Clin Oral Invest DOI 10.1007/s00784-013-1128-7
ORIGINAL ARTICLE
Polymerization kinetic calculations in dental composites: a method comparison analysis Nicoleta Ilie & Jürgen Durner
Received: 25 June 2013 / Accepted: 23 October 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Objectives This study aimed to describe the polymerization process and to quantify the parameters of influence in two bulk-fill resin-based composites by comparing two real-time methods: the Fourier transform infrared (FTIR) spectroscopy and the visible light transmission spectrometry. Materials and methods The degree of conversion (DC) was recorded in real time for 5 min using the attenuated total reflectance FTIR spectroscopy (n =6) on the lower surface of 2, 4, and 6 mm thick samples irradiated for 20 s. The variation in irradiance was recorded during material irradiation at the bottom of the samples (n =5). Results were statistically analyzed using one-way and multiple-way ANOVAs with Tukey HSD post hoc test (α =0.05), partial eta-squared statistics, and Pearson correlation. Results No significant difference was found in DC in any materials as a function of incremental thickness, whereas the irradiance passing the specimens differed consistently within both analyzed increments and materials. These data could be described by the superposition of two exponential functions, the first being attributed to the gel phase and the second to the glass phase, resulting in an exponential sum function. DC data were able to calculate the end of the gel phase and the beginning of the glass phase, whereas irradiance measurements were able to detect only the last phase. The polymerization kinetics in the glass phase was less material-dependent as in the gel phase. Conclusions The irradiance measurements were more sensitive to variation in thickness, meaning that translucency is
N. Ilie (*) : J. Durner Department of Operative/Restorative Dentistry, Periodontology and Pedodontics, Ludwig-Maximilians-Universität München, Goethestr. 70, 80336 Munich, Germany e-mail: [email protected]
continuing to change as a function of thickness at a higher extent than DC. Clinical relevance Knowing the impact of the modulation factors describing the calculated sum exponential function allows the manipulation of the polymerization process at different stages to tailor material properties. Keywords Polymerization kinetics . Degree of conversion . Irradiance . Bulk-fill resin-based composites . FTIR spectroscopy . Spectrometry
Introduction Methacrylate-based dental restoratives contain monomers with at least two carbon-carbon (C–C) double bonds, such as the Bowen monomer bisphenol-A–glycidyldimethacrylate (BisGMA) or the triethylene glycol dimethacrylate (TEGDMA), able to build a three-dimensional polymer network [1]. Although the kinetics of monofunctional methyl methacrylates are widely analyzed under various conditions (influence of temperature and amount of starter radical [2, 3]), less is known about the polymerization kinetics in more complex matrices, such as resin-based composites (RBCs), which are based on monomers with two or more methacrylate functions. As generally known, the light-induced polymerization process is a radical one and described by the following phases: (a) initiation, characterized by the formation of a free radical able to start the polymerization process; (b) propagation, directed by the radical attack on methacrylic monomers, leading to a larger molecule (chain growth) by preserving the free radical; and (c) termination, described by different mechanisms to stop the polymerization process such as “coupling,” when two free radicals join, or “disproportionation,” when one molecule
Clin Oral Invest
abstracts a hydrogen atom from another, forming a C–C double bond [4]. In RBCs, the propagation is an essential stage, allowing to manipulate the polymerization process to reduce shrinkage stress. The polymer chain grows during propagation. This phase is subdivided into three phases: the quasistatic process, the gel phase, and the glass phase. After normal chain growth (quasistatic process), the number of high-molecular-weight chains increases and, consequently, also the viscosity of the reaction solution. During the gel phase, the flexibility and the diffusion rate of the growing polymer chains decrease. The reaction rate increases compared with the quasistatic process because of a decreasing number of terminations as a result of a viscosity increase in the reaction solution (the gel phase or the Trommsdorff– Norrish effect) [5]. At a further progress of the polymerization process, the reaction solution becomes a gelatinous state and the reaction rate decreases (glass effect) [6]. In dental RBCs, however, because of the presence of filler particles and highly viscose basic monomers such as BisGMA, the viscosity of the material is extremely high. Therefore, it can be expected that the polymerization process became increasingly complex compared with the polymerization process described by lower molecular weight methacrylate polymerization theory [7, 8]. It was already shown that the polymerization kinetics in RBCs can be described by an exponential function [9]. Because the propagation phases of the polymerization process are described by different subphases, as mentioned previously, it is of great interest to verify if the decrease of C–C double bonds measured by Fourier transform infrared (FTIR) spectroscopy during polymerization can be described by a superposition of two exponential functions, each of them characterizing one subphase, thus resulting in an exponential sum function. To test these assumptions, a new class of RBCs—the bulk-fill RBCs—is of interest because in vitro studies confirmed that the degree of conversion (DC), and the micromechanical properties are maintained within larger increments (at least 4 mm at an irradiation time of up to 20 s; SDR, Dentsply DeTrey, Konstanz, Germany; Venus® Bulk Fill, Heraeus Kulzer, Hanau, Germany) [10]. Although advertised as a new material category, bulkfill RBCs seem not to differ essentially in their chemical composition from regular nano- and microhybrid RBCs [11], which contain regular methacrylate monomers and filler systems. In SureFil SDR flow, the first bulk-fill material on the market, the organic matrix additionally comprises a patent-registered urethane dimethacrylate with incorporated photoactive groups able to control polymerization kinetics [12] (SDR™
technology=stress decreasing resin). With one exception (Tetric EvoCeram Bulk Fill), no changes in the polymerization initiating system were specified; thus, the enlarged depth of cure must have been regulated by enhancing the material’s translucency. The variation in translucency during polymerization can be monitored in real time as well and might contribute to elucidate the polymerization process. Therefore, the aim of our study was to pursuit in real time and to describe the polymerization process in two bulk-fill RBCs at different incremental thicknesses, using two different methods: assessing the variation of the C–C double bonds by FTIR spectroscopy and evaluating the changes in irradiance while passing the material by visible light transmission spectrometry. The work hypotheses were as follows: (1) there would be a correlation between both methods, and (2) the methods and/or correlated approximate functions will determine the gel phase and the glass phase in the analyzed materials.
Methods Two bulk-fill RBCs, Venus Bulk Fill (Heraeus Kulzer, Hanau, Germany) and SDR (Dentsply De Trey, Konstanz, Germany), were investigated (Table 1).
Degree of conversion The measurements of the DC (n =6) were made in real time with an FTIR spectrometer with an attenuated total reflectance (ATR) accessory (Nexus, Thermo Nicolet, Madison, USA). Therefore, the unpolymerized composite paste was put directly on the diamond ATR crystal in 2, 4, and 6 mm high moulds, respectively, with a diameter of 3 mm. The moulds were filled in one increment, covered by a transparent matrix strip, and the curing unit (20 s, LED light source Freelight 2; 3 M ESPE, Seefeld, Germany) was applied directly on the sample’s surface. The irradiance of the curing unit (1,706 mW/cm2 when applied directly on the sensor) was measured on a laboratorygrade NIST-referenced USB4000 spectrometer, described in the next section. The FTIR spectra were recorded in real time for 5 min on the lower surface of the samples (number of sample scans=2, sampling interval=0.42 s, mirror velocity= 6.3290 cm/s). The diameter of the measured surface was 800 μm; the wave number range of the spectrum was 4,000–650 cm−1, and the FTIR spectra were recorded with four scans at a resolution of 8 cm−1. To determine the percentage of the remaining unreacted C– C double bonds, the DC was assessed as the variation of the absorbance intensities peak height ratio of the methacrylate
Clin Oral Invest Table1 Materials, manufacturer, chemical composition of matrix, and filler as well as filler content by weight (wt.) and volume (vol.) % Bulk fill RBCs
Manufacturer, color, batch
Resin matrix
Filler
Filler wt/vol
Venus® bulk fill, nano-hybrid RBC SureFil® SDR™ flow, flowable base RBC
Heraeus Kulzer, Universal, 010026
UDMA, EBPDMA
Ba–Al–F–Si–glass, SiO2
65/38
Dentsply Caulk, Universal, 100407
Modified UDMA, TEGDMA, EBPDMA
Ba–Al–F–B–Si–glass and Sr–Al–F–Si–glass
68/44
Data are provided by manufacturers EBPDMA ethoxylated bisphenol-A–dimethacrylate, TEGDMA triethyleneglycol dimethacrylate, UDMA urethane dimethacrylate
C–C double bond (peak=1,634 cm−1) and those of an internal standard (aromatic C–C double bond; peak=1, 608 cm−1) during polymerization in relation to the uncured material: DCPeak
height %
¼ ½1 −
ð1634 cm−1 =1608 cm−1 Þ Peak
height after curing
ð1634 cm−1 =1608 cm−1 Þ Peak
height before curing
100
In each sample, the increase of DC (decrease of the C–C double bonds) is described by the superposition of two exponential functions. The correlation function is represented by the sum of two exponential functions as follows: y ¼ y0 þ a* 1−e−bx þ c* 1−e−dx The term y 0 presents the y-intercept, depending on the thickness of the specimen and the composition of the material. Parameters a, b, c , and d (Table 2, FTIR spectrometry) are modulation factors of the exponential function to optimize the double exponential function on the measured curve. The measuring points were plotted on a time–DC curve. A line of best fit was inserted through the first and the last points of this curve, respectively. Their intercept point was calculated and presented as t 1-DC. The slope of these lines is the velocity of the C–C double bond decrease, which was calculated and plotted on a time–velocity DC curve. Lines of best fit were inserted through the first and the last points of this curve, and their intercept point was calculated (t 2-DC). The slopes of these lines represent the acceleration of the C–C double bond decrease.
Visible light transmission spectrometry The analysis of the variation in irradiance was performed on a laboratory-grade NIST-referenced USB4000 spectrometer (Managing Accurate Resin Curing [MARC] System; BlueLight Analytics Inc., Halifax, Canada) (n =5). The miniature fiber optic USB4000 spectrometer uses a 3,648-element Toshiba linear CCD array detector and high-speed electronics.
The spectrometer has been spectroradiometrically calibrated using Ocean Optics’ NIST-traceable light source (300–1,050 nm). The system uses a CC3-UV cosine corrector to collect radiation more than 180° field of view, thus mitigating the effects of optical interference associated with light collection sampling geometry. Similar to the DC measurements, the materials were applied in molds of 2, 4, and 6 mm, separated from the sensor and the curing unit by a transparent matrix strip. The irradiance (wavelength ranging from 360 to 540 nm) passing the specimens was measured at the bottom of the specimens with a velocity of 16 records/s. The increase in irradiance (increase in translucency) during polymerization is described, similar to the FTIR data, by the superposition of two exponential functions resulting in an exponential sum function, described by the above-mentioned parameters y 0, a , b , c , and d (Table 2, Visible light transmission spectrometry data). Similar to the DC measurements, the measured irradiance values were plotted on a time–irradiance curve, and t 1-Ir and t 2-Ir were calculated (Fig. 1).
Calculations and statistics The results are presented as means±standard deviation. For DC, the statistical significance (p
ORIGINAL ARTICLE
Polymerization kinetic calculations in dental composites: a method comparison analysis Nicoleta Ilie & Jürgen Durner
Received: 25 June 2013 / Accepted: 23 October 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract Objectives This study aimed to describe the polymerization process and to quantify the parameters of influence in two bulk-fill resin-based composites by comparing two real-time methods: the Fourier transform infrared (FTIR) spectroscopy and the visible light transmission spectrometry. Materials and methods The degree of conversion (DC) was recorded in real time for 5 min using the attenuated total reflectance FTIR spectroscopy (n =6) on the lower surface of 2, 4, and 6 mm thick samples irradiated for 20 s. The variation in irradiance was recorded during material irradiation at the bottom of the samples (n =5). Results were statistically analyzed using one-way and multiple-way ANOVAs with Tukey HSD post hoc test (α =0.05), partial eta-squared statistics, and Pearson correlation. Results No significant difference was found in DC in any materials as a function of incremental thickness, whereas the irradiance passing the specimens differed consistently within both analyzed increments and materials. These data could be described by the superposition of two exponential functions, the first being attributed to the gel phase and the second to the glass phase, resulting in an exponential sum function. DC data were able to calculate the end of the gel phase and the beginning of the glass phase, whereas irradiance measurements were able to detect only the last phase. The polymerization kinetics in the glass phase was less material-dependent as in the gel phase. Conclusions The irradiance measurements were more sensitive to variation in thickness, meaning that translucency is
N. Ilie (*) : J. Durner Department of Operative/Restorative Dentistry, Periodontology and Pedodontics, Ludwig-Maximilians-Universität München, Goethestr. 70, 80336 Munich, Germany e-mail: [email protected]
continuing to change as a function of thickness at a higher extent than DC. Clinical relevance Knowing the impact of the modulation factors describing the calculated sum exponential function allows the manipulation of the polymerization process at different stages to tailor material properties. Keywords Polymerization kinetics . Degree of conversion . Irradiance . Bulk-fill resin-based composites . FTIR spectroscopy . Spectrometry
Introduction Methacrylate-based dental restoratives contain monomers with at least two carbon-carbon (C–C) double bonds, such as the Bowen monomer bisphenol-A–glycidyldimethacrylate (BisGMA) or the triethylene glycol dimethacrylate (TEGDMA), able to build a three-dimensional polymer network [1]. Although the kinetics of monofunctional methyl methacrylates are widely analyzed under various conditions (influence of temperature and amount of starter radical [2, 3]), less is known about the polymerization kinetics in more complex matrices, such as resin-based composites (RBCs), which are based on monomers with two or more methacrylate functions. As generally known, the light-induced polymerization process is a radical one and described by the following phases: (a) initiation, characterized by the formation of a free radical able to start the polymerization process; (b) propagation, directed by the radical attack on methacrylic monomers, leading to a larger molecule (chain growth) by preserving the free radical; and (c) termination, described by different mechanisms to stop the polymerization process such as “coupling,” when two free radicals join, or “disproportionation,” when one molecule
Clin Oral Invest
abstracts a hydrogen atom from another, forming a C–C double bond [4]. In RBCs, the propagation is an essential stage, allowing to manipulate the polymerization process to reduce shrinkage stress. The polymer chain grows during propagation. This phase is subdivided into three phases: the quasistatic process, the gel phase, and the glass phase. After normal chain growth (quasistatic process), the number of high-molecular-weight chains increases and, consequently, also the viscosity of the reaction solution. During the gel phase, the flexibility and the diffusion rate of the growing polymer chains decrease. The reaction rate increases compared with the quasistatic process because of a decreasing number of terminations as a result of a viscosity increase in the reaction solution (the gel phase or the Trommsdorff– Norrish effect) [5]. At a further progress of the polymerization process, the reaction solution becomes a gelatinous state and the reaction rate decreases (glass effect) [6]. In dental RBCs, however, because of the presence of filler particles and highly viscose basic monomers such as BisGMA, the viscosity of the material is extremely high. Therefore, it can be expected that the polymerization process became increasingly complex compared with the polymerization process described by lower molecular weight methacrylate polymerization theory [7, 8]. It was already shown that the polymerization kinetics in RBCs can be described by an exponential function [9]. Because the propagation phases of the polymerization process are described by different subphases, as mentioned previously, it is of great interest to verify if the decrease of C–C double bonds measured by Fourier transform infrared (FTIR) spectroscopy during polymerization can be described by a superposition of two exponential functions, each of them characterizing one subphase, thus resulting in an exponential sum function. To test these assumptions, a new class of RBCs—the bulk-fill RBCs—is of interest because in vitro studies confirmed that the degree of conversion (DC), and the micromechanical properties are maintained within larger increments (at least 4 mm at an irradiation time of up to 20 s; SDR, Dentsply DeTrey, Konstanz, Germany; Venus® Bulk Fill, Heraeus Kulzer, Hanau, Germany) [10]. Although advertised as a new material category, bulkfill RBCs seem not to differ essentially in their chemical composition from regular nano- and microhybrid RBCs [11], which contain regular methacrylate monomers and filler systems. In SureFil SDR flow, the first bulk-fill material on the market, the organic matrix additionally comprises a patent-registered urethane dimethacrylate with incorporated photoactive groups able to control polymerization kinetics [12] (SDR™
technology=stress decreasing resin). With one exception (Tetric EvoCeram Bulk Fill), no changes in the polymerization initiating system were specified; thus, the enlarged depth of cure must have been regulated by enhancing the material’s translucency. The variation in translucency during polymerization can be monitored in real time as well and might contribute to elucidate the polymerization process. Therefore, the aim of our study was to pursuit in real time and to describe the polymerization process in two bulk-fill RBCs at different incremental thicknesses, using two different methods: assessing the variation of the C–C double bonds by FTIR spectroscopy and evaluating the changes in irradiance while passing the material by visible light transmission spectrometry. The work hypotheses were as follows: (1) there would be a correlation between both methods, and (2) the methods and/or correlated approximate functions will determine the gel phase and the glass phase in the analyzed materials.
Methods Two bulk-fill RBCs, Venus Bulk Fill (Heraeus Kulzer, Hanau, Germany) and SDR (Dentsply De Trey, Konstanz, Germany), were investigated (Table 1).
Degree of conversion The measurements of the DC (n =6) were made in real time with an FTIR spectrometer with an attenuated total reflectance (ATR) accessory (Nexus, Thermo Nicolet, Madison, USA). Therefore, the unpolymerized composite paste was put directly on the diamond ATR crystal in 2, 4, and 6 mm high moulds, respectively, with a diameter of 3 mm. The moulds were filled in one increment, covered by a transparent matrix strip, and the curing unit (20 s, LED light source Freelight 2; 3 M ESPE, Seefeld, Germany) was applied directly on the sample’s surface. The irradiance of the curing unit (1,706 mW/cm2 when applied directly on the sensor) was measured on a laboratorygrade NIST-referenced USB4000 spectrometer, described in the next section. The FTIR spectra were recorded in real time for 5 min on the lower surface of the samples (number of sample scans=2, sampling interval=0.42 s, mirror velocity= 6.3290 cm/s). The diameter of the measured surface was 800 μm; the wave number range of the spectrum was 4,000–650 cm−1, and the FTIR spectra were recorded with four scans at a resolution of 8 cm−1. To determine the percentage of the remaining unreacted C– C double bonds, the DC was assessed as the variation of the absorbance intensities peak height ratio of the methacrylate
Clin Oral Invest Table1 Materials, manufacturer, chemical composition of matrix, and filler as well as filler content by weight (wt.) and volume (vol.) % Bulk fill RBCs
Manufacturer, color, batch
Resin matrix
Filler
Filler wt/vol
Venus® bulk fill, nano-hybrid RBC SureFil® SDR™ flow, flowable base RBC
Heraeus Kulzer, Universal, 010026
UDMA, EBPDMA
Ba–Al–F–Si–glass, SiO2
65/38
Dentsply Caulk, Universal, 100407
Modified UDMA, TEGDMA, EBPDMA
Ba–Al–F–B–Si–glass and Sr–Al–F–Si–glass
68/44
Data are provided by manufacturers EBPDMA ethoxylated bisphenol-A–dimethacrylate, TEGDMA triethyleneglycol dimethacrylate, UDMA urethane dimethacrylate
C–C double bond (peak=1,634 cm−1) and those of an internal standard (aromatic C–C double bond; peak=1, 608 cm−1) during polymerization in relation to the uncured material: DCPeak
height %
¼ ½1 −
ð1634 cm−1 =1608 cm−1 Þ Peak
height after curing
ð1634 cm−1 =1608 cm−1 Þ Peak
height before curing
100
In each sample, the increase of DC (decrease of the C–C double bonds) is described by the superposition of two exponential functions. The correlation function is represented by the sum of two exponential functions as follows: y ¼ y0 þ a* 1−e−bx þ c* 1−e−dx The term y 0 presents the y-intercept, depending on the thickness of the specimen and the composition of the material. Parameters a, b, c , and d (Table 2, FTIR spectrometry) are modulation factors of the exponential function to optimize the double exponential function on the measured curve. The measuring points were plotted on a time–DC curve. A line of best fit was inserted through the first and the last points of this curve, respectively. Their intercept point was calculated and presented as t 1-DC. The slope of these lines is the velocity of the C–C double bond decrease, which was calculated and plotted on a time–velocity DC curve. Lines of best fit were inserted through the first and the last points of this curve, and their intercept point was calculated (t 2-DC). The slopes of these lines represent the acceleration of the C–C double bond decrease.
Visible light transmission spectrometry The analysis of the variation in irradiance was performed on a laboratory-grade NIST-referenced USB4000 spectrometer (Managing Accurate Resin Curing [MARC] System; BlueLight Analytics Inc., Halifax, Canada) (n =5). The miniature fiber optic USB4000 spectrometer uses a 3,648-element Toshiba linear CCD array detector and high-speed electronics.
The spectrometer has been spectroradiometrically calibrated using Ocean Optics’ NIST-traceable light source (300–1,050 nm). The system uses a CC3-UV cosine corrector to collect radiation more than 180° field of view, thus mitigating the effects of optical interference associated with light collection sampling geometry. Similar to the DC measurements, the materials were applied in molds of 2, 4, and 6 mm, separated from the sensor and the curing unit by a transparent matrix strip. The irradiance (wavelength ranging from 360 to 540 nm) passing the specimens was measured at the bottom of the specimens with a velocity of 16 records/s. The increase in irradiance (increase in translucency) during polymerization is described, similar to the FTIR data, by the superposition of two exponential functions resulting in an exponential sum function, described by the above-mentioned parameters y 0, a , b , c , and d (Table 2, Visible light transmission spectrometry data). Similar to the DC measurements, the measured irradiance values were plotted on a time–irradiance curve, and t 1-Ir and t 2-Ir were calculated (Fig. 1).
Calculations and statistics The results are presented as means±standard deviation. For DC, the statistical significance (p
