(354d) Life Cycle Assessment for Polyamide 6: Production, Recycling, Disposal

Conventionally, polymers are produced from fossil resources via a myriad of intermediate monomers. At the end of life, they are disposed in landfill or incinerated. The resulting emissions contribute to anthropogenic climate change, and the process is unfavorable with respect to other environmental KPI. Circular economy aims at transforming the existing routes by replacing raw materials with wastes, and closing the cycle of disposal and production. Various steps on the circular economy ladder have been described (Keijer et al., 2019), ranging from reuse and redistribution to recycle and recover. Material can be recycled either through the biosphere or technosphere (Tóth et al., 2022; Meys et al., 2021). Hence, several options exist to supply polymers to the economy sustainably. They include mechanical recycling, chemical recycling, bio-based synthesis, and derivation from captured carbon. Combinations of synthesis, recycling, and end-of-life options differ in cost, carbon and water footprint, land use, as well as overall sustainability. In this work, we perform life cycle assessment (LCA) to compare the various combination for polyamide 6 (nylon 6) with respect to economic and environmental indicators.

For some polymers, mechanical recycling, i.e., melting the polymer, not changing its chemical structure, is possible and cost-effective. However, it requires the waste polymer to be sorted carefully; any impurities can destroy the structure of the polymer. This is a major complication, considering many products contain combinations of polymers. The melting process can also change the properties of the polymer with every cycle, yielding sub-standard product (Coates et al., 2020; Achilias et al., 2014). As a result, chemical recycling, specifically the depolymerization of the polymer to its monomer, has been a research focus (Coates et al., 2020; Achilias et al., 2014). It enables the separation of the monomer and re-polymerization to pure, high-quality polymer. Subjecting the waste polymer to pyrolysis and synthesizing the monomer from pyrolysis oil also constitutes a chemical recycling pathway (Somoza-Tornos et al., 2020). Another option is the recycling of material through the biosphere. When the raw material for polymer production is sustainably sourced biomass, it can be incinerated at the end of life without incurring net emissions to the atmosphere. Alternatively, the cycle can be closed following the incineration by capturing CO₂ and using it as carbon source for the synthesis, a concept referred to as carbon capture and utilization (CCU) (Meys et al., 2021). If the polymer remains sourced from fossil resources, the combustion emissions can be abated by capturing the CO₂ and sequestering it (CCS). CCS could be combined with bio-based synthesis, potentially yielding net negative emissions.

Polyamides (PA) represent 8% of current polymer production at ~7 Mt/a (Rihko-Struckmann, 2022) and are utilized in the automotive industry, for electrics and electronics, consumer goods, healthcare, and construction. They are at present produced from fossil benzene and toluene via multiple monomers. Efforts to collect and recycle PA at scale both by mechanical and chemical processes have been realized commercially by DuPont since the 1990s (Kasserra, 1998) and also practiced by other companies (Coates et al., 2020). Several options for depolymerization as well as research on pyrolysis of PA exist (Deulkar, 2022; Minor et al., 2023). Still, only a small fraction of polyamide is currently recycled. Biological decomposition of PA appears extremely difficult.

In this work, we examine production, recycling, and disposal options for polyamide 6, which is synthesized from the monomer caprolactam. We include as virgin production routes the synthesis from fossil benzene, and synthesis from agricultural waste via hydroxymethylfurfural and adipic acid. Recycling options are mechanical recycling, and chemical recycling back to caprolactam by hydrolysis using phosphoric acid (Minor et al., 2023). In case of recycling via CO₂, the synthesis comprises methanol synthesis, methanol-to-aromatics, and the conventional step from benzene to caprolactam (Rihko-Struckmann, 2022; Deulkar, 2022). Incineration and landfill are considered as end-of-life treatment for the amount of PA-6 which is not recycled.

We gather LCAs from literature wherever available, and perform LCA using ECOINVENT 3.10 based on techno-economic analyses in literature for the individual processes. Then, we compare combinations of pathways and technologies. Each case combines virgin production, recycling option, and end-of-life treatment. For instance, fossil production combined with mechanical recycling, incineration, and CCS, or bio-based production without recycling plus landfill. Thereby we identify optimal routes with respect to cost, emissions, and other LCA indicators including resource depletion and biodiversity. Further, we vary recycling rates to evaluate how the impacts may evolve over time if ambitions for circularity in plastics are realized, and identify the lowest possible environmental impact PA utilization can have. It is of interest to quantify the trade-offs between the environmental benefits and economic drawbacks of recycling options. Recent work on modelling circular economy for plastics suggests similar emissions reductions can be achieved with input of large quantities of either biomass or (low-carbon) electricity (Meys et al., 2021). Hence, the optimal strategy may depend on the national context including natural resources as well as decarbonization and circular economy agenda. Further, estimating the cost of a low-carbon system, and the (additional) cost of a circular system could help inform public policy needed to achieve the transition. Detailed analysis of the distinct routes could also identify potential pitfalls in designing green PA production as well as key technologies and future research needs.

References

Tom Keijer, Vincent Bakker, and J. Chris Slootweg. Circular chemistry to enable a circular economy. Nature Chemistry, 11(3):190–195, 2019. doi: 10.1038/s41557-019-0226-9.

András József Tóth, Dániel Fózer, Péter Mizsey, Petar Sabev Varbanov, and Jirí Jaromír Klemeš. Physicochemical methods for process wastewater treatment: powerful tools for circular economy in the chemical industry. Reviews in Chemical Engineering, 0(0), 2022. doi: 10.1515/revce-2021-0094.

Raoul Meys, Arne Katelhon, Marvin Bachmann, Benedikt Winter, Christian Zibunas, Sangwon Suh, and André Bardow. Achieving net-zero greenhouse gas emission plastics by a circular carbon economy. Science, 374(6563):71–76, 2021.

Geoffrey W Coates and Yutan DYL Getzler. Chemical recycling to monomer for an ideal, circular polymer economy. Nature Reviews Materials, 5(7):501–516, 2020.

Dimitris S Achilias, Lefteris Andriotis, Ioannis A. Koutsidis, Dimitra A. Louka, Nikolaos P. Nianias, Panoraia Siafaka, Ioannis Tsagkalias, and Georgia Tsintzou. Recent advances in the chemical recycling of polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA), chapter 1, pages 3–64. IntechOpen, 2012.

Liisa Rihko-Struckmann and Kai Sundmacher. Closing the circular cycles for polyamides. Smart Process Systems Engineering Symposium 2022: Chemical and Biological Decomposition Processes, 2022.

H Peter Kasserra. Recycling of polyamide 66 and 6. Science and Technology of Polymers and Advanced Materials: Emerging Technologies and Business Opportunities, pages 629–635, 1998.

Ankush Kishor Deulkar. Material flow analysis of polyamide in Germany. MSc thesis, Otto von Guericke University Magdeburg, 2022.

Ann-Joelle Minor, Ruben Goldhahn, Liisa Rihko-Struckmann, and Kai Sundmacher. Opportunities for chemical recycling processes of nylon 6 to caprolactam: Review and techno-economic assessment. Chemical Engineering Journal: 145333, 2023.

Ana Somoza-Tornos, Andres Gonzalez-Garay, Carlos Pozo, Moisès Graells, Antonio Espuña, and Gonzalo Guillén-Gosálbez. Realizing the potential high benefits of circular economy in the chemical industry: ethylene monomer recovery via polyethylene pyrolysis. ACS Sustainable Chemistry & Engineering, 8(9):3561–3572, 2020.