Effect of Photopolymerization Variables on Composite Hardness

Abstract

The purpose of this study was to characterize the relation between total light energy and the final hardness of 4 composites polymerized under varying conditions. Four commercial composite restorative resins (Tetric Ceram, Ivoclar Vivadent; Admira, Voco; Arabesk, Voco; Surefil, Dentsply) were used to prepare 5 disk-shaped specimens (10 mm x 2 mm and 10 mm x 4mm) for each experimental condition. Photoactivation was carried out with a light device. The light-to-material distance was varied: (1) the light was maintained for 40 seconds with the end of the light source in contact (0 distance); or (2) the light was maintained for 40 seconds with the end of the light source is 10 mm away. Immediately after polymerization, Vickers hardness and surface roughness were determined on specimens surface that had been exposed to the light and on the opposing surface also. The specimens then were examined in the scanning electron microscopy (SEM) in different sections. The results were analyzed statistically. The Vickers hardness values of Surefil, light applicated by 0 distance and 10 mm away, were significantly higher compared to other materials (p<0.001). The opposite surface of the surface exposed to light showed lower Vickers hardness values in all groups with the thickness of 2 mm and 4 mm. Introduction Light cured composites have become almost universal in modern clinical dentistry. A visible light source is employed to produce the work, that converts a monomer or oligomer into polymers and produces transformation of a viscous paste into a reasonably solid final product. The factors affecting the intensity of the light radiation are the power of the measuring device, the time the device is operated and the surface area over which the light is applied [1]. The problem associated with photopolymerization is that the amount of light available to excite the photoinitiator decreases form the top surface inward as a result of light absorption and scattering [2]. There are many studies investigating the efect of light intensity on depth of polymerization, polymerization conversion, material properties, gap formation, shrinkage, and adhesion [3,4]. Variations in light-polymerizing parameters may effect the physical characteristics of polymerized resin. This study was performed to investigate the physical characteristics (microhardness and surface roughness) of four dental restorative resins under varying polymerization distance and restorative thickness. Materials and Methods Materials used in this investigation are listed in Table 1. Table 1. Materials used in this investigation. Material Material type Filler size Filler content Manufacturer Admira TM Ormocer based restorative 0.7 μm 78 wt% Voco, Germany Surefil TM Packable composite 0.8-0.10 μm 82 wt% Dentsply,USA Arabesk TM Microhybrid composite 0.7 μm 56 wt% Voco, Germany Tetric Ceram TM Hybrid composite 0.04-3.0 μm 80 wt% Vivadent, Liechtenstein Five disk-shaped specimens were fabricated for each experimental condition. The specimens of the various restoratives were prepared in 10 mm diameter x 2 mm depth for one group; and 10 mm Key Engineering Materials Vols. 264-268 (2004) pp. 2027-2030 online at http://www.scientific.net © (2004) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 160.75.44.253-19/09/06,09:44:10) diameter x 4 mm depth for the other group. Light polymerization was performed by using a lightcuring unit (Hilux) with a light intensity of 600 mW/cm 2 . Polymerization: was maintained for 40 seconds with the end of the light source in contact with the sample (0 distance); or the end of the light source is 10 mm away. All the specimens were stored in distilled water for 24 h at 37 °C. After the storage period, the specimens were blotted dry and subjected to hardness testing. Vickers hardness measurements were used as an indicator of the effects of varied polymerization distance and restorative thickness on four dental restorative resins. The specimens were positioned beneath the indenter of the tester (Tukon, USA) and a 250 g load was applied through the indenter with a dwell time of 45 s. The hardness test was performed both on the surfaces that had been directly exposed to the light and on the opposing surface. Five measurements were made on each surface of the specimens, and their average was recorded as the final hardness value of the surface. Following the hardness tests, the specimens were subjected to surface roughness test. The surface roughness test was performed both on the surfaces that had been directly exposed to the light and on the opposing surface. Finally, the specimens were carbon coated under vacuum and were examined using SEM (JEOL JSM-T330) to investigate the differences between the two surfaces (the surface directly exposed to light and the opposing surface) of the restoratives. The results were analyzed statistically with Kruskal Wallis, Mann Whitney-U and one way ANOVA. The groups for each material in our investigation were as following: aupper surface, or the surface exposed directly to light, bbottom surface, or the opposing surface not facing the light source, Ispecimen thickness: 2 mm; end of the light source in contact or 0 distance, IIspecimen thickness: 2 mm; end of the light source 10 mm away from the surface. IIIspecimen thickness: 4 mm; end of the light source in contact or 0 distance, IVspecimen thickness: 4 mm; end of the light source 10 mm away from the surface. Results and Discussion The hardness values of the investigated materials are given in Table 2a and b. Table 2a Hardness values of the investigated materials (upper surface) (*p<0.001) Admira Surefil Arabesk Tetric Ceram Groups Mean SD Mean SD Mean SD Mean SD I-a 50.06 1.42 82.50* 2.60 52.55 1.27 63.88 1.51 II-a 47.90 2.96 75.92* 1.51 45.65 .84 42.86 1.37 III-a 46.00 1.52 75.24* 1.75 46.93 1.65 50.06 1.42 IV-a 46.30 1.96 80.98* 1.99 45.36 1.93 40.47 2.63 Table 2b Hardness values of the investigated materials (bottom surface) (*p<0.001) Admira Surefil Arabesk Tetric Ceram Groups Mean SD Mean SD Mean SD Mean SD I-b 33.48 2.99 68.87* 2.18 41.64 1.56 44.28 .56 II-b 33.67 1.70 59.90* 1.52 34.63 1.15 38.70 1.19 III-b 30.62 1.15 36.00 1.58 41.60 2.41 34.48 2.49 IV-b 21.21 1.96 26.96 3.07 21.33* 1.18 27.97 1.39 The upper surfaces of the Surefil group were significantly harder (p<0.001) than those of the other groups, which were exposed to light by 0-10 mm. There is a decrease in the hardness values of the bottom surfaces of all materials. The highest hardness values belonged to Surefil samples, which were followed by Tetric Ceram and Arabesk. The lowest hardness values were found in the 4-mm thick samples. The difference between the hardness values of the upper surfaces was found to be significant in Tetric Ceram and Arabesk samples (I-a/ III-a) with regard to p<0,05. Euro Ceramics VIII 2028