# Influence of Food Pigments and Thermal Aging on the Color Stability of Denture Base Resins
## 4 Discussion
Self-cured resins contain a higher amount of residual monomer, which acts as a plasticizer and makes the surface more porous and rough, thereby decreasing chromatic stability. Also, the heat-cured acrylic presents a more compact resin due to their higher polymerization rate and the presence of cross linking agents. Water absorption in these resins is greater than in heat-cured resins, making them more susceptible to pigmentation. Additionally, their composition includes an activator, tertiary amine (N,N-dimethyl-p-toludiine), whose oxidation leads to the production of chromatic components [5]. It is extremely important to note that, after specimen preparation, bubbles were observed in the specimens of the S V4 resin. Bubble formation resulting from chemical activation is one of the major disadvantages of self-cured resins, which may explain why chromatic changes were greater in this resin. The presence of bubbles makes the resin surface more porous and susceptible to chromatic alterations. However, in the present study, no statistically significant differences were found among the resins, and thus, the third null hypothesis cannot be rejected. All resins studied contained intrinsic nanoparticulate pigments designed to mimic gingival tissue. [PERSON] et al. [9] found that the addition of 7% pigment (organic particles) improves the chromatic stability of acrylic resins: increasing the organic content results in the formation of stronger cross-links, which reduce water absorption and, consequently, diminish chromatic changes in acrylic resin. Therefore, the intrinsic pigmentation of the resins in this study may explain why no statistically significant differences were observed among them, as well as why their chromatic changes remained within clinically acceptable values. This study demonstrated the influence of contact with pigments from daily dietary intake, thermal aging, and the polymerization type of the resins on their chromatic stability. Thermocycling acted as an intrinsic factor in material discoloration, causing changes in the polymer matrix when subjected to different chemical and physical conditions. The absorption and adsorption of pigments from beverages served as extrinsic factors responsible for the chromatic changes observed  
It is important to highlight that there are other factors, either isolated or combined, that may also be responsible for the chromatic instability of resins, such as poor prosthesis hygiene, the use of chemical products or mouthwashes [1; 2; 27; 29], material porosity and roughness, dehydration, oxidation, infiltration, and the formation of pigments due to material degradation [2; 14; 17; 29; 39]. Although all necessary procedures were performed to closely mimic the conditions of the oral cavity, in vitro studies inherently present some limitations. Color evaluation can be very sensible to extrinsic factors, such as room light or the equipment inclination, which can easily cause dispersion of the results. In spite of various studies which have used similar numbers of specimens per group [31], this should be increased to a sample size that can provide strong power to the results. This adequate sample size should be calculated in future research. Factors such as prosthesis brushing, pigment dilution in saliva [22], abrasion resulting from use [3], the wide variety of beverages (tea) and foods ingested (red berries, turmeric, tomato sauce, etc. ), the influence of enzymes and proteins present in saliva, pH changes in saliva [29; 38], and functional and parafunctional forces [24] were not considered in this study, but they significantly impact the color stability of dental prostheses. Although the 7 days of immersion in beverages was intended to simulate 7 months of prosthesis use, it is important to emphasize that this is merely an estimate and that habits differ between populations. For this study, the habits of the Portuguese population were considered, where coffee is consumed in smaller quantities but at higher concentrations.