Effects of an Acidic Environment on the Strength and Chemical Changes of Resin-based Composites
The objective of this study was to evaluate the aging effects of long-term exposure to acidic beverages on the flexural strength (FS) and chemical reactions of two resin-based composites (RBCs) and one giomer. The FS of composite specimen bars (2 mm × 2 mm × 25 mm) was measured using a universal testing machine at various levels of thermocycling (TC; 0, 10,000, 50,000, and 100,000 cycles) in two beverages with different pH values (distilled water [DW], pH 7.0; Coca-Cola, pH 2.4-2.8). The FS data were analyzed using three-way analysis of variance with the post hoc Tukey test and t-test at a significance level of a=0.05. In DW, the FS of an RBC and a giomer did not decrease until 10,000 cycles. The other RBC, Z250, decreased rapidly until 50,000 cycles (p<0.05), followed by no additional decrease until 100,000 cycles. In Coca-Cola, the FS of two RBCs and a giomer decreased more rapidly than in DW from 10,000 cycles (t-test, p<0.05). In Coca-Cola, the increased porosity observed in scanning electron microscopy (SEM) images, the changes of the hydroxyl peak at 3340 cm−1 and ester peak at 1730-1700 cm−1 in Fourier-transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR) spectra, and the continuous increase of the Si-O/Si-C peak height ratio from 10,000 cycles to 100,000 cycles in X-ray photoelectron spectroscopy (XPS) suggested an increased loss of silane-carbon bond between the matrix and fillers of the Z250 RBC, compared to those in DW. In conclusion, when TC was performed in DW, unreacted monomers and a coupling agent were washed out, which caused porosity and reduced FS. In Coca-Cola, acidic conditions accelerated the removal of the matrix through the hydrolysis reaction at the ester groups, resulting in more porosity and a faster decrease in FS than in DW.SUMMARY
Schematic illustration of light-curing of specimens in a polyethylene mold (2 mm wide × 2 mm high × 25 mm long).
Changes in the scanning electron microscopic images of Beautifil II with prolonged thermocycling. After 100,000 cycles in Coca-Cola, noticeable pores are observed. (a) 10,000 cycles in DW; (b) 50,000 cycles in DW; (c) 100,000 cycles in DW; (A) 10,000 cycles in Coca-Cola; (B) 50,000 cycles in Coca-Cola; (C) 100,000 cycles in Coca-Cola. (0) shows the specimen surface before thermocycling, ie, immediate after curing. Abbreviations: DW, distilled water.
Changes in the scanning electron microscopic images of Tetric N-Ceram Bulk fill with prolonged thermocycling. After 100,000 cycles in Coca-Cola, noticeable pores are observed. (a) 10,000 cycles in DW; (b) 50,000 cycles in DW; (c) 100,000 cycles in DW; (A) 10,000 cycles in Coca-Cola; (B) 50,000 cycles in Coca-Cola; (C) 100,000 cycles in Coca-Cola. (0) shows the specimen surface before thermocycling, ie, immediate after curing. Abbreviations: DW, distilled water.
Changes in the scanning electron microscopic images of Z250 with prolonged thermocycling. (a) 10,000 cycles in DW; (b) 50,000 cycles in DW; (c) 100,000 cycles in DW; (A) 10,000 cycles in Coca-Cola; (B) 50,000 cycles in Coca-Cola; (C) 100,000 cycles in Coca-Cola. (0) shows the specimen surface before thermocycling, ie, immediate after curing. Abbreviations: DW, distilled water.
FTIR-ATR peaks of Z250 specimen in DW. The ester peak in Bis-GMA also has a carboxylic peak in addition to the ester peak; as a result, the peak (1718 cm−1) appears broad from 1730 cm−1 and 1700 cm−1. The OH peak in Bis-GMA can be observed around 3340 cm−1. Abbreviations: DW, distilled water.
FTIR-ATR peaks of Z250 specimen in Coca-Cola. The -OH peak observed at 3340 cm−1 in DW (red in the left box) was lowered and shifted to 3270 cm−1 (green). The 1718 cm−1 peak, which appeared relatively wide in DW (red in the right box), was shifted to 1733 cm−1 and became sharper (green). Abbreviations: DW, distilled water.
Deconvoluted C1s peaks showing their component peaks in Z250 resin-based composite. When the number of cycles increased, the intensity of the C1s (C=C) peak decreased relatively compared to that of the C1s (C-O) peak in both conditions. In the DW conditions, the intensity of the C-O peak relative to that of C=C peak gradually increased with the number of cycles; however, in Coca-Cola conditions, that of the C-O bond relative to that of C=C peak increased rapidly. Abbreviations: DW, distilled water; Coke, Coca-Cola.
Deconvoluted Si2p peaks showing their component peaks in Z250 resin-based composite. Abbreviations: DW, distilled water; Coke, Coca-Cola.
Schematic illustration of the shape and shift of the hydroxyl and ester peaks, wherein multiple bonding energies may be merged to form a peak (Upper illustration). Middle illustration: Methacrylic acid (MA, orange) is washed out and removed from the surface, which narrows and shifts the ester peak (from red to green in the upper-right illustration). In contrast, the other end of the hydrolyzed Bis-GMA (Bis-HPPP) changes to an -OH group (blue) and adds another -OH peak to the spectrum, making the peak shift and broaden (from red to green in upper-left illustration). Lower illustration: Mechanism of hydrolysis of Bis-GMA. During thermocycling at 55°C, the large amount of H+ contained in the Coca-Cola can hydrolyze Bis-GMA into bishydroxy-propoxy-phenyl-propane (Bis-HPPP) and MA.
Silanization process of SiO2 with γ-MPS. In XPS, the bonds shown in red are observed as Si-C peaks and those shown in blue are observed as Si-O peaks.
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