Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction

Objectives: To evaluate the shrinkage-strain, degree of conversion (DC) and percent filler of a range of flowable resin-composites. Methods: Several commercially available flowable resin-composites were selected. The bonded-disk technique was used to measure shrinkage-strain kinetics. Measurements were obtained continuously for 60 min by irradiation for 40 s at 750 mW/cm² at two initial temperatures of 23 and 37 °C. Three repetitions (n = 3) were made at each temperature per material. The DC was measured by FTIR spectroscopy immediately post-cure. The percent filler was determined by combustion of specimens for 1 h at 400 °C. Results: The final values of shrinkage-strain ranged from 2.61 to 6.25% at 23 °C and from 3.88 to 6.56% at 37 °C. Statistically significant differences (p < 0.05) were found in shrinkage-strain between the flowable resin-composites at both temperatures 23 and 37 °C. Significantly higher values were obtained at 37 °C (p < 0.05). A strong inverse correlation was found between the percent filler and the shrinkage-strain at 37 and 23 °C (r² = 0.94 and 0.87, respectively). No correlation was found between shrinkage-strain and DC (p > 0.05). Conclusions: Flowable resin-composites vary widely in shrinkage-strain magnitude and the inverse relationship between filler percent and shrinkage-strain is explained by the corresponding decrease in volume fraction of monomers present to undergo polymerisation.