(154d) Hydrothermal Water and Benign Solvent Mixtures for Recycling of Multilayer Plastic Films

Polymer based multi-layer films are commonly used to provide the combined performance of multiple plastics to their application. Typically used in the food, biomedical and packaging industries, these films are not commonly recycled due to contamination and difficulties in separating the layers of these films. Typically adhered via coextrusion or lamination, these films cannot be easily separated, and our current recycling processes cannot handle mixed waste products. Current research uses a targeted solvent process to dissolve one part of these systems, capture the material in a filter and subject it to an anti-solvent to recover the polymer. While potentially effective, these methods utilize many toxic solvents, generating unnecessary waste. This work proposes an alternative of using supercritical water and green solvent mixtures. 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 (HSP) of hydrothermal, near critical and supercritical water were calculated as a function of temperature and pressure using different published methods of calculation. HSPs can then be used to select conditions for solubilization of different polymers, with the objective being selective fractionation of multilayer polymer films. A second set of calculations was then performed for mixtures of water with acetone, ethanol, methanol, 2-methyltetrehydrofuran (MeTHF), and ethyl acetate as representative green solvents. These solvents were chosen because they can solubilize some of the selected polymers at room temperature and have low environmental impact. These calculations were performed to find the relative energy distance (RED), solubility conditions and guide experimental work.

The relative energy distance or RED for selected solvent blends and supercritical water can be seen below in Figure 1. A lower RED is indicative the polymer will solubilize in the selected solvent. Typically, the RED at room temperature should be below one however, at elevated temperatures solubility can increase due to increased entropy. From Figure 1, supercritical water at 450 °C and beyond shows a similar RED to the 15% solvent blends. The solvent blend with the lowest RED value is 85% acetone in water. These preliminary results show that the solvation properties of supercritical water can be tuned with temperature which can solubilize polymers with sufficiently low RED values.

Based off the results of Figure 1, 85% acetone in water can be assumed to be the best solvent. Figure 2 shows the solubility sphere of 85% acetone and the nine selected polymers. Figure 2 indicates that polymers inside the sphere are considered soluble with the solvent, polymers on the edge will likely experience swelling, and polymers outside the sphere are not soluble with the solvent. From Figure 2, many common polymers will interact with the water-acetone mixture sufficiently to swell, if not completely solubilize. Solvent-plastic interaction will allow for selective fractionation of films depending on solvent properties. While the acetone-water mixture still requires an organic solvent, it has the potential for tuned polymer miscibility and fractionation. Open questions include the reactivity of polymers at these conditions. Tests are ongoing to evaluate reactivity and solubility in water-acetone mixtures near the critical point.