(619d) Screening Green Solvents for Multilayer Plastic Film Separation

The increasing global demand for specialized product packaging materials beyond the capabilities of single-layer plastics has driven the development of multilayer plastic films. These multilayer plastic films often consist of two to several polymers, including Polyethylene (PE), Polyethylene Terephthalate (PET), Ethylene Vinyl Alcohol (EVOH), and Polyvinyl Chloride (PVC), with each polymer offering diverse functions towards enhancing product safety, shelf life, and handling (Alias et al. 2022; Barry, 2022). However, the intricate mix of these polymers increases the difficulty in recycling multilayer plastic films via mechanical processes, leaving their end-of-life fate to landfills and incineration (Hutchings et al. 2021; Milbrandt et al. 2022; Rudolph et al. 2020). Solvent-based separation technologies are promising for re/up-cycling these multilayer plastic films (Zhao et al. 2018). A manifestation of this approach is the so-called Solvent Targeted Recovery And Precipitation (STRAPTM) process, which is a technology that sequentially separates polymer components of multilayer plastic films through a series of solvent washes using selective solvents (Walker et al. 2020). The STRAPTM process has exhibited potential in the recovery of high-quality resins from multilayer plastic film wastes (Zhao et al. 2018) and offers environmental (Munguía-López et al. 2023) and economic (Sánchez-Rivera at al. 2021) advantages over fossil-based plastic films. A critical step in the actualization of the STRAP™ process involves the identification of solvents capable of selectively dissolving a polymer layer while leaving the other layers undissolved. Molecular-scale models have been harnessed to predict the temperature-dependent solubilities of diverse polymers in organic solvents (Zhou et al. 2021). These models have been integrated with experimentally derived solubility data to improve the accuracy of polymer solubility predictions on a large scale across ~1000 solvents (Zhou et al. 2023). Although the STRAP™ process shows promise for revolutionizing the plastic recycling sector, significant challenges persist. While solvents typically remain unconsumed in solvent-mediated recycling technologies, repeated usage can lead to minute quantities escaping into the environment or their intentional disposal, potentially generating new waste streams. The ecological impact of such waste streams can be significant if the solvents in question possess toxic properties. Moreover, like other solvent-mediated processes, there is a heightened risk of prolonged exposure to toxic solvents among operational personnel. Therefore, beyond prioritizing selective solubility, it is critical to assess the greenness of solvents during screening, contributing to a more sustainable process design – an essential facet of green chemistry (Gao et al. 2020).

This study introduces a multistep green solvent selection framework for separating multilayer plastic films into their constituent resins. This framework, comprising eight sequential steps, focuses on evaluating polymer selective solubility (steps 1-5), solvent greenness (steps 6-7), and cost of greenness (step 8). This framework begins with employing molecular scale models to quickly predict temperature-dependent solubilities of the multilayer plastic film components (i.e., polymers) in ~1000 solvents. Utilizing short oligomers to represent polymers, molecular dynamic (MD) simulations, and the COnductor-like Screening MOdel for Real Solvents (COSMO-RS), grounded in quantum chemistry principles, enables accurate solubility predictions. Subsequently, a fast solvent screening process is employed to identify solvents capable of selectively dissolving and precipitating a target polymer under standard atmospheric pressure. To assess the greenness of the selected solvents, various criteria, including the operational energy required for the dissolution process, octanol-water partition coefficients (LogP), and greenness levels of solvents using the industrial solvent selection guidelines provided by GlaxoSmithKline (GSK), are employed. However, these guidelines do not contain information about most of the solvents in our study. Therefore, solvent regulatory standards such as the Registration, Evaluation, Authorization, and Restriction of Chemical Regulation (REACH) initiative and the US Environmental Protection Agency (EPA) are consulted to identify the potential hazards and any regulatory restrictions associated with each solvent on their industrial usage. The cost of greenness is evaluated by performing a techno-economic and life cycle analysis on the STRAPâ„¢ process using each of the candidate solvents to explore the tradeoffs between solubility, cost, and solvent greenness, utilizing the open-source software BioSTEAM (Cortes-Pena et al. 2020), and OpenLCA (Ciroth et al. 2020). Subsequently, this framework is applied to an industrial multilayer plastic film containing PE, EVOH, and PET to identify green solvents for its separation and associated tradeoffs.

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