Understanding the Phase Behavior of Ethylene Vinyl Alcohol Copolymer Under Isobaric and Isothermal Conditions to Recycle Multilayered Plastics

Plastic production has skyrocketed in recent decades, resulting in billions of tons of plastic pollution across the world. This plastic culminates in landfills, fragments into microplastics, and contaminates our ecosystems. Plastic does not readily degrade; therefore, it persists in nature for hundreds of years and pollutes every environment. As a result, studying plastic recycling techniques is essential to create a demand for sustainable consumption of everyday commodities. The research objective is to explore the feasibility of recycling multilayered plastics (MLP) using dissolution techniques. Multilayered plastics reduce recyclability because they are composed of multiple polymeric layers with specific barrier properties adhered to tie layers. Therefore, applying standard recycling techniques such as float/sink or air sortation methods would be difficult to employ. Instead, selective dissolution techniques can be used to separate the polymer layers by taking advantage of each specific property layer. Therefore, the phase behavior of EVOH (ethylene vinyl alcohol) first must be quantified to later understand its recovery after dissolution and overall feasibility to recycle in multilayered plastics.

To separate EVOH from the MLP system, DMSO (dimethyl sulfoxide) was chosen as the target solvent because of its high compatibility with EVOH. Three grades of EVOH (27 mol%, 32 mol% and 48 mol% ethylene content respectively) were subjected to isobaric conditions to understand cloud point behavior. This first set of experiments proved that the relationship between the volume fraction of DMSO and temperature are inverse; therefore, dissolving EVOH in DMSO requires either a higher temperature with less solvent or a lower temperature with more solvent. Understanding this one-phase system is critical to effectively separate EVOH from its multilayered counterparts and later transport it to an antisolvent system for recovery. The second set of experiments used isothermal conditions to understand the pressure ranges at which EVOH in DMSO would phase separate with the inclusion of supercritical carbon dioxide as an antisolvent. Because DMSO has a stronger affinity for supercritical CO₂ (carbon dioxide) over EVOH, the EVOH will phase separate under specific pressure ranges and be capable of recovery for further studies. The experiments were conducted in a small-scale variable volume reactor with a viewing cell. Noteworthy findings from this data show that the lower solution temperature of the highest EVOH grade was similar to the higher solution temperature of the lowest EVOH grade, creating a generalized miscibility zone analogous to all three grades. These tests characterized the optimal conditions to precipitate EVOH in DMSO and supercritical CO₂ to later maximize EVOH yields in future recovery tests. The overarching goal of this research is to quantify EVOH retention from multilayered plastics dissolution and determine if chemical recycling is effective in industrial settings.