Synthesis of bisphenol-A monomer by the hydrothermal depolymerisation of poly(bisphenol-A)-carbonate
A significant challenge of our age is finding the harmony between the need for constant economic growth and our environment's limited resources. Plastics are characterised by their various unique properties. However, their decomposition rate in the environment is often extremely slow. Microplastics, which can be derived from this decomposition process, also present a broad spectrum of environmental and human health risks. Recycling creates an opportunity to channel plastic waste back as raw material into the economy's circulation. A crucial tool, besides immediate reuse, is the area of conversion technologies. Conversion technology means the chemical conversion of plastic waste into materials the industry can use. Hydrothermal depolymerisation is thermal conversion technology in which we harvest the plastic's monomer or its other depolymerisation products by reacting the polymer in high-pressure, high-temperature water.
Poly(bisphenol-A)-carbonate is an engineering plastic. Its unique properties are why it is an excellent material for specialised applications ranging from optical data storage units to the automotive industry. In the literature batch hydrothermal depolymerisations of poly(bisphenol-A) can be found yielding a high variety of dissolved products with 5-8.24% of bisphenol-A yields at full conversion of the polymer at typical reaction conditions of 250-350 °C temperature and 1-2 hours reaction time. [1-2]
My current work aims to develop a process that yields bisphenol-A by hydrothermal depolymerisation of poly(bisphenol-A)-carbonate. The designed and built semi-continuous lab plant could reach the total conversion of the polymer in under one hour with the adequately regulated process parameters. These are average retention time (1-3 minutes), temperature (300°C), pressure (12 MPa), and auxiliary stream (NaOH solution with a concentration of 0.45 mol/dm3). By combining the optimised depolymerisation with a subsequent crystallisation, 55% preparative yield of bisphenol-A was achieved with excellent reproducibility. The final product is crystalline bisphenol-A with a purity of 94% without requiring any additional operation.
[1] H. Tagaya, K. Katoh, J. I. Kadokawa, and K. Chiba, Polym. Degrad. Stab., 1999, 64, 289–292.
[2] X. Zhao, L. Zhan, B. Xie, and B. Gao, Chemosphere, 2018, 207, 742–752.
