Application of silica-sol investment to dental castable ceramics

Abstract

In order to decrease the adhering area between high-fusing alloys and investment moulds, dental investments have been improved by adding glycerol to the liquid-powder mixture including silica, magnesia and phosphate powders [1, 2], and adding nitr!de compound to the investment powder [3]. The adhering oxides were related to chemical reactions between phosphate or gypsum (a bonding agent in the investment) and the metal elements (alloys) [4-6]. Thus, newly-designed investment moulds with no phosphate or gypsum are needed to cast the currently available castable ceramics. In experimental investment (powder) and mixture (liquid), effectiveness is determined by examining setting behaviour using thermal analysis [7]. Also the investment, when set, expanded during heating (thermal expansion). As the expansion behaviour was exothermic [7-9], temperature increase associated with sol-gel reaction was measured after the powder was mixed with water or silica-sol liquid. In a manner similar to that used to investigate the polymerization of composite resins [9], differential scanning calorimetry (DSC) is used to analyse thermal change during the setting of mixed investment. The purpose of this preliminary report is to examine setting behaviours and compression strength of silica-sol investment powders mixed with the silica-sol liquid including ammonium carbonate solution (AC), and to use the investment mould for casting dental ceramics. The materials tested were the investment powder (o~-cristobalite; 55wt% (15/xm median size) and ol-quartz powders; 45 wt % (5/xm)) and the investment mixed with the silica-sol liquid which was 100% hydrolysed, or diluted with distilled water (H1; p H = 1.1; HAS-l , or H2; pH = 1.2; HAS-6, Colcoat Co., Tokyo). The mixture, comprising AC solution (1.1 ml) added to silica-sol liquid (16.0 ml), was used as the liquid for mixing the powder at a ratio of liquid powder (0.34). To observe the interface between the investment mould and the dental ceramic [10], electron probe X-ray microanalysis (EPMA; Shimadzu EPMA-8705, Kyoto) was used (ceramics: 20CAO10P2Os-10MgO-50SiO2 with very small amounts of other additives included). The properties examined by DSC analysis of the silica-sol investments (peak time (P), final setting time (F) and heat for setting (H)) . P is the time from mixing until the DSC curve reaches a peak [11], and F is the time from the start of mixing until the DSC baseline reaches the horizontal. The H value in the DSC curve represents the heat for