(151c) Optimization of the Solvent-Targeted Recovery and Precipitation (STRAP) Process for Multilayer Plastic Recycling

Plastic films have been widely used to package food. Particularly, multilayer films represent a significant fraction of the food packaging that we buy every day. Around 20% of food packaging in Europe is multilayer films and this percentage is expected to increase due to the growing demand for food products [1]. Multilayer films are composites of different polymers [2] that are combined to achieve certain properties such as oxygen and water barriers, temperature resistance, and structural integrity [3]. These properties allow having products with a long shelf-life to avoid food waste. Moreover, multilayer films help reduce costs because larger amounts of single-layer materials would be required to attain some of these properties [4]. To achieve the desired properties, 2 to 17 layers of different polymers are commonly combined [5,6]. However, it is possible to find multilayer films with 2 to several hundred individual layers [7]. Some of the polymers that are typically combined to make the films are polyethylene (PE), ethylene vinyl alcohol (EVOH), and polyethylene terephthalate (PET).

When the films are produced, large amounts of waste are generated because of inefficiencies and manufacturing errors. This waste is known as post-industrial waste (PIW). Up to 40 % of the million tons of multilayer films production ends up as PIW. PIW is not currently recycled because of a lack of commercially available technologies [8]. For instance, mechanical recycling cannot be used because the layers are chemically incompatible [9]. Therefore, only single-component plastics can be recovered and most of the PIW films end up in landfills [10].

An approach to deconstructing multilayer films into their constituent polymers has been recently reported [11]. This new technology is denominated as the STRAP process, and it uses a series of solvent washes that are guided by thermodynamic calculations of polymer solubility. Two STRAP processes have been proposed to recover the constituent polymers of two post-industrial films manufactured by Amcor (A1 and A2). [12]. Since these multilayer films have similar compositions, the STRAP processes used are also similar; however, the design of the STRAP process can vary significantly, depending on the composition of the film (including the type of polymers and the number of layers).

Currently, the STRAP processing pathway is guided by polymer-solvent dependencies; however, the economic performance and environmental impacts of the STRAP process change depending on the film that is recycled and on the chosen processing pathway. Furthermore, the polymer-solvent dependencies can result in several possible alternatives which increase with the number of layers of the film. Therefore, we propose an optimization approach for the design of a flexible STRAP process. The modeling framework considers economic and environmental objectives to find the optimal STRAP processing pathway for different sets of multilayer films. The optimal pathway will include the solvents and equipment that minimize the costs and the environmental impacts of the process.

References:

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[11] T. W. Walker, N. Frelka, Z. Shen, A. K. Chew, J. Banick, S. Grey, M. S. Kim, J. A. Dumesic, R. C. Van Lehn, and G. W. Huber. Recycling of multilayer plastic packaging materials by solvent-targeted recovery and precipitation. Science advances, 6(47):eaba7599, 2020.

[12] K. L. Sánchez-Rivera, P. Zhou, M. S. Kim, L. D. González Chávez, S. Grey, K. Nelson, S. C. Wang, I. Hermans, V. M. Zavala, R. C. Van Lehn, and G. W. Huber. Reducing Antisolvent Use in the STRAP Process by Enabling a Temperature-Controlled Polymer Dissolution and Precipitation for the Recycling of Multilayer Plastic Films. ChemSusChem, 14(19):4317-29, 2021.