Abstract
Objective
1) to determine the moment during the redox polymerization reaction of dual cure cements at which to photo-activate the material in order to reduce the polymerization stress, and 2) to evaluate possible synergistic effects between adding chain transfer agents and delayed photo-activation.
Methods
The two pastes of an experimental dual-cure material were mixed, and the polymerization kinetics of the redox phase was followed. The moment when the material reached its maximum rate of redox polymerization (MRRP) of cement was determined. The degree of conversion (DC) and maximum rates of polymerization (Rp max ) were assessed for materials where: the photoactivation immediately followed material mixing, at MRRP, 1 min before and 1 min after MRRP. Thio-urethane (TU) additives were synthesized and added to the cement (20% wt), which was then cured under the same conditions. The polymerization kinetics was evaluated for both cements photo-activated immediately or at MRRP, followed by measurements of polymerization stress, flexural strength (FS) and elastic modulus (EM). Knoop hardness was measured before and after ethanol storage.
Results
Photo-activating the cement at or after MRRP reduced the Rp max and the polymerization stress. Addition of TU promoted additional and more significant reduction, while not affecting the Rp max . Greater hardness loss was observed for cements with TU, but the final hardness was similar for all experimental conditions. Addition of TU slightly reduced the EM and did not affect the FS.
Conclusion
Delayed photo-activation and addition of TU significantly reduce the polymerization stress of dual-cured cements.
Clinical implications
The addition of thio-urethanes to dual-cured resin cements significantly reduced the polymerization stress without compromise to the conversion and mechanical properties of the material. The use of this additive combined with delayed photo-activation further improved these parameters. Clinically, this has the potential to minimize the effects of the extreme confinement imposed to the cement layer, at least in the regions of the restoration where light penetration is effective, and where stress develops rapidly after photoactivation.
1
Introduction
Dual-cured resin cements are largely used for cementation of indirect restorations due to a combination of the favorable characteristics of self- and light-cured agents. For example, those cements still offer some control of working time, and ensure that the margins of the restoration can be polymerized to relatively high degrees of conversion right after cementation, to avoid premature dissolution − those all being features one can harness from the photo-activated components . However, photo-activation cannot guarantee high conversion values in areas that are inaccessible to light, either due to reflection, absorption or scattering phenomena through thick and opaque indirect restorative materials . In those areas, the chemical [or redox] components ensure that the conversion progresses to the extent necessary to provide acceptable mechanical properties. One drawback that is still present with dual cure cements is the polymerization stress that may results in cracks on the cement line or the restorative material itself, gaps on margins and post-operative sensitivity, compromising the longevity of restorative procedures . Polymerization stress is closely related to the C-factor, defined as the ratio between bonded and non-bonded surface. In general, the higher C-factor, the greater the polymerization stress . Resin cements are used in the worst scenario in terms of C-factor, since only the margins of the cement layer are able to deform and accommodate part of the polymerization stress . Thus, clinical approaches or materials able to reduce the polymerization stress can be envisioned to improve the longevity of restorations.
Usually, dual-cured cements are photo-activated by clinicians immediately after restoration placement and removal of excess cement. However, some studies evaluating three commercially available dual-cured resin cements suggest that delaying the photo-activation might be able to reduce the polymerization stress caused by the resin cement . It was speculated that differences on crosslinking density affecting the elastic modulus would explain the results . In those prior studies, compositional differences among the cements might have affected the speed of the chemically activated reaction to some extent. With that being said, using the same delay period (3 or 5 min) for all materials, Faria-e-Silva et al. observed differences on maximum rate of polymerization (Rp max ) and degree of conversion after 30 min (maximum period of evaluation) among the cements evaluated when photo-activation was delayed. Others have attempted to tackle the stress issue by using compositions with different polymerization mechanisms, including the use of thiol-ene polymerizations and/or the incorporation chain transfers agents (thiols) into methacrylate polymerizations . When photo-activation is used, thiol-ene polymerizations progress through a radically assisted step-growth mechanism, in which diffusion limitations occur at higher DC or, in other words, gelation and vitrification are delayed to higher values of conversion . The same effect is observed in thiol-methacrylate polymerizations, even at off-stoichiometry ratios . Therefore, the stiffness of the network does not build up until later in conversion, which in turn allows dimensional changes (and stress) to be accommodated with significantly lower stress generation . A recent study demonstrated that adding thiol-terminated thio-urethane oligomers to dual-cured resin cement was able to significantly reduce the polymerization stress when compared to a control . However, in that study, both experimental (containing thio-urethane) and control cements were photo-activated immediately after mixing.
There might be advantages to combining delayed gelation/vitrification strategies with delayed photoactivation protocols in dual-cure cements to ultimately reduce polymerization stress. These two strategies are more useful to relieve/avoid part of the stress that is developed in regions of the cement/restoration that are effectively reached by light. In the regions where polymerization progresses “in the dark”, the redox mechanism prevails, and there is limited stress generation . However, except for the free margin where the material can comply to the deformation imposed by polymerization, stresses develop rapidly under confinement in the regions where the photoactivation is efficient. Thus, the aims of the present study were 1) to determine the moment during the redox polymerization reaction of dual cure cements at which to photo-activate the material in order to reduce the polymerization stress, and 2) to evaluate possible synergistic effects between adding chain transfer agents and delayed photo-activation of dual-cured cements in terms of on polymerization stress, reaction kinetics, mechanical properties and degradation after storage in ethanol. The hypothesis was that delaying the photo-activation of dual-cured cements containing thio-urethane results in additional reduction of polymerization stress.
1
Introduction
Dual-cured resin cements are largely used for cementation of indirect restorations due to a combination of the favorable characteristics of self- and light-cured agents. For example, those cements still offer some control of working time, and ensure that the margins of the restoration can be polymerized to relatively high degrees of conversion right after cementation, to avoid premature dissolution − those all being features one can harness from the photo-activated components . However, photo-activation cannot guarantee high conversion values in areas that are inaccessible to light, either due to reflection, absorption or scattering phenomena through thick and opaque indirect restorative materials . In those areas, the chemical [or redox] components ensure that the conversion progresses to the extent necessary to provide acceptable mechanical properties. One drawback that is still present with dual cure cements is the polymerization stress that may results in cracks on the cement line or the restorative material itself, gaps on margins and post-operative sensitivity, compromising the longevity of restorative procedures . Polymerization stress is closely related to the C-factor, defined as the ratio between bonded and non-bonded surface. In general, the higher C-factor, the greater the polymerization stress . Resin cements are used in the worst scenario in terms of C-factor, since only the margins of the cement layer are able to deform and accommodate part of the polymerization stress . Thus, clinical approaches or materials able to reduce the polymerization stress can be envisioned to improve the longevity of restorations.
Usually, dual-cured cements are photo-activated by clinicians immediately after restoration placement and removal of excess cement. However, some studies evaluating three commercially available dual-cured resin cements suggest that delaying the photo-activation might be able to reduce the polymerization stress caused by the resin cement . It was speculated that differences on crosslinking density affecting the elastic modulus would explain the results . In those prior studies, compositional differences among the cements might have affected the speed of the chemically activated reaction to some extent. With that being said, using the same delay period (3 or 5 min) for all materials, Faria-e-Silva et al. observed differences on maximum rate of polymerization (Rp max ) and degree of conversion after 30 min (maximum period of evaluation) among the cements evaluated when photo-activation was delayed. Others have attempted to tackle the stress issue by using compositions with different polymerization mechanisms, including the use of thiol-ene polymerizations and/or the incorporation chain transfers agents (thiols) into methacrylate polymerizations . When photo-activation is used, thiol-ene polymerizations progress through a radically assisted step-growth mechanism, in which diffusion limitations occur at higher DC or, in other words, gelation and vitrification are delayed to higher values of conversion . The same effect is observed in thiol-methacrylate polymerizations, even at off-stoichiometry ratios . Therefore, the stiffness of the network does not build up until later in conversion, which in turn allows dimensional changes (and stress) to be accommodated with significantly lower stress generation . A recent study demonstrated that adding thiol-terminated thio-urethane oligomers to dual-cured resin cement was able to significantly reduce the polymerization stress when compared to a control . However, in that study, both experimental (containing thio-urethane) and control cements were photo-activated immediately after mixing.
There might be advantages to combining delayed gelation/vitrification strategies with delayed photoactivation protocols in dual-cure cements to ultimately reduce polymerization stress. These two strategies are more useful to relieve/avoid part of the stress that is developed in regions of the cement/restoration that are effectively reached by light. In the regions where polymerization progresses “in the dark”, the redox mechanism prevails, and there is limited stress generation . However, except for the free margin where the material can comply to the deformation imposed by polymerization, stresses develop rapidly under confinement in the regions where the photoactivation is efficient. Thus, the aims of the present study were 1) to determine the moment during the redox polymerization reaction of dual cure cements at which to photo-activate the material in order to reduce the polymerization stress, and 2) to evaluate possible synergistic effects between adding chain transfer agents and delayed photo-activation of dual-cured cements in terms of on polymerization stress, reaction kinetics, mechanical properties and degradation after storage in ethanol. The hypothesis was that delaying the photo-activation of dual-cured cements containing thio-urethane results in additional reduction of polymerization stress.
2
Material and methods
This study was divided in two aims: 1) to determine if delaying the photo-activation of a dual cure cement can lead to reduction in polymerization stress; 2) to determine whether the addition of a stress-reducing thio-urethane oligomer would act synergistically with the delayed photo-activation of dual-cured cements. Both types of materials, in the several delayed activation protocols tested, were evaluated for polymerization stress, reaction kinetics, mechanical properties and degradation after storage in ethanol. The statistical analysis was conducted separately for each aim, and the presentation of the results aligns with that design.
2.1
Cement formulation
An experimental 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%. To the base paste, 0.2 wt% of dl-camphoroquinone, 3.5 wt% of a tertiary amine (EDMAB [ethyl 4-dimethylaminobenzoate]), and 0.1 wt% of inhibitor (BHT [2,6-di- tert -butyl-4-methylphenol] were added. To the catalyst paste, 3.5 wt% of benzoyl-peroxide was added. Those concentrations were determined to produce polymerization levels of 30% or more in 20 min, according to the results of a pilot study. We acknowledge the poor performance of EDMAB as a redox pair for BPO, as previously demonstrated . However, we opted for keeping due to its good performance as a co-initiator for CQ, as the photoinitiated reaction was the one to more likely benefit from the inclusion of the TU additive. Previously published results show that the high concentration of amine is not detrimental to the polymerization initiated by the CQ/EDMAB pair, as long as the concentration of CQ was kept above 0.2 wt% . All initiators were obtained from SigmaAldrich (St. Louis, MO, USA).
For the materials containing thio-urethane, thiol-functionalized oligomers were synthesized by combining 1,6-hexanediol-diissocyante (aliphatic) with pentaerythritol tetra-3-mercaptopropionate (PETMP), according to previously reported methods (13). The same cement composition describe above was used in aim 1 was used here, except that thio-urethane oligomers were added to the base paste at the concentration of 40 wt%. This resulted in a 20 wt% thio-urethane concentration after mixing base and catalyst pastes.
2.2
Pilot study on the purely redox activation of cements with and without thio-urethane oligomer
To define the delay period between the end of the spatulation of the dual-cured cement and the start of light exposure, a pilot study was conducted. A portion of the experimental cements, with and without thio-urethane, was mixed and placed in the specimen holder of the infra-red spectrometer (Nicolet 6700, Thermo Scientific, Pittsburgh, PA, USA) equipped with an InGaAs detector, 3 min after spatulation. Spectra were collected in real-time for 20 min, at 4 cm −1 resolution, 2 Hz acquisition rate, following the same protocol described in detail in a subsequent section (DC and polymerization kinetics). This had the objective of obtaining the polymerization kinetics profile of the redox-activated reaction only. From those curves (n = 3) shown in Fig. 1 , the maximum rate of polymerization (Rp max ) and the degree of conversion (DC) at Rp max and after 10 min or 24 h were determined. Then, the times required to reach those points were determined. Those times were then chosen to delay the photoactivation after mixing the dual-cure cement, in four levels: immediately after spatulation, during autoacceleration of the redox reaction, at the maximum rate or during deceleration of redox activated polymerization reaction. For the cement without thio-urethane, Rp max (0.13%/s) was observed at around 420 s, and this time was used as a reference for selection of the different times to start the photo-activation: immediately (5 s); 1 min before the maximum rate of redox polymerization (MRRP) (360 s); at MRRP (420 s) and 1 min after MRRP (480 s). For the cement containing thio-urethane, the polymerization was fast from the beginning of spatulation, and by the time the material was placed in the spectrometer’s sample compartment, the reaction kinetics was already in the acceleration region. Therefore, the Rp max value for the redox reaction for this material is likely underestimated at 0.24%/s, recorded at approximately 70 s ( Fig. 1 ). However, since the mixing and sample preparation procedures were standardized, we adopted the 70 s time as the MRRP.
2.3
DC and polymerization kinetics
Equal amounts of each paste were mixed for 15 s and inserted in cylindrical silicone rubber molds (n = 3; diameter = 6 mm; thickness = 0.8 mm), sandwiched between two glass slides. Polymerization kinetics was monitored in near-infrared using a spectrometer (Nicolet 6700). The standard delay between the end of sample preparation and the start of FTIR spectra collection was 3 min, as determined in the pilot study, which allowed sufficient time for specimen preparation and placement on the IR holder. The monitoring of polymerization kinetics, therefore, started 3 min after the beginning of cement mixing, and was followed for 10 min. Cements were then photoactivated according to the delay times described above (5, 360, 420 or 480 s) using a LED-based light-curing unit Demi (Kerr, Orange, CA, USA) for 20 s. DC (2 scans/spectrum, 4 cm −1 resolution, >2 Hz data acquisition rate) was calculated based on the area of the methacrylate vinyl overtone at 6165 cm −1 . The polymerization rate (Rp) was calculated as the first derivative of the conversion vs. time curve. The DC at the start of photo-activation, at Rp max and after 10 min were recorded. Polymerized specimens were stored dry for 24 h and the DC was measured again.
2.4
Polymerization stress
Polymerization stress development (in MPa) was followed in real-time for 20 min using the Bioman . Standardized mass of mixed resin cement (around 0.8 g) was placed over a silane-treated (Ceramic Primer, 3M-ESPE) glass plate held to the device with a cylindrical bolt, where the tip of light-curing unit (Demi) was positioned. A metallic rod (5.6 mm of diameter) was then placed over the cement to produce disc specimens with 0.75 mm thickness (configuration factor = 3.73). The light-curing was performed according to light-curing protocols defined above (n = 4).
2.5
Knoop hardness (following experimental design for aim 2 only)
After DC measurement, Knoop micro-hardness (KHN) was tested on the same specimens using a hardness tester (Duramin, Struers, Cleveland, OH, USA) under a load of 490 N (50 gf) for 5 s. In each sample, five measurements were obtained and the Knoop micro-hardness values (KHN: kg/mm2) were recorded as the average of the five indentations per specimen. The specimens were then immersed in absolute ethanol for 24 h to evaluate KHN loss after solvation.
2.6
3-point bending test (following experimental design for aim 2 only)
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 was modified in this study to allow for a single exposure per sample. This was done to keep the same exposure protocols used for the other tests in the mechanical properties evaluation. Resins cements were inserted in rubber molds measuring 14 × 2 × 1 mm, and then photo-activated immediately or at the moment when the redox activated reaction reached its maximum rate. The dimensions of the bars were checked with a digital caliper accurate to 0.01 mm (Mitutoyo Corporation, Tokyo, Japan). The specimens were positioned in a 3-point bending device coupled to a mechanical testing system (Q-test, MTS Inc., Orono, ME, USA). The distance between supports was 10 mm and the load was applied to the center of specimen. The diameter of both supports and of the loading rod was 2 mm. The tests were performed at a crosshead speed of 1.0 mm/min until failure, and the load was monitored by the testing machine software (TestWorks v.3.08, MTS Inc., Orono, ME, USA.). To calculate flexural strength (σf), in MPa, the following equation was used:
σ f = 3 FI 2 bh 2
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