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Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth Khold Al-Ahdal a,b , Nicoleta Ilie c,∗ , Nick Silikas a , David C. Watts a,d a
Biomaterials Science Research Group, School of Dentistry, University of Manchester, UK College of Dentistry, King Saud University, Riyadh, Saudi Arabia c Ludwig-Maximilians-University of Munich, Germany d Photon Science Institute, University of Manchester, UK b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Since bulk-fill (BF) resin composites should cure efficiently to a depth up to 4 mm,
Received 15 June 2015
the aim of the study was to determine the time-dependence of degree of conversion (DC) at
Received in revised form
that depth during 24 h post-irradiation.
28 July 2015
Methods. Eight representative BF resin composites were studied [x-tra base (XTB), Venus
Accepted 28 July 2015
Bulk Fill (VBF), Tetric EvoCeram Bulk Fill (TECBF), Sonic Fill (SF), Filtek Bulk Fill (FBF), everX
Available online xxx
Posterior (eXP), Beautifil-Bulk Flowable (BBF), Beautifil-Bulk Restorative (BBR)]. Specimens were fabricated in white Delrin moulds of 4 mm height and 5 mm internal diameter directly
Keywords:
on an attenuated total reflectance (ATR) accessory attachment of an (FTIR) spectrometer
Bulk-fill
(Nicolet iS50, Thermo Fisher, Madison, USA). Upper specimen surfaces were irradiated in situ
Resin-composite
for 20 s with an LED curing unit (Elipar S10 with average tip irradiance of 1200 mW/cm2 ).
Degree of conversion
Spectra from the lower surface were recorded continuously in real-time for 5 min and then
Polymerization kinetics
at 30 and 60 min and 24 h post irradiation.
Depth of cure
Results. Mean ranges of DC4mm (%) of the materials at 4 mm depth were 39–67; 48–75; 45–74; and 50–72 at 5, 30 and 60 min and 24 h respectively. DCs for XTB, VBF, TECBF, FBF, BBR increased significantly 30 min after irradiation (p < 0.05) and were not affected by subsequent time up to 24 h (p > 0.05). DC for SF was not affected by subsequent time after 5 min (p > 0.05). For eXP and BBF, DC increased 24 h after irradiation (p < 0.05). The data were described by the superposition of two exponential functions characterising the gel phase (described by parameters a, b) and the glass phase (described by parameters c and d). Significance. Post polymerization impact of bulk-fill composites is material dependent. Five materials exhibited their maximum DC4mm already 30 min after starting the irradiation while DC4mm for two materials optimized after 24 h. BF materials were found to exhibit after 24 h a DC between 50 and 72% at 4 mm depth under the stated irradiation conditions. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author at: Department of Operative Dentistry and Periodontology Ludwig-Maximilians-University Munich, Germany. E-mail address: [email protected] (N. Ilie).
http://dx.doi.org/10.1016/j.dental.2015.07.004 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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1.
Introduction
The adequate conversion of resin composite materials is essential in determining their mechanical performance. Ideally, during the polymerization reaction all the monomer in the resin composite material would have been converted to polymer. However, dimethacrylate monomers exhibit residual double carbon bonds in the final material, with the Degree of Conversion (DC) ranging from 55 to 75% [1–3]. An ideal resin composite should exhibit a high degree of conversion (DC) and only a minimal polymerization shrinkage [4]. There are many factors that can affect the final degree of conversion: these include intrinsic factors such as the chemical structure and composition of the dimethacrylate monomer and the concentration of the photo-initiator. The DC is also affected by extrinsic factors such as the polymerization temperature [5,6]. Resin composites are conventionally applied in increments of 2 mm thickness [6,7]. However, this is time consuming, especially when applied in deep posterior cavities. Therefore, a new category of resin composite termed “bulk-fill” has been introduced which can be cured in 4 mm thick increments. Compositional modifications have been made to allow such bulk placement that may also change the polymerization kinetics. Dental resin composites are mostly based on dimethacrylate resins; where the polymerization process is usually activated by applying visible light [8,9]. This free-radical polymerization process is fast in the early stages where the monomer molecules are mobile and able to reach the reactive sites easily. However, the polymerization rate decreases afterwards as the degree of conversion increases and hinders the mobility of monomer molecules to reach the reactive sites [10]. The majority of the polymerization process occurs during the first few minutes after irradiation [6]. Bulk fill composites are receiving attention mainly because they can be placed in 4 mm increments without adverse effects on polymerization shrinkage, cavity adaptation or the degree of conversion (DC) [11]. Additionally their polymerization shrinkage may be lower than conventional composites, so that post-operative problems of gap formation and subsequent caries recurrence may be reduced [12]. A comparison between Surefil® SDRTM flow and Venus® bulk fill showed that Surefil® SDRTM flow exhibited higher mechanical properties despite lower DC values than Venus® bulk fill [11]. Some studies have established and compared the degree of conversion of conventional resin composites and bulk-fill resin composites. Previous research [13] has shown that the surface DC values of bulk-fill resin composites are comparable to those of conventional resin composites and that there is difference in DC between 5 min and 24 h. However, this was done on thin films rather than with 4 mm thick specimens. Li et al. [14] compared the DC of bulk-fill and conventional composites and showed that Filtek Bulk Fill Flowable and everX Posterior showed the highest DC followed by SDR then Tetric EvoCeram Bulk Fill and Herculite XRV Ultra. The precise nature of the initial photo-cure of resincomposites has an essential role in the post-cure polymerization process. The polymerization reaction starts rapidly after
applying the irradiation source, which causes internal mobility restrictions within the growing polymer matrix network, which in turn causes reduction in the polymerization rate [15]. Consequently, the free radicals have reduced movement within the matrix and so the polymerization continues at a slower rate [16]. Some studies reported that the post-cure polymerization continues up to or beyond 24 h after irradiation [17,18]. As long as there are free radicals and reactants, the polymerization process will continue but as the quantity of the free radicals decreases, the polymerization rate decreases. Moreover, postcure polymerization has been detected up to 1 month after irradiation [19]. Optimum clinical properties of dental composites are significantly affected by their composition. However, properties also depend upon the effective polymerization of the material during, and following, clinical placement. The degree of conversion (DC) of resin composite materials is a key measure of effective polymerization and crosslinking of the multifunctional monomers that are used to create the matrix. Good conversion is thus essential to their long-term functionality [2,20,21] and inadequate DC can be detrimental to the success of dental restorations. Thus several factors affect the DC of resin composites when applied clinically. These include operator-related factors, such as thickness of each layer, irradiance of the light curing unit, and proximity of light curing tip to the restoration [22–24]. The DC is nevertheless influenced by composite formulation, as determined by the manufacturer, through the type of resin-matrix, filler type, size and loading [25–27]. Since ‘bulk fill’ (BF) resin composites are generally claimed to be suitable for placement in increments of up to 4 mm, it is useful to denote ‘the degree of conversion measured at 4 mm’ as DC4mm . Measuring this particular parameter was the focus of this investigation. The aim of this study was to assess and compare the degree of conversion (DC4mm ) of some bulk-fill resin composite materials using real time Fourier transform infrared spectroscopy (FTIR) up to 30 min, 60 min and 24 h post cure. Our null hypothesis was that for each composite there is no difference between degree of conversion (DC4mm ) at 5 min, 30 min, 60 min and 24 h post cure.
2.
Materials and methods
Eight bulk fill resin composites were investigated (Table 1). The DC measurements were conducted using a Fourier transform infrared spectroscopy (FTIR) with an attenuated total reflectance (ATR) accessory (Nicolet iS50, Thermo Fisher, Madison, USA).
2.1.
Real time measurement.
The un-polymerized composite pastes (n = 6) were placed directly on the diamond ATR crystal in molds of 4 mm height and 3 mm internal diameter, filled in one increment. The specimens were covered with plastic matrix strips (Frasaco, Tettnang, Germany) and the light curing unit (LCU), with measured average tip irradiance of 1200 mW/cm2 , was applied
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
DENTAL-2595; No. of Pages 7
Code
Material
Shade
Manufacturer
Lot. no.
Resin Matrix
Filler Composition
Load (wt%/vol %)
Flowable bulk-fill Filtek Bulk Fill
Universal
3 M ESPE, ESPE, St. Paul, MN, USA
N390563
Bis-GMA, bis-EMA, UDMA
VBF
Venus Bulk Fill
Universal
010102
UDMA, EBPDMA
BBF
Beautifil-Bulk Flowable
Dentin
Heraeus Kulzer GmbH, Hanau, Germany Shofu Inc. Kyoto, Japan
121302
Bis-GMA, UDMA, Bis-MPEPP, TEGDMA
Xtb
x-tra base
Universal
Voco, Cuxhaven, Germany
1315357
Aliphatic di-methacrylate (UDMA), Bis-EMA
Restorative bulk-fill eXP ever XPosterior
Universal
1308261
TECBF
Tetric EvoCeram Bulk Fill
IV A
GC Corporation Tokyo, Japan Ivoclar Vivadent AG, Schaan, Liechtenstein
bis-GMA, TEGDMA, PMMA Bis-GMA, UDMA
SF
Sonic Fill
A2
4427299
BBR
Beautifil-Bulk Restorative
A
Kerr Corp., Orange, CA, USA Shofu Inc. Kyoto, Japan
S09220
021402
Bis-GMA, TEGDMA, EBPDMA Bis-GMA, UDMA, Bis-MPEPP, TEGDMA
A combination of ytterbium trifluoride filler (sizes range from 0.1 to 5.0 m) and zirconia/silica (size range of 0.01 to 3.5 m) Ba-Al-F-Si glass, SiO2
64.5/42.5
S-PRG filler based on fluoroboroaluminosilicate glass, polymerization initiator. Barium glass ceramic, fumed silica mean size 3.5 m
73/n.a
Hybrid filler fractions & E-glass fibers Ba-Al-Si glass, prepolymer filler (monomer, glass filler, and ytterbium fluoride), spherical mixed oxide SiO2 , glass, oxide
74.2/53.6
S-PRG filler based on fluoroboroaluminosilicate glass, polymerization initiator
65/38
75/58
79-81/60-61
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FBF
83.5/n.a. 87/74.5
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Table 1 – Resin composites studied, ordered in flowable bulk-fill and restorative bulk-fill categories.
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directly above the sample surface for 20 s (Elipar S10, 3 M Espe, Seefeld, Germany). The FTIR spectra were recorded continuously in real time for 5 min taking spectra from the lower surface of the specimens in direct contact with the ATR crystal.
y = a × (1 − e−bx ) + c × (1 − e−dx )
2.2. Measurements at 30 min, 60 min and 24 h of post-cure DC4mm Further test specimens (n = 6, for each material) were made by placing a mold (4 mm height; 5 mm internal diameter) on a plastic matrix strip covering a glass microscope slide. Each mold was filled in one increment and covered with a plastic matrix strip. The specimen was then light cured with the LCU being directly applied on the sample surface for 20 s. After 5 min the samples were stored dry at 37 ◦ C. After 30 min, 60 min and 24 h, each specimen was carefully placed on the ATR crystal plate and pressed with a clamp to sustain good contact between the sample and the ATR crystal. 100 FTIR spectra were then rapidly collected for each specimen and averaged.
2.3.
Calculation of DC
Calculation of the DC for the dimethacrylate based resin composites was done by comparing the height of particular peaks in the spectra derived from the unpolymerized and polymerized resin. There were two peaks of interest. The first peak corresponded to a group actively involved in polymerization, while the second peak functioned as an internal standard. The change during polymerization was calculated using both peak heights. The percentage of unreacted carbon double bonds C C was obtained from the peak height ratio of the methacrylate C C (at 1637 cm−1 ) and those of an internal standard aromatic carbon double bond (at 1608 cm−1 ) during polymerization, in relation to the uncured material. The percentage DC was calculated for each sample DCPeak %
= 1−
(1634 cm−1 /1608 cm−1 )Peak
height after curing
(1634 cm−1 /1608 cm−1 )Peak
height before curing
compared to the absorption values of a separate single precure sample of each material. For each material, the increase in DC4mm during 24 h was described by the superposition of two exponential functions.
× 100 (1)
In the real time investigation, DC was calculated using the pre-cure and post-cure absorption values of the same sample. For the 30 min, 60 min and 24 h post-cure investigation DC was calculated using the absorption values of each sample
(2)
The parameters a, b, c, d were modulation factors of the exponential function to optimize the fit of the double exponential function to the measured DC4mm versus time curves.
2.4.
Statistical analysis
Data were analyzed using statistical software (SPSS ver.20, IBM Corp, Armonk, NY, USA) at (p < 0.05) significance level. The Kolmogorov–Smirnov Test indicated that all DC data for the materials was normally distributed except for eXP and BBF. A Kurskal–Wallis non-parametric test was used for these two materials and subsequent paired comparisons were made following a Mann–Whitney non-parametric test. The other materials were analyzed by oneway ANOVA and the Bonferroni post-hoc test, to compare DC4mm between different times within each resin composite.
3.
Results
The degree of conversion data measured at a material depth of 4 mm is presented in Table 2. Post-cure time affected DC for all materials except SF (p > 0.05). The measured DC at 4 mm depth (DC4mm ) of SF 5 min after photo-initiation did not increase after 30 min, 60 min or 24 h. DC4mm significantly increased (p < 0.05) after 30 min for XTB, VBF, TECBF, FBF and BBR. The DC4mm of eXP and BBF significantly only increased by 24 h post-cure (p < 0.05). The experimental DC data were well described by the superposition of two exponential functions manifest by a correlation factor: r2 > 0.88 defined for the parameters a, b, c, and d, as presented in Table 3 and Fig. 1
4.
Discussion
This study aimed to compare DC4mm of several contemporary bulk-fill resin composites. These composites varied in their composition in terms of filler content (type and loading) and resin matrices. Statistically significant differences in the degree of conversion (DC4mm ), for each resin composite, were found between different times post-cure. Hence our null hypothesis was rejected.
Table 2 – Mean (SD) degrees of conversion, at 4 mm depth, of resin composites at different times post-cure. Materials 5 min 30 min 60 min 24 h
XTB
VBF
TECBF
SF
FBF
eXP
BBF
BBR
49.4 a (1.89) 57.6 b (2.92) 56.2 b (2.85) 57.7 b (3.14)
66.6 a (0.89) 74.8 b (2.56) 73.6 b (1.98) 71.9 b (2.00)
46.9 a (4.53) 53.0 b (1.06) 54.8 b (4.56) 54.5 b (1.53)
66.6 a (4.96) 70.2 a (1.48) 68.6 a (2.16) 71.6 a (3.98)
55.8 a (1.06) 62.3 b (2.68) 61.0 b (1.79) 58.5 a, b (3.50)
54.4 a (0.64) 59.9 a, b (4.52) 63.9 b, c (3.36) 66.2 c (2.38)
56.3 a (1.73) 58.4 a, b (7.57) 63.7 b, c (1.12) 65.7 c (1.65)
38.9 a (4.57) 47.6 b (6.07) 45.3 a, b (5.44) 49.7 b (4.98)
Within each material, similar superscript letters indicate homogenous subsets among the time periods.
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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Table 3 – Confidence intervals for pre-exponential (a, c) and exponential (b, d) parameters describing the kinetic functions (Eq. (2)) that fit the data with coefficients of determination: r2 > 0.88. RBC
r2
a
b
c
BBR TECBF XTB FBF BBF eXP VBF SF
0.90 0.93 0.95 0.95 0.96 0.92 0.95 0.89
32.78–33.11 40.63–40.97 44.64–44.85 48.19–48.67 49.63–49.96 49.79–50.10 60.31–60.71 61.87–62.48
0.14–0.15 0.14–0.14 0.19–0.20 0.20–0.20 0.16–0.17 0.19–0.20 0.16–0.17 0.13–0.13
13.62–15.32 11.30–12.98 11.05–12.27 8.43–9.06 9.58–10.72 11.76–13.60 10.63–12.25 6.60–9.80
Furthermore, the curing time undoubtedly had a significant effect on the DC, depending upon the light irradiance level used and the total energy required by the material for polymerization. Durner et al. and Frauscher et al. concluded that adequate curing time (20 s, 40 s) with a moderate irradiance (ca. 1000 mW/cm2 ) is essential to ensure that the resin composite is adequately polymerized; as an extended curing time (more than 40 s) resulted in a significant increase in DC and a short curing time (5 s, 10 s) resulted in a lower DC [31,32]. DC4mm is a valuable property in characterising changes within one material. A direct comparison of the DC among the materials is certainly only possible by considering the kinetic parameters. All analyzed materials were methacrylatebased dental restoratives containing monomers with at least two carbon-carbon (C C) double bonds, such as bisphenol-A-glycidyldimethacrylate (BisGMA) or triethylene glycol dimethacrylate (TEGDMA), able to build a threedimensional polymer network. As previously shown, the polymerization kinetics in resin composites can be very well described by an exponential sum function [33]. This was confirmed by the present experimental data, which were well described for all materials (r2 > 0.88) by the superposition of two exponential functions, the first exponential function characterizing the gel phase (described by the parameters a and b) and the second exponential function characterizing the glass phase (described by parameters c and d) (Table 3). It was previously shown that parameter “a” is a materialdependent pre-exponential term for the polymerization kinetics in the gel phase, while parameter “b” is dependent on the specimen’s thickness, demonstrating that the decrease
Fig. 1 – DC at 4 mm depth from 0 to 300 s for eight bulk-fill resin composites.
d 0.19–0.23 × 10−2 0.23–0.26 × 10−2 0.18–0.20 × 10−2 0.53–0.67 × 10−2 0.30–0.38 × 10−2 0.16–0.20 × 10−2 0.27–0.35 × 10−2 0.13–0.25 × 10−2
of the C C double bonds in the gel phase is faster in thinner layers. As for the parameters “c” and “d”, they are comparable for all analyzed materials. Parameter “c” varied amongst the analyzed materials, being between 8.20 and 14.47 compared to 32.9–62.18 for the parameter “a”. However, parameter “d” was similar in all materials and of less relevance because of its low value in the exponential function. This suggests that that the reaction kinetics in the glass phase is less material dependent compared with the gel phase. In the first few minutes up to 1 h after light curing the resin composites, the polymerization progressed [34,35] and then it continued slowly up to a maximum measured at 24 h [36]. In this study, all the materials (except SF), showed a significant increase of DC at 30 min, 60 min and at 24 h post-irradiation with storage at 37 ◦ C when compared to results obtained 5 min post-cure. The DC4mm of XTB, VBF, TECBF, FBF and BBR increased significantly at 30 min post-irradiation. While eXP and BBF increased significantly at 24 h post-irradiation. These variations in reaching the maximum DC could be due to differences in the resin matrix compositions amongst these resin composites [37]. eXP and BBF reached their maximum DC in 24 h post-cure. Both of them have similar filler loading (74.2% and 73% by weight respectively). In addition all the resin composites which reached their maximum DC at 30 min post-cure contained UDMA in their resin matrix which is a less viscous monomer than bis-GMA [38]. UDMA incorporates an imino group ( NH ) group which is responsible for the characteristic chain transfer reactions that provide an alternative path for the continuation of polymerization. On the other hand, SF reached its maximum DC in the first 5 min. This might also be explained by the polymerization kinetics, since for SF the largest parameter “a” was identified, corresponding to a very fast start of polymerization. Sideridou et al. [38] reported that TEGDMA increased the polymerization rate when added to bis-GMA. This mixture increased the rate of polymerization from the start of photo-cure compared to resin composites with bis-GMA alone. As TEGDMA is a low viscous monomer and acts as a diluent it results in rapid propagation of the polymerization reaction. The filler content also has an effect on the DC and setting kinetics of resin composite materials. Beautifil Bulk Restorative (BBR) and Beautifil Bulk Flowable (BBF) have a similar composition, as the both contain the surface pre-reacted glass-ionomer (S-PRG) filler, which is an original type of filler that releases fluoride and some ions such as Al3+ , B− , and Sr2+ [28–30]. It is also claimed that the resinous coating material containing S-PRG fillers is able to prevent caries as it increase
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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the mineralization. However, these two products vary in their filler loading as BBR has 87 wt% and BBF 73 wt%; DC4mm significantly increased after 30 min for BBR while DC4mm of BBF significantly increased 24 h post-cure. Therefore it appears that as the filler loading increased, the DC4mm reached its maximum in a shorter post-cure period. The DC of some resin composites have been assessed in previous studies. Czasch and Ilie measured the DC after 5 min of some bulk-fill resin composites at different depths and polymerization times using a curing light with irradiance of 1226 mW/cm2 . They measured VBF and the value was 66.1% [11] which is comparable to our value after 5 min post cure (66.6%). This is due to the similarity in the irradiance and curing time. Another study by Alshali et al. [13] reported DC values of 54%, 56% and 50% for XTB, VBF and FBF respectively. These results were achieved immediately post-cure using a light cure with an irradiance of 600 mW/cm2 . In this present study similar materials were assessed 5 min post-cure after irradiation at 1200 mW/cm2 . The DC values were 49%, 67% and 56% for XTB, VBF and FBF respectively. Taking into consideration variation in curing light irradiance and post-irradiation time, these values can be considered comparable.
5.
Conclusion
DC4mm and the kinetic parameters for cure of resin composites depend greatly on their composition when light curing irradiance and time are fixed.
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[35] Watts DC, Amer OM, Combe EC. Surface hardness development in light-cured composites. Dent Mater 1987;3(5):265–9. [36] Pilo R, Cardash HS. Post-irradiation polymerization of different anterior and posterior visible light-activated resin composites. Dent Mater 1992;8(5):299–304. [37] Ilie N, Stark K. Curing behaviour of high-viscosity bulk-fill composites. J Dent 2014;42(8):977–85. [38] Sideridou I, Tserki V, Papanastasiou G. Effect of chemical structure on degree of conversion in light-cured dimethacrylate-based dental resins. Biomaterials 2002;23(8):1819–29.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth Khold Al-Ahdal a,b , Nicoleta Ilie c,∗ , Nick Silikas a , David C. Watts a,d a
Biomaterials Science Research Group, School of Dentistry, University of Manchester, UK College of Dentistry, King Saud University, Riyadh, Saudi Arabia c Ludwig-Maximilians-University of Munich, Germany d Photon Science Institute, University of Manchester, UK b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Since bulk-fill (BF) resin composites should cure efficiently to a depth up to 4 mm,
Received 15 June 2015
the aim of the study was to determine the time-dependence of degree of conversion (DC) at
Received in revised form
that depth during 24 h post-irradiation.
28 July 2015
Methods. Eight representative BF resin composites were studied [x-tra base (XTB), Venus
Accepted 28 July 2015
Bulk Fill (VBF), Tetric EvoCeram Bulk Fill (TECBF), Sonic Fill (SF), Filtek Bulk Fill (FBF), everX
Available online xxx
Posterior (eXP), Beautifil-Bulk Flowable (BBF), Beautifil-Bulk Restorative (BBR)]. Specimens were fabricated in white Delrin moulds of 4 mm height and 5 mm internal diameter directly
Keywords:
on an attenuated total reflectance (ATR) accessory attachment of an (FTIR) spectrometer
Bulk-fill
(Nicolet iS50, Thermo Fisher, Madison, USA). Upper specimen surfaces were irradiated in situ
Resin-composite
for 20 s with an LED curing unit (Elipar S10 with average tip irradiance of 1200 mW/cm2 ).
Degree of conversion
Spectra from the lower surface were recorded continuously in real-time for 5 min and then
Polymerization kinetics
at 30 and 60 min and 24 h post irradiation.
Depth of cure
Results. Mean ranges of DC4mm (%) of the materials at 4 mm depth were 39–67; 48–75; 45–74; and 50–72 at 5, 30 and 60 min and 24 h respectively. DCs for XTB, VBF, TECBF, FBF, BBR increased significantly 30 min after irradiation (p < 0.05) and were not affected by subsequent time up to 24 h (p > 0.05). DC for SF was not affected by subsequent time after 5 min (p > 0.05). For eXP and BBF, DC increased 24 h after irradiation (p < 0.05). The data were described by the superposition of two exponential functions characterising the gel phase (described by parameters a, b) and the glass phase (described by parameters c and d). Significance. Post polymerization impact of bulk-fill composites is material dependent. Five materials exhibited their maximum DC4mm already 30 min after starting the irradiation while DC4mm for two materials optimized after 24 h. BF materials were found to exhibit after 24 h a DC between 50 and 72% at 4 mm depth under the stated irradiation conditions. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author at: Department of Operative Dentistry and Periodontology Ludwig-Maximilians-University Munich, Germany. E-mail address: [email protected] (N. Ilie).
http://dx.doi.org/10.1016/j.dental.2015.07.004 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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1.
Introduction
The adequate conversion of resin composite materials is essential in determining their mechanical performance. Ideally, during the polymerization reaction all the monomer in the resin composite material would have been converted to polymer. However, dimethacrylate monomers exhibit residual double carbon bonds in the final material, with the Degree of Conversion (DC) ranging from 55 to 75% [1–3]. An ideal resin composite should exhibit a high degree of conversion (DC) and only a minimal polymerization shrinkage [4]. There are many factors that can affect the final degree of conversion: these include intrinsic factors such as the chemical structure and composition of the dimethacrylate monomer and the concentration of the photo-initiator. The DC is also affected by extrinsic factors such as the polymerization temperature [5,6]. Resin composites are conventionally applied in increments of 2 mm thickness [6,7]. However, this is time consuming, especially when applied in deep posterior cavities. Therefore, a new category of resin composite termed “bulk-fill” has been introduced which can be cured in 4 mm thick increments. Compositional modifications have been made to allow such bulk placement that may also change the polymerization kinetics. Dental resin composites are mostly based on dimethacrylate resins; where the polymerization process is usually activated by applying visible light [8,9]. This free-radical polymerization process is fast in the early stages where the monomer molecules are mobile and able to reach the reactive sites easily. However, the polymerization rate decreases afterwards as the degree of conversion increases and hinders the mobility of monomer molecules to reach the reactive sites [10]. The majority of the polymerization process occurs during the first few minutes after irradiation [6]. Bulk fill composites are receiving attention mainly because they can be placed in 4 mm increments without adverse effects on polymerization shrinkage, cavity adaptation or the degree of conversion (DC) [11]. Additionally their polymerization shrinkage may be lower than conventional composites, so that post-operative problems of gap formation and subsequent caries recurrence may be reduced [12]. A comparison between Surefil® SDRTM flow and Venus® bulk fill showed that Surefil® SDRTM flow exhibited higher mechanical properties despite lower DC values than Venus® bulk fill [11]. Some studies have established and compared the degree of conversion of conventional resin composites and bulk-fill resin composites. Previous research [13] has shown that the surface DC values of bulk-fill resin composites are comparable to those of conventional resin composites and that there is difference in DC between 5 min and 24 h. However, this was done on thin films rather than with 4 mm thick specimens. Li et al. [14] compared the DC of bulk-fill and conventional composites and showed that Filtek Bulk Fill Flowable and everX Posterior showed the highest DC followed by SDR then Tetric EvoCeram Bulk Fill and Herculite XRV Ultra. The precise nature of the initial photo-cure of resincomposites has an essential role in the post-cure polymerization process. The polymerization reaction starts rapidly after
applying the irradiation source, which causes internal mobility restrictions within the growing polymer matrix network, which in turn causes reduction in the polymerization rate [15]. Consequently, the free radicals have reduced movement within the matrix and so the polymerization continues at a slower rate [16]. Some studies reported that the post-cure polymerization continues up to or beyond 24 h after irradiation [17,18]. As long as there are free radicals and reactants, the polymerization process will continue but as the quantity of the free radicals decreases, the polymerization rate decreases. Moreover, postcure polymerization has been detected up to 1 month after irradiation [19]. Optimum clinical properties of dental composites are significantly affected by their composition. However, properties also depend upon the effective polymerization of the material during, and following, clinical placement. The degree of conversion (DC) of resin composite materials is a key measure of effective polymerization and crosslinking of the multifunctional monomers that are used to create the matrix. Good conversion is thus essential to their long-term functionality [2,20,21] and inadequate DC can be detrimental to the success of dental restorations. Thus several factors affect the DC of resin composites when applied clinically. These include operator-related factors, such as thickness of each layer, irradiance of the light curing unit, and proximity of light curing tip to the restoration [22–24]. The DC is nevertheless influenced by composite formulation, as determined by the manufacturer, through the type of resin-matrix, filler type, size and loading [25–27]. Since ‘bulk fill’ (BF) resin composites are generally claimed to be suitable for placement in increments of up to 4 mm, it is useful to denote ‘the degree of conversion measured at 4 mm’ as DC4mm . Measuring this particular parameter was the focus of this investigation. The aim of this study was to assess and compare the degree of conversion (DC4mm ) of some bulk-fill resin composite materials using real time Fourier transform infrared spectroscopy (FTIR) up to 30 min, 60 min and 24 h post cure. Our null hypothesis was that for each composite there is no difference between degree of conversion (DC4mm ) at 5 min, 30 min, 60 min and 24 h post cure.
2.
Materials and methods
Eight bulk fill resin composites were investigated (Table 1). The DC measurements were conducted using a Fourier transform infrared spectroscopy (FTIR) with an attenuated total reflectance (ATR) accessory (Nicolet iS50, Thermo Fisher, Madison, USA).
2.1.
Real time measurement.
The un-polymerized composite pastes (n = 6) were placed directly on the diamond ATR crystal in molds of 4 mm height and 3 mm internal diameter, filled in one increment. The specimens were covered with plastic matrix strips (Frasaco, Tettnang, Germany) and the light curing unit (LCU), with measured average tip irradiance of 1200 mW/cm2 , was applied
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
DENTAL-2595; No. of Pages 7
Code
Material
Shade
Manufacturer
Lot. no.
Resin Matrix
Filler Composition
Load (wt%/vol %)
Flowable bulk-fill Filtek Bulk Fill
Universal
3 M ESPE, ESPE, St. Paul, MN, USA
N390563
Bis-GMA, bis-EMA, UDMA
VBF
Venus Bulk Fill
Universal
010102
UDMA, EBPDMA
BBF
Beautifil-Bulk Flowable
Dentin
Heraeus Kulzer GmbH, Hanau, Germany Shofu Inc. Kyoto, Japan
121302
Bis-GMA, UDMA, Bis-MPEPP, TEGDMA
Xtb
x-tra base
Universal
Voco, Cuxhaven, Germany
1315357
Aliphatic di-methacrylate (UDMA), Bis-EMA
Restorative bulk-fill eXP ever XPosterior
Universal
1308261
TECBF
Tetric EvoCeram Bulk Fill
IV A
GC Corporation Tokyo, Japan Ivoclar Vivadent AG, Schaan, Liechtenstein
bis-GMA, TEGDMA, PMMA Bis-GMA, UDMA
SF
Sonic Fill
A2
4427299
BBR
Beautifil-Bulk Restorative
A
Kerr Corp., Orange, CA, USA Shofu Inc. Kyoto, Japan
S09220
021402
Bis-GMA, TEGDMA, EBPDMA Bis-GMA, UDMA, Bis-MPEPP, TEGDMA
A combination of ytterbium trifluoride filler (sizes range from 0.1 to 5.0 m) and zirconia/silica (size range of 0.01 to 3.5 m) Ba-Al-F-Si glass, SiO2
64.5/42.5
S-PRG filler based on fluoroboroaluminosilicate glass, polymerization initiator. Barium glass ceramic, fumed silica mean size 3.5 m
73/n.a
Hybrid filler fractions & E-glass fibers Ba-Al-Si glass, prepolymer filler (monomer, glass filler, and ytterbium fluoride), spherical mixed oxide SiO2 , glass, oxide
74.2/53.6
S-PRG filler based on fluoroboroaluminosilicate glass, polymerization initiator
65/38
75/58
79-81/60-61
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FBF
83.5/n.a. 87/74.5
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Table 1 – Resin composites studied, ordered in flowable bulk-fill and restorative bulk-fill categories.
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directly above the sample surface for 20 s (Elipar S10, 3 M Espe, Seefeld, Germany). The FTIR spectra were recorded continuously in real time for 5 min taking spectra from the lower surface of the specimens in direct contact with the ATR crystal.
y = a × (1 − e−bx ) + c × (1 − e−dx )
2.2. Measurements at 30 min, 60 min and 24 h of post-cure DC4mm Further test specimens (n = 6, for each material) were made by placing a mold (4 mm height; 5 mm internal diameter) on a plastic matrix strip covering a glass microscope slide. Each mold was filled in one increment and covered with a plastic matrix strip. The specimen was then light cured with the LCU being directly applied on the sample surface for 20 s. After 5 min the samples were stored dry at 37 ◦ C. After 30 min, 60 min and 24 h, each specimen was carefully placed on the ATR crystal plate and pressed with a clamp to sustain good contact between the sample and the ATR crystal. 100 FTIR spectra were then rapidly collected for each specimen and averaged.
2.3.
Calculation of DC
Calculation of the DC for the dimethacrylate based resin composites was done by comparing the height of particular peaks in the spectra derived from the unpolymerized and polymerized resin. There were two peaks of interest. The first peak corresponded to a group actively involved in polymerization, while the second peak functioned as an internal standard. The change during polymerization was calculated using both peak heights. The percentage of unreacted carbon double bonds C C was obtained from the peak height ratio of the methacrylate C C (at 1637 cm−1 ) and those of an internal standard aromatic carbon double bond (at 1608 cm−1 ) during polymerization, in relation to the uncured material. The percentage DC was calculated for each sample DCPeak %
= 1−
(1634 cm−1 /1608 cm−1 )Peak
height after curing
(1634 cm−1 /1608 cm−1 )Peak
height before curing
compared to the absorption values of a separate single precure sample of each material. For each material, the increase in DC4mm during 24 h was described by the superposition of two exponential functions.
× 100 (1)
In the real time investigation, DC was calculated using the pre-cure and post-cure absorption values of the same sample. For the 30 min, 60 min and 24 h post-cure investigation DC was calculated using the absorption values of each sample
(2)
The parameters a, b, c, d were modulation factors of the exponential function to optimize the fit of the double exponential function to the measured DC4mm versus time curves.
2.4.
Statistical analysis
Data were analyzed using statistical software (SPSS ver.20, IBM Corp, Armonk, NY, USA) at (p < 0.05) significance level. The Kolmogorov–Smirnov Test indicated that all DC data for the materials was normally distributed except for eXP and BBF. A Kurskal–Wallis non-parametric test was used for these two materials and subsequent paired comparisons were made following a Mann–Whitney non-parametric test. The other materials were analyzed by oneway ANOVA and the Bonferroni post-hoc test, to compare DC4mm between different times within each resin composite.
3.
Results
The degree of conversion data measured at a material depth of 4 mm is presented in Table 2. Post-cure time affected DC for all materials except SF (p > 0.05). The measured DC at 4 mm depth (DC4mm ) of SF 5 min after photo-initiation did not increase after 30 min, 60 min or 24 h. DC4mm significantly increased (p < 0.05) after 30 min for XTB, VBF, TECBF, FBF and BBR. The DC4mm of eXP and BBF significantly only increased by 24 h post-cure (p < 0.05). The experimental DC data were well described by the superposition of two exponential functions manifest by a correlation factor: r2 > 0.88 defined for the parameters a, b, c, and d, as presented in Table 3 and Fig. 1
4.
Discussion
This study aimed to compare DC4mm of several contemporary bulk-fill resin composites. These composites varied in their composition in terms of filler content (type and loading) and resin matrices. Statistically significant differences in the degree of conversion (DC4mm ), for each resin composite, were found between different times post-cure. Hence our null hypothesis was rejected.
Table 2 – Mean (SD) degrees of conversion, at 4 mm depth, of resin composites at different times post-cure. Materials 5 min 30 min 60 min 24 h
XTB
VBF
TECBF
SF
FBF
eXP
BBF
BBR
49.4 a (1.89) 57.6 b (2.92) 56.2 b (2.85) 57.7 b (3.14)
66.6 a (0.89) 74.8 b (2.56) 73.6 b (1.98) 71.9 b (2.00)
46.9 a (4.53) 53.0 b (1.06) 54.8 b (4.56) 54.5 b (1.53)
66.6 a (4.96) 70.2 a (1.48) 68.6 a (2.16) 71.6 a (3.98)
55.8 a (1.06) 62.3 b (2.68) 61.0 b (1.79) 58.5 a, b (3.50)
54.4 a (0.64) 59.9 a, b (4.52) 63.9 b, c (3.36) 66.2 c (2.38)
56.3 a (1.73) 58.4 a, b (7.57) 63.7 b, c (1.12) 65.7 c (1.65)
38.9 a (4.57) 47.6 b (6.07) 45.3 a, b (5.44) 49.7 b (4.98)
Within each material, similar superscript letters indicate homogenous subsets among the time periods.
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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Table 3 – Confidence intervals for pre-exponential (a, c) and exponential (b, d) parameters describing the kinetic functions (Eq. (2)) that fit the data with coefficients of determination: r2 > 0.88. RBC
r2
a
b
c
BBR TECBF XTB FBF BBF eXP VBF SF
0.90 0.93 0.95 0.95 0.96 0.92 0.95 0.89
32.78–33.11 40.63–40.97 44.64–44.85 48.19–48.67 49.63–49.96 49.79–50.10 60.31–60.71 61.87–62.48
0.14–0.15 0.14–0.14 0.19–0.20 0.20–0.20 0.16–0.17 0.19–0.20 0.16–0.17 0.13–0.13
13.62–15.32 11.30–12.98 11.05–12.27 8.43–9.06 9.58–10.72 11.76–13.60 10.63–12.25 6.60–9.80
Furthermore, the curing time undoubtedly had a significant effect on the DC, depending upon the light irradiance level used and the total energy required by the material for polymerization. Durner et al. and Frauscher et al. concluded that adequate curing time (20 s, 40 s) with a moderate irradiance (ca. 1000 mW/cm2 ) is essential to ensure that the resin composite is adequately polymerized; as an extended curing time (more than 40 s) resulted in a significant increase in DC and a short curing time (5 s, 10 s) resulted in a lower DC [31,32]. DC4mm is a valuable property in characterising changes within one material. A direct comparison of the DC among the materials is certainly only possible by considering the kinetic parameters. All analyzed materials were methacrylatebased dental restoratives containing monomers with at least two carbon-carbon (C C) double bonds, such as bisphenol-A-glycidyldimethacrylate (BisGMA) or triethylene glycol dimethacrylate (TEGDMA), able to build a threedimensional polymer network. As previously shown, the polymerization kinetics in resin composites can be very well described by an exponential sum function [33]. This was confirmed by the present experimental data, which were well described for all materials (r2 > 0.88) by the superposition of two exponential functions, the first exponential function characterizing the gel phase (described by the parameters a and b) and the second exponential function characterizing the glass phase (described by parameters c and d) (Table 3). It was previously shown that parameter “a” is a materialdependent pre-exponential term for the polymerization kinetics in the gel phase, while parameter “b” is dependent on the specimen’s thickness, demonstrating that the decrease
Fig. 1 – DC at 4 mm depth from 0 to 300 s for eight bulk-fill resin composites.
d 0.19–0.23 × 10−2 0.23–0.26 × 10−2 0.18–0.20 × 10−2 0.53–0.67 × 10−2 0.30–0.38 × 10−2 0.16–0.20 × 10−2 0.27–0.35 × 10−2 0.13–0.25 × 10−2
of the C C double bonds in the gel phase is faster in thinner layers. As for the parameters “c” and “d”, they are comparable for all analyzed materials. Parameter “c” varied amongst the analyzed materials, being between 8.20 and 14.47 compared to 32.9–62.18 for the parameter “a”. However, parameter “d” was similar in all materials and of less relevance because of its low value in the exponential function. This suggests that that the reaction kinetics in the glass phase is less material dependent compared with the gel phase. In the first few minutes up to 1 h after light curing the resin composites, the polymerization progressed [34,35] and then it continued slowly up to a maximum measured at 24 h [36]. In this study, all the materials (except SF), showed a significant increase of DC at 30 min, 60 min and at 24 h post-irradiation with storage at 37 ◦ C when compared to results obtained 5 min post-cure. The DC4mm of XTB, VBF, TECBF, FBF and BBR increased significantly at 30 min post-irradiation. While eXP and BBF increased significantly at 24 h post-irradiation. These variations in reaching the maximum DC could be due to differences in the resin matrix compositions amongst these resin composites [37]. eXP and BBF reached their maximum DC in 24 h post-cure. Both of them have similar filler loading (74.2% and 73% by weight respectively). In addition all the resin composites which reached their maximum DC at 30 min post-cure contained UDMA in their resin matrix which is a less viscous monomer than bis-GMA [38]. UDMA incorporates an imino group ( NH ) group which is responsible for the characteristic chain transfer reactions that provide an alternative path for the continuation of polymerization. On the other hand, SF reached its maximum DC in the first 5 min. This might also be explained by the polymerization kinetics, since for SF the largest parameter “a” was identified, corresponding to a very fast start of polymerization. Sideridou et al. [38] reported that TEGDMA increased the polymerization rate when added to bis-GMA. This mixture increased the rate of polymerization from the start of photo-cure compared to resin composites with bis-GMA alone. As TEGDMA is a low viscous monomer and acts as a diluent it results in rapid propagation of the polymerization reaction. The filler content also has an effect on the DC and setting kinetics of resin composite materials. Beautifil Bulk Restorative (BBR) and Beautifil Bulk Flowable (BBF) have a similar composition, as the both contain the surface pre-reacted glass-ionomer (S-PRG) filler, which is an original type of filler that releases fluoride and some ions such as Al3+ , B− , and Sr2+ [28–30]. It is also claimed that the resinous coating material containing S-PRG fillers is able to prevent caries as it increase
Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004
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the mineralization. However, these two products vary in their filler loading as BBR has 87 wt% and BBF 73 wt%; DC4mm significantly increased after 30 min for BBR while DC4mm of BBF significantly increased 24 h post-cure. Therefore it appears that as the filler loading increased, the DC4mm reached its maximum in a shorter post-cure period. The DC of some resin composites have been assessed in previous studies. Czasch and Ilie measured the DC after 5 min of some bulk-fill resin composites at different depths and polymerization times using a curing light with irradiance of 1226 mW/cm2 . They measured VBF and the value was 66.1% [11] which is comparable to our value after 5 min post cure (66.6%). This is due to the similarity in the irradiance and curing time. Another study by Alshali et al. [13] reported DC values of 54%, 56% and 50% for XTB, VBF and FBF respectively. These results were achieved immediately post-cure using a light cure with an irradiance of 600 mW/cm2 . In this present study similar materials were assessed 5 min post-cure after irradiation at 1200 mW/cm2 . The DC values were 49%, 67% and 56% for XTB, VBF and FBF respectively. Taking into consideration variation in curing light irradiance and post-irradiation time, these values can be considered comparable.
5.
Conclusion
DC4mm and the kinetic parameters for cure of resin composites depend greatly on their composition when light curing irradiance and time are fixed.
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Please cite this article in press as: Al-Ahdal K, et al. Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.07.004