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Effect of photopolymerization variables on composite hardness.

STATEMENT OF PROBLEM
Variations in light-polymerizing parameters, such as light intensity and light-to-material distance, may affect the physical characteristics of polymerized resin.


PURPOSE
The purpose of this study was to characterize the relation between total light energy and the final hardness of 4 composites polymerized under varying conditions.


MATERIAL AND METHODS
Four commercial composite restorative resins (Z100, Filtek A110, Tetric Ceram, and Tetric Flow) were used to prepare 4 disk-shaped specimens (6 x 2 mm) for each experimental condition. Photoactivation was carried out with a light device and energy of 22.6, 15.7, 9.0, or 6.7 J/cm(2). Either the light-to-material distance (0, 5, 10, 15 mm) or activation time (40, 28, 16, 12 seconds) was varied. Immediately after polymerization, Barcol hardness was determined on the specimen surface that had been exposed to the light. Analysis of variance (P<.05) and regression analysis were used to examine the data.


RESULTS
No significance (P>.05) was found for the overall effect of the experimental variables (polymerization time and distance), but significant differences (P<.01) were found among materials and energy levels. Regression equations for each product and polymerization condition were calculated for hardness as a function of energy. No significant differences were found for slopes within each material. Hardness values were 3.0 to 3.5 for Tetric Ceram, Tetric Flow, and Filtek A110 and approximately 4.9 for Z100.


CONCLUSION
The hardness of the products analyzed was related to the total energy used for activation. The effect was independent of the manner in which the amount of energy was modified (light-to-material distance or activation time).