Fluoropolymers (FPs) are attractive for special applications owing to their unique properties, including excellent combustion and chemical resistance and low refractive indices. These properties are derived from high fluorine electronegativity and strong carbon–fluorine bond and are generally uncommon among most organic compounds. Among the existing FPs, poly(tetrafluoroethylene) (PTFE), popularly known as Teflon™, is widely used in many industries. This can be attributed to its extraordinary properties, among them being the best non-stick coating performance. However, it is difficult to process PTFE using conventional extrusion and injection molding methods due to its high melting point and viscosity.
With the increased application of FPs and PTFEs, several approaches for effective and efficient processing of these materials have been developed. Despite the remarkable progress, the disposal of produced and consumed FP-based materials presents another challenge in their application. Burning these polymers is not only expensive but also produces toxic gases and substances that are harmful to human health and the ecosystem. Additionally, landfilling as waste, which is a common FP waste disposal method, further exacerbates the existing environmental pollution menace.
Recently, chemical recycling has emerged as an alternative solution. Unfortunately, there is a high possibility that FPs may not be suitable for chemical recycling owing to their chemical inertness, considering that they are olefinic polymers. Although the degradation of some FPs has been extensively studied, most of these studies only concentrated on their aging characteristics and thermal stabilities. To date, there are limited studies on mineralization – a concept that is based on using FPs to create inorganic compounds that are comparable or even better than minerals in terms of properties. This concept is deemed to be particularly suitable for PTFE, which is rarely subjected to chemical recycling.
On this account, Professor Naohisa Yanagihara and his student Takahiro Katoh from Teikyo University proposed a facile and environmentally friendly mineralization approach based on molten alkaline sodium hydroxide to facilitate the chemical recycling of PFTE and other FPs. This was a two-step approach. The first step involved using molten alkaline hydroxide to mineralize FPs to soluble alkaline fluorides via degradation at elevated atmospheric pressure and temperature of 500 °C for three hours. In the second and final step, the former aqueous solution was treated with CaCl2 to form CaF2 – an important starting material for nearly all organofluorine compounds. The work is currently published in the journal, Green Chemistry.
Professor Yanagihara and Dr. Katoh reported efficient and successful mineralization. CaF2, the final mineralization product, was obtained with a 78.3% yield. However, when potassium hydroxide was used in place of sodium hydroxide, a low CaF2 yield was obtained. By applying the same mineralization process under the same experimental conditions to PCTFE, PVD and poly(VDF-co-HFP), successful mineralization was achieved, producing CaF2 yields of 52.3, 83.57 and 84.0% with respect to the initial amounts, respectively.
Although the temperature for the individual processes was not necessarily proportional to the corresponding FP, poly(VDFco-HFP) produced the highest CaF2 yield despite having a significantly low melting point. Moreover, PTFE produced the highest CaF2 yield at a temperature of 500 °C despite being thermally stable with the highest melting point. The mineralization mechanism appeared to involve a synergistic effect between polymer pyrolysis and nucleophilic attack by O2- generated by the molten NaOH. FP mineralization by molten sodium hydroxide could not be successfully achieved without these two processes.
In summary, this is the first study to report the chemical recycling of PTFE via mineralization. Based on the findings, the reaction involving molten sodium hydroxide, just like FP mineralization reaction, could be potentially used to treat other general-purpose polymers without generating exhaust gas. In a statement to Advances in Engineering, the first and corresponding author Professor Naohisa Yanagihara explained that the study would contribute to developing novel processes to aid in efficient and environmentally friendly treatment of discarded fluoropolymers.
Yanagihara, N., & Katoh, T. (2022). Mineralization of poly(tetrafluoroethylene) and other fluoropolymers using molten sodium hydroxide. Green Chemistry, 24(16), 6255-6263.