(8d) Recycling of Polyolefins Using Green Solvents and Water: A Thermodynamic Approach

With over 13,000 additives available for plastics, the end-use of mechanical recyclate is severely limited. Recent advances in chemical recycling have proposed a new method for additive-removal. Using a high solvent loading; plastics are solubilized, then additives are filtered from the effluent, and plastics are selectively recrystallized, leading to a purified, near-virgin quality plastic. While these processes are successful, the authors hypothesize that similar purification of plastic waste can be obtained by swelling plastics using green solvents.

Using Hansen Solubility Parameters, we can create a thermodynamic roadmap for solvent tuning. As seen in Figure 1, the solvents relative energy distance changes with temperature meaning we can use this parameter to find conditions that will selectively swell plastics for additive removal. As it is the most abundant solvent, the effects of water treatment on a PE/PP blend can be seen in Figure 2. There is a significant reduction in volatile organic compounds (VOC) in the hydrothermal sample, indicating a high percentage of additives were extracted to the water phase. Looking at Figure 2b, the hydrothermal sample retains a higher Mw and Mn despite harsher processing conditions. We expect this is due to the lower shear environment of our process, and water effectively diluting mid-chain radical intermediates to prevent β C–C scission.

In summary, treatment of PE/PP in water is an effective method to remove VOCs and maintain molecular weights. The results presented here establish a novel, sustainable pathway for purifying plastic waste. Additional green solvents are currently being tested and additive removal will be further quantified by identifying molecular compounds that remain in the solvent phase and measuring color removal. Our results thus far have proven the existence of a novel reaction regime in which polymers do not have to be fully dissolved in a solvent for additives to be removed.