Calibration of FTIR conversion analysis of contemporary dental resin composites F.A. Rueggeberg D.T. Hashinger C.W. Fairhurst Department of Dental Materials School of Dentistry Medical College of Georgia Augusta, GA 30912 Received October 4, 1989 Accepted December 14, 1989 This research was supported by a grant from the Medical College of Georgia Research Institute, Augusta, GA 30912. Dent Mater 6:241-249, October, 1990

Abstract-Composite restorative material has undergone gradual change in composition since its introduction in the 1960's. Early commercial resins were mixtures of BIS-GMAand TEGDMA. Today, these mixtures are still present, but urethane dimethacrylates and large oligomeric structures of BIS-GMAurethanes exist. Because of these changes in composition, the past methods of calculating monomer conversion by means of infrared spectroscopy may need modification. This research investigatesdifferent methods used to formulate calibration curves for determination of monomer conversion by infrared spectroscopy of contemporary commercial composites containing aromatic structures. Conversion calibration procedures using various baseline methods with BIS-GMA, BisphenoI-A/rEGDMA, and a hydrogenated bonding resin were established. Both peak and area infrared absorptions were determined. One particular baseline method proved the best fit to the Beer-Lambert law. BisphenoI-A was found unsuitable as an infrared calibration model for resin composites. The BIS-GMA/TEGDMA calibration model closely simulates conversion values obtained when a hydrogenated commercial resin model was used.

here have been many studies determining the degree of conversion zn dental composite restorative materials by means of infrared analysis (Asmussen, 1982a,b; Ban et al., 1983; Eliades et al., 1987; Ferracane and Greener, 1984; Ferracane, 1985; Rueggeberg and Craig, 1988; Ruyter and Gyorosi, 1976; Ruyter and Svendsen, 1978; Ruyter, 1982; Ruyter and ~ysaed, 1982, 1987; Vankerckhoven et al., 1982). The resin formulation of commercial composite systems has evolved from simple mixtures of BIS-GMA (2,2Bis-[4-(2-hydroxy-3-methacryloxypropyloxy)phenyl] propane) and TEGDMA (triethyleneglycol dimethacrylate) (Asmussen, 1975; Ruyter and Sj~vik, 1981) to very complex oligomers (Ruyter and ~ysaed, 1987). These developments in resin formulation require that the methods used for determining monomer conversion also remain relevant. The methodology of conversion analysis by infrared methods relies upon calculation of the ratio of the aliphatic carbon-to-carbon (C=C) absorption at 1640 cm -1 to the aromatic C = C absorption at 1608 cm -1. The aromatic absorption functions as an internal standard, eliminating the need for determination of cell-path length or control of the contact area of material when attenuated total reflectance (ATR) is used (Conley, 1966; Medeck, 1968; Rabek, 1980). Three chemical combinations have been used as calibration standards to establish the relationship between various concentrations of aliphatic and aromatic C = C groups and their corresponding infrared absorption ratio. These solutions are mixtures of TEGDMA and Bisphenol-A (Ruyter and Svendsen, 1978; Vankerckhoven et al., 1982), TEGDMA and BIS-GMA (Ban et al., 1982; Ruyter and Svendsen, 1978; Vankerckhoven et al., 1982), and proportions of h y d r o g e n a t e d and

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unhydrogenated commercial bonding resin (Vankerckhoven et al., 1982). The infrared spectrum in the region of analysis of these C=C peaks contains many overlapping absorption peaks that may lead to anomalous absorption values as a result of additive peak interaction (Conley, 1966; Rabek, 1980). When C = C peak intensities are determined, the standard baseline technique has been used (Conley, 1966; Rabek, 1980). However, baselines can be drawn in a number of locations (Conley, 1966; Rabek, 1980), and not all researchers draw baselines in a similar manner. Because of the influence of overlapping absorption peaks and differences in establishment of a baseline, there exists the possibility that the relationship between the C = C concentration ratio and the infrared C = C absorption ratio may not be truly linear. For accurate quantitative analysis, either linearity must be maintained in this relationship or the exact functional correspondence must be determined (Conley, 1966; Willard et al., 1965). If solution concentration and infrared absorption are not linearly related, i.e., do not follow Beer's law, the resulting monomer conversion calculation may be inaccurate to the degree the relationship deviates from linearity. If deviation occurs, it may be compensated for by establishment of the precise mathematical nature of the relationship between concentration and absorption, but this necessitates the use of polynomial equations. Conversion calculation is made much more expedient if a linear relationship is obtained, especially one that regresses naturally through the origin (Rabek, 1980). By use of different baseline methods, there also exists the possibility that conversion results may not only be inaccurate, but also may be imprecise when inter-laboratory results are compared. The purpose of this paper is to de-

Dental Materials/October 1990 241

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