Authors:J. L. Martín, J. M. Salla, A. Cadenato, and X. Ramis
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.
Authors:J. M. Salla, J. M. Morancho, X. Ramis, and A. Cadenato
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.
Authors:S. García, A. Serra, X. Ramis, and J. Suay
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.
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
Authors:A. Azzouz, A. Kotbi, P. Niquette, T. Sajin, A. Ursu, A. Rami, F. Monette, and R. Hausler
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.
Authors:J. Salla, X. Fernández-Francos, X. Ramis, C. Mas, A. Mantecón, and A. Serra
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.
Authors:A. Cadenato, J. Morancho, X. Fernández-Francos, J. Salla, and X. Ramis
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.