Abstract
The purpose of this study was to investigate the influence of the light-irradiating times on the color changes of light-cured composite resins over specified periods of time.
Composite resin disks which were 10 mm in diameter and 1 mm in thickness, and made of the 3M Co. light-cured composite resin Silux Plus (shades: U, Y, DY, L, G, UO, YO) were made with 20 seconds (20 second group),40 seconds (40second group), and 120 seconds (120 secoud group) of light exposure through a l mm thick glass plate. The specimens were stored in artificial saliva at 37°C. Colorimetric values of the specimens immediately after curing and every 6 months for 3 years were measured without background color, backed by a white plate which was used to substitute for the lining material, using a fast spectrophotometer.
The following results were obtained.
1) Both in the cases of with and without background color, the ΔE* ab of all shades increased for 2 years 6 months proportionately to the time elapsed in the 20,40 and 120 second groups. However, the ΔE* ab of many shades decreased between 2 years and 6 months and 3 years, and the tendency was significant in the 20 second group.
2) For specimens without background color, compared to the ΔE* ab immediately after curing, the three-year ΔE*ab values ranged between 2.82 (DY) and 4.75(L) in the 20 second group,3.95 (YO) and 5.08 (L) in the 40 second group, and 3.82 (YO) and 6.06 (L) in the 120 second group.
3) For specimens measured against a white plate, compared to the ΔE*ab immediately after curing, the three-year ΔE*ab values ranged between 3.88 (DY)and 5.51 (L) in the 20 second group,5.01 (YO) and 6.76 (L) in the 40 second group, and 4.91 (YO) and 7.04 (L) in the 120 second group.
4) Compared to the ΔE*ab immediately after curing, three-year ΔE*ab values showed a tendency to increase in the longer light irradiation time group.
5) In the 20 second group and the 40 second group, the color of the resin disks showed a tendency to change to a chalky white color. It is supposed that the transparency changes of the composite resin might influence the color changes.
