Probing the origins and control of shrinkage stress in dental resin composites. II. Novel method of simultaneous measurement of polymerization shrinkage stress and conversion.

This study probes the interrelationships between polymerization shrinkage stress development and the polymerization progress with a novel experimental technique. This technique is capable of real time, simultaneous measurement of double-bond conversion and shrinkage stress with the use of a noninvasive near-infrared fiber-optic system, along with a cantilever beam-based tensometer. The results from both filled and unfilled bis-GMA/TEGDMA (70:30 mass ratio) systems showed that the shrinkage stress buildup was concentrated in the latter stages of polymerization, with its dramatic increase linked to the asymptotic approach of conversion to its limiting value. The monotonic increase of shrinkage stress with conversion in the vitrified state is attributed to the dramatic increase of the sample's elastic modulus during the vitrification stage and a certain amount of cooling stress as the sample cools down from the temperature rise caused by the exothermic polymerization and light absorption. Excellent reproducibility of both the polymerization kinetics assessment and the shrinkage stress measurement has been achieved.