The expansion of phosphate bonded investments: Part I--Setting expansion

Less expensive silver and palladium (AgPd) dental casting alloys are popular, but require phosphate bonded investments. The physical properties of such materials have not been thoroughly investigated. This study determined whether three different phosphate bonded investments could provide adequate expansion to compensate for the casting shrinkage of AgPd. The investments were mixed with distilled water or their unique special liquids provided by the manufacturers and allowed setting times of 1 hour or 24 hours. Setting expansions were measured with a vertical dilatometer. When mixed with special liquid, material I had a mean setting (1 hour) expansion of 0.19% +/- 0.01%; material II, 0.14% +/- 0.03%; and material III, 1.17% +/- 0.08%. Twenty-four hours of setting did not significantly increase the setting expansion (p > 0.05). Mixing the three investments with distilled water drastically reduced setting expansions. A three-way analysis of variance was computed to evaluate the data and investigate significant interactive and main effects. The two-way interaction (material x liquid) was significant. The results were consistent with the concept of a higher silica-containing special liquid for material III compared with the other materials.