R. GUPTA, KBR Technology, Gurugram, India
Nylons are one of the most widely used thermoplastic condensation polymers in the contemporary world. Most applications are found in the automobile sector, engineering plastics, textiles and electrical and electronics industries. There has been tremendous growth in non-textile related sectors for nylon applications (FIG. 1) in the last couple of decades, and a 5.95% compound annual growth rate (CAGR) is predicted by market research groups.1 These rising trends are putting more stress on the environment due to the increasing volume of end-of-life nylon polymers entering landfills and oceans. According to a European Parliament estimate, fishing nets made of nylon account for 27% of all ocean plastic pollution, and the Great Pacific Garbage Patch comprises 46% fishing nets. There is an urgent need to address this issue, and nylon recycling is one potential solution.
Nylon compounds are also called polyamides, which are typically synthesized by reacting diamines with dicarboxylic acids. According to literature,2 the global polyamide market is around 8 MMtpy and is expected to reach 10.4 MMtpy by 2027, with China being the largest producer of Nylon 6. Very little of this quantity is currently being recycled and reused: for example, only 2% of global polyamide production utilizes recycled material.2 Compared to other commonly used plastics like polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC), nylon recycling is relatively less commercialized.
Many methods are under research to recycle polyamides. Among them, chemical recycling methods like solvolysis, enzymatic recycling and pyrolysis are discussed in this study along with their industrial scalability. Condensation polymers like PET and nylon can be recycled using solvolysis methods, whereas plastics like PE and PP, which have only a single repetitive monomer unit, cannot be recycled using solvolysis.
The main solvolysis methods discussed in this study are hydrolysis, glycolysis and ammonolysis.
A wide range of nylon compounds can be synthesized based on applications by varying their chemical composition and manufacturing processes. Out of all types of nylon, Nylon 6 and Nylon 6,6 are the most commonly used polymers (FIG. 2). The nomenclature of nylon is based on the number of carbon atoms in the monomer chain. Nylon 6 has six carbon atoms in its repeating unit, while Nylon 6,6 has six carbon atoms on each side of the amide group. These nylons are primarily used in the textile and automobile industries. Nylon 6 is formed from caprolactam, and Nylon 6,6 is formed by reacting adipic acid with hexamethylenediamine. These are the individual monomers that can be recovered from recycling polyamides through solvolysis.
SOLVOLYSIS METHODS
Hydrolysis. Polyamides react with water under high pressure and temperature conditions to form respective monomers. Acids and bases are used as catalysts for faster reaction rates. In literature,2 all the lab-scale hydrolysis processes attempted in the study are consolidated and presented. The average temperature range is 250°C–350°C (482°F–662°F).
Phosphoric acid (H₃PO4) as a catalyst is observed to have the highest yield for Nylon 6 recycling, with a yield greater than 90% forming ε-aminocaproic acid as the product in 20 minutes. ε-aminocaproic acid can be converted to caprolactam through cyclization. Hydrochloric acid/amide as a catalyst is observed to achieve complete depolymerization of Nylon 6,6 with the highest yield of 90%, forming adipic acid and hexamethylenediamine (FIG. 3).
Among the chemical recycling methods of nylon, acid hydrolysis is the only method that is currently operated on an industrial scale, with H₃PO4 as the acidic catalyst.3 The main challenges in acidic hydrolysis are the corrosion caused by the acid catalysts and, in the case of H₃PO4, the formation of phosphorous-containing byproducts.
Glycolysis. Glycols, most commonly ethylene glycol, react with polyamides. The hydroxyl (OH) group of the glycol reacts with the carbonyl group of the polyamide to form an intermediate, which after rearrangement forms a carboxylic acid derivative and an amine derivative. The main challenges in the glycolysis process are low yield and significant formation of oligomers (FIG. 4). Focused research on developing suitable catalysts that can minimize the energy required and increase the yield will help enhance the process. In literature,4 excess glycol is used with 2% diammonium hydrogen phosphate as a catalyst; the operating conditions of the reaction are reduced, forming low molecular weight liquids. Similarly, research must be conducted to find catalysts to obtain the required products. At the time of this publication, no known commercial glycolysis plant is in operation to recycle polyamides.
Ammonolysis. Polyamides react with ammonia (NH3) to obtain respective monomers. Electron-rich NH3 attacks the carbonyl carbon of the polyamide, leading to the formation of monomers. At 300°C–350°C (572°F–662°F) with a pressure of ~68 barg in the presence of an ammonium phosphate catalyst, ε-caprolactam, 6-aminocapronitrile, and 6-aminocaproamide are formed from Nylon 6 (FIG. 5). Catalysts like ruthenium on alumina (Ru/Al₂O3), H₃PO4 and crude glycerol have been tried and tested in literature to attain higher yields with lower operating conditions.5 Among these, the reaction with crude glycerol as the catalyst can operate at 1 atm and 200°C (392°F), but the reaction time is 20 hrs. The requirements of high temperature and pressure are the major challenges in the ammonolysis of polyamides. Process optimization, suitable catalyst development, heat integration and research on optimum reactor design and operating conditions can help in overcoming these challenges.
Pyrolysis. Heating polyamides in the absence of oxygen produces pyrolytic oil and gases like hydrogen cyanide (HCN), carbon monoxide (CO), carbon dioxide (CO2), NH3, and nitrogen oxides (NOx ).2 For a circular economy, where recycled nylon is produced from nylon waste, pyrolysis is not a suitable option. The monomers, such as ε-caprolactam or other required monomers, would need to be produced again from fossil feedstock to manufacture new nylon, which leads to economic loss and breaks the circular economy loop. Therefore, pyrolysis does not support nylon-to-nylon recycling and is incompatible with true circularity for polyamides.
Enzymatic recycling. Enzymatic recycling of Nylon 6 is an innovative, sustainable approach to depolymerizing synthetic polyamides into their monomeric form primarily ε-caprolactam (FIG. 6). This method uses specially engineered enzymes, such as nylon hydrolases or amidases, to cleave the polymer's amide bonds under mild conditions (ambient to moderate temperatures and atmospheric pressure), offering a low-energy and low-emissions alternative to traditional chemical recycling.
General challenges. Nylon is predominantly used in the textile industry, and most post-consumer nylon waste exists in blended forms with other fibers such as cotton and polyester. This blending makes segregation and recycling challenging. To address this issue, advanced, low-energy segregation methods must be developed, such as chemical treatments designed to selectively remove impurities and separate nylon effectively.6 Additionally, many textiles undergo specialized chemical treatments (e.g., for water repellency and other functional properties) during manufacturing. These treatments can further complicate recycling by altering the nylon's recoverability. A collaborative approach between the textile and recycling industries is essential to minimize or substitute chemicals that negatively impact recyclability and to develop advanced purification and chemical removal techniques to enhance nylon recovery. Such advancements will significantly improve the efficiency and sustainability of nylon recycling.
Takeaways. Among the existing recycling methods, hydrolysis and ammonolysis are currently the only techniques operated at a relatively large industrial scale for nylon recycling. In contrast, pyrolysis and glycolysis, although widely explored for other polymers such as PET, have no reported commercial-scale implementation for nylon recycling according to the available literature.
A promising recent advancement is the enzymatic recycling of Nylon 6, which utilizes biologically derived enzymes to depolymerize nylon under milder and more environmentally friendly conditions.
Overall, the development of nylon recycling technologies remains in a nascent stage. Each method has its own advantages and limitations. Going forward, efforts in process optimization, catalyst/enzyme engineering and feedstock segregation will be essential to improve efficiency and scalability. Moreover, lifecycle assessment of these processes can provide critical insights into environmental impact, guiding future investments and development in sustainable polyamide recycling. HP
LITERATURE CITED
Global Nylon Market Research, “Global nylon market size & outlook, 2026–2033,” Report ID: 19466, 2024, online: https://www.grandviewresearch.com/horizon/outlook/nylon-market-size/global
Hirschberg, V. and D. Rodrigue, “Recycling of polyamides: Processes and conditions,” Journal of Polymer Science, Vol. 61, Iss. 17, September 2023, pp. 1937–1958, online: https://onlinelibrary.wiley.com/doi/full/10.1002/pol.20230154
Tonsi, G., C. Maesani, S. Alini, M. A. Ortenzi and C. Pirola, “Nylon recycling processes: A brief overview,” Chemical Engineering Transactions, Vol. 100, pp. 727–732, June 2023, online: https://www.cetjournal.it/index.php/cet/article/view/CET23100122
Chanda, M., “Chemical aspects of polymer recycling,” Advanced Industrial and Engineering Polymer Research, Vol. 4, Iss. 3, pp. 133–150, July 2021, online: https://www.sciencedirect.com/science/article/pii/S2542504821000336
Achilias, D. S., L. Andriotis, I. A. Koutsidis and D. A. Louka, “Recent advances in the chemical recycling of polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA),” Material Recycling—Trends and Perspectives, March 2012, online: https://www.researchgate.net/publication/221929069_Recent_Advances_in_the_Chemical_Recycling_of_Polymers_PP_PS_LDPE_HDPE_PVC_PC_Nylon_PMMA
Datta, J., K. Błażek, M. Włoch and R. Bukowski, “A new approach to chemical recycling of polyamide 6.6 and synthesis of polyurethanes with recovered intermediates,” Journal of Polymers and the Environment, Vol. 26, pp. 4415–4429, 2018, online: https://link.springer.com/article/10.1007/s10924-018-1314-4
Rajendra Gupta is a Technical Professional Leader at KBR.