Polymerization kinetics in dental acrylics

Comparison of chemical analysis of residual monomer in a chemical-cured dental acrylic material to an FTIR method

OBJECTIVE

The purpose of this work was to perform quantitative analysis of residual monomer in chemical-cured acrylic using an infrared spectroscopic method and to compare it to an accepted form of quantitative chemical analysis. Identical samples of acrylic were analyzed and compared using Fourier transform infrared (FTIR) spectroscopy with multiple standard additions vs. "wet" chemical analysis of bromination followed by titration.

METHODS

Two 6 g disks from a single mix of cold-cured acrylic (Lang Dental Mfg. Co., Inc.) were prepared and cured in room air for 1 hr using a ratio of 12 g of powder to 8 mL of liquid. One of the cold-cured acrylic disks was dissolved in glacial acetic acid and analyzed according to the "wet" technique described by Smith and Bains (Smith and Bains, 1956). The other disk was dissolved in methyl isobutyrate (MIBT). Five aliquots, zero plus four incremental additions of pure methyl methacrylate (MMA), were prepared from the MIBT solution. Absorption spectra were collected for all five aliquots. The data were plotted with the ratio of the mass of methyl methacrylate added to the mass of aliquot of MIBT solution as the independent variable, and the absorption, corrected for dilution, as the dependent variable. Least squares fit of the data yielded the slope and intercept. The ratio of intercept to slope divided by the weight fraction of the acrylic disk dissolved in MIBT yielded the concentration of methyl methacrylate in the disk.

RESULTS

The plot of absorbance as a function of mass ratio of MMA added to MIBT solution was linear over the concentration range covered (r = 0.999). Analysis of the spectroscopic data revealed a concentration of residual C = C double bonds of 0.32 +/- 0.01 mol/kg. "Wet" chemical analysis of the acrylic yielded a concentration of residual C = C double bonds of 0.288 +/- 0.003 mol/kg. There was significant difference (students t-test, 99% confidence level) between the two techniques with respect to the amount of residual monomer calculated.

SIGNIFICANCE

The multiple standard additions technique works well for the quantitative analysis of small amounts of residual double bonds in the poly(methyl methacrylate) chemical-cured acrylic polymer system, eliminating the need for an internal standard or external calibration.

Effect of amine activators on the properties of chemical cured dental composites

The purpose of this study was to evaluate the reactivity and the effect of concentration of three tertiary amines upon the mechanical properties of a chemical curing dental composite. Chemical cured composite pastes were prepared by keeping peroxide concentration constant at 1 wt% (by weight of resin mixture) and by varying the amine/peroxide molar ratio from 0.25 to 1.5. Composite samples were prepared for all three amine pastes aged for 1, 15, 30, 45, and 60 d stored at 8, 22, and 37 degrees C. The loss in activity of the tertiary amine with time was measured. Changes in compressive strength, diametral tensile strength, and microhardness were also measured. A sharp decrease in working and setting times corresponding to increased activity was noticed with an increased amine content. The activity was found to vary in the order N,N-dimethyl p-toluidine (DMPT) > 2-(4-dimethylaminophenyl)ethanol (DMAPEA) > N,Ndiethanol p-toluidine (DEPT). DMPT is found to be more temperature sensitive than DMAPEA and DEPT. However, DEPT is found to provide better storage stability out of all three amines tested. Each amine was found to possess optimum concentrations at which the mechanical properties showed maximum values. DEPT is preferred for long-term storage stability in chemical-cured dental composites where aging tends to reduce the activity of the amine.

Effects of two amine reducing agents on the degree of conversion and physical properties of an unfilled light-cured resin

The influence of varying concentrations of two amine reducing agents commonly used in commercial light-cured composites, N,N-dimethyl-p-toluidine (DMPT) and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA), on the degree of conversion (DC), Knoop hardness, and compressive and transverse strengths of an unfilled light-cured resin was investigated. The DC obtained from employing two different internal standard peaks, carbonyl (C=O) at 1730 cm-1 and urethane (N-H) at 3350 cm-1, was compared. The resin consisted of 50 wt% triethyleneglycol dimethacrylate (TEGDMA) and 50 wt% 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimeth ylhexane (UDMA) activated with 0.5 mol% camphorquinone (CQ) and varying concentrations of either DMPT or DMAEMA. The DC calculated by use of either C=O or N-H absorption peak as an internal reference showed similar values. For both amines, the physical properties were directly related to DC and appeared to reach maximal values at an amine/CQ molar ratio of 4.0. The DC, however, appeared to reach a maximum at an amine/CQ molar ratio of 3.0. Generally, for the same amine/CQ molar ratios, the polymers formulated with DMAEMA, had greater DCs and better physical properties than those formulated with DMPT.