## 4 Discussion  
The bonding between fiber-reinforced composite posts and core resin is an important factor in successful restoration of endodontically-treated teeth, which has insufficient coronal tooth structure to retain a core for the definitive restoration [1], and therefore the objective of this study was to use recently highlighted NTAPP for application on epoxy resin-based fiber post to improve bonding with core resin. First, the contact angle was measured as an indicator of surfaced energy on epoxy resin. The water contact angle has been used to evaluate the surface hydrophilicity [30; 31; 32], but also diiodomethane \begin{table}
\begin{tabular}{c c} \hline \hline
**Group** & **Push-Out Shear Bond Strength (Mean \(\pm\) SD)** \\ \hline
1 & \(19.36\pm 1.06^{\text{a}}\) \\
2 & \(26.36\pm 0.97^{\text{c}}\) \\
3 & \(21.86\pm 1.17^{\text{b}}\) \\
4 & \(22.66\pm 1.30^{\text{b}}\) \\
5 & \(26.83\pm 2.76^{\text{c}}\) \\ \hline \hline \end{tabular} Different superscript letters mean they are significantly different statistically (\(p<0.05\)).  
\end{table} 
Table 2: The result of push-out shear bond strength between epoxy resin-based fiber post and core resin (MPa). Figure 8: Push-out shear bond strength between group with no surface treatment (1) NTAPP treatment for 1 min; (2) treated with 4% hydrofluoric acid gel for 60 s and then rinsed with running water for 10 s; (3) treated with silane using micro-brush for 60 s; (4) and treated with 4% hydrofluoric acid followed by silane as described earlier; (5) different lowercase letters above the bar indicate a significant difference at \(p<0.05\) for each group. and resin monomer mixture were additionally used in this experiment to evaluate the contact angle of these liquids on the epoxy resin surface following NTAPP treatment. This is because they will be more clinically relevant as they are the component liquids that will be in contact with epoxy resin in clinical situations. Surface energy has been considered in this study using the contact angle measurement of both polar and non-polar liquid, as it is a parameter that is linked to the wettability of the surface. Increased wettability is an important feature of dental materials, especially during the adhesion as improved wettability of the dental material surface would result in larger area occupied by liquid-like adherent, resulting in improved adhesions [12; 20]. The results showed that the contact angles decreased for each of the three different liquids used following NTAPP treatment on epoxy resin (Figure 4a) and consequent surface energy increased with NTAPP treatment (Figure 4b). Such effects were in agreement with our previous studies that considered application of same type of NTAPP on polyvinyl siloxane based dental impression materials and dental wax [12; 20] that resulted in the increase of the surface energy. It is also in agreement with study by [PERSON] et al. who reported the change of surface energy against distilled water as well as diiodomethane on polyethylene (PE) film after dielectric barrier discharge plasma treatment [33]. Additionally, in a similar study where fiber post bonding with resin cement was considered along with effects of silane and hydrofluoric acid, the results showed water contact angles of 56.1\({}^{\circ}\) and 71.2\({}^{\circ}\) for silane and hydrofluoric acid treated fiber post, respectively [11]. Also, by combining silane and hydrofluoric acid treatment on fiber post, the contact angle was 59.4\({}^{\circ}\) in same study. Hence, it is indicated that treatment as short as 30 s with NTAPP in this study would have resulted in contact angle lower than surface treated with hydrofluoric acid, while treatment of 1 min or more would be required for the lowering of contact angle similar to silane or silane and hydrofluoric acid treatment combined. The increased surface energy by NTAPP would result in larger area of interaction between the epoxy resin-based fiber post and core resin, allowing improved adhesion between two surfaces.