## 4 Discussion
The near-neutral effect of pH is shown to be undamaging on the resin matrix system of the restorative composite materials. The data of the elastic modulus are interesting and interpretive, and they corroborate well with the nanoharness results. The restorative composites with high elastic modulus perform better clinically [28]. The significant decrease in elastic modulus of the microhybrid composite strongly indicates the depletion due to the pH or diffusion gradient of the Opalescence Regular components with lower molecular weight into the resin matrix system of the restorative composite material [29]. However, the bleaching session from each bleaching product was limited to 10 min for seven consecutive days and the samples were incubated in distilled water at 37 \({}^{\circ}\)C after each bleaching session until the next bleaching procedure. Depending on the polarity of the molecular structure, the restorative composites absorbed water to varying degrees [29]. The water uptake might have affected the elastic modulus of the restorative composite. The insignificant elastic modulus reduction in the remaining three groups, in particular to HiSmile(tm) bleaching on the microhybrid composite, might suggest a mild and innocuous effect of this new peroxide-free bleaching product. Recent studies also claim that the presence of nano-hydroxyapatite in the formulation helps in remineralizing the enamel surface more effectively [30, 31]. In the case of the resin matrix, these nano-hydroxyapatite fillers might have filled in the tiny pits created due to the bleaching process and made the resin matrix resilient and stiff. No study group exhibited a significant decrease in flexural strength of the restorative composite. We can assume that the bleaching treatment did not affect the bulk properties of the restorative composites. The obtained results suggest Opalescence Regular had a deleterious effect on the flexural properties of the microhybrid composite. This could be because of the higher vulnerability to water sorption and material disintegration due to the higher amount of organic resin matrix in the microhybrid composite [32, 33]. A \(p\)-value close to 0.05 in the Opalescence Regular microhybrid composite may argue that unprotected double bonds at the resin-filler interface were oxidized by free radicals generated due to the bleaching process [15, 34]. In contrast, the flexural strength of the restorative composites using HiSmile(tm) was less affected. We believe that since the surface properties were mildly affected using this novel bleaching system, the effect on bulk properties was insignificant and diminutive only, while incubation of samples in distilled water at 37 \({}^{\circ}\)C after each bleaching session of 7 consecutive days might have caused water sorption at the filler-resininterface. Consequently, the flexural strength and integrity of these materials were reduced in each study group. The findings of our study are in line with the previous studies that have advocated the insignificant effect of home-bleaching gels on the flexural properties of the dental composites [7; 35]. Similarly, no significant difference in the evaluation of diametral tensile strength was observed between the tested bleaching groups. This might suggest that the effect of CP is confined to the material's surface only, resulting in only a significant change in surface properties such as roughness, nanohardness, and the elastic modulus of the material, while the insignificant decrease in flexural and diametral tensile strength tests might hint toward undamaged bulk material due to the shorter duration of the bleaching procedure. Phthalimidoperoxycaproic acid (PAP+), the new bleaching agent under investigation, is an organic acid with a high oxidation potential. The bleaching process requires oxidation because it breaks down the organic double bonds that result in tooth discolorations. PAP+, however, does not depend on HP being released [18].