Charles S. Pinzino

Elements of Light-cured Epoxy-based Dental Polymer Systems

The greatest problem with current dental composite systems is their polymerization shrinkage. Extensive work is being done by many investigators to alleviate this problem. Our approach has been to examine epoxy- and spiro-orthocarbonate (SOC)-based resins. The hypothesis to be tested in this study was that the cure characteristics of experimental visible-light-cured epoxy resin systems are governed by the types and concentrations of co-reactants and activators. Resin samples containing onium salt initiators and a thiozanthone sensitizer were successfully cured by means of either an experimental visible-light irradiation system or a commercially available dental lamp. Test resins consisted of di-epoxies alone or in combination, epoxy mixtures in combination with an SOC, or an epoxy in combination with a caprolactone-derived polyol. Significant findings were as follows: (a) Resins containing the SOC had longer cure times than their counterparts; (b) the optimum ratios of epoxy to polyol for most rapid cure were 50:50 or 60:40 under conditions tested; (c) resins containing TONE 305 polyol generally were faster to cure than those containing no polyol, or TONES 201 or 310; and (d) a resin mixture was found that had a cure time of 1 to 3 min when irradiated with a commercial dental lamp. Based on this exploratory study, it should be possible for clinically relevant cure times to be achieved for visible-light-cured epoxy-based resins by careful manipulation and optimization of key elements.

Design and Development of Isocyanatoacrylates as Dental Adhesives

During the last 12 years, significant progress has been made in the development of dental adhesive systems. Some of the more promising systems are based on multifunctional structures that contain polymerizable vinyl double bonds and reactive isocyanate groups. The utility of compounds with such structures as adhesives arises in part because their isocyanate functionality is available for reaction independently, without compromising the reactivity of the vinyl groups. The hypotheses tested in this investigation were: (1) that the monomer reactivity ratios (r1, r2) for the free-radical-initiated copolymerization of ethyl a-isocyanatoacrylate (a-EIA) and 2-isocyanatoethyl methacrylate (IEM) with selected vinyl monomers can be determined; (2) that these reactivity ratios can be used to establish Q (reactivity) and e (polarity) values for a-EIA and IEM; and (3) that these reactivity parameters can be useful in designing copolymers with controlled compositions for dental adhesive applications. The free-radical copolymerization characteristics of a-EIA and IEM were studied. The isocyanate monomers were copolymerized at seven comonomer ratios with n-butyl acrylate (NBA), methyl methacrylate (MMA), and styrene (STY). Reactivity ratios, r1 and r2, were calculated for each of the copolymer systems, giving: IEM (r1) = 0.38 and STY (r2) = 0.44; IEM (r1) = 1.19 and MMA (r2) = 0.84; IEM (r1) = 2.50 and NBA (r2) = 0.40; a-EIA (r1) = 2.20 and STY (r2) = 0.06; α-EIA (r1) = 7.00 and MMA (r2) = 0.10; and a-EIA (r1) = 23.50 and NBA (r2) = 0.04. The Q (reactivity) and e (polarity) values for IEM and a-EIA were calculated from r1 and r2 with use of the Alfrey-Price equations, giving, for IEM, Q = 0.89 and e = 0.60, and, for a-EIA, Q = 7.64 and e = 0.74. These reactivity parameters are useful for tailoring copolymers with controlled compositions and properties. Based on these calculated reactivity parameters, several copolymers of IEM [for example, IEM/2-hydroxyethyl methacrylate (HEMA)] are currently being prepared and evaluated as adhesives.

Photoreactivity of expanding monomers and epoxy‐based matrix resin systems

The relative photoreactivity of five expanding monomers (EMs) in homopolymerization, and as comonomers in a candidate low shrinkage dental matrix resin, were evaluated. The EMs were 2,8-dimethyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DM-TOSU); 3,9-diethyl-3,9-dipropionyloxymethyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DEDPM-TOSU); 1,3-dioxane-2-one (DOO); 4-methyl-1,3-dioxane-2-one (M-DOO); and 5,5-diethyl-1,3-dioxane-2-thione (DE-DOT). The candidate low shrinkage resin system was an 80/20 mixture of UVR-6105 epoxide/polytetrahydrofuran (Mn ≈ 250). All reaction mixtures contained a diaryliodonium salt as a photoinitiator and camphorquinone as photosensitizer. Reactivities were evaluated using photodifferential scanning calorimetry. For homopolymerizations, the reactivity ranking (based on time to exotherm peak and total enthalpy) was DE-DOT ≫ DM-TOSU > DOO > M-DOO ≥ DEDPM-TOSU. In the comonomer system, the reactivity ranking was M-DOO > DEDPM- TOSU > DM-TOSU > DOO ≥ DE-DOT. This experimental work was substantiated and extended by molecular modeling studies employing the AM1 semiempirical method. Heats of formation of protonated EM structures, and heats of formation and potential energies of possible polymerization pathways were estimated. The relative reactivities of EM-based polymerization systems are related to chemical structure and the dominance of the most favored reaction mechanism.

Design, formulation, and evaluation of isocyanatoacrylate copolymer dental adhesives

Experiments have recently been completed to explore the development of isocyanatoacrylate copolymers as new dental adhesives. A main goal of this work was to test the utility of solubility parameter differences between the candidate adhesives and etched dentin as a predictor of relative bond strength. All candidate adhesive mixtures contained 2-isocyanatoethyl methacrylate (IEM), a selected amount of tri-n-butylborane oxide (TBBO) initiator, and one of 13 methacrylate comonomers. Reactivity ratios were computed for comonomer pairs as indicators of relative reactivity. The concentration of TBBO was optimized for each comonomer mixture to obtain working times of 2-6 min and setting times of 6-10 min. The solubility parameter difference Deltadelta (J/cm(3))(1/2) was calculated for each test mixture with respect to an etched dentin substrate, as an approximation of wetting ability. Using standard techniques for shear bond strength evaluation, mean shear bond strength values ranging between 7-15.5 MPa were obtained for comonomer adhesives in bonding Z-100 composite to treated dentin. Shear bond strength values showed a good correlation (r = -0.612, P </= 0.05) with solubility parameter differences. This study illustrates the usefulness of reactivity ratios and solubility parameters in the design and development of effective dentin bonding agents.

Bulk polymerization and characterization of isocyanatoacrylate copolymers

The purpose of this investigation was to prepare by bulk polymerization six new isocyanatoacrylate copolymers and to characterize them. The isocyanatoacrylate copolymers, which were prepared by tri-n-butylborane oxide (TBBO)-initiated free-radical polymerization, were formed from 1 : 1 mol mixtures of 2-isocyanatoethyl methacrylate (IEM) with 2-hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate (HPMA), 2-hydroxyethyl thiomethacrylate (HETMA), ethylthioethyl methacrylate (ETEMA), 2-(acetoxyacetoxy)ethyl methacrylate (AAEMA), and tetrahydrofurfuryl methacrylate (THFMA). These six copolymers were compared to the homopolymer of IEM, which was polymerized in an identical fashion. The bulk polymers were fractionated into their acetone-soluble and acetone-insoluble components. Physical characterization via photoacoustic infrared (PASIR) spectroscopy showed vast differences in residual isocyanate content. Differential scanning calorimetry (DSC) thermal analysis was carried out on all polymers. Elemental analysis (nitrogen) determined the ratio of IEM to the comonomer and boron analysis showed whether the initiator stayed in the acetone-insoluble fraction or was extracted into the acetone-soluble fraction. In conclusion, we found that the composition of the copolymers correlated well with the predicted design.