Authors:L. Barral, J. Cano, A. López, P. Nogueira, and C. Ramírez
Using dynamic mechanical analysis (DMA) we have studied the variation with the frequency of the dynamic mechanical properties
(storages modulus,E'; loss modulus,E'' and loss tangent or tan σ) for a system containg a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane
(1,3-BAC). These properties were measured both in the glass transition and β transition regions. An increase in frequency
caused a shift of tan σ peak positions in both regions toward higher temperature. Finally, we report the activation energies
of a DGEBA/1,3-BAC expoxy system for α and β transitions.
Authors:M. R. Granados-Uribe, F. J. Lona-Ramírez, C. Pérez-Pérez, J. Barajas-Fernández, V. Rico Ramírez, and G. González-Alatorre
The nitrosation of 1,3-dialkylureas was carried out in the presence of carboxylic acids and halides in an aqueous perchloric medium. The aim of this work was to validate the proposed mechanism for the nitrosation of such substrates. In accordance with the rate limiting step of the proposed mechanism, the protonic transfer to the solvent, basic catalysis and an absence of catalysis by halides should be observed. The Br⊘nsted parameters were determined by basic general catalysis.
Authors:L. Barral, J. Cano, J. López, P. Nogueira, M. Abad, and C. Ramírez
Differential scanning calorimetry (DSC) was applied to study the cure kinetics of an epoxy system containing both tetraglycidyl
4,4′-diaminodiphenylmethane (TGDDM) and a multifunctional Novolac glycidyl ether resin, cured with 4,4′-diaminodiphenylsulfone
(DDS). The experimental data were analyzed in terms of a mechanistic model proposed by Cole, which includes the etherification
reaction. The kinetics can be completely described in terms of three rate constants, which obey the Arrhenius relationship.
This model gives a good description of the cure kinetics up to the onset of vitrification. The effect of diffusion control
was incorporated to describe the cure in the later stages. By combining the model and a diffusion factor, it was possible
to predict the cure kinetics over the whole range of conversion, including an analysis of the evolution of different chemical
species during the curing process. Good agreement with the experimental DSC data was achieved with this mechanistic model
over the whole range of cure when the etherification reaction was assumed to be of first order with respect to the concentrations
of epoxide groups, hydroxy groups, and the tertiary amine groups formed in the epoxide amine reaction.
Authors:L. Barral, J. Cano, J. López, I. López-Bueno, P. Nogueira, C. Ramírez, and M. Abad
A study of an epoxy-cycloaliphatic amine system has been realized using a thermogravimetric technique (TG). Isothermal and
non-isothermal (dynamic) methods were employed to determine the kinetic data of this system.
Five methods were used for determining the activation energies of this system in the dynamic heating experiments. In two of
them (Flynn-Wall-Ozawa, and Kissinger) it is not necessary to have a prior knowledge of the reaction mechanism of the degradation
behaviour for this system. In the other ones (Coats and Redfern, Horowitz and Metzger, and Van Krevelen et al.) it is necessary
to know this reaction mechanism, besides Criado et al. method was used for determining it.
The results have shown that good agreement between the activation energies obtained from all methods can be achieved if it
is assumed that the degradation behaviour of this system is of sigmoidal-rate type.
Authors:C. Ramírez, M. Abad, L. Barral, J. Cano, F. Díez, and J. López
Using differential scanning calorimetry (DSC) we have studied the physical aging of an epoxy resin based on the diglycidyl
ether of bisphenol A (DGEBA) modified by two different contents of an acrylonitrile-butadiene-styrene (ABS) and cured with
1,3-bisaminomethylcyclohexane (1,3-BAC). Samples fully cured were annealed at temperature of 125C for periods of time of
72 and 120 h, to determine the process of physical aging. The apparent activation energy for the enthalpy relaxation, Dh*, is determined as the sample is heated at 10C min-1 following cooling at various rates through the glass transition region. DSC studies suggested that the presence of thermoplastic
inhibits the process of relaxation.
Authors:L. Barral, J. Cano, A. López, J. López, P. Nogueira, and C. Ramírez
Using dynamic mechanical analysis (DMA) we have studied thermal degradation for a system containing a diglycidyl ether of
bisphenol A (DGEBA) and 1,3-bisaminomethylcylohexane (1,3-BAC). The changes of dynamic mechanical properties during thermal
degradation indicated a shift of the glass transition temperature (Tg) to higher temperatures and a decrease in the peak value of the dynamic loss factor (tan δ) with an increasing of aging time.
The value of dynamic storage modulus (E′) at the rubbery state showed an increase with aging time, whiteE′ at the glassy state only underwent a moderate change with increased thermal degradation. From these results it can be argued
that thermal degradation during the stage prior to the onset of the severe degradation involves structural changes in the
epoxy system, as further crosslinking and loss of dangling chains in the crosslinked network.
Authors:L. Barral, J. Cano, A. J. López, J. López, P. Nogueira, and C. Ramírez
The diffusive and dynamic mechanical behavior of the DGEBA/1,3-BAC epoxy resin system was studied during water absorption. The diffusion of water was investigated at 100% relative humidity, by immersion of specimens in water at 60, 80 and 100°C. In all absorption experiments, water diffusion followed Fick's law. Diffusion coefficients and saturated water concentrations are given for these temperatures. The activation energy for diffusion was determined from the relationship between the diffusion coefficient and the reciprocal of the absolute temperature. The value obtained was 31.2 kJ mol−1. Dynamic mechanical analysis of samples immersed in 100°C water and with various water contents showed both a shift of Tg, defined by thetanδ peak, to lower temperatures and a slight decrease in the dynamic modulus in the presence of water. These effects are probably a result of plasticization.