In vitro Comparative Study of Compressive Strength of Different Types of Composite Resins in Different Periods of Time

     To evaluate the ultimate compressive strength of five composite resins after 1 hour, 24 hours, 7 days and 1 month. Twenty four cylindrical 4 mm×8mm specimens of each commonly used composite resins in posterior region (Nulite-F , Z250 , P60, Biscore, Tetric ceram HB) were prepared. Each group of composite resins were divided into four time groups of 1 h, 24 h, 7 days and 1 month. All of specimens stored in an incubator with 37 °C. After each period of time all of the specimens were tested by Zwick/Roell Z020 (Germany) for ultimate compressive strength at cross head speed of 0.5 cm/min. Results were analyzed by ANOVA and Scheffe test. P60 and Z250 had the highest and Nulite-F and Tetric ceram HB had the lowest compressive strength at all the times. The difference between these two groups was statistically significant (p < 0.05). The results of the Biscore was steady among the other groups. It is suggested to use Z250 or P60 in posterior restorations instead of the other composite resins tested.