Preparation of presintered zirconia blocks for dental restorations through colloidal dispersion and cold isostatic pressing

Abstract

This work proposes an effective method for dispersion of zirconia suspension for dental block preparation and optimizes the cold isostatic pressing (CIP) pressure to improve the densification of slip-casted zirconia blocks. Two batches of 44wt% zirconia suspension were prepared using distilled water in a pH 2 medium containing 0.5wt% polyethyleneimine as dispersant. The first batch was sonicated for different durations (from 5min to 30min), and the second batch was dispersed through ball milling at rotational speeds of 200, 300, and 400rpm for 60, 90, and 120min. All suspensions were subjected to sedimentation test and particle size measurement. Results revealed that the optimum ultrasonication duration was 10min, which yielded the smallest particle size of 133nm. Ball milling at 300rpm for 120min achieved the maximum dispersion of particles, with an average size of 75nm. Under the optimum conditions of ultrasonication duration, ball milling duration, and ball milling speed, the particle size decreased to 48nm, which is close to the primary particle size. These dispersion techniques and parameters were selected for preparing a suspension to be consolidated into blocks through slip casting and were enhanced through CIP at pressure ranging from 100MPa to 300MPa. CIP compaction at 250MPa significantly increased the shrinkage percentage of green zirconia blocks, with pore radius decreased to 18nm. The density of zirconia pressed at 250MPa and presintered at a low temperature of 950°C was 59% of the theoretical density and was higher than that of commercial presintered blocks. Thus, CIP should be conducted under a compaction pressure of 250MPa to produce dense and homogeneous zirconia blocks.