Pre-heating mitigates composite degradation

Abstract

Dental composites cured at high temperatures show improved properties and higher degrees of conversion; however, there is no information available about the effect of pre-heating on material degradation. Objectives This study evaluated the effect of pre-heating on the degradation of composites, based on the analysis of radiopacity and silver penetration using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). Material and Methods Thirty specimens were fabricated using a metallic matrix (2x8 mm) and the composites Durafill VS (Heraeus Kulzer), Z-250 (3M/ESPE), and Z-350 (3M/ESPE), cured at 25°C (no pre-heating) or 60°C (pre-heating). Specimens were stored sequentially in the following solutions: 1) water for 7 days (60°C), plus 0.1 N sodium hydroxide (NaOH) for 14 days (60°C); 2) 50% silver nitrate (AgNO3) for 10 days (60°C). Specimens were radiographed at baseline and after each storage time, and the images were evaluated in gray scale. After the storage protocol, samples were analyzed using SEM/EDS to check the depth of silver penetration. Radiopacity and silver penetration data were analyzed using ANOVA and Tukey's tests (α=5%). Results Radiopacity levels were as follows: Durafill VS<Z-350<Z-250 (p<0.05). The depth of silver penetration into the composites ranked as follows: Durafill VS>Z-350>Z-250 (p<0.05). After storage in water/NaOH, pre-heated specimens presented higher radiopacity values than non-pre-heated specimens (p<0.05). There was a lower penetration of silver in pre-heated specimens (p<0.05). Conclusions Pre-heating at 60°C mitigated the degradation of composites based on analysis of radiopacity and silver penetration depth.

Figure 1. Composition of the assessed composite resins

Figure 2. Mean radiopacity values obtained for the three composite resins assessed a) according to storage solution (Tukey’s test); b) according to curing temperature and storage solution (ANOVA); and c) according to storage solution only (Tukey’s test for paired data). Different letters indicate statistically significant differences (p<0.05)

Figure 3. Mean values obtained for depth of silver penetration: a) according to temperature (ANOVA); and b) according to type of composite resin (Tukey’s test). Different letters indicate statistically significant differences (p<0.05)

Figure 4. Patterns of silver penetration obtained via linear analysis with EDS, in association with different temperatures and composites. The length of the waved portion of the yellow line determines the depth of silver penetration. Red arrows indicate the direction of the quantitative digital linear scanning across each specimen’s surface

Figure 5. Backscattered electron micrographs of Durafill VS resin impregnated with silver. a) Extensive silver penetration over the first µm of the surface (x2,700). Note penetration of silver particles around fillers (*) and inside pre-polymerized filler particles (arrows). b) Higher magnification of the same area (x10,000)

Figure 6. Backscattered electron micrographs of Z-250 resin impregnated with silver. a) Extensive silver penetration over approximately 11 µm of the surface, reaching 26.803 μm (x1,000). The arrow shows silver surrounding a filler particle. b) Higher magnification of the same area (x10,000). The arrow shows silver surrounding a filler particle; the asterisk (*) shows no evidence of silver within the particle

Figure 7. Backscattered electron micrographs of Z-350 resin impregnated with silver. a) Extensive silver penetration over approximately 68.40 µm into the subsurface. The metal is concentrated at specific sites (x1000). b) Another specimen of the same resin group impregnated with silver. The asterisk (*) shows metal surrounding nanoclustered fillers; the arrow shows metal inside nanoclusters (x10000)