Mineralization of poly(tetrafluoroethylene) and other fluoropolymers using molten sodium hydroxide

Abstract

A simple, easy, and environmentally benign new technology consisting of a two-step process is proposed for the chemical recycling of fluoropolymers. In the first step, fluoropolymers were mineralized to soluble alkaline fluorides via degradation using a molten alkaline hydroxide at elevated temperatures and atmospheric pressure. In the subsequent step, CaF₂, which is an indispensable starting material for almost all organofluorine compounds, was obtained following treatment of the former aqueous solution with CaCl₂. Poly(tetrafluoroethylene) (PTFE), otherwise known as Teflon™, was subjected to mineralization using this new technique, and poly(vinylidene fluoride) (PVDF), poly(chlorotrifluoroethylene) (PCTFE), and the poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (poly(VDF-co-HFP)) were also successfully mineralized. When PTFE was heated with a large excess of NaOH at 500 °C for 3 h, 73.8% yield of CaF₂ was obtained with respect to the initial amount of PTFE. However, when KOH was used instead of NaOH, a low CaF₂ yield was obtained (i.e., 28%). For the other fluoropolymers, under the same experimental conditions, using NaOH, the yields of CaF₂ were 83.7% for PVDF, 52.3% for PCTFE, and 84.0% for poly(VDF-co-HFP). Finally, a mechanism based on the interdependence of polymer pyrolysis and nucleophilic attack by O²⁻ generated by the molten NaOH was developed.