Recycling of thermoplastic waste: Phase equilibrium in polystyrene‐PVC‐polyolefin solvent systems

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Abstract
The contribution of plastics to the total amount of waste produced is increasing significantly. Not only do plastics present disposal difficulties, but traditional disposal techniques constitute a waste of valuable resources. A process to recycle plastic waste economically would provide an attractive solution to both problems. Approximately 70 percent of the U.S. plastics production is comprised of only three families of plastics; polyethylene, PVC and polystyrene. Mixtures of these three polymers invariably have poor properties because of their thermodynamic incompatibility, so a recycle scheme will have to achieve separation into nearly pure components. A process is proposed which takes advantage of the thermodynamic incompatibility of polymers in solution to effect such a separation. The equilibration of a simulated waste mix containing 4 parts polyolefin (polyethylene and polypropylene), 1 part polystyrene and I part PVC was investigated in a variety of solvents at total polymer concentrations of up to 15 percent, in the temperature range 115 to 125°C, using differential refractometry. In an 85 percent xylene, 15 percent cyclohexanone solvent, the polyolefins (polypropylene and the various densities of polyethylene) coexisted in a single phase, but the three phases obtained—polyolefin, polystyrene and PVC—routinely contained 99 + percent pure polymer, indicating that excellent separations of the major thermoplastic components of a waste mix can indeed be obtained.

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