As a result of esthetic demands, restorations in the mouth may also be exposed to agents during the bleaching process, which has become a frequently applied procedure. During the bleaching process, agents affect the optical properties of restorative materials, which are important for esthetics [9, 23, 24]. Since natural teeth are translucent, maintaining the transparency of restorative materials is an important criterion. Therefore, this study aimed to investigate the effect of different bleaching procedures on the translucency and contrast ratio of resin-containing restorative materials. In the present study, it was observed that the translucency and opacity values ​​of the tested materials with the translucency calculation method (TP) and contrast ratio (CR) varied in a wide range and there were significant differences in terms of TP and CR values ​​between both bleaching methods and materials (p < 0.05). While the bleaching process caused an increase in the CR values ​​of the materials, it caused a decrease in the TP values. It was found that Opalescence PF caused a less decrease in TP values ​​compared to the office group. It was reported that the materials that caused the most change in TP and CR values ​​were composite materials. As a result, both hypotheses were rejected.

It has been reported in the literature that L* and b* values ​​are more decisive than other parameters in determining translucency, that translucency is mainly affected by changes in brightness, and that the yellow-blue coordinate (CIE b*) also plays an important role [17]. The decrease in TP values ​​in most of our samples may be due to the materials becoming opaque after bleaching. In the present study, an increase in the CR values of the materials was detected. The increase in CR values also favors the opacity of the materials. Higher CR values and lower TP values correspond to more opaque materials, whereas lower CR and higher TP correspond to materials with higher translucency. The CR ranges from 0 to 1, with 0 indicating a completely transparent material and 1 indicating complete opacity [11]. It has been shown that bleaching causes surface dissolution in materials. In addition to the bleaching effect, it has been reported that small pores on the surface caused by matrix dissolution in the materials and differences in optical properties between air and water may have increased the reflection on the surface, which may have led to a partial increase in L* values ​​as in the present study [25]. The decrease in b* values ​​can also be associated with a decrease in yellowish color along with a tendency towards opacification.

Natural teeth have a translucent structure and restorative materials are expected to have translucent properties similar to natural teeth. For this reason, it is important not only for the material to be translucent but also to preserve the translucency of the material. It is known that bleaching affects the physical properties of restorative materials. Previous studies have reported that the effect of bleaching treatment on restorative materials depends on the concentration of the bleach, the exposure time, and the amount of resin matrix of the restorative materials [20, 25,26,27]. The results of the present study revealed that different bleaching methods have different effects on the translucency of resin-containing materials. Previous studies have shown that bleaching agents increase the surface roughness and reduce the translucency of resin composites [8, 26]. In the present study, bleaching agents containing 16% carbamide peroxide and 40% hydrogen peroxide were used and a decrease in TP values was observed. After home bleaching, 16% carbamide peroxide decomposes into 5.8% hydrogen peroxide, urea, ammonia and carbon dioxide [28]. Yılmaz et al. [29] found that bleaching with 10% carbamide peroxide did not affect the translucency of resin composites. In the present study, it was found that office bleaching caused a greater decrease in TP values than home bleaching.

In studies examining the effects of bleaching on restorative materials, it has been reported that hydrogen peroxide may interact with carbon-carbon single or double bonds in the materials and cause deterioration in the polymer structure of the composite [12, 17]. It has been reported that as a result of the deterioration in the structure of the composite, solvents can penetrate the structure of the material and the diffusion of the monomers contained in the composite will also increase [12, 20].

The aqueous environment to which the materials are exposed in the mouth is an environment between water and more aggressive solutions such as ethanol, methanol and acetonitrile. A 75% ethanol-water solution was used to mimic this environment in vitro. The United States Federal Drug Administration has also reported that a 75% ethanol-water solution can be used to mimic the oral environment [30]. Based on this, 75% ethanol-water solution was used as the storage solution in the present study. The fact that office bleaching caused a greater decrease in translucency in the present study may be due to the fact that office bleaching disrupts the structure of the materials more due to its high content of HP and, as a result, the material absorbs more ethanol.

Translucency also depends on thickness, and in this study, a thickness of 2 mm was used because restorative materials are clinically applied at a thickness of 2 mm. A higher value for TP represents greater translucency; if the material is completely opaque, the TP value is zero [31]. TP has been used in several studies to evaluate the translucency of restorative materials [1, 11, 13, 21, 22, 32,33,34].

The fact that the polymerization depth of light-cured composites is limited to 2 mm and that layered application takes time is a significant disadvantage today. Bulk-fill composites produced to eliminate this disadvantage can be applied in thick layers due to improving photoinitiator dynamics and increasing translucency properties [35]. This can be explained by the fact that bulk fill composite materials were found to be the most translucent material among composite materials in the present study. Günal and Ulusoy [36] reported TP (CIELAB) values of 18.64 and 17.93 for Cerasmart and Lava Ultimate samples, respectively, similar to the findings of the present study. Günal Abduljalil et al. [37] reported that Cerasmart is the most translucent material compared to Voco Grandio, Brilliant Crios and Lava Ultimate. Cerasmart’s higher TP value may be due to its lack of opacifying compounds and its composition of aluminum barium silicate particles embedded in a polymer network that increases light transmission [38]. As the crystal content increases, material durability increases. However, this also causes increased opacity [11]. In the present study, the reason why VITA Enamic has lower translucency than other resin-ceramic hybrid materials may be related to its Al2O3 content (about 23% by weight).

As a result of the present study, the largest decrease in TP values was found in the composite materials. Several common monomers found in the composite resin matrix have hydrophilic properties and can often cause discoloration due to excess water absorption. It has also been reported in the literature that the presence of low TEGDMA content may limit water uptake and consequently color changes caused by the absorption of the staining solution [39]. TEGDMA content of composite materials may have caused more fluid uptake and decreased translucency. The decrease in TP after bleaching was statistically significant in the Beautifil II, Filtek Bulk Fill Posterior, Filtek Bulk Fill Flowable and Lava Ultimate groups (p < 0.05). Beautifil II (Giomer) is a hybrid aesthetic restorative material based on pre-reacted glass ionomer “PRG” technology, using pre-reacted glass ionomer cements as fillers [40]. This significant decrease in the TP value of Beautifil II may be due to its nano-hybrid gyomer filler content.

The production of Ormocers is based on hydrolysis and polycondensation reactions (sol-gel processing) to form a molecule with a long-chain inorganic silica backbone and lateral organic chains [41]. It is claimed that Ormocer-containing composites exhibit higher conversion, minimum polymerization shrinkage, color stability, toughness and increased surface hardness as a result of the formation of a more highly cross-linked polymer network. Another advantage of Ormocer is that the amount of free unreacted monomers in the polymer network decreases with the increase in the number of chemical bonds between methacrylate groups [42]. In this case, our expectation was that the translucency change would be less than the composites after bleaching, depending on the structure of the material. However, in this study, when the resin composites and Ormocer values ​​were examined, no statistically significant difference was found between the initial and post-bleaching TP and CR values.

In the present study, it was found that the change in TP and CR was less in hybrid ceramic groups than in composite groups. During the production of ceramic materials in an industrial environment, subjecting them to processes such as ceramic powder sintering, resin composite polymerization, etc. minimizes the defects in the structure of the material and makes the material more homogeneous and durable [43]. It also has a high reducing capacity by producing peroxides, free radicals and oxides [44]. Peroxides have been reported to induce oxidative cleavage of polymer chains [45]. Tight polymer chains limit the effect of peroxide. If a material contains tight cross-links formed by high molecular weight polymer molecules, more time will be required for the bleach to diffuse into the material [46]. The differences observed between resin matrix ceramics and composite resin groups may be related to the structures of the materials and the production technique.

As a result, the translucency of materials is affected by both the structure of the material and the bleaching method applied. There are limited studies in the literature examining the changes in translucency as a result of bleaching of materials. This study is important in terms of simultaneously examining a wide range of materials used for restorative purposes and comparing the results.

The limitation of this study is that since it was studied in vitro, the thermal, mechanical and chemical effects that the restorative material is exposed to in the mouth cannot be reflected exactly on the material. Studies on materials are generally conducted in vitro, tending to focus on material properties and procedures. Although this has contributed to the development process of materials, it should not be forgotten that in vivo studies and evaluation of follow-up procedures can provide valuable information to the literature.