Comparison between microwave and thermal curing of glass fiber–epoxy composites: Effect of microwave‐heating cycle on mechanical properties

The objective of this study was to compare the mechanical properties between epoxy composites cured by thermal heating and microwave heating. Epoxy‐anhydride resins reinforced with glass fiber were cured in a domestic microwave oven and in a thermal oven. Hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Microwave curing was carried out at various conditions, including 1‐, 2‐, and 3‐step heating cycle, whereby each cycle employed different power level and time. Mechanical properties were tested according to ASTM standards. It is found that the microwave‐cured composites produced mechanical properties as good as the thermally cured composites. The 2‐ and 3‐step heating cycle used in the microwave curing process produced better mechanical properties higher than those obtained from the microwaved 1‐step and thermally curing process. This is attributed to the slow increase in temperature during the beginning of the microwave curing process whereby the very low power level was applied in the first cycle of the multistep heating process. This affected the slower rate of viscosity increment, resulting in better wettability of the glass fiber with enhanced interfacial adhesion between the fibers and the resins. The viscosity of resins affected the homogeneity of the crosslinked structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1059–1070, 2006     

Zinc lactate‐catalyzed ring‐opening polymerization of trimethylene carbonate under microwave irradiation

Microwave‐assisted ring‐opening polymerization (MROP) of trimethylene carbonate (TMC) was carried out with four different types of zinc lactate, as the catalyst. Poly(trimethylene carbonate)s (PTMC) with a number–average molar mass ranging from 2990 to 75,410 g/mol and a TMC conversion ranging from 85.2% to 98.2% were synthesized effectively in 30 min at 120°C under microwave irradiation. The effects of the catalyst type, catalyst concentration, and microwave irradiation time on the MROP of TMC were studied. The MROP of TMC was much faster than that under conventional heating conditions. Thermal analysis suggested that PTMC with higher molar mass exhibited higher glass transition temperature (T_g) and thermal stability. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Rapid Salt‐Assisted Microwave Demulsification of Oil‐Rich Emulsion Obtained by Aqueous Enzymatic Extraction of Peanut Seeds

Aqueous enzymatic extraction (AEE) is an environmentally friendly edible‐oil‐extraction process that can also provide edible protein. However, the AEE process may form a stable emulsion in most cases, which seriously limits the large‐scale industry applications for producing vegetable oils. In this study, the salt‐assisted microwave radiation demulsification of the oil‐rich emulsion prepared with AEE from peanuts is investigated. The microwave demulsification method is compared with other conventional demulsification methods, including heating, and freezing–thawing. The salt‐assisted microwave demulsification of the emulsions shows a greater free oil yield than conventional heating demulsification. Moreover, the microwave demulsification shows a similar free oil yield in less time than freezing–thawing method. Under the optimal operating conditions of demulsification, the free oil yield can reach 92.3% with CaCl₂‐assisted microwave demulsification for only 2 min. In addition, the oxidative properties and the fatty acid compositions of the demulsified peanut oil are investigated. No significant difference in the fatty acid composition is observed among salt‐assisted microwave, freezing–thawing, and heating demulsified oil. The oxidative properties of the salt‐assisted microwave demulsified peanut oil is better than the conventional heating demulsified oil. Thus, salt‐assisted microwave demulsification provides a quick and effective demulsification method to obtain vegetable oils with high quality. Practical Applications: Aqueous enzymatic extraction (AEE) is an environmentally friendly edible‐oil‐extraction process. To solve the problem of stable emulsion formed during AEE process, the salt‐assisted microwave demulsification of the oil‐rich emulsion prepared with AEE is developed with high efficiency (demulsification for 2 min). In addition, the oxidative properties of the microwave demulsified oil is better than the conventional heating demulsified oil.     

Comparison of the curing kinetics of a DGEBA/acid anhydride epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol‐A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave‐heated power‐compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared. The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2054–2063, 2007     

Microwave irradiated and thermally heated olive stone activated carbon for nickel adsorption from synthetic wastewater: A comparative study

In attempt to compare the removal efficiency and yield of the activated carbon prepared using the conventional and microwave‐assisted heating is the focus of this work. Toward this olive stone (a biomass precursor) is activated using the popular activating agent potassium hydroxide. The process optimization exercise is carried out by using the standard full factorial statistical design of experiments (response surface methodology). The activated carbons prepared under the optimized conditions are compared based on the adsorption capacity and yield. The adsorption capacity was found higher using microwave heating as compared with conventional heating. The microwave heating requires significantly lesser holding time as compared to conventional heating method to produce activated carbon of comparable quality, with higher yield. The BET surface area of carbon using microwave heating is significantly higher than the conventional heating. Although the mesopore surface area of carbon is not vary significantly, the activation time, power, and nitrogen gas consumption are significantly lower than the conventional heating rendering that the activation process via microwave is more economical than that via conventional heating. The adsorption isotherm data fitted the Langmuir isotherm well and the monolayer adsorption capacity was found to be 12.0 and 8.42 mg/g for microwave and thermally heated activated carbon, respectively. Regeneration studies showed that microwave‐irradiated and thermally heated olive stone could be used several times by desorption with an HCl reagent. Both carbons can be used for the efficient removal of Ni²⁺ (>99%) from contaminated wastewater. © 2013 American Institute of Chemical Engineers AIChE J, 60: 237–250, 2014     

Microwave‐irradiated ring‐opening polymerization of D,L‐lactide under atmosphere

Poly(D ,L ‐lactide) (PDLLA) was synthesized by microwave‐irradiated ring‐opening polymerization catalyzed by stannous octoate (Sn(Oct)₂) under atmosphere. The effects of heating medium, monomer purity, catalyst concentration, microwave irradiation time, and vacuum level were discussed. Under the appropriate conditions such as carborundum (SiC) as heating‐medium, 0.15% catalyst, lactide with purity above 99.9%, 450 W microwave power, 30 min irradiation time, and atmosphere, PDLLA with a viscosity–average molecular weight (M_η) over 2.0 × 10⁵ and a yield over 85% was obtained. The dismission of vacuum to ring‐opening polymerization of D ,L ‐lactide (DLLA) under microwave irradiation simplified the process greatly. The temperature under microwave irradiation and conventional heating was compared. The largely enhanced ring‐opening polymerization rate of DLLA under microwave irradiation was the coeffect of thermal effects and microwave effects. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2244–2247, 2006     

Compensation of microwave‐source heating attenuation into epoxy–glass composites: Experimental results

The final goal of this study was to manufacture an epoxy–glass leaf spring by microwave processing. The physical properties of the final part to be manufactured, in particular, the mechanical properties, were directly related at the repartition of microwave‐source heating during the treatment. The major problem in microwave processing, however, is the attenuation of the microwave source. Here, we propose a dielectric effect of attenuation inversion of the electromagnetic waves as a new method for the uniform treatment of epoxy–glass by microwave energy. This solution used the dielectric properties of the mold to control the microwave‐heat‐source attenuation into the composite to be treated. Many experiments were carried out to validate the proposed solution. The results show that the microwave‐source heating attenuation could controlled and inversed. We demonstrated the uniform treatment of epoxy–glass parts about 100 cm long by means of the compensation of microwave‐source attenuation. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39908.     

Kinetics of microwave‐assisted polymerization of ϵ‐caprolactone

The kinetics of polymerization of ?‐caprolactone (CL) in bulk was studied by irradiating with microwave of 350 W and frequency of 2.45 GHz with different cycle‐heating periods (30–50 s). The molecular weight distributions were determined as a function of reaction time by gel permeation chromatography. Because the temperature of the system continuously varied with reaction time, a model based on continuous distribution kinetics with time/temperature‐dependent rate coefficients was proposed. To quantify the effect of microwave on polymerization, experiments were conducted under thermal heating. The polymerization was also investigated with thermal and microwave heating in the presence of zinc catalyst. The activation energies determined from temperature‐dependent rate coefficients for pure thermal heating, thermally aided catalytic polymerization, and microwave‐aided catalytic polymerization were 24.3, 13.4, and 5.7 kcal/mol, respectively. This indicates that microwaves increase the polymerization rate by lowering the activation energy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1450–1456, 2004     

Synthesis of new polyethers derived from isoidide under phase‐transfer catalysis: Reactivity and selectivity under microwaves and classical heating

Microwave irradiation was applied to the synthesis of polyethers from 1,8‐dibromo‐ and 1,8‐dimesyl‐octane under solid–liquid phase‐transfer catalytic conditions. To evaluate the influence of microwaves on the reaction rates and polyether properties, we carried out the polymerization reactions under similar conventional conditions (oil bath) with the same temperature profiles. First, the microwave‐assisted syntheses proceeded more rapidly in comparison with conventional heating, and the reaction time was reduced from 24 h to 30 min with higher yields of polyethers. Second, the structure of the polymers strictly depended on the activation mode. Under microwave conditions, the polyethers were characterized by higher molecular weights with better homogeneity. Third, the mechanism of chain termination was different under microwave and conventional conditions. The polyethers prepared with conventional heating possessed shorter chains with mainly hydroxylated ends, whereas under microwave irradiation, the polymer chains were longer with mainly ethylenic group ends. In fact, under microwave irradiation, ethylenic group ends were formed rather rapidly and set up a hindrance to further polymer growth. In contrast, under conventional conditions terminations were essentially constituted by hydroxyl functions; however, further polymerization was terminated as well. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1255–1266, 2003     

Soluble new optically active polyamides derived from 5‐(4‐methyl‐2‐phthalimidylpentanoylamino)isophthalic acid and different diisocyanates under microwave irradiation in molten ionic liquid

Ionic liquids (IL)s have been generating increasing attention over the last decade. ILs were originally introduced as alternative green reaction media owing to their distinctive chemical and physical properties of nonvolatility, nonflammability, thermal stability, and controlled miscibility. In this investigation, 5‐(4‐methyl‐2‐phthalimidyl‐pentanoylamino)isophthalic acid ( 6 ), as a bulky monomer, containing phthalimide and flexible chiral groups, has been synthesized. The direct polycondensation of this diacid monomer with several aromatic and aliphatic diisocyanates, such as 4,4′‐methylenebis(phenyl isocyanate), toluylene‐2,4‐diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate, was carried out in tetrabutylammonium bromide as a molten IL in the presence of different catalysts under microwave irradiation as well as conventional heating. The resulting polyamides (PA)s were characterized by FTIR and ¹H NMR spectroscopy, inherent viscosity measurements, thermal and elemental analysis. The obtained PAs showed high yields and moderate inherent viscosities in a range of 0.32–0.57 dL g⁻¹. The PAs were soluble in aprotic polar solvents. Thermogravimetric analysis showed that PAs are thermally stable, 10% weight loss temperatures in excess of 240 and 245°C, and char yields at 600°C in nitrogen higher than 14%. Since toxic and volatile solvent such as NMP was eliminated, this process was safe and green. It is very important to note that, because of high polarizability of ILs, they are very good solvents for absorbing microwaves. The combination of IL and microwave irradiation leads to large reductions in reaction times, very high heating rate with various benefits of the eco‐friendly approach, named green chemistry. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

One-pot synthesis and modification of silica nanoparticles with 3-chloropropyl-trimethoxysilane assisted by microwave irradiation

Industry and academia have shown great interest in the synthesis of organic–inorganic hybrid material because the modification of inorganic supports with organic groups increases the options for using these materials. Microwave irradiation as a heat source is an alternative tool compared with conventional heating (oil bath or furnace) to reduce the reaction times during the synthesis of these materials. Thus, the purpose of this work was to synthesize an organic–inorganic hybrid material, more specifically silica nanoparticles modified with 3-chloropropyl-trimethoxysilane, using microwave irradiation as the heat source. The hybrid materials were synthesized using the sol–gel method, with microwave irradiation for 10, 25, and 40?min, at 300?W of power and temperature of 40, 60, and 80°C. Elemental analyses, FTIR, and N₂ adsorption–desorption isotherms were developed to characterize the materials. It can be concluded that when microwave irradiation is used as a heat source, the reaction rate is accelerated and the surface area of hybrid materials increases considerably.     

Efficient and rapid synthesis of optically active polyamides in the presence of tetrabutylammonium bromide as ionic liquids under microwave irradiation

There is increasing interest in using ionic liquids (ILs) as solvents for polymerization processes. The use of an inexpensive and readily available IL such as tetrabutylammonium bromide (TBAB) as a solvent for clean synthesis and catalytic processes is becoming widely recognized and accepted. This article reports an extension of a microwave method, describing the synthesis of optically active polyamides (PAs) in TBAB. Polycondensation reactions of diacid 5‐(3‐methyl‐2‐phthalimidylpentanoylamino) isophthalic acid with different diisocyanates such as 4, 4′‐methylenebis(phenyl isocyanate), toluylene‐2,4‐diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate were carried out in the presence of TBAB as a molten salt under microwave irradiation and with a conventional heating method, and they were compared with polymerizations in a traditional solvent such as 1‐methyl‐2‐pyrrolidone. A series of optically active PAs with high yields and inherent viscosities ranging from 0.20 to 0.60 dL/g were obtained. These PAs were characterized with Fourier transform infrared spectroscopy, specific rotation measurements, ¹H‐NMR, elemental analysis, thermogravimetric analysis, and differential scanning calorimetry. All data agreed with the proposed structures. Some physical properties and structural characterizations of these PAs are reported. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microwave and conventional curing of thick‐section thermoset composite laminates: Experiment and simulation

In conventional processing, thermal gradients cause differential curing of thick laminates and undesirable outside‐in solidification. To reduce thermal gradients, thick laminates are processed at lower cure temperatures and heated with slow heating rates, resulting in excessive cure times. Microwaves can transmit energy volumetrically and instantaneously through direct interaction of materials with applied electromagnetic fields. The more efficient energy transfer of microwaves can alleviate the problems associated with differential curing, and the preferred inside‐out solidification can be obtained. In this work, both microwave curing and thermal curing of 24.5 mm (1 inch) thick‐section glass/epoxy laminates are investigated through the development of a numerical process simulation and conducting experiments in processing thick laminates in a conventional autoclave and a microwave furnace. Outside‐in curing of the autoclave‐processed laminate resulted in visible matrix cracks, while cracks were not visible in the microwave‐processed laminate. Both numerical and experimental results show that volumetric heating due to microwaves promotes an inside‐out cure and can dramatically reduce the overall processing time.     

The effects of microwave heating on the kinetics of isothermal dehydration of equilibrium swollen poly(acrylic‐co‐methacrylic acid) hydrogel

The isothermal dehydration of poly(acrylic‐co‐methacrylic acid) (PAM) hydrogel under microwave heating (MWH) was investigated. The isothermal kinetics curves of the PAM hydrogel dehydration at temperature range from 293 K to 333 K were recorded. Based on the differential isoconversion method it was concluded that the microwave dehydration of poly(acrylic‐co‐methacrylic acid) hydrogel is an elementary kinetics process. Applying the model‐fitting method it was established that the kinetics of microwave isothermal hydrogel dehydration can be described by the kinetics model of the phase‐boundary controlled process (contracting area). The values of the kinetics parameters (activation energy (E_a) and preexponential factor (lnA)) of the dehydration process under microwave heating are lower than the values for conventional heating (CH). The established influence of MWH on the kinetics of hydrogel dehydration is explained with a specific activation mechanism of water molecules for dehydration and with the increase in the value of the energy of the ground level of the resonant oscillator of water molecule (v = 837 cm^?1) due to the absorption of microwave energy. POLYM. ENG. SCI., 56:87–96, 2016. © 2015 Society of Plastics Engineers     

Polyurethane modified with 3‐aminopropyltriethoxysilane as wool antifelting agent

A wool antifelting agent containing polyurethane modified with 3‐aminopropyltriethoxysilane (APTES) was synthesized. Firstly an isocyanate ( NCO) group ended prepolymer was gained by a hydrogen shift reaction between isophorone diisocyanate (IPDI) and poly (propylene oxide triol) (PPT), and then NCO‐group of the prepolymer was blocked by 3‐aminopropyltriethoxysilane to form a precursor PPT‐[Si(OEt)₃]₃. After that a sol of the precursor was prepared for an antifelting agent. During heating the sol gelled and formed inorganic–organic hybrid film that was characterized by using FTIR and TGA. And then the wool fabric samples were treated with sol by pad‐dry‐cure process at different curing temperature (120, 140, and 160°C). The antifelting effects of different curing temperatures were compared. The results indicated that the new agent could endow a better antifelting effect with the low curing temperature at 120°C and 3 min. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Comparison of microwave and thermal cure of epoxy–anhydride resins: Mechanical properties and dynamic characteristics

The objective of this work was to compare the mechanical properties of epoxy resins cured by thermal heating and microwave heating. Epoxy–anhydride (100:80) resins were cured in a domestic microwave oven and in a thermal oven. The hardening agents included methyl tetrahydrophthalic anhydride and methyl hexahydrophthalic anhydride. Three types of accelerators were employed. Thermal curing was performed at 150°C for 20 and 14 min for resins containing 1 and 4% accelerator, respectively. Microwave curing was carried out at a low power (207 or 276 W) for 10, 14, and 20 min. All cured resins were investigated with respect to their tensile properties, notched Izod impact resistance, and flexural properties (three‐point bending) according to ASTM standards. The tan δ and activation energy values were investigated with dynamic mechanical thermal analysis, and the extent of conversion was determined with differential scanning calorimetry. The differences in the mechanical properties of the thermally cured and microwave‐cured samples depended on the resin formulation and properties. Equivalent or better mechanical properties were obtained by microwave curing, in comparison with those obtained by thermal curing. Microwave curing also provided a shorter cure time and an equivalent degree of conversion. The glass‐transition temperatures (tan δ) of the thermally and microwave‐cured resins were comparable, and their activation energies were in the range of 327–521 kJ/mol. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1442–1461, 2005     

微波加热在化学反应中的应用进展

介绍了微波加热的基本原理,并就微波加热在有机合成、高分子合成及加工、无机合成、天然气转化等方面的最新应用情况进行了综述分析,指出应加强微波对化学反应作用机理的研究。     

Kinetic study of the nonthermal effect of the esterification of octenyl succinic anhydride modified starch treated by microwave radiation

With cassava starch as a raw material and octenyl succinic anhydride as an esterifying agent, octenyl succinic anhydride modified starch (OSA–starch) was prepared in an aqueous medium and treated by water‐bath heating and microwave radiation at a certain temperature, respectively. The reaction kinetics of esterification were studied. The structural analysis and synthesis mechanism of OSA–starch were investigated by means of scanning electron microscopy and Fourier transform infrared spectroscopy. The differences in the esterification reaction kinetics between starches treated with water‐bath heating and microwave radiation were observed. Under the condition of water‐bath heating, the apparent activation energy of the esterification reaction was 52.22 ± 1.21 kJ/mol, and the pre‐exponential factor was 9018.20/min^?1. Under the condition of microwave radiation, the apparent activation energy of the esterification reaction was 50.13 ± 1.16 kJ/mol, and the pre‐exponential factor was 4510.21/min^?1. We found that microwave radiation could reduce both the activation energy of the reaction and the pre‐exponential factor. The lowering effect of the apparent activation energy was greater than that of the pre‐exponential factor under the condition of microwave radiation, and this resulted in increased reaction rates. The change in the esterification reaction kinetics was a nonthermal effect of microwave radiation on the esterification of cassava starch. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43909.     

Synthesis,characterization and properties evaluation of copolymers of 2,3,4,5,6‐pentafluorostyrene and N‐phenylmaleimide

Copolymers of 2,3,4,5,6‐pentafluorostyrene (PFS) having a combination of high hydrophobicity and high glass transition temperature (T_g) are reported here for the first time. The copolymerization was carried out using N‐phenylmaleimide (NPM) as the comonomer and azobisisobutyronitrile (AIBN) as the initiator under both conventional thermal heating and microwave heating. The initial copolymerization rate was found to be higher under microwave heating than under thermal heating. The copolymerization parameters were determined using the Fineman–Ross method and were found to be r₁ (NPM) = 0.28 and r₂ (PFS) = 0.86. Increased incorporation of NPM in the copolymers led to an increase in T_g of the copolymers without significantly affecting the hydrophobicity of poly(2,3,4,5,6‐pentafluorostyrene). Thermal stability of the copolymers is also reported. Copyright © 2005 Society of Chemical Industry     

Preparation and physical properties of transparent organic–inorganic nanohybrid materials based on urethane dimethacrylate

The aim of this study was to prepare transparent organic–inorganic nanohybrid materials with improved physical properties in comparison with the matrix polymer. Polymerizable silica nanoparticles were synthesized via the reaction of silanol groups on the surface of silica nanoparticles (particle diameter ≈ 12 nm) with isocyanate groups of 2‐(methacryloyloxy)ethyl isocyanate (MOI) in ethyl acetate. In addition, the matrix monomer, urethane dimethacrylate, was prepared by the reaction of an MOI isocyanate group with the hydroxyl group of 2‐hydroxyethyl methacrylate, and novel organic–inorganic nanohybrid materials were obtained at various silica contents with bulk polymerization. The surface treatment of the silica nanoparticles and preparation of the matrix monomer were carried out in a one‐pot reaction. The prepared hybrid materials retained high transparency, and the elastic modulus and surface hardness improved with increasing silica content. Moreover, the strength of the material containing 20 wt % silica was up to 30 MPa higher than that of the matrix polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008