Study On The Setting Chemistry Of Glass Ionomer Cement

The subjects of the thesis was to investigate SiO₂·Al₂O₃·CaO·CaF₂·P₂O₅ system glass using for glass ionomer cements. The influence of composition and structure of the glass on the properties of the glass ionomer cement were considered initially, but the focus of the thesis was the setting reaction mechanism and the mature process of the glass ionomer cement.The structure of SiO₂·Al₂O₃·CaO·CaF₂·P₂O₅ system glass was studied by means of FTIR, DSC/TG and X-ray. After acid-etched, the FTIR spectral had a little change for fluoride-free in the glass composition, while for fluoride-contain glass, the spectral band of Si-0 stretching and contracting vibrate at 1006-1024 cm^-1 separated into 1086cm^-1 and 996cm^-1～1024cm^-1, and absorbing strength changed significantly. It indicates that differ reactivity phase exists in the fluoride-contained glass and the setting time delayed is due to the silica gel layer partly screen the glass particle after acid-etched. Except phase separate and crystallization, loss weight constantly and continuously from 180℃to 800℃in the heat treatment process also exist significant effective on the reaction rate resulting from changes of composition and structure of glasses.The setting chemistry was studied by comparing the short-tern mechanical properties of set cement stored in silicone oil, air and the distilled water respectively firstly, and then the corresponding samples were investigated by measurement of IR spectra, differential scanning calorimetry (DSC/TG) and scanning electronic microscope (SEM). The strength of the sample exposed in the air reached the maximum after 24h, then decreased up to about 210Mpa and kept constant; The strength of the sample in water or silicon oil increased continuously, however the increasing rate of sample stored in silicone oil was much higher than that in the water. It indicated that the water play a key role on the strength changes with aging, which can maintain, delay or abandon the setting reaction process. The primary strength increased significantly with increasing fluoride content of the glass, while for phosphate content in glass, it prolonged working time and the compressive strength increased to a maximum at intermediate phosphate content, then decreasing with increasing phosphate content.The results of IR spectra and DSC/TG of corresponding sample showed that the structure changes of the matrix and silica gel-interface layer during the predetermined ntervals studied. It has been found that the band intensity around 1413 cm^-1 due to the vibration of polyacrylate salt increased with aging, and the shoulder band at 950 cm^-1 due to the stretching vibration of Si-OH still appeared during the periods studied. The heat decomposition process of the matrix suggested that the inorganic substance existed and presented over throughout matrix, however it is not enough to prove the existence of inorganic network. The results of SEM showed that the a great numbers of hemispherical hole occurring at primary stage, which formed as water volatile at high vacuum. It indicated that there is unreacted or less-reacted phase in the set cement and the set cement was heterogeneous. As the time going, the hemisphere hole decreasing progressively, which means Ca²⁺ and Al³⁺ diffuse into the phase and react with polyacrylic acid, resulted in crosslink density increasing and the set cement get homogeneous.All of these results, which are consistent with that of mechanical determination of compressive strength, suggested that the crosslink density increase resulting from the slow diffusion of Ca²⁺ and Al³⁺ and react with polyacrylic acid is responsible for compressive strength increasing with aging, and formation and maturing of interface layer comprising of silica gel may also play an important role on compressive strength.