Investigation of UV-Absorber Concentrations in Experimental Composite Pastes

Objectives: To investigate effects of 2,4-dihydroxybenzophenone (2,4-DHB), a UV-absorber used to prevent light-degradation of polymers, on color and physical properties of experimental dental composite (EDC) pastes.
Methods: Six EDC formulations of 0.00wt%-1.00wt% 2,4-DHB (Sigma-Aldrich Inc., Missouri, USA) of the resin content were made. The filler-to-resin ratio was 75wt% to 25wt%. The camphorquinone-based photoinitiator system was held constant. CIE L*a*b* values of EDC pastes were measured by spectrophotometer (Ci6X, X-RITE, Michigan, USA) uncured and after light-curing (20 seconds per sample face, 1,000 mW/cm², VALO, Ultradent, Utah, USA). These samples (n=3 per paste, 12.0mm diameter, 2.3mm thickness), flexural strength (FS, 25mm by 2.0mm by 2.0mm), and compressive strength (CS, 4mm diameter, 6mm thickness) samples were subjected to 150,000 lux light in water for 24 hours and 240 hours.
Results: Higher 2,4-DHB in uncured EDC pastes showed lower a* values (greener) and higher b* values (yellower). Curing reduced all b* values, but higher 2,4-DHB samples retained lower a* values and higher b* values. After 24 hours of light exposure, all L* values increased (more luminous), all a* values increased, and for concentrations of 0.50wt% and above, b* values increased. After 240 hours, all a* values increased, and all b* values decreased, however, higher 2,4-DHB samples still retained lower a* values and higher b* values comparatively. After 24 hours of light exposure, FS appeared slightly diminished by higher 2,4-DHB. After 240 hours, while most FS values fell, higher 2,4-DHB EDCs were impacted less. After 24 hours of light exposure, CS value was highest for the 0.50wt% 2,4-DHB EDC and lowest for the 1.00wt% 2,4-DHB EDC. This trend held after 240 hours.
Conclusions: 2,4-DHB affects inherent color and strength after light exposure of the EDCs in this study. 2,4-DHB showed UV protection of physical strength to an extent, but higher concentrations resulted in undesirable yellow-green colors for dental materials.