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Biomaterials & Bioengineering |
Oxygen Inhibition in Dental Resins
M.A. Gauthier1,
I. Stangel2,
T.H. Ellis1 and
X.X. Zhu1,*
1 Department of Chemistry, Université de Montréal, POB 6128, Downtown Station, Montreal, Quebec, H3C3J7, Canada; and
2 BioMat Sciences, 9700 Great Seneca Hwy. #180, Rockville, MD 20850, USA;
Correspondence: * corresponding author, julian.zhu{at}umontreal.ca
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ABSTRACT
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Oxygen inhibits free radical polymerization and yields polymers with uncured surfaces. This is a concern when thin layers of resin are being polymerized, or in circumstances where conventional means of eliminating inhibition are inappropriate. In this study, we tested the hypothesis that viscosity, filler content, and polymerization temperature modify oxygen diffusion in the resin or the reactivity of radical species, and affect the degree of conversion near the surface. Confocal Raman micro-spectroscopy was used to measure monomer conversion from the surface to the bulk of cured resins. Increased viscosity was shown to limit oxygen diffusion and increase conversion near the surface, without necessarily modifying the depth of inhibition. The filler material was shown to increase, simultaneously, oxygen diffusivity and the viscosity of the resin, which have opposite effects on conversion. Polymerization at a temperature above ~ 110°C was shown to eliminate oxygen inhibition.
Key Words: oxygen inhibition • confocal Raman spectroscopy • visible-light cure • filler content • viscosity
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INTRODUCTION
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Typically, commercial dental composites are random copolymers of 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA), filled with various types of inorganic particles. Bis-GMA and TEGDMA are bi-functional methacrylate monomers that harden following a free-radical-induced polymerization reaction. While this type of reaction has several distinct advantages (fast reaction rates, high degrees of monomer conversion, and absence of solvents), it is strongly inhibited by free-radical scavengers such as oxygen (Xia and Cook, 2003). The inhibition resulting from oxygen diffusing from the atmosphere into curing resins is responsible for the inhibited surface layers commonly found on freshly polymerized unfilled resins (Finger et al., 1996; Vallittu, 1999; Yatabe et al., 2001). This is due to the oxidation of radicals into stable species known as peroxides (Reaction 1) (Andrzejewska et al., 1998; Schulze and Vogel, 1998), which have low reactivity toward monomers.
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It is hypothesized that intrinsic resin parameters such as viscosity, filler content, and polymerization temperature may modify how oxygen diffuses into the resin and therefore affect oxygen inhibition.
Since inhibition is proportional to the quantity of oxygen present in the resin during curing, the degree of conversion can therefore be used to quantify inhibition. In the past, Raman spectroscopy has been successfully used to quantify the conversion of the methacrylate group during polymerization (Shin et al., 1993; Claybourn et al., 1994; Pianelli et al., 1999). Profiles of conversion vs. depth are recorded by confocal Raman micro-spectroscopy, which, to our knowledge, is the only non-invasive method that is sufficiently spatially resolved and sensitive to record the profiles shown in this study. We believe the use of this method to be innovative and to provide a more complete picture of inhibition in comparison with other methods (Ruyter, 1981; Finger et al., 1996), due to its ability to quantify conversion from the surface to the bulk, rather than measuring the thickness of inhibition optically.
We prepared filled and unfilled model resins to distinguish the individual contributions of viscosity and filler content to oxygen inhibition, as well as the relative importance of these parameters in oxygen diffusion. The effect of curing temperature on oxygen inhibition was investigated for visible-light and microwave curing, the latter being an emerging method for polymerizing dental resins (Thomas and Webb, 1995; Urabe et al., 1999; Shull et al., 2000). Conventional thermal curing was performed as a control. The impact of these factors on oxygen inhibition will be interpreted through Fickian diffusion of oxygen in the resin.
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MATERIALS & METHODS
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Materials
Bis-GMA (Polysciences, Warrington, PA, USA), benzoyl peroxide, camphorquinone and N,N-dimethyl-aminoethyl methacrylate, TEGDMA, and unsilanized silica (with particle sizes ranging between 0.5 and 10 µm, 80% between 1 and 5 µm) were obtained from Aldrich (Milwaukee, WI, USA) and used as received.
Temperature
The temperature of a Bis-GMA:TEGDMA (1:1, mol/mol) mix containing 0.3 wt% camphorquinone and 0.7 wt% N,N-dimethyl-aminoethyl methacrylate was monitored during visible-light irradiation (cylindrical mould; d x h = 7 x 7 mm) for 60 sec with the use of an Optilux 401 visible-light gun with 400 mW.cm–2 received by the sample. Bis-GMA:TEGDMA (1:1, mol/mol) mixtures containing 0.5 wt% benzoyl peroxide were placed in silicone moulds (5 x 5 x 10 mm), and their temperature monitored during microwave curing (5 min at 450 W, with the magnetron operating at half-duty cycle) and conventional heat-curing at 120°C in an air oven. The temperature of the monomer mixtures during visible-light and thermal curing (n = 4) was monitored with a thermocouple inserted directly into the resin. We monitored the temperature of the microwave-cured sample by stopping the microwave at set intervals and inserting the thermocouple into the resin. In all cases, the initiator was dissolved in chloroform and added to the monomer resin. The chloroform was removed in vacuo, and the resin was transferred to the appropriate mould. The upper surface of the mould was left open to the atmosphere to ensure an inhibited surface on the material when polymerized. All polymers for which conversion profiles were measured were prepared in this fashion. We performed two-way ANOVA (Tukey, p < 0.05; curing method and time set as independent factors) to establish differences in the temperature profiles for the different curing methods. The maximum temperature values for each curing method were compared by one-way ANOVA (Tukey, p < 0.05).
Resin Viscosity
Bis-GMA:TEGDMA mixtures containing 20, 33, 66, and 80 mol% (of total monomers) Bis-GMA were prepared. Filled Bis-GMA:TEGDMA resins (1:1, mol/mol) containing 10, 20, 30, 40, and 50 wt% (of total mass) silica filler were also prepared. The viscosity of the uncured mixtures was measured with a TA Instruments AR2000 Rheometer, with a 40-mm-diameter cone-and-plate geometry (2° cone angle; 55-micron gap, ~ 5 times larger than the greatest filler particle size). The temperature was controlled via an integrated Peltier unit.
Raman Spectra
Raman spectra were acquired on a Renishaw Ramascope (System 3000) equipped with a charge-coupled device detector (CCD), an 1800-lines/mm grating, and a He-Ne laser at 632.8 nm. The spectrometer was coupled with a DMLM Leica microscope with a motorized stage (precision of ± 0.1 µm) and a 100X metallurgic objective. The Ramascope is put into confocal mode with 10-µm slits and a CCD area of 2 x 575 pixels. Depth resolution with this arrangement was found, by a standard method, to be 1.16 µm in air (Hajatdoost and Yarwood, 1996).
Conversion Profiles
We established the degree of conversion at different depths by measuring the decrease of the vinyl stretching band (1640 cm–1) relative to its value in the uncured mixture. The quadrant-stretching vibration of the aromatic ring on Bis-GMA (1609 cm–1) served as an internal standard. This method has been described elsewhere (Shin et al., 1993). Conversion profiles were recorded for 3 samples from each group (n = 3). For each profile, we analyzed the mean conversion values at each depth by one-way ANOVA (Tukey, p < 0.05), to determine the depth at which no further evolution of the degree of conversion occurred. This depth was taken as the transition point between inhibited and bulk regions (thickness of inhibited layer). We then performed two separate two-way ANOVAs (Tukey, p < 0.05; curing method, viscosity, or filler content and depth set as independent factors) to compare the cure profiles in the 0- to 15-micron (inclusive) and 20- to 50-micron regions. These regions were selected based on the one-way ANOVA, which showed the maximum depth of inhibition to be 15 microns.
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RESULTS
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Temperature
The temperature was monitored during visible-light, microwave, and conventional heat-curing (Fig. 1A ). All temperature profiles were found to be statistically different (p < 0.001). The temperature of the visible-light-cured sample rose rapidly to a maximum of 85 ± 3°C, due to the exotherm of polymerization. Microwave curing caused rapid heating, to a maximum temperature of 100 ± 30°C within 30–40 sec into the irradiation. The large uncertainty associated with the sampling method is responsible for the larger error as well as the broadening of the maximum. The increase of temperature leveled off due to the lesser mobility of the molecules once cured (De Clerck, 1987). The temperature rose gradually during conventional heating to a maximum of 138 ± 7°C, due to the inefficient thermal contact between the sample and the oven. The maximum temperatures reached by thermal and microwave curing were equivalent (p  0.05), but differed from that obtained by visible-light-curing (p > 0.05).
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Figure 1. Effect of curing temperature on the degree of monomer conversion near the surface. (A) Temperature measured during microwave (open circles), visible-light (closed squares), and thermal (closed circles) curing (mean ± SD; n = 4 for each method) of an unfilled 50:50 (mol:mol) Bis-GMA:TEGDMA resin. (B) Degree of conversion as a function of depth for the polymers obtained by the preceding 3 methods (mean ± SD; n = 3 for each group).
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Conversion profiles of the polymers obtained by the 3 curing methods (Fig. 1B ) were all statistically different (p < 0.001). The visible-light-cured polymer possessed a surface degree of conversion of 25 ± 5% and a bulk conversion of 90 ± 2%, reached at a depth of 15 microns. Microwave- and thermally cured polymers had constant degrees of conversion of 61 ± 5% and 66 ± 5%, respectively, from the surface through to the bulk.
For the uncured monomer mixtures, viscosity decreased with heating and became the same order of magnitude past 70°C (Fig. 2). The subsequent increase of viscosity around 110–120°C was due to auto-polymerization. The substantial error observed beyond this temperature was attributed to the differences in the reactivities of the oxidized radicals (oxidized inhibitors, chains in growth, etc.) contributing to polymerization.
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Figure 2. Change in viscosity as a function of temperature associated with the re-activation of oxidized radicals for unfilled monomer mixtures containing 20 to 80 mol% Bis-GMA (mean ± SD; n = 3 for each mixture).
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Viscosity
The viscosity of the unfilled monomer mixtures was found to range from 0.04 to 31 Pa.s (Fig. 3A). For the filled mixes, substantial shear-thinning was observed (Fig. 3B). The maximum viscosity value for each plot was considered as the material’s zero-shear viscosity. The filled monomer mixtures possessed viscosities ranging from 0.8 to 1324 Pa.s.
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Figure 3. Viscosity at 25°C measured as a function of shear rate for (A) unfilled monomer mixtures containing 20 to 80 mol% Bis-GMA and (B) filled monomer mixes containing 50 mol% Bis-GMA and 10 to 50 wt% SiO2 (mean ± SD; n = 3 for each mixture).
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Degree of Cure
The transition from inhibited to bulk regions was located at a depth of 15 microns for all unfilled polymers, and for those composites containing 30 wt% filler and less. The composites containing 40 and 50 wt% filler reached their bulk regions at depths of 10 and 5 microns, respectively.
Statistical differences (p 0.05) in the 0- to 15-micron region between the unfilled resins containing 20% and 80% Bis-GMA showed an increase of conversion with viscosity (Fig. 4A). A very slight but non-systematic change of bulk conversion with viscosity (p 0.05) was observed for these polymers. Over an equivalent range of viscosity, the composites (0–30 wt% filler) showed no statistically significant (p > 0.15) differences of conversion in the 0- to 15- or in the 20- to 50-micron regions (Fig. 4B). Above 30 wt% filler, statistically significant differences (p < 0.001) showed an increase of conversion with filler content in the 0- to 15-micron region, but a decrease of bulk conversion.
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Figure 4. Degree of conversion as a function of depth for polymers prepared by photo-polymerizing (A) unfilled polymers containing 20 to 80 mol% Bis-GMA and (B) filled polymers containing 50 mol% Bis-GMA and 0 to 50 wt% SiO2 (mean ± SD; n = 3 for each group).
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DISCUSSION
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Confocal Raman Depth Profiling
Raman depth profiling is seriously affected by refraction and diffraction, within the sample, which decrease spatial resolution with depth. Past studies have concluded that the most accurate way of depth-profiling a large sample would be to section the latter vertically and then scan the microscope laterally along the cut (Everall, 2000a,b). However, at relatively shallow depths (80–90 µm), profiles recorded by depth profiling and sectioning have shown good agreement (Reinecke et al., 2001). We have tested this conclusion for our system, and can therefore state that, for these materials, within the depth range of interest, the loss of spatial resolution with depth did not adversely affect the Raman depth profiles of the investigated materials (see Appendix).
Thermal Activation of Radicals
The activation of oxidized radicals at high temperatures is well-known (Andrzejewska et al., 1998; Schulze and Vogel, 1998) and has been shown to start for Bis-GMA:TEGDMA mixture at 110–120°C by rheology, a substantially more sensitive technique than calorimetry. Of the 3 polymerization methods investigated in this study, visible-light-curing was the only method which was unable to reach the threshold temperature for auto-polymerization (Fig. 1A). This is due to the fact that visible-light-curing does not directly heat the sample, as is the case for the other methods. The lack of inhibition for polymers whose temperatures reached 110°C strongly suggests that post-curing visible-light-cured materials may eliminate surface inhibition. This is an effective means of eliminating oxygen inhibition without modifying the chemical composition of the resin.
Viscosity and Inhibition
For a system at rest, dissolved oxygen is in dynamic equilibrium with the atmosphere. If this equilibrium is displaced by consumption of oxygen (polymerization), diffusion from the atmosphere will occur to re-establish the preceding equilibrium. In previous studies, oxygen-impermeable barriers (glass slides, Mylar strips, etc.) have been used to block the resin/atmosphere interface, thereby eliminating oxygen inhibition (Urabe et al., 1999; Palin et al., 2003). This indicates that the quantity of dissolved oxygen at equilibrium is insufficient to provoke inhibition, and that the replenishment of oxygen during curing causes inhibition. The increase of conversion with viscosity observed for the unfilled resins (Fig. 4A) was a consequence of the lesser mobility of molecules within the material, which lowers the rate of oxygen replenishment in accordance with Fickian diffusion.
The bulk degrees of conversion of ~ 90% for these polymers were higher than those typically found for similar unfilled resins (60–85%; Rueggeberg et al., 1997; Jancar et al., 2000; Halvorson et al., 2003). This was thought to arise from the use of optimal polymerization conditions (initiator concentration, light intensity, irradiation time, sample size), aging of the polymer (Lee et al., 2004), as well as a slight systematic error in the method used to measure conversion, which may be as high as 5% when conversion is high.
Filler Content and Inhibition
Filler material added to the uncured resin complicates the modeling of oxygen diffusion, in that several effects are expected. Filler particles may act as obstacles to oxygen diffusion (Odrobina et al., 2001), may adsorb oxygen onto their surface, but may also allow for diffusion along their surface, thereby providing new pathways for oxygen diffusion, depending on the specific polymer/filler interactions (Lu et al., 2001). Furthermore, filler content is known to affect viscosity dramatically (Fig. 3B).
The lack of statistically significant change of conversion in the inhibited region for the composites containing 30% and less filler—despite an increase of viscosity (Fig. 3), which was shown to provoke a change for the unfilled resins—indicates that a competing phenomenon is acting to reduce conversion in this region. This may be attributed to an increased rate of oxygen diffusion, due to the transport along the filler’s surface (James et al., 1985). Increased oxygen solubility of the uncured resin, due to adsorption of oxygen onto the surfaces of filler particles, may also provoke a decrease of conversion at the composite/atmosphere interface, in accordance with Henry’s law (Kamiya et al., 1990).
The increased conversion and lower depth of inhibition of the composites containing 40 wt% and more filler indicate that, past this point, the polymer/filler interactions which are responsible for the increase in viscosity [and subsequently reduced oxygen transport (Long and Lequeux, 2001; Sereda et al., 2003)] become more pronounced than those phenomena which facilitate oxygen transport, and inhibition is reduced.
While filler content was shown to reduce the depth of inhibition, conversion in the inhibited region was not necessarily modified. This is because the thickness of inhibition was simultaneously affected by the increase of conversion near the surface and its decrease in the bulk. Consequently, the accurate characterization of conversion from the surface to the bulk provided by confocal Raman micro-spectroscopy is useful for characterizing oxygen inhibition and allows for the effects of temperature, viscosity, and the dual (and opposite) effects of filler material to be interpreted in a more profound manner. The insight gained through this study is significant for the preparation of materials with tailored properties in situations where conventional means of oxygen inhibition are inappropriate.
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ACKNOWLEDGMENTS
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This study was supported by FQRNT of Québec and NSERC of Canada. M.A. Gauthier acknowledges graduate scholarships awarded by the preceding organizations.
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FOOTNOTES
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A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org
Received for publication February 23, 2004.
Revision received April 12, 2005.
Accepted for publication May 13, 2005.
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Journal of Dental Research, Vol. 84, No. 8,
725-729 (2005)
DOI: 10.1177/154405910508400808
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ABSTRACT
INTRODUCTION