Heterocoagulation of Inorganic Pigments for Solvent-Based Recycling of Polystyrene

When recycling plastics, obtaining colorless recyclates increases their market value, whilst simultaneously reducing the risk of any impairment to the mechanical properties of the finished product. Hence, the removal of pigments is a pivotal factor in enhancing the circularity of plastics. In this study the heterocoagulation of three different inorganic pigments (carbon black, red, and yellow) was investigated, as alternative solid-liquid separation technique in solvent-based recycling. One specific polymer (PS), was selected and two different solvents were tested (xylene and cyclohexanone). The best-performing heterocoagulants were cationic polyacrylamide, poly(ethylene glycol), and sodium dodecyl sulfate, which resulted in turbidity reduction higher than 99%. Tests were performed also in polymer solutions of PS, where red and yellow pigments reached turbidity reduction values higher than 90% at 5 wt% polymer concentration. Carbon black revealed to be more difficult to treat at 5 wt% PS, achieving a successful separation (turbidity reduction up to 98%) at lower polymer concentration (1 wt%). Finally, the selective recovery of the heterocoagulants was demonstrated possible by washing the sediment with water, wich dissolves the more polar heterocoagulants without dissolving the polymer residues.