## 4 Discussion  
The aim of the present study was to investigate the effects of thermal aging and chemical disinfection protocols on the microhardness and flexural strength of two flexible nylon-based resins used as a prosthetic base for removable dentures. In addition, the mechanical behavior of the two flexible resins exposed to physical or chemical environmental influences was compared with a commonly used PMMA-based resin. It was found that thermal aging only reduced the microhardness of the flexible resins, and the flexural strength of all resins was maintained. However, compared to the acrylic resins, the flexible resins exhibited lower flexural strength. Chemical disinfection with alkaline peroxides affected the mechanical properties of the resins, depending on their chemical composition. Regarding the effects of thermal cycling on the mechanical properties of the resins, a decrease in microhardness was observed in the flexible resins Deflex Classic SR and Supra SF after thermal aging, which is consistent with previous studies showing that water absorption can plasticize resins [13; 29; 30; 31; 32]. During the thermal aging process, the test specimens are immersed in water, which can act as a plasticizer that changes the dimensions and mechanical properties of the resins and reduces the forces between the polymer chains. Some authors claim that acrylic resins have lower water absorption compared to flexible resins and argue that water absorption is greater in polyamide resins due to the amide groups in the main polymer chain [33; 34; 35; 32]. This fact could explain the differences in flexible resins, because the more water molecules are absorbed, the more flexible the material becomes, and consequently the lower its microhardness [13; 36; 37; 38; 33]. In contrast, in the present study, the flexural strength values in all groups of specimens did not change after thermal aging. This is probably because only 1000 thermal aging cycles were performed, which corresponds to 6 weeks of biodegradation in the oral cavity and does not affect the core of the materials, which is responsible for flexural strength [30; 35]. These results allow us to partially reject the first hypothesis, which states that thermocycling has no effect on mechanical properties, since it was found that thermal aging affects the microhardness of flexible resins, but not their flexural strength. Regarding the second hypothesis, which refers to the influence of the type of resin tested, the ProBase Hot specimens showed higher flexural strength values than the flexible resins both before and after thermal aging. The results are consistent with other studies that have also found higher flexural strength values for conventional resins [10; 12; 38; 31; 32; 39; 40; 41; 42; 38; 42]. This can be attributed to the different composition of the resins, as conventional acrylic resins consist mainly of PMMA with a higher monomer/polymer ratio as well as a higher number of crosslinking agents, in contrast to the flexible resins, which not only have a lower number of these agents but also a higher fiber content [8; 38]. The two flexible resins tested had a microhardness of less than 15 KHN, a value recommended by the American Dental Association (ADA) in its Specification No. 12. On the other hand, they had a flexural strength of less than 65 MPa, a value mentioned in the specifications of the ISO standards as the limit for clinical acceptability. However, it should be noted that there are no standards for any of these properties for polyamide resins, so we cannot make direct comparisons with these reference values, which leads us to the need to establish standards that include this family of resins. Since no differences in microhardness were found when comparing the three resins, the second hypothesis can be partially rejected, as the type of resin influences the flexural strength of the resins but not the microhardness.