University of Wisconsin-Madison
We will produce food grade resins from multi- and mono-layer plastic films using Solvent Targeted Recovery and Precipitation (STRAP). We will obtain 2,000+ hours time on stream in a 25 kg/hr STRAP system and design a 500 kg/hr commercial STRAP system at Convergen Energy in Green Bay, WI.
The general principle of STRAP process is that physicochemical differences between different polymers allow the selective dissolution of a single polymer layer in a solvent system in which the targeted polymer layer is soluble, but the others are not. The solubilized layer is then separated by filtration and precipitated by cooling that renders the dissolved polymer insoluble. The solvent is re-used and the targeted polymer layer is recovered as a dry, pure solid. This process is then sequentially repeated for each of the polymer layers resulting in a few segregated streams that can then be recycled.
Two key elements are essential to point out (1), the selectivity of the resin extracted, i.e., extracting one resin, and (2) the purity of the extracted resin, i.e., no colourants and no solvents.
To aid in solvent selection, we developed a first-principles molecular modeling approach to predict temperature-dependent polymer solubilities in thousands of solvents. We have demonstrated the process with a variety of films, post-industrial printed flexibles and post-consumer films obtaining near quantitative recovery of the pure resins. The resulting solids were analyzed using attenuated total reflectance infrared spectroscopy (ATR-FTIR) and head space GC-FID indicating that they are chemically pure polymers (LDPE, HDPE, PP, EVOH, PET, etc.). We have sent these resins to Amcor Flexibles who have indicated that the resins can be used in food grade applications. This indicates that the STRAP technology can recycle mono and multilayer flexible films into the pure resins without degrading resin properties.
Producing resins without ink or colourants is important for the food quality required and selling price of the resins. The dissolved ink and colourants do not precipitate with the plastics during cooling but remain in the solvent. Solvents are purified by distillation.