## 4 Discussion
The SFE is calculated based on water and DIM contact angle measurements. Our SFE measured data were between 45.65 and 49.07 mJ/m\({}^{2}\) regardless of surface treatment. The SFE of the tested composite were very close to each other and significantly did not differ from each other, which is in good agreement with the previous result [23; 27; 28]. The range of change was small among the composite and due to the surface treatment. Numerically, the resin-based composite can be considered a low SFE material [30]. Since the SFE is a calculated number from water and DIM contact angle measurements, it can be divided into two parts: the polar part that represents the behaviors of the surface against water and the dispersed part that has a relationship with DIM contact angles. The dispersed parts accounted for a larger part of SFE of all tested composites, similarly in good agreement with previous results. The high dispersive forces with low polar forces indicated the strong hydrophobicity of the tested composite meaning that polar liquids cannot uniformly spread and wet surfaces until the apolar liquid has good wetting ability on such surfaces [31]. In the aspect of surface free energy, the Ba-Al borosilicate and silicon dioxide filler particles are high surface energy particles; when they get close to the surface, they reduce the contact angle [29]. The wettability of dental adhesives was different on the polished and sandblasted composite surfaces. Generally, all tested dental adhesives wet the composite surfaces based on the measured low contact angles that were between 15\({}^{\circ}\) and 35\({}^{\circ}\). The polished surfaces were more wettable for adhesives than the sandblasted surface. As previously mentioned, the sandblasting increases the Ra, which gives more opportunity for the formation of air bubbles and voids in the interface layer. These entrapped air bubbles unfavorably influenced the wetting ability and penetration of adhesives. The contact angle measurements revealed that the tested composite had a hydrophobic surface, and the surface roughening provided an increased surface area for dental adhesive. Therefore, HB, which is an apolar, solvent-free resin, spreads very well on the composite surface. The other adhesives contained different solvents, which have a more hydrophilic nature. These solvents are mostly water and alcohol, which are polar solvents, and acetone, which has an apolar character. Ethanol and water solvents can be found in TBF II, SU, and CL. The GP contained acetone next to water. Based on the contact angle of dental adhesives, it was found that the solvent-free adhesive spread better on the blasted surface than on the polished surface (Figure 8). Moreover, the solvent containing dental adhesives showed a higher contact angle due to the sandblasting, and the same pattern (TBF II \(>\) SU \(>\) CL \(>\) GP) was observed regardless of the different chemical compositions of tested RBCs. These differences can be explained by different chemical compositions of adhesives that influence their surface tension differently. Based on the measured data, solvent-free adhesives can be recommended for luting to sandblasted composite to improve micromechanical retention in the clinical situations of direct composite restoration repair and luting adhesively bonded indirect restorations. The bond strength can increase by the application of functional monomers (e.g., 10-MDP) that can form chemical bonds with components of RBCs. The other solvent-containing dental adhesives are constructed for dentin application that is a hydrophilic surface. Since the chemical bond provided by the functional molecules has a positive influencing effect, these solvent-based adhesives are used in composite repair that are designed for dentin surfaces. In this study, one drop of dental adhesive was placed on the surface without any active application (rubbing and air blowing), which provided information about the first contact between the dental adhesive and composite surface.