| Up to now, resin luting cements are so popularly used in clinic, due to the remarkable mechanical, physical and bonding properties, and the excellent esthetic effects. Resin luting cement can be divided into three kinds according to the curing modes:self-cured (chemical cured), light cured, and dual cured resin (light cured & chemical cured) cement. Light cured resin cement can be used for bonding ceramic or composite restoratives. When the restoratives are seated and then light cured, the light intensity can be attenuated by the restorative materials, and the degree of conversion of resin cement will be reduced and large amount of unconverted "c=c" will be residual. Low degree of conversion can lead to weakness of strength and hardness and the lower color stability of resin luting cement, and can causing high solubility, and disturbing the biological properties. The cytotoxicity of uncured resin luting cement will be more severe than the cured ones. To solve all the problems cause by insufficient cure of light cured resin cements, dual cured resin luting cements were introduced. After light curing to the dual-cured resin luting cement, the self-curing will also be generated and thus compensate the insufficient curing caused by light curing only. However, previous studies has confirmed that if the dual cured resin luting cement is not light cured, the degree of conversion will reduced. But, there is no evidence to support the theory that large quality of chemical curing can occur after light curing. And it is uncertain whether the curing method would affect the degree of conversion.The purpose of this present study was to examine the effects of light curing, prolonged curing time and soft-started curing mode on the degree of conversion (DC) of two dual-cured resin luting cements: Panavia F & Clearfil SA-LUTING resin cement.In experiment 1, the effect of light curing on the polymerization properties of dual-cured resin cements has been evaluated. The observation periods were selected as 20min, 24h and 1mo.. We could obtain the results as the following:â‘ Within 20min, the two tested materials without light curing didn't cured; without light curing, the DC of PF group was unchanged, and that of SA group increased slowly;â‘¡Within 24hs, the DC of two materials increased with time;â‘¢Within 1 month, the DC of two materials increased with time, and after 24hs, the trend of DC increased gently;â‘£For every time tip, the DC of SA was higher than that of PF, and DC of light cured group was higher than the group without light curing.In experiment 2, the effect of prolonged light curing on the DC was observed. The results showed that:â‘ Within 20min, the DC of PF groups of light cured for 20s and 40s groups could increase quickly at the beginning and kept in flat later, the DC of group being light cured for 60s increased slowly; and for SA group, when light cured for 20s, 40s and 60s, its DC raised slowly;â‘¡within 24hs, the DC of two materials increased with time;â‘¢within 1mo., the DC of two materials increased with time, and the DC increased gently after 24hs;â‘£for the same material, prolonged curing time could increase the DC significantly.In experiment 3, the effect of soft-started light curing on the DC was determined. The results showed that:â‘ for both the materials, within 20min, 24h and 1mo., the DC increased with time;â‘¡with the same treatment, the DC of SA was higher than that of PF;â‘¢In the same observation tip, the DC of soft-started group was higher than that of the normally cured group regardless of the type of the cement.According to the results from this present study, we could draw the conclusions as the following:1. Light curing should be one initiator of the curing process of dual-cured resin cement. If the dual-cured resin cement was not exposed to light irradiation, no polymerization would be induced during the polymerization process. For groups with no light irradiation, although polymerization happened after 20min, the DC of light cured groups were higher than the groups without light curing during all the curing process. Thus, light curing should be provided to the dual-cured resin cement to activate and promote the reaction of polymerization.2. Light curing time might affect the DC of resin cement. Among all the observing period, prolonging the curing time could increase the DC. And for group with 60s'light curing, the polymerization at the first period could go to the pit quickly. Thus, 60s is the recommended curing time from this study. 3. The light intensity can affect the DC of dual-cured resin cement. In soft-started light curing group, the light intensity received was lower than that of normally cured group and would affect the DC of the resin cement. So, considering the complete DC of dual-cured resin cement, the soft-started curing mode sould not be recommended to the clinic.
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