Effects of indentation load and immersion period in water on Vickers hardness of denture base resins

The purpose of this study was to examine the effects of indentation load and storage in water on Vickers hardness and viscoelastic recovery of indentation of denture base resins. Two types of acrylic denture base materials were investigated; Vertex (VT) and Acryshot (AS). Five resin disks each were prepared in 20 mm diameter and 1.8 mm in thickness) and highly polished according to manufactures recommendations. Vickers hardness test was conducted at a load of 25, 250, 500, 1000, 2000 gf with a dwell time of 30 s from one specimen. Vickers hardness (HV) and maximum indentation depth values at each load were averaged from three measurements. Vickers diagonal length of each indentation was re-measured after water storage of 1, 3, and 7 days, and HV values of specimens were recalculated. There were no significant differences in HV between indention loads (p>0.05), indicating no load dependency in HV value of specimens. Moreover, the HV value of specimens did not show statistically significant change after water storage up to 7 days. However, the indentation depth (D_m) values were higher than the indentation depth (D_c ) calculated from Vickers indentaiton geometry. The difference between D_m and D_c was increased with increase in indentation load, indicating some load dependency. This phenomenon was probably due to immediate viscoelastic recovery of denture base resins. Vickers hardness with indentation depth measurement can be a useful tool to investigate mechanical behavior of denture base resins.