Polymerization shrinkage of index and pattern acrylic resins.

Inadequate dimensional stability caused by polymerization shrinkage has been reported concerning the various applications of acrylic resins. The objective of the study was to evaluate dimensional changes of two self-curing acrylic resins marketed as pattern and index material. Early volumetric changes were measured with a dilatometer and late linear changes were recorded with an inductive transducer. After 24 hours the volumetric shrinkage was 7.9% for Duralay resin and 6.5% for Palavit G resin; 80% of the change appears before 17 minutes at room temperature. Shrinkage was significantly increased when the proportion of powder in the mix was diminished. Results suggest that these resins should be used with some method to compensate for the shrinkage, when used as index material. However, the dimensional change might provide significant advantages for intracoronal castings.