Influence of the processing technique on the flexural fatigue strength of denture base resins: an in vitro investigation.

BACKGROUND: Though acrylic resins possess many desirable properties, denture fracture due to flexural fatigue or impact failure is a common problem. One major factor influencing the flexural fatigue strength of denture base resins is the processing technique used. AIM: To measure the flexural fatigue strength of denture base resins polymerized using short and long curing cycles using water bath, pressure cooker, and microwave polymerization techniques. MATERIALS AND METHODS: Flexural fatigue strength of 60 samples (n=10) were measured using a cyclic 3-point loading method on a dynamic universal testing machine. Data were analyzed using a Student 't' test. RESULTS: Comparative evaluation using Student's 't' test of mean flexural fatigue strength of samples processed by water bath processing (660.6) and the microwave technique (893.6) showed statistically significant (P < 0.01) result with microwave processing being higher. Comparison of water bath (660.6) and pressure cooker (740.6) processing and microwave (893.6) and pressure cooker (740.6) processing using Student's 't' test was not statistically significant (P > 0.05). In the intra-group analysis, it was found that there was statistically significant difference in samples processed using the short and long curing cycle, the latter being better in all groups, P-values being < 0.05, < 0.001, and < 0.001 for water bath, microwave, and pressure cooker polymerization techniques, respectively. CONCLUSION: The polymerization procedure plays an important role in influencing the flexural fatigue strength of denture base resins, and the microwave long curing processing technique produced denture bases with highest flexural fatigue strength.

Fabrication and performance of fiber electrophoresis microchips.

A method based on the in situ polymerization of methyl methacrylate (MMA) has been developed for the rapid fabrication of a novel separation platform, fiber electrophoresis microchip. To demonstrate the concept, prepolymerized MMA molding solution containing a UV initiator was sandwiched between a poly(methyl methacrylate) (PMMA) cover plate and a PMMA base plate bearing glycerol-permeated fiberglass bundles and was exposed to UV light. During the UV-initiated polymerization, the fiberglass bundles were embedded in the PMMA substrate to form fiberglass-packed microchannels. When the glycerol in the fiberglass bundles was flushed away with water, the obtained porous fiberglass-packed channels could be employed to perform electrophoresis separation. Scanning electron micrographs (SEMs) and microscopic images offered insights into the fiber electrophoresis microchip. The analytical performance of the novel microchip has been demonstrated by separating and detecting dopamine and catechol in connection with end-column amperometric detection. The fiber-based microchips can be fabricated by the new approach without the need for complicated and expensive lithography-based microfabrication techniques, indicating great promise for the low-cost production of microchips, and should find a wide range of applications.

Electron-beam irradiation of experimental denture base polymers.

OBJECTIVE: Since the properties of polymers can be influenced using electron-beam irradiation, the aim of this study was to investigate whether electron-beam post-curing can improve the mechanical properties of experimental denture base polymers. MATERIAL AND METHODS: Rectangular specimens of different experimental polymeric blends were electron-beam irradiated (post-cured) with 25 kGy and 200 kGy using an electron accelerator of 4.5 MeV. Fracture toughness, work of fracture, Vickers hardness and colour changes were measured and compared in non-irradiated specimens. RESULTS: The mechanical properties of all the investigated polymers seemed to benefit from low-energy electron-beam irradiation (25 kGy). Using an energy dose of 200 kGy, all blends showed deteriorated mechanical properties resulting from chain breakage. Nevertheless, all investigated polymers had undesirable colour changes after electron-beam irradiation. CONCLUSIONS: Mechanical properties of experimental polymethyl-methacrylate could be changed using electron-beam irradiation. Because of discolorations caused by the irradiation levels investigated, these levels cannot be recommended for practical applications.

Oxidation of methyl methacrylate from semiconductor wastewater by O3 and O3/UV processes.

This study investigated the oxidation of methyl methacrylate (MMA) by sole ozonation and ozone/UV treatments. The semi-batch ozonation experiments were proceeded under different reaction conditions to study the effects of ozone dosage and UV radiation on the oxidation of MMA. The experimental results indicated that both the oxidation of MMA by the sole ozonation and O3/UV processes can completely decompose MMA to form the following intermediates within 30 min reaction time. To increase the applied ozone dosage can significantly raise the removal efficiency of MMA. However, the mineralization of MMA via the direct oxidation reaction of molecular ozone was slow, while introducing the UV radiation can promote the mineralization rate of MMA. In addition, the pH value of the oxidized solution in the O3/UV treatment decreased lower than that in the sole ozonation treatment of about 1 unit. The possible scheme of the decomposition pathway of MMA under the ozonation process is proposed in this study. Formic acid and acetic acid were found to be the main ozonated intermediates.

Graft copolymerization of methyl methacrylate onto Bombyx mori initiated by semiconductor-based photocatalyst.

Methyl methacrylate (MMA) was graft copolymerized onto Bombyx mori fibre (natural silk). The graft copolymerization was carried out by photopolymerization of MMA using semiconductor particles (CdS) as photocatalyst in the presence of visible light. The effect of additives like triethylamine (Et(3)N) and ethylene glycol on graft copolymerization was studied. We have achieved 2-10% graft conversion with 10-20% homopolymer formation. After removal of the homopolymer, the graft copolymer (grafted fibre) was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC). The chemical resistance of grafted fibre was compared with virgin one.

Dendrimer/methyl methacrylate co-polymers: residual methyl methacrylate and degree of conversion.

Dendrimer/methyl methacrylate co-polymers were studied for use in dental composites. The aim was to determine the effects of methyl methacrylate concentration in the resin mixture and polymerization method on the degree of conversion and residual monomer content of the copolymers. Two dendrimers were studied, D12 with 12 reactive methacrylate groups and D24 with 24 reactive groups. The concentration of methyl methacrylate varied from 20 wt% to 50 wt% of monomers. Camphorquinone (CQ) was used as the light-activation initiator and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) as the activator, both in the quantity of 3.0 wt%. Three polymerization methods were used: photo-polymerization, photo-polymerized immediately followed by post-polymerization at 120 degrees C for 15 min, and photo-polymerization followed by postpolymerization after 7 days. The degree of conversion was determined using FT-IR. Residual monomers were extracted with tetrahydrofuran and methanol and analyzed with HPLC. The highest degrees of conversion were 65 and 62%, and the lowest residual monomer contents 1.0 and 1.5% for D12 and D24, respectively. These were measured after heat-induced post-polymerization. For D12, increasing the proportion of methyl methacrylate decreased the degree of conversion and increased the residual monomer content after photo-polymerization. Post-polymerization enhanced the polymerization of the dendrimer co-polymers in respect of degree of conversion and residual monomer content. The present study suggested that the tested dendrimer/methyl methacrylate copolymers require heat-induced polymerization to reach the generally accepted levels of degree of conversion and residual monomers.

Degree of conversion and flexural properties of a dendrimer/methyl methacrylate copolymer: design of experiments and statistical screening.

OBJECTIVES: The aim of this study was to investigate the effect of photosensitive initiator and activator concentrations on the degree of conversion and flexural properties of an experimental photopolymerized dental copolymer containing dendrimer. METHODS: The experimental resin system consisted of a dendrimer with 12 methacrylate groups and methyl methacrylate in a mass ratio of 80:20. The initiator and activator used were camphorquinone and 2-(N,N-dimethylamino)ethyl methacrylate, whose concentrations varied individually from 1 to 4 wt%. The degree of conversion was determined with FTIR spectroscopy, and flexural strength and flexural modulus with the three-point bending test. The experiments were designed and analyzed, and the results plotted with Modde 5.0 software. RESULTS: The highest degree of conversion was obtained with 2.5 wt% initiator and activator concentrations, the highest flexural strength with 2.5 wt% initiator and 1.0 wt% activator concentration, and the highest flexural modulus with 1.0 wt% initiator and activator concentrations. SIGNIFICANCE: The results indicated that the degree of conversion increased, and flexural strength and flexural modulus decreased, with increasing initiator and activator concentrations. The high concentrations of initiator probably inhibited the transmittance of the active wavelengths to the depths of the samples (the inner filter effect), resulting in inhomogeneous conversion and thus decreased mechanical properties.

Influence of different acid conditioners on the tensile bond strength of 4-META/MMA-TBB resin to Er:YAG laser-irradiated bovine dentin.

PURPOSE: This study evaluated the effect of acid conditioners on resin bonding to dentin following irradiation with an Er:YAG laser and investigated the characteristics of resin bonding to the laser-treated dentin. MATERIALS AND METHODS: Extracted bovine teeth were cervically sectioned to expose a dentin surface. After polishing, the dentin was irradiated with an Er:YAG laser. Aqueous solutions of 10% citric acid (10-0) or 10% citric acid/3% ferric chloride (10-3) were then applied to the laser-treated surface as acid conditioners. After the acid treatment, a PMMA rod was bonded to the irradiated dentin using 4-META/MMA-TBB resin, and miniaturized dumbbell-shaped bonded specimens were prepared. These specimens were subjected to tensile testing, and fractured surfaces were observed with field-emission scanning electron microscopy (FE-SEM) to determine the mode of fracture. Additionally, the resin-dentin interfaces were observed under FE-SEM. RESULTS: The tensile bond strength of acid-conditioned bonded specimens was lower than that of specimens not subjected to acid treatment (11.1 MPa) in the laser-irradiated group. No significant difference was observed between 10-0 and 10-3 treatments. 10-3 treatment yielded the highest bond strength (24.6 MPa) in the nonirradiated group, as opposed to only 7.7 MPa in the laser-treated group. Cohesive failure in the dentin was observed in almost all specimens in the irradiated group. Furthermore, a 10- to 30-micron-thick resin-penetrated layer was observed at the interface between the dentin and resin. CONCLUSION: These results suggest that the effect of acid conditioners on resin bonding to dentin differs according to whether the dentin has been laser irradiated or not.