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  • Author or Editor: A. Rami x
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It is known that experimental parameters may affect peak characteristics in DSC studies. Kinetic parameters calculated from isothermal and dynamic runs, can also be affected by the choice of experimental conditions.

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Summary Thermogravimetry was used to study the kinetics of isothermal degradation of an epoxy thermoset powder coating in a nitrogen atmosphere and in oxidizing atmospheres of air and pure oxygen. An integral isoconversional procedure was used to analyse how the activation energy varies depending on the degree of conversion and depending on the atmospheres used. In the case of degradation in a nitrogen atmosphere, in addition to the activation energy, the kinetic triplet was completed using an Avrami reaction model and the pre-exponential factor. With this atmosphere, the conclusion was reached that the isothermal and non-isothermal kinetics are equivalent. It was shown that the thermooxidative degradation process is more complex and consists of a two-stage process. The first stage of degradation is similar whether nitrogen, oxygen or air are present. Chain scission occurs and it seems that there is formation of thermally more stable compounds. The second stage of degradation, involving several phenomena, occurs only in the presence of oxygen or air and leads to the total disappearance of the organic material by thermooxidation. These stages are very similar under non-isothermal or isothermal conditions.

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Abstract  

Solid bisphenol-A epoxy resin (DGEBA) of medium molecular mass was cured using o-tolylbiguanide (TBG) as cross-linking agent. In order to improve the kinetics of the reactive system, two Lewis acid catalysts (erbium(III) and ytterbium(III) trifluoromethanesulfonates) were added in proportions of 1 phr. The kinetic study was performed by dynamic scanning calorimetry (DSC) and the complete kinetic triplet (E, A and g(α)) determined. The kinetic analysis was performed with an integral isoconversional procedure (model-free), and the kinetic model was determined by the Coats-Redfern method and through the compensation effect (IKR). All the systems followed the m=1.5/n=0.5 isothermal curing model simulated from non-isothermal experiments. The addition of a little proportion of ytterbium or erbium triflates accelerated the curing process. In order to extract further information about the role of the lanthanide triflates added to epoxy/TBG systems, the kinetic results were compared with our previous kinetic studies made on DGEBA/lanthanide triflates initiated systems.

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Abstract  

Solid bisphenol-A epoxy resin of medium molecular mass was cured using a Lewis acid initiator (ytterbium(III) trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr). A kinetic study was performed in a differential scanning calorimeter. The complete kinetic triplet was determined (activation energy, pre-exponential factor, and integral function of the degree of conversion) for each system. A kinetic analysis was performed with an integral isoconversional procedure (free model), and the kinetic model was determined both with the Coats-Redfern method (the obtained isoconversional value being accepted as the effective activation energy) and through the compensation effect. All the systems followed the same isothermal curing model simulated from non-isothermal ones. The growth-of-nuclei Avrami kinetic model A3/2 has been proposed as the polymerization kinetic model. The addition of initiator accelerated the reaction especially when 2 phr was added. 0.5 and 1 phr showed very few kinetic differences between them.

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Abstract  

Ion-exchanged montmorillonite-rich materials (ca. 96% purity) like NaMt, Fe(II)Mt, Co(II)Mt, Ni(II)Mt and Cu(II)Mt showed catalytic activity in the ozonation of oxalic acid in water at room temperature, in the pH range 3.4–6.0. The conversion of oxalic acid exceeds 95% after 180 min of ozone bubbling in the presence of Fe(II)Mt. The oxalic acid removal efficiency was found to increase swiftly with the acid character of the clay surface up to a certain level, but decreases gently with excessive surface acidity. The pH exerts a strong influence on the catalyst efficiency, because it induces changes in the composition of both the liquid media and catalyst. The synergic action of ozone and clay catalysts at acidic pH seems to involve ozone adsorption and interaction between cation and adsorbed oxalate. The negative effect of increasing pH between 3.44 and 6.0 is discussed in terms of a decrease in the amount and mobility of the cation in the vicinity of the clay surface, and of a decay in the clay surface area available to ozonation.

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Non-isothermal differential scanning calorimetry (DSC) experiments were performed to study the kinetics of the curing process of mixtures of diglycidylether of bisphenol A (DGEBA) and γ-butyrolactone (γ-BL) with ytterbium triflate as an initiator. It can be deduced that the cured material consists of epoxide homopolymers with incorporated poly(ether-ester) unities, which come from the lactone incorporated into the network. The kinetic parameters, obtained using the non-isothermal isoconversional procedure, show not only the importance of the proportion of initiator but also the influence of γ-butyrolactone on the polymerization of DGEBA. The homopolymerization of DGEBA catalyzed by ytterbium triflate has an activation energy of 85.3 kJ mol−1, which decreases to 68.2 kJ mol−1 in the presence of γ-butyrolactone forming copolymers. Analysis from DSC and FTIR data showed that, when the proportion of ytterbium triflate was increased, the reaction process accelerated and the mechanism of the cationic non-linear polymerization named activated monomer (AM) became more evident than the activated chain-end mechanism (ACE). Finally, the activation energies and the pre-exponential factors were determined for both mechanisms.

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Abstract  

The thermal polymerization kinetics of dimethacrylate monomers was studied by differential calorimetry using non-isothermal experiments. The kinetic analysis compared the following procedures: isoconversional method (model-free method), reduced master curves, the isokinetic relationship (IKR), the invariant kinetic parameters (IKP) method, the Coats-Redfern method and composite integral method I. Although the study focused on the integral methods, we compared them to differential methods. We saw that even relatively complex processes (in which the variations in the kinetic parameters were only slight) can be described reasonably well using a single kinetic model, so long as the mean value of the activation energy is known (E). It is also shown the usefulness of isoconversional kinetic methods, which provide with reliable kinetic information suitable for adequately choosing the kinetic model which best describes the curing process. For the system studied, we obtained the following kinetic triplet: f(α)=α0.6(1−α)2.4, E=120.9 kJ mol−1 and lnA=38.28 min−1.

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