Application and optimization of alkaline hydrolysis in recycling clear aligners

An increasing number of patients with malocclusion are choosing clear aligners for treatment, leading to a substantial rise in their production. Given the potential environmental impact of plastic products, recycling clear aligners is crucial. This study employed alkaline hydrolysis to recover the degradation products of clear aligners. The aligners were subjected to hydrolysis under various reaction conditions, including different concentrations of sodium hydroxide (NaOH), temperature, reaction time, ethanol-to-water mass ratio, and stirring rate. After the reaction, the organic components were isolated using thin-layer chromatography (TLC) plates, and their structure was characterized by ¹H-NMR spectroscopy and mass spectrometry. The degradation product was identified as 4,4'-methylenedianiline (MDA). The optimal reaction conditions for achieving the maximum degradation yield of MDA were determined to be stirring at 300 rpm at 130°C in a solution containing 5 wt% NaOH and a 1:1 mass ratio of ethanol to water for 24 h. This method demonstrates that clear aligners can be effectively recycled through alkaline hydrolysis, supporting global efforts to reduce environmental emissions.