Influence of storage temperature on vickers microhardness of resin composite

Abstract

Introduction: The purpose of this study is to evaluate the effect of three storage temperatures on microhardness of high and low viscosity bulk-fill materials and compare them with conventional resin-based composite materials. Materials and method: Six composite resin-based materials were used in this study (TN, TNB, TNF, FZ250, FB and FBF) samples were subdivided into three groups based on the pre-curing storage temperature (5°C, 23°C and 37°C). Light polymerization for each material was performed based on the manufacturer’s recommendation using Bluephase G2 curing unit (Ivoclar Vivadent, Schaan, Liechtenstein) in a high-intensity mode with an irradiance of 1200 mW/cm2. Vickers hardness values of top and the bottom surfaces of each sample were evaluated using (NOVA 130 series, Vickers and Knoop hardness testing instrument) under a 200 g load with a dwell time of 10 sec. Also three indentations with the random distance of 1 mm were taken from the top and the bottom surfaces of each sample and a mean Vickers hardness (VHN) value were calculated (n=18 top and n=18 bottom). The mean bottom/top ratio was calculated by dividing VHN of the bottom surface by VHN of the top surface. Results: When the tested materials were stored at room temperature (23°C) before testing in the present study, they failed to reach the minimum 80% of the mean bottom to top hardness value ratio except for FZ250 and FBF, where they reached 97.8% and 83.2% respectively. Where in samples that were stored refrigerated at 5°C all the materials have reached the minimum 80% of the mean bottom to top hardness value ratio except for FBF (77.3%) and TB (77.2%). On the other hand, the only material that reached the minimum 80% of the mean bottom to top hardness value ratio when the materials were stored at 37°C was FZ250 (93.5%). Conclusion: Despite the promising results from this preliminary study, regarding improvement of microhardness with refrigerated composite resins, further research has to be conducted. The enhancement of hardness values associated with preheated composites could be beneficial in countries with warm climate such as Saudi Arabia. The association of precooled composite resin and the use of the LED curing units could be recommended to improve resin-based composite hardness. Further research is needed to evaluate the other mechanical properties and whether or not they are influenced by storage temperature.