Influence of Food Pigments and Thermal Aging on the Color Stability of Denture Base Resins

Abstract

Color stability of acrylic resins is essential for preserving the aesthetic appearance of denture bases over time. This study explores how food pigments and thermal changes affect the color stability of commonly used denture base resins. Four acrylic resins were tested: three heat-cured acrylic resins with different characteristics (Zhermack^® Villacryl H Plus V2, H Plus V4, and H Rapid FN V4) and one self-cured acrylic resin (Zhermack^® Villacryl S V4). To simulate the oral environment, the resins underwent 1000 thermal cycles between 5 °C and 55 °C, followed by a 7-day immersion period in beverages such as coffee, red wine, a caramel-colored soft drink (cola), and distilled water (control), forming sixteen group of specimens (n = 5). Color changes (∆E) were measured using the VITA Easyshade V^® spectrophotometer, following the CIEDE2000 standard. The findings revealed that thermal aging caused noticeable color changes in all resins (p < 0.001). Red wine led to the most intense discoloration, followed by coffee. The caramel-colored soft drink caused moderate staining, while distilled water had a negligible effect. The type of polymerization did not affect the degree of discoloration, as no significant differences were found between the resins after exposure to beverages (p > 0.05). Overall, this study highlights how both internal and external factors impact the appearance of acrylic resins. Thermal aging can accelerate polymer degradation, while pigments in beverages cause visible staining. Among the tested beverages, red wine proved to be the most aggressive due to its high pigment concentration and low pH. These findings emphasize the need for improved material formulations to enhance the longevity and aesthetic performance of dentures.

1. Introduction

The loss of dental structures is a common reality today, prompting many patients to seek functional and aesthetic solutions from dental professionals. Among the various treatment options available, removable prostheses often emerge as the most practical solution for rehabilitating multiple missing teeth and surrounding tissues due to their affordability and straightforward fabrication process [1].

Acrylic resins have long been considered the gold standard for fabricating removable dentures [2,3]. These resins are primarily composed of poly(methyl methacrylate) (PMMA), a polymer that minimizes polymerization shrinkage while enhancing the mechanical strength of the final product. Depending on the type of the activator, acrylic resins can be heat-cured (heat activated), self-cured (chemically activated), or light-cured (light-activated), exposing different internal properties [4].

Most commercially available acrylic denture base resins used in conventional fabrication of total or partial dentures come in a powder–liquid system. The powder contains PMMA and a benzoyl peroxide initiator that triggers the polymerization process when mixed with the liquid. The liquid component includes methyl methacrylate (MMA) and stabilizers such as hydroquinone, which prolong the material’s working time [5].

Acrylic resins are favored due to their ease of manipulation, affordable cost, biocompatibility, and aesthetically pleasing results [3,4]. However, these benefits may be compromised over time as the material undergoes changes when exposed to the oral environment. Factors such as reduced elasticity, increased porosity, and water absorption can affect its structural stability, leading to fractures and deformation [2,6]. Additionally, prolonged contact with moisture, pigments from foods and beverages, and varying temperatures can cause significant color changes [7].

Color stability, defined as the ability of a material to maintain its original shade despite environmental exposure, is one of the most critical clinical properties of acrylic resins [2,8,9]. Color changes can be influenced by intrinsic factors, such as the resin’s natural aging due to physical and chemical reactions, and extrinsic factors, such as staining from food pigments, mouth rinses, and poor oral hygiene [10,11,12,13]. The accumulation of stains, artificial food dyes, and high surface roughness caused by inadequate polishing during manufacturing can further exacerbate discoloration [13,14,15].

It is essential to distinguish between discoloration caused by aging and pigments intentionally added for aesthetic customization. Acrylic resin powders often contain pigments like mercury sulfide, cadmium sulfide, cadmium selenide, and iron oxide, which provide a range of natural-looking shades. These pigments can be embedded in the material during production or added later by dental technicians using specialized paints to replicate the natural appearance of the gums. Zinc and titanium oxides are also used to enhance opacity [4].

Given that dental prostheses remain in continuous contact with oral fluids, food, and beverages, extended exposure increases the risk of color changes over time [12]. Numerous studies have investigated how different beverages [2,3,16,17,18,19,20,21,22,23,24,25,26], mouth rinses [1,2,27,28], and thermocycling procedures [2,13,17,23,27] affect the color stability of acrylic resins. To simulate real-life conditions in the laboratory, aging methods such as thermocycling, ultraviolet light exposure, and water storage have been employed. These techniques provide valuable insights into the long-term behavior of acrylic materials used in dental prosthetics [13].

In this context, the present study aims to evaluate the influence of food pigments present in beverages and thermal changes on the color stability of acrylic denture resins with different characteristics of the curing process and of the intrinsic pigments. To address these research questions, three hypotheses were formulated: that food pigments from beverages influence the color stability of acrylic resins, that thermocycling has a significant effect on color stability, and that the type of resin affects the overall color resistance of the materials. Investigating these factors will contribute to a deeper understanding of how environmental and material-related conditions impact the longevity and appearance of acrylic resins used in oral rehabilitation.

2. Materials and Methods

In the present study, PMMA-based resins, commonly employed in dental laboratory procedures, were selected according to the characteristics of the polymerization process (type of activator of the polymerization reaction: heat- or chemically activated polymerization; type of heat-activated curing cycle: slow or rapid) and of intrinsic pigments that provide differently colored heat-cured resins: light pink (V2) or dark pink (V4). This selection was made to ensure a comprehensive analysis of the physical properties relevant to their clinical performance.

Specifically, three heat-polymerizing Villacryl resins H Plus V2, H Plus V4, H Rapid FN V4) and one self-polymerizing Villacryl resin (S V4) were used, all from the Zhermack^® brand and based on PMMA (Figure 1). The resins’ characteristics are provided in

Table 1. Characteristics of the acrylic resins used in the study.

Characteristics	Villacryl H Plus V2 and V4	Villacryl H Rapid FN V4	Villacryl S V4
Powder/Liquid Ratio (g/mL)	24/10.5	22/10	10/5.3
Setting Time at 23 °C (min)	20–25	8–10	-
Working Time (min)	25–30	20	8
Curing cycle (min)	60 °C → 100 °C (30 min), 100 °C (30 min), cool to 30 °C (30 min)	80 °C → 100 °C (10 min), 100 °C (20 min), cool to 30 °C (15 min)	50–60 °C (15 min, 2 bar)
Flexural Strength	>65 MPa	>65 MPa	>60 MPa
Solubility	0.8 µg/mm3 (<1.6 µg/mm3)	0.8 µg/mm3 (<1.6 µg/mm3)	0.7 µg/mm3 (<8 µg/mm3)
Water Absorption	19.1 µg/mm3 (<32 µg/mm3)	18.7 µg/mm3 (<32 µg/mm3)	21.8 µg/mm3 (<32 µg/mm3)
Colours	V4 Pink-veined and V2 Milk Pink-veined	V4 Pink-veined	V4 Pink-veined

.

2.1. Preparation of Specimens

A total of 80 specimens were prepared—20 from each acrylic resin, with diameters of 12 mm and a thicknesses of 2 mm, following the ADA Specification No. 17. The specimens were created using 2 mm pink wax sheets (Metrowax No. 2, Metrodent, Huddersfield, UK). A caliper (Porcelain Sampler No. 7015, Smile Line, St-Imier, Switzerland) was used to shape the specimens into circular forms. One surface of each specimen was marked to identify the side where chromatic measurement would be performed (Figure 2).

Next, a flask was lubricated with petroleum jelly and filled with Type II gypsum (Pro-solid, Saint-Gobain Formula, Newark, UK). After waiting for 1 min, the wax patterns were positioned on the gypsum and gently pressed to ensure that only their marked surfaces remained visible (Figure 3a). Once the setting reaction was complete, the flask was closed, and the counter-flask was filled with type III gypsum (Pro-solid Super, Saint-Gobain Formula, UK). After the type III gypsum had set, the flask was placed in a wax elimination machine (Mestra, Biskaia, Spain) at 80 °C for 10 min. Subsequently, the flask was opened, wax residues were removed, and two layers of separating medium (Ivoclar Vivadent, Schaan, Liechtenstein) were applied to both sides of the flask (Figure 3b).

The preparation of acrylic resins was carried out according to the manufacturer’s instructions (Table 1).

The acrylic mixture of each resin was placed in a sealed container (Figure 4a) and left to rest until ready for use, with periodic checks. Next, the mixture was kneaded and applied onto the flask (Figure 4b). The flask was closed and placed in a hydromatic press HMP 1251-4 (Reco Dental, Wiesbaden, Germany^®) at a pressure of 5000 kg for 15 min (Figure 4c).

Subsequently, the flask was clamped and immersed in preheated water of a polymerization unit (Reco Dental, Wiesbaden, Germany) (Figure 4d), then heated until the temperature, time, and pressure recommended by the manufacturer were reached (Table 1). Finally, the flask was removed and cooled to room temperature, and the specimens were carefully extracted.

After obtaining all the specimens, the excess material was removed using a tungsten carbide bur mounted on a handpiece (Figure 5a,b). The sides of the specimens were then polished with 500-grit and 1000-grit sandpaper using a manual polishing machine Lunn Major (Struers, Ballerup, Denmark) under constant water cooling (Figure 5c). The correct thickness was confirmed with a digital micrometer (Fischer Darex, Le Chambon Feugerolles, France).

All specimens were polished by the same operator and within the same laboratory timeframe to minimize bias. The polishing process began with pumice (SPD, Lisbon, Lot No. 00185/17) and a polishing brush No. 48 (Bredent, Senden, Germany) mounted on a polishing machine Kavo EWL (Kavo Dental, Biberach, Germany) at a speed of 1500 rpm for 15 s. Subsequently, the specimens were cleaned with distilled water, and the polishing was completed using a brush (Dentaurum, Ispringen, Germany) and polishing paste Universal Polishing Paste (Ivoclar Vivadent, Schaan, Liechtenstein) applied with a low-speed handpiece for 10 s per specimen (Figure 6).

To remove pumice and polishing paste residues, the specimens were placed in distilled water and immersed in an ultrasonic bath for 3 min, followed by air drying. Finally, all specimens were stored in distilled water at 37 °C for 24 h.

2.2. Color Measurement

After polishing, the specimens were stored in individual compartments, each assigned a unique number (Figure 7a). The color of the specimens was recorded using the VITA Easyshade V^® (VITA, Bad Säckingen, Germany) in restoration mode, which registers values in the L (lightness), C (chroma), and h (huge) coordinates. Measurements were consistently taken from the same marked surface of each specimen, with three measurements performed to obtain an average value. The device was calibrated, and its tip was cleaned after every six measurements.

Measurements were conducted over a pink cardboard background to simulate mucosal color, and a black PVC device was used to neutralize ambient light during the measurement process (Figure 7b). This device was placed over each specimen, and the VITA Easyshade V^® tip was inserted through its opening to ensure that consistent readings were taken from the specimen’s center (Figure 7c,d). The pink cardboard color was defined as the baseline for all measurements.

The recorded values indicated the chromatic variation between the acrylic resins and the baseline color. The L, C, and h values were registered for each measurement, and the ΔE was calculated using the CIEDE2000 system, with parametric factors set to 1.

Throughout this study, three chromatic measurements were performed: an initial measurement after specimen preparation (T0), a measurement after thermocycling (T1), and a final measurement after 7 days of immersion in pigment-containing beverages (T2).

The ∆E values were then converted to National Bureau of Standards (NBS) as ∆E* × 0.92, which expresses the color differences from a clinical perspective as extremely slight change (NBS < 0.5), slight change (0.5 < NBS < 1.5), perceivable change (1.5 < NBS < 3), appreciable change (3 < NBS < 6), much appreciable change (6 < NBS < 12), and change to another color (NBS > 12) [29].

2.3. Thermal Aging

The specimens were sorted into mesh bags of different colors and properly labeled using colored rubber bands (Figure 8a). All specimens underwent thermal aging through 1000 alternating cycles in distilled water baths at temperatures ranging from 5 °C to 55 °C, with a 20-s immersion in each bath, using a thermocycling machine Refri 200-E ( Aralab, Cascais, Portugal) (Figure 8b).

2.4. Immersion in Beverages

Specimens from the four acrylic resins were placed back into their respective individual boxes and divided into four subgroups (n = 5):

• Subgroup C—specimens immersed in instant coffee prepared according to standard brewing instructions (12 g of ground coffee dissolved in 200 mL of hot water) (Figure 9a), containing caffeine in a concentration of approximately 35 mg /100 mL.
• Subgroup V—specimens immersed in red wine from a commercially available brand (Figure 9b), containing tannins (anthocyanins and derivates) in a concentration of approximately 54.4 mg/100 mL.
• Subgroup CC—specimens immersed in a carbonated cola-flavored beverage (Figure 9c), containing caffeine in a concentration of approximately 10 mg per 100 mL.
• Subgroup A—specimens immersed in distilled water (control group).

All specimens were immersed in 4 mL of their respective beverages (Figure 10a,b) at room temperature for a period of 7 days (168 h). The beverages were replaced daily at the same time. During this period, the specimens were stored in an oven at 37 °C and wrapped in aluminum foil to minimize light interference (Figure 10c).

After the 7-day period, the specimens were rinsed under running distilled water for 5 s and wiped with paper sheets to remove pigment residues. A new chromatic reading was then performed for each specimen. The experimental protocol is outlined in Figure 11. Following the final chromatic reading, the color changes of a randomly selected specimen from each group were photographically recorded (Figure 12).

2.5. Statistical Analysis

The statistical analysis and graphical representations were performed using SPSS software version 25 (IBM, Armonk, NY, USA).

The descriptive analysis included the calculation of means ($\overline{x}$) and standard deviations (σ) of ΔE by resin and beverage categories, corresponding to chromatic differences caused by thermocycling (T1–T0) and  pigment immersion (T2–T1).

After assessing normality and homogeneity of variance (Shapiro–Wilk and Levene tests), p < 0.05 non-parametric tests were applied. The Wilcoxon signed-rank test for a single sample was used to evaluate whether thermocycling caused changes in the initial color of the resins (H0: T1 − T0 = 0). The Kruskal–Wallis test was employed to assess ΔE differences between resin and beverage categories. Subsequent multiple comparisons with Bonferroni correction were automatically generated by the software. The significance level used throughout the analysis was set at 0.05.

3. Results

Comparisons of ΔE among resins after thermocycling (between T0 and T1) are summarized in

Table 2. Comparisons of ΔE (T1–T0) between Villacryl resin categories after thermocycling ($\overline{x}$: sample mean, s: sample standard deviation. Wilcoxon signed-rank test for a single sample (H0: T1 − T0 = 0): p < 0.001).

Villacryl Resin	ΔE (T1–T0) $\overline{\mathit{x}}$ (s)	p
H Plus V2	1.64 (0.3)	0.376
H Plus V4	1.74 (0.39)
H Rapid V4	1.56 (0.31)
S V4	1.82 (0.63)

and Figure 13. Regarding the effect of thermocycling on the color changes of specimens, statistically significant color differences were observed regardless of the resin type (p < 0.001), as shown in Table 2. However, comparisons among resins did not reveal statistically significant differences (p = 0.376) (Figure 13).

The color changes resulting from the subsequent immersion of the specimens in different beverages can be observed in detail in

Table 3. Comparisons of ΔE (T2–T1) between Villacryl resin and beverage categories after immersion in the beverages ($\overline{x}$: sample mean, s: sample standard deviation. C: coffee, W: wine, CC: carbonated soft drink, A: water).

ΔE (T2–T1) $\overline{\mathit{x}}$ (s)
Beverage	Plus V2	Plus V4	Rapid V4	S V4	p
Coffee (C)	1.08 (0.66)	1.19 (0.52)	1.51 (0.57)	1.21 (0.34)	0.493
Red wine (W)	2.54 (0.69)	2.98 (0.4)	2.18 (0.31)	2.11 (0.47)	0.064
Carbonated soft drink (CC)	0.61 (0.24)	1.02 (0.41)	0.61 (0.54)	0.66 (0.18)	0.187
Water (A)	0.36 (0.22)	0.93 (0.48)	0.35 (0.28)	0.64 (0.25)	0.051
p	0.004	0.013	0.002	0.002
Multiple comparisons	A-W	A-W	A-W, CC-W	A-W, CC-W

and Figure 14. In Table 3, the multiple comparisons (MCs) row includes only the pairs of categories with statistically significant differences. It was observed that comparisons between beverages showed statistically significant differences, regardless of the resin type (p < 0.05).

Regarding the multiple comparisons between beverages, a similar trend was observed across the various resin groups. Immersion in red wine caused significantly greater color changes compared to water (p < 0.05) for all resins and compared to the carbonated soft drink (p < 0.05) for the Rapid V4 and S V4 resins. No statistically significant differences were observed between coffee and red wine. However, specimens immersed in red wine presented the highest ΔE values, regardless of the resin type.

With respect to comparisons between resins within each beverage category, no statistically significant differences were observed, although the results approached the significance level for specimens immersed in water (p = 0.051) and red wine (p = 0.064). In both red wine-immersed and water-immersed specimens, the Plus V4 resin recorded the highest color changes (2.98 ± 0.4 and 0.93 ± 0.48, respectively).

Table 4. NBS units and color changes according to resin category after thermocycling (T0–T1).

Villacryl Resin	NBS Units	Color Changes
Plus V2	1.51	perceivable
Plus V4	1.6	perceivable
Rapid V4	1.44	Slight
S V4	1.67	perceivable

presents the NBS units corresponding to the color differences observed after thermocycling (between T0 and T1). All specimens exhibited perceivable differences, except for the specimens of the Rapid V4 resin, where the color difference was classified as “slight”.

The NBS units corresponding to color changes after immersion in beverages (between T1 and T2) and their critical points are presented in

Table 5. NBS units and color changes in specimens according to beverage and resin category after 7 days of immersion in the beverages (T1–T2).

Beverage	Plus V2 (NBS Units)	Plus V2 (Color Change)	Plus V4 (NBS Units)	Plus V4 (Color Change)	Rapid V4 (NBS Units)	Rapid V4 (Color Change)	S V4 (NBS Units)	S V4 (Color Change)
Coffee	0.99	Slight	1.09	Slight	1.39	Slight	1.11	Slight
Red wine	2.34	Perceivable	2.74	Perceivable	2.01	Perceivable	1.94	Perceivable
Carbonated soft drink	0.56	Slight	0.94	Slight	0.56	Slight	0.61	Slight
Water	0.33	Extremely Slight	0.86	Slight	0.32	Extremely Slight	0.59	Slight

. It was observed that all specimens immersed in wine exhibited “perceivable” color changes, regardless of the resin type. Some groups showed “extremely slight” color changes, specifically the specimens immersed in water from the Plus V2 and Rapid V4 resins. The remaining specimens exhibited “slight” color changes.

4. Discussion

The present laboratory study aimed to evaluate the color change effects of immersion in food pigments such as coffee, red wine, and a carbonated soft drink on three different heat-polymerized Villacryl acrylic resins (Villacryl H Plus V2, V4, and H Rapid FN V4) and one self-polymerized Villacryl acrylic resin (Villacryl S V4), as well as to assess the influence of thermal aging on these resins. It was concluded that thermal aging and red wine significantly affected all resins, and that that effect did not depend on the type of resin studied.

Acrylic resins are frequently used in dentistry, particularly as a base for dentures, due to their numerous advantages [26]. However, after their insertion into the oral cavity, a process of biodegradation begins in their structure, leading to color changes that are aesthetically unsatisfactory and a reflection of unrealistic aging. These changes are caused by extrinsic factors, particularly the pigments present in food and beverages that discolor the resin through adsorption or absorption phenomena [11,12]. After penetrating, these pigments often cannot be removed through cleaning or polishing, potentially necessitating a denture replacement, which increases cost and patient dissatisfaction [24].

To minimize the color instability of acrylic resins, it is important to understand which pigments are present in the daily diets of patients that may alter the color of these materials. Coffee, red wine, and carbonated soft drinks were selected for the present study, since they showed a high consumption frequency among the general population, as well as due to their high pigmentation capacity.

To mimic the oral cavity, the specimens were stored at a temperature of 37 °C and wrapped in aluminum foil to minimize the influence of natural light. The beverages were replaced daily at the same time to avoid pigment concentration resulting from water evaporation.

The type of beverage, its specific pigments and concentration, and the frequency of ingestion determine the amount of pigment and the level of resin pigmentation [17,23].

It is estimated that each person consumes an average of 3.2 cups of coffee per day, with each cup taking approximately 15 min to drink. Thus, the 7 days of immersion in the present study was aimed to simulate 7 months of prosthesis use [12].

We currently have devices available that help us quantify color changes in dental materials. The Vita Easyshade equipment was selected for being a cordless, small, portable and contact-type spectrophotometer that is able to provide enough shade information for the color analysis process and is usually used for research on clinical ambience [30,31]. These devices are based on the CIE Lab* system or the CIEDE2000 system and allow for the calculation of the color difference (ΔE) between the measured object and a baseline value using the coordinates L, a, and b or L, C, and h, respectively. For this study, the ΔE value was obtained using the CIEDE2000 system, as it quantifies coordinates that are critical for the visual perception of an object, such as chroma and hue. The Vita Easyshade mode for restoration color verification operates in L*C*h, while tooth color measurement operates in L*a*b*. The L*C*h color space correlates well with how the human eye perceives color and has the same diagram as the L*a*b* color space, but uses cylindrical coordinates instead of rectangular coordinates. Nevertheless, L*C*h values can easily be converted to L*a*b* values using mathematical formulas [30,31,32]. A high ΔE value indicates a significant change in the chromatic properties of the resin, which could lead to patient dissatisfaction. Several studies indicate that ΔE values greater than 3.3 are not clinically acceptable [3,7,8,22,33].

All specimens in this study demonstrated statistically significant changes after 7 days of immersion in the beverages, which allows us to reject the first null hypothesis. Multiple comparisons revealed a similar trend across the various resin groups. Immersion of the specimens in red wine caused significantly higher changes compared to water (p < 0.05) for all resins and compared to the carbonated soft drink (p < 0.05) in the Rapid FN V4 and S V4 resins.

Wine was the beverage that caused the greatest color changes, regardless of the resin type, was as has been presented in previous studies [2,8,12,20,23,24].

Red wine contains darker pigments called chromogens (anthocyanins and derivatives) that adhere to the surface of dentures and promote discoloration over time [2]. Also have polyphenols, called tannins, that help the pigments bind to the proteins present in saliva, forming a tannin–protein complex that precipitates on the surface of resins and enamel, altering their color. Another aspect that favors the discoloration is that red wine is acidic, which makes the denture more porous, thus capturing more staining. Additionally, the presence of ethanol softens the resin surfaces, making them more susceptible to pigmentation [32].

The surface adsorption and absorption of pigments by the organic phases of acrylic resins can lead to alterations in their color [12,24]. The fact that they are hydrophilic materials makes them more prone to absorbing pigments from beverages with which they come into contact. Some studies have shown that caramel, tannin acid, caffeine, and caffeic acid, which are present in coffee, may be responsible for the pigmentation shown in the present study [8,13,17,20,21,23,25,26]. Coffee seems to maximally stain the resins due to tannic acid solutions allowing the yellow/brownish colorant in caffeine to be compatible with the polymer phase [24]. Silva et al. [13] found that after 7 days of immersion in coffee, there was a reduction in luminosity and an increase in hue for the resins studied.

Regarding the carbonated soft drink, in addition to being an acidic beverage, it contains phosphoric acid, which can affect the surface integrity of the material through erosion [22]. According to Al-Noori [16], its low pH increases the solubility of the resin, and as immersion time increases, water absorption becomes the main cause of discoloration. This explains the lower color change values of specimens immersed in this beverage compared to the others, which has also been seen in previous studies [2,8,13,17,22,26]

It is important to highlight that the control group also demonstrated color changes. As mentioned earlier, acrylic resins are hydrophilic materials, making water absorption the primary cause of the observed chromatic changes. The same finding was reported in the studies by Rutkunas et al. [24] and Goiato et al. [2].

For the present study, ΔE values greater than 3.3 were considered clinically unacceptable [3,7,8,22,34], and ΔE values between 1 and 3 were considered perceptible to the human eye [3,35]. All specimens exhibited clinically acceptable values (ΔE < 3.3). At T1, all color changes were perceptible to the human eye (ΔE > 1), whereas at T2, only the color changes of specimens immersed in coffee and red wine were perceptible, regardless of resin type. The NBS units demonstrated that no specimen exhibited color changes above the “perceptible” level (between 1.5 and 3.0), meaning that, in a clinical scenario, these changes would be difficult for the patient to detect, as they would require a color alteration across the entire prosthesis surface. According to Hong et al. [28], patients cannot detect chromatic changes below the “slight” level, that is, below 6 NBS units. Thus, the chromatic stability of the resins under study, after immersion in pigments from the beverages, was satisfactory, as it fell within clinically acceptable values and would hardly be detected by patients.

Since pigments do not bind to the internal composition of resins; diffusion and degradation of the pigments can occur in the acrylic resin material. 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 limitations of an in vitro study when extrapolating the results obtained. As such, after specimen preparation, they were stored in distilled water at 37 °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 °C and 55 °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 Goiato et al. [2], Abou-Obaid [27], and Altinci et al. [17], whose studies revealed that thermal aging of acrylic resins through thermocycling induced significant color changes. Furthermore, other authors, such as Silva et al. [13] and Macedo 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].

Abou-Obaid [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 ± 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 ΔE values for self-cured resins compared to heat-cured resins [2,4,26,27,28]. 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-toluidine), 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. Goiato 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. The type, amount, and temperature of the beverage, as well as the type and concentration of pigment present, influence resin pigmentation. All these variables were kept constant throughout this study, unlike what occurs in the oral cavity.

Several studies have shown that the longer the immersion time in a pigment, the greater the observed color changes [16,19,22]. Additionally, only one of the many brands of acrylic resins available on the market was evaluated. Therefore, further studies are needed to evaluate a greater diversity of resins with longer immersion times while also considering other important factors, such as daily prosthesis brushing.

In addition to color stability, internal morphologic and chemical composition (using FTIR and Raman analysis), as well as surface roughness studies of specimens exposed to beverages, should be considered in future research to understand the effect of pigments on the internal and external surface of resins with different chemical compositions and polymer arrangements.

5. Conclusions

Based on the findings of this study, it can be concluded that the color stability of denture acrylic resins is influenced by external factors such as beverage pigments and thermal variations, rather than by the type of resin used. Among the beverages tested, red wine caused the most pronounced chromatic changes due to its acidic pH, dark pigment, and ethanol content, which increased the resin’s susceptibility to discoloration. Coffee and the carbonated soft drink also caused chromatic changes, albeit to a lower extent, while water, used as a control, caused minimal alterations due to the hydrophilic nature of the resins.

Thermal aging through thermocycling emerged as a significant factor affecting color stability in the present study. The simulation of one month of clinical use with 1000 temperature-alternating cycles probably led to microfractures, polymer matrix degradation, and increased water absorption, all of which contributed to pigment penetration and discoloration. This underscores the importance of considering thermal stress when evaluating the long-term performance of acrylic resins.

The results showed no statistically significant differences between resin types after thermocycling and beverage immersion. Although the self-cured resin S V4 demonstrated a tendency for greater chromatic changes due to its higher residual monomer content and increased surface porosity, this trend did not reach statistical significance. The intrinsic pigmentation of the resins may have played a protective role, reducing pigment absorption and contributing to the absence of significant differences.

Despite the inherent limitations of an in vitro study, including the absence of factors like brushing, abrasion, and enzyme activity, this research highlights the key mechanisms of chromatic instability in acrylic resins. Future studies should explore longer immersion periods, additional resin brands, and more realistic conditions to provide a more comprehensive understanding of the long-term performance of dental prostheses in clinical settings.