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Biomaterials & Bioengineering |
Effect of TEGDMA/BisGMA Ratio on Stress Development and Viscoelastic Properties of Experimental Two-paste Composites
A.J. Feilzer1,* and
B.S. Dauvillier2
1 ACTA, Department of Dental Materials Science, Louwesweg 1, 1066 EA Amsterdam, The Netherlands; and
2 Universiteit Utrecht-Debye Institute, Department of Inorganic Chemistry and Catalysis, Sorbonnelaan 16, 3508 TB Utrecht, Netherlands;
Correspondence: *corresponding author, a.feilzer{at}acta.nl
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ABSTRACT
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In this study, we explored the reduction of shrinkage stresses in restored teeth by stimulating viscous flow of adhesive restoratives during curing, by increasing the TEGDMA/BisGMA ratio in the resin of composite restoratives. We studied a series of experimental two-paste composites with different amounts of TEGDMA (30, 50, 70 wt%, respectively) in the resin by mechanical testing, infrared spectroscopy, and dilatometry, to determine how comonomer composition affects the mechanical and chemical properties of the composite during curing. It was found that the polymerization rate of BisGMA-TEGDMA composites is indicative of the viscoelastic behavior during curing: The higher the reactivity, the higher the stiffness and viscosity development. Composites with 50 wt% TEGDMA in the resin displayed the highest maximum polymerization rate. High amounts of TEGDMA in the resin only modestly increased the pre-gel viscous flow (= lowered viscosity) property of composites. Of these composites, high post-gel shrinkage is the decisive factor in high shrinkage stress development.
Key Words: viscoelasticity • shrinkage • stress-strain • composites
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INTRODUCTION
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The clinical success of an adhesive restoration depends on the achievement and maintenance of a tight seal to prevent microleakage. Since developing shrinkage stress may have a detrimental effect on this seal, many attempts are made to minimize these stresses. Shrinkage stress development in the restoration can be low when shrinkage is accompanied by predominant (pre-gel) viscous flow property of the material (Davidson and de Gee, 1984). Previous studies showed that the mechanical behavior of resin-based dental materials during curing is viscoelastic by nature (Hübsch, 1995; Dauvillier et al., 2000, 2001). This behavior is provided by the resin phase as it changes from predominant viscous flow behavior before to predominant solid behavior after curing.
Many of today’s commercially available dental resin composite materials utilize Bisphenol-A-glycidyldimethacrylate (BisGMA) as major monomer in the resin. This viscous, bulky bifunctional monomer has a high reactivity, high molecular weight, undergoes low polymerization shrinkage, and produces a cross-linked, three-dimensional resin network (Peutzfeldt, 1997). Due to the high viscosity of BisGMA, the resin phase of dental composites has to be diluted to enhance the handling of composite pastes. The monomer most often used for this purpose is triethylene glycol dimethacrylate (TEGDMA), a conventional glycol dimethacrylate. However, diluting the composite with TEGDMA has been shown to have less desirable effects on the properties of the resin, since it increases water sorption and polymerization shrinkage (Kalachandra and Kusy, 1991; Kalachandra et al., 1993). On the other hand, the relative ease of flow of diluted resin composites during the early stage of curing may cause lower polymerization shrinkage stresses. It was hypothesized that an increase of the TEGDMA/BisGMA ratio in a resin composite will lead to a more favorable viscous flow property.
The aim of this study was to evaluate the influence of the TEGDMA/BisGMA ratio on the curing shrinkage-stress development, the degree of conversion, and viscoelastic property of experimental two-paste composites restoratives during curing.
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MATERIALS & METHODS
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Preparation of Resin Composites
The experimental two-paste resin composites were made according to the composition given in Table 1 . The chemicals for the preparation of the composites were used as received. The specific composition was chosen to mimic, as close as possible, commercially available conventional dimethacrylate composites. The resins were de-gassed for 15 min under vacuum prior to the determination of density by pycnometry. Silanated glass filler (LAB 7373, Schott glas, Landshut, Germany) was used to load the resin to a content of 70 wt% (50.4 ± 0.4 vol%). The glass filler was silanated by Heraeus Kulzer GmbH & Co. (Wehrheim, Germany) by means of a dry-blending process (Erdrich and Grundler, personal communications, 1999). Density was equal to 2.64 g/cm3 (23°C), and mean particle size was equal to 1.53 µm.
Dynamic Test: Oscillatory Strain Measurement
The mechanical properties of the resin composites during curing were measured with the use of the oscillatory strain test on an automated universal testing machine (H10KM, Hounsfield, Salfords, UK) as described in a previous study (Dauvillier et al., 2001). The freshly mixed resin composite (1:1 w/w) was bonded between opposing steel disks with diameter (d) of 5.4 mm and separated by a distance (h) of 5.0 mm, creating a configuration factor of 0.5 (C = d/2h). During the measurement, the upper disk performed an oscillating sinusoidal deformation with an amplitude of 1.0 µm (= 0.02% strain) and frequency of 0.1 Hz around the original height of the specimen. As composite shrinkage in the axial direction was hindered, the stress response on the oscillating deformation is superimposed on the continuous shrinkage stress development of the composite.
One hour after the start of the experiment, the resin composite was subjected to tensile loading (5 µm/min) until fracture. The measurements were repeated 3 times at room temperature (23 ± 1°C). During the measurement, the data (time, load, and displacement signal) were collected simultaneously (sample rate = 18 datapoints/sec). The measurements were started within 2 min after the composites were mixed.
Volumetric Shrinkage
Volumetric shrinkage measurements (n = 3) were performed by mercury dilatometry at 23.0 ± 0.1°C (De Gee et al., 1981). The conversion of volumetric shrinkage to axial shrinkage was carried out according to the conversion factor as obtained in a previous study (Feilzer et al., 1989).
Parameter Identification on Stress-Strain Data
It was assumed that the mechanical properties of the composites are isotropic, and that the viscoelastic behavior of the composites in the early stage of curing can be described by the Maxwell model (Dauvillier et al., 2001). The axial load and displacement data from the oscillatory strain measurement were converted to normal stress ( ) and strain ( ) data by the use of equations 1 and 2, respectively:
 | (1) |
 | (2) |
in which A is the cross-sectional area of the cylindrical specimen (m2), F the recorded load response of the specimen (N),  L is the displacement recorded by the LVDT transducers at the level of the specimen (m), and L0 the height of the specimen before being cured (m). The shrinkage and dynamic components in the stress data were isolated with the standard Fast Fourier Transform (FFT) smoothing procedure in Origin (version 5.0, Microcal, Northampton, MA, USA). We determined Maxwell’s model (Lakes, 1999) parameters (E = Young’s modulus and  = viscosity) by applying a parameter identification procedure, as described in the Appendix (www.dentalresearch.org), on small time intervals (approx. 10 sec) in the stress-strain data. We calculated the strain for the small time intervals by adding the oscillatory strain of the dynamic experiment to the shrinkage strain, which was considered as a linear function:
 | (3) |
in which (t0) is the strain at the beginning of the small interval, A is the slope of the shrinkage strain (1/s), B is the amplitude, and  the angular frequency (rad/s) of the applied oscillatory strain.
After parameter identification, the Maxwell model was loaded with the calculated parameters and shrinkage strain of the composite so that we could evaluate where in curing time the model simulates the real behavior of the composite. The stress relaxation time ( , = /E), which is the time required for the stress to decrease to 1/e (  37%) of its initial value, was calculated for each material as a function of curing time.
Infrared Spectroscopy
The degree of double-bond conversion in the composites during curing was measured on a Fourier transform infrared (FT-IR) spectrometer (model 165, Biorad, Cambridge, MA, USA), equipped with a Deuterated TriGlycine Sulphate (DTGS) detector. All specimens were measured in a Teflon mold (d = 5.4 mm, h = 5.0 mm) placed directly on the crystal (Golden Gate Single Reflection Diamond ATR, 1050 0 series, Graseby, Smyrna, TN, USA). Absorbance spectra were taken before (individual pastes) and during (mixed pastes) the curing of the resin composites. The kinetic scan was started within 60 sec after the composites were mixed. The double-bond conversion ( ) for each spectrum was determined by the following equation:
 | (4) |
where ct is the ratio of the peak height of the methacrylate double-bond stretch vibration at 1636 cm–1 to the peak height of the internal reference (1,4-disubstituted phenylene) stretch vibration at 1582 cm–1 (Rueggeberg et al., 1990) at time t, and u is the same (mean) ratio for the individual pastes. The static and kinetic scans were measured 3 and 5 times, respectively.
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RESULTS
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The Fig. shows the volumetric shrinkage strain and axial shrinkage stress development during curing for the experimental composites with various BisGMA-TEGDMA weights in the resin. There was no premature debonding from the steel disks, because during tensile loading the fracture always occurred in the specimen, at a stress level approximately 10 times the maximum shrinkage stress, as recorded after 1 hr of curing. The pre-gel/total shrinkage (vol%) ratios for the 30, 50, and 70 wt% TEGDMA composites were, respectively, 0.3/3.5, 0.6/4.6, and 1.1/5.7.
The infrared results are summarized in Table 2. The double-bond conversion rate was determined from the time derivative of the mean conversion-time data. Student’s t test with pooled variance (p < 0.05) demonstrated that the maximum amount of double-bond conversion after 1 hr of curing was similar for the 50 and 70 wt% TEGDMA composites.
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Table 2. Summary of the Infrared Results for Two-paste Composites in Which the Weight Composition of BisGMA-TEGDMA in the Resin was Varied
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Table 3 lists the mechanical parameters of the experimental composites. The Maxwell model was used up to 15 min after specimens were mixed, because the model had been proven valid during this curing phase (Fig.).
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Table 3. Mechanical Properties of Experimental Two-paste Resin Composites with Different BisGMA-TEGDMA Composition in the Resin (mean ± SD)
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DISCUSSION
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Shrinkage Stress-Strain
It is often difficult, on the basis of the published literature, to know what has caused measured differences in mechanical properties, because the investigated dental restorative composites were different in several physical and chemical aspects. In this study, only one aspect was varied, i.e., the weight composition of BisGMA and TEGDMA in the resin. The more BisGMA was replaced by TEGDMA, the higher the composite shrinkage development (Fig.). This observation is in agreement with expectations, because the increase of TEGDMA results in the increase of double-bond content in the resin (Table 1 ). Although the presence of more TEGDMA in the resin leads to more flowable composites, as noticeable during handling, it does not provide the desired prolonged predominant viscous state of the composite. At maximum flowability, only 20% of the total shrinkage had been completed before a stress response was registered.
Infrared Spectroscopy
The increase of double-bond conversion with TEGDMA (Table 2) is in agreement with that reported in the literature (Ferracane and Greener, 1986; Venhoven et al., 1993). The conversion after 1 hr of curing was limited to values below 60%, which is in line with the range of 55-80% observed for light-activated dental dimethacrylate resins (Barron et al., 1992; Lovell et al., 1999).
The rise of the double-bond conversion rate to a maximum, even though the amount of monomer is diminishing, is quite common in multifunctional dimethacrylate systems, and is known as auto-acceleration (Challa, 1993). A close look at the maximum conversion rates (Table 2) reveals that an increase of TEGDMA in the resin causes the maximum conversion rate to shift significantly (p < 0.05) to higher values, with an optimum for the composite with 50 wt% TEGDMA in the resin. This is in agreement with results found in experimental light-activated resins, wherein the highest maximum rate is observed in the range of 25 to 50 wt% TEGDMA (Barron et al., 1992; Lovell et al., 1999). The occurrence of a maximum rate at this specific resin composition is attributed to the excellent diluent property of TEGDMA, and the high reactivity of BisGMA (Moore, 1976; Cowperthwaite et al., 1981). In this specific resin composition range, sufficient TEGDMA is present to reduce the initial viscosity of the composite, thereby increasing the mobility of the reactive BisGMA. When the composite contains too much TEGDMA in the resin, then the less-reactive TEGDMA dominates the reaction, resulting in a slower polymerization reaction.
It is striking that the point of curing time at maximum conversion rate is related not to the value of the maximum conversion rate (Table 2), but to the amount of TEGDMA in the resin. The explanation for this observation may be that the composites did not have the same inhibition periods, due to different amounts of inhibitor. Since these monomers were used as received (Table 1), the total inhibitor concentration decreased when BisGMA was replaced by TEGDMA, resulting in shorter inhibiton time periods.
Comparison of the point of time of the maximum conversion rate (Table 2) with the start point of shrinkage stress development (Fig. 1) reveals that the transition from auto-acceleration to auto-deceleration (Challa, 1993) occurs in the pre-gel curing phase of the composite. It was after 200 sec, in which 17% or more of the measured double-bond conversion had been completed, that a stress response was registered. Although in the early stage of polymerization the conversion of monomers into polymeric chains is dominant (Rabek, 1993), it may be expected that up to 17% double-bond conversion, some cross-linking of the polymer chains may have occurred, since both TEGDMA and BisGMA are cross-linking agents. The fact that the composite structure is still capable of flowing predominantly up to 17% double-bond conversion indicates that the build-up of the resin network may proceed with the formation of isolated cross-linked polymer segments, which can slip along one another.
Maxwell Model
The evaluation results of the Maxwell model (Fig.) show that even in the post-gel phase of curing, the composites can still flow permanently for a considerable period of time. This may irreversibly indicate that the interpolymer cross-linking reaction proceeds more slowly than the intrapolymer cross-linking reaction. Diffusion limitation may be the cause of the slow cross-linking reaction between growing polymer segments, as the material becomes stiffer. As soon as all polymer segments are connected to each other, viscous flow is highly restricted to processes such as local re-arrangement in the polymer network and movement of the unreacted vinyl groups, and to porosity in the composites, introduced by mixing and the formation of carbon dioxide (Challa, 1993). From this point in curing time, Maxwell’s model is no longer valid, because it predicts permanent viscous flow. In this study, this point is reached at approximately 15-20 min (Fig.), wherein 40% or more double-bonds have been converted.
Parameter Identification
The analogy between the polymerization rate of BisGMA-TEGDMA composites (Table 2) with their stiffness and viscosity development during curing (Table 3) can be explained by the nature of the monomer subunits in the polymer network. BisGMA is stiffer than TEGDMA, because the aromatic group in the central part of the molecule causes much larger barriers to free rotation about the bonds, while the ether (C-O-C) linkages in the TEGDMA molecule give rise to only slight barriers to free rotation about the bonds (Peutzfeldt, 1997). Furthermore, BisGMA molecules are capable of forming hydrogen bonding, which restricts sliding of polymer chains, thereby increasing the viscosity of the system.
The calculated E, , and values were all positive and increased with curing time (Table 3). No explanation was found for the high relaxation time value for the 30 wt% TEGDMA composite at 240 sec of curing.
The curves in the Fig. and the stress-relaxation time values listed in Table 3 show that for the most reactive BisGMA-TEGDMA composites, post-gel shrinkage is the decisive factor in the shrinkage stress development of the composite. Too much TEGDMA in the resin leads to a higher concentration of double-bond conversion, and thus higher post-gel shrinkage. Since the stress relaxation time values are only slightly lower than for the 50 wt% TEGDMA composite, ultimate shrinkage stress development is higher.
The composite with the lowest content of TEGDMA undergoes low post-gel shrinkage and develops low stress relaxation time values. As a consequence, the shrinkage stress is favorably low, which gives the bond to the cavity wall a chance to form and remain intact. However, due to the restricted mobility of the BisGMA monomer, fewer double-bonds are converted. As a result, relatively more unreacted monomer will be present in the composite. The presence of unreacted monomers is undesirable, because these monomers will slowly leach into the surrounding medium, resulting in deleterious effects on the mechanical stability and biocompatibility of the restoration (Ferracane, 1994; Geurtsen, 1998).
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Figure. (a) Volumetric shrinkage strain and (b) axial shrinkage stress development (— measured,  Maxwell model) of experimental resin composites (C-factor = 0.5) during curing. The onset in shrinkage stress (= gel-point) for the 30, 50, and 70 wt% TEGDMA composites was 192, 180, and 190 sec, respectively. Error bars indicate the standard deviations of the calculated mean (n = 3).
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ACKNOWLEDGMENTS
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The authors are grateful to Prem Pallav for the design and production of the ACTAIntense and to Joost Buijs for writing the application software. This investigation was financially supported by the Netherlands Institute for Dental Sciences.
Received for publication October 15, 2002.
Revision received April 29, 2003.
Accepted for publication June 27, 2003.
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Journal of Dental Research, Vol. 82, No. 10,
824-828 (2003)
DOI: 10.1177/154405910308201012
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ABSTRACT
INTRODUCTION