In Search of the Driving Factor for the Microwave Curing of Epoxy Adhesives and for the Protection of the Base Substrate against Thermal Damage

This study used controlled microwaves to elucidate the response of adhesive components to microwaves and examined the advantages of microwave radiation in curing epoxy adhesives. Curing of adhesives with microwaves proceeded very rapidly, even though each component of the adhesive was not efficiently heated by the microwaves. The reason the adhesive cured rapidly is that microwave heating was enhanced by the electrically charged (ionic) intermediates produced by the curing reaction. In contrast, the cured adhesive displayed lower microwave absorption and lower heating efficiency, suggesting that the cured adhesive stopped heating even if it continued to be exposed to microwaves. This is a definite advantage in the curing of adhesives with microwaves, as, for example, adhesives dropped onto polystyrene could be cured using microwave heating without degrading the polystyrene base substrate.     

Green Enzymatic Synthesis of L-Ascorbyl Fatty Acid Ester: An Antioxidant

In green chemistry, biocatalysis using microwaves is a very attractive tool for various regioselective syntheses. L-Ascorbyl fatty acid esters were synthesized by an immobilized lipase from Bulkholderia multivorans using microwaves, with a dynamically enhanced rate of reaction and appreciable yield of around 80%. Microwave radiation had no effect on enzymic inactivation, however, synergism between microwaves and biocatalyst was observed.     

微波干介质反应

微波干介质反应是一门新兴的绿色化学合成技术,已成功地应用于多种有机反应。运用微波干介质反应技术进行一步法或多组分反应研究是当前库合成的研究热点。无溶剂反应具有成本低、操作简便、选择性好等优点,是绿色化学的重要研究方向之一。本文主要介绍了官能团的转化、氧化、还原、杂环化合物的合成、缩合及多组分反应等微波干介质反应。     

The importance of agitation and fill volume in small scale scientific microwave reactors

The effect of agitation and fill volume in a microwave tube on the rate of homogenous reaction solutions under both stirred and unstirred conditions is reported. Significant increases in rate are found in two different microwave instruments when the homogenous reaction mixture is unstirred. These results have significant implications for the continuing use of domestic or kitchen type microwaves in synthetic applications; and in scientific microwaves where stirring may be compromised, for example in the presence of heavy slurries or viscous liquids.     

The analysis of animal bioelectric brain activity influenced by microwaves or by the introduction of strychnine

The widespread impact made by technology has raised concerns about the safety of human exposure to electromagnetic radiation in the environment. The brain is especially sensitive to the influence of microwaves. The most effective method for estimation of the organism's functional states is an analysis of electroencephalograms. The statistical and spectral methods are usually used for analysis of animal electrocorticograms. The information obtained in such way is the integrated character and it is sometimes insufficient for identification of the brain state charging caused by various factors, especially microwaves altering the ecological situation. The nonlinear dynamics method is used in our work concurrent with the spectral correlation method for animal electrocorticogram processing. The correlation dimensionality represents a numerical criterion allowing for comparative investigation of various dynamic states of the system. In the process of investigation, it has been found that the nonlinear dynamics method may be used to analyze the electrocorticograms of experimental animal in different functional states being confirmed by increasing parameter of the correlation dimensionality in electrocorticograms of animal irradiated by microwaves or subjected to the introduction of strychnine.     

Microwave enhanced non-alkaline bleaching of mechanical pulps: A new solution to an old problem

Since the granting of our first US patent on microwave induced catalytic decomposition of PCBs some 18 years ago, microwave technology has slowly begun to attract some industrial attention in the last few years. There was no doubt that microwaves had been applied to facilitate the conventional bleaching process of wood pulps in the past but with little or no success. The myth that microwaves can be regarded and manipulated as a rapid heat source probably caused most of the failures of unsuccessful microwave applications. It is therefore necessary to understand the real potential of microwaves as an energy source and to identify the many critical electromagnetic properties of the load (chemical systems), before a final choice of a proper microwave system be designed and installed. In this preliminary report, we demonstrate the real potential of pulsed microwave radiation for non-alkaline bleaching of mechanical pulps. Typically, very high consistency TMP up to 93% can be treated by pulsed microwave in the order of 90 seconds with an average increase of 20 to 25 points in brightness.     

Microwave Flow Chemistry as a Methodology in Organic Syntheses,Enzymatic Reactions,and Nanoparticle Syntheses

Several studies have used microwaves as a heat source for carrying out various types of reactions employing circulation reaction vessels. The microwave flow chemical synthesis methodology is most appropriate in the use of microwaves in chemical syntheses. It can attenuate the problem of microwave heating (non‐uniform heating and penetration depth) and maximize the benefits (rapid heating and first temperature adjustments). In this brief review, we examine and explain some of the relevant features of microwave heating with applicative examples of the usage of microwave flow chemistry equipment in carrying out organic syntheses, enzymatic reactions, and (not least) nanoparticle syntheses.     

Combined protocol for the analysis of polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBs) from sediments using focussed microwave assisted (FMW) extraction at atmospheric pressure

A microwave-assisted protocol has been developed using focussed microwaves at atmospheric pressure for the extraction of PAHs and PCBs from sediments. It combines extraction and purification assisted by microwaves. This protocol has been applied to the quantification of 12 individual PAHs, 8 individual PCBs and 8 PCB coeluted mixtures from two Standard Reference Materials and one natural sediment. The results for both classes of compounds (PAHs and PCBs) are good in terms of recoveries, which are always greater than 70% and in most cases around 100%. The reproducibility is also good with coefficients of variation below 10% in most cases. This protocol has the great advantage of saving time, the time dedicated to the preparation / extraction assisted by microwaves being reduced to less than half an hour.     

Microwaves and sorption on oxides: a surface temperature investigation

Microwave heating is not the same as conventional heating, and it is believed that this difference, the "microwave effect," may be interpreted to be due to selective, local heating. The temperature at the surface where sorption occurs is "effectively" greater than the measured solid or gas temperature. In these studies, measurements of the amounts of adsorption as functions of the partial pressures of a specific adsorbate in the presence of microwave irradiation were related to the conventional adsorption isotherms. Equating the adsorbate pressure required to achieve a specific coverage (an isostere) in the presence of microwave irradiation to the amount adsorbed for a conventional isotherm allowed for an estimate of the "effective" surface temperature in the presence of microwaves. It was found that the effective surface temperature increased when using adsorbates having a higher permittivity or when increasing the microwave power. The implication of this change in the surface energy for specific species in the presence of microwaves is discussed.     

Combined protocol for the analysis of polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBx) from sediments using focussed microwave assisted (FMW) extraction at atmospheric pressure

A microwave-assisted protocol has been developed using focussed microwaves at atmospheric pressure for the extraction of PAHs and PCBs from sediments. It combines extraction and purification assisted by microwaves. This protocol has been applied to the quantification of 12 individual PAHs, 8 individual PCBs and 8 PCB coeluted mixtures from two Standard Reference Materials and one natural sediment. The results for both classes of compounds (PAHs and PCBs) are good in terms of recoveries, which are always greater than 70% and in most cases around 100%. The reproducibility is also good with coefficients of variation below 10% in most cases. This protocol has the great advantage of saving time, the time dedicated to the preparation / extraction assisted by microwaves being reduced to less than half an hour.     

Porous Calcium Hydroxyapatite as an Efficient Catalyst for Synthesis of Pyrazolines via 1,3‐Dipolar Cycoloaddition Under Solvent‐Free Microwave Irradiation

Adsorbed on porous calcium hydroxyapatite, 1,3‐dipolar cycloaddition of diphenylnitrilimine on olefins is readily catalyzed under solvent‐free microwaves irradiation. The pyrazolines are obtained in few minutes with high yields. Specific surface of porous calcium hydroxyapatite and microwaves effects are discussed.     

Analytical capabilities of the microwave-coupled hollow-cathode discharge in emission spectroscopy

The intensity of spectra emitted by hollow copper cathodes with and without the superposition of 2450-MHz microwaves was studied under different operating conditions with argon as the filler gas. The hollow-cathode discharge was operated at 25–200 mA, with argon pressures ranging from 166 to 690 Pa. The intensities of the copper spectral lines are usually higher in the presence of microwaves, whereas the argon lines are weaker. The differences in the copper spectral lines with and without microwaves increase with increasing pressure of the filler gas and decreasing current. Analogous conclusions were reached for aqueous solutions of gold and silver ions placed in the cathode cavity and dried prior to discharge. The suitability of the method for improving the detection limits obtainable with the hollow-cathode discharge could thus be demonstrated.     

Microwave‐activated Synthesis of Pyrroles: A Short Review

In the current scenario for continuous requirement of better drugs, medicinal chemists must visage the challenging task of preparing novel patentable compounds, combining high activity and selectivity, good drug‐likeness, and pharmacokinetic properties. There is an acute need of new molecules in both lead identification and lead optimization. Chemists thoroughly scrutinize ways to simplify synthetic protocols using green chemistry approaches, for example, microwaves. Reduction in the reaction time by the use of microwaves has emerged as a promising method for bringing out new leads in organic synthesis. Among the azaheterocyclic systems, synthesis of pyrrole moiety has been a highly attractive and an inspiring proposition. It has been found to be an inbuilt pharmacophore of various momentous pharmacologically active compounds in medicinal chemistry. This article outlines the basic principles of microwave technology and its use in the synthesis of pyrrole derivatives wherein this technology has made a tremendous impact.     

Microwaves-assisted solvent-free synthesis of N-acetamides by amidation or aminolysis

The preparation of acetamides directly from amines and an acetyl donor under microwaves without any catalyst is described. The inexpensive, solvent free, and fast reaction conditions are the important features of this procedure.     

Can "microwave effects" be explained by enhanced diffusion?

The "microwave effect" or non-thermal effects due to microwaves have been the subject of intense debate. This paper explores the following hypothesis: if the transport of an active species is a rate limiting step in a reaction, and if microwaves enhance the diffusion of that species, then the overall reaction rate would change under microwave heating compared with conventional heating. If the hypothesis is correct then it should be possible to pick those reactions that would speed up, slow down or stay the same, under microwave irradiation. One consequence of the hypothesis is that the equilibrium states (end point of the reactions) remain unchanged by microwave irradiation. The measurements and theory presented here strongly suggest that this hypothesis is correct.     

Microwave heating of water, ice, and saline solution: molecular dynamics study

In order to study the heating process of water by the microwaves of 2.5-20 GHz frequencies, the authors have performed molecular dynamics simulations by adopting a nonpolarizable water model that has fixed point charges on a rigid-body geometry. All runs are started from the equilibrated states derived from the I(c) ice with given density and temperature. In the presence of microwaves, the molecules of liquid water exhibit rotational motion whose average phase is delayed from the microwave electric field. Microwave energy is transferred to the kinetic and intermolecular energies of water, where one-third of the absorbed microwave energy is stored as the latter energy. The water in ice phase is scarcely heated by microwaves because of the tight hydrogen-bonded network of water molecules. Dilute salt water is significantly more heated than pure water because of the field-induced motion of salt ions, especially that of large-size ions, by the microwave electric field and energy transfer to water molecules by collisions.     

Developing rapid detection of mycobacteria using microwaves

In this paper, we describe the development of a culture-based biochip device for rapid detection of mycobacteria in environmental samples. Individual biochips rely upon the unique paraffinophilic nature of mycobacteria to rapidly and selectively adhere to the surface of the device. We used prototype biochips to experimentally demonstrate the concept of rapid and selective detection of mycobacteria by testing pure cultures and using epifluorescence microscopy to visualize microorganisms on the surface. As an alternative, rapid approach for identifying the biomass on the biochip surface, we used microwaves in the 10 to 26 GHz frequency range. The results of this study indicate that different microorganisms are responsible for specific shifts in resonance frequencies of a microwave cavity. By combing the semi-selective paraffin surface of the biochip with the microorganism-specific response to the microwaves, we have developed an improved analytical system with the potential to rapidly identify and enumerate mycobacteria in environmental samples in as little as 2 h.     

The synthesis of unsubstituted cyclic imides using hydroxylamine under microwave irradiation

Unsubstituted cyclic imides were synthesized from a series of cyclic anhydrides, hydroxylamine hydrochloride (NH2OH.HCl), and 4-N,N-dimethylamino-pyridine (DMAP, base catalyst) under microwave irradiation in monomode and multimode microwaves. This novel microwave synthesis produced high yields of the unsubstituted cyclic imides for both the monomode (61 - 81%) and multimode (84 - 97%) microwaves.     

Liquid-phase dehydration of sorbitol under microwave irradiation in the presence of acidic resin catalysts

Liquid-phase dehydration of sorbitol has been investigated in wide reaction conditions especially under microwave irradiation in the presence of acidic resin catalysts. From the selectivity for sorbitan and isosorbide, it can be understood that the dehydration is a consecutive reaction (sorbitol to sorbitan, and finally to isosorbide) and that the sorbitan is an intermediate of the dehydration. By using microwave irradiation, the dehydration can be accelerated by around 20?C34 times compared with the rate by conventional electric heating at the same temperature, or the reaction temperature can be decreased by around 40 °C for the comparable conversion in a similar reaction time. However, the microwaves do not have noticeable effects on the selectivity for isosorbide or sorbitan. The accelerated dehydration under microwaves is mainly due to decreased activation energy.     

Graphitized carbon black in quartz tubes for the sampling of indoor air nicotine and analysis by microwave thermal desorption-capillary gas chromatography

Nicotine in a smoky indoor air environment can be determined using graphitized carbon black as a solid sorbent in quartz tubes. The temperature stability, high purity, and heat absorption characteristics of the sorbent, as well as the permeability of the quartz tubes to microwaves, enable the thermal desorption by means of microwaves after active sampling. Permeation and dynamic dilution procedures for the generation of nicotine in the vapor phase at low and high concentrations are used to evaluate the performances of the sampler. Tube preparation is described and the microwave desorption temperature is measured. Breakthrough volume is determined to allow sampling at 0.1-1 L/min for definite periods of time. The procedure is tested for the determination of gas and paticulate phase nicotine in sidestream smoke produced in an experimental chamber.