1

Introduction

The use of resin-based restorative materials in dentistry has increased exponentially over the last few decades with hardly a single dental procedure today being accomplished without their use. These materials are tooth colored, durable in the oral environment, able to adhere to tooth structure, and have the ability to be placed directly within the prepared tooth. Unfortunately, the issue of initial color, color-matching, and color stability after long-term intraoral exposure still remains . A significant shift in color may be sufficient reason to merit replacement of the restoration , which can increase the risk for further irreversible damage to the remaining tooth structure .

Color stability of composites is influenced by a variety of extrinsic and intrinsic factors . While extrinsic factors, such as surface staining, environmental exposure and changes in surface morphology due to wear are clinically manageable using proper finishing and polishing techniques , intrinsic factors, such as composition of the resin matrix, conversion rate , water sorption rate , and type of initiator system used, are not readily manageable through adjustments in clinical technique .

Light-cured resins generally have acceptable color and color stability after polymerization. However, color stability has become more of a problem recently due to the rise in popularity of bleaching and esthetic restorative resins. The inclusion of colorless onium-ion compounds (e.g., p-octyloxy phenyl-phenyl iodonium hexafluoroantimonate; OPPI) and other cationic photoinitiators in the photoinitiator system have been suggested as a means to improve degree of cure, lower initial color (to make it less yellow, i.e., whiter), and improve color stability. Such photo-co-initiators would reduce the need for those components that contribute to oxidative color changes such as the ubiquitous blue-light-absorbing photosensitizer camphorquinone (CQ) and an amine initiator .

Self-cure and dual-cure resins have a much darker initial yellow color and a larger color-shift (Δ E *) to a still darker shade of yellow after polymerization, than do light-cured resins. This causes these resins to be relegated mainly for use as cements and luting agents, where esthetics is not as crucial. Nonetheless, their color-shift is clinically unacceptable especially when used with thin veneers. A major culprit of this high initial color and color shift is the initiator system. Both the tertiary aromatic amine used as the self-cure activator (e.g., N,N-dimethyl- p -toluidine) and self-cure initiators such as benzoyl peroxide produce colored oxidation products that contribute to the discoloration of resins over time . Since tertiary aromatic amines are more likely to oxidize than the aliphatic amines used in light-cure resins, color changes in self-cure and dual-cure resins are more obvious than those in light cured resins .

To combat this, Temin et al. investigated the use of a hydroperoxide oxidizing agent and a substituted thiourea reducing agent as an alternative catalyst redox system in self-cured resins to provide enhanced color stability and greater shelf life . However, in a pilot study to verify the results reported by Temin, et al., we found that such self-cured systems had typically lower degrees of cure and, while they are more color-stable than self-cured systems based on benzoyl peroxide, they still produced significantly greater color-shift than current light-cured systems.

Thus, the goal of this work was to develop a novel dual-cure (DC) initiator system with both low initial color (less yellow) and a smaller color shift. We investigated the effect of using a redox catalyst self-cure initiator system in combination with an onium-ion photo-co-initiator in a commercially available dual-cure resin cement (Permaflo DC, Ultradent Products, Inc.) on initial color, color shift, degree of cure, and mechanical properties.

2

Materials and methods

Initially, the effect of replacing the chemical-cure initiator system in PermaFlo DC with an allylthiourea (T, Acros Organics)/cumyl hydroperoxide (CH, Sigma–Aldrich) redox self-cure initiator system on resin color, color stability and degree of cure was investigated. Once the most effective T/CH combination for reducing color and color shift (Δ E *) after polymerization and maintaining degree of cure was identified, the effect of adding an onium-ion photo-co-initiator, p-octyloxy phenyl-phenyl iodonium hexafluoroantimonate (OPPI, Spectra Group Ltd., Inc.), to the light-cure initiator system on resin color, color stability and degree of cure was investigated. Finally, the mechanical properties and degree of conversion (DoC) of the most effective dual-cure initiator system were determined.