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Raj, B.; Rahul, J.; Singh, P.K.; Rao, V.V.L.K.; Kumar, J.; Dwivedi, N.; Kumar, P.; Singh, D.; Strzałkowski, K. Polyethylene Terephthalate Recycling Techniques. Encyclopedia. Available online: https://encyclopedia.pub/entry/48042 (accessed on 19 May 2024).
Raj B, Rahul J, Singh PK, Rao VVLK, Kumar J, Dwivedi N, et al. Polyethylene Terephthalate Recycling Techniques. Encyclopedia. Available at: https://encyclopedia.pub/entry/48042. Accessed May 19, 2024.
Raj, Beenu, Jitin Rahul, Pramod K. Singh, Velidandi V. L. Kanta Rao, Jagdish Kumar, Neetu Dwivedi, Pravita Kumar, Diksha Singh, Karol Strzałkowski. "Polyethylene Terephthalate Recycling Techniques" Encyclopedia, https://encyclopedia.pub/entry/48042 (accessed May 19, 2024).
Raj, B., Rahul, J., Singh, P.K., Rao, V.V.L.K., Kumar, J., Dwivedi, N., Kumar, P., Singh, D., & Strzałkowski, K. (2023, August 14). Polyethylene Terephthalate Recycling Techniques. In Encyclopedia. https://encyclopedia.pub/entry/48042
Raj, Beenu, et al. "Polyethylene Terephthalate Recycling Techniques." Encyclopedia. Web. 14 August, 2023.
Polyethylene Terephthalate Recycling Techniques
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This entry is adapted from the peer-reviewed paper 10.3390/su151612269

Polyethylene terephthalate (PET) is a widely used thermoplastic with excellent properties, making it a popular choice for various packaging and single-use plastic products. Its exceptional tensile strength, processability, transparency, thermal stability, barrier properties, toughness-to-weight ratio, and chemical resistance have contributed to its widespread use. However, the significant consumption of PET has led to a substantial amount of PET waste, especially in the form of single-use packaging. This has raised concerns about the environmental impact of plastic waste, such as marine pollution and landfill accumulation.

polyethylene terephthalate PET recycling plastic waste mechanical recycling chemical recycling

1. Introduction

To reduce the environmental impact of plastic waste, conserve natural resources and energy, there is a growing global movement to reduce single-use plastics and increase PET recycling efforts. Various strategies have been proposed, including promoting reusable products, exploring sustainable packaging options like bioplastics, implementing deposit–return systems for plastic bottles and other packaging, and investing in recycling programs and infrastructure. These efforts aim to minimize plastic consumption, increase recycling rates, and encourage the adoption of sustainable materials in the packaging industry.
Sustainable packaging solutions have gained momentum in recent years due to consumer awareness and environmental considerations. PET has emerged as a leading candidate in the pursuit of eco-friendly packaging materials due to its recyclability and potential for a circular economy. The ability to collect, process, and transform used PET products into new packaging materials offers a significant opportunity to reduce reliance on virgin resources and minimize waste. PET’s transparency also plays a crucial role, allowing vibrant colors and product visibility, enhancing brand recognition, and engaging consumers. Furthermore, PET’s exceptional barrier properties protect sensitive contents, extending product freshness and shelf life.
As consumer lifestyles evolve, PET packaging continues to meet the demands for convenience and on-the-go products. Its lightweight nature reduces material waste and provides portability and ease of use for single-serve applications.
PET waste can be recycled using both mechanical and chemical recycling techniques. Mechanical recycling involves collecting and sorting PET waste, followed by shredding, washing, melting, and reprocessing the waste into new plastic pellets or fibers. This method is widely used and helps to produce various PET-based products. On the other hand, chemical recycling utilizes different processes such as glycolysis, hydrolysis, methanolysis, or solvolytic depolymerization to break down PET waste into its monomer building blocks, or other valuable chemicals. Chemical recycling offers the potential to recover high-quality monomers for PET production and address challenges associated with complex PET products (Table 1). Continued research and development efforts are being made to optimize both mechanical and chemical recycling techniques for PET waste recycling.
Table 1. Different glycolysis methods for PET recycling.
S. No PET Glycolysis Methods References
1 Kinetics of PET waste glycolytic degradation. [1][2][3][4][5][6][7][8][9]
2 Theoretical and simulation research on PET glycolysis depolymerization. [10]
3 Microwave glycolysis depolymerization of PET. [9][11][12]
4 PET glycolysis depolymerization catalysts Organometallic catalysts (heavy metal salt) [2][13][14][15][16]
Non-toxic metal salt (ionic liquid) [16][17][18]
Deep eutectic solvent catalyst [19]
Regenerable or recoverable catalysts [20]
Large-surface-area catalysts [21]
Other types of catalysts [22]

2. Mechanical Recycling

Mechanical recycling is the most common and widely employed method for recycling PET waste. It involves several steps:
  • Collection—PET waste, such as used PET bottles, is collected through recycling programs, waste management systems, or dedicated collection points. The proper collection and segregation of PET waste are crucial for effective recycling;
  • Sorting and cleaning—Collected PET waste undergoes sorting to separate it from other types of plastic and non-recyclable materials. The sorted PET waste is then thoroughly cleaned to remove contaminants like labels, caps, and residual liquids;
  • Shredding and granulating—The cleaned PET waste is shredded into smaller pieces or flakes. The flakes are then further processed into granules or pellets. Shredding and granulation increase the surface area of the PET material, making it easier to handle during subsequent processing;
  • Melting and purification—The PET flakes or granules are melted down to a liquid state in high-temperature extruders. During this process, any remaining impurities, such as dyes or additives, are filtered or removed. The purified molten PET is then cooled and solidified;
  • Reprocessing—The solidified PET material is usually cut into small pellets or chips, which can be used as feedstock in various manufacturing processes. These processes can include injection molding, blow molding, or extrusion to produce a wide range of PET-based products, such as fibers, films, sheets, bottles, containers, and packaging materials.

3. Chemical Recycling

The following Table 1 represents various methods of glycolysis used for the treatment of polyethylene terephthalate (PET) plastic waste. This table highlights different approaches used in glycolysis, such as kinetics of PET waste glycolytic degradation, the microwave glycolysis depolymerization of PET, etc., and Table 2 compares them based on key parameters including reaction conditions, catalysts used, reaction time, yield of monomers and temperature. The table provides a comprehensive overview of the different methods of glycolysis, enabling a comparative analysis of their effectiveness and feasibility for PET plastic waste management.

Relative Advantages of Glycolysis over Methanolysis and Hydrolysis

Glycolysis of PET is an area of widespread research because of the advantages of this process over methanolysis and hydrolysis, which include its suppleness, simplicity, low capital costs [23], eco-friendliness, and high yield. It also has a lower reaction time, and the process can be simply adapted to the conventional plants used for PET production. It stands out as the best PET recycling process over the other methods for the reason that it is carried out in an extensive series of temperatures from 180 °C to 240 °C [24], and achieves the uppermost proficiency and eminence of the product when a catalyst is used [25][26]. Another additional benefit of glycolysis is that the BHET can be mixed with fresh BHET, and the combination can be used for other (DMT-based or TPA-based) PET production lines [27][28].
On the other hand, hydrolysis is comparatively easy compared to glycolysis and methanolsis. Among the three depolymerizing agents used in these three processes, i.e., water (hydrolysis), methanol (methanolysis), and ethylene glycol (glycolysis), water is the weakest nucleophile [29]. The other disadvantage of hydrolysis is the use of higher temperatures (200–250 °C) and pressures (1.4–2 MPa), besides a longer duration needed for depolymerization. Commercially, hydrolysis is not widely used to produce food-grade recycled PET because of the cost associated with the purification of the TPA produced during the process [6].
The main disadvantage of the methanolysis method is the high cost associated with the separation and refining of the mixture of the reaction products (glycols, alcohols, and phthalate derivatives). Furthermore, the water formed during the process causes a toxic effect on the catalyst, also causing the formation of various azeotropes. Also, with the existing inclination to use TPA, instead of DMT, as the raw material for the production of PET, the DMT produced by methanolysis must to be converted into TPA, which significantly adds to the cost of the methanolysis process [30].
Mechanical vs. Chemical Recycling.
The relative advantages and disadvantages of mechanical and chemical recycling are presented in Table 2.
Table 2. Advantages and disadvantages of recycling methods.
Mechanical Recycling
Advantages Disadvantages
Recycling PET by melt reprocessing is relatively simple, requires lower investments, utilizes established equipment [31][32], is flexible in relation to feedstock volume, and has little adverse environmental effect.
(a)
The foremost disadvantage faced in the melt reprocessing of any PET is a reduction in melt viscosity, which is caused by thermal and hydrolytic degradation [14].
(b)
Melt reclaiming can produce cyclic and linear oligomers, which can affect the final product’s features, such as printability or dyeability [33].
(c)
Impurities such as polyvinylidene chloride (PVDC), PVC, glues, paper, ethylene-vinyl acetate (EVA), etc., produce acidic mixtures, which catalyze the hydrolysis of the PET’s linkages of ester during thermal reprocessing [34].
(d)
The yellowing of recycled post-consumer PET is another disadvantage. It could be due to intra-molecular cross-linking. Yellowing is sometimes a significant hindrance to the production of transparent recycled PET products.
Chemical Recycling
Advantages Disadvantages
(a)
It does not involve the use of huge machines.
(b)
It may be more cost-effective than mechanical recycling.
(c)
Compared with mechanical recycling, chemical recycling conforms more to the principles of sustainability because it produces original raw materials.
(a)
The initial developing cost can be higher.
(b)
To be economically viable, chemical recycling methods need to be on a larger scale.
(c)
No published reports are available on the utilization of chemical recycling on an industrial scale.

References

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  13. Imran, M.; Kim, D.H.; Al-Masry, W.A.; Mahmood, A.; Hassan, A.; Haider, S.; Ramay, S.M. Manganese-, cobalt-, and zinc-based mixed-oxide spinels as novel catalysts for the chemical recycling of poly(ethylene terephthalate) via glycolysis. Polym. Degrad. Stab. 2013, 98, 904–915.
  14. Lopez-Fonseca, R.; Duque-Ingunza, I.; De Rivas, B.; Arnaiz, S.; Gutierrez-Ortiz, J.I. Chemical recycling of post-consumer PET wastes by glycolysis in the presence of metal salts. Polym. Degrad. Stab. 2010, 95, 1022–1028.
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  16. Al-Sabagh, A.M.; Yehia, F.Z.; Eshaq, G.; Elmetwally, A.E. Ionic liquid coordinated ferrous acetate complex immobilized on bentonite as a novel separable catalyst for PET glycolysis. Ind. Eng. Chem. Res. 2015, 54, 12474–12481.
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Subjects: Environmental Sciences; Public, Environmental & Occupational Health; Green & Sustainable Science & Technology
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Beenu Raj
,
Jitin Rahul
,
Pramod K. Singh
,
Velidandi V. L. Kanta Rao
,
Jagdish Kumar
,
Neetu Dwivedi
,
Pravita Kumar
,
Diksha Singh
,
Karol Strzałkowski
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Entry Collection: Environmental Sciences
Revisions: 2 times (View History)
Update Date: 14 Aug 2023
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