Effect of radiant exposure and UV accelerated aging on physico-chemical and mechanical properties of composite resins

Abstract Currently, there is no consensus in terms of defining the minimum radiant exposure values necessary for achieving adequate properties of composite resin. In addition, the long-term influence that radiant exposure has on the properties of composite resins is still questionable. Objective: The objective of this study was to evaluate the effect of radiant exposure and UV accelerated aging on the physico-chemical and mechanical properties of micro-hybrid and nanofilled composite resins. Material and Methods: A nanofilled (Filtek Supreme; 3M ESPE) and a micro-hybrid composite resin (Filtek Z250; 3M ESPE) were investigated under different radiant exposures (3.75, 9, and 24 J/cm2) and UV accelerated aging protocols (0, 500, 1000, and 1500 aging hours). The degree of conversion (DC), flexural strength (FS), modulus (M), water sorption (WS), and solubility (WL) were evaluated. The results obtained were analyzed using two-way ANOVA and Tukey's test. Comparisons were performed using a significance level of α=0.05. Results: The DC, FS, and M were found to be significantly influenced by both radiant exposure and accelerated aging time. The DC and EM increased with radiant exposure in the no-aging group (0-hour aging) for both micro-hybrid and nanofilled composites, whereas no correlation was found after accelerated aging protocols. WS and WL of micro-hybrid and nanofilled composite resins were scarcely affected by radiant exposure (p>0.05), whereas they were significantly reduced by accelerated aging (p<0.001). Conclusions: Although increasing radiant exposure affected the degree of conversion and mechanical properties of micro-hybrid and nanofilled composites, no influence on the hydrolytic degradation of the material was observed. In contrast, UV accelerated aging affected both the physico-chemical and mechanical properties of the composites.

For the study of the behavior and longevity of dental restorative materials, accelerated aging under standardized laboratory conditions is an alternative because it allows the simulation of clinical longterm conditions in a short time period. Methods like water or water-ethanol immersion, artificial saliva immersion, hydrothermal cycling and ultraviolet (UV) accelerated aging system have been widely used for this purpose. 11 In this study, we aimed to evaluate

Accelerated aging protocol
The specimens of composite resin were exposed to UV accelerated aging for 500, 1000, and 1500 hours, which represent approximately 1, 2, and 3 years of clinical use, respectively. 12     Water sorption and solubility The water sorption (WS) and solubility (WL) were evaluated according to ISO 4049. 13 For such, composite disks (n=10) were prepared (15 mm in diameter and 1 mm in thickness), and the samples were polymerized following the curing protocol described above and subjected to the different UV accelerated aging processes.
Once the UV accelerated aging protocol was concluded, the samples were stored inside a desiccator and their masses were monitored daily until a constant value m 1 was attained, which was considered when the variation of two weights was less than 0.1 mg. After that, the diameter and thickness of the specimens were measured to obtain the volume of each one (V). Then, these specimens were immersed in distilled water at 37°C for 7 days and, after this time, the specimens were air-dried for 15 s and weighted to obtain the m 2 mass. Finally, the samples were stored again in a desiccator and were monitored until a constant mass was acquired (m 3 ). The values of W sp and W sl were calculated using the following equations:

Environmental scanning electron microscopy
Three cylindrical specimens (15x1 mm) for each group were fabricated and exposed to UV accelerated aging process according to section 2.3. After aging processes, its surface morphology was analyzed using

Results
The degree of conversion from the composite resins was found to be significantly influenced by both radiant exposure (p<0.001) and accelerated aging time (p<0.001), and an interaction between these two variables was observed (p<0.001) (  Although both the FS and modulus of the composite resins were significantly affected by radiant exposure and aging time, the interaction between these two factors was only significant for the modulus in both composites (p<0.001) (Figure 4). An increase in the modulus with radiant exposure was observed in the 0-hour aging group for both micro-hybrid (r=0.83; p<0.001) and nanofilled composite (r=0.88;  p<0.001), but no correlation was found after 500, 1000, and 1500 aging hours.
Water sorption and solubility of micro-hybrid and nanofilled composite resins were scarcely affected by radiant exposure (p>0.05). In contrast, these parameters were significantly reduced when the UV accelerated aging time increased in 3.75, 9, and 24 J/ cm 2 radiant exposures for both micro-hybrid (r≥0.80; p<0.001) and nanofilled (r≥0.60; p≤0.02) composites ( Figure 5).   Figure 6B). This becomes more evident after aging for 1000 ( Figure 6C) and 1500 hours ( Figure   6D). The nanocomposite Filtek Supreme presented a surface with apparently increased roughness after aging for 500 hours ( Figure 7B). The degradation of the organic matrix is evident after aging of 1000 and 1500 hours, since the filler particles seem to be more exposed ( Figure 7C-D).

Discussion
An improvement in the physico-chemical and mechanical properties of resin-based materials has been reported to take place upon increased radiant exposure. [3][4][5] However, the findings of this study indicate that the degree of conversion and the mechanical properties of micro-hybrid and nanofilled composite resins depend on both radiant exposure and UV accelerated aging time, whereas the hydrolytic degradation of the material depends only on the latter. Thus, the null hypothesis was rejected.
The degree of conversion increased with radiant exposure in the 0-hour aging group for both microhybrid and nanofilled composites, but no correlation was found after aging for 500, 1000, and 1500 hours.
These results corroborate with other studies that have also demonstrated an initial direct relation between radiant exposure and double-bond conversion. 14,15 Polymerization kinetics theory of light-activated composite resins suggest that the production of free radicals increases with radiant exposure, which is accompanied by an increment in the formation of multiple growth centers and a subsequent increase in the tendency to form a high cross-linked polymer. 16 Also, when materials are subjected to high levels of irradiance, the resulting rise in exothermic heat within the polymeric matrix could provide polymer chains with greater mobility, which would account for a higher degree of conversion observed in composites irradiated with 24 J/cm 2 . Achieving high values of degree of conversion is not only important to improve the mechanical properties of the material, but also has an important effect on the biological response of the pulp-dentinal complex, since unreacted monomers could be released into the medium and may irritate the pulp, generating an inflammatory response. 17 Despite the initial positive effect, it is worth mentioning that the influence of radiant exposure on the degree of conversion was found to disappear after 500, 1000 and 1500 hours of accelerated aging.
This behavior could be attributed to the effects of post-cure polymerization, heat, and UV irradiation.
It is well known that the polymerization reaction of light-activated composites continues even after the exposure to visible light irradiation is interrupted. 18 Although, during initial polymerization, free radicals located at the functional groups of unreacted molecules can be quenched within the polymer network due to rapid increase in viscosity, 19 our findings suggest that even low radiant exposures to visible light can provide the unreacted molecules with sufficient molecular mobility that allows for additional polymerization in the organic matrix to proceed.
The maximum values of conversion were achieved after aging for 1000 hours, which is likely due to the effect of heat and UV irradiation on the methacrylate groups. Heat and UV radiation are commonly used to initiate the polymerization of acrylate monomers. 20 Accordingly, photoinitiators that absorb light at the ultraviolet spectral wavelength region can be added to resin-based dental composites. 21 When a monomer is exposed to UV irradiation in the presence of initiators, large amounts of free radicals are immediately generated and strongly cross-linked polymer networks can be formed. 22 On the other hand, the increase in the internal temperature of the material allows for molecular relaxation and, as a consequence, an increase in the polymer chain mobility. 23 In such a condition, trapped free radicals could react with unpolymerized monomers, favoring additional crosslinking. 24 We observed a decrease in the degree of conversion after aging for 1500 hours, which could be attributed to some kind of degradation of the organic matrix. This degradation could be caused by the

Conflict of interest
This research is free of conflict of interest.