# Degree of Conversion and Mechanical Properties of Modern Self-Adhesive Luting Agents
## 4 Discussion  
The results of the present study showed that the dual-curing mode demonstrated superior results in comparison with the self-curing mode in all the material properties tested. Significant differences were found between materials within the same curing mode for HM (SC), C\({}_{\rm{IT}}\) (DC), n\({}_{\rm{IT}}\), DC% (DC and SC). Moreover, differences were registered between the structure and elemental composition of the materials tested. Therefore, the null hypothesis should be rejected for these properties. The self-adhesive luting agents tested were representative of the most current commercially available types based on the characteristics stated by their manufacturers. PSP is a typical dual-cured material, PSU is the only material of this category incorporating a silane, SPC is marketed as a self-curing material with a light-curing option and THC as a Ca-releasing material. A composite luting agent, without adhesive monomers (PV5) was used as a control in all experiments. All materials demonstrated a typical structure of particle-filled composites, with differences in particle size and mean atomic number (greyscale level of BE images). The greatest particles were found in SPC with a maximum size of approximately 10 \(\upmu\)m, which may correspond to pre-polymerized particles, based on their greyscale level. The high-contrast (bright) submicron size particle phase found mainly in SPC and THC is attributed to Yb (salts or glasses), which enhance radiopacity and in the case of YbF\({}_{3}\) may serve as a source of slow fluoride release [18]. From the series of Panavia products, PSP showed the highest Al content, although no information for the presence of Al is given in the manufacturer's files. The P/Si ratios of PSP and PSU were six times greater than PV5, which is an adhesive-free monomer composite. This difference may be assigned to the presence of MDP or/and P\({}_{2}\)O\({}_{5}\) in the glass fillers of the former. SPC and THC resulted in the highest Ca/Si and P/Si ratios, suggesting that these materials may exert a bioactive effect. Their P/Si ratios were 6.6 and 3.3 times greater than PSP and PSU respectively, implying that P should be also part of the filler fraction. The composition of THC includes Portland cement powder, with major constituents Ca- and Al-silicates. This has been considered by the manufacturer as a major Ca-source for two important reactions: First, the fast neutralization of unreacted acidic phosphate monomers upon setting, and second, the Ca-releasing capacity at the vicinity, which may reduce or inhibit dental hard tissue demineralization after exposure to acidic challenges, assisted by the synergistic effect of F- released from YbF\({}_{3}\) particles. However, surprisingly, SPC exhibited a 3.7 times greater Ca/Si ratio than THC, anticipating a stronger inhibitory effect, which is further assisted by the 20 times greater F content of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Product** & **Al/Si** & **Ca/Si** & **P/Si** & **Ba/Si** & **Yb/Si** \\ \hline PSP & 0.49 & - & 0.03 & 0.26 & - \\ \hline PSU & 0.35 & - & 0.03 & 0.24 & - \\ \hline PV5 & 0.35 & - & 0.005 & 0.27 & - \\ \hline SPC & 0.52 & 0.84 & 0.2 & 0.48 & 0.34 \\ \hline THC & 0.24 & 0.23 & 0.1 & 0.03 & 0.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Atomic ratios of the major elements in the products tested.