Variables affecting stress development and resin conversion in light-cured dental composites
Abstract: The general aim of this dissertation was to identify and investigate factors that can be used to minimize stress development in light cured dental resins without compromising the conversion level of the polymer. Modulus of elasticity, polymerization contraction strain, degree of conversion and shrinkage of light-cure dental composites were determined after curing with three different light power densities where total irradiated energy (J/cm2) kept constant. FT-Raman spectroscopy was employed to determine the degree of conversion. The cure kinetic of light cured resins was studied by use of photocalorimetry (photo-DSC). Dynamic mechanical thermal (DMTA) analysis was used to investigate how different light curing methods affected glass transition and tangent delta of light curable dental resins when the temperature changed from 0 to 200°C. Optical properties of dental composites were studied. Three different filler types, two different surface treatments and eight different filler fractions per filler type and surface treatment were investigated. Light transmission was measured for the different composite compositions at sample thicknesses of 1 to 5 mm by use of a universal power meter. As long as the total light energy remained the same, the modulus of elasticity remained constant for each composite, even though the power density differed. Composite thickness, irradiance time, composition of the light cure composite and irradiation value had significant impact on degree of conversion. The irradiance value did not significantly affect on the transition temperature value. Initiator, co-initiators and light irradiance value had all significant impact on cure behavior. Different filler types and filler surface treatments had significant effects on light absorption. In general, light absorption increased linearly with filler fraction and sample thickness of the cured composites. Conclusion: Low rather than high light irradiance values decrease stress levels in composites, and comparable conversion levels are reached as long as the total light energy value remains the same for low versus high irradiance. By increasing the composite thickness above 2 mm but not exceeding 6 mm, energy levels exceeding 30 J are needed to achieve acceptable levels of degree of conversion. Different irradiance values do not affect the final Tg of tested composites as long as the total light energy remains the same. By using appropriate photo initiator/co-initiator combination and soft-start curing it is possible to achieve slow curing and high DC within a 40 s. As expected, different filler particle properties have significant effects on light absorption during curing making it important to consider these differences when one tries to develop a general light curing strategy for light curable dental resins.
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