## 4 Discussion
As already mentioned in the results of [PERSON] et al., the study instead analyzed the physical and chemical properties of five transparent thermoplastic materials. Most of them are amorphous polymers, such that the transparency of the materials decreases when their structure changes from amorphous to crystalline [12]. We can therefore divide these materials into three main groups. The first group is based on PETG or PCT, comprising ST (PCT), Ace (PETG), and DX30 (PETG). The second group consists of ST (TPU) and Zendura (TPU). Finally, the third group consists of C+ (PP-EPR). PETG and PCT normally exhibit excellent clarity as they have an amorphous structure (Figure 4); this aspect is important to guarantee an acceptable aesthetic result for the patient [12]. As for the flexion modules, Zendura is a TPU material without any soft segments. The flexural modulus of Ace is  
Figure 3: Digital orthodontic workflow of 3D-printed aligners. higher than that of DX30. However, as mentioned in the results, the flexural modulus of ST was between Ace and DX30 [12]. Normally, we would prefer a material with high and stable moduli. This means that it can provide a high enough force to the teeth even with a thinner, more invisible aligner. However, the high stiffness could cause problems when a patient transitions to a new set of aligners (the aligner may not fit as well) [12]. A new material (TC-85) was then compared with the PETG materials in the [PERSON] et al. study for the analysis of thermo-mechanical and viscoelastic properties. It was found that conventional thermoplastics show thickness changes after thermoforming depending on size and shape [10]. In clinical practice, since the clear aligners are produced using dental models with variability in anatomical structures, irregular thicknesses can occur. Since the thickness of the aligners affects the orthodontic force applied to the teeth, the uneven thickness of clear aligners makes it difficult for clinicians to predict performance and treatment outcomes [10; 36]. The yield strength and elastic modulus were higher in PETG than in TC-85, while the elastic range was wider in TC-85 than in PETG. Thanks to the greater flexibility and the wider elastic range of the TC-85 aligners, it is possible to perform more tooth movements without permanent deformations [10; 37]. Furthermore, TC-85 showed a slight stress decay, while it was faster for PETG [3]. Thanks to the shape memory property (bending test) of the TC-85, the aligners can consistently apply orthodontic forces to the teeth under normal temperature conditions without force decay caused by deformation. However, further studies such as voltage testing are needed in the future [10; 38]. In the [PERSON] et al. study, only one type of thermoplastic material, PETG, was compared as a control. Therefore, it is difficult to conclude that TC-85 is superior to all conventional materials [10]. [PERSON] highlighted material thickness reduction and geometry modification following thermoforming by altering an aligner's ability to transmit controlled forces and moments onto a tooth and decreasing mechanical properties. This decrease can be attributed to partial changes from the amorphous to the crystalline structure of the material during thermoforming, where the crystalline phase could affect the elastic properties of the material. Both PETG and PU materials were tested [11]. Moreover, it was then possible to observe a decrease in the Young's modulus for all thermoformed materials [11]. [PERSON] et al. showed a change in mechanical properties after heat treatment. Taking into account that the luminaires are heat-molded to obtain the desired shape, the order proposed in this study to use these marks is C+, ACE 035, Crystal 0.75, and Crystal 1.0, suggesting that manufacturers should indicate these values after thermal processing [17; 18]. Figure 4: (**a**) Polymer chain arrangement in amorphous and semi-crystalline polymers. (**b**) The microscopic mechanism of gas dissolution and diffusion in an amorphous polymer.