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

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.