Rheometric measurement of polymerization shrinkage and stress of restorative materials

Objectives: The aim of this study was to demonstrate a simple and quick rheology-based test method to characterize polymerization shrinkage and contraction stress during setting of self-cured restorative materials utilizing normal force and gap control capabilities of a modern rheometer.

Methods: For both shrinkage and contraction stress measurement,  time sweeps were conducted with 1 Hz frequency using 8 mm diameter parallel plates with 0.7 mm gap at 23 ēC.  In polymerization shrinkage measurement, constant compressive normal force of 0.2 N was applied and gap changes were monitored to measure linear polymerization shrinkage.  Gap at cross-over point of G′ and G″, which is considered to be starting point of gelation, was compared with subsequently monitored gap changes.  For contraction stress measurement, gap between two parallel plates was set to fixed value (0.7 mm) and stress build-up due to polymerization shrinkage was monitored using a built-in normal force transducer.  0.05% oscillation shear strain was applied in both experiments to minimize disruption of polymerization process and all experiments were conducted while avoiding ambient light. Tested materials in this study were experimental dual-cured luting cement (A), dual-cured core material (B), self-cured cement (C), and self-cured composite (D).  

Results: Calculated shrinkage percentages based on linear shrinkage measurements in this study were well agreed with volumetric shrinkage percentages measured using video imaging-based AcuVol.  For all test specimens, shrinkage contraction stress was increased rapidly right after G′ and G″ cross-over point and decreased slightly after maximum possibly due to material relaxation. Experimental dual-cured cement produced highest shrinkage and contraction stress as expected.  Lowest contraction stress was observed in self-cured composite originally developed for reducing polymerization shrinkage stress.

Conclusion: In this study, polymerization shrinkage and shrinkage stress were measured in same experimental configuration so that relationship between shrinkage strain and stress can be correlated more directly.