Surface hardness
In this study, significant effects on surface hardness were observed according to materials, storage medium and time (p ≤ 0.05). Thus, the first null hypothesis was rejected. Filtek One showed the highest VHN, followed by Surefil One, Fuji II LC and Activa.
Hardness indicates the resistance of a material to permanent indentation, which is an important parameter to consider when comparing different materials. An adequate surface hardness is one of the main requirements for restorative materials, especially in posterior-stress-bearing areas [
17]. Surface hardness of composite restorations is a direct reflection of the curing quality. The process of polymerization has a notable influence on the hardness of materials, and it has been observed that hardness tends to increase as the degree of conversion during polymerization increases [
18]. In the realm of dental restorations, dual-cured composites have garnered attention due to their superior polymerization efficiency, which is expected to result in enhanced mechanical properties. By combining both light-activated and self-curing mechanisms, these composites ensure thorough polymerization even in areas that may be difficult to reach with light curing alone. This comprehensive polymerization is believed to contribute to improved material performance, particularly in terms of hardness and strength [
19]. However, hardness is not an isolated material property. It is interconnected with other key characteristics, including the modulus of elasticity and viscosity [
20]. Hence, hardness can serve as an indicator of the material's resistance to deformation and wear. Studies have revealed a strong correlation between low surface hardness and poor wear resistance, which can significantly impact the longevity of dental restorations [
21,
22].
Various factors related to the composition of the material have been identified as influential in determining surface hardness, including monomer type and ratio, photoinitiators and the degree of polymer crosslinking [
17]. Moreover, surface hardness has been found to be strongly influenced by filler load, size, morphology and distribution [
23,
24]. Each of these factors can influence the degree of conversion during polymerization, ultimately affecting the resulting hardness of the material. This could explain the different VHN values recorded among the materials tested in this study.
Several studies confirmed a positive correlation between VHN and filler loading; as the filler load increased, higher hardness values were observed [
25,
26]. In this study, a positive correlation was confirmed between filler load and hardness. However, the interpretation of the results when considering a single factor, such as filler load, can be deceptive due to the complex nature of the tested materials.
Filtek One showed the highest VHN despite having a filler load comparable with that of Surefil One, which might be attributed to the nano-sized fillers in Filtek One. This is in accordance with previous studies showing that small-sized filler particles have improved hardness. The mean distance between neighboring particles decreases with small-sized fillers, which in turn increases the number of particles at the surface [
20,
27]. Surefil One presents a higher filler load and overall smaller and rounder filler when compared to Fuji II LC which could explain the difference in VHN [
28]. One study reported that composites with round particles showed improved hardness and flexural strength properties compared with those containing irregular-shaped particles [
29].
Activa had the lowest hardness value at the baseline (1 h) measurement and after storage in both solvents. This result is consistent with recent studies comparing Activa with a conventional resin composite and GIC materials [
16,
30,
31]. This could be due to its low filler loading (56%) which is mostly composed of bioactive glass fillers that have been showed to reduce surface hardness in previous studies [
32,
33]. A study evaluated different ion-releasing materials including Activa showed a reduction in the mechanical properties after long-term storage [
34].
The surface hardness of dental composites may be affected by both solvent absorption and contact time with liquids. Several studies revealed a significant influence of different aging media and aging time on the mechanical properties of RBCs [
2,
16]. In this study, the results show an increase in VHN for all the materials after 1 d of storage in distilled water when compared to the baseline hardness. This finding is consistent with previous studies and can be attributed mainly to post-irradiation polymerization as well as ongoing acid–base reaction, or self-cure process in dual-cure materials [
35,
36].
For Filtek One and Surefil One, the increase in hardness continued for up to 30 days which might suggests that the continuous crosslinking reaction hinder the plasticizing effect of the absorbed water. These results also indicate that these materials require a certain period of time to achieve their maximum degree of polymerization. On the contrary, Activa showed a reduction in VHN after 30 d of storage which could be related to the solubility and the leaching out of the bioactive glass fillers that are not tightly adhered to the matrix [
37,
38].
Fuji II LC required 7 days to achieve its maximum hardness. Kanchanavasita et al. found that when RMGICs were immersed in distilled water, 90% of the equilibrium water uptake occurred within 7 days [
39]. This was followed by VHN reduction due to water acting as a plasticizing molecule [
4].
Regarding storage in ethanol, a significant reduction in VHN after 1d, which continued for over 30 days, was observed for all tested materials. This finding is in agreement with a previous studies that reported a significant reduction in hardness after 1 d of storage in ethanol [
2,
5]. Ethanol causes softening of the resin composite surface as it easily penetrates the resin matrix, generating stress at the matrix–filler interface and thereby increasing diffusion and leaching of fillers [
39]. Activa showed the highest reduction in VHN after 30 d of ethanol storage. This might be a reflection of filler-matrix debonding as a result of weakened filler surface due to ion leaching [
40]. Filtek One showed the smallest reduction (19.76%) in VHN among all the tested materials after storage in ethanol. This might be attributed to its high filler load as well as its hydrophobic resin matrix compared to the other materials. A study evaluated the hardness of 11 bulk-fill RBCs after ethanol storage reported that filler content is a critical factor to determine the resistance to ethanol softening [
41]. The high filler ratio in high-viscosity bulk fills such as Filtek One reduces the amount of resin affected by ethanol. The presence of fillers extends the diffusion path length, thus reducing the diffusion coefficient of ethanol [
42].
Biaxial flexural strength
The degradation of mechanical properties in the oral environment alongside growth and accumulation of cracks results in catastrophic failure of restorations [
6]. RBCs must have high strength to withstand repeated chewing forces. In vitro flexural testing has been shown to be an appropriate method for assessing the strength of a restorative material [
17].In this study, the BFS of the tested materials was evaluated after storage in water for different time intervals (1, 7 and 30 d). Filtek One showed the highest BFS followed by Activa, Fuji II LC and Surefil One. A significant increase in BFS after 7 d of storage was observed for all the tested materials. Thus, the null hypothesis that there would be no significant difference in BFS between tested materials and within each material at the different storage times was rejected.
Flexural strength can be influenced by matrix type, filler size and load and their salinization. Kim et al. observed a significant influence of the filler rate and morphology on the flexural strength, elastic modulus and microhardness of the composites evaluated [
24]. Furthermore, filler size was positively correlated to higher flexural strength in seventeen commercial resin composites [
26].
Resin composites should ideally be chemically stable, and their mechanical properties should not exhibit significant deterioration as they age. Ferracane et al. investigated the mechanical properties of experimental composites after 2 years of water exposure. It was concluded that water had little influence on flexural strength and modulus [
43].
Filtek One, showed the highest BFS with no significant difference between 1 and 30 d of storage. This result agrees with a previous study where Filtek one showed the highest flexural strength and modulus among different bulk-fill resin composites [
44].This was expected due to the high proportion of inorganic nanosized fillers. Tanimoto et al. evaluated the effect of filler size on flexural strength; the results showed a reduction in flexural strength with increasing filler particle size. Finite element analysis showed that stress concentration at the filler–matrix interface increased with increasing filler particle size. Smaller filler sizes increase the filler surface area, which increases the uniformity of stress distribution through the material and decreases the stress concentration at the filler–matrix interface [
45].
Surefil One showed the lowest BFS values despite the high filler load. This could be attributed to difference in resin-matrix composition and filler characteristics. Surefil one is described as a self-adhesive ion releasing material. In order to allow for adhesion and ion releasing, this material was formulated with a modified polyacid system that possess hydrophilic properties [
46]. In the SEM analysis, Surefil One presented obvious extended cracks which could explain its low BFS. The mechanism of microcrack formation was explained by Soderholm et al. as a result of an increased osmotic pressure at the matrix-filler interface due to filler degradation and ion leaching [
47].
Activa demonstrated superior flexural strength compared to Surefil One and Fuji II LC which is in accordance with previous studies [
30,
48,
49]. According to the manufacturer report, Activa contains a rubberized resin matrix with energy-absorbing elastomeric components (a blend of diurethane and methacrylates with modified polyacrylic acid). UDMA has a relatively high molecular weight (MW = 470 g/mol) and low viscosity with high flexibility. UDMA polymers had significantly higher rates of conversion and crosslinking, resulting in improved flexural strength [
50]. Previous studies have reported a notable bend in Acitva specimens before fracture [
16,
51]. This was attributed to its low flexural modulus due to the energy-absorbing property rendering the material more flexible [
48,
51]. However, higher distortion is expected in materials with low modulus of elasticity. Occlusal load on flexible restorative materials may cause lateral expansion and effect tooth integrity. Therefore, high modulus is necessary for restorative materials in stress-bearing areas to prevent distortion and marginal failure [
52].
Despite low initial BFS, Fuji II LC presented the highest increase after 30d (62.3%). This gradual increase in flexural strength over storage time is in agreement with a previous study [
53]. Maturation of RMGICs is a complex phenomenon that occurs over time and involves a variety of mechanisms. This development in strength can be attributed to the ongoing acid–base reaction as well as post-irradiation monomer conversion and crosslinking [
14].
Based on the results of this study, both dual-cured composites (Surefil One and Activa) showed lower VHN and BFS when compared to the light-cured bulk-fill material. Surefil One showed a high hardness yet demonstrated the lowest strength which suggests that this material should not be recommended in stress-bearing areas.
Activa on the other hand had the lowest hardness but showed significantly high strength. However, considering the flexibility of this material, it can be recommended for restoring cervical lesions as it can flex during function, which in turn minimize the stresses at the tooth/restoration interface and reduce the chances of failure [
54].
Laboratory aging provides an indication of materials’ long-term performance. However, this may not be directly reflected in clinical conditions. In this study, water storage did not lead to a significant reduction in the VHN and BFS of dual-cured bulk-fill materials. The 30-day storage period might not be long enough to elicit deterioration of the mechanical properties. Therefore, further studies including longer aging periods are recommended. Moreover, considering the ion-releasing property of these materials, a study investigating the interaction between ion leaching and mechanical properties is recommended.