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

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. 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. 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. 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.