Polymeric clear aligners are increasingly used in orthodontics due to their aesthetic appeal and comfort. When placed in the oral cavity, they are continuously exposed to challenging conditions such as temperature fluctuations and changes in pH. The mechanical properties of these aligners are strongly influenced by temperature and liquid absorption. This study employs Dynamic Mechanical Analysis (DMA) to evaluate the effect of a saline solution (0.9 % NaCl) on the polyurethane-based material Tera Harz TC-85.

DMA was performed over a temperature range of 20 °C to 88 °C, measuring storage modulus (E′), loss factor (tan δ), and glass transition temperature (Tg). Aging the material under laboratory conditions for three months showed no significant effect on its mechanical properties. However, exposure to the saline solution led to a marked decrease in E′, tan δ, and Tg, indicating that the saline solution acted as a plasticizer. At 35 °C, the modulus of soaked samples decreased by 59 % compared to as-prepared samples. Further soaking beyond five days did not produce additional changes. The plasticization process was reversible; after three days of drying, the material's properties were almost fully restored.

Saline solution acts as a reversible plasticizer for Tera Harz TC-85, reducing its stiffness and Tg. Given that the average oral cavity temperature (34 °C) is near or above the Tg of soaked samples (30 °C – 31.5 °C), their mechanical properties degrade during use. This can lead to reduced effectiveness of orthodontic aligners.