Search Results

You are looking at 61 - 70 of 275 items for :

  • "reaction kinetics" x
  • All content x
Clear All

Abstract  

Some specific features of the thermochemistry of epoxy-amine curing at the later stages of the reaction are considered. Possible mechanism of cross-linking and the question about the driving force leading to the infinite network are discussed. The coupling of the reaction kinetics and rearrangement of the chains crosslinked into the rigid supramolecular structure is the essential feature of epoxy-amine vitrified system. It has been proposed that owing to the contribution from the side process, different curing temperatures can result in the structures with different T g. It was also established that reaction of epoxy ring opening alone is not responsible for the residual curing. The latter is the result of the side processes. As compared with the reaction of epoxy ring opening the side processes are strongly dependent on the geometrical aspects.

Restricted access

Abstract

This article described the synthesis and mesomorphic behavior transition of a novel liquid crystalline (LC) epoxy resin 4-(2,3-epoxypropoxy)biphenyl,4″-(2,3-epoxypropoxy)phenyl-4′carboxylate (EBEPC), which combined a hydroxyl benzoic aromatic ester and biphenol rigid-rod group. EBEPC showed a clear nematic schlieren texture under curtain conditions. The reaction kinetics of EBEPC cured by 4,4′-diaminodiphenyl-methane (DDM) was studied by using an isoconversional method under isothermal conditions with differential scanning calorimetry (DSC). The isothermal DSC data can be fitted reasonably by an autocatalytic curing model. Smectic phases had been observed in the EBEPC/DDM curing system. The results of DSC showed that the formation of the LC phase had pronounced influence on the curing reaction.

Restricted access

Abstract  

The thermal decomposition of ammonium perchlorate (AP)/hydroxyl-terminated-polybutadiene (HTPB), the AP/HTPB solid propellant, was studied at different heating rates in dynamic nitrogen atmosphere. The exothermic reaction kinetics was studied by differential scanning calorimetry (DSC) in non-isothermal conditions. The Arrhenius parameters were estimated according to the Ozawa method. The calculated activation energy was 134.5 kJ mol-1, the pre-exponential factor, A, was 2.04×1010 min-1 and the reaction order for the global composite decomposition was estimated in 0.7 by the kinetic Shimadzu software based on the Ozawa method. The Kissinger method for obtaining the activation energy value was also used for comparison. These results are discussed here.

Restricted access

Abstract  

Aspects of the theories that are conventionally and widely used for the kinetic analyses of thermal decompositions of solids, crystolysis reactions, are discussed critically. Particular emphasis is placed on shortcomings which arise because reaction models, originally developed for simple homogeneous reactions, have been extended, without adequate justification, to represent heterogeneous breakdowns of crystalline reactants. A further difficulty in the mechanistic interpretation of kinetic data obtained for solid-state reactions is that these rate measurements are often influenced by secondary controls. These include: (i) variations of reactant properties (particle sizes, reactant imperfections, nucleation and growth steps, etc.), (ii) the effects of reaction reversibility, of self-cooling, etc. and (iii) complex reaction mechanisms (concurrent and/or consecutive reactions, melting, etc.). A consequence of the contributions from these secondary rate controls is that the magnitudes of many reported kinetic parameters are empirical and results of chemical significance are not necessarily obtained by the most frequently used methods of rate data interpretation. Insights into the chemistry, controls and mechanisms of solid-state decompositions, in general, require more detailed and more extensive kinetic observations than are usually made. The value of complementary investigations, including microscopy, diffraction, etc., in interpreting measured rate data is also emphasized. Three different approaches to the formulation of theory generally applicable to crystolysis reactions are distinguished in the literature. These are: (i) acceptance that the concepts of homogeneous reaction kinetics are (approximately) applicable (assumed by many researchers), (ii) detailed examination of all experimentally accessible aspects of reaction chemistry, but with reduced emphasis on reaction kinetics (Boldyrev) and (iii) identification of rate control with a reactant vaporization step (L’vov). From the literature it appears that, while the foundations of the widely used model (i) remain unsatisfactory, the alternatives, (ii) and (iii), have not yet found favour. Currently, there appears to be no interest in, or discernible effort being directed towards, resolving this unsustainable situation in which three alternative theories remain available to account for the same phenomena. Surely, this is an unacceptable and unsustainable situation in a scientific discipline and requires urgent resolution?

Restricted access

A thermo magnetic analysis (TMA) apparatus is used to follow reactions under controlled conditions of temperature and pressure with the Faraday method. Relations giving the conversion degree λχ at a given timet as a function of the sample susceptibility are presented. Methods for studying the effects of the magnetic field on the reaction kinetics are considered. In particular, the kinetic curves obtained for the reaction

Restricted access

Abstract

Hydrogen phosphate (HPO4 2−) or poly(acrylic acid) (PAA) stabilized cobalt(0) nanoclusters were in situ generated from the reduction of cobalt(II) chloride during the catalytic hydrolysis of sodium borohydride (NaBH4) in the presence of stabilizers, HPO4 2− or PAA. Cobalt(0) nanoclusters stabilized by HPO4 2− or PAA were characterized by using UV–Visible spectroscopy, TEM, XPS and FT-IR techniques. They were employed as catalysts in the hydrolysis of NaBH4 to examine the effect of stabilizer type on their catalytic activity and stability. Detailed reaction kinetics of the hydrolysis of NaBH4 in the presence of both catalysts was studied depending on catalyst concentration, substrate concentration and temperature. PAA stabilized cobalt(0) nanoclusters provided higher total turnover number (TTON = 6,600) than that of HPO4 2− stabilized cobalt(0) nanoclusters (1,285 turnovers). However, the HPO4 2− stabilized cobalt(0) nanoclusters provided a lower activation energy (E a = 53 ± 2 kJ mol−1) than the PAA stabilized cobalt(0) nanoclusters (E a = 58 ± 2 kJ mol−1) for the hydrolysis of NaBH4. The use of two types of stabilizers in the preparation of the same metal(0) nanoclusters following the same methodology enables us to compare the electrostatic and steric stabilization in terms of the catalytic activity and stability of metal(0) nanoclusters.

Restricted access

Summary  

Sand filters are used as a filter bed in many ground water treatment plants to remove the physical contaminants and oxidation products. A build-up of radioactivity may take place on the granules, where iron and manganese oxides are deposited and form thin films on the surface of sand filter. The oxides of iron and manganese play an important role in adsorbing radium from ground water. The disposal of those granules makes a significant problem. A batch technique is used for solubilization of radium from sand filters in the presence of some organic acids, which act as reducing agents. These acids are formic acid, acetic acid, benzoic acid, succinic acid, oxalic acid, phthalic acid, and adipic acid. The data were obtained as a function of acidity, temperature, contact time and liquid/solid ratio particle size and shaking speed. It was found that oxalic acid was the best for radium removal. The effectiveness of these acids on radium removal was as follows: oxalic acid > phthalic acid > adipic acid > succinic acid > formic acid > acetic acid. The maximum removal obtained was 69.9% at 1M oxalic acid at 8 ml/g ratio. Reaction kinetics and mechanism parameters of the dissolution process were studied and compared with other published data.

Restricted access

Abstract  

A Mangelsdorf's approach to modeling the epoxy-amine cure kinetics has been developed. Analysis of the data by means of Mangelsdorf's approach makes it possible not only to determine the reaction rate constant and the heat of epoxy ring opening, but also to elucidate the reaction mechanism. However, to model the kinetic curves obtained by the calorimetric method for the complicated reaction should be derived an equation expressing the rate of change of the heat with time, as a function of the reaction rate and the extent of conversion. In a detailed examination the thermokinetic data, we found that glassy state transition is kinetically feasible. Using data available in literature, the kinetic model for epoxy-amine cure reaction was developed. Our treatment of glass formation is based on the picture of the reaction system as a miscible mixture of two structurally different liquids. This approach is similar to that presented by Bendler and Shlesinger as a Two-Fluid model. In the application of this model to reaction kinetics, we believe the explanation of glass structure formation lies in the splitting of the homogeneous mixture into two liquid phases.

Restricted access

Abstract  

Differential scanning calorimetry was employed to investigate the reaction of diglycidyl ethers of bisphenol A (DGEBA) of mean molecular mass 348–480 Da, with collagen hydrolysate of chrome-tanned leather waste in a solvent-free environment. The reaction leads to biodegradable polymers that might facilitate recycling of plastic parts in products of the automotive and/or aeronautics industry provided with protective films on this basis. The reaction proceeds in a temperature interval of 205–220°C, at temperatures approx. 30–40°C below temperature of thermal degradation of collagen hydrolysate. The found value of reaction enthalpy, 519.19 J g−1 (= 101.24 kJ mol−1 of epoxide groups) corresponds with currently found enthalpy values of the reaction of oxirane ring with amino groups. Reaction heat depends on the composition of reaction mixture (or on mass fraction of diglycidyl ethers in the reaction mixture); proving the dependence of kinetic parameters of the reaction (Arrhenius pre-exponential factor A (min−1) and activation energy E a (kJ mol−1)) did not succeed. Obtained values of kinetic parameters are on a level corresponding to the assumption that reaction kinetics is determined by diffusion.

Restricted access

The reaction between copper(I) sulphide and excess copper(II) sulphate in the temperature range 600–750 K was investigated by methods of thermal analysis as well as by measuring the phase composition as a function of the fractional conversions. The reaction proceeds in four stages. The transient products are Cu2S, a Cu2SO2 phase and CU2SO4, and the final product is CU2O with the non-defect structure. The initial composition of the substrate mixture strongly influence the reaction kinetics.

Restricted access