Influence Of Long-term Water Immersion On The Coefficient Of Thermal Expansion Of Low Shrinkage Composite Resins

Light-cured composite resins have been widely used both in the dental treatment and the prosthodontics field because of the simulation of aesthetics features, great physical and chemical properties and the characteristics of easy operation. The thermal expansion is one of the most important properties of light-cured composite resins. As an evaluation index, coefficient of thermal expansion(CTE) has a significant effect on the bond strength between the composite resins and the teeth.When the oral temperature has greatly changed, the difference of CTE between composite resins and teeth might result in adverse effects such as microleakage,secondary caries, postoperative sensitivity and even pulp damage etc. These complications would lead an undesirable influence on the long-term restorative effects.The ideal CTE of light-cured composite resins should be close to that of the dentin and enamel’s which are 17×10-6/℃ and 11×10-6/℃ from zero to 60 degree. However,the commercial light-cured composite resins whose CTE are much higher than the enamel and dentine’s CTE will induce negative effects on long-term restoration.At present, there are few researches on the changes of the CTE ofcomposite resins after water aging There is hardly any research on the CTE of the low shrinkage of composite resins which have had a long-term water immersion.Objective:This experiment investigated the change of the CTE of composite resins during long-term artificial aging process, the change rule and influence factor of the composite resins’ CTE.Methods:This experiment used six composite resins(4 kinds of low shrinkage composite resins: Clearfil Majesty Posterior, Kalore GC, Filtek? LS and Admira;2 kinds of universal composite resins: Filtek? Z250,Neofil Nano). Used the ThermomechanicalAnalyzer(TMA) to measure the CTE of the six composite resins respectively after they were immersed in 37℃ distilled water for about 24 hours,1month,3months and6 months.Then made a statistical analysis on all the datum. The coefficient of thermal expansion for each specimen was calculated according to the formula: CTE=△L/L0△T.Results:Clearfil Majesty Posterior, Filtek? Z250, Admira and Neofil Nano have been showed that after immersed in the water for four different times, the CTEs increased first and then declined, and the CTEs showed a maximum when they were immersed in the water for 3 months(P<0.05). When they were immersed in water for 6 months,the CTEs had no significant different between the CTEs which were immersed in water for 24 hours and 1 month(P>0.05). The CTEs of Kalore GC and Filtek? LS had no significant difference after immersed in water for four times(P>0.05).Conclusion:With the increasing of the immersing time and temperature, the CTE of the light-cured composite resins changed. The changes were relative to the composition and structure of the material. The kind of prepolymerized composites,and silorane-based composites in low shrinkage composite resins have better thermal stability. The CTE of the composite resins rise as the temperature increasing.