Dual-cure (DC) resins are mainly used as cements due to high initial color (generally yellow) and large color shift (Δ E *) 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.
To develop a novel DC initiator system with both lower initial color (less yellow, i.e., whiter) and smaller Δ E *.
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 Δ E * 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.
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 (Δ E * = 27 vs. 8). With OPPI, hardness in the experimental groups increased significantly and several were comparable to the controls. Initial color and Δ E * increased slightly (Δ E * = 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.
DC resins that use the T/CH initiator system are weaker but have extremely low color and Δ E *. The addition of OPPI increases DoC and mechanical properties to clinically acceptable levels and maintains extremely low color and Δ E *.
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.
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.
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.
A total of 25 combinations were prepared ( n = 5) that contained 50% (w/w) of the light-cure formulation of PermaFlo DC (designated PermaFlo LC) and 50% (w/w) of the self-cure formulation of PermaFlo DC with the self-cure initiator system replaced with various concentrations of T and CH as shown in Table 1 . Controls included PermaFlo DC (negative control) and PermaFlo LC (positive control used as a representative example of a light-cure resin). The two parts were mixed and placed in disc-shaped molds (3/8 in. diameter × 1/16 in. thick) between two glass slides and light-cured for 20 s on each side using an Ultra Lume 5 LED lamp (Ultradent Products, Inc.).
|1CH (3.74%) a||2CH||3CH||4CH||5CH|
|1T (2.1%) a||1T:1CH||1T:2CH||1T:3CH||1T:4CH||1T:5CH|
Once the most effective T/CH formulation was determined, OPPI was incorporated to further reduce color and color shift, and enhance cure properties. To determine the most effective light-cure initiator system, varying concentrations of OPPI, camphorquinone (CQ; Sigma–Aldrich), and dimethylaminoethylmethacrylate (DMAEMA; Sigma–Aldrich) as shown in Table 2 were used in the light cure side and investigated.
|Concentration (%OPPI/% CQ/% DMAEMA) %w/w of filled monomer in the light-cure part of PermaFlo DC|
|Group 1||0.15% OPPI/0.15% CQ/0.21% DMAEMA|
|Group 2||0.15% OPPI/0.15% CQ/0.30% DMAEMA|
|Group 3||0.15% OPPI/0.15% CQ/0.60% DMAEMA|
|Group 4||0.15% OPPI/0.21% CQ/0.21% DMAEMA|
|Group 5||0.15% OPPI/0.21% CQ/0.30% DMAEMA|
|Group 6||0.15% OPPI/0.21% CQ/0.60% DMAEMA|
|Group 7||0.15% OPPI/0.30% CQ/0.30% DMAEMA|
|Group 8||0.15% OPPI/0.30% CQ/0.60% DMAEMA|
|Group 9||0.21% OPPI/0.15% CQ/0.21% DMAEMA|
|Group 10||0.21% OPPI/0.15% CQ/0.30% DMAEMA|
|Group 11||0.21% OPPI/0.15% CQ/0.60% DMAEMA|
|Group 12||0.21% OPPI/0.21% CQ/0.21% DMAEMA|
|Group 13||0.21% OPPI/0.21% CQ/0.30% DMAEMA|
|Group 14||0.21% OPPI/0.21% CQ/0.60% DMAEMA|
|Group 15||0.21% OPPI/0.30% CQ/0.30% DMAEMA|
|Group 16||0.21% OPPI/0.30% CQ/0.60% DMAEMA|
|Group 17||0.30% OPPI/0.15% CQ/0.21% DMAEMA|
|Group 18||0.30% OPPI/0.15% CQ/0.30% DMAEMA|
|Group 19||0.30% OPPI/0.15% CQ/0.60% DMAEMA|
|Group 20||0.30% OPPI/0.21% CQ/0.21% DMAEMA|
|Group 21||0.30% OPPI/0.21% CQ/0.30% DMAEMA|
|Group 22||0.30% OPPI/0.21% CQ/0.60% DMAEMA|
|Group 23||0.30% OPPI/0.30% CQ/0.30% DMAEMA|
|Group 24||0.30% OPPI/0.30% CQ/0.60% DMAEMA|
Commercially available PermaFlo DC contains 0.21% (w/w) CQ and 0.21% (w/w) DMAEMA in the light-cure side. However, there was difficulty dissolving the CQ provided by Ultradent Products into the resin system using in-house equipment, so CQ from Sigma–Aldrich was used instead. However these specimens were significantly more yellow and underwent significantly greater color shift. Thus, in this phase of the work, the following controls were used: PermaFlo DC, HSC LC (PermaFlo LC formulated using CQ from Sigma–Aldrich), and HSC LC+1T:5CH (dual cure resin formulated with HSC LC and the most color-stable T/CH self-cure initiator formulation from the first part of this work) for a more appropriate comparison. Disc-shaped specimens were prepared as described above.
Initial color and color stability were evaluated using a Konica Minolta Chroma Meter CR-400 to record CIE L * a * b * color parameters during accelerated aging in which samples were stored in water at 75 °C for up to 6 weeks . Since chemical reactions, including color shift, generally have an Arrhenius relationship to temperature, at 75 °C, aging is estimated to be accelerated 16 times that at mouth temperature (35 °C). Thus, 6 weeks is estimated to correspond to about 96 weeks (almost 2 years) in the mouth. Changes in overall color, Δ E *, and in the yellow–blue coordinate, b *, were determined weekly. b * values were used instead of all three values of L * a * b * for convenient comparison since most of the color shift was observed in the b * range.
24 h after light curing, Rockwell 15T hardness (Wilson 3JR Rockwell Hardness Tester) ( n = 5) was determined as a measure of degree of cure using a 15T 1/16 in. ballpoint indenter with a 15 kg force. Homogeneity of cure was assessed by taking three separate indentations in various locations on each specimen.
Three point bending flexural test
The dual-cure (DC) formulation with OPPI that was most effective in reducing color and color shift, and which had at least comparable hardness to PermaFlo DC, i.e., the “Optimal” sample, along with control groups PermaFlo DC, HSC LC, and HSC LC + 1T:5CH were characterized mechanically using a three point bending test to determine modulus and ultimate transverse strength of cured specimens (40 mm × 4 mm × 2 mm specimens; n = 5) using an Instron/MTS 1125 ReNew universal mechanical test instrument at a crosshead speed of 1 mm/min.
Fourier transform infrared spectroscopy degree of conversion (DoC)
DoC was determined using FTIR (Midac 101025) with an ATR (Pike MIRacle ATR) single bounce reflectance prism. The change in ratio between the aliphatic and aromatic carbon double-bond absorptions (1638 cm −1 and 1608 cm −1 , respectively) were determined prior to and after 60 s of light-curing, and again after 24 h to measure continued polymerization due to the self-cure mode.
Rockwell 15T hardness, initial and 6 week b *, Δ b *, Δ E *, elastic modulus, ultimate transverse strength and DoC were all compared between groups using analysis of variance (ANOVA) with Neuman Keuls post hoc test at a significance level of p < 0.05.