Effect of microwave disinfection procedures on torsional bond strengths of two hard chairside denture reline materials

Purpose: This study evaluated the potential effects of denture base resin water storage time and an effective denture disinfection method (microwave irradiation at 650 W for 6 minutes) on the torsional bond strength between two hard chairside reline resins (GC Reline and New Truliner) and one heat-polymerizing denture base acrylic resin (Lucitone 199). Materials and Methods: Cylindrical (30 x 3.9 mm) denture base specimens (n = 160) were stored in water at 37°C (2 or 30 days) before bonding. A section (3.0 mm) was removed from the center of the specimens, surfaces prepared, and the reline materials packed into the space. After polymerization, specimens were divided into four groups (n = 10): Group 1 (G1) - tests performed after bonding; Group 2 (G2) - specimens immersed in water (200 ml) and irradiated twice (650 W for 6 minutes); Group 3 (G3) - specimens irradiated daily until seven cycles of disinfection; Group 4 (G4) - specimens immersed in water (37°C) for 7 days. Specimens were submitted to a torsional test (0.1 Nm/min), and the torsional strengths (MPa) and the mode of failure were recorded. Data from each reline material were analyzed by a two-way analysis of variance, followed by Neuman-Keuls test (p = 0.05). Results: For both Lucitone 199 water storage periods, before bonding to GC Reline resin, the mean torsional strengths of G2 (2 days - 138 MPa; 30 days - 132 MPa), G3 (2 days - 126 MPa; 30 days - 130 MPa), and G4 (2 days - 130 MPa; 30 days - 137 MPa) were significantly higher (p < 0.05) than G1 (2 days - 108 MPa; 30 days - 115 MPa). Similar results were found for Lucitone 199 specimens bonded to New Truliner resin, with G1 specimens (2 days - 73 MPa; 30 days - 71 MPa) exhibiting significantly lower mean torsional bond strength (p < 0.05) than G2 (2 day - 86 MPa; 30 days - 90 MPa), G3 (2 days - 82 MPa; 30 days - 82 MPa), and G4 specimens (2 days - 78 MPa; 30 days - 79 MPa). The adhesion of both materials was not affected by water storage time of Lucitone 199 (p > 0.05). GC reline showed a mixed mode of failure (adhesive/cohesive) and New Truliner failed adhesively. Conclusions: Up to seven microwave disinfection cycles did not decrease the torsional bond strengths between the hard reline resins, GC Reline and New Truliner to the denture base resin Lucitone 199. The effect of additional disinfection cycles on reline material may be clinically significant and requires further study. Copyright © 2006 by The American College of Prosthodontists.

Effect of microwave disinfection on physical and mechanical properties of acrylic resins

This study evaluated the effect of microwave energy on the hardness, impact strength and flexural strength of the Clássico, Onda-Cryl and QC-20 acrylic resins. Aluminum die were embedded in metallic or plastic flasks with type III dental stone, in accordance with the traditional packing technique. A mixing powder/liquid ratio was used according to the manufacturer's instructions. After polymerization in water batch at 74°C for 9 h, boiling water for 20 min or microwave energy at 900 W for 10 min, the specimens were deflasked after flask cooling at room temperature, and submitted to finishing. Specimens non-disinfected and disinfected by microwave irradiation were submitted to hardness, impact and flexural strength tests. Each specimen was immersed in distilled water and disinfected in a microwave oven calibrated to 650 W for 3 min. Knoop hardness test was performed with 25 g load for 10 s, impact test was carried out using the Charpy system with 40 kpcm, and 3-point bending test with a crosshead speed of 0.5 mm/min until fracture. Data were submitted to statistical analysis by ANOVA and Tukey's test (α=0.05). Disinfection by microwave energy decreased the hardness of Clássico and Onda-Cryl acrylic resins, but no effect was observed on the impact and flexural strength of all tested resins.

Pretreatment of sugarcane bagasse with microwaves irradiation and its effects on the structure and on enzymatic hydrolysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Evaluation of microwave-assisted pretreatment of lignocellulosic biomass immersed in alkaline glycerol for fermentable sugars production

A pretreatment with microwave irradiation was applied to enhance enzyme hydrolysis of corn straw and rice husk immersed in water, aqueous glycerol or alkaline glycerol. Native and pretreated solids underwent enzyme hydrolysis using the extract obtained from the fermentation of Myceliophthora heterothallica, comparing its efficiency with that of the commercial cellulose cocktail Celluclast (R). The highest saccharification yields, for both corn straw and rice husk, were attained when biomass was pretreated in alkaline glycerol, method that has not been previously reported in literature. Moreover, FTIR, TG and SEM analysis revealed a more significant modification in the structure of corn straw subjected to this pretreatment.Highest global yields were attained with the crude enzyme extract, which might be the result of its content in a great variety of hydrolytic enzymes, as revealed zymogram analysis. Moreover, its hydrolysis efficiency can be improved by its supplementation with commercial beta-glucosidase.

Microwave-Assisted Phospha-Michael Reactions with Internal Alkenes

The atom efficient phospha-Michael reaction between bis 4-methylphenyl phosphine oxide and several activated internal alkenes has been shown to occur under microwave irradiation without added solvent or catalyst. The alkenes used for this study were ethyl 4-nitrocinnamate, two chalcones ((E)-3-(4-methoxy-phenyl)-1-(4- nitrophenyl)-prop-2-en-1-one and (E)-1-(4-methoxyphenyl)-3-(3-nitro-phenyl)-prop-2- en-1-one), and 2-phenylmethylene-propanedinitrile. In the case of ethyl 4-nitrocinnamate, reaction with bis 4-methylphenyl phosphine oxide for sixty minutes at 130 °C yielded the desired phospha-Michael product in a 55% yield after purification. Varying the location of the nitro group on the phenyl rings of the chalcones did not seem to have a large effect on their reactivity. By NMR, both chalcones seemed to react to the same extent when the reaction times and temperatures were held constant. Interestingly, a phospha-Michael reaction was observed at a reaction temperature of 65°C for experiments involving 2- phenyl-methylene-propanedinitrile while the other substrates required a reaction temperature of 130 °C. Similar experiments were carried out with bis mesityl phosphine oxide and two internal alkenes: 2-phenylmethylene-propanedinitrile and ethyl-2-cyano-3- methyl-2-butenoate. These experiments did not yield any of the predicted phospha- Michael products, which suggest steric limitations to the Michael donor for this reaction.

Microwave-Assisted Bond Forming Reactions

ABSTRACT FOR PART I: PHOSPHA-MICHAEL ADDITIONS TO ACTIVATED INTERNAL ALKENES: STERIC AND ELECTRONIC EFFECTS A method for the phospha-Michael addition of bis(4-methyl)phenyl phosphine oxide to activated internal alkenes has been developed. Michael acceptors including cinnamates, crotonates, chalcones, and internal alkenes containing multiple activating groups were all successfully utilized in this reaction. The reaction was fairly tolerant of electron-donating and electron-withdrawing substituents on the Michael acceptor, and moderate to excellent yields (49-96%) of the adducts were isolated. When steric bulk was increased by a second substituent on the -position of the Michael-acceptor the reaction was suppressed. This was usually overcome by adding a second activating substituent to the -position. ABSTRACT FOR PART II: MICROWAVE-ASSISTED ARYLGOLD BOND FORMATION A microwave-assisted method was developed for the formation of arylgold complexes containing (2-Biphenyl)di-tert-butylphosphine (JohnPhos) as the supporting phosphine ligand. Arylboronic acids with increasingly bulky aromatic groups were screened to determine the steric limitations of the reaction. Arylgold complexes (JohnPhos)Au(p-methoxyphenyl), (JohnPhos)Au(2,4,6-trimethylphenyl), and (JohnPhos)Au(4-bromo-10-anthracene) were all synthesized by microwave irradiation at 70ºC in the presence of Cs2CO3 in either THF or iPrOH. Reactions performed with arylboronic acids containing unhindered ortho positions were carried out in THF. Arylboronic acids with substituents on the ortho position required iPrOH as the reaction solvent. Arylboronic acids with extreme steric hindrance on the ortho position of the aryl substituent, 2,4,6-triisopropylpphenylboronic acid, were unreactive. It was determined that increasing the irradiation temperature increased the formation of side products, therefore to promote conversion to the arylgold complex the duration of the reaction time was increased while maintaining a temperature of 70ºC. Arylgold complexes (JohnPhos)Au(p-methoxyphenyl), (JohnPhos)Au(2,4,6-trimethylphenyl), and (JohnPhos)Au(4-bromo-10-anthracene) were synthesized with moderate yields (40-69%).

Heat transfer in microwave heating

Heat transfer is considered as one of the most critical issues for design and implement of large-scale microwave heating systems, in which improvement of the microwave absorption of materials and suppression of uneven temperature distribution are the two main objectives. The present work focuses on the analysis of heat transfer in microwave heating for achieving highly efficient microwave assisted steelmaking through the investigations on the following aspects: (1) characterization of microwave dissipation using the derived equations, (2) quantification of magnetic loss, (3) determination of microwave absorption properties of materials, (4) modeling of microwave propagation, (5) simulation of heat transfer, and (6) improvement of microwave absorption and heating uniformity. Microwave heating is attributed to the heat generation in materials, which depends on the microwave dissipation. To theoretically characterize microwave heating, simplified equations for determining the transverse electromagnetic mode (TEM) power penetration depth, microwave field attenuation length, and half-power depth of microwaves in materials having both magnetic and dielectric responses were derived. It was followed by developing a simplified equation for quantifying magnetic loss in materials under microwave irradiation to demonstrate the importance of magnetic loss in microwave heating. The permittivity and permeability measurements of various materials, namely, hematite, magnetite concentrate, wüstite, and coal were performed. Microwave loss calculations for these materials were carried out. It is suggested that magnetic loss can play a major role in the heating of magnetic dielectrics. Microwave propagation in various media was predicted using the finite-difference time-domain method. For lossy magnetic dielectrics, the dissipation of microwaves in the medium is ascribed to the decay of both electric and magnetic fields. The heat transfer process in microwave heating of magnetite, which is a typical magnetic dielectric, was simulated by using an explicit finite-difference approach. It is demonstrated that the heat generation due to microwave irradiation dominates the initial temperature rise in the heating and the heat radiation heavily affects the temperature distribution, giving rise to a hot spot in the predicted temperature profile. Microwave heating at 915 MHz exhibits better heating homogeneity than that at 2450 MHz due to larger microwave penetration depth. To minimize/avoid temperature nonuniformity during microwave heating the optimization of object dimension should be considered. The calculated reflection loss over the temperature range of heating is found to be useful for obtaining a rapid optimization of absorber dimension, which increases microwave absorption and achieves relatively uniform heating. To further improve the heating effectiveness, a function for evaluating absorber impedance matching in microwave heating was proposed. It is found that the maximum absorption is associated with perfect impedance matching, which can be achieved by either selecting a reasonable sample dimension or modifying the microwave parameters of the sample.

Microwave-Promoted Copper-Free Sonogashira–Hagihara Couplings of Aryl Imidazolylsulfonates in Water

Aryl imidazol-1-ylsulfonates have been efficiently cross-coupled with aryl-, alkyl-, and silylacetylenes in neat water under copper-free conditions at 110 °C assisted by microwave irradiation. Using 0.5 mol% of an oxime palladacycle as precatalyst, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (SPhos, 2 mol%) as ligand, hexadecyltrimethylammonium bromide (CTAB) as additive, and triethylamine (TEA) as base, a wide array of disubstituted alkynes has been prepared in good to high yields in only 30 min.

Microwave-assisted palladium-catalyzed highly regio- and stereoselective head to head dimerization of terminal aryl alkynes in water

A highly regio- and stereoselective oxime palladacycle/imidazolinium-catalyzed head to head dimerization of terminal aryl alkynes in water is presented. The reaction, which is carried out at 130 °C under microwave irradiation in the presence of 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride as ligand, triethylamine as base, and TBAB as surfactant, allows the synthesis of (E)-1,4-enynes as single stereoisomers in good isolated yields.

Microwave-Enhanced Asymmetric Transfer Hydrogenation of N-(tert-Butylsulfinyl)imines

Microwave irradiation has considerably enhanced the efficiency of the asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)imines in isopropyl alcohol catalyzed by a ruthenium complex bearing the achiral ligand 2-amino-2-methylpropan-1-ol. In addition to shortening reaction times for the transfer hydrogenation processes to only 30 min, the amounts of ruthenium catalyst and isopropyl alcohol can be considerably reduced in comparison with our previous procedure assisted by conventional heating, which diminishes the environmental impact of this new protocol. This methodology can be applied to aromatic, heteroaromatic and aliphatic N-(tert-butylsulfinyl)ketimines, leading, after desulfinylation, to the expected primary amines in excellent yields and with enantiomeric excesses of up to 96 %.

Study of the kinetics and mechanism of rapid self-assembly in block copolymer thin films during solvo-microwave annealing

Microwave annealing is an emerging technique for achieving ordered patterns of block copolymer films on substrates. Little is understood about the mechanisms of microphase separation during the microwave annealing process and how it promotes the microphase separation of the blocks. Here, we use controlled power microwave irradiation in the presence of tetrahydrofuran (THF) solvent, to achieve lateral microphase separation in high- lamellar-forming poly(styrene-b-lactic acid) PS-b-PLA. A highly ordered line pattern was formed within seconds on silicon, germanium and silicon on insulator (SOI) substrates. In-situ temperature measurement of the silicon substrate coupled to condition changes during "solvo-microwave" annealing allowed understanding of the processes to be attained. Our results suggest that the substrate has little effect on the ordering process and is essentially microwave transparent but rather, it is direct heating of the polar THF molecules that causes microphase separation. It is postulated that the rapid interaction of THF with microwaves and the resultant temperature increase to 55 degrees C within seconds causes an increase of the vapor pressure of the solvent from 19.8 to 70 kPa. This enriched vapor environment increases the plasticity of both PS and PLA chains and leads to the fast self-assembly kinetics. Comparing the patterns formed on silicon, germanium and silicon on insulator (SOI) and also an in situ temperature measurement of silicon in the oven confirms the significance of the solvent over the role of substrate heating during "solvo-microwave" annealing. Besides the short annealing time which has technological importance, the coherence length is on a micron scale and dewetting is not observed after annealing. The etched pattern (PLA was removed by an Ar/O-2 reactive ion etch) was transferred to the underlying silicon substrate fabricating sub-20 nm silicon nanowires over large areas demonstrating that the morphology is consistent both across and through the film.

Highly efficient and reusable CNT supported iron(II) catalyst for microwave assisted alcohol oxidation

The highly efficient eco-friendly synthesis of ketones (yields over 99%) from secondary alcohols is achieved by combination of [FeCl2{eta(3)-HC(pz)(3)}] (pz = pyrazol-1-yl) supported on functionalized multi-walled carbon nanotubes and microwave irradiation, in a solvent-free medium. The carbon homoscorpionate iron(II) complex is the first one of this class to be used as catalyst for the oxidation of alcohols.

Enhancement of the total phenolic compounds and antioxidant activity of aqueous Citrus limon L. pomace extracts using microwave pre-treatment on the dry powder

The effect of microwave pre-treatment on the levels of total phenolic compounds, flavonoids, proanthocyanidins and individual major compounds as well as the total antioxidant activity of the dried lemon pomace was investigated. The results showed that microwave pre-treatment significantly affected all the examined parameters. The total phenolic content, total flavonoids, proanthocyanidins, as well as the total antioxidant activity significantly increased as the microwave radiation time and power increased (e.g., 2.5 folds for phenolics, 1.4 folds for flavonoids and 5.5 folds for proanthocyanidins), however irradiation more than 480 W for 5 min resulted in the decrease of these parameters. These findings indicate that microwave irradiation time and power may enhance higher levels of the phenolic compounds as well as the antioxidant capacity of the dried lemon pomace powder. However, higher and longer irradiation may lead to a degradation of phenolic compounds and lower the antioxidant capacity of the dried lemon pomace.

THERMOGRAVIMETRIC ALTERATIONS OF CASSAVA STARCH GRANULES INDUCED BY HYDROGEN PEROXIDE, UV RAYS AND MICROWAVE

Starches are applied in several fields of industry. Amylose and amylopectin (natural polymers) constitute the starch in vegetable cells. In some processes native starches cannot support high stress conditions (high temperatures/acidity). Then, modification methods are developed aiming the improving of starch technological utilization. Oxidative modification with H2O2 has been the subject of many researches. UV rays as well microwave irradiation can be used. The aim was to confirm possible thermogravimetric alterations in native cassava starch (A) granules due to a double starch modification: 1st step) H2O2 standard solutions 0.1 mol L-1 (B), 0.2 mol L-1 (C) and 0.3 mol L-1 (D) and UV rays exposure for 1h; 2nd step) microwave irradiation for 5 min. The results of thermogravimetric curves (TG-DTA) show that the behaviors of the starch proprieties were modified. Highlighting, the modified samples C and D showed a decrease on the thermal stability step. This alteration turned them suitable to many field of industry like the paper one.

The conversion of lignocellulosics to levulinic acid

Biomass represents an abundant and relatively low cost carbon resource that can be utilized to produce platform chemicals such as levulinic acid. Current processing technology limits the cost-effective production of levulinic acid in commercial quantities from biomass. The key to improving the yield and effi ciency of levulinic acid production from biomass lies in the ability to optimize and isolate the intermediate products at each step of the reaction pathway and reduce re-polymerization and side reactions. New technologies (including the use of microwave irradiation and ionic liquids) and the development of highly selective catalysts would provide the necessary step change for the optimization of key reactions. A processing environment that allows the use of biphasic systems and/or continuous extraction of products would increase reaction rates, yields and product quality. This review outlines the chemistry of levulinic acid synthesis and discusses current and potential technologies for producing levulinic acid from lignocellulosics.