Effect of oxirane groups on curing behavior and thermal stability of vinyl ester resins

The effects of oxirane groups in vinyl ester (VE) resin and reactive diluent on curing characteristics and thermal behavior of cured resins are described. Stoichiometric (0.5:1, sample A) as well as nonstoichiometric (0.5:0.85, sample B) ratios of the diglycidyl ether of bisphenol-A (DGEBA) and methacrylic acid (MA) were used for the synthesis of VE resins. Resin sample B had more residual epoxy groups because of the stoichiometric imbalance of the reactants. VE resins thus obtained were diluted with methyl methacrylate (MMA; 1:1, w/w), and controlled quantities of epoxy groups were introduced by partial replacement of MMA with glycidyl methacrylate (GMA), keeping the overall ratio of resin and reactive diluent constant. Increase of GMA content in resin A or B resulted in a decrease in gel time, indicating that the curing reaction is facilitated by the presence of epoxy groups. An increase in initiator content also reduced the gel time. In the differential scanning calorimetry (DSC) scans, a sharp curing exotherm was observed in the temperature range 107 ± 3–150 ± 1 °C. The onset temperature (T_onset) and peak exotherm temperature (T_exo) decreased with increase in GMA content. Heat of curing (ΔH) also increased with increase in GMA content. A broad exotherm was observed after the initial sharp exotherm that was attributed to the etherification reaction. Cured VE resins were stable up to 250–260 °C, and started losing weight above this temperature. Rapid decomposition was observed in the temperature range 400–500 °C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 416–423, 2001     

Separation of phenylacetic acid using tri-n-butyl phosphate in hexanol: Equilibrium and kinetics

Equilibrium and kinetics studies are required to design the continuous extraction process for the acid-extraction system. In the present study, an attempt has been made to investigate the equilibrium and kinetics parameters for the reactive extraction of phenylacetic acid (PAA) with tri-n-butyl phosphate (TBP) in hexanol. The equilibrium results show that the formation of the (1:1) PAA–TBP complex in the organic phase with an overall equilibrium complexation constant (K_e) was 78.74 and 29.15 m³.kmol^?1 for TBP concentrations of 0.734 and 1.464 kmol.m^?3, respectively. The mass transfer coefficients (k_L) for PAA were found to be in the range of 3.7 × 10^–5–6.2 × 10^–5 m.s^?1. Based on the Hatta number (H_a = 8.48), the reaction was found to be fast chemical reaction (regime 3) with the order of reaction as 0.77 and 0.36 with respect to PAA and TBP, respectively. The rate constant of the reaction was obtained as 0.017 kmol.m^?3.s^?1.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: H2 yield and process characteristics

We have recently demonstrated that boric acid (H₃BO₃, BA) is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for thermolysis of ammonia borane (NH₃BH₃, AB). The observations suggest that tetrahydroxyborate ion released by heating BA serves as Lewis acid and catalyzes AB dehydrogenation. Using this approach, we obtained high H₂ yield at 85°C, along with rapid kinetics. Various operating conditions were investigated, such as reactor temperature, AB wt %, and particle size of BA. Even in the presence of 10 wt % BA, high H₂ yield (13 wt %) and trace amount of ammonia (10–20 ppm) were obtained at 80°C, proton exchange membrane (PEM) fuel cell operating temperature. To our knowledge, such H₂ yield value is higher than from any other method using AB with additive or catalyst at PEM fuel cell operating temperatures. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3359–3364, 2013     

Effect of isomerization on thermal behaviour of bisitaconimides

The effect of isomerization of N,N′‐bisitaconimido‐4,4′‐diphenyl ether to the corresponding biscitraconimide on the curing characteristics and thermal stability of cured resins is described. Resins having bisitaconimide:biscitraconimide ratios of 23:77–93:7 were prepared by reacting 4,4′‐diaminodiphenyl ether with itaconic anhydride in solvents of different polarities and under different reaction conditions. Resins containing a higher proportion of citraconimide had a lower melting point (191 vs 208 °C). The curing exotherm was observed immediately after melting in all the resins and exothermic peak temperature reduced with increase in citraconimide content. Resins having a higher proportion of citraconimide on isothermal curing (200 °C, 2 h) and subsequent heating in nitrogen atmosphere degraded at a slightly lower temperature. However, the char yield at 800 °C did not show any systematic dependence on citraconimide content. © 2002 Society of Chemical Industry     

Effect of organic chelates on the performance of hybrid sol–gel coated AA 2024-T3 aluminium alloys

Sol–gels are organic–inorganic polymers formed by hydrolysis/condensation reactions of alkoxide precursors, primarily silanes, which have found applications as electronic, optical and protective coatings. These coatings possess important characteristics such as chemical stability, physical strength and scratch resistance. Further performance improvement is achieved through the incorporation of zirconium and titanium based nanoparticles, also formed through the sol–gel process. However due to the inherent difference in the reactivity of the precursors, the hydrolysis of each precursor must be carried out separately before being combined for final condensation. Zirconium precursors are commonly chelated using acetic acids, prior to hydrolysis, to lower the hydrolysis rate.In this body of work various ligands such as organic acids, acetyl acetone (AcAc) and 2,2′-bipyridine (Bipy) were used to control the zirconium hydrolysis reaction and form nanoparticles within the silane sol matrix.Nanoparticle modified coatings formed from the silane sol on AA 2024-T3 aluminium were characterised spectroscopically, electrochemically and calorimetrically to evaluate the potential effect of the different chelates on the final film properties while neutral salt spray tests were performed to study their anti-corrosion performance. Results indicate that the acid ligand modified coatings provided the best performance followed by AcAc, while Bipy was the poorest. In all cases the zirconium nanoparticle improved the protective properties of the sol–gel coating.     

Effect of pulsing on reaction outcome in a gas–liquid catalytic packed-bed reactor

A novel experiment is described for studying the effect of flow regime on reaction outcome for a consecutive-parallel reaction. By taking advantage of the convective nature of disturbances that grow into pulses in gas–liquid packed-bed reactors, it is shown that it is possible to compare reaction behavior for pulsing and trickling at the same flow rates. This contrasts previous studies where effects of regime were found, but at different flow rates. This experiment is accomplished by packing the column with mostly inert particles and confining the catalytically active region either near the inlet, where pulses have not yet formed, or near the end where they have developed. It is found that for the reaction of phenylacetylene to styrene and ethylbenzene over a platinum/alumina catalyst, where pulses are present in the bottom of the reactor but not at the top, about a 15% increase in styrene concentration, as an intermediate, occurs under pulsing conditions.     

Anomalous Dielectric Behaviour in Melt Quenched Lithium Borate Glasses

Transparent glasses of lithium tetraborate (Li₂B₄O₇) were prepared by a splat quenching technique. X-ray powder diffraction (XRD) and high resolution transmission electron microscopic studies revealed the amorphous nature of the as-quenched samples. The glassy nature of these samples was confirmed by differential thermal analysis. Physical properties such as density, dielectric and ac conductivity have been studied. Glasses of Li₂B₄O₇ exhibit a dielectric anomaly close to the crystallization temperature which is attributed to the interfacial polarization caused by Li⁺ ion motion within the borate network.     

Optimization of conditions for hydrogen production from complex dairy wastewater by anaerobic sludge using desirability function approach

Present study deals with the multiple-response optimization for biohydrogen production using anaerobic sludge and outstanding approach to overcome the drawbacks of conventional response surface methodology (RSM). Dairy wastewater was used as source in batch fermentation was followed for this study. Response surface methodology (RSM), based on a three level, four variable Box–Behnken design, was employed to obtain the best possible combination of substrate concentration, pH, COD/N ratio and COD/P ratio for maximum H₂ yield (HY) and specific hydrogen production rate (SHPR). Experimental data were evaluated by applying RSM integrating a desirability function approach. The optimum H₂ yield and SHPR conditions were: substrate concentration 15.3 g COD/L, pH 5.5, COD/N ratio 100.5 and COD/P ratio 120 with maximum overall desirability D of 0.94. The confirmation experiment under these optimal condition showed a HY and SHPR of 13.54 mmol H₂/g COD and 29.91 mmol H₂/g-VSS.d, respectively. This was only 0.22% and 0.20%, respectively, different from the predicted values, suggesting that the desirability function approach with RSM was a useful technique to get the maximum H₂ yield and SHPR simultaneously.     

Characterization of the L41 gene in Cryptococcus neoformans: its application as a selectable transformation marker for cycloheximide resistance

A transformation system using resistance to the antibiotic cycloheximide as a dominant selectable marker was developed for the pathogenic yeast Cryptococcus neoformans. A 3.5 kb DNA fragment containing a gene encoding the ribosomal protein L41 was cloned from a wild-type strain of C. neoformans which is sensitive to cycloheximide. The open reading frame of the L41 gene contains five introns and encodes a protein of 107 amino acids, which is similar to those reported for other yeasts. The cycloheximide resistance gene to be used as a marker was constructed by replacing a DNA segment of the wild-type L41 gene, which contained the amino acid proline at its 56th position with a homologous DNA segment from a mutant strain resistant to cycloheximide that contained leucine in that position. Cycloheximide resistant transformants were obtained by electroporation on YEPD plates, supplemented with 10-20 microg/ml cycloheximide, at a maximum efficiency of 300 transformants/microg plasmid DNA. While with other genes, most transformants of serotype D in C. neoformans maintain the transforming DNA as episomes, the cycloheximide-resistant transformants were all the result of ectopic genomic integration events.