# Degree of Conversion and Mechanical Properties of Modern Self-Adhesive Luting Agents
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Product** & **O** & **F** & **Na** & **Al** & **Si** & **P** & **S** & **Cl** & **K** & **Ca** & **Ba** & **Yb** \\ \hline PSP & 54.99 & - & 1.19 & 11.09 & 24.65 & 0.78 & 0.39 & 0.30 & 0.20 & - & 6.41 & - \\ \hline PSU & 56.11 & - & 1.06 & 9.18 & 25.87 & 0.83 & 0.19 & 0.25 & 0.29 & - & 6.21 & - \\ \hline PV5 & 58.62 & 0.44 & 1.14 & 9.13 & 23.34 & 0.12 & 0.42 & 0.30 & - & - & 6.21 & - \\ \hline SPC & 45.00 & 20.5 & 1.42 & 6.60 & 12.61 & 2.46 & - & 0.14 & 0.13 & 5.57 & 1.28 & 4.28 \\ \hline THC & 59.40 & 0.89 & 1.98 & 5.88 & 24.91 & 2.60 & 0.76 & 0.49 & 0.23 & 1.35 & 0.63 & 0.87 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The results of the elemental analysis by EDX (atomic %). the former. Although no further information is given by the manufacturer for the filler content, it seems that SPC may demonstrate increased bioactivity. The ATR-FTIR spectra recorded confirmed the absence of aromatic monomers in SPC. The dual-curing irradiation mode provided higher conversion in all materials, including SPC, which showed the highest self-curing conversion. Despite the developments in the self-curing catalysis, the materials tested failed to reach the levels of dual-curing, a finding already documented in previous studies for most dual-cured luting agents [3; 4; 5; 6]. Although quantitative data for the monomer content is missing, some assumptions for the effects of specific monomers can be made. PSP and PSU showed lower conversion from PV5 in the dual-curing mode; the latter, though, was used without the dedicated tooth-primer which facilitates conversion via a touch-cure mechanism [19]. A possible explanation may be given based on the acidic nature of the mixed PSP and PSU pastes, due to the presence of acidic monomers (such as MDP), known to interfere with conversion [2]. An interesting finding was that PSU, processing the same chemistry with PSP with the exemption of the long-chain silane, demonstrated better self-curing conversion from PSP, reaching the levels of the control (PV5). This might be explained by the presence of the long CH\({}_{2}\) chain hydrophobic silane. The flexible chain may increase the segmental mobility of terminal methacrylate groups and enhance slow-setting conversion via copolymerization. It should be noted that it was not possible to identify characteristic peaks of the long-chain silane in the spectra of PSU (presence of methoxy at 2835 cm\({}^{-1}\) or silanol groups at 904 cm\({}^{-1}\)) to assess the silane status in the material (unset, dual- or self-cured state), possibly due to the low silane content. The high conversion values manifested by SPC in both curing modes should be assigned to the presence of fast-reacting aliphatic dimethacrylate monomers, which show less residual unsaturation than stiff dimethacrylate aromatic monomers, such as BisGMA, due to steric hindrance phenomena [20]. On the other hand, the presence of Ca-containing fillers may neutralize the acidic monomers upon mixing, creating more favorable conditions for enhanced C=C conversion, due to pH balance and molecular orientation effects [21]. This is especially important for the slow-setting self-curing mode [7]. The same mechanism may explain the curing performance of the BisGMA-containing THC. The high THC conversion in the dual-curing mode should be additionally ascribed to the presence of the fast-reacting hydrophilic mono-methacrylate HEMA, known to improve conversion when used as a BisGMA comonomer [22]. Nevertheless, the self-curing conversion, of THC was significantly lower than SPC, reflecting a lower self-curing activation capacity of the material. The IIT-derived mechanical properties tested may have a significant clinical impact on the performance of the luting agents. Hardness is associated with wear resistance and may affect the marginally exposed part of the luting agent [23].