Novel chemical recycling of polycarbonate (PC) waste into bis-hydroxyalkyl ethers of bisphenol A for use as PU raw materials

Abstract

An efficient process for the chemical recycling of polycarbonate (PC) waste into diols of bis-phenol A (BPA) for use as raw materials in PU production has been developed. Digestion of PC waste in alkylene glycols, i.e., ethylene glycol (EG) and propylene glycol (PG), with a catalytic amount of sodium carbonate at 180 °C under normal atmospheric pressure afforded partial alkoxylation products of BPA. This initial product mixture was found to consist of BPA (28%) and monohydroxyethyl ether (MHE-BPA, 40%) and bishydroxyethyl ether of bisphenol A (BHE-BPA, 25%) when digested in EG solution. Whereas, in PG digestion solution, the corresponding digestion products consist of monohydroxypropyl ether of BPA (MHP-BPA, 53%) and bishydroxypropyl ether of BPA (BHP-BPA, 21%), with the rest being un-propoxylated BPA. When these digested solutions were further treated with molar excess of urea with a catalytic amount of zinc oxide at 180 °C, BHE from EG or BHP-BPA from PG was produced in high yields. In both digestion processes, cyclic alkylene carbonates, i.e.ethylene carbonate (EC) or propylene carbonate (PPC), were observed as the transient intermediates which eventually disappear during the prolonged alkoxylation conditions. ZnO was added to the second step as the catalyst to accelerate urea's trans-esterification process. Both BHE and BHP products have been sucessfully utilized in making PU polymers with good mechanical properties. This chemical approach thus provides a viable alternative for PC recycling.