Linear shrinkage of photo-activated composite resins during setting

The linear shrinkages during setting of five commercially available photo-activated composite resins were measured using a developed weight-loaded plate plastometer. Six specimens of thickness ranging from 0.25-2.00 mm were used and the linear shrinkage was measured in three stages: under no-load or load of 200 or 400 g. Two light-irradiation times of 20 and 60 s were chosen for each specimen. All measurements were carried out in an air cabinet maintained at either 23 degrees C or 37 degrees C. The measuring time of linear shrinkage was 11 min from the start of light-irradiation. The ratio of apparent linear shrinkage decreased according to the increase in specimen thickness and decreased when load was added during setting. Furthermore, the ratio of linear shrinkage of the specimens increased according to the increase in light-irradiation time and the ambient temperature of the specimens. When the adsorptivity or stickiness of material to a tooth was high, detectable initial time (Ti) of the linear shrinkage became longer and the magnitude of linear shrinkage to the adherent direction decreased.