Introduction: The characteristics of a suitable restoration material is having acceptable mechanical properties, protecting teeth against decay and ease of use in clinics. Diet can affect properties of restorative materials in the mouth. Since amongst important properties of composite restorations are mechanical properties such as hardness, the aim of this study was evaluation of the effect of Coca-Cola Beverages on surface hardness of a silorane based (p90) and methyl methacrylate (p60) composites.

Methods: In this experimental study, thirty disc-shaped specimens (5 × 2 mm) of each of methyl methacrylate based (Filtek p60, 3M Dental Products, USA) and a silorane based (Filtek p90, 3M Dental Products, USA) composite resins were prepared, according to manufacturers' instructions. Specimens were randomly divided to four groups as follows (N = 15): G1: Filtek p90 without exposure to Coca-Cola, G2: Filtek p90 with exposure to Coca-Cola, G3: Filtek p60 without expure to Coca-Cola, G4: Filtek p60 with expure to Coca-Cola. The specimens were exposed to regular soft drinks Coca-Cola, (Khoshgovar, Tehran, Iran) at room temperature for seven days with a frequency of three times daily for 20 minutes at a time. In the remaining times of the day, they were kept in distilled water. Then, micro hardness measurements were made for each specimen with a Vickers hardness testing machine (Buehler, Lake Bluff, IL, USA) under 500 g of force for 15 seconds. Data were analyzed using SPSS 18 and independent t-test at a significance level of 0.05

Results: Micro hardness values of four groups were G1: 68.28 ± 2.65; G2:59.56 ± 6.61; G3: 93.5 ± 2.38; and G4:86.76 ± 5.47, respectively. The results of this study showed that Coca-Cola reduces the surface hardness of the two composite materials (P < 0.05).

Conclusions: The results showed the hardness of both Filtek p90 and Filtek p60 composite significantly decreases with Coca-Cola yet the reduction was equal.