In vitro hydrolytic and photooxidative stability of polyester-based materials for clear orthodontic aligners

• High pH solutions and photolysis degrade the polymer more. • The temperature of the hot beverages amplifies material modifications. • The stiffness of the material was significantly reduced. • Surface becomes much more hydrophilic. • Leaching was favored because of degradation and solvent interactions. . This study evaluated the stability of PETG material used in clear orthodontic aligners (COAs). The material was subjected to hydrolysis, oxidation, photolysis, and hot aqueous environments. The objectives were to reproduce different situations that can occur during the intraoral aging of COAs when patients fail to follow the orthodontist's instructions or clean their aligner, and to evaluate the impact on the material properties and the compounds leached under these conditions, two factors that can have consequences for the performance and safety of the device. Changes in surface chemical groups were analyzed using Fourier Transform Infrared (ATR-FTIR) spectroscopy. Morphology was examined using scanning electron microscopy (SEM), while surface hydrophobicity was assessed by measuring the water contact angle. Monitoring of leaching products was performed using High-Performance Liquid Chromatography (HPLC). Furthermore, the thermomechanical properties were studied using several techniques, including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The most significant impact on the device was observed in an alkaline environment, during photooxidation, and after contact with hot or alcoholic beverages. These alterations manifested in different ways: the PETG chains can be degraded, resulting in the formation of small degradation products, whose leaching can be promoted by the interaction with the liquid simulant. The released products differ significantly depending on the storage conditions and may have consequences for patient safety. Interactions with environment can also modify the thermo-mechanical behavior, and alter the surface properties. The surface of the device can be modified through the formation of a rougher surface during hydrolysis, the creation of cavities during photolysis and oxidation, or the formation of coatings. A significant decrease in hydrophobicity was also observed. In addition, thermomechanical properties, particularly the static and dynamic glass transition temperatures, can decrease after storage due to chain reorganization and solvent sorption. The stiffness of the device is also impacted, with a decrease in the storage modulus, particularly for alkaline solutions and photooxidation, suggesting potential consequences for the treatment efficacy.