Water-absorption affects the basic properties of resin-modified glass-ionomer cements (RMGICs). Fick's law is usually invoked to explain the absorption process. The purpose of this study is to show that the absorption in accordance with the Fickian model cannot be extended to the whole of the specimen, and that microcrack formation is the main degradation mechanism for specimens cured in a closed environment. For this purpose, flat disk-shaped paste specimens 1.5 mm thick (aspect ratio 4), irradiated in closed conditions between two glass slides and stored in water for approximately 20 months, were analyzed periodically gravimetrically and under confocal fluorescence microscopy, with absorbed eosin used as the fluorescent probe. At pH 7.0, the specimen surface (10-20 micrometers in depth) absorbed water rapidly, swelled, and disintegrated in 20-40 days. Long-term storage produced isolated cracks and grains, no progress in the swelling, and a slow weight decrease. A lower pH (pH 3.5) produced a significant increase of the number of microcracks. The decrease in the irradiation time (30 s or less) enhanced the erosion process, producing very broad cracks. It was concluded that the prevalent mechanism of long-term hydrolytic degradation was based on the slow formation of cracks, whereas only in the early stage of storage did absorption occur quickly in accordance with the Fickian diffusion.
Linear thermal contraction of impression materials has been measured in the 40-18 ~ temperature range immediately after the polymerization shrinkage. Samples of 2 cm in length have been analysed by a scanned laser beam micrometer and the coefficient of thermal contraction has been calculated. Silicone-based specimens of different viscosities have been tested. The temperature-induced shrinkage has been compared with the time-dependent shrinkage.PACS 87.90 -Other topics in biophysics, medical physics, and biomedical engineering. PACS 81.70 -Materials testing. PACS 65.70 -Theoretical expansion and thermomechanical effects.
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