Dynamic mechanical and thermal properties of clear aligners after thermoforming and aging

Abstract

Background Based on the role of properties of aligner materials on their efficiency, we aimed to assess their thermomechanical properties after thermoforming and simulated aging. Methods In this experimental study, 96 samples of polyethylene terephthalate glycol (PETG) aligners (Duran and Erkodur) were prepared and divided to three groups: control (C), after thermoforming (T), after thermoforming and aging (TA). Thermoforming was done through 3D-printed molds, and aging was exerted by 200 thermal cycles after immersion in 37°C distilled water for 24h. Flexural modulus, hardness, glass transition temperature (Tg), elastic and viscous modulus, and loss factor were evaluated. Two-way ANOVA, T-independent, and Tukey HSD tests were done for statistical analysis and significance level was set to 0.05. Results In both materials, flexural modulus decreased significantly after thermoforming, 88% in Duran and 70% in Erkodur, but did not change significantly after aging. After thermoforming, hardness decreased significantly in both materials (22% in Duran and 7.6% in Erkodur). Dynamic Tg was significantly lower in T and TA in both materials. At all temperatures (25, 37, 55°C) in Duran, the elastic modulus difference was only significant between C and TA, but in Erkodur, it decreased significantly in T, and there was no significant change after aging. Viscous modulus and loss factor showed the same change patterns at all temperatures. In both materials, they increased after thermoforming, but did not change significantly after aging. Conclusion Thermoforming had more prominent role than aging in diminishing of thermomechanical properties. In general, Duran had greater thermomechanical stability than Erkodur.

Clinical Performances and Biological Features of Clear Aligners Materials in Orthodontics

“…This study contrasts with Delaie's study (Dalaie et al, 2021), which investigates the thermomechanical properties of two PET-G aligners of two different thicknesses, 1 and 0.8 mm, in response to seeing in both types, the hardness decreases by about 7.6% after thermoforming.…”

“…The study of Dalaie et al investigated the thermomechanical properties of two PET-G aligners of two different thicknesses, 1 and 0.8 mm, in response to thermoforming; it has been seen that in both types, the hardness decreases by about 7.6% after thermoforming, but there are no significant differences in hardness into the two types of aligners. The reduction of thickness decreases after thermoforming in both types of aligners, which justifies the reduction of flexural modulus (Dalaie et al, 2021). Comparing the flexural modulus of Duran with a thickness of 0.75 and Duran 1mm, the first modulus increases and the second decrease, meaning that with increasing thickness of the Pet-G sheets, the bending modulus decreases (Dalaie et al, 2021).…”

“…The reduction of thickness decreases after thermoforming in both types of aligners, which justifies the reduction of flexural modulus (Dalaie et al, 2021). Comparing the flexural modulus of Duran with a thickness of 0.75 and Duran 1mm, the first modulus increases and the second decrease, meaning that with increasing thickness of the Pet-G sheets, the bending modulus decreases (Dalaie et al, 2021).…”

“…Such temperature increases can affect the mechanical properties of thermoplastics (Iijima et al, 2015). As evidence of this, there are several in vivo and in vitro studies (Ryokawa et al, 2006;Iijima et al, 2015;Bucci et al, 2019;Dalaie et al, 2021).…”

“…The in vitro study by Dalaie (Dalaie et al, 2021) simulated the temperature variations that occur in the oral environment through intermittent thermocycles, 22 h per day for 14 days and studied how the hardness, thickness and flexural modulus of two aligners (in Pet-G foil with thicknesses of 1 and 0.8 mm) vary in contact of the oral environment.…”