Comparative mechanical performance of thermoplastic materials for clear aligners under simulated oral conditions

Objective: To assess the suitability of polycyclohexylenedimethylene terephthalate glycol (PCTG) for orthodontic aligners and to compare the mechanical properties and 14-day stress relaxation behavior of different materials under simulated oral conditions, offering clinical guidance for material selection. Methods: Materials used were Maxflex, Scheu, and Fusion Align, consisting of thermoplastic polyurethane (TPU), polyethylene terephthalate glycol (PETG), and PCTG, respectively. The following experiments were conducted: (1) tensile testing at 23°C and under simulated oral conditions; (2) prediction of 14-day stress relaxation using the time–temperature superposition principle; (3) right-angle tear strength testing; and (4) Shore D hardness testing. Results: Mechanical properties followed the trend TPU > PETG > PCTG in elastic modulus, yield strength, right-angle tear strength, and Shore D hardness (P < 0.05); PETG > PCTG > TPU in yield strain (P < 0.05). TPU showed higher elongation at break than PETG and PCTG (P < 0.05), with no significant difference between PETG and PCTG (P > 0.05). Stress relaxation behavior was predicted based on the time–temperature superposition principle. With prolonged duration, the stress ranking progressively shifted from TPU > PETG > PCTG to PCTG > PETG > TPU. Conclusions: PCTG exhibits light and sustained force, but its tear resistance and hardness are relatively low. In contrast, TPU demonstrates excellent tear resistance and hardness but experiences rapid force decay.