Greening the physical recycling of HDPE: dissolution precipitation with natural solvents

Polyolefins comprise some of the most abundant plastics, but their recycling faces several difficulties. This work focuses on the recycling of high-density polyethylene (HDPE) through the dissolution-precipitation method. Although few works have been published on this topic, this is an extremely promising plastic recycling strategy. However, for this kind of process to become a viable alternative, the replacement of conventional solvents by green solvents and the minimization of energy usage need to be implemented. In this work, several natural terpenes were screened based on COSMO-SAC predictions for HDPE solubility, and three were tested: limonene, alpha-pinene, and p-cymene. These solvents showed a good capability of dissolving HDPE, confirming the computational predictions. The dissolution and precipitation conditions were optimized based on the results for limonene, with the optimal values being 3 wt% initial concentration of the polymer in the solvent, a dissolution period of 10 min at a temperature of 110 degrees C, and an antisolvent/solvent ratio of 2/1. Comparing both HDPE and solvent recoveries for the three selected solvents, the best results were obtained for alpha-pinene as the solvent and ethanol as the antisolvent. The recovered HDPE samples were analyzed by TGA, FTIR, and DSC, showing little modification between the recycled and pristine materials. Scaling up of the process and solvent reuse were also investigated. Through process simulation, it was concluded that solvent recovery and reuse is a crucial step for economic viability. HDPE dissolution-precipitation with alpha-pinene was found to be a sustainable process from preliminary sustainability analysis, and the separation of HDPE and PET (polyethylene terephthalate) was successfully carried out. The selective dissolution of HDPE with a green solvent allows its simple and sustainable separation from other polymers, e.g. PET.