Development of a low-color, color stable, dual cure dental resin

Dual-cure (DC) resins are mainly used as cements due to high initial color (generally yellow) and large color shift (ΔE*) after polymerization as compared to light-cured resins. However, even as cements, this color shift is clinically unacceptable, especially when used to cement thin veneers. Objective: To develop a novel DC initiator system with both lower initial color (less yellow, i.e., whiter) and smaller ΔE*. Methods: The effect of using an allyl thiourea (T)/cumene hydroperoxide (CH) self-cure (SC) initiator system in combination with a photo-co-initiator, p-octyloxy-phenyl-phenyl iodonium hexafluoroantimonate (OPPI), in a commercial DC resin cement (PermaFlo DC, Ultradent Products, Inc.) was investigated. Initial color and ΔE* were assessed for 6 weeks in vitro under accelerated aging conditions (75 °C water bath). Rockwell_15T hardness was used to assess degree of cure (DoC) and the three-point bending test was used to assess mechanical properties. Results: PermaFlo DC (control) was significantly harder than all experimental groups without OPPI but had up to three times higher initial color and four times greater color shift (ΔE* = 27 vs. 8). With OPPI, hardness in the experimental groups increased significantly and several were comparable to the controls. Initial color and ΔE* increased slightly (ΔE* = 9), but was still 3 times less than that of PermaFlo DC. DC samples containing OPPI had comparable modulus and ultimate transverse strengths to those of the controls. Conclusions: DC resins that use the T/CH initiator system are weaker but have extremely low color and ΔE*. The addition of OPPI increases DoC and mechanical properties to clinically acceptable levels and maintains extremely low color and ΔE*. Significance: With this novel initiator system, DC resins potentially can now have comparable color and color stability to light-cure resins and be used in broader esthetic dental applications to improve color stability and reduce shrinkage stress in restorative composites.