytical techniques for quality control of products and to check for the presence of contaminants. It can also be used to help encourage the use of recycled plastics in the manufacturing process.4
Spectroscopy can identify which polymeric species may be part of a recycled blend and as a result, if the material's behavior of the mixture is well-characterized, be used to identify which blends would be most suited to certain applications.
It can also be used for starting material quality control to minimize manufacturing waste by identifying which materials are unlikely to be able to achieve the required final properties and may be better suited to other applications. When combined with laser cutting and processing techniques such as 3D printing, both of which can help reduce the amount of starting material required in manufacturing, spectroscopy has an important place in reducing plastic waste in manufacturing.
While attempts to reduce plastic use by industries and individuals will undoubtedly have an impact on overall plastic waste generation, there are some applications for which there are few suitable materials for replacing plastics at present. This includes sterile medical packaging and applications that make use of the high strength to weight ratios that polymers offer.
Spectroscopy is one of the key tools used in understanding how even very long-lived plastics degrade and how the chemical structure of the polymer gives rise to the desired final properties.5 This includes looking at how processes such as UV irradiation and mechanical wear lead to breakdown.
This knowledge can be used for future polymer design that still achieves many of the excellent material properties required for a given application but can incorporate some degree of biodegradability.
By incorporating weaker bonds or those that can be readily broken down by bacteria, there has already been significant progress towards the development of antimicrobial materials that are also biodegradable on reasonable timescales.6
References and Further Reading
- SrivBorrelle, S. B., Ringma, J., Law, K. L., Monnahan, C. C., Lebreton, L., McGivern, A., ... & Rochman, C. M. (2020). Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science, 369(6510), 1515-1518. https://doi.org/10.1126/science.aba3656
- Hale, R. C., Seeley, M. E., La Guardia, M. J., Mai, L., & Zeng, E. Y. (2020). A Global Perspective on Microplastics. Journal of Geophysical Research: Oceans, 125(1), 1–40. https://doi.org/10.1029/2018JC014719
- Neo, E. R. K., Yeo, Z., Low, J. S. C., Goodship, V., & Debattista, K. (2022). A review on chemometric techniques with infrared, Raman and laser-induced breakdown spectroscopy for sorting plastic waste in the recycling industry. Resources, Conservation and Recycling, 180, 106217. https://doi.org/10.1016/j.resconrec.2022.106217
- Vilaplana, F., & Karlsson, S. (2008). Quality concepts for the improved use of recycled polymeric materials: a review. Macromolecular Materials and Engineering, 293(4), 274-297. https://doi.org/10.1002/mame.200700393
- Agamuthu, P., & Faizura, P. N. (2005). Biodegradability of degradable plastic waste. Waste Management & Research, 23(2), 95–100. https://doi.org/10.1177/0734242X05051045
- Zhong, Y., Godwin, P., Jin, Y., & Xiao, H. (2020). Biodegradable polymers and green-based antimicrobial packaging materials: A mini-review. Advanced Industrial and Engineering Polymer Research, 3(1), 27-35. https://doi.org/10.1016/j.aiepr.2019.11.002