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Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
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Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites Amel Amirouche-Korichi a,b , Mohamed Mouzali b,∗ , David C. Watts c a b c
CRAPC, BP 248, Alger RP 16004, Algiers, Algeria LPCMAE, Faculty of Chemistry, BP32 El-Alia, Algiers, Algeria Biomaterials Science Research Group, School of Dentistry, University of Manchester, Manchester M15 6FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The degree of conversion (DC) and polymerization shrinkage of resin compos-
Received 18 January 2009
ites are closely related manifestations of the same process. Ideal dental composite would
Accepted 4 June 2009
show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and
Keywords:
monomer ratios on the DC and the shrinkage-strain of experimental composites based on
Restorative composites
(BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the
Opaque mineral fillers
shrinkage-strain and the degree of conversion values was also investigated. The radiopacity
Polymerization Shrinkage-strain
of these experimental composites has been investigated in a previous paper.
Degree of conversion
Methods. Experimental resin composites were prepared by mixing different monomer
Viscosity
ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacry-
BisGMA
late (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La2 O3 ,
TEGDMA
BaO, BaSO4 , SrO and ZrO2 at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm2 , was measured using the bonded-disk technique at room temperature with respect to time. Results. The result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated. Significance. The nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites. © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +213 21247951; fax: +213 21247951. E-mail address: [email protected] (M. Mouzali). 0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2009.06.009
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1.
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
Introduction
Visible light-curable polymeric composites are now routinely used as filling materials for dental restorations. Generally, they consist of mineral fillers dispersed in an organic resin matrix which consists of a multifunctional methacrylate, a dimethacrylate diluent, and a photo-initiating system [1]. Dental composites have many advantages such as mechanical properties equivalent to commercial dental amalgams and dental ceramics, an excellent esthetic quality and the ability to bond to enamel surface [2]. Properties of the dental composites are greatly influenced not only by the properties of their fillers but also by the chemical structure of the monomers used in the matrix phase [2,3]. If the organic matrix is used unfilled, the restoration would exhibit a low wear resistance. Thus, the increased filler content tends to improve mechanical properties and to reduce curing shrinkage and the thermal expansion coefficient [4]. The 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy) phenyl] propane: BisGMA is most commonly used as the organic phase of dental restorative materials because of its high strength and hardness [5]. The rigid aromatic backbone structure of BisGMA, based on bisphenol A, provides different desirable properties such as less shrinkage, higher modulus and reduced toxicity due to its lower volatility and diffusivity into tissues [4]. However, the drawback of this monomer is its very high viscosity, owing to hydrogen bondings between the hydroxyl groups, which limited the incorporation of inorganic fillers and hence a low final DC [6]. Therefore, a reactive diluent, such as triethylene glycol dimethacrylate (TEGDMA) is added to improve the viscosity, reactivity and the final conversion of the matrix phase [1]. Nevertheless, because of favorable stereochemistry of TEGDMA and, its relatively low molecular weight, the polymerization of the corresponding material results in high shrinkage and degree of conversion as well as low mechanical properties [3]. Since BisGMA has greater viscosity than TEGDMA because of the stiff central core and hydroxyl groups in its chemical structure, the relative concentration of each monomer can have an important effect on the mobility and thus kinetics of the reaction. It has been found [7] that the ratio of BisGMA in the organic matrix has a significant effect on the mechanical properties of the resin composites. During the polymerization process, monomers react to form a covalent bond. The distance between the molecules is reduced leading to free volume reduction, which results in volumetric shrinkage in the final polymer network [8]. Shrinkage-strain of restorative materials constitutes a major challenge to polymer chemists and dental scientists [9]. The initial shrinkage-strain rises rapidly during the initial solidification process causing failures in the dental composite restorations [7,3]. Clinically, the shrinkage-strain, manifests as stress, may damage the bonding or cause deflection of the surrounding tooth structure [10]. It may also cause fractures of the tooth that results in microleakage and recurrent caries [10–12]. The volumetric shrinkage of a dental composite is determined by its filler volume fractions and the composition of the resin matrix [8]. The measurement of the polymerization shrinkage of dental composites can be obtained using different methods
such as dilatometry; optical; bonded-disk; linear displacement; strain gage [13]. The bonded-disk method, which measures dimensional change along a single axis, does not require extensive and expensive instrumentation. The shrinkage is measured indirectly by monitoring the deflection of a glass coverslip, in contact with the surface of the composite [13,14]. Theoretically, during the polymerization reaction, all the molecules of monomers would be converted to polymer. However, dimethacrylate monomers exhibit important residual (C C) bonds in the final product, with DC ranging from 55 to 75% under conventional irradiation conditions [11,12]. Different studies [3,15,16] found that the release of unreacted monomers that remains in the restoration may stimulate the growth of bacteria around the restoration and promote allergic reactions in some patients. They may also act as a plasticizer and decrease the mechanical properties of the system [5]. The volumetric shrinkage of composites has been shown to be proportional to its degree of conversion [8,17]. It has been found [18] that an ideal composite exhibits a minimal polymerization shrinkage with an optimal degree of conversion. As an increase of monomer conversion leads to the increase of polymerization shrinkage, these seem to be antagonistic. DC, which is associated with many factors like cure time, matrix composition, light intensity, filler content, diluent concentration and initiator concentration, has a large role in determining the physical and mechanical properties of the material such as hardness, tensile strength, compressive strength, etc. [19,20]. Among several methods to determine DC of dental composites, Fourier transform infrared spectroscopy (FTIR) has been widely used as a reliable method due to the availability of equipment and numerous sampling techniques. This method detects the (C C) stretching vibrations, centered around 1638 cm−1 , directly before and after curing of materials [21,22]. The objective of this study was to provide an experimental opaque materials and to examine the effect of the nature, volume and concentration of five radiopacifying fillers: La2 O3 , BaO, BaSO4 , SrO and ZrO2 on the degree of conversion and the shrinkage-strain of experimental composites based on the mixture (BisGMA/TEGDMA) at different ratios: (50/50), (25/75) and (75/25). The radiopacity of these experimental composites was investigated in a previous study [23]. The aim of this work was also focused on obtaining an optimal formulation of (Bis-GMA/TEGDMA) mixture, regarding the DC [24] and the shrinkage-strain of the experimental composites studied.
2.
Materials and methods
2.1.
Materials
2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl] propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA), were obtained from Aldrich (France). Camphoroquinone (CQ) and N,N -dimethyl aminoethyl methacrylate (DMAEMA) were purchased from Fluka (Germany). Five different radiopacifying agents were used: lanthanum oxide (La2 O3 ), barium oxide (BaO), barium sulfonate (BaSO4 ), zirconium oxide (ZrO2 ), and stronsium oxide (SrO), were obtained from Aldrich (France).
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
2.2.
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Methods
Experimental filler composites were prepared by mixing (Bis-GMA/TEGDMA) at different ratios by weight: (50/50), (75/25) and (25/75) as matrix phase. 0.5 wt.% CQ and 0.5 wt.% DMAEMA, as a photo-initiator system, were added to the mixture. The radiopacifying filler powders were then added, in various proportions, into the mixture to provide loadings ranging from 0 to 80 wt.%. Each experimental composite was well blended to obtain a homogenous mixture.
2.2.1.
Degree of conversion measurement
The degree of conversion (DC%) was measured using FTIR spectroscopy (Nicolet 360 Avatar 360 FTIR spectrometer) with an attenuated total reflectance (ATR) sampling accessory (Type Pike miracle ATR, Diamond/ZnSeW/Pike technology). The absorbance peaks of the cured and uncured samples were obtained using 32 scans at a resolution of 4 cm−1 . Three specimens (n = 3) were used for each ratio of each material. Each specimen was light polymerized for 40 s using a visible light source (LA 500 Blue light curing light, 500 mW/cm2 , 450–490 nm, Aposa Enterprise). The spectrum of each sample after curing was also obtained. The DC of each specimen was determined from the ratio of absorbance intensities of aliphatic (C C), peak at 1638 cm−1 and normalized against the aromatic (C C) at 1608 cm−1 , as follows [6]:
DC (%) = 1 −
Acaliphatic /Acaromatic
Aualiphatic /Auaromatic
× 100
(1)
where Acaliphatic is the absorbance peak at 1638 cm−1 of the cured specimen, Acaromatic is the absorbance peak at 1608 cm−1 of the cured specimen, Aualiphatic is the absorbance peak at 1638 cm−1 of the uncured specimen and Auaromatic is the absorbance peak at 1608 cm−1 of the uncured specimen.
2.2.2.
Shrinkage-strain measurement
The shrinkage-strain was performed at room temperature using the bonded-disk technique [25]. This method measures an axial shrinkage which was restricted to the vertical thickness dimension only, but with the disc-shape of the specimen, which is bonded to one side of the rigid glass surface; it was suggested [25–27] that the central deflection of the disc is representative of contraction over the top surface of the specimen. So, the fractional linear shrinkage measured is approximately equivalent to the fractional volumetric shrinkage: L ∼ V = L0 V0
Fig. 1 – Degree of conversion vs. mole number of BisGMA in the composite (BisGMA/TEGDMA), containing 0% opaque filler.
3.
Results
The final values with standard deviations (SD) of the degree of conversion and the shrinkage-strain after 1 h of experimental composites based on (BisGMA/TEGDMA): (50/50), (75/25), (25/75), containing the photo-initiator system and mixed with five different radiopacifying agents: La2 O3 , BaO, BaSO4 , BaO, SrO, ZrO2 at different weight fractions, ranging from 0 to 80 wt.% are summarized in Tables 1 and 2. Figs. 1 and 2 show, respectively, the degree of conversion and the final shrinkage-strain as a function of Bis-GMA content in experimental composites without containing fillers. These figures indicate a significant decrease in shrinkagestrain and DC when the BisGMA concentration increases in the (BisGMA/TEGDMA) mixture. Fig. 3 shows the typical shrinkage-strain curves for the experimental composites (Bis-GMA/TEGDMA): (50/50) with different La2 O3 filler contents. The data showed that the final shrinkage-strain decreased by increasing quantity of opaque
(2)
The specimens were light cured for 40 s using a visible light source (LA 500 Blue light curing light, 500 mW/cm2 , 450–490 nm, Aposa Enterprise). The shrinkage-strain was measured continuously and total shrinkage-strain data of each sample was recorded with respect to time (1 h). Three repetitions (n = 3) were made per specimen.
Fig. 2 – Shrinkage-strain vs. mole number of BisGMA in composite (BisGMA/TEGDMA), containing 0% opaque filler.
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Table 1 – Degree of conversion (%) (standard deviation in parentheses) of samples based on (Bis-GMA/TEGDMA) blended with five radiopacifying agents (salts): BaO, BaSO4 , La2 O3 , ZrO2 , and SrO ranging from 0 to 80 wt.%. Opaque fillers (BisGMA/ TEGDMA) salt (wt.%)
BaO
BaSO4
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
75.73 (0.1) 74.09 (0.10) 73.19 (0.02) 72.54 (0.03) 70.51 (0.17) 68.21 (0.01)
68.94 (0.03) 70.21 (0.15) 67.07 (0.03) 66.23 (0.01) 65.55 (0.02) 64.88 (0.10)
66.07 (0.08) 63.20 (0.06) 61.48 (0.14) 58.26 (0.16) 54.46 (0.07) 50.99 (0.13)
75.73 (0.1) 74.32 (0.08) 68.98 (0.17) 67.67 (0.05) 66.17 (0.01) 66.14 (0.18)
68.94 (0.03) 67.47 (0.17) 66.63 (0.10) 66.22 (0.11) 66.07 (0.13) 63.77 (0.22)
66.07 (0.08) 60.77 (0.1) 60.07 (0.01) 58.29 (0.21) 58.07 (0.18) 56.53 (0.09)
75.73 (0.1) 74.24 (0.15) 72.11 (0.15) 70.55 (0.17) 69.10 (0.09) 66.06 (0.04)
(50/50)
68.94 (0.03) 67.33 (0.02) 67.23 (0.03) 64.24 (0.12) 62.19 (0.1) 61 (0.11)
SrO
ZrO2
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
66.07 (0.08) 61.88 (0.08) 61.01 (0.05) 58.09 (0.15) 57.52 (0.01) 51.04 (0.15)
75.73 (0.1) 74.78 (0.01) 72.98 (0.07) 70.19 (0.04) 68.63 (0.03) 66.44 (0.13)
68.94 (0.03) 68.30 (0.13) 68.0 (0.13) 67.58 (0.07) 66.35 (0.13) 65.80 (0.10)
66.07 (0.08) 63.92 (0.1) 61.93 (0.12) 61.50 (0.13) 60.7 (0.15) 59.43 (0.14)
75.73 (0.1) 73.97 (0.11) 72.46 (0.20) 72.13 (0.07) 71.50 (0.16) 66.11 (0.07)
68.94 (0.03) 67.96 (0.04) 64.25 (0.15) 63.51 (0.16) 58.27 (0.05) 56.90 (0.02)
66.07 (0.08) 64.89 (0.01) 62.89 (0.17) 61.50 (0.14) 57.49 (0.07) 55.66 (0.10)
Table 2 – Shrinkage-strain (%) (standard deviation in parentheses) of samples based on (Bis-GMA/TEGDMA) blended with five radiopacifying agents (salts): BaO, BaSO4 , La2 O3 , ZrO2 , and SrO ranging from 0 to 80 wt.%. Opaque fillers
0 10 20 40 60 80
La2 O3
BaO
BaSO4
SrO
ZrO2
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
9.46 (0.13) 8.5 (0.05) 8.11 (0.03) 7.93 (0.10) 7.93 (0.12) 7.41 (0.08)
8.05 (0.1) 7.41 (0.17) 7.09 (0.17) 6.58 (0.02) 6.10 (0.02) 5.87 (0.13)
5.21 (0.08) 4.70 (0.01) 4.60 (0.04) 4.37 (0.14) 4.09 (0.18) 4.07 (0.10)
9.46 (0.13) 8.46 (0.13) 8.21 (0.10) 8.13 (0.07) 8.12 (0.04) 7.89 (0.13)
8.05 (0.1) 7.69 (0.09) 7.42 (0.15) 6.65 (0.1) 6.24 (0.03) 5.88 (0.04)
5.21 (0.08) 5.13 (0.1) 4.79 (0.03) 4.70 (0.01) 4.41 (0.13) 4.32 (0.06)
9.46 (0.13) 8.43 (0.02) 8.17 (0.01) 8.16 (0.13) 8.11 (0.06) 7.98 (0.17)
8.05 (0.1) 7.63 (0.15) 7.12 (0.14) 6.60 (0.15) 6.5 (0.11) 5.87 (0.15)
5.21 (0.08) 4.92 (0.17) 4.86 (0.01) 4.73 (0.05) 4.40 (0.21) 4.33 (0.09)
9.46 (0.13) 8.6 (0.08) 8.54 (0.2) 8.12 (0.12) 8.08 (0.17) 7.84 (0.16)
8.05 (0.1) 7.45 (0.01) 6.76 (0.09) 6.62 (0.10) 6.13 (0.01) 5.85 (0.13)
5.21 (0.08) 5.16 (0.22) 5.15 (0.13) 5.08 (0.18) 4.73 (0.03) 3.83 (0.1)
9.46 (0.13) 8.46 (0.14) 8.24 (0.11) 8.12 (0.17) 8.10 (0.01) 7.90 (0.01)
8.05 (0.1) 7.54 (0.04) 6.62 (0.06) 6.56 (0.07) 6.40 (0.13) 6.14 (0.12)
5.21 (0.08) 5.04 (0.12) 4.92 (0.13) 4.84 (0.14) 4.63 (0.05) 3.58 (0.13)
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
0 10 20 40 60 80
La2 O3
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
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Fig. 3 – Typical shrinkage-strain curves for composite (BisGMA/TEGDMA): (50/50) with varying La2 O3 filler content (vol.%).
Fig. 5 – Correlation between shrinkage-strain and La2 O3 filler volume fraction in the composite (BisGMA/TEGDMA) at different monomer ratios.
fillers. Similar results have been observed with all the investigated composites. Figs. 4 and 5 show, respectively, the variation of the degree of conversion and the shrinkage-strain with filler volume fraction of investigated composites at different monomer ratios. The results indicate that the DC and the shrinkage-strain were linearly related to the filler contents. A same result has been obtained with all experimentally studied opaque filler. Figs. 6 and 7 represent, respectively, the variation of the degree of conversion and the final shrinkage-strain of composites containing different opaque fillers at various volume fractions added to a mixture (BisGMA/TEGDMA): (50/50). There is no significant difference among the shrinkage values of the composites with different opaque fillers having different kind of heavy metals. Similar results have been observed with monomer ratios of (25/75) and (75/25). Figs. 8 and 9 illustrate the correlation between the final shrinkage-strain and DC of the experimental composite con-
Fig. 6 – Degree of conversion vs. filler volume fraction of composite (BisGMA/TEGDMA), containing different opaque fillers.
Fig. 4 – Degree of conversion vs. La2 O3 filler volume fraction in the composite (BisGMA/TEGDMA) at different monomer ratios.
Fig. 7 – Shrinkage-strain vs. filler volume fraction of composite (BisGMA/TEGDMA) with varying different opaque fillers.
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d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
Fig. 8 – Correlation between shrinkage-strain and degree of conversion of composite (BisGMA/TEGDMA), La2 O3 at various weight fraction fillers ranging from 0 to 80 wt.% and at different monomer ratios.
taining (BisGMA/TEGDMA) at different monomer ratios, mixed with La2 O3 filler at various volume fraction contents. Similar results have been obtained with all investigated experimental composites containing different opaque fillers salts.
4.
Discussion
This study revealed the effect of radiopacifying agents containing different kinds of heavy metals added to a standard organic matrix on the degree of conversion and the shrinkagestrain of composite resin. As it has been shown in Figs. 1 and 2, the degree of conversion and the shrinkage-strain decreased as BisGMA concentration was increased. It has been suggested [28] that varying the relative amounts of the matrix monomers has a significant effect on the shrinkage-strain and consequently on the mechanical properties of dental composites. The reduction in shrinkage-strain with the increasing of the
Fig. 9 – Shrinkage-strain/degree of conversion vs. La2 O3 volume fraction content of composite (BisGMA/TEGDMA), La2 O3 with varying monomer ratios.
BisGMA concentration could be attributed to the fact that BisGMA has a high molecular weight with hydroxyl groups and a rigid aromatic group: bisphenol A in the central part section of its chemical structure which causes much barriers to free rotation around the bonds, while the ether (C–O–C) linkages in the TEGDMA molecule give rise to only slight barriers to free rotation around the bonds [29]. The hydroxyl groups of BisGMA molecules are also capable to form intermolecular hydrogen bonding which restricts sliding of polymer chains, fewer double-bonds are converted, thereby increasing gradually the viscosity of the system with the increasing of BisGMA content in the composite [8]. As a consequence, this restriction of the growing macro-radicals and monomers mobility affects the propagation of the free radicals and thus decreasing the degree of conversion. TEGDMA has a molecular weight lower than BisGMA, in its unpolymerized state. So the presence of more TEGDMA in the mixture leads to more flowable composites and more covalent bonds are created during polymerization. Consequently, the degree of conversion and the shrinkagestrain increase with an increasing content of TEGDMA [18,28]. It has been shown [15,16,20,30] that the presence of unreacted monomers is undesirable, because, they will leach into the surrounding medium, resulting in deterious effects on the mechanical stability, causing allergic reactions and stimulate bacterial growth around restoration. A linear correlation of the degree of and conversion shrinkage-strain with filler volume fraction of investigated composites at different monomer ratios has been shown in Figs. 4 and 5. The results indicate that the progressive decreases of DC and shrinkage-strain were linearly related to the filler contents. Our results are in agreement with those of Garoushi and Atai [6,31]. The mobility of monomer-chain can be restricted by the incorporation of the fillers, leading to decreasing the monomers and radical mobility resulting then in lower conversion and shrinkage. The results indicate also that for a given filler content, the DC and shrinkage increase significantly with the increasing of TEGDMA concentration in the organic matrix. It can be suggested that the filler molecules distribution is more favorable in a flowable system because of the flexibility of polymer chain, so the filler will not take place an important volume in the matrix. As a consequence, DC final increase as well as the total shrinkage-strain. The volume occupied by fillers in the matrix can be observed in Figs. 4 and 6. It can be seen that this volume increases with the increasing of BisGMA concentration. However, Figs. 4 and 5 show that the decrease of DC and shrinkage is not significant with the increasing of filler loading in the composites. It has been suggested [6] that the effect of fillers contents on the mobility of radicals is very low and that the influence of fillers on the conversion is more related to filler particle-size and surface area than filler loadings. The fillers used in our study are all microfillers. As it is shown in the Figs.6 and 7, there is no significant difference in the values of DC and of the final shrinkage strain of the composites with various opaque fillers which have different kinds of heavy metal, although the increase in volume occupied by these fillers, because of the increase in the heavy metal atomic number of the salt, is significant. Similar results have been observed with monomer ratios of (25/75) and (75/25). So, the results indicate that the nature and volume of
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
opaque fillers does not affect the DC and the shrinkage-strain of the resulting composites. These results are in agreement with the work of Atai and Watts [6]. The correlation between the final shrinkage-strain and DC of the experimental composite containing (BisGMA/TEGDMA) at different monomer ratios, mixed with La2 O3 filler at various volume fraction contents, has been shown in Figs. 8 and 9. These results indicate a close relationship between DC and shrinkage-strain, with taking care of the antagonistic clinical implications. Similar results have been obtained with all investigated experimental composites containing different opaque fillers salts. It generally has been observed that the higher the DC in resin composites, the higher the shrinkagestrain [17] was obtained. The present work supports this correlation. Fig. 9 confirms that DC and the shrinkage-strain vary with the same proportion.
5.
Conclusions
The degree of conversion and the shrinkage-strain progressively decrease linearly with the increasing of opaque filler contents. However, the effect of the nature and the volume of the opaque fillers on DC and the shrinkage of experimental composites investigated were not significant. The DC and the final shrinkage decreased significantly with increasing BisGMA content in the organic matrix due to it’s higher molecular weight, and viscosity. A linear correlation between the shrinkage-strain and DC of the investigated composites has been obtained. Then we confirmed that direct relationships linked these two properties. In perspective, the effect of the filler loading and monomer concentrations on other properties such as hardness, glass transition temperature (Tg ) and shrinkage-stress will be carried out in future works.
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[1] Asmussen E, Peutzfeldt A. Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites. Dent Mater 1998;14:51–6. [2] Ahn KD, Chung CM, Kim YH. Synthesis and photopolymerization of multifunctional methacrylates derived from Bis-GMA for dental applications. J Appl Polym Sci 1999;71:2033–7. [3] Sideridou I, Tserki V, Papanastasiou G. Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins. Biomaterials 2003;24:655–65. [4] Sideridou I, Tserki V, Papanastasiou G. Effect of chemical structure on degree of conversion in light-cured dimethacrylate-based dental resins. Biomaterials 2002;23:1819–29. [5] Lovell LG, Stansbury JW, Syrpes DC, Bowman CN. Effects of composition and reactivity on the reaction kinetics of dimethacrylate/dimethacrylate copolymerizations. Macromolecules 1999;32:3913–21. [6] Atai M, Watts DC. A new kinetic model for the photopolymerization shrinkage-strain of dental composites and resins. Dent Mater 2006;22:1–7. [7] Atai M, Watts DC, Atai Z. Shrinkage strain-rates of dental resin-monomer and composites Systems. Biomaterials 2005;26:5015–20.
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[8] Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review. Dent Mater 2005;21:962–70. [9] Watts DC, Hindi AA. Intrinsic ‘soft-start’ polymerization shrinkage-kinetics in an acrylate-besed resin-composite. Dent Mater 1999;15:39–45. [10] Gayosso CA, Santana FB, Ibarra JG, Espinola GS, Martinez MAC. Calculation of contraction rates due to shrinkage in light-cured composites. Dent Mater 2004;20:228–35. [11] Baroudi K, Saleh AM, Silikas N, Watts DC. Shrinkage behaviour of flowable resin-composites related to conversion and filler-fraction. J Dent 2007;35:651–5. [12] Stansbury JW, Dickens SH. Determination of double bond conversion in dental resins by near infrared spectroscopy. Dent Mater 2001;17:71–9. [13] Sakaguchi RL, Wiltbank BD, Shah NC. Critical configuration analysis of four methods for measuring polymerization shrinkage strain of composites. Dent Mater 2004;20:388– 96. [14] Watts DC, Cash AJ. Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 1991;7:281–7. [15] Carmichael AJ, Gibson JJ, Walls AW. Allergic contact dermatitis to bisphenol-A-glycidyldimethacrylate (Bis-GMA) dental resin associated with sensitivity to epoxy resin. Br Dent J 1997;8:8–297. [16] Hansel C, Leyhausen G, Mai UE, Geurtsen W. Effetcs of various resin composite (co) monomers and extracts on two caries-associated micro-organisms in vitro. J Dent Res 1998;1:7–60. [17] Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6. [18] Dewaele M, Truffier-Boutry D, Leloup G, Devaux J. Volume contraction in photocured dental resins: the shrinkage–conversion relationship revised. Dent Mater 2006;22:359–65. [19] Kim S, Jang J. The measurement of degree of conversion for BisGMA/silica composites by FT-IR spectroscopy. Polym Test 1996;15:559–71. [20] Ferracane JL, Greener EH. Fourier transform infrared analysis of degree of polymerization in unfilled resins—methods comparison. J Dent Res 1984;8:1093– 5. [21] Stansbury JW, Dickens SH. Determination of double bound conversion in dental resins by near infrared spectroscopy. Dent Mat 2001;17:71–9. [22] Imazato S, McCabe JF, Tarumi H, Ehara A, Ebisu S. Degree of conversion of composites measured by DTA and FTIR. Dent Mater 2001;17:178–83. [23] Amirouche A, Mouzali M, Watts DC. Radiopacity Evaluation of BisGMA/TEGDMA/opaque mineral filler dental composites. J Appl Polym Sci 2007;104:1632–9. [24] Halvorson RH, Erickson RL, Davidson CL. The effect of filler and silane content on conversion of resin-based composite. Dent Mater 2003;19:327–33. [25] Watts DC, Cash AJ. Kinetic measurements of photo-polymerization contraction in resins and composites. Meas Sci Technol 1991;2:788–94. [26] Watts DC, Marouf AS. Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials. Dent Mater 2000;16:447–51. [27] Weinmann W, Thalacker C, Guggenberger R. Siloranes in dental composites. Dent Mater 2005;21:68–74. [28] Atai M, Nekoomanesh M, Hashemi SA, Amani S. Physical and mechanical properties of an experimental dental composite based on a new. Dent Mater 2004;20:663–8.
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[29] Peutzfeldt A. Resin composites in dentistry: the monomer system. Eur J Oral Sci 1997;105:97–116. [30] Gertsten W. Substances released from dental resin composites and glass ionomer cements. Eur J Oral Sci 1998;106:687–95.
[31] Garoushi S, Vallitu PK, Watts DC, Lassila LVJ. Effects of nanofiller fractions and temperature on polymerization shrinkage on glass fiber reinforced filling material. Dent Mater 2007;24:606–10.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites Amel Amirouche-Korichi a,b , Mohamed Mouzali b,∗ , David C. Watts c a b c
CRAPC, BP 248, Alger RP 16004, Algiers, Algeria LPCMAE, Faculty of Chemistry, BP32 El-Alia, Algiers, Algeria Biomaterials Science Research Group, School of Dentistry, University of Manchester, Manchester M15 6FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The degree of conversion (DC) and polymerization shrinkage of resin compos-
Received 18 January 2009
ites are closely related manifestations of the same process. Ideal dental composite would
Accepted 4 June 2009
show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and
Keywords:
monomer ratios on the DC and the shrinkage-strain of experimental composites based on
Restorative composites
(BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the
Opaque mineral fillers
shrinkage-strain and the degree of conversion values was also investigated. The radiopacity
Polymerization Shrinkage-strain
of these experimental composites has been investigated in a previous paper.
Degree of conversion
Methods. Experimental resin composites were prepared by mixing different monomer
Viscosity
ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacry-
BisGMA
late (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La2 O3 ,
TEGDMA
BaO, BaSO4 , SrO and ZrO2 at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm2 , was measured using the bonded-disk technique at room temperature with respect to time. Results. The result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated. Significance. The nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites. © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +213 21247951; fax: +213 21247951. E-mail address: [email protected] (M. Mouzali). 0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2009.06.009
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1.
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
Introduction
Visible light-curable polymeric composites are now routinely used as filling materials for dental restorations. Generally, they consist of mineral fillers dispersed in an organic resin matrix which consists of a multifunctional methacrylate, a dimethacrylate diluent, and a photo-initiating system [1]. Dental composites have many advantages such as mechanical properties equivalent to commercial dental amalgams and dental ceramics, an excellent esthetic quality and the ability to bond to enamel surface [2]. Properties of the dental composites are greatly influenced not only by the properties of their fillers but also by the chemical structure of the monomers used in the matrix phase [2,3]. If the organic matrix is used unfilled, the restoration would exhibit a low wear resistance. Thus, the increased filler content tends to improve mechanical properties and to reduce curing shrinkage and the thermal expansion coefficient [4]. The 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy) phenyl] propane: BisGMA is most commonly used as the organic phase of dental restorative materials because of its high strength and hardness [5]. The rigid aromatic backbone structure of BisGMA, based on bisphenol A, provides different desirable properties such as less shrinkage, higher modulus and reduced toxicity due to its lower volatility and diffusivity into tissues [4]. However, the drawback of this monomer is its very high viscosity, owing to hydrogen bondings between the hydroxyl groups, which limited the incorporation of inorganic fillers and hence a low final DC [6]. Therefore, a reactive diluent, such as triethylene glycol dimethacrylate (TEGDMA) is added to improve the viscosity, reactivity and the final conversion of the matrix phase [1]. Nevertheless, because of favorable stereochemistry of TEGDMA and, its relatively low molecular weight, the polymerization of the corresponding material results in high shrinkage and degree of conversion as well as low mechanical properties [3]. Since BisGMA has greater viscosity than TEGDMA because of the stiff central core and hydroxyl groups in its chemical structure, the relative concentration of each monomer can have an important effect on the mobility and thus kinetics of the reaction. It has been found [7] that the ratio of BisGMA in the organic matrix has a significant effect on the mechanical properties of the resin composites. During the polymerization process, monomers react to form a covalent bond. The distance between the molecules is reduced leading to free volume reduction, which results in volumetric shrinkage in the final polymer network [8]. Shrinkage-strain of restorative materials constitutes a major challenge to polymer chemists and dental scientists [9]. The initial shrinkage-strain rises rapidly during the initial solidification process causing failures in the dental composite restorations [7,3]. Clinically, the shrinkage-strain, manifests as stress, may damage the bonding or cause deflection of the surrounding tooth structure [10]. It may also cause fractures of the tooth that results in microleakage and recurrent caries [10–12]. The volumetric shrinkage of a dental composite is determined by its filler volume fractions and the composition of the resin matrix [8]. The measurement of the polymerization shrinkage of dental composites can be obtained using different methods
such as dilatometry; optical; bonded-disk; linear displacement; strain gage [13]. The bonded-disk method, which measures dimensional change along a single axis, does not require extensive and expensive instrumentation. The shrinkage is measured indirectly by monitoring the deflection of a glass coverslip, in contact with the surface of the composite [13,14]. Theoretically, during the polymerization reaction, all the molecules of monomers would be converted to polymer. However, dimethacrylate monomers exhibit important residual (C C) bonds in the final product, with DC ranging from 55 to 75% under conventional irradiation conditions [11,12]. Different studies [3,15,16] found that the release of unreacted monomers that remains in the restoration may stimulate the growth of bacteria around the restoration and promote allergic reactions in some patients. They may also act as a plasticizer and decrease the mechanical properties of the system [5]. The volumetric shrinkage of composites has been shown to be proportional to its degree of conversion [8,17]. It has been found [18] that an ideal composite exhibits a minimal polymerization shrinkage with an optimal degree of conversion. As an increase of monomer conversion leads to the increase of polymerization shrinkage, these seem to be antagonistic. DC, which is associated with many factors like cure time, matrix composition, light intensity, filler content, diluent concentration and initiator concentration, has a large role in determining the physical and mechanical properties of the material such as hardness, tensile strength, compressive strength, etc. [19,20]. Among several methods to determine DC of dental composites, Fourier transform infrared spectroscopy (FTIR) has been widely used as a reliable method due to the availability of equipment and numerous sampling techniques. This method detects the (C C) stretching vibrations, centered around 1638 cm−1 , directly before and after curing of materials [21,22]. The objective of this study was to provide an experimental opaque materials and to examine the effect of the nature, volume and concentration of five radiopacifying fillers: La2 O3 , BaO, BaSO4 , SrO and ZrO2 on the degree of conversion and the shrinkage-strain of experimental composites based on the mixture (BisGMA/TEGDMA) at different ratios: (50/50), (25/75) and (75/25). The radiopacity of these experimental composites was investigated in a previous study [23]. The aim of this work was also focused on obtaining an optimal formulation of (Bis-GMA/TEGDMA) mixture, regarding the DC [24] and the shrinkage-strain of the experimental composites studied.
2.
Materials and methods
2.1.
Materials
2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl] propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA), were obtained from Aldrich (France). Camphoroquinone (CQ) and N,N -dimethyl aminoethyl methacrylate (DMAEMA) were purchased from Fluka (Germany). Five different radiopacifying agents were used: lanthanum oxide (La2 O3 ), barium oxide (BaO), barium sulfonate (BaSO4 ), zirconium oxide (ZrO2 ), and stronsium oxide (SrO), were obtained from Aldrich (France).
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
2.2.
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Methods
Experimental filler composites were prepared by mixing (Bis-GMA/TEGDMA) at different ratios by weight: (50/50), (75/25) and (25/75) as matrix phase. 0.5 wt.% CQ and 0.5 wt.% DMAEMA, as a photo-initiator system, were added to the mixture. The radiopacifying filler powders were then added, in various proportions, into the mixture to provide loadings ranging from 0 to 80 wt.%. Each experimental composite was well blended to obtain a homogenous mixture.
2.2.1.
Degree of conversion measurement
The degree of conversion (DC%) was measured using FTIR spectroscopy (Nicolet 360 Avatar 360 FTIR spectrometer) with an attenuated total reflectance (ATR) sampling accessory (Type Pike miracle ATR, Diamond/ZnSeW/Pike technology). The absorbance peaks of the cured and uncured samples were obtained using 32 scans at a resolution of 4 cm−1 . Three specimens (n = 3) were used for each ratio of each material. Each specimen was light polymerized for 40 s using a visible light source (LA 500 Blue light curing light, 500 mW/cm2 , 450–490 nm, Aposa Enterprise). The spectrum of each sample after curing was also obtained. The DC of each specimen was determined from the ratio of absorbance intensities of aliphatic (C C), peak at 1638 cm−1 and normalized against the aromatic (C C) at 1608 cm−1 , as follows [6]:
DC (%) = 1 −
Acaliphatic /Acaromatic
Aualiphatic /Auaromatic
× 100
(1)
where Acaliphatic is the absorbance peak at 1638 cm−1 of the cured specimen, Acaromatic is the absorbance peak at 1608 cm−1 of the cured specimen, Aualiphatic is the absorbance peak at 1638 cm−1 of the uncured specimen and Auaromatic is the absorbance peak at 1608 cm−1 of the uncured specimen.
2.2.2.
Shrinkage-strain measurement
The shrinkage-strain was performed at room temperature using the bonded-disk technique [25]. This method measures an axial shrinkage which was restricted to the vertical thickness dimension only, but with the disc-shape of the specimen, which is bonded to one side of the rigid glass surface; it was suggested [25–27] that the central deflection of the disc is representative of contraction over the top surface of the specimen. So, the fractional linear shrinkage measured is approximately equivalent to the fractional volumetric shrinkage: L ∼ V = L0 V0
Fig. 1 – Degree of conversion vs. mole number of BisGMA in the composite (BisGMA/TEGDMA), containing 0% opaque filler.
3.
Results
The final values with standard deviations (SD) of the degree of conversion and the shrinkage-strain after 1 h of experimental composites based on (BisGMA/TEGDMA): (50/50), (75/25), (25/75), containing the photo-initiator system and mixed with five different radiopacifying agents: La2 O3 , BaO, BaSO4 , BaO, SrO, ZrO2 at different weight fractions, ranging from 0 to 80 wt.% are summarized in Tables 1 and 2. Figs. 1 and 2 show, respectively, the degree of conversion and the final shrinkage-strain as a function of Bis-GMA content in experimental composites without containing fillers. These figures indicate a significant decrease in shrinkagestrain and DC when the BisGMA concentration increases in the (BisGMA/TEGDMA) mixture. Fig. 3 shows the typical shrinkage-strain curves for the experimental composites (Bis-GMA/TEGDMA): (50/50) with different La2 O3 filler contents. The data showed that the final shrinkage-strain decreased by increasing quantity of opaque
(2)
The specimens were light cured for 40 s using a visible light source (LA 500 Blue light curing light, 500 mW/cm2 , 450–490 nm, Aposa Enterprise). The shrinkage-strain was measured continuously and total shrinkage-strain data of each sample was recorded with respect to time (1 h). Three repetitions (n = 3) were made per specimen.
Fig. 2 – Shrinkage-strain vs. mole number of BisGMA in composite (BisGMA/TEGDMA), containing 0% opaque filler.
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Table 1 – Degree of conversion (%) (standard deviation in parentheses) of samples based on (Bis-GMA/TEGDMA) blended with five radiopacifying agents (salts): BaO, BaSO4 , La2 O3 , ZrO2 , and SrO ranging from 0 to 80 wt.%. Opaque fillers (BisGMA/ TEGDMA) salt (wt.%)
BaO
BaSO4
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
75.73 (0.1) 74.09 (0.10) 73.19 (0.02) 72.54 (0.03) 70.51 (0.17) 68.21 (0.01)
68.94 (0.03) 70.21 (0.15) 67.07 (0.03) 66.23 (0.01) 65.55 (0.02) 64.88 (0.10)
66.07 (0.08) 63.20 (0.06) 61.48 (0.14) 58.26 (0.16) 54.46 (0.07) 50.99 (0.13)
75.73 (0.1) 74.32 (0.08) 68.98 (0.17) 67.67 (0.05) 66.17 (0.01) 66.14 (0.18)
68.94 (0.03) 67.47 (0.17) 66.63 (0.10) 66.22 (0.11) 66.07 (0.13) 63.77 (0.22)
66.07 (0.08) 60.77 (0.1) 60.07 (0.01) 58.29 (0.21) 58.07 (0.18) 56.53 (0.09)
75.73 (0.1) 74.24 (0.15) 72.11 (0.15) 70.55 (0.17) 69.10 (0.09) 66.06 (0.04)
(50/50)
68.94 (0.03) 67.33 (0.02) 67.23 (0.03) 64.24 (0.12) 62.19 (0.1) 61 (0.11)
SrO
ZrO2
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
66.07 (0.08) 61.88 (0.08) 61.01 (0.05) 58.09 (0.15) 57.52 (0.01) 51.04 (0.15)
75.73 (0.1) 74.78 (0.01) 72.98 (0.07) 70.19 (0.04) 68.63 (0.03) 66.44 (0.13)
68.94 (0.03) 68.30 (0.13) 68.0 (0.13) 67.58 (0.07) 66.35 (0.13) 65.80 (0.10)
66.07 (0.08) 63.92 (0.1) 61.93 (0.12) 61.50 (0.13) 60.7 (0.15) 59.43 (0.14)
75.73 (0.1) 73.97 (0.11) 72.46 (0.20) 72.13 (0.07) 71.50 (0.16) 66.11 (0.07)
68.94 (0.03) 67.96 (0.04) 64.25 (0.15) 63.51 (0.16) 58.27 (0.05) 56.90 (0.02)
66.07 (0.08) 64.89 (0.01) 62.89 (0.17) 61.50 (0.14) 57.49 (0.07) 55.66 (0.10)
Table 2 – Shrinkage-strain (%) (standard deviation in parentheses) of samples based on (Bis-GMA/TEGDMA) blended with five radiopacifying agents (salts): BaO, BaSO4 , La2 O3 , ZrO2 , and SrO ranging from 0 to 80 wt.%. Opaque fillers
0 10 20 40 60 80
La2 O3
BaO
BaSO4
SrO
ZrO2
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
(25/75)
(50/50)
(75/25)
9.46 (0.13) 8.5 (0.05) 8.11 (0.03) 7.93 (0.10) 7.93 (0.12) 7.41 (0.08)
8.05 (0.1) 7.41 (0.17) 7.09 (0.17) 6.58 (0.02) 6.10 (0.02) 5.87 (0.13)
5.21 (0.08) 4.70 (0.01) 4.60 (0.04) 4.37 (0.14) 4.09 (0.18) 4.07 (0.10)
9.46 (0.13) 8.46 (0.13) 8.21 (0.10) 8.13 (0.07) 8.12 (0.04) 7.89 (0.13)
8.05 (0.1) 7.69 (0.09) 7.42 (0.15) 6.65 (0.1) 6.24 (0.03) 5.88 (0.04)
5.21 (0.08) 5.13 (0.1) 4.79 (0.03) 4.70 (0.01) 4.41 (0.13) 4.32 (0.06)
9.46 (0.13) 8.43 (0.02) 8.17 (0.01) 8.16 (0.13) 8.11 (0.06) 7.98 (0.17)
8.05 (0.1) 7.63 (0.15) 7.12 (0.14) 6.60 (0.15) 6.5 (0.11) 5.87 (0.15)
5.21 (0.08) 4.92 (0.17) 4.86 (0.01) 4.73 (0.05) 4.40 (0.21) 4.33 (0.09)
9.46 (0.13) 8.6 (0.08) 8.54 (0.2) 8.12 (0.12) 8.08 (0.17) 7.84 (0.16)
8.05 (0.1) 7.45 (0.01) 6.76 (0.09) 6.62 (0.10) 6.13 (0.01) 5.85 (0.13)
5.21 (0.08) 5.16 (0.22) 5.15 (0.13) 5.08 (0.18) 4.73 (0.03) 3.83 (0.1)
9.46 (0.13) 8.46 (0.14) 8.24 (0.11) 8.12 (0.17) 8.10 (0.01) 7.90 (0.01)
8.05 (0.1) 7.54 (0.04) 6.62 (0.06) 6.56 (0.07) 6.40 (0.13) 6.14 (0.12)
5.21 (0.08) 5.04 (0.12) 4.92 (0.13) 4.84 (0.14) 4.63 (0.05) 3.58 (0.13)
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
0 10 20 40 60 80
La2 O3
d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1411–1418
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Fig. 3 – Typical shrinkage-strain curves for composite (BisGMA/TEGDMA): (50/50) with varying La2 O3 filler content (vol.%).
Fig. 5 – Correlation between shrinkage-strain and La2 O3 filler volume fraction in the composite (BisGMA/TEGDMA) at different monomer ratios.
fillers. Similar results have been observed with all the investigated composites. Figs. 4 and 5 show, respectively, the variation of the degree of conversion and the shrinkage-strain with filler volume fraction of investigated composites at different monomer ratios. The results indicate that the DC and the shrinkage-strain were linearly related to the filler contents. A same result has been obtained with all experimentally studied opaque filler. Figs. 6 and 7 represent, respectively, the variation of the degree of conversion and the final shrinkage-strain of composites containing different opaque fillers at various volume fractions added to a mixture (BisGMA/TEGDMA): (50/50). There is no significant difference among the shrinkage values of the composites with different opaque fillers having different kind of heavy metals. Similar results have been observed with monomer ratios of (25/75) and (75/25). Figs. 8 and 9 illustrate the correlation between the final shrinkage-strain and DC of the experimental composite con-
Fig. 6 – Degree of conversion vs. filler volume fraction of composite (BisGMA/TEGDMA), containing different opaque fillers.
Fig. 4 – Degree of conversion vs. La2 O3 filler volume fraction in the composite (BisGMA/TEGDMA) at different monomer ratios.
Fig. 7 – Shrinkage-strain vs. filler volume fraction of composite (BisGMA/TEGDMA) with varying different opaque fillers.
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Fig. 8 – Correlation between shrinkage-strain and degree of conversion of composite (BisGMA/TEGDMA), La2 O3 at various weight fraction fillers ranging from 0 to 80 wt.% and at different monomer ratios.
taining (BisGMA/TEGDMA) at different monomer ratios, mixed with La2 O3 filler at various volume fraction contents. Similar results have been obtained with all investigated experimental composites containing different opaque fillers salts.
4.
Discussion
This study revealed the effect of radiopacifying agents containing different kinds of heavy metals added to a standard organic matrix on the degree of conversion and the shrinkagestrain of composite resin. As it has been shown in Figs. 1 and 2, the degree of conversion and the shrinkage-strain decreased as BisGMA concentration was increased. It has been suggested [28] that varying the relative amounts of the matrix monomers has a significant effect on the shrinkage-strain and consequently on the mechanical properties of dental composites. The reduction in shrinkage-strain with the increasing of the
Fig. 9 – Shrinkage-strain/degree of conversion vs. La2 O3 volume fraction content of composite (BisGMA/TEGDMA), La2 O3 with varying monomer ratios.
BisGMA concentration could be attributed to the fact that BisGMA has a high molecular weight with hydroxyl groups and a rigid aromatic group: bisphenol A in the central part section of its chemical structure which causes much barriers to free rotation around the bonds, while the ether (C–O–C) linkages in the TEGDMA molecule give rise to only slight barriers to free rotation around the bonds [29]. The hydroxyl groups of BisGMA molecules are also capable to form intermolecular hydrogen bonding which restricts sliding of polymer chains, fewer double-bonds are converted, thereby increasing gradually the viscosity of the system with the increasing of BisGMA content in the composite [8]. As a consequence, this restriction of the growing macro-radicals and monomers mobility affects the propagation of the free radicals and thus decreasing the degree of conversion. TEGDMA has a molecular weight lower than BisGMA, in its unpolymerized state. So the presence of more TEGDMA in the mixture leads to more flowable composites and more covalent bonds are created during polymerization. Consequently, the degree of conversion and the shrinkagestrain increase with an increasing content of TEGDMA [18,28]. It has been shown [15,16,20,30] that the presence of unreacted monomers is undesirable, because, they will leach into the surrounding medium, resulting in deterious effects on the mechanical stability, causing allergic reactions and stimulate bacterial growth around restoration. A linear correlation of the degree of and conversion shrinkage-strain with filler volume fraction of investigated composites at different monomer ratios has been shown in Figs. 4 and 5. The results indicate that the progressive decreases of DC and shrinkage-strain were linearly related to the filler contents. Our results are in agreement with those of Garoushi and Atai [6,31]. The mobility of monomer-chain can be restricted by the incorporation of the fillers, leading to decreasing the monomers and radical mobility resulting then in lower conversion and shrinkage. The results indicate also that for a given filler content, the DC and shrinkage increase significantly with the increasing of TEGDMA concentration in the organic matrix. It can be suggested that the filler molecules distribution is more favorable in a flowable system because of the flexibility of polymer chain, so the filler will not take place an important volume in the matrix. As a consequence, DC final increase as well as the total shrinkage-strain. The volume occupied by fillers in the matrix can be observed in Figs. 4 and 6. It can be seen that this volume increases with the increasing of BisGMA concentration. However, Figs. 4 and 5 show that the decrease of DC and shrinkage is not significant with the increasing of filler loading in the composites. It has been suggested [6] that the effect of fillers contents on the mobility of radicals is very low and that the influence of fillers on the conversion is more related to filler particle-size and surface area than filler loadings. The fillers used in our study are all microfillers. As it is shown in the Figs.6 and 7, there is no significant difference in the values of DC and of the final shrinkage strain of the composites with various opaque fillers which have different kinds of heavy metal, although the increase in volume occupied by these fillers, because of the increase in the heavy metal atomic number of the salt, is significant. Similar results have been observed with monomer ratios of (25/75) and (75/25). So, the results indicate that the nature and volume of
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opaque fillers does not affect the DC and the shrinkage-strain of the resulting composites. These results are in agreement with the work of Atai and Watts [6]. The correlation between the final shrinkage-strain and DC of the experimental composite containing (BisGMA/TEGDMA) at different monomer ratios, mixed with La2 O3 filler at various volume fraction contents, has been shown in Figs. 8 and 9. These results indicate a close relationship between DC and shrinkage-strain, with taking care of the antagonistic clinical implications. Similar results have been obtained with all investigated experimental composites containing different opaque fillers salts. It generally has been observed that the higher the DC in resin composites, the higher the shrinkagestrain [17] was obtained. The present work supports this correlation. Fig. 9 confirms that DC and the shrinkage-strain vary with the same proportion.
5.
Conclusions
The degree of conversion and the shrinkage-strain progressively decrease linearly with the increasing of opaque filler contents. However, the effect of the nature and the volume of the opaque fillers on DC and the shrinkage of experimental composites investigated were not significant. The DC and the final shrinkage decreased significantly with increasing BisGMA content in the organic matrix due to it’s higher molecular weight, and viscosity. A linear correlation between the shrinkage-strain and DC of the investigated composites has been obtained. Then we confirmed that direct relationships linked these two properties. In perspective, the effect of the filler loading and monomer concentrations on other properties such as hardness, glass transition temperature (Tg ) and shrinkage-stress will be carried out in future works.
references
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