(532y) Near and Supercritical Water As a Tunable Solvent for Recycling Multilayer Plastic Films

Polymer based multilayer films are commonly used to provide the combined performance of multiple materials. These polymers are adhered to each other by coextrusion or glues, which makes recycling these multicomponent materials very difficult_­­¹. Several processes have been proposed using different solvents to dissolve one part of these systems, capture the material in a filter and subject it to an anti-solvent to recover the polymer^2-4. Solvent selection is based on sequential tuning of solvation properties for selective dissolution of the film, layer by layer. The fractionated polymers can then undergo recycling, chemical recycling, or upcycling, as appropriate. While potentially effective, these methods rely on many toxic solvents, generating waste. A proposed alternative is using supercritical water at varying temperatures to target separation of different layers of multilayer films. Water is non-toxic and abundant, unlike the organic solvents proposed in previous work. Moreover, the solvation properties of water can be tuned by adjusting the temperature or by addition of environmentally benign cosolvents. The challenge is selecting conditions for polymer solubility that do not result in polymer degradation. In this work, the Hansen solubility parameters of hydrothermal, near critical and supercritical water were calculated as a function of temperature using different published methods of calculation⁵. Hansen solubility parameters can then be used to selection conditions for solubilization of different polymers, with the objective being selective fractionation of multilayer polymer films. Following calculations with water, a second set of calculations was performed for mixtures of water with ethanol, propanol, and ethyl acetate as representative green solvents. This work forms the thermodynamic basis for development of hydrothermal and supercritical water as a tunable solvent for dissolution of multilayer plastic films.