Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins.

OBJECTIVE: The aim of this study was to compare the photobleaching rate of CQ in different dental resins. METHODS: The photodecomposition rate of CQ/amine system in bis-GMA/TEGDMA, bis-EMA and UDMA polymerizing monomers was evaluated at different light intensities. The photobleaching of the CQ was studied by monitoring the decrease in light absorption as a function of continuous irradiation time. The absorption changes were assessed by recording the transmitted light that passed through samples of monomers containing CQ/amine. RESULTS: Complete photobleaching of CQ was observed in all the monomer tested and the rate constant for the photobleaching was proportional to the radiation intensity. Hydrogen abstraction from amines by the excited CQ state via electron transfer and direct hydrogen abstraction from monomer structures were involved in the CQ photoreduction. CQ was photobleached in the absence of coinitiator in a dimethacrylate monomer containing a carbamate functional group (UDMA). This behavior was attributed to the presence of labile hydrogen atoms in the UDMA monomer. The CQ photobleaching rate constant in UDMA containing CQ/amine was similar to that in UDMA in the absence of amine. Moreover, the efficiency of CQ to photoinitiate the polymerization of UDMA in the absence of amine demonstrated that the radicals derived from the UDMA monomer via hydrogen abstraction are highly reactive toward double bonds. SIGNIFICANCE: CQ photoinitiates the polymerization of the UDMA monomer in the absence of amine and the efficiency of this process is comparable to that of traditional bis-GMA and bis-EMA monomers activated with CQ/amine.

Influence of thermal expansion on shrinkage during photopolymerization of dental resins based on bis-GMA/TEGDMA.

OBJECTIVE: The aim of this study was to assess volume changes that occur during photopolymerization of unfilled dental resins based on bis-GMA-TEGDMA. METHODS: The resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with dimethylamino ethylmethacrylate (DMAEMA) or ethyl-4-dimethyl aminobenzoate (EDMAB). A fibre-optic sensing method based on a Fizeau-type interferometric scheme was employed for monitoring contraction during photopolymerization. Measurements were carried out on 10mm diameter specimens of different thicknesses (1 and 2mm). RESULTS: The high exothermic nature of the polymerization resulted in volume expansion during the heating, and this effect was more pronounced when the sample thickness increased. Two approaches to assess volume changes due to thermal effects are presented. Due to the difference in thermal expansion coefficients between the rubbery and glassy resins, the increase of volume due to thermal expansion was greater than the decrease in volume due to thermal contraction. As a result, the volume of the vitrified resins was greater than that calculated from polymerization contraction. The observed trends of shrinkage versus sample thickness are explained in terms of light attenuation across the path length during photopolymerization. SIGNIFICANCE: Results obtained in this research highlight the inherent interlinking of non-isothermal photopolymerization and volumetric changes in bulk polymerizing systems.

Photopolymerization of N,N-dimethylaminobenzyl alcohol as amine co-initiator for light-cured dental resins.

OBJECTIVE: The present study was carried out in order to assess the suitability of N,N-dimethylaminobenzyl alcohol (DMOH) as co-initiator of camphorquinone (CQ) and 1-phenyl-1,2-propanedione (PPD) in light-cured dental resins. METHODS: DMOH was synthesized and used as co-initiator for the photopolymerization of a model resin based on {2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane} (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA). Experimental formulations containing CQ or PPD in combination with DMOH at different concentrations were studied. The photopolymerization was carried out by means of a commercial light-emitting diode (LED) curing unit. The evolution of double bonds consumption versus irradiation time was followed by near-infrared spectroscopy (NIR). The photon absorption efficiency (PAE) of the photopolymerization process was calculated from the spectral distribution of the LED unit and the molar absorption coefficient distributions of PPD and CQ. RESULTS: DMOH is an efficient photoreducer of CQ and PPD resulting in higher polymerization rate and higher double bond conversion compared with dimethylaminoethylmethacrylate. The PAE for PPD was higher than that for CQ. However, the polymerization initiated by PPD progressed at a lower rate and exhibited lower values of final conversion compared with the resins containing CQ. This observation indicates that the lower polymerization rate of the PPD/amine system should be explained in terms of the mechanism of generating primary radicals by PPD, which is less efficient compared with CQ. SIGNIFICANCE: The DMOH/benzoyl peroxide redox system, has recently been proposed as a more biocompatible accelerator for the polymerization of bone cements based on poly(methyl methacrylate), because cytotoxity tests have demonstrated that DMOH possesses better biocompatibility properties compared with traditional tertiary amines. The results obtained in the present study reveal the suitability of the CQ/DMOH initiator system for the polymerization of light-cured dental composites.

Effect of different photoinitiator systems on conversion profiles of a model unfilled light-cured resin.

OBJECTIVE: The aim of this study was to compare the efficiency of different camphorquinone (CQ)/amine photo-initiating systems for the photopolymerization of a model dental resin based in Bis-GMA/TEGDMA. METHODS: The monomer conversion versus exposure time was measured in resins containing different types and concentrations of photoinitiators and subjected to different irradiation procedures. The conversion profiles during photopolymerization were investigated using near-infrared spectroscopy (NIR). CQ was used in combination with dimethylaminoethylmethacrylate (DMAEMA), ethyl-4-dimethylaminobenzoate (EDMAB), 4-(N,N-dimethylamino)phenethyl alcohol (DMPOH), and N,N-3,5-tetramethylaniline (TMA) at different concentrations. RESULTS: From the conversion profiles, the approximate order of the accelerating ability of the respective amines were EDMAB approximately DMPOH>TMA>DMAEMA. The conversion profiles of all resins were sensitive to the illumination period. For a given exposure time, samples cured by sequential illumination resulted in lower conversion than samples cured by continuous irradiation. These results were attributed to a combination of both photo and thermal effects. NIR demonstrated to be a convenient method to follow the evolution of the monomer conversion during the photopolymerization of dental resins and, consequently, it is of great utility as a method for determining the relative efficacy of different photoinitiator. SIGNIFICANCE: The DMPOH/benzoyl peroxide redox system, has been recently proposed as a more biocompatible accelerator for the polymerization of acrylic resins. The results obtained in the present study reveal the potential usefulness of the DMPOH amine in light-cured dental composites.

Properties of acrylic bone cements formulated with Bis-GMA.

Experimental cement formulations were prepared by replacing part of the methylmethacrylate (MMA) liquid phase of a conventional surgical cement with an equivalent weight of 2,2-bis [4(2-hydroxy-3-methacryloxypropoxy) phenyl] propane (Bis-GMA), which is the reaction product of diglycidyl ether of bisphenol A and methacrylic acid. It was found that up to 50 wt % of the MMA could be replaced by Bis-GMA without reductions in flow characteristics of the precured polymers. Cements containing 20, 30, 40, and 50 wt % of Bis-GMA in the liquid component were prepared. Over this range of Bis-GMA wt %, it was found that, relative to the unmodified cement, the volumetric shrinkage (DV), the peak temperature reached during the polymerization reaction (Tp), and the flexural strength (obtained in three-point bend tests) were each significantly reduced, the flexural modulus (obtained in three-point bend tests) increased significantly, the compressive strength increased slightly, while there were no significant effects on any of the other properties determined, namely, degree of conversion of the monomer during the polymerization reaction and the glass transition temperature. The drops in D(V) and Tp indicate that cements whose liquid monomers are modified using Bis-GMA hold promise for use in anchoring total joint replacements. The increase in the crosslinking density with increasing amount of Bis-GMA renders the polymer matrix more brittle. This feature was considered responsible for the reduced flexural strength.

Theoretical prediction and experimental determination of the effect of mold characteristics on temperature and monomer conversion fraction profiles during polymerization of a PMMA-based bone cement.

The present work is concerned with applications of a kinetic model for free-radical polymerization of a polymethylmethacrylate-based bone cement. Autocatalytic behavior at the first part of the reaction as well as a diffusion control phenomenon near vitrification are described by the model. Comparison of theoretical computations with experimental measurements for the temperature evolution during batch casting demonstrated the capacity of the proposed model to represent the kinetic behavior of the polymerization reaction. Temperature evolution and monomer conversion were simulated for the cure of the cement in molds made of different materials. The maximum monomer conversion fraction was markedly influenced by the physical properties of the mold material. The unreacted monomer acts as a plasticizer that influences the mechanical behavior of the cement. Hence, the same cement formulation cured in molds of different materials may result in different mechanical response because of the differences in the amounts of residual monomer. Standardization of the mold type to prepare specimens for the mechanical characterization of bone cements is recommended. Theoretical prediction of temperature evolution during hip replacement indicated that for cement thickness lower than 6 mm the peak temperature at the bone-cement interface was below the limit stated for thermal injury (50 degrees C for more than 1 min). The use of thin cement layers is recommended to diminish the risk of thermal injury; however, it is accompanied by an increase in the amount of unreacted monomer present in the cured material.

Flexural strength distribution of a PMMA-based bone cement.

Polymethylmethacrylate bone cement containing either no added antibiotic or 0.5 g of Gentamicin was prepared and stored either in air at room temperature or in a 37 degree C water bath for 48 h. An additive-free cement stored in air at room temperature was also tested for purposes of comparison. Following storage the specimens were tested in flexure. Weibull statistics demonstrated to fit the flexural strength distribution of all the materials tested with regression coefficients of at least 0.98. The presence of a BaSO(4) radiopacifier markedly reduced the mean flexural strength and increased the data scatter in the air-stored specimens. On the other hand, the flexural strength of both impregnated and nonimpregnated antibiotic increased when those materials were stored in water at 37 degree C, compared with the same material stored in air, as a consequence of the water ingress. The water-stored antibiotic-impregnated cement displayed lower flexural strength, increased data scatter, and a remarkably higher number of weak specimens compared with the antibiotic-free cement. The influence of the load type on the flexural behavior was studied by testing the air-stored specimens in three-point bending and four-point bending. Cements tested in four-point bending resulted in lower flexural strength than that tested in three-point bending. The ratio of mean strength measured in the different load arrangements was satisfactory, as predicted by the Weibull model.

Influence of pressurization on flexural strength distributions of PMMA-based bone cements.

Flexural strength distributions of standard viscosity and low viscosity bone cements based on Polymethylmethacrylate were obtained by testing the materials in four-point bending according to the ISO 5833 protocol. The cement dough was poured into a mold and was allowed to cure at atmospheric pressure. An additional set of specimens of the standard viscosity cement was prepared under pressure while the cement dough was polymerizing in the mold. Following preparation, test specimens were stored in a 37 degrees C water bath for 48 h. The two-parameter Weibull model, which was used to analyze the data, gave a good representation of the fracture loads distribution. Low viscosity cement displayed a higher mean flexural strength and a slightly lower data scatter than standard viscosity cement. The mean flexural strength of the cement increased about 60% when pressure was applied compared with the same material cured at atmospheric pressure. The Weibull modulus, m, characterizes the scattering in the measured values of strength. For the cement prepared at atmospheric pressure the m value was 8.6 while for the cement cured under pressure it was 12.3, which reveals a reduction in the data scatter. The cement tested in four-point bending displayed lower mean flexural strength compared with the cement tested in three-point bending. The influence of the load type upon the mean flexural strength was satisfactory predicted by Weibull model.