Effect of pre-heating on the viscosity and microhardness of a resin composite

The effect of pre-heating resin composite on pre-cured viscosity and post-cured surface hardness was evaluated. Groups of uncured specimens were heated to 60 °C and compared with control groups (24 °C) with respect to viscosity and surface hardness. Mean (SD) viscosities of the pre-heated specimens (n = 15) were in the range of 285 (13)-377 (11) (Pa) compared with 642 (35)-800 (23) (Pa) at ambient temperature. There was a statistically significant difference between the two groups (P < 0·001). Mean (SD) Vickers microhardness (VHN) of the pre-heated group (n = 15) was 68·6 (2·3) for the top surface and 68·7 (1·8) for the bottom surface measured at 24 h post curing (specimen thickness = 1·5 mm). The corresponding values for the room temperature group were 60·6 (1·4) and 59·0 (3·5). There was a statistically significant difference between corresponding measurements taken at the top and bottom for the pre-heated and room temperature groups (P < 0·001). There was no significant difference between top and bottom measurements within each group. Pre-heating resin composite reduces its pre-cured viscosity and enhances its subsequent surface hardness. These effects may translate as easier placement together with an increased degree of polymerization and depth-of-cure.