Stability of initiation systems in acidic photopolymerizable dental material

Abstract

Objectives

This study simulated the shelf life to evaluate the stability of initiation systems on acidic photopolymerizable dental material, through an experimental self-adhering flowable composite resin (SACR).

Methods

An SACR model was formulated with monomers Bis-GMA, TEGDMA, acidic monomer (GDMA-P), and inorganic fillers. Initiation system combinations of camphorquinone (CQ), tertiary amine (EDAB), diphenyl phosphine oxide (TPO), phenylbis phosphine oxide (BAPO), and the diphenyliodonium hexafluorophosphate (DPIHFP) were tested. Five SACRs were evaluated, varying the initiation system: SACR CQ , SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , SACR BAPO and SACR TPO . The SARC were stored at 23 °C and, according to shelf life, were evaluated for degree of conversion (DC), polymerization rate (Rp) and microtensile bond strength (μTBS). The DC was evaluated after 0, 1, 2, 4, 8, 12, 24 and 48 storage weeks of SACRs. Bonding to dentin was performed after 0, 4, 8, 12 and 24 storage weeks of SACRs and tested immediately (24 h) and after 6 and 12 months by μTBS. Halogen light curing unit (3M ESPE) was used for photoactivation procedures.

Results

SACR CQ+EDAB+DPIHFP showed higher Rp. The DC of SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , and SACR BAPO were similar (40%) and higher than SACR TPO and SACR CQ (20 and 10%, respectively), staying stable up to 48 weeks. The SACR CQ , SACR CQ+EDAB , and SACR TPO had pre-testing failure on μTBS. The shelf life of SACRs did not affect the μTBS of the groups that obtained adhesion. Furthermore, the immediate μTBS (MPa) SACR CQ+EDAB+DPIHFP (8.4) was similar to SACR BAPO (10.5); however, after 12 months, only SACR CQ+EDAB+DPIHFP maintained the μTBS.

Significance

The choice of a suitable initiation system is crucial to the performance and stability of acidic photopolymerizable dental material.

Introduction

Self-etch/adhesive resin or self-adhering materials were developed in dentistry to simplify the restorative procedure by reducing the number of bonding steps, the application time, and technique sensitivity for the operator . This class of dental materials can be commercially found as one-bottle self-etch adhesive systems, self-adhesive resin cements, or as a more recent product, the self-adhering flowable composite resin .

The approach of self-adhering materials has been claimed to be more user friendly; however, mixing all ingredients into one bottle has caused some problems for these materials regarding phase separation, reduced rate of polymerization, and reduced shelf life . These should be attributed to the difficulty of obtaining stable materials in the complex mixture of monomers (monofunctional, crosslinking, and strongly acidic ones), inorganic fillers, solvents, and photoinitiator systems. The most commonly used photoinitiator system in dental polymers is based on the visible-light photosensitizer camphorquinone (CQ), a Norrish type II photoinitiator. It requires a co-initiator, such as the tertiary amine, which donates hydrogen to promote an efficient polymerization reaction . However, some studies have demonstrated that the tertiary amine is chemically unstable in acidic materials , as with the self-adhesive ones and the all-in-one materials. In addition, it was suggested that the acid–base reaction of the acidic monomers with tertiary amines in the materials that are chemical polymerizable can decrease the shelf life stability of these materials and may affect the polymerization reactivity and adhesion process .

Since then, some studies have proposed using unitary initiation systems that are co-initiator-free, such as a Norrish type I photoinitiator; or CQ-based binary initiation systems with alternative co-initiators as amine substitutes; or CQ-based ternary initiation systems would also present better color stability, degree of conversion, rate of polymerization, and mechanical properties and would have shelf life stability. Today, the self-adhering materials are completely, or at least in part, photosensitive; hence, it is important to evaluate shelf life stability and its influence on polymerization reaction, adhesion process, and the mechanical properties of the CQ/amine-based materials in a visible-light polymerization reaction.

Despite the possible effects of the initiation system added as a constituent of acidic materials, there is no evidence in the literature regarding the shelf life of this component on the formulation stability and properties of acidic photopolymerizable dental material, such as self-adhering flowable composite resin (SACR). Therefore, the aim of this study was to evaluate the influence of shelf life on a model SACR, in terms of its polymerization reaction and bonding effectiveness to dentin, when different initiation systems are added to this visible-light curing acidic material. The hypotheses tested were that the initiation systems are unstable in acidic photopolymerizable dental material and that the shelf life affects the SACR properties.

Materials and methods

Reagents

The bisphenol A glycidyl dimethacrylate (Bis-GMA; Esstech Inc., Essignton, PA, USA) and triethylene glycol dimethacrylate (TEGDMA; Esstech), were used as received. The acid phosphate monomer 1,3 glycerol dimethacrylate phosphate (GDMA-P) was synthesized as previously described . Silanized inorganic particles (barium borosilicate glass, 0.7 μm average size and 9% silane) were used as fillers (Esstech). The initiators camphorquinone (CQ; Esstech), ethyl 4-dimethylaminobenzoate (EDAB; Aldrich Chemical Co., Milwaukee, WI, USA), diphenyliodonium hexafluorphosphate (DPIHFP; Aldrich Chemical), diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide (TPO; Aldrich Chemical), and phenylbis (2,4,6-trimethylbenzoyl)-phosphine oxide (BAPO; Aldrich Chemical) were used to render the materials photocurable.

Formulation of the experimental groups

A model SACR was prepared through the intensive mixture of 30 wt% of Bis-GMA, 10 wt% TEGDMA, 20 wt% acidic phosphate monomer GDMA-P, and 40 wt% inorganic fillers.

An optimal photoinitiators and co-initiators concentration of 0.5 mol% CQ , 1 mol% EDAB , 1 mol% DPIHFP , 1 mol% TPO , and 1 mol% BAPO were established from the standpoint of degree of conversion according to previous studies and were added to the model SACR according to the monomer moles. Five SACR were evaluated, varying the initiation system: SACR CQ , SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , SACR BAPO , and SACR TPO . The structural formula and the molecular weight of the initiators evaluated are presented in Table 1 . No radical scavenger was added to avoid interference with the polymerization kinetics. Photoactivation procedures were carried out using a halogen light curing unit (XL 3000; 3M ESPE, St. Paul, MN, USA) with 400 mW/cm 2 irradiance confirmed by a digital power meter (Ophir Optronics, Danvers, MA, USA).

Evaluation of SACR stability and storage time conditions

To evaluate the stability of initiator system on SACR in long term, the shelf life was simulated. After formulation the SACRs were stored in the incubator at 23 °C and, according to the storage time, were evaluated for degree of conversion (DC), rate of polymerization (Rp), and immediate and longitudinal microtensile bond strength (μTBS) following the procedures described below.

Analysis of Rp was performed immediately after SACRs formulation. The DC was evaluated after 0, 1, 2, 4, 8, 12, 24, and 48 storage weeks of SACRs. Bonding to dentin was performed after 0, 4, 8, 12, and 24 storage weeks of SACRs and tested by μTBS after 24 h and 6 and 12 months.

Rate of polymerization and degree of conversion

The reaction of polymerization of the SACR ( n = 3) was evaluated using real-time Fourier transform mid-infrared (FTIR) spectroscopy (Prestige 21; Shimadzu, Tokyo, Japan) with an attenuated total reflectance device (ZnSe crystal). The SACR (∼3 μl) was directly dispensed on the ZnSe crystal.

To DC and Rp evaluation, a reading for the material was taken using setup monitoring scan mode by IRsolution software (SHIMADZU, Columbia, MD, USA), 1 scan per second and 4 cm −1 resolution. The sample scanning was performed simultaneously with the photoactivation for 60 s. The DC (%) and Rp (s −1 ) was calculated as previously described , considering the intensity of C C stretching vibration at 1635 cm −1 an using, as an internal standard, the symmetric ring stretching at 1610 cm −1 from the polymerized and unpolymerized samples. The data curve fitting was performed by Hill’s three-parameter nonlinear regression and the Rp was calculated as the DC at time t subtracted from DC at time t −1 .

For DC evaluation, a preliminary reading for the unpolimerized material (monomer) was taken using 32 scans co-addition and 4 cm −1 resolution. The SACR was photoactivated for 20 s, and a reading was carried out (polymer). The percentage of C C conversion was calculated as previously described considering the intensity of C C stretching vibration at 1635 cm −1 an using, as an internal standard, the symmetric ring stretching at 1610 cm −1 from the polymerized and unpolymerized samples. The data were subjected to one-way ANOVA. All pair-wise multiple comparison procedures were performed using the Holm–Sidak method ( p < 0.05).

Microtensile bond strength testing on dentin

Two hundred fifty bovine incisors, which had the root portion removed, were cleaned and stored in a 0.5% chloramine T solution for seven days. Their buccal faces were wet-ground to create a flat surface. To standardize the smear layer, the dentin was wet-polished with 600-grit SiC papers for 1 min at 100 rpm (Aropol-E; Arotec S.A. Indústria e Comércio, Cotia, SP, Brazil). Ten teeth were randomly allocated to each group. Dentin moisture was controlled with absorbent paper until no surface water was observed. The experimental SARC were vigorously applied on the prepared dentin surfaces for 20 s using a microbrush and light activated for 20 s. Two increments of resin composite (Filtek Z250; 3M ESPE, St. Paul, USA) were inserted to cover the dentin surface completely. The increments were light activated for 20 s each.

After storage for 24 h in distilled water at 37 °C, the teeth were sectioned perpendicular to the bonded interface with a refrigerated low-speed diamond saw (ISOMET 1000; Buheler, Lake Bluff, IL, USA), producing micro-specimens with a cross-sectional surface area of about 0.7 mm 2 . For each tooth, 9 central beams were used and randomly separated into three storage times (24 h, 6 and 12 months).

After the predetermined storage period, the specimens were fixed to the grips of a microtensile device and tested on a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The μTBS values were obtained in MPa. The data were submitted to one-way ANOVA. All pair-wise multiple comparison procedures were performed using the Holm–Sidak method ( p < 0.05).

Failure analyses for all micro-specimens tested were analyzed using stereomicroscope at magnification of 100× and 500×. The modes of failure were classified as pre-testing failure, adhesive failure, cohesive failure within resin, cohesive failure within dentin and mixed failure, when there is more than one type of failure.

Materials and methods

Reagents

The bisphenol A glycidyl dimethacrylate (Bis-GMA; Esstech Inc., Essignton, PA, USA) and triethylene glycol dimethacrylate (TEGDMA; Esstech), were used as received. The acid phosphate monomer 1,3 glycerol dimethacrylate phosphate (GDMA-P) was synthesized as previously described . Silanized inorganic particles (barium borosilicate glass, 0.7 μm average size and 9% silane) were used as fillers (Esstech). The initiators camphorquinone (CQ; Esstech), ethyl 4-dimethylaminobenzoate (EDAB; Aldrich Chemical Co., Milwaukee, WI, USA), diphenyliodonium hexafluorphosphate (DPIHFP; Aldrich Chemical), diphenyl (2,4,6-trimethylbenzoyl)-phosphine oxide (TPO; Aldrich Chemical), and phenylbis (2,4,6-trimethylbenzoyl)-phosphine oxide (BAPO; Aldrich Chemical) were used to render the materials photocurable.

Formulation of the experimental groups

A model SACR was prepared through the intensive mixture of 30 wt% of Bis-GMA, 10 wt% TEGDMA, 20 wt% acidic phosphate monomer GDMA-P, and 40 wt% inorganic fillers.

An optimal photoinitiators and co-initiators concentration of 0.5 mol% CQ , 1 mol% EDAB , 1 mol% DPIHFP , 1 mol% TPO , and 1 mol% BAPO were established from the standpoint of degree of conversion according to previous studies and were added to the model SACR according to the monomer moles. Five SACR were evaluated, varying the initiation system: SACR CQ , SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , SACR BAPO , and SACR TPO . The structural formula and the molecular weight of the initiators evaluated are presented in Table 1 . No radical scavenger was added to avoid interference with the polymerization kinetics. Photoactivation procedures were carried out using a halogen light curing unit (XL 3000; 3M ESPE, St. Paul, MN, USA) with 400 mW/cm 2 irradiance confirmed by a digital power meter (Ophir Optronics, Danvers, MA, USA).

Evaluation of SACR stability and storage time conditions

To evaluate the stability of initiator system on SACR in long term, the shelf life was simulated. After formulation the SACRs were stored in the incubator at 23 °C and, according to the storage time, were evaluated for degree of conversion (DC), rate of polymerization (Rp), and immediate and longitudinal microtensile bond strength (μTBS) following the procedures described below.

Analysis of Rp was performed immediately after SACRs formulation. The DC was evaluated after 0, 1, 2, 4, 8, 12, 24, and 48 storage weeks of SACRs. Bonding to dentin was performed after 0, 4, 8, 12, and 24 storage weeks of SACRs and tested by μTBS after 24 h and 6 and 12 months.

Rate of polymerization and degree of conversion

The reaction of polymerization of the SACR ( n = 3) was evaluated using real-time Fourier transform mid-infrared (FTIR) spectroscopy (Prestige 21; Shimadzu, Tokyo, Japan) with an attenuated total reflectance device (ZnSe crystal). The SACR (∼3 μl) was directly dispensed on the ZnSe crystal.

To DC and Rp evaluation, a reading for the material was taken using setup monitoring scan mode by IRsolution software (SHIMADZU, Columbia, MD, USA), 1 scan per second and 4 cm −1 resolution. The sample scanning was performed simultaneously with the photoactivation for 60 s. The DC (%) and Rp (s −1 ) was calculated as previously described , considering the intensity of C C stretching vibration at 1635 cm −1 an using, as an internal standard, the symmetric ring stretching at 1610 cm −1 from the polymerized and unpolymerized samples. The data curve fitting was performed by Hill’s three-parameter nonlinear regression and the Rp was calculated as the DC at time t subtracted from DC at time t −1 .

For DC evaluation, a preliminary reading for the unpolimerized material (monomer) was taken using 32 scans co-addition and 4 cm −1 resolution. The SACR was photoactivated for 20 s, and a reading was carried out (polymer). The percentage of C C conversion was calculated as previously described considering the intensity of C C stretching vibration at 1635 cm −1 an using, as an internal standard, the symmetric ring stretching at 1610 cm −1 from the polymerized and unpolymerized samples. The data were subjected to one-way ANOVA. All pair-wise multiple comparison procedures were performed using the Holm–Sidak method ( p < 0.05).

Microtensile bond strength testing on dentin

Two hundred fifty bovine incisors, which had the root portion removed, were cleaned and stored in a 0.5% chloramine T solution for seven days. Their buccal faces were wet-ground to create a flat surface. To standardize the smear layer, the dentin was wet-polished with 600-grit SiC papers for 1 min at 100 rpm (Aropol-E; Arotec S.A. Indústria e Comércio, Cotia, SP, Brazil). Ten teeth were randomly allocated to each group. Dentin moisture was controlled with absorbent paper until no surface water was observed. The experimental SARC were vigorously applied on the prepared dentin surfaces for 20 s using a microbrush and light activated for 20 s. Two increments of resin composite (Filtek Z250; 3M ESPE, St. Paul, USA) were inserted to cover the dentin surface completely. The increments were light activated for 20 s each.

After storage for 24 h in distilled water at 37 °C, the teeth were sectioned perpendicular to the bonded interface with a refrigerated low-speed diamond saw (ISOMET 1000; Buheler, Lake Bluff, IL, USA), producing micro-specimens with a cross-sectional surface area of about 0.7 mm 2 . For each tooth, 9 central beams were used and randomly separated into three storage times (24 h, 6 and 12 months).

After the predetermined storage period, the specimens were fixed to the grips of a microtensile device and tested on a mechanical testing machine (DL500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min until failure. The μTBS values were obtained in MPa. The data were submitted to one-way ANOVA. All pair-wise multiple comparison procedures were performed using the Holm–Sidak method ( p < 0.05).

Failure analyses for all micro-specimens tested were analyzed using stereomicroscope at magnification of 100× and 500×. The modes of failure were classified as pre-testing failure, adhesive failure, cohesive failure within resin, cohesive failure within dentin and mixed failure, when there is more than one type of failure.

Results

The kinetics of polymerization in real time of the experimental SACR immediately after formulation with different initiation systems are shown in Fig. 1 . When evaluating only CQ (SACR CQ ), very low final values of degree of conversion (DC) were observed, <5%. However, the addition of amine (EDAB) caused an increase in DC, shown in the binary initiation system SACR EDAB+CQ DC values >30%. The SACR CQ+EDAB+DPIHFP and SACR BAPO obtained final DC values similar to those of SACR CQ+EDAB , shown in Fig. 1 A. The addition of DPIHFP, besides not affecting the conversion of monomers, helped to increase the maximum rate of polymerization (Rp max ), exhibiting that the SACR CQ+EDAB+DPIHFP ternary initiation system had a Rp max higher than the SACR CQ+EDAB initiation system commonly used in commercial dental products ( Fig. 1 B). The group formed by the TPO (SACR TPO ) showed values of DC < 15% and a very low rate of polymerization up to 10 s of photoactivation (not detectable), Fig. 1 (A and B).

Fig. 1
(A) Degree of conversion and (B) rate of polymerization of experimental SACRs with different photoinitiation systems. Data obtained immediately after formulation.

The results for the DC of SACRs containing different initiation systems according to the shelf life are shown in Figs. 2 and 3 . The statistical analysis showed that the “initiation system” and “shelf life” factors were statistically significant ( p < 0.05).

Fig. 2
Degree of conversion of experimental SACRs with different photoinitiation systems after each storage week. Data obtained after photoactivation for 20 s.

Fig. 3
Degree of conversion of experimental SACRs with different photoinitiation systems after shelf life. Data obtained after photoactivation for 20 s.

Fig. 2 shows the comparison of the DC values of the SACRs in each shelf life conditions. The DC values of SACR CQ+EDAB were statistically similar to SACR CQ+EDAB+DPIHFP and SACR BAPO after 0, 4, 12, 24, and 48 weeks of shelf life storage. The approximate DC value of these SACRs at week 0 was 30%, and after 1, 2, 4, 8, 12, 24, and 48 weeks, it was around 40%. The DC values of SACR CQ and SACR TPO were statistically lower than those of SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , and SACR BAPO after most shelf life storage (1, 2, 4, 8, 24, and 48 weeks).

Fig. 3 shows the DC stability of the SACR under different initiation systems after shelf life storage. The statistical analysis showed that for SACR CQ ( p = 0.49) and SACR BAPO ( p > 0.05), the “shelf life” factor was not statistically significant, and these initiation systems show a stable DC in acidic photopolymerizable dental material. SACR CQ+EDAB , SACR CQ+EDAB+DPIHFP , and SACR TPO did not reveal a significantly decreased DC up to 48 weeks of shelf life storage.

The μTBS values of experimental SACRs, in which potential interference of shelf life versus an initiator system on the bonding effectiveness to dentin was tested, are shown in Table 2 . The statistical analysis showed that the factors “initiator system,” “shelf life,” and “storage period of SACRs-dentin bonds” were both significant. All specimens of the SACR CQ , SACR TPO , and SACR CQ+EDAB had pre-testing failure, assigning them a bond strength value of zero. When evaluating the influence of shelf life on bonding effectiveness, it was observed that shelf life had not damaged the bonding effectiveness for SACRs tested. The SACR CQ+EDAB+DPIHFP and SACR BAPO did not reveal a significantly decreased μTBS up to 24 weeks of shelf life storage. When evaluating the longevity of SACRs-dentin bonds (24 h, 6 and 12 months), it was observed that the type initiator system affected bonding longevity. The BAPO revealed a significantly lower μTBS at 6 and 12 months of aging than the immediate (24 h) at all shelf life lengths. In contrast, the SACR CQ+EDAB+DPIHFP showed no significant decrease in the values of long-term bond strength in all shelf life lengths.

Table 2
Microtensile bond strength values (mean (standard deviation) in MPa) of the experimental SACRs after shelf life and longevity of SACRs-dentin bonds.
Longevity of the bond strength
Shelf life CQ CQ + EDAB CQ + EDAB + DPIHFP BAPO TPO
24 h 6 m 12 m 24 h 6 m 12 m 24 h 6 m 12 m 24 h 6 m 12 m 24 h 6 m 12 m
0 week 0 0 0 0 0 0 8.2 (4.3) B 6.0 (3.5) AB 5.1 (4.1) AB 3.8 (5.1) B 0.9 (1.5) A 0.2 (0.5)* A 0 0 0
4 weeks 0 0 0 0 0 0 8.1 (3.7) B 3.8 (5.9) B 4.2 (6.0) B 10.7 (7.0) AB 4.3 (8.0) A 1.5 (2.9)* A 0 0 0
8 weeks 0 0 0 0 0 0 19.2 (9.2) A 12.7 (9.8) A 11.1 (10.4) A 12.6 (7.1) A 4.6 (4.3)* A 2.9 (3.5)* A 0 0 0
12 weeks 0 0 0 0 0 0 8.4 (4.7) B 4.9 (2.0) A 4.3 (2.4) AB 10.5 (5.2) AB 3.5 (5.2)* A 3.0 (2.3)* A 0 0 0
24 weeks 0 0 0 0 0 0 10.7 (6.1) B 6.8 (3.9) A 5.2 (6.1) AB 7.0 (3.6) AB 1.8 (2.2)* A 2.2 (2.7)* A 0 0 0
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Stability of initiation systems in acidic photopolymerizable dental material
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