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, I. López-Bueno, P. Nogueira, M. Abad and C. Ramírez
The physical aging of a system containing tetraglycidyl-4-4′-diaminodiphenylmethane (TGDDM), with a multifunctional novolac
glycidyl ether resin hardened by 4,4′-diaminodiphenylsulphone (DDS) has been investigated by differential scanning calorimetry
(DSC) and dynamic mechanical analysis (DMA). Samples fully cured were aged at temperatures between 200 and 250C, during periods
of time from 1 to a maximum of 336 h. Furthermore, the dynamic mechanical relaxation behaviour annealed at temperature of
220C, was studied, aging during 24 and 168 h. The effect of the enthalpy relaxation during DSC heating scan is shown by the
presence of an endothermic peak whose position and intensity depends on the aging conditions, both temperature and time. DSC
studies suggest that enthalpy relaxation increases gradually with aging time to a limiting value for each temperature where
structural equilibrium is reached. DMA results show that the effect of aging is to cause chain stiffening and a decrease in
the height of the peak value of the loss factor.
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:C. Ramírez, M. Rico, J. M. L. Vilariño, L. Barral, M. Ladra and B. Montero
Summary Cubic silsesquioxanes offer access to organic/inorganic hybrids. A system formed by a polyhedral oligomeric silsesquioxane (POSS) with eight epoxy reactive groups per molecule, octaepoxycyclohexyldimethylsilyl-POSS, cured with an aliphatic diamine (bisaniline) was studied in different ratios. The characterization of the POSS was carried out by differential scanning calorimetry (DSC) and thermogravimetry (TG). The distribution of species in the reaction of the mixtures POSS/diamine was followed by gel permeation chromatography (GPC). The degradation was studied by thermogravimetric analysis.
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. 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.