The addition of thiourethane to dual-cured cements reduces their working time.
Low concentrations of redox initiators were required in cements with thiourethane.
High conversion was observed even in absence of light-curing for these cements.
Cements with thiourethane presented reduced polymerization stress.
The moment of light-activation did not affect cement properties and stress.
This study evaluated the properties of experimental dual-cured cements containing thiourethane (TU) and low concentrations of p-Tolyldiethanolamnie (DHEPT) and benzoyl peroxide (BPO) as chemical initiators.
BisGMA/TEGDMA-based dual-cured cement was formulated with 1.0 wt% DHEPT and 0.75 wt% BPO as initiators and used as control. The concentration of BPO was adjusted to 0.1 wt% in catalyst paste of experimental cements, and two base pastes containing TU and 0.5 wt% or 0.25 wt% of DHEPT were formulated. The rheological behavior and kinetics of polymerization of cements were assessed in the absence of light activation. The kinetics of polymerization was also evaluated for cements light-activated immediately or 5 min after the start of mixing. Polymerization stress, flexural strength and elastic modulus (n = 5) were also evaluated under these conditions.
Cements with TU presented lower viscosity than the control, improved working time (0.25% DHEPT > 0.5% DHEPT) and higher conversion in the absence of light-activation. Delaying the light-activation reduced the maximum rate of polymerization (Rp max ) but did not affect the conversion or stress. The addition of TU increased the Rp max and conversion, and reduced the stress when compared to the control, without affecting the flexural strength. Except for the control with delayed light-activation (highest values), the other experimental conditions yielded similar modulus.
Adding TU and using a low concentration of DHEPT/BPO resulted in dual-cured cements with longer working time, reduced polymerization stress and increased conversion even in the absence of light, with no significant effect on the mechanical properties.
Dual-cured resin cements are widely used to lute indirect restorations because they combine the advantages of light-activation, which allows for the on-command stabilization of the restoration, and of chemical-activation, which is necessary to ensure adequate polymerizations in regions not accessible to the light [ ]. However, an increased concentration of redox initiators is necessary to ensure high conversion of the cement in those regions, which in turn reduces working time and increases the potential for discoloration, ultimately compromising the luting procedures [ ]. Thus, some resin cements manufacturers have added primer/ adhesive solutions to be applied onto the substrate prior to the cement insertion to improve its chemical-activated polymerization [ , , ]. The rationale is that co-initiators such as sodium sulfinate salts from these primer/ adhesives would increase the concentration of free-radicals to accelerate the redox-based polymerization of cements containing a low concentration of initiators (i.e., amines and benzoyl peroxide) [ ]. However, despite allowing the use of cements with increased working time even when the light-activation is compromised, the faster cement polymerization obtained using this approach can impair the luting procedure in some clinical situations. One example is radicular post cementation [ ].
Another important concern which remains over the adhesive cementation procedures is the polymerization stress. In addition to the volume of material and compliance of substrates, the cavity configuration factor (C-factor) also affects the stress developed during the polymerization of resin-based materials [ ]. Therefore, resin cements are used in a critical scenario in terms of C-factor, since only the margins of the cement layer can deform and accommodate part of the polymerization stress. Prior studies have demonstrated that delaying the light-activation in dual-cure systems might reduce the polymerization stress by allowing part of the conversion to take place under less stringent diffusional limitations [ ]. However, delaying the light-activation of the cement increases the chair time, and faster and simpler procedures are preferred by clinicians. Another approach for reducing the polymerization stress is to add chain transfers agents (i.e. thiourethane) into methacrylate polymerizations [ ]. In fact, a prior study demonstrated the effectiveness of thiourethane in reducing the polymerization stress, but the dual-cured experimental resin cement containing this oligomer had short working time since the thiol moieties can also work as reducing agents and react with benzoyl peroxide [ ]. Hence, the use of more efficient co-initiators for the chemically-activated reaction than the amine used in that prior study (ethyl 4-dimethylaminobenzoate) can allow for the addition of lower concentration of benzoyl peroxide to the catalyst paste and improve the cement working time.
Therefore, the aim of this study was to evaluate the rheological behavior, polymerization kinetics, polymerization stress, and mechanical properties of experimental dual-cured cements containing thiourethane and low concentrations of p-Tolyldiethanolamine (DHEPT) and benzoyl peroxide as the redox initiator system. The hypothesis of the study was that adding thiourethane to experimental dual-cured cements with low concentrations of DHEPT and benzoyl peroxide would improve the working time and conversion, and reduce the polymerization stress of cement, without compromising its mechanical properties.
Material and Methods
The control dual-cured resin cement was formulated using the monomers BisGMA (bis-phenol A diglycidyl dimethacrylate) and TEGDMA (tri-ethylene glycol dimethacrylate) at a 2:1 mass ratio, all from Esstech (Essington, PA, USA). Fillers of 0.7-μm silane-treated Barium glass (Esstech) were added at 60 wt%. Bis-acylphosphine oxide (BAPO) at 2.0 wt% was added as a photo-initiator to the base paste, and the amine DHEPT was also added to this paste at 1.0 wt%. The catalyst paste contained 0.75 wt% of benzoyl peroxide (BPO). The inhibitor (BHT [2,6-di- tert -butyl-4-methylphenol] was added to both pastes at 0.1 wt%. DHEPT was obtained from TCI America (Portland, OR, USA) and the other initiators from SigmaAldrich (St. Louis, MO, USA). Those concentrations of amine and BPO were determined to obtain a working time around 5 min according to the results of a pilot study.
Prior to the formulation of experimental cements containing thiourethane, thiol-functionalized oligomers were synthesized by combining trimethylol-tris-3-mercaptopropionate (TMP) with 4,4′-Diisocyanato-methylenedicyclohexane (HMDI), according to previously reported methods [ ]. Since the thiourethane can only be added to base paste [ ], the oligomer was added at the concentration of 40 wt%, resulting in a 20 wt% thiourethane concentration after mixing base and catalyst pastes. The monomeric formulation of the catalyst paste was the same used for the control, but the proportion between BisGMA and TEGDMA was adjusted to 1:1 in the base paste to compensate for the increased viscosity caused by the addition of the oligomer thiourethane. For cements containing thiourethane, the concentrations of BAPO and BHT were the same used for the control. However, using the same concentrations of amines and BPO as in the control resulted in experimental cements with short working time, as demonstrated previously [ ]. Therefore, a pilot study defined two base pastes containing thiourethane, which were formulated by adding DHEPT at 0.5 wt% or 0.25 wt% to be used with a catalyst paste with a reduced amount of BPO (0.1 wt%). These initiators concentrations were determined to obtain a dual-cured cement with working time similar to that observed for the control, and another with a longer working time. Therefore, the rationale for the different monomer and initiator concentrations for the experimental materials and the control was based in clinically-relevant aspects, rather than compositional uniformity.
Rheologic properties of the cements
Equal amounts of base and catalyst pastes of each cement were mixed and sandwiched between two 20 mm parallel acrylic disc plates, attached to a rheometer (DHR-1, TA Instruments, New Castle, DE, USA). The specimens were tested in oscillatory shear at a frequency of 1.0 Hz with 1.0% strain, using a gap of 300 μm without any irradiation for 15 min (n = 5). The time elapsed between the beginning of the cement mixing and the start of data collection was standardized at 60 s, and this time was added to the results. The curve of the complex viscosity development as a function of the time was plotted and the working time calculated based on the tangent line touching the curve ( Fig. 1 ). The time for cements to reach the gel point was estimated based on the crossover between G′ (storage modulus) and G″ (loss modulus), when the Tan δ was 1.0 ( Fig. 2 ).
DC and polymerization kinetics
Equal amounts of each paste were mixed and inserted in cylindrical silicone rubber molds (n = 3; diameter = 6 mm; thickness = 0.8 mm), sandwiched between two glass slides (n = 5). Polymerization kinetics was monitored in an infra-red spectrometer (Nicolet 6700, Thermo Scientific, Pittsburgh, PA, USA) equipped with an InGaAs detector. Spectra were collected in real-time for 20 min, at 4 cm −1 resolution, 2 Hz acquisition rate. The monitoring of polymerization kinetics started 3 min after the beginning of cement mixing and was followed for 30 min in the absence of any light-activation. Polymerization kinetics of cements used at dual-cured mode was also monitored for 10 min after light activation. The light activation was performed with a LED-based unit (Elipar Deepcure-S, 3M ESPE, St. Paul, MN, USA) delivering approximately 1450 mW/cm 2 to the surface of the specimen for 20 s, and the light-activation started immediately or after 5 min (delayed) from the beginning of kinetics measurement. Light intensity was checked daily using portable spectrometer-based instrument (CheckMARC, BlueLight Analytics, Halifax, NS, Canada). Degree of conversion (DC) was calculated based on the area of the methacrylate vinyl overtone at 6165 cm –1 [ ]. The polymerization rate was calculated as the first derivative of the conversion vs. time curve. Polymerized specimens were stored dry for 72 h and the degree of conversion was measured again.
Standardized mass of each paste of cements were mixed and placed over a silane-treated glass plate held to the Bioman device [ ]. The tip of the of light-curing unit (Elipar Deepcure-S) was positioned under the glass plate into a cylindrical bolt. The cement was covered with a metallic rod (5.6 mm of diameter) treated with metal primer (Z-Prime Plus, Bisco Inc, Bisco, Schaumburg, IL, USA) placed 0.75 mm from the glass yielding a C-factor = 3.73. The time between the beginning of the cement mixing and to start the data collection was standardized in 3 min (the same of kinetics polymerization analysis) and the light-activation was performed for 20 s according to the protocols defined above. Polymerization stress development (in MPa) was followed in real-time for 20 min (n = 5).
3-Point bending test
The dimensions of bars required by the International Organization for Standardization (ISO) specification 4049, which establishes the standards for performing the 3-point bending test for polymer-based dental materials were modified in this study to allow for a single exposure per sample. Mixed cements were inserted in rubber molds with 14 × 2 × 1 mm, which still complies with the proportions that allow for the application of beam theory [ ]. Specimens were then light-activated following the same conditions described previously. The delay time between to start the cement mixing and light-activation was also standardized accordingly. The specimens were stored dry for 24 h in dark containers, and then positioned in a 3-point bending device coupled to a mechanical testing system over two rods with diameter of 2 mm spanning 10 mm between them. A compressive load was applied to the center of specimen with a 2-mm of diameter tip and a crosshead speed of 1.0 mm/min until failure. The following equation was used to calculate flexural strength (σf) in MPa:
where F is the maximum load (N) exerted on the specimen, l is the distance (mm) between the supports, and b is the width (mm) and h the height (mm) at the center of the specimen.
The Elastic modulus (E) was calculated using the following equation: