# Influence of Food Pigments and Thermal Aging on the Color Stability of Denture Base Resins
## 2 Materials and Methods
### 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 (Metrowas 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 \({}^{\circ}\)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). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Characteristics**} & \multicolumn{1}{c}{**Villacryl H Plus V2 and V4**} & \multicolumn{1}{c}{**Villacryl H Rapid FN V4**} & \multicolumn{1}{c}{**Villacryl S V4**} \\ \hline Powder/Liquid Ratio (g/mL) & 24/10.5 & 22/10 & 10/5.3 \\ \hline Setting Time at 23 \({}^{\circ}\)C (min) & 20–25 & 8–10 & - \\ \hline Working Time (min) & 25–30 & 20 & 8 \\ \hline \multirow{2}{*}{Curing cycle (min)} & 60 \({}^{\circ}\)C \(\rightarrow\) 100 \({}^{\circ}\)C (30 min), 100 \({}^{\circ}\)C & 80 \({}^{\circ}\)C \(\rightarrow\) 100 \({}^{\circ}\)C (10 min), 100 \({}^{\circ}\)C & 50–60 \({}^{\circ}\)C (15 min, 2 bar) \\  & (30 min), cool to 30 \({}^{\circ}\)C (30 min) & (20 min), cool to 30 \({}^{\circ}\)C (15 min) & \\ \hline Flexural Strength & \(>\)65 MPa & \(>\)65 MPa & \(>\)60 MPa \\ \hline Solubility & 0.8 \(\upmu\)g/mm\({}^{3}\) (\(<\)1.6 \(\upmu\)g/mm\({}^{3}\)) & 0.8 \(\upmu\)g/mm\({}^{3}\) (\(<\)1.6 \(\upmu\)g/mm\({}^{3}\)) & 0.7 \(\upmu\)g/mm\({}^{3}\) (\(<\)8 \(\upmu\)g/mm\({}^{3}\)) \\ \hline Water Absorption & 19.1 \(\upmu\)g/mm\({}^{3}\) (\(<\)32 \(\upmu\)g/mm\({}^{3}\)) & 18.7 \(\upmu\)g/mm\({}^{3}\) (\(<\)32 \(\upmu\)g/mm\({}^{3}\)) & 21.8 \(\upmu\)g/mm\({}^{3}\) (\(<\)32 \(\upmu\)g/mm\({}^{3}\)) \\ \hline \multirow{2}{*}{Colours} & V4 Pink-veined and V2 Milk & \multirow{2}{*}{V4 Pink-veined} & \multirow{2}{*}{V4 Pink-veined} \\  & Pink-veined & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the acrylic resins used in the study. Figure 1: Zhermack® acrylic resins. (**a**) Villacryl H Plus V4, (**b**) Villacryl H Rapid FN, and (**c**) Villacryl S.  
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(r)) at a pressure of 5000 kg for 15 min (Figure 4c). Figure 4: (**a**) Acrylic mass in the sealed container; (**b**) acrylic mass on the flask; (**c**) flask in the press under 5000 kg pressure; (**d**) flask in the polymerization unit. Figure 3: (**a**) Wax patterns on type III gypsum; (**b**) application of the separating medium on the flask after removal of the wax patterns. Figure 2: Wax mold fabrication: (**a**,**b**) pressing the calibrator onto the wax sheet; (**c**) calibrator and wax mold; (**d**) wax molds marked on one side.