Lasers Med Sci DOI 10.1007/s10103-014-1572-x
ORIGINAL ARTICLE
Influence of light-activation protocol on methacrylate resin-composite evaluated by dynamic mechanical analysis and degree of conversion Maria Cecília C. Giorgi & Vinícius Pistor & Raquel S. Mauler & Débora A. N. L. Lima & Giselle M. Marchi & Flávio H. B. Aguiar
Received: 14 November 2013 / Accepted: 30 March 2014 # Springer-Verlag London 2014
Abstract The aim of this study was to evaluate the degree of conversion (DC) and to identify the viscoelastic properties: storage modulus (E’), loss modulus (E”), tangent delta (tan δ), and glass transition temperature (Tg) of a microhybrid resincomposite light-activated by three different protocols. A Filtek Z250 (3 M ESPE) shade A3 was inserted in a Teflon mold (21 mm×5 mm×1 mm for viscoelastic properties; and 5 mm× 1 mm for DC) and light-activated according to the following light-activation protocols: (S) 1,000 mW/cm2 ×19 s, (HP) 1,400 mW/cm2 ×14 s, and (PE) 3,200 mW/cm2 ×6 s, all set up to deliver 19 J/cm2. Viscoelastic properties was assessed by dynamic mechanical analysis (DMA) (n=3), performed in single cantilever clamped mode. DC (n=5) was measured by FTIR on top (T) and bottom (B) surfaces, and the data was submitted to a split-plot one-way ANOVA. For DC, there was a significant effect for surface factor and light-activation protocols factor. Top surface showed higher DC than B in all experimental conditions. Light-activation protocols S and HP resulted in higher DC than PE and were similar between them. Viscoelastic properties (E’, E”, tan δ, Tg) were not affected by light-activation protocols. It could be concluded that lightactivation protocols influenced DC but not influenced the viscoelastic properties.
M. C. C. Giorgi (*) : D. A. N. L. Lima : G. M. Marchi : F. H. B. Aguiar Department of Restorative Dentistry, Piracicaba Dental School, University of Campinas (UNICAMP), Avenida Limeira, 901 Bairro Areião, Piracicaba, SP 13414-903, Brazil e-mail: [email protected] V. Pistor : R. S. Mauler Instituto de Química IQ/PGCIMAT, Universidade do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Campus do Vale, Porto Alegre, RS 91540-000, Brazil
Keywords Polymers . Composite resins . Physical properties . Curing time . Irradiance
Introduction The polymerization process of light-activated resincomposites is a reaction triggered by free radicals, which are generated by irradiation of a photoinitiator and open the double bond of methacrylate groups (C=C), generating a chain reaction [1]. From the development of this material, studies have been conducted to evaluate the effectiveness of polymerization since this is related to the longevity of the tooth restoration. In theory, the polymerization reaction can occur indefinitely; however, the material is not fully polymerized and small amount of residual free monomers still remains and the polymer structure harbors considerable quantities of pendant double bonds [2]. The degree of conversion (DC) is an important feature and is correlated with other material characteristics, such as mechanical properties, wear resistance, and shrinkage, which influence the longevity of the restoration. Light-curing units (LCUs) are used to irradiate the resincomposite and depending on light-activation protocol, it may influence the structure of the formed polymer. Typically, the greater the degree of conversion, the better the mechanical properties and the greater will be the restoration longevity. On the other hand, the polymerization shrinkage tends to be higher, which can challenge the adhesive interface and lead to gap formation resulting in microleakage, postoperative sensitivity, and secondary caries [3]. This contradiction reveals the complexity of the process and justifies the need to research protocols that may present a more favorable scenario. One strategy used to mitigate the effects of polymerization shrinkage, and at the same time achieve an acceptable degree
Lasers Med Sci
of conversion is the use of soft-start light exposure protocol [4]. This soft-start protocol involves an initial short low level output that allows the material to flow and release part of the polymerization stress in the first step. After that, an increase of irradiance is performed in order to improve the conversion of monomer into polymer. Nevertheless, concern about the effectiveness of polymerization resulted in a tendency to develop powerful LCUs especially designed to overcome everyday challenges for light to reach difficult areas, as interproximal box of class II cavity or during luting of indirect restorations. These powerful LCUs can be set up to deliver the same fluence with different combinations of irradiance and exposure duration and this could result in different polymer structure. For a given composite, different light-activation protocols may even result in polymers with similar degree of conversion but with distinct crosslink density [5]. The degree of conversion in spite of being an important factor does not provide a complete characterization of the polymer structure [6]. The extent of crosslinking plays an important role in the properties of the polymer and has been associated with increased fracture resistance. Polymeric systems with low crosslink density tend to be flexibilized while polymers with higher crosslinking densities are harder, less flexible, and more resistant to heat [6]. This study aimed to evaluate a methacrylate resincomposite light-activated by different light-activation protocols that delivered light in continuous mode. The working hypothesis was that light exposure protocols providing the same fluence in continuous mode but differing in irradiance and exposure time would result in different network structure and viscoelastic properties but not different in degree of conversion.
Materials and methods The light-curable microhybrid composite Filtek Z250 (3 M ESPE, St Paul, USA) (34.5 wt% bis-EMA; 34.5 wt% UEDMA; 6 % wt% TEGDMA; 60 % Silica/Zirconia; batch number 1108500200) shade A3 was selected for this study. The light exposure protocols were as follows: third LED generation Valo (Ultradent Products, South Jordan, USA) in three modes—standard (S) (1,000 mW/cm2, 19 s), high power (HP) (1,400 mW/cm2, 14 s), and plasma emulation (PE) (3,200 mW/cm2, 6 s); all set up to deliver approximately 19 J/cm2. Sample preparation For DMA test, bar samples were prepared by filling a Teflon mold measuring 21×5×1 mm. For DC test, cylindrical samples were prepared by filling a Teflon mold measuring 5 mm×1 mm. The lower surface of the mold was overlaid with a transparent polyester strip, and the resincomposite was inserted in one increment. The upper surface
was covered with another polyester strip, and a glass slide was positioned over the polyester strip and was pressed for 30 s in order to obtain a flat surface and to eliminate a possible oxygeninhibited layer. After that, the glass slide was removed and the sample was light-activated according to the experimental groups. For DMA test, the resin-composite was light-activated in three steps, each step involving one third of the sample and the light-curing tip was positioned close to the polyester strip without touching it. While one area has been irradiated, the others were covered to avoid receiving extra energy. For DC test, the light-activation was performed in one step. Degree of conversion test (n=5) The DC measurements were recorded in absorbance mode with FTIR spectrometer (Spectrum 100 FTIR, PerkinElmer, São Paulo, SP, Brazil) coupled to a zinc selenide multiple (six) reflection attenuated total reflection (ATR) accessory, refraction index of 2.4 at 1,000 cm−1 (PerkinElmer, Walthman, MA, USA), operating under the following conditions: 650–4,000 cm−1 wavelength, 4 cm−1 resolution, and 16 scans. The percentage of unreacted carbon-carbon double bonds (C=C) was determined from the ratio of absorbance intensities of aliphatic C=C (peak at 1,638 cm−1) against the internal standard (aromatic C-C, peak at 1,608 cm−1). The degree of conversion was determined by subtracting the percentage C=C from 100 %, according to the following formula: Rpolymerized DC ð%Þ ¼ 100 1− ð1Þ Rnonpolymerized where R is the ratio of the band height at 1,638 cm−1 to the band height at 1,608 cm−1. Data were submitted to split-plot ANOVA, followed by Tukey’s test (α=0.05). Light-activation protocol factor was considered as plot and surface factor as split-plot. Dynamic mechanical analysis test (n=3) Q800 AT DMA multi-frequency-strain (TA Instruments, New Castle, DE, USA) measured the viscoelastic properties. Bending loads were applied at 1 Hz frequency and deformation tension of 0.08 %, in a single cantilever clamped mode. The samples were heated within the −50 to 250 °C temperature range in a heating rate of 3 °C/min. Data of glass transition temperature (Tg), storage modulus (E’), loss modulus (E”), and tangent delta (tan δ) were plotted against the temperature over this period.
Results For DC test, split-plot one-way ANOVA showed significant effect for surface factor (p≤0.05) and for light-activation protocol (p≤0.05). The top surface presented higher DC with
Lasers Med Sci
significant difference to bottom surface in all experimental conditions (Table 1). For light-activation protocols, S and HP showed similar DC values and PE exhibit the lower DC values, with significant statistical difference to the others (Table 2). The E’ and E” obtained by DMA are shown in Fig. 1. Three analyzes were performed for each sample, and an average curve is generated to show the behavior of the different light exposure protocol. By the E’ curves, in Fig. 2, the values of modulus were collected at 25 and 140 °C corresponding to the vitreous and elastomeric regions, respectively as a function of the time and dependent also of the fluence applied. The results of the modulus did not present a significant variation. The tan δ was achieved by the ratio of E” and E’. Tan δ curves showed the loss of energy associated to the increase in mobility with the increase in temperature. The tan δ curves obtained for the different light-activation protocols are shown in Fig. 3. The parameters obtained by tan δ for all experimental groups can be observed in Table 3. The analysis of the values shows the height and width at half height of the peaks and the Tg. The Tg did not change with the different light-activation protocols, and there were no changes related to the homogeneity of the three-dimensional network in the polymer.
Discussion The aim of the study was to evaluate through degree of conversion and viscoelastic properties, a methacrylate resincomposite light-activated by different light-activation protocols. The working hypothesis was that light-activation protocols providing the same fluence but differing in irradiance and exposure time would result in similar degree of conversion but in different network structure. For DC test, the surface factor presented statistical significant effect, and top surface showed higher DC than bottom surface. Studies had shown that when light passes through the material, it suffers absorption and scattering [7]. Due to this, the light intensity gradually decreases and there is a reduction in the potential of polymerization in deeper areas [8]. As a consequence, even when powerful LCUs are used, bottom surface receive less energy to sensitize the photoinitiators and,
Table 2 Results of degree of conversion for light-activation protocol Light-activation protocol
DC (%)
S (1,000 mW/cm2 ×19 s) HP (1,400 mW/cm2 ×14 s) PE (3,200 mW/cm2 ×6 s)
51.95 a 51.03 a 46.75 b
Mean values with different lower case letter were statistically different (p
ORIGINAL ARTICLE
Influence of light-activation protocol on methacrylate resin-composite evaluated by dynamic mechanical analysis and degree of conversion Maria Cecília C. Giorgi & Vinícius Pistor & Raquel S. Mauler & Débora A. N. L. Lima & Giselle M. Marchi & Flávio H. B. Aguiar
Received: 14 November 2013 / Accepted: 30 March 2014 # Springer-Verlag London 2014
Abstract The aim of this study was to evaluate the degree of conversion (DC) and to identify the viscoelastic properties: storage modulus (E’), loss modulus (E”), tangent delta (tan δ), and glass transition temperature (Tg) of a microhybrid resincomposite light-activated by three different protocols. A Filtek Z250 (3 M ESPE) shade A3 was inserted in a Teflon mold (21 mm×5 mm×1 mm for viscoelastic properties; and 5 mm× 1 mm for DC) and light-activated according to the following light-activation protocols: (S) 1,000 mW/cm2 ×19 s, (HP) 1,400 mW/cm2 ×14 s, and (PE) 3,200 mW/cm2 ×6 s, all set up to deliver 19 J/cm2. Viscoelastic properties was assessed by dynamic mechanical analysis (DMA) (n=3), performed in single cantilever clamped mode. DC (n=5) was measured by FTIR on top (T) and bottom (B) surfaces, and the data was submitted to a split-plot one-way ANOVA. For DC, there was a significant effect for surface factor and light-activation protocols factor. Top surface showed higher DC than B in all experimental conditions. Light-activation protocols S and HP resulted in higher DC than PE and were similar between them. Viscoelastic properties (E’, E”, tan δ, Tg) were not affected by light-activation protocols. It could be concluded that lightactivation protocols influenced DC but not influenced the viscoelastic properties.
M. C. C. Giorgi (*) : D. A. N. L. Lima : G. M. Marchi : F. H. B. Aguiar Department of Restorative Dentistry, Piracicaba Dental School, University of Campinas (UNICAMP), Avenida Limeira, 901 Bairro Areião, Piracicaba, SP 13414-903, Brazil e-mail: [email protected] V. Pistor : R. S. Mauler Instituto de Química IQ/PGCIMAT, Universidade do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Campus do Vale, Porto Alegre, RS 91540-000, Brazil
Keywords Polymers . Composite resins . Physical properties . Curing time . Irradiance
Introduction The polymerization process of light-activated resincomposites is a reaction triggered by free radicals, which are generated by irradiation of a photoinitiator and open the double bond of methacrylate groups (C=C), generating a chain reaction [1]. From the development of this material, studies have been conducted to evaluate the effectiveness of polymerization since this is related to the longevity of the tooth restoration. In theory, the polymerization reaction can occur indefinitely; however, the material is not fully polymerized and small amount of residual free monomers still remains and the polymer structure harbors considerable quantities of pendant double bonds [2]. The degree of conversion (DC) is an important feature and is correlated with other material characteristics, such as mechanical properties, wear resistance, and shrinkage, which influence the longevity of the restoration. Light-curing units (LCUs) are used to irradiate the resincomposite and depending on light-activation protocol, it may influence the structure of the formed polymer. Typically, the greater the degree of conversion, the better the mechanical properties and the greater will be the restoration longevity. On the other hand, the polymerization shrinkage tends to be higher, which can challenge the adhesive interface and lead to gap formation resulting in microleakage, postoperative sensitivity, and secondary caries [3]. This contradiction reveals the complexity of the process and justifies the need to research protocols that may present a more favorable scenario. One strategy used to mitigate the effects of polymerization shrinkage, and at the same time achieve an acceptable degree
Lasers Med Sci
of conversion is the use of soft-start light exposure protocol [4]. This soft-start protocol involves an initial short low level output that allows the material to flow and release part of the polymerization stress in the first step. After that, an increase of irradiance is performed in order to improve the conversion of monomer into polymer. Nevertheless, concern about the effectiveness of polymerization resulted in a tendency to develop powerful LCUs especially designed to overcome everyday challenges for light to reach difficult areas, as interproximal box of class II cavity or during luting of indirect restorations. These powerful LCUs can be set up to deliver the same fluence with different combinations of irradiance and exposure duration and this could result in different polymer structure. For a given composite, different light-activation protocols may even result in polymers with similar degree of conversion but with distinct crosslink density [5]. The degree of conversion in spite of being an important factor does not provide a complete characterization of the polymer structure [6]. The extent of crosslinking plays an important role in the properties of the polymer and has been associated with increased fracture resistance. Polymeric systems with low crosslink density tend to be flexibilized while polymers with higher crosslinking densities are harder, less flexible, and more resistant to heat [6]. This study aimed to evaluate a methacrylate resincomposite light-activated by different light-activation protocols that delivered light in continuous mode. The working hypothesis was that light exposure protocols providing the same fluence in continuous mode but differing in irradiance and exposure time would result in different network structure and viscoelastic properties but not different in degree of conversion.
Materials and methods The light-curable microhybrid composite Filtek Z250 (3 M ESPE, St Paul, USA) (34.5 wt% bis-EMA; 34.5 wt% UEDMA; 6 % wt% TEGDMA; 60 % Silica/Zirconia; batch number 1108500200) shade A3 was selected for this study. The light exposure protocols were as follows: third LED generation Valo (Ultradent Products, South Jordan, USA) in three modes—standard (S) (1,000 mW/cm2, 19 s), high power (HP) (1,400 mW/cm2, 14 s), and plasma emulation (PE) (3,200 mW/cm2, 6 s); all set up to deliver approximately 19 J/cm2. Sample preparation For DMA test, bar samples were prepared by filling a Teflon mold measuring 21×5×1 mm. For DC test, cylindrical samples were prepared by filling a Teflon mold measuring 5 mm×1 mm. The lower surface of the mold was overlaid with a transparent polyester strip, and the resincomposite was inserted in one increment. The upper surface
was covered with another polyester strip, and a glass slide was positioned over the polyester strip and was pressed for 30 s in order to obtain a flat surface and to eliminate a possible oxygeninhibited layer. After that, the glass slide was removed and the sample was light-activated according to the experimental groups. For DMA test, the resin-composite was light-activated in three steps, each step involving one third of the sample and the light-curing tip was positioned close to the polyester strip without touching it. While one area has been irradiated, the others were covered to avoid receiving extra energy. For DC test, the light-activation was performed in one step. Degree of conversion test (n=5) The DC measurements were recorded in absorbance mode with FTIR spectrometer (Spectrum 100 FTIR, PerkinElmer, São Paulo, SP, Brazil) coupled to a zinc selenide multiple (six) reflection attenuated total reflection (ATR) accessory, refraction index of 2.4 at 1,000 cm−1 (PerkinElmer, Walthman, MA, USA), operating under the following conditions: 650–4,000 cm−1 wavelength, 4 cm−1 resolution, and 16 scans. The percentage of unreacted carbon-carbon double bonds (C=C) was determined from the ratio of absorbance intensities of aliphatic C=C (peak at 1,638 cm−1) against the internal standard (aromatic C-C, peak at 1,608 cm−1). The degree of conversion was determined by subtracting the percentage C=C from 100 %, according to the following formula: Rpolymerized DC ð%Þ ¼ 100 1− ð1Þ Rnonpolymerized where R is the ratio of the band height at 1,638 cm−1 to the band height at 1,608 cm−1. Data were submitted to split-plot ANOVA, followed by Tukey’s test (α=0.05). Light-activation protocol factor was considered as plot and surface factor as split-plot. Dynamic mechanical analysis test (n=3) Q800 AT DMA multi-frequency-strain (TA Instruments, New Castle, DE, USA) measured the viscoelastic properties. Bending loads were applied at 1 Hz frequency and deformation tension of 0.08 %, in a single cantilever clamped mode. The samples were heated within the −50 to 250 °C temperature range in a heating rate of 3 °C/min. Data of glass transition temperature (Tg), storage modulus (E’), loss modulus (E”), and tangent delta (tan δ) were plotted against the temperature over this period.
Results For DC test, split-plot one-way ANOVA showed significant effect for surface factor (p≤0.05) and for light-activation protocol (p≤0.05). The top surface presented higher DC with
Lasers Med Sci
significant difference to bottom surface in all experimental conditions (Table 1). For light-activation protocols, S and HP showed similar DC values and PE exhibit the lower DC values, with significant statistical difference to the others (Table 2). The E’ and E” obtained by DMA are shown in Fig. 1. Three analyzes were performed for each sample, and an average curve is generated to show the behavior of the different light exposure protocol. By the E’ curves, in Fig. 2, the values of modulus were collected at 25 and 140 °C corresponding to the vitreous and elastomeric regions, respectively as a function of the time and dependent also of the fluence applied. The results of the modulus did not present a significant variation. The tan δ was achieved by the ratio of E” and E’. Tan δ curves showed the loss of energy associated to the increase in mobility with the increase in temperature. The tan δ curves obtained for the different light-activation protocols are shown in Fig. 3. The parameters obtained by tan δ for all experimental groups can be observed in Table 3. The analysis of the values shows the height and width at half height of the peaks and the Tg. The Tg did not change with the different light-activation protocols, and there were no changes related to the homogeneity of the three-dimensional network in the polymer.
Discussion The aim of the study was to evaluate through degree of conversion and viscoelastic properties, a methacrylate resincomposite light-activated by different light-activation protocols. The working hypothesis was that light-activation protocols providing the same fluence but differing in irradiance and exposure time would result in similar degree of conversion but in different network structure. For DC test, the surface factor presented statistical significant effect, and top surface showed higher DC than bottom surface. Studies had shown that when light passes through the material, it suffers absorption and scattering [7]. Due to this, the light intensity gradually decreases and there is a reduction in the potential of polymerization in deeper areas [8]. As a consequence, even when powerful LCUs are used, bottom surface receive less energy to sensitize the photoinitiators and,
Table 2 Results of degree of conversion for light-activation protocol Light-activation protocol
DC (%)
S (1,000 mW/cm2 ×19 s) HP (1,400 mW/cm2 ×14 s) PE (3,200 mW/cm2 ×6 s)
51.95 a 51.03 a 46.75 b
Mean values with different lower case letter were statistically different (p
