Degree of conversion and microhardness of dental composite resin materials

Degree of conversion and microhardness of dental composite resin materials

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Journal of Molecular Structure 1044 (2013) 299–302

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Degree of conversion and microhardness of dental composite resin materials D. Marovic a,⇑, V. Panduric a, Z. Tarle a, M. Ristic b, K. Sariri c, N. Demoli c, E. Klaric a, B. Jankovic a, K. Prskalo a a

Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Gunduliceva 5, 10000 Zagreb, Croatia Division of Materials Chemistry, Institute Rudjer Boskovic, Bijenicka 54, Zagreb, Croatia c Laboratory for Coherent Optics, Institute of Physics, Bijenicka 46, Croatia b

h i g h l i g h t s " Light cured dental composite materials are tested. " Degree of conversion of resin monomers to polymer network is measured. " Fourier transform infrared spectroscopy and microhardness are compared. " Microhardness could not substitute Fourier transform infrared spectroscopy.

a r t i c l e

i n f o

Article history: Available online 8 November 2012 Keywords: Composite resins Fourier transform infrared spectroscopy Degree of conversion

a b s t r a c t Dental composite resins (CRs) are commonly used materials for the replacement of hard dental tissues. Degree of conversion (DC) of CR measures the amount of the un-polymerized monomers in CR, which can cause adverse biological reactions and weakening of the mechanical properties. In the past, studies have determined the positive correlation of DC values determined by Fourier transform infrared spectroscopy (FT-IR) and microhardness (MH) values. The aim of this study was to establish whether MH can replace FTIR for the determination of DC of contemporary CR. Two nano-hybrid CR: Tetric EvoCeram (TEC; Ivoclar Vivadent, Liechtenstein) and IPS Empress Direct (ED; Ivoclar Vivadent) and one submicron-hybrid CR – Charisma Opal (CO; Heraeus Kulzer, Germany) were tested. DC was determined by using FT-IR (n = 10) and Vickers MH (n = 10) was measured using Leitz Miniload 2 Microhardness Tester (Leitz, Germany). The data were analyzed using ANOVA and Tukey’s post hoc test (p < 0.05). CO was the highest polymerized material (62.20%) in comparison to TEC (58.85%) and ED (58.78%). Opposite, ED was significantly hardest material (24.49) when compared to CO (17.81) and TEC (20.05). Since the CO was the material with the highest DC, but also with the lowest MH, it can be concluded that the DC of new CR formulations cannot be estimated through the MH data. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Dental composite resins (CRs) are most commonly used materials for the replacement of lost hard dental tissues. The resin matrix in their composition gives them plasticity and good handling properties, whereas filler particles are mostly responsible for the hardness, strength and other mechanical properties needed for the longevity of the material in the demanding conditions found in the oral cavity [1]. The material is in the plastic phase and its hardening occurs due to the visible light initiated cross-linking of resin monomers into a three-dimensional polymer network [2].

⇑ Corresponding author. Tel.: +385 1 4899 203; fax: +385 1 4802 159. E-mail address: [email protected] (D. Marovic). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.10.062

A high degree of composite polymerization is essential for the optimal physical properties [3] and the biocompatibility [4]. The conversion of monomers to the polymer is never complete and amounts up to 75% [5]. At the beginning of light irradiation, photo-initiators are activated and turn into free-radicals. The collision of free-radical initiators activates the monomers, which, in turn, activate other monomers and form covalent bonds between carbon atoms and form long-chain polymers. The lengthening and the interaction of the polymer chains cause an increase in the viscosity and the rigidity of the composite paste. The bridges of covalent bonds link the chains and form a cross-linked network. Within a rapidly stiffening structure, certain unreacted monomers remain trapped [6]. Residual unconverted methacrylate groups which may reside in lower parts of poorly polymerized composite fillings present not only cytotoxic and genotoxic risks [7–9], but

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also their solubility might cause the formation of voids and the occurrence of secondary caries [7,10]. Therefore, the degree of conversion (DC) is the property which is often tested in vitro. For the determination of the DC of dental CR, Fourier transform infrared spectroscopy (FT-IR) [2,11–14] and microhardness (MH) [3,15–17] are often used. Besides, other methods, such as Raman spectroscopy [11,14,18,19], differential scanning calorimetry [20], differential thermal analysis [14] and high performance liquid chromatography [21] are also used in dental research, but FT-IR remains the most often used technique. Infrared spectroscopy is used to determine the DC by the proportion of the remaining concentration of the aliphatic [email protected] double bonds in a cured composite sample relative to the total number of [email protected] bonds in the uncured material [22]. For dental CR based on methacrylates, which still represent the majority of CR, mid- (MIR) or near-infrared regions (NIR) can be used. In MIR, the intensity or area of the methacrylate stretch band at 1638 cm 1 is measured, which is correlated to the internal reference peak, whose intensity remains unaltered during polymerization process. This is the internal standard for normalization, which does not require measuring of the sample thickness [22]. Generally, composites based on methacrylates present aromatic bands at 1537 [23] 1583, 1608 [12] and 4623 cm 1 which can be used as internal standards [22]. In NIR, there are two aliphatic bands that can be used, at 6165 cm 1 (overtone = CH2) and at 4743 cm 1 (@CAH), but the latter is not recommended due to the instability of the adjacent peaks [18]. The peak at the 6165 cm 1 does not require the internal standard and gives bulk polymer conversion data on sample geometry, unlike the measurements in MIR, which need extremely thin samples for measurement [18]. Another property of CR that may be important to consider is hardness. Hardness of composite materials is a property that enables it to resist plastic deformation, penetration, indentation and scratching. The microhardness of dental composite materials is usually used to predict their abrasion resistance if used as restorations in functional areas [24,25]. A positive correlation of volume fraction of filler and the Knoop hardness was found [26], as well as between mass fraction of fillers and Vickers microhardness [27,28]. Regarding the size of the fillers, it was found that composites containing nanofillers exhibit higher microhardness values than conventional composites due to more intimate contact of nanofillers with resin matrix than microfillers [28]. In past, many studies have determined the positive correlation of DC values determined by MH and FT-IR measurements [15,29], but there are also contrary reports [30,31]. The co-dependence of DC and microhardness is long known [29,32] and it was often used to indirectly assess the composites’ depth of cure, which was defined as the deepest hardness value found equivalent to that at 0.5 mm depth [33]. Another method employed the difference in microhardness between the upper and lower surfaces of cured composite samples [34–36]. However, no correlation was found between DC and surface hardness [30,31]. It was observed that opaque materials and materials with high filler load, which exhibit stronger light scattering, consequently had lower DC and lower

microhardness. Conversely, translucent shades were not influenced and they exhibited high DC and microhardness [37]. The aim of the study was to measure the DC and MH of contemporary CR and to establish whether MH values can be used instead of FTIR for determination of DC. 2. Experimental 2.1. Materials Two nano-hybrid CR: Tetric EvoCeram (TEC; Ivoclar Vivadent, Liechtenstein) and IPS Empress Direct (ED; Ivoclar Vivadent) and one submicron-hybrid CR – Charisma Opal (CO; Heraeus Kulzer, Germany). Their composition, as stated by the manufacturers, is given in Table 1. 2.2. Methods For FT-IR measurements (n = 10), a half of a rice grain amount of each CR was placed between two Mylar sheets and pressed under 107 Pa (1 cm in diameter, 0.1 mm thickness). The samples were polymerized using a Bluephase G2 LED polymerization device (Ivoclar Vivadent) in high power polymerization mode (1200 mW/ cm2) for 30 s. The uncured samples were pressed into KBr pellets (d = 1 cm) using spectroscopically pure KBr (Merck, Germany) with a Carver press. DC of polymerized samples was determined by Fourier transform spectrometer Mo. 2000 (Perkin Elmer, UK). Recording and processing of absorption spectra of composite specimens were carried out with Spectrum v5.3.1 software (Perkin Elmer, UK). Spectra of paired un-polymerized and polymerized composite specimens were recorded in a transmission mode at room temperature, corrected by subtracting the background and then converted into the absorbance mode (Fig. 1.). A total of 22 scans per sample were measured at a resolution of 4 cm 1. DC (%) was calculated from the equivalent aliphatic (1638 cm 1)/aromatic (1608 cm 1) molar ratios of cured (C) and uncured (U) samples according to the following expression [12]: DC = (1 C/U)  100 (%). MH samples (n = 10) were placed between two Mylar films and pressed between two steel plates to 0.85 mm thickness. Vickers MH was measured using Leitz Miniload 2 Microhardness Tester (Leitz, Germany) with the load of 5 or 10 g, 3 measurements for each load per sample. The Vickers microhardness was calculated according to the formula: HV = 1.854 P/d2, where P is the applied load in kg and d is the indentation in mm. The data of DC and MH are expressed as means and standard deviations, and were analyzed using ANOVA and Tukey’s post hoc test (p < 0.05). For the correlation of the DC and MH for each material, Pearson Correlation was used (p < 0.05). 3. Results Fig. 1. shows the infrared spectra of tested composite materials in the region 1670–1580 cm 1. Bands at 1638 cm 1 represent the

Table 1 The composition of tested materials according to the manufacturers data. Bis-GMA: bisphenol A-glycidyl methacrylate; UDMA: urethane dimethacrylate; Bis-EMA: Bisphenol A polyethylene glycol diether dimethacrylate; TCDMMA – Tricyclodocandimethanoldimethacrylat, YT3 – ytterbium trifluoride; PFP – prepolymerized filler particles. Material

Resin (vol.%)

Filler (vol.%)

Manufacturer

Shade, batch and expiration date

Tetric EvoCeram (TEC) IPS Empress Direct (ED) Charisma Opal (CO)

45–47% Bis-GMA, UDMA, Bis-EMA 41–48% UDMA, TCDMMA, Bis-GMA 42% Bis-GMA based matrix

53–55% Barium glass, YT3, mixed oxide, PFP (550 lm)

Ivoclar Vivadent, Schaan, Liechtenstein Ivoclar Vivadent, Schaan, Liechtenstein Heraeus Kulzer GmbH, Hanau, Germany

A3; LOT N36895; Exp. 2014-05 A3 enamel; LOT N32078; Exp. 2014-03 A3; LOT 010026; Exp. 2013–12

52–59% Barium glass, YT3, mixed oxide, silicon dioxide and copolymer (40–3000 lm) 58% Barium aluminium glass (0.02–2 lm) and highly dispersive silica (0.02–0.07 lm)

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301

Tetric EvoCeram

1610

1638

Polymerized

1638 1610

Unpolymerized

IPS Empress Direct Fig. 2. Degree of conversion of tested materials. Different letters indicate the statistically significant difference between tested composite resins. TEC – Tetric EvoCeram; ED – IPS Empress Direct; CO – Charisma Opal.

Absorbance / a.u.

1638 1610

Polymerized

1638 1610

Charisma Opal

Unpolymerized

1610 1638

Polymerized

1638 1610

Unpolymerized

Fig. 3. Vickers microhardness of tested materials. Different letters indicate the statistically significant difference between tested composite resins. TEC – Tetric EvoCeram; ED – IPS Empress Direct; CO – Charisma Opal.

1660

1640

1620

1600

1580

Wave number / cm -1 Fig. 1. FT-IR spectra of polymerized and unpolymerized composite samples.

aliphatic stretching vibrations of resin matrix, in these materials mostly influenced by the Bis-GMA, UDMA and Bis-EMA. Aromatic bands at 1610 cm 1 were taken as internal standard. As can be seen by comparing the spectra of polymerized and unpolimerized material samples, the aliphatic band shows the reduction in the absorbance after being polymerized. Fig. 2. shows the results of the FT-IR measurement of DC, which indicate that CO was the highest polymerized material (62.20%) in comparison to TEC (58.85%) and ED (58.78%). Opposite, as demonstrated in Fig 3, ED was significantly hardest material (24.49) when compared to CO (17.81) and TEC (20.05). Pearson Correlation demonstrated that there is no correlation between the DC and MH values for tested composite materials with the level of significance set to 0.05 (r2 = 0.14 (TEC); r2 = 0.29 (CO); r2 = 0.01 (ED)).

4. Discussion The present study was aimed to investigate and compare the DC and MH of three modern dental composite materials. The second

aim was to assess the validity of the earlier statement that DC can be accurately measured by means of MH instead of spectroscopic methods [15,29,32]. Vickers microhardness and the DC were measured and the comparison of the data demonstrated that there was no correlation between them. The properties of dental composite resins are reflections of their intricate composition and they are mostly affected by the interplay between their major constituents: the resin composition and the type and the amount of fillers. The degree of polymerization of dental composite resins is primarily influenced by the nature and the amount of individual monomers in their composition [5,38]. The predominant base monomer used in commercial dental composites is bisphenol A-glycidyl methacrylate (Bis-GMA), which is also present in all materials tested in this study. Because of its large molecular size and rigid structure, Bis-GMA has high viscosity, providing lower polymerization shrinkage, more rapid hardening and production of stronger and stiffer resins [5]. On the other side, its low mobility does not allow it to achieve high DC values. Therefore, Bis-GMA has to be mixed with diluent monomers of low viscosity, such as triethyleneglycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA) or bisphenol A polyethylene glycol diether dimethacrylate (Bis-EMA) in order to achieve acceptable levels of polymerization [39]. UDMA and Bis-EMA also serve as base monomers in some systems, like ED in this study. Although UDMA has much lower viscosity than Bis-GMA, it seems that the amount used in ED was not sufficiently high to achieve improvement in DC in comparison

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to TEC and CO. Besides, this material has the highest filler volume of tested materials, which has probably contributed to increased light scattering, higher viscosity of the composite paste, therefore lower monomer mobility and consequently lower DC [40–42]. Unfortunately, the exact resin composition of the highest polymerized material, CO, is unknown. High DC in the Bis-GMA base systems can be achieved by increasing the content of TEGDMA, a reactive diluents of relatively small molecular weight and high mobility [38]. In a Bis-GMA:TEGDMA comonomer system with high TEGDMA ratio, the conversion will be increased, while the hardness remains unaffected, but at the same time it will make the material brittle and prone to fracture [5,38,43]. This could be the explanation of low hardness of the CO material in the present study. In conventional composite materials DC is highly correlated to the hardness of the material [15,26,29], explained by the higher density achieved in the densely compacted cross-network of dental CRs. However, this is not the only factor influencing the hardness of dental composite resins and fillers are recognized as more influential. The exceptions are microfilled composites with prepolymerized filler particles and higher amount of organic matrix [39], similar to TEC material to in this study. Although it contains nanofiller particles, which characterizes it as a nanohybrid composite resin, it also contains prepolymerized resin fillers, which are consisted of fillers embedded in resin, polymerized and milled to obtain a desired particle size. Therefore, prepolymerized fillers never achieve as high MH values as the composites without prepolymerized particles [17], which is in agreement with our results. Nano-hybid material ED without prepolymerized fillers and with certain amount of particles in nano-size was the hardest material in this study. It was the material with the highest volume filler loading (59 vol.%), whose correlation to the hardness has been long established [35,44]. The studies examining the hardness of the nanofilled composites postulated that these fillers achieve high hardness and good polishability [45,46]. It is considered that nanofillers can achieve a more intimate contact to resin than the microfillers, which is confirmed by our results [28]. Although the results of a recent study have questioned these assumptions [47], the advantage of ED in terms of hardness over TEC and CO is that the resin matrix is based on UDMA, which has shown greater flexibility than Bis-GMA and higher toughness and strength of resin composites based on this monomer [48]. Since the CO was the material with the highest DC, but also with the lowest MH, it can be concluded that the DC of new CR formulations with the addition of fillers in nano-range cannot be estimated through the surface MH data. Previous studies have proven the correlation between DC and MH [3,14,15,26,29], especially the correlation between the DC and the ratio of MH at the top and the bottom surface of composite samples, but they were performed on either unfilled resins, a single material or within the same group of materials. This study compared two nano-hybrid materials, one with and one without prepolymerized filler paricles, and a submicron-hybrid CR. The introduction of new nano-sized fillers in dental composites has changed the existing rules and it should be emphasized that microhardness is more affected by filler content than by the DC [27]. Within the limitations of the present study, it can be concluded that the correlation of DC and surface HV does not apply to the contemporary dental composites with nano-fillers, but the correlation of volume filler load and HV is valid.

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