الفهرس 
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Abstract
This study was conducted to evaluate the effect of different aging protocols on some properties of 2 commercially widely used dental resin composite materials; a nano and a nano hybrid. The selected aging protocols were mechanical aging, thermocycling, chemical aging and combination of all aging methods. The properties to be evaluated were flexural strength, surface micro-hardness, color stability and translucency. Surface morphology by SEM was also evaluated.
A total of 210 samples of the two different types of composites were prepared; 105 each. Fifty five rectangular shaped samples from each type were prepared for flexure strength test and 50 cylindrical samples from each type for micro-hardness, optical and surface morphology tests. The samples were divided into five groups according the type of aging protocol to which they were exposed; Gp1 (non aged control group), Gp2 (mechanically aged samples for 6000 loading cycles), Gp3(thermally aged samples for 6000 thermocycles at temperature between 5oC and 55oC with dwell time 25 sec.), Gp4(chemically aged samples by storage in specially prepared acidic solution with pH 3.6 for 8 days) and Gp5 (samples were treated with all previous aging protocols).
For transverse (flexural) strength test, 110 rectangular samples with dimensions 25 mm in length and 2 mm in both thickness and width were subjected to 3 point bending test after different adopted aging protocols. Flexural strength was calculated in MPa according the following equation: σ=3FL/(2bd^2 ) , where σ is the flexural strength, F is the maxim load in Newton, L is the distance between two supporting points, b and d are the width and the thickness of the sample respectively.
Regarding surface micro-hardness test, it was determined by using Digital Display Vickers Micro-hardness Tester. Micro-hardness was obtained using the
following equation: HV=1.854 P/d^2 , where, HV is the Vickers hardness in Kgf/mm2, P is the load in Kgf and d is the length of the diagonals in mm.
Using a Reflective spectrophotometer, the samples’ colors were measured according to the CIE L*a*b* color space relative to the CIE standard illuminant D65. The color difference (ΔE) of the disc shaped samples was evaluated using the following formula: ΔECIELAB = (∆L*2 + ∆a*2 + ∆b*2) ½, where: L* = lightness (0-100), a* = change the color of the axis red/green and b* = color variation axis yellow/blue. The translucency parameters (TP) values were obtained by calculating the color difference of the disc shaped samples over standard black and white backgrounds.
For surface morphology, a disc sample from each group was subjected to gold sputtering by sputtering machine, and then was examined by scanning electron microscope (SEM).
The obtained results were submitted for statistical analysis, for transverse (flexural) strength results, Gp3 and Gp5 showed the lower mean values than those of Gp2 and Gp4.group 1 recorded the highest mean value. Mechanically and combined aged Nano filled composite (Gps 2B & 5B) showed significant reduction in transverse (flexural) strength compared to nano hybrid type (Gps 2A & 5A) after same aging methods.
For surface micro-hardness, it showed a significant decrease after all aging protocol, especially chemically aged group (Gp4) which recorded the lowest mean value. Regarding the effect of different aging protocols on both tested composite types, there were no significant differences in micro-hardness values between the two tested materials after different aging protocols.
Regarding color stability and translucency, the color change (ΔE) was significant of the two composites after all aging protocols, leading to clinically unaccepted color change. Generally, there was an increase in translucency compared to control group (Gp 1). Thermally and combined aged groups (Gps 3&5) showed alterations below perceivable limit, while mechanically and chemically aged groups (Gps 2&4) caused perceivable changes.
For surface morphology by SEM, the surfaces of aged samples showed changes compared to control samples.
Conclusions:
Within the limitations of this study, the following can be concluded:
The followed mechanical aging and chemical aging protocols of tested resin composites can be used interchangeably when their transverse strength is to be evaluated.
Simple thermal aging would be sufficient aging protocol compared to more complex combined aging protocol of tested resin composites before their 3-point loading test.
All aging protocols resulted in significant reduction in transverse strength of both tested materials.
The applied mechanical aging, thermocycling or combined protocol can be used interchangeably when micro hardness of tested composites is considered, however, mechanical or thermocycling protocols are much simpler than combined one.
Mechanical aging and combined aging protocols could produce a significant higher change in color of nano-filled composite than nano-hybrid one. In contrast, thermal aging resulted in a significant higher color change (ΔE) of nano-hybrid composite.
Chemical aging protocol produced comparable results in the color change (ΔE) of nano-hybrid and nano-filled composites.
Change in translucency to values above the perceivable limit occurred after mechanical and chemical aging protocols.
Mechanical aging and combined aging protocols resulted in comparable results of translucency between the two tested composite materials. On the other hand, thermal aging and chemical aging protocols effects were significantly different between both types of composites.
Surface morphology altered after all aging protocols.