While the durability of thermoplastic aligners has been the subject of numerous studies, the durability of thermoplastic retainers has received significantly less attention. Patients are often advised to wear their thermoplastic retainers indefinitely, so the durability of the materials used in their fabrication is crucial to determining whether they are worth the cost. Limited studies have evaluated the properties of thermoplastic retainer materials and the effects of thermocycling on their mechanical properties. Thus, this study aimed to examine six thermoplastic retainer materials after thermoforming with and without thermocycling. The materials’ flexural modulus, hardness, and surface roughness values were measured after thermoforming (Group 1) and after thermoforming with subsequent thermocycling for 10,000 cycles (Group 2). After thermoforming, there was a significant difference in flexural modulus and hardness values between most of the materials. However, their surface roughness was not significantly different (p < 0.05). After thermocycling, the flexural modulus and hardness increased significantly for most tested materials (p < 0.05) compared to Group 1. Concerning the surface roughness, only two materials showed significantly higher values after thermocycling than Group 1. Thus, all the mechanical properties of the evaluated materials differed after thermoforming, except the surface roughness. Moreover, while thermocycling made the materials stiffer and harder in general, it also made some of them rougher.

Due to the fact that retainers are often recommended for a lifetime, their endurance and longevity are directly related to the quality of the materials used in their production. Our study examined the water sorption, water solubility, and Fourier transform infrared (FTIR) spectroscopy of six commercially available thermoplastic retainer materials (Essix Plus, Zendura, Duran Plus, Tru-Tain DX, Imprelon S pd, and Essix ACE). Moreover, this study evaluated the effect of thermoforming and thermocycling on the water sorption and solubility and surface molecular composition of the tested materials. The present study found that the type of retainer material affected water sorption and solubility capabilities. In addition, the aging methods employed significantly affected some retainer materials’ water sorption and solubility. Moreover, the surface molecular composition evaluated by FTIR spectroscopy revealed that most of the evaluated materials had similar FTIR spectra except for Zendura. All materials had a spectrum that resembled polyethylene terephthalate glycol (PETG) while Zendura had a spectrum similar to semi-rigid polyurethane (PU). Only Zendura had relatively unstable surface structural composition evaluated under the effects of (thermoforming and thermocycling) compared to all tested materials.