Physical and chemical changes of clear aligners after thermoforming and intraoral exposure

Abstract

This study assessed the physical and chemical changes of clear aligners after thermoforming and intraoral exposure. This ex vivo study evaluated raw sheets, thermoformed sheets, and aligners used for 2 weeks from two commercial brands of Erkodur and Shinseki (n = 8 in each group). The surface hardness (by the Vickers hardness test), thickness (by a digital caliper), and glass transition temperature (Tg) (by differential scanning calorimetry) of the specimens were measured, and they underwent X-ray diffraction (XRD) to assess their degree of crystallinity and attenuated total reflectance-Fourier-transform infrared spectroscopy (ATR-FTIR) for analysis of their molecular composition. Data were analyzed by one-sample, paired and independent t-tests, and Mann–Whitney U and Wilcoxon signed-rank tests (alpha = 0.05). The XRD and ATR-FTIR spectra of Erkodur and Shinseki aligners matched those of polyethylene terephthalate glycol polymer and did not change after thermoforming or intraoral exposure. Unlike intraoral exposure (P > 0.05), thermoforming significantly decreased the hardness of both brands of aligners (P < 0.05). Intraoral exposure significantly decreased the thickness of Erkodur aligners at some points and significantly increased the thickness of Shinseki aligners (P < 0.05). Thermoforming significantly decreased the Tg of both brands of aligners (P < 0.05). Intraoral exposure significantly decreased the Tg of Erkodur and increased the Tg of Shinseki aligners (P = 0.000). Erkodur and Shinseki aligners showed high chemical and structural stability after thermoforming and 2 weeks of intraoral exposure. However, thermoforming affected the thermal and physical properties of both brands of aligners. Erkodur aligners had higher dimensional stability than Shinseki aligners.