## 4 Discussion
### Water sorption and Solubility  
Essix Plus had the lowest water sorption, and Zendura had the highest water sorption in all three groups. In a previous study, Zendura as received sheets were found to have the highest water sorption value of (15 ug/mm\({}^{3}\)) (after one week of immersion in water at 37 \({}^{\circ}\)C) compared to three different aligner materials (Erkoadur, Ghost Aligner, Essix Plastic) [26], while in the current study, it was found to be (15.61 ug/mm\({}^{3}\)) on average. The effect of thermoforming and both thermoforming and subsequent thermocycling on water sorption was only significant in Zendura. The water sorption of Zendura had substantially increased after thermoforming. However, thermocycling significantly decreased the water sorption of Zendura when compared to the water sorption of thermoprofmed specimens. The results reported by [PERSON] et al. [27] show that the water absorption capacity increased for all four types of aligner materials after thermoforming compared to before thermoforming. Moreover, [PERSON] et al. [28] reported that thermocycled specimens (1000 cycles) absorbed 48% more water compared to their immersed counterparts. Whereas our experimental data showed a negligible effect of thermocycling on the water sorption behavior. This could be due to the high number of cycles used in this study (10,000 cycles) prior to water sorption evaluation which probably consumed the material's potential to absorb more water. In the current study, the water sorption of thermoplastic retainer materials before thermoforming (Group 1) was significantly different between all the materials except Essix ACE and Tru-Tain DX. Moreover, in Group 2 and Group 3, there was a significant difference in water sorption between all thermoplastic retainer materials except between Imprelon S pd and Essix Plus and between Essix ACE and Tru-Tain DX. Thus, we rejected the first, second, and third null hypotheses for water sorption. We failed to reject the fourth and fifth null hypotheses for all materials except for Zendura in terms of water sorption. The negative results of water solubility may indicate that thermoplastic retainer materials absorb more water, leading to an increased weight that masks their true solubility. Before and after thermoforming, Zendura had the lowest water solubility, and Essix Plus showed the highest water solubility. After thermoforming and subsequent thermocycling, Zendura had the highest water solubility, and Essix Plus had the lowest water solubility. Zendura was the only material that showed a statistically significant difference in water solubility between the three groups. As the effects of aging intensified, water solubility increased in Zendura. [PERSON] et al. [26] found that the water solubility of Zendura as received sheets after 14 days of immersion in water at 37 \({}^{\circ}\)C was (\(-\)0.6 ug/mm\({}^{3}\)) [26], whilein the current study, it was found to be (\(-3\)\(\upmu\)g/mm\({}^{3}\)) (after one week of water immersion at the same temperature). In this work, water solubility decreased slightly (statistically not significant) after thermoforming in Imprelon S pd, Tru-Tain DX, Essix ACE, and Essix Plus compared to before thermoforming. Whereas [PERSON] at al. [27] have reported that the water solubility was significantly greater after thermoforming than before thermoforming for most of their tested aligner materials. This could be because of variations in the experiment methods adopted or the materials used. Thermocycling significantly increased the water solubility for all the materials except Essix Plus when compared to thermoforming effects only. When we compared the water solubility of thermocycled specimens to raw as received specimens, it increased significantly for all the materials except Essix Plus and Tru-Tain DX. There was a significant difference in water solubility between the different types of retainer materials among the three groups. Thus, we rejected the first, second, and third null hypotheses regarding water solubility.