Degree of conversion and monomer elution of CQ/amine and TPO adhesives

Abstract

Objective

To evaluate the effect of photo-initiator on the degree of conversion (DC) and elution of Bis-GMA and HEMA for 8 one-step adhesive formulations.

Methods

We used Scotchbond Universal (‘SBU-CQ/amine_4.0’, 3M ESPE), containing about 2 wt% camphorquinone (CQ) and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), an experimental ‘SBU-TPO_2.1’ version, containing 2.1 wt% diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and 6 experimental LUB adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, respectively containing 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO. DC was measured using micro-Raman spectroscopy. Additional specimens were immersed in ethanol for 24 h to determine the elution of Bis-GMA and HEMA using HPLC.

Results

DC of the respective SBU and LUB adhesives was alike at high photo-initiator concentration. At low concentration, TPO was significantly more efficient than CQ/amine (LUB adhesives only). A statistically significant positive photo-initiator concentration effect on DC was noted for both CQ/amine and TPO (LUB adhesives only). A statistically significant inverse photo-initiator concentration effect on HEMA elution was noted for both the CQ/amine- and TPO-containing LUB adhesives. A significantly strong correlation was found between DC and Bis-GMA elution ( R 2 = 0.744, p = 0.026), and between DC and HEMA elution ( R 2 = 0.913, p = 0.002) for the LUB adhesives.

Significance

The photo-initiator kind and concentration affect DC and the Bis-GMA/HEMA elution. TPO can be used as an alternative photo-initiator for CQ/amine.

Introduction

Camphorquinone (CQ) combined with tertiary amine as co-initiator is the most generally used photo-initiator system in dental adhesives and composites. Although the tertiary amine co-initiator boosts polymerization , the low pH of especially one-step self-etch adhesives may neutralize the tertiary amine by acid-based reaction . Also water, present in self-etch adhesives to ionize acidic monomers, and the hydrophilic nature of one-step self-etch adhesives, thereby also attracting water through osmosis from the host dentin , may make the rather hydrophobic CQ less efficient . Therefore, as an alternative photo-initiator to CQ/amine, the polymerization efficiency of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (commonly being referred to as TPO) has been studied ; TPO has already been used in some commercially available products. TPO was used alone without any co-initiator , or added as second initiator . Studies showed the advantage of TPO over CQ regarding polymerization efficiency, water compatibility and color stability . A possible drawback may be the different light-absorption spectrum of TPO (380–425 nm) versus that of CQ/amine (470 nm), necessitating the use of a polywave LED-curing unit .

Previous studies revealed that the photo-initiator concentration influences DC, the latter obviously of importance to many material properties, such as hardness, water sorption, bond strength, and degradation resistance . A resin-based material should be optimized for the photo-initiator concentration so that maximum physical properties are obtained with a minimum amount of photo-initiator that otherwise may leach out and be potentially toxic .

Bisphenol A glycidyl dimethacrylate (Bis-GMA) and 2-hydroxyethyl methacrylate (HEMA) are two common monomers used in dental adhesives. Both monomers can elute when they get insufficiently immobilized in the 3D resin matrix upon polymerization; leached monomers may then come into contact with the adjacent ‘soft’ gingival, mucosal and/or pulpal tissues, and so have been associated with biocompatibility issues . Several studies have indeed shown that the monomers themselves or their degradation products may induce cytotoxic, genotoxic, mutagenic, apoptosis, and estrogenic effects .

The purpose of this study was to evaluate the effect of two photo-initiator systems on DC and elution of Bis-GMA and HEMA for 1 commercial one-step adhesive and 7 experimental one-step adhesive formulations. The null hypothesis tested was that the kind and concentration of the photo-initiator systems CQ/amine and TPO do not affect DC, nor have an effect on Bis-GMA/HEMA elution.

Materials and methods

Eight different adhesive formulations were used in this study. The commercially available adhesive Scotchbond Universal (referred further in this manuscript to as ‘SBU-CQ/amine_4.0’; 3M ESPE), used in a self-etch mode and containing according to the manufacturer about 2 wt% CQ and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), and an experimental derivative, referred to as ‘SBU-TPO_2.1’ and containing 2.1 wt% TPO, were, respectively, provided and experimentally prepared by the manufacturer. We additionally used 6 experimental ‘LUB’ adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, with the photo-initiator respectively consisting of 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO ( Table 1 ). The experimental LUB adhesive was provided by the manufacturer without photo-initiator. The photo-initiators CQ and TPO, and the co-initiator EDMAB were purchased from Sigma–Aldrich (Sigma–Aldrich Chemie, Steinheim, Germany). The respective amounts of CQ, TPO and EDMAB were measured on an analytical balance with 0.01 mg accuracy (AB304-S’ analytic balance, Mettler-Toledo, Greifensee, Switzerland) and added to the LUB adhesive in amber vials wrapped with aluminum foil to protect them from light. The photo-initiators were dissolved in the adhesive using a closed container immersed in an ultrasonic bath for 1 min.

Table 1
Composition of the SBU and LUB adhesive formulations with the different photo-initiators in various concentrations.
Adhesive Composition a Initiators
SBU-CQ/amine_4.0 b Bis-GMA 15–25 wt%, HEMA 15–25 wt%, DMDMA 5–15 wt%, MDP, ethanol 10–15 wt%, water 10–15 wt%, silica filler 8–12 wt%, copolymer of acrylic and itaconic acid 1–5 wt%, toluene < 0.02 wt% CQ ∼2 wt%, EDMAB <2 wt%
SBU-TPO_2.1 TPO 2.1 wt%
LUB-CQ/amine_0.7 Bis-GMA 15 wt%, HEMA 30 wt%, MDP 10 wt%, TEGDMA 15 wt%, ethanol 15 wt%, water 15 wt%, colloidal silica 10 wt%, stabilizers (minute amount) CQ 0.35 wt%, EDMAB 0.35 wt%
LUB-CQ/amine_1.4 CQ 0.7 wt%, EDMAB 0.7 wt%
LUB-CQ/amine_4.0 CQ 2 wt%, EDMAB 2 wt%
LUB-TPO_0.35 TPO 0.35 wt%
LUB-TPO_0.7 TPO 7 wt%
LUB-TPO_2.0 TPO 2 wt%

a Abbreviations : Bis-GMA, bisphenol A glycidyl dimethacrylate; CQ, camphorquinone; DMDMA, decamethylene dimethacrylate; EDMAB, ethyl 4-(dimethylamino)benzoate; HEMA, 2-hydroxyethyl methacrylate; MDP, 10-methacryloyloxydecyl dihydrogenphosphate; TEGDMA, triethylene dimethacrylate; TPO, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide.

b Commercialized as Scotchbond Universal (Lot: 468651, 3M ESPE).

Non-caries human third molars (collected following informed consent approved by the Commission for Medical Ethics of KU Leuven) were stored in 0.5% chloramine/water at 4 °C and used within 3 months after extraction. Each adhesive (5 μl) was applied to 12 mid-coronal human dentin discs, covered by a glass slide and light-cured for 10 sec using a Bluephase 20i LED light-curing unit (Ivoclar-Vivadent, Schaan, Liechtenstein) with an output of around 1100 mW/cm 2 . The curing parameters, the light-intensity output and emission spectrum, were monitored using a MARC Patient Simulator (Bluelight Analytics, Halifax, NS, Canada). The specimen-preparation procedure is graphically presented in Fig. 1 . DC of 4 specimens was immediately measured using micro-Raman spectroscopy (Senterra, Bruker, Ettlingen, Germany), using a near-infrared (785 nm) laser with a power output of 100 mW, a 50× microscope objective and 50-μm pin-hole aperture. The collected spectra ranged from 50 to 3500 cm −1 with a resolution of around 9–15 cm −1 . The integration time was set to 20 s with 2 co-additions. The CCD detector possessed a 1024 × 256 pixel resolution, and was cooled down thermo-electrically to a temperature of −65 °C. The results were processed using OPUS 7.0 software (Bruker, Ettlingen, Germany), while the laser was calibrated using Sure-cal software (Bruker). DC was calculated as the ratio of peak intensities of the aliphatic 1639 cm −1 and aromatic 1609 cm −1 peaks in cured and uncured materials.

Fig. 1
Specimen preparation procedure to measure degree of conversion and monomer elution.

The monomer elution was measured using the 8 remaining specimens, which were immediately immersed in 1 ml absolute ethanol (99.99%; CAS: 64-17-5, VWR, Haasrode, Belgium) at 37 °C for 24 h, after which the Bis-GMA and HEMA concentration in ethanol were determined using high-performance liquid chromatography (HPLC-DAD, Agilent 1200); the instrument was equipped with an XDB-C18 column with a length of 250 mm, a diameter of 3.0 mm, and a particle size of 5 μm (Zorbax Eclipse, Agilent Technologies, Santa Clara, CA, USA). The mobile phase consisted of a mixture of water and acetonitrile (CH 3 CN), applied successively in a concentration of 10% CH 3 CN (0–7 min), 60% CH 3 CN (7–10 min), 100% CH 3 CN (10–17 min) and 10% CH 3 CN (17–20 min). The flow rate was 1 ml/min and the injection volume was 10 μl. The diode array detector was set to 210 nm for monitoring Bis-GMA and HEMA monomer. Both monomers were identified and quantified by comparison with reference compounds with known composition measured under the same HPLC conditions. Reference standards of Bis-GMA and HEMA (Sigma–Aldrich Chemie, Steinheim, Germany) were used as obtained to produce stock solutions of 1000 μg/ml each. These stock solutions were diluted with absolute ethanol to produce the calibration solutions: 3.125, 6.25, 12.5, 25, 50, 100 and 200 μg/ml. The peak area for each monomer was determined and plotted versus concentration using linear regression analysis for Bis-GMA ( R 2 = 0.9996) and HEMA ( R 2 = 0.9999), and used to quantify monomer concentration in the sample solutions.

Statistical analysis

The DC and monomer elution data were analyzed by one-way ANOVA and Tukey’s multiple comparison test ( α = 0.05). Any correlation between DC and monomer elution was also calculated (R 2.13.2, R Foundation for Statistical Computing, Vienna, Austria).

Materials and methods

Eight different adhesive formulations were used in this study. The commercially available adhesive Scotchbond Universal (referred further in this manuscript to as ‘SBU-CQ/amine_4.0’; 3M ESPE), used in a self-etch mode and containing according to the manufacturer about 2 wt% CQ and 2 wt% ethyl-4-dimethylamino benzoate (EDMAB), and an experimental derivative, referred to as ‘SBU-TPO_2.1’ and containing 2.1 wt% TPO, were, respectively, provided and experimentally prepared by the manufacturer. We additionally used 6 experimental ‘LUB’ adhesives (Kuraray Noritake), namely ‘LUB-CQ/amine_0.7’, ‘LUB-CQ/amine_1.4’, ‘LUB-CQ/amine_4.0’, ‘LUB-TPO_0.35’, ‘LUB-TPO_0.7’ and ‘LUB-TPO_2.0’, with the photo-initiator respectively consisting of 0.35 wt% CQ and 0.35 wt% EDMAB, 0.7 wt% CQ and 0.7 wt% EDMAB, 2.0 wt% CQ and 2.0 wt% EDMAB, 0.35 wt% TPO, 0.7 wt% TPO, and 2.0 wt% TPO ( Table 1 ). The experimental LUB adhesive was provided by the manufacturer without photo-initiator. The photo-initiators CQ and TPO, and the co-initiator EDMAB were purchased from Sigma–Aldrich (Sigma–Aldrich Chemie, Steinheim, Germany). The respective amounts of CQ, TPO and EDMAB were measured on an analytical balance with 0.01 mg accuracy (AB304-S’ analytic balance, Mettler-Toledo, Greifensee, Switzerland) and added to the LUB adhesive in amber vials wrapped with aluminum foil to protect them from light. The photo-initiators were dissolved in the adhesive using a closed container immersed in an ultrasonic bath for 1 min.

Table 1
Composition of the SBU and LUB adhesive formulations with the different photo-initiators in various concentrations.
Adhesive Composition a Initiators
SBU-CQ/amine_4.0 b Bis-GMA 15–25 wt%, HEMA 15–25 wt%, DMDMA 5–15 wt%, MDP, ethanol 10–15 wt%, water 10–15 wt%, silica filler 8–12 wt%, copolymer of acrylic and itaconic acid 1–5 wt%, toluene < 0.02 wt% CQ ∼2 wt%, EDMAB <2 wt%
SBU-TPO_2.1 TPO 2.1 wt%
LUB-CQ/amine_0.7 Bis-GMA 15 wt%, HEMA 30 wt%, MDP 10 wt%, TEGDMA 15 wt%, ethanol 15 wt%, water 15 wt%, colloidal silica 10 wt%, stabilizers (minute amount) CQ 0.35 wt%, EDMAB 0.35 wt%
LUB-CQ/amine_1.4 CQ 0.7 wt%, EDMAB 0.7 wt%
LUB-CQ/amine_4.0 CQ 2 wt%, EDMAB 2 wt%
LUB-TPO_0.35 TPO 0.35 wt%
LUB-TPO_0.7 TPO 7 wt%
LUB-TPO_2.0 TPO 2 wt%
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Degree of conversion and monomer elution of CQ/amine and TPO adhesives
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