The Influence Of Accelerated Aging And Toothbrushing On The Color And Microstructure Of Light-Curing Resin Composites

Objective The aim of this investigation was to examine the effect after aging, immersion and toothbrushing on the color stability, surface microstucture and inner chemical bonds of four light-curing resin composites, and the relation the color differences with the microstucture and chemical bonds. Methods The composites tested in the investigation were P60,Z100,Z250,Z350. Discs were prepared with a diameter of 12 mm and a thickness of 2.0±0.05 mm. The groups of aging and immersing had five specimens of each composites respectively, and the group of ESEM+FTIR had 3 specimens of each composites. All tested samples were stored in distilled water at 37℃for 24h.The color of the specimens of the aging and immersing groups were measured by CIE L*a*b* with a spectrophotometer. The groups of aging and ESEM+FTIR were accelerated aged for 24h and 96h. The immersing groups were storied in distilled water at 37℃for 24h and 96h as the control. Then the specimens of aging and immersing groups after treatment for 96h were subjected to toothbrushing. The color of aging and immersing groups were measured at the different internals. Examining the specimen of each composites after different treatments used an ESEM and a FTIR spectrophotometer. The color change (△E*ab) was calculated and analyzed statistically by repeated-measures 2-way analysis of variance after aging for 24h and 96h. The statistical analysis of△E*ab was also carried out with a two-sided T-test for the samples between the aging and immersing groups at the same stage. The relationship between△E*ab and△L*,△a*,△b* was analyzed statistically by liner regression ,liner correlation and multiple linear regression. The level of significance was defined asα=0.05. Results The materials showed statistically significant differences in color between the aging and immersing groups (P<0.05).No significant differences could be found between the four kinds of materials after immersing (P>0.05). The△E*ab of the aging and immersing groups after toothbrushing had not statistically significant differences (P>0.05). The absorbance of FTIR after aging for 24h was stronger than for 96h. And the absorbance of FTIR of the toothbrushing after the aging specimens was stronger than that of the immersing. The chemical bonds of the specimens were weaken or broken after aging or toothbrushing. The matrix appeared some concaves and pores and the filler particles exposed after aging .The microstructure was changed seriously after aging and toothbrushing. Conclusion The color differences after aging were bigger than that after immersing at the same time. Among the three treatment stages, the greatest color changes occurred during the first aging cycle for 24h. The hybrid filled composites had the best color stability, the microfilled composites had the worst color stability, and the nanofilled composites had the medium ones. Going with aging, the composites of the wear resistance was declined after toothbrushing. The roughnees of the composites surfaces of the aging groups were more serious than that of the immersing groups. The energy of chemical bonds were weaken or broken under the aging and toothbrushing. The unsaturated polymer reacted again ,and caused the color change of the resin composites.