Optimum particle size distribution for reduced sintering shrinkage of a dental porcelain

The purpose of this investigation was to develop a systematic method for obtaining an optimum particle size distribution for a given dental porcelain and compaction method that had a low sintering shrinkage.

Particles of coarse, medium and fine size were obtained from a commercial dental porcelain using a sedimentation method. Particle sizes were chosen so that the smaller particles would fit into the interstices between the next larger size. The different-sized powders were mixed in various proportions and compacted into a cylindrical mold. The specimens were fired and their linear firing shrinkages determined. An equation was developed to predict optimum proportions. The shrinkage of these mixed powders was compared to the shrinkage of the component powders, to the as-received commercial porcelain, and to predictions based on theoretical equations. These comparisons were statistically analyzed using analysis of variance (ANOVA) and Fisher's Protected Least Significant Differences (PLSD).

Mixing specific proporations of different-sized particles significantly reduced sintering shrinkage (p = 0.01) for all mixtures except for one trimodal mixture having the smallest particles. These reductions in shrinkage were obtained for the proportions predicted by the developed equation. Sintering shrinkages for predicted proportions of a three-component mixture and two two-component mixtures were significantly less than that of the original powder (p = 0.01).

A systematic method was developed for producing dental frits with low firing shrinkage. Mixing of different-sized particles produced frits with lower sintering shrinkage. The optimum proportions of the sized particles were given by an equation. The results of this study suggest that optimum particle size distributions for frits are dependent upon compaction methods.