Chemical recycling of polycarbonate in dilute aqueous ammonia solution under hydrothermal conditions

Polycarbonate (PC) pellets were subjected to dilute aqueous ammonia solution under hydrothermal conditions in a semi-batch reactor at temperatures ranging from 433 to 463 K and at a pressure of 10 MPa. The PC pellets were almost completely converted to bisphenol A (BPA). During an initial certain period, referred to as an induction time, neither BPA nor total organic carbon in solution were detected, and the BPA yield increased with time. The monomer yield was well represented by a surface reaction model, two-thirds-order reaction with respect to the mass of unreacted PC. The overall rate constant of the reaction in 0.6 mol/kg aqueous ammonia solution at 433 K was about 15 times greater than that in 0.6 mol/kg NaOH solution. The rate constant at 433 K was proportional to the ammonia or NaOH concentration. There was a correlation between the induction time and temperature, as well as the ammonia or NaOH concentration. By carrying out the reaction in aqueous mixtures of (NH₄)₂SO₄ and NaOH at various concentrations of NaOH, ammonia was confirmed not to function as an alkaline reagent, but as a nucleophile reagent.