Conventional resin-based composites have been successfully used in the field of aesthetic restorative dentistry for many years, however, their limited depth of cure into small increments of 2 mm thickness is still considered a critical point that limits their performance, rendering the conventional incremental composite placement technique sensitive and time-consuming (1, 2). Recently, a novel category of resin-based composites, known as “bulk-fill” composites, has been introduced to the dental market aiming at reducing the cost and saving time (3). The distinctive feature of these novel materials is their ability to be cured in a single step and placed in bulks of 4 mm thickness, in contrast to the conventional incremental placement technique, without affecting the degree of conversion, polymerization shrinkage, or their adaptation to the cavity walls and margins (4).

Bulk-fill resin composites are available in two different viscosities which highly influence their application efficiency and cavity wall adaptation. Low-viscosity bulk-fill resin composites are flowable in consistency which enables their adaptation to the cavity floors even in deeper and less accessible areas, this reduced viscosity is associated with decreased filler loading which reduces their surface wear resistance, and therefore, these low-viscosity forms must be capped with a layer of conventional composite materials. on the other hand, high-viscosity bulk-fill resin composites are designed to be used without a capping layer, as they have higher filler loading and enhanced mechanical properties, the higher filler loading and higher viscosity of the material reduce their adaptability to the cavity walls and margins and creates difficulty in sculpting the surface layer (5) (6). Accordingly, preheating the composite resin before its application is currently gaining popularity among dental practitioners due to its potential to enhance the extrusion and flow of high-viscosity composites, as well as for its potential to reduce microleakage and marginal adaptation (7). It has been demonstrated that the polymerization process is optimized and the degree of conversion is increased by increasing the temperature before the polymerization (8, 9).

In addition to enhancing the sculpting and handling characteristics of bulk-fill composite resins, it is anticipated that these direct restorative materials exhibit good aesthetic qualities and durability (10). The consumed oral beverages remain a critical point that affects the oral health (11). Despite advancements in the organic matrix and particle size of composite resins, color stability and gloss retention remain a prevalent issue for both patients and dentists (12, 13). The exposure of bulk-fill composite resins to different beverages in the oral environment may lead to color alteration due to intrinsic or external causes as reported in previous studies (14, 15). Besides, acidic beverages with low pH are also known to degrade the matrix structure of resins (16). Previous studies showed the negative effect of acidic beverages on the surface hardness of bulk-fill composite resin, which reflects the ability of the material to resist abrasion during function. Consequently, the restoration's durability may be influenced by the factors that impact its hardness (17–19).

Despite the reported advantages of preheating the composite resin materials on enhanced degree of conversion and improved marginal adaptation due to reduced viscosity (8, 20, 21) there is insufficient data about the preheating effect on optical characteristics and surface hardness of highly viscous bulk-fill composites. Thus, the current investigation aims to explore the influence of preheating two highly viscous bulk-fill composite resins on their color stability, surface gloss and microhardness after immersion in a commercial carbonated beverage. The first null hypothesis states that there will be no difference between the two materials regarding the tested properties. The second null hypothesis is that there will be no difference between the two materials in preheated and non-preheated state, and the third null hypothesis states that there will be no difference in the tested properties after immersion in the carbonated beverage.

The current in vitro study investigated the effect of a carbonated Cola beverage on the color stability, surface gloss and microhardness of two bulk-fill composite resins in preheated and non-preheated conditions before their polymerization. The first and third null hypotheses were rejected due to significant differences in the tested properties between the two resin composites and the impact of the carbonated beverage on all properties. The second null hypothesis was partially rejected, as preheating had no significant effect on the color change of BB.

The two investigated bulk-fill restorative materials in the current study were selected as they have different formulations both in their organic resin matrix and inorganic filler, they can be used in anterior and posterior restorations according to their manufacturer's claims. Beautifil-Bulk restorative is based on surface pre-reacted glass (S-PRG) fillers technology which imparts sustained fluoride release with an anti-plaque effect, this makes it beneficial for cases with high caries risk due to its ability to minimize secondary caries formation. Filtek One bulk-fill contains innovative monomers that reduce shrinkage and shrinkage stress, they also possess nano-filler technology which provides superior aesthetic properties. Both composites also have different depth of cure levels, 4 and 5 mm respectively, thus they have a wide range of applications among dental practitioners.

Preheating of the bulk-fill composites was compared to room temperature polymerization in the current study since it became a popular technique and gained attention lately. The flow of composite polymers can be enhanced by increasing their temperature, as demonstrated by recent literature (7). Some of the potential advantages of composite preheating include enhanced marginal adaptation, better handling, and a higher degree of monomer conversion. A previous study by Kampanas (28) found that the average preheating temperature of bulk-fill composites which was considered safe and not harmful to the pulp tissues was between the range of 54°C and 68°C, thus, the preheating temperature in the current investigation was set to 68°C to maximize the benefits of the preheating technique.

Another critical factor to consider when preheating is the sufficient time to achieve optimal flow and enhance the properties of the restorative material. Previous studies that referenced the required time for material heating revealed a wide range of minimum and maximum times. Nevertheless, a clinical time of approximately 15 min is considered reasonable, as per the findings of Mohammadi, et al., and Karacan and Ozyurt (29, 30). Accordingly, 15 min of preheating time was considered in the current investigation.

It is crucial to ascertain whether the behaviour of the investigated materials has clinical implications and to consider its clinical acceptability during function. Therefore, the color stability results in the current study were statistically analyzed to assess the significant differences between groups, besides, it was further assessed based on the 50:50% acceptability threshold (AT) and the perceptibility threshold (PT) in CIEDE2000 which were 0.8 and 1.8, respectively, as reported by Paravina, et al. (31) and as aligned with ISO standards (25). The susceptibility of the composite resins to staining may be related to their hydrophilicity, the degree of conversion, as well as to the water sorption of the resin matrix (16, 32). Although the color change of both tested materials was beyond the 50/50% acceptability limit (>1.8), the color change of BB composite was significantly higher than FOB in both preheated in non-preheated states, this may be related to the difference in resin matrix composition between both materials. While both composites contain UDMA, which may reduce hydrophilicity and water absorption thus enhancing the color stability of composite resins, BB contains the more hydrophilic monomers Bis-GMA and TEGDMA, which are reported to increase the discoloration of composites by Ren, et al. (33), and Ozera, et al. (16). Moreover, BB is a bulk-fill form of giomer composites, representing a distinct category of bioactive resin-based materials capable of releasing fluoride as a preventive mechanism. However, the fluoride-release process may potentially create voids within the resin matrix, which in turn can reduce color stability due to enhanced pigment retention (34). Consequently, a more significant color shift with the exposure to the Cola beverage occurred in BB composite. The presence of these hydrophilic monomers and the bioactive characteristics also explain the current finding that preheating of composites was not able to stabilize the color of BB, this finding is in line with the study by Daneshpooy, et al, who found a significant color change of giomer composite after preheating in tea solution compared to microhybrid and nanohybrid composites (35). On the other hand, preheating of FOB in the current study improved it's the color stability after the Cola beverage attack, this is probably due to the presence of the unique additive fragmentation monomer (AFM), which increases the formation of cross-links between adjacent polymer chains, the preheating seems to improve the reactivity of this monomer and increase the degree of conversion which in turn improves the color stability by reducing the free monomer chains exposed to the colorant solution as reported in studies by Sousa, et al. (36) and Darabi, et al. (37). The current study result, however, is not in line with a study by Abdulmajeed, et al. (22) who reported in their study that preheating had no effect on stabilizing the color of the FOB composite, probably due to the strong staining potential of the coffee utilized in their study (13).

Surface gloss is an optical feature that is determined by the intensity of light reflection. The angle of incident light, the refractive index of the material components, and surface features are several elements that influence gloss (38). This study employed a 60° angle of incident light as per ISO 2813:2014 standards for medium gloss materials (26), rendering the gloss values contingent upon the surface topography and the material's refractive index. Irregular surface topography disperses light rather than reflecting it, hence diminishing the gloss value (39). There is no definitive threshold for gloss values of dental composites; however, it is advised to maintain a gloss value within the 40–60 GU range (40).

A recently published systematic review concluded that surface gloss is a reliable indicator for evaluating the effectiveness of finishing and polishing procedures on composite resin surfaces, as it directly correlates with surface roughness (41). Moreover, previous studies have demonstrated that the smoothest composite surfaces are obtained when polymerized against Mylar strips, without undergoing mechanical finishing (42, 43). In line with this evidence, the specimens in the current study were polymerized against polyester celluloid strips without additional finishing or polishing. This approach was intended to eliminate operator-dependent variability associated with polishing procedures, ensuring that gloss measurements reflect the intrinsic surface characteristics of the composite materials in their preheated and non-preheated states, thereby revealing their baseline performance without alterations induced by polishing. In the current study, both materials exhibited satisfactory gloss values before the immersion in Cola beverage, with FOB showing superior gloss compared to BB, this may be attributed to the difference in filler load as well as the filler size and distribution in both materials, FOB filler is composed of agglomerated and non-agglomerated nanosized filler with total volume of 58.4% which is less than the 74.5% SPG-R filler of BB. It was reported in previous studies by Alhassan, et al., and (44) that composites having a filler size between 5 and 20 nm are associated with an increase in gloss and light reflection, as these fillers are smaller than the visible light 400–800 nm wavelength. The high filler loading is also associated with less degree of conversion as suggested by Dionysopoulos, et al. (45) and Bucuta and Ilie (46), and this explains the improved gloss values of BB composites after preheating, which can be attributed to the improvement in the degree of conversion of the resin matrix secondary to its preheating. Although preheating reduced the gloss values of FOB, its gloss values remained clinically acceptable, this reduction may be related to stress build-up within the resin matrix secondary to the fastened polymerization rate after preheating as reported in Deb, et al. (21), and El-Korashy (47) studies, which may alter the bonding between the filler particles and resin matrix, thus changing the light reflection and subsequently reducing the surface gloss. On the Other hand, the gloss values reduced dramatically to be clinically unacceptable in all groups upon Cola exposure, which is in line with previous studies by Zovko, et al. (48) and Ozera, et al. (16). Such decrease in gloss is thought to be due to the presence of phosphoric acid in the composition of the Cola drink, which softens the organic matrix and thus leads to a change in light refraction and consequently a decrease in surface gloss.

The microhardness values in the current study was generally material dependent, the higher VHN recorded for FOB in all groups compared to BB is more likely related to the presence of the nano-sized filler particles which reduces the interstitial spaces, thus improving the surface hardness as reported in a study by Yap, et al. (49). The preheating showed a significant increase in microhardness of both tested composite resins in the current investigation compared to non-preheated groups without the acidic challenge, this finding is in line with previous studies (50–53). The degree of conversion of carbon double bonds in the composite resin matrix is often reflected in its hardness values, thus increased hardness following preheating is highly related to the improvement in the degree of conversion and therefore greater cross-linking in the monomer chains (54). According to Trujillo, et al. (55), and Daronch, et al. (56) the rate of monomer-to-polymer conversion can be enhanced by warming a composite resin at biologically compatible conditions. However, a significant reduction in microhardness values occurred in both materials after exposure to the Cola drink in the preheated and non-preheated state, the deterioration effect of acidic drinks on composites microhardness is in line with Borges, et al. (17), and Poggio, et al. (57), studies. This is more likely due to softening of the organic matrix by the action of phosphoric acid on of both composites, hence changing the bonding between the silane coupling agent and filler particles, however, the reduction in hardness was less in FOB, probably due to its harder zirconia filler content compared to the surface pre-reacted glass filler (SPG-R) in BB restorative composite. Besides, the inclusion of nanoclusters alongside nanoparticles in FOB filler diminishes the interstitial space, hence enhancing the physical properties and the surface hardness.

The findings of the current study are limited to the characteristics of in vitro studies, some oral environmental conditions were not included, such as the change in intraoral temperature, the effect of other beverages and the diluent effect of saliva. Future research should explore a broader range of preheating temperatures and investigate the effects of different beverages on composite materials. Additionally, further studies are recommended to assess other clinically relevant outcomes such as mechanical properties, and the long-term durability of preheated bulk-fill composites, particularly after thermal cycling and under clinical conditions.

Given the constraints of the present studies, it can be concluded that preheating improved the color stability of FOB and the gloss of BB, it also improved the microhardness of both bulk-fill composites. Conversely, exposure to Cola adversely affected all properties, resulting in clinically unacceptable color alterations.