Abstract
Chemical recycling of waste plastics has been getting greater attention in recent years, and polycarbonate (PC) is one of such polymers seeking appropriate recycling systems. We investigated the decomposition of polycarbonate using subcritical and supercritical water to obtain bis-phenol A (BPA), raw material of polycarbonate, focusing on its significant variation of solvent properties such as density, dielectric constant, and so on. In addition, the decomposition of PC/ABS (polycarbonate/ acrylonitrile-butadiene-styrene) alloy compound was also carried out to obtain bis-phenol A selectively from that alloy, by taking advantage of the difference of decomposition rate between polycondensation polymers and addition polymerization polymers.
Batch reactions were conducted at temperatures ranging from 573 to 723 K, and under pressures of 25 MPa and 30 MPa, with water density ranging from 0.15 to 0.80 g/cm3. The rate of polycarbonate decomposition increased with temperature, but the decomposition of bis-phenol A also proceeded rapidly under as high temperature as 723 K. Almost complete decomposition of polycarbonate to bis-phenol A was achieved at 673 K under 25 MPa (water density = 0.17 g/cm3) in a few minutes, with no generation of byproducts. The dielectric constant under this condition was 2.4, which is as low as organic solvent. This result indicates that low dielectric constant of supercritical water is important, which has both effects of accelerating the polycarbonate decomposition and protecting bis-phenol A. It can be said that such properties of supercritical water as variation of the dielectric constant, cage effect, and high mobility and diffusibility were all matched to this reaction system. In addition, polycarbonate contained in PC/ABS alloy could be decomposed and 70% of BPA selectivity was obtained under the same condition. These results suggest the great possibility of polymer recycling system using water, which is undoubtedly the least expensive and non-toxic solvent.
