| Sign In to gain access to subscriptions and/or personal tools. |
Resin Composite Properties and Energy Density of Light CureDepartment of Dental Materials, School of Dentistry, University of Copenhagen, 20 Nørre Allé, 2200 Copenhagen N, Denmark; Correspondence: * corresponding author, apz{at}odont.ku.dk
According to the ‘total energy concept’, properties of light-cured resin composites are determined only by energy density because of reciprocity between power density and exposure duration. The kinetics of polymerization is complex, and it was hypothesized that degree of cure, flexural strength, and flexural modulus were influenced not only by energy density, but also by power density per se. A conventional resin composite was cured at 3 energy densities (4, 8, and 16 J/cm2) by 6 combinations of power density (50, 100, 200, 400, 800, and 1000 mW/cm2) and exposure durations. Degree of cure, flexural strength, and flexural modulus increased with increasing energy density. For each energy density, degree of cure decreased with increasing power density. Flexural strength and modulus showed a maximum at intermediate power density. Within clinically relevant power densities, not only energy density but also power density per se had significant influence on resin composite properties.
Key Words: degree of cure • exposure duration • mechanical properties • power density
It is well-documented that energy density (power density x exposure duration) of the light cure influences the degree of cure, depth of cure, and mechanical properties of a resin composite (Nomoto et al., 1994; Rueggeberg et al., 1994; Ferracane and Berge, 1995; Unterbrink and Muessner, 1995; Davidson-Kaban et al., 1997; Sakaguchi and Ferracane, 2001; Yap and Seneviratne, 2001; Halvorson et al., 2002; Emami and Söderholm, 2003). Thus, at the same exposure duration, an increase in power density leads to improved cure, as does an increase in exposure duration at the same power density. A given energy density can be delivered with different combinations of power density and exposure duration. Limited and contradictory data have been published from which the effect of variation in this combination may be inferred (Nomoto et al., 1994; Miyazaki et al., 1996; Asmussen and Peutzfeldt, 2001a,b, 2003, 2004; Lovell et al., 2001; Sakaguchi and Ferracane, 2001; Yap and Seneviratne, 2001; Halvorson et al., 2002; Emami and Söderholm, 2003; Musanje and Darvell, 2003). Based on degree-of-cure measurements, one study concluded that a reciprocal relationship exists between power density and exposure duration (Halvorson et al., 2002). However, close examination of the results reveals a statistically significant effect of the combination of power density and exposure duration for each of the 4 energy density levels studied. Moreover, for a given resin composite, different curing modes may result in similar degrees of cure but in polymer networks that differ with respect to cross-link density, and thus with respect to mechanical properties (Asmussen and Peutzfeldt, 2001a, 2003). For photo-initiated free radical systems, it is a result of theoretical kinetics that degree of cure depends on the product of exposure duration raised to the power of 1 and power density raised to the power of 0.5–0.6 (Cook, 1992; Lovell et al., 1999). A power of 0.5–0.6 implies that simple reciprocity between power density and exposure duration does not exist. The hypothesis tested in the present study, therefore, was that degree of cure, flexural strength, and flexural modulus would be influenced by energy density, as well as by the combination of power density and exposure duration. This led to the aim of determining degree of cure, flexural strength, and flexural modulus of a resin composite cured at different energy densities and with different combinations of power density and exposure duration.
Light Exposure The investigation used a commercially available resin composite (Tetric Ceram A3 lot# F59052, Ivoclar Vivadent, Schaan, Liechtenstein) based on conventional monomers (bisGMA, urethane dimethacrylate, and TEGDMA) and a conventional, camphorquinone-amine initiator system. The resin composite was cured with a quartz-tungsten-halogen light source (Optilux 501, SDS Kerr, Danbury, CT, USA). The input voltage to the light source was regulated by means of a rheostat, so that we could obtain 6 different power densities (50, 100, 200, 400, 800, and 1000 mW/cm2) as monitored with a handheld dental-curing radiometer (Model 100, Demetron Research Corporation, Danbury, CT, USA). For each power density, the exposure duration was varied such that 3 different energy densities were obtained (4, 8, and 16 J/cm2). The Table  shows the 18 resulting experimental groups.
Degree of Cure The degree of cure was measured by means of FTIR spectroscopy as previously described (Peutzfeldt et al., 2000). In brief, with the window of the light guide positioned directly on the upper glass plate, a thin layer of resin composite was irradiated between 2 glass plates, according to the above-mentioned curing conditions. The thickness of the resulting films was approximately 40 µm. The diameter of the films was 5 mm, and thus less than the diameter of the light guide, so that the amount of light received by the films was well-defined. The films were stored in air at 37°C at ambient humidity for 1 wk prior to spectroscopy. The absorbance of the C=C double bond at 1640 cm–1 and the absorbance of the aromatic bond at 1610 cm–1 were determined before and after polymerization of the films. By taking the ratio between the 2 absorbances, we could calculate the degree of cure. There were 5 specimens in each experimental group.
Flexural Strength and Modulus Before the test, the height (a, mm) and width (b, mm) of each specimen were measured with a micrometer, and the specimen was then subjected to three-point loading with l = 3 mm between the supports. The radii of supports and plunger were 0.5 mm. The cross-head speed of the testing machine (Instron Universal Testing Machine, High Wycombe, England) was 1 mm/min. Flexural strength (S, MPa) was calculated as
where F in N is the force at fracture (Peyton and Craig, 1971). Flexural modulus (E, GPa) was calculated as
where β is the slope in N/mm of the straight-line relationship between force and deflection of the resulting flexural curve (Peyton and Craig, 1971). Ten specimens were made in each experimental group.
Statistics
For each of the 3 dependent variables (degree of cure, flexural strength, and flexural modulus), the 18 mean values were found to differ with statistical significance (Table  ). Two-dimensional regression analyses—performed between energy density, on the one hand, and degree of cure (r = 0.78), flexural strength (r = 0.87), or flexural modulus (r = 0.76), on the other—found statistically significant regression coefficients (p < 0.0005). For each energy density, degree of cure decreased with increasing power density (p < 0.0005) (Table). For an energy density of 4 J/m2, the lowest and the highest power density resulted in a significantly lower flexural strength than did intermediary power densities (p < 0.0005) (Table). For energy densities of 8 and 16 J/m2, respectively, there were no statistically significant differences among the 6 mean values of flexural strength (p > 0.05) (Table). For each energy density, flexural modulus showed a maximum at intermediate power density (p < 0.0005) (Table).
Higher levels of degree of cure and of flexural strength and modulus were found with increasing energy density, i.e., the higher the energy density, the higher the degree of cure and mechanical properties. This influence of energy density on degree of cure and mechanical properties is in corroboration with the findings of previous studies (Nomoto et al., 1994; Rueggeberg et al., 1994; Ferracane and Berge, 1995; Unterbrink and Muessner, 1995; Davidson-Kaban et al., 1997; Sakaguchi and Ferracane, 2001; Yap and Seneviratne, 2001; Halvorson et al., 2002; Emami and Söderholm, 2003), and with the first part of our hypothesis. The study also found that for each energy density, the combination of power density and exposure duration had a significant influence. When power density was increased, the degree of cure decreased linearly, whereas flexural strength and flexural modulus showed a parabolic relationship, i.e., a maximum was found at an intermediary power density. The finding that the combination of power density and exposure duration had significant influence is in agreement with the results of Musanje and Darvell (2003), and with results that may be inferred from previous studies (Asmussen and Peutzfeldt, 2001a,b, 2003, 2004; Lovell et al., 2001; Yap and Seneviratne, 2001; Halvorson et al., 2002), and leads to acceptance of the second part of our hypothesis. However, the finding is in apparent contrast to several other, previously published, studies. For one study, this discrepancy lies in the fact that the authors failed to acknowledge the significant influence of the combination of power density and exposure duration actually found (Nomoto et al., 1994). In another study, the significant influence of the combination of power density and exposure duration found was explained by inadequate resolution of the measurement of unattenuated power density of the curing unit within the first 10 sec (Halvorson et al., 2002). Other studies included only 1 energy density and fewer combinations of power density and exposure duration than the present study. The restricted number of experimental groups may explain why these studies failed to reveal a significant influence of the combination of power density and exposure duration (Miyazaki et al., 1996; Sakaguchi and Ferracane, 2001; Emami and Söderholm, 2003). Finally, a study concluded that polymer structure and glass transition temperature (Tg) depend on degree of cure and not on the method or rate of cure (Lovell et al., 2001). However, the Tg values determined following different methods of cure (e.g., different combinations of power density and exposure time), although quite close in magnitude, appear to differ with statistical significance, despite similar degrees of cure. It may be that such variations in Tg would correspond to significant variations in mechanical properties, such as found in the present study. The kinetics of polymerization has been found to be highly complex, and a simple reciprocal relationship between power density and exposure duration does not exist. Indeed, degree of cure depends on the product of exposure duration raised to the power of 1 and power density raised to the power of 0.5–0.6 (Cook, 1992; Lovell et al., 1999). This implies that, for a given energy density, long exposure durations at low power density lead to a higher degree of cure than short durations at high power density. The results of this study are in corroboration with this predicted relationship. Mechanical properties are highly influenced by the degree of cross-linking (Sideridou et al., 2003). As power density is increased and exposure duration simultaneously decreased, degree of cure decreases, as discussed above, kinetic chain length also decreases, but frequency of cross-linking increases (Lovell et al., 2001). The effect on mechanical properties of degree of cure and frequency of cross-linking is of the opposite direction, and may explain the parabolic relationship found between power density and flexural strength, and between power density and flexural modulus.
It may be argued that energy density is more important than the combination of power density and exposure duration. However, as is evident from the Table One finding of particular current interest is that high power density used for short exposures led to lower degree of cure, and lower flexural strength and modulus, than did curing with intermediary power densities for longer exposures. This indicates that the curing units representative of the 1990s, which were usually combined with 40-second exposures, provided resin composites with optimal properties. It may be that the high-power-density curing units of today save the clinician and the patient some time, but these curing units may also be expected to result in less than optimally cured resin composite restorations. We used a rheostat to vary input voltage and thus power density. However, varying input voltage not only influences peak intensity (power density) (Cook, 1982; Fan et al., 1987), but also causes a shift in the spectral distribution of the emitted light (see Appendix). For the efficiency of a light-cure protocol to be evaluated, the emission spectrum of the light must be related to the absorption spectrum of the initiator—in this case, camphorquinone. Considering the absorption spectrum of camphorquinone and the shifts in spectral distribution (Fouassier, 1995; Halvorson et al., 2002), the efficiency at high voltage may be somewhat underestimated in the present study; the higher the power density, the more pronounced the underestimation (see Appendix). These underestimations imply that the energy densities obtained by multiplication of power density and exposure duration are more efficient at higher power densities. Another limitation of this study is the tendency of quartz-tungsten-halogen light sources to decrease gradually in power density during functioning (Halvorson et al., 2002). A gradual drop in power density would have greater consequence for long exposure durations than for short exposure durations. This implies that the energy densities calculated for light exposures that use long exposures and low power density may be somewhat overestimated. However, a correction for any effect of the limitations discussed would tend to accentuate the influence of the combination of power density and exposure duration: Compensation for an overestimation of the efficiency of the light exposure at higher power densities would lead to degree-of-cure and flexural properties values even lower than those measured, and compensation for an underestimation of the energy density at lower power densities would lead to degree-of-cure and flexural properties values even higher than those measured, possibly outbalancing the measurable effect of low cross-link density on flexural properties.
This investigation was supported by the School of Dentistry, University of Copenhagen.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication February 27, 2004. Revision received February 22, 2005. Accepted for publication April 22, 2005.
Journal of Dental Research, Vol. 84, No. 7,
659-662 (2005) This article has been cited by other articles:
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
= 0.05. 
