# Influence of Food Pigments and Thermal Aging on the Color Stability of Denture Base Resins
## 4 Discussion
Furthermore, denture color stability can be achieved through proper cleaning. Mechanical cleaning is the most frequently used due to its convenience and reasonableness, but dentures can be cleaned chemically by immersing them in a chemical solution like sodium perborate that also exerts micromechanical cleaning action through oxygen bubbles [36]. All steps of the present laboratory study were performed by the same operator to minimize the introduction of bias. At each stage of the laboratory process, efforts were made to closely mimic the conditions of the oral cavity, thereby reducing the limitationsof an in vitro study when extrapolating the results obtained. As such, after specimen preparation, they were stored in distilled water at 37 \({}^{\circ}\)C for 24 h to simulate the rehydration process of the resins that occurs during the first day in the oral cavity [3; 8; 9; 20; 22; 27]. This step also helped reduce the content of residual monomer. To further recreate in vivo conditions, all specimens were subjected to thermal aging prior to immersion in pigments to mimic the temperature fluctuations the oral cavity experiences daily with the ingestion of different beverages and foods. This was achieved through thermocycling of the specimens for 1000 alternating cycles between water baths at 5 \({}^{\circ}\)C and 55 \({}^{\circ}\)C, simulating 1 month of denture use [37]. Thermal aging caused statistically significant color changes in the specimens, regardless of the resin type, which allows for the rejection of the second null hypothesis. All specimens exhibited significant chromatic changes after 1000 cycles of thermocycling. The results obtained align with the findings of [PERSON] et al. [2], [PERSON] [27], and [PERSON] et al. [17], whose studies revealed that thermal aging of acrylic resins through thermocycling induced significant color changes. Furthermore, other authors, such as [PERSON] et al. [13] and [PERSON] et al. [23], obtained similar results after thermally aging bis-acrylic resins or reline resins [29]. Thermocycling causes constant volumetric expansion and contraction, which results in thermal stress and, consequently, the degradation and chromatic alteration of materials [38]. Water plays a significant role in the chemical degradation of resins: adsorption and absorption of water molecules lead to their penetration into the spaces between polymer chains, causing separation and resulting in the formation of micro-fractures [2; 23; 39; 40]. Water acts as a plasticizer and promotes the hydrolytic degradation of the polymer and the deterioration of its structure. These micro-fractures allow for the penetration of extrinsic pigments, leading to chromatic alterations in the resin [25]. In addition to accelerating oxidation and hydrolysis processes [41; 42], thermocycling causes temperature fluctuations that result in dimensional changes and surface modifications in acrylic resins [35; 39]. [PERSON] [27] found that, after 5000 cycles of thermocycling, the color of self-cured resins was altered significantly compared to heat-cured resins. In the present study, although thermocycling significantly altered the color of all resin types, no statistically significant differences were observed between them, which may be related to the smaller number of cycles performed using the thermocycling equipment. However, the specimens of self-polymerized resin (Villacryl S V4) showed the highest chromatic alteration values after thermocycling (1.82 \(\pm\) 0.63) compared to the other resins, indicating a trend associated with the polymerization type. Comparisons between resins within each beverage category also did not reveal statistically significant differences among resins, although the results approached the significance level for specimens immersed in water (\(p\) = 0.051) and wine (\(p\) = 0.064). However, the trend for higher color changes in the Villacryl S V4 resin aligns with the literature, which reports significantly higher \(\Delta\)E values for self-cured resins compared to heat-cured resins [2; 4; 26; 27; 28].