## 4 Discussion  
Considering the results obtained, the hypothesis tested was partially rejected: The surface properties of the tested composite materials were affected by CP-based Opalescence Regular bleaching gel, while the bulk properties were insignificantly affected when using Opalescence Regular gel. In contrast, PAP+based bleaching gel did not affect the surface and bulk properties of the composite materials. To avoid plaque accumulation, a smooth restorative surface is vital [20]. However, we observed that the active and other added agents in the Opalescence regular bleaching system may compromise the structural integrity of the restorative composite surface. A significant increase in surface roughness of the microhybrid restorative composite using Opalescence Regular insinuates that the resin matrix of the composite is susceptible to chemical erosion due to multi-component \begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**Bleaching Gel** & **Composite** & **Treatment** & **Mean** & **Std. Dev.** & \(p\) \\ \hline \multirow{4}{*}{Opalescence Regular} & \multirow{2}{*}{Microhybrid} & Control & 81.18 & 4.50 & \multirow{2}{*}{0.056} \\  & & experimental & 75.36 & 5.14 & \\ \cline{2-5}  & \multirow{2}{*}{Nanohybrid} & Control & 75.92 & 3.49 & \multirow{2}{*}{0.067} \\  & & experimental & 71.75 & 4.75 & \\ \hline \multirow{4}{*}{Hismile} & \multirow{2}{*}{Microhybrid} & control & 82.18 & 5.22 & \multirow{2}{*}{0.082} \\  & & experimental & 77.86 & 3.30 & \\ \cline{1-1} \cline{2-5}  & \multirow{2}{*}{Nanohybrid} & control & 74.94 & 2.96 & \multirow{2}{*}{0.092} \\  & & experimental & 72.32 & 3.75 & \\ \hline \hline \end{tabular} Note: \(p\) < 0.05, MPa = megapascal.  
\end{table} 
Table 4: Mean flexural strength (in MPa) of the two composite materials distributed according to two different treatments. Figure 3: Graphical representation of diametral tensile strength of the control and experimental groups: (**A**) Diametral tensile strength using Opalescence Regular bleaching gel on microhybrid and nanohybrid restorative composites; (**B**) depicts the diametral tensile strength using HiSmileTM bleaching gel on microhybrid and nanohybrid restorative composites. acidic and oxidizing bleaching chemicals [24]. The irreversible changes in the restorative composite might have an effect on the durability and clinical life of the restorative material. In contrast, the insignificant increase in surface roughness after bleaching in the HiSmile(tm) group might hint towards a mild effect on the resin matrix system of both restorative composite systems. The nanoindentation test is considered a reliable and valid method for gauging any changes at the nanoscale [21, 25]. A significant decrease in nanoharness of the resin matrix with Opalescence Regular using the microhybrid composite indicates the detrimental effect of carbamide peroxide on the resin matrix of the microhybrid composite. Carbamide peroxide breaks down into HP and urea. HP is a powerful oxidant that produces free radicals and causes oxidation and has high reactivity and extensive ability to diffuse into an organic component of the composite [26]. Free radicals have the potential to impact the resin-filler interface and result in filler-matrix debonding [27]. In contrast, a statistically insignificant nanoharness reduction in the nanohybrid composite group might be explained by the lower resin matrix ratio and higher content of inorganic fillers in the composition. Although the peroxide-free experimental bleaching product reduced the nanoharness both in the microhybrid and nanohybrid composite groups, the reduction was not statistically significant. This diminutive reduction could be due to the vulnerability of the resin matrix to chemical alteration. Another cause might be that the resin matrix of the composite material absorbs water, which transforms into plasticizing molecules inside the resin matrix. The presence of hydroxyapatite and potassium citrate for buffering in formulation maintains a near-neutral pH during application [18].