Summary Polymers have a great interest for the study and design of new materials. Among these materials are epoxy resins, that have good properties, such as low shrinkage during cure, good adhesion, high water and chemical resistance, etc. They have also fast and easy cure in a broad range of temperatures. TTT diagrams are very helpful to design new epoxy materials as they allow the search for very important final properties, such as thermal stability, conversion or glass transition temperature of a material cured through a selected curing cycle. In this work the dependence of the thermal stability on the selected curing cycle for a DGEBA/1,2 DCH system was studied.
Authors:M. Núñez, M. Villanueva, B. Rial, and L. Núñez-Regueira
The thermal degradation of the epoxy systems diglycidyl ether of bisphenol A (BADGE n=0)/1, 2 diamine cyclohexane (DCH) and diglycidyl ether of bisphenol A (BADGE n=0)/1, 2 diaminecyclohexane (DCH) containing calcium carbonate filler immersed and not immersed in hydrochloric acid have
been studied by thermogravimetric analysis in order to compare their decomposition processes and to determine the reaction
mechanism of the degradation processes. The value of the activation energies, necessary for this study, were calculated using
various integral and differential methods. Analysis of the results suggests that hydrochloric acid does not affect the decomposition
of the epoxy network and that the reaction mechanisms produce sigmoidal-type curves for the systems not immersed in HCl and
deceleration curves for the same systems immersed.
Authors:Lisardo Núñez Regueira, M. Núñez, M. Villanueva, and B. Rial
The influence of agents originated in a municipal landfill on the thermal degradation of a polymeric system composed of a
diglycidyl ether of bisphenol A (n=0) and 1,2-diaminecyclohexane was studied by thermogravimetric analysis (TG) in order to obtain the lifetime of this material
before and after being attacked. The different data obtained were analyzed to check the resistance of these materials to chemical
attack and the possibility of their use as coating materials in plants where those reagents were present. At the optimum temperature
of service for this material, 373.16 K, the lifetimes obtained from the experimental results were 2633 years and 2135 years,
Authors:Lisardo Núñez-Regueira, M. Villanueva, and I. Fraga-Rivas
Differential scanning calorimetry was used to study the influence of
an epoxy reactive diluent, vinylcyclohexane dioxide, on the curing reaction
of a polymeric system composed of diglycidyl ether of bisphenol A (n=0) and 1,2-diaminecyclohexane (DCH). Heat evolution
and glass transition temperature, were measured in terms of the added diluent
percentage. Experimental results show that both the curing degree and the
glass transition temperature of the polymeric system decrease with an increase
in the diluent percentage.
Dynamic mechanical analysis of several
samples also showed that Tg
decreases with the increase of diluent percentage, thus corroborating DSC
Authors:L. Núñez-Regueira, M. Villanueva, and I. Fraga-Rivas
study of the degradation of a polymer is important because it can determine
the upper temperature limit, the mechanism of a solid-state process, and the
life-time for this system. Since the behavior of thermosets is affected by
the selection of the curing cycle, it is important to investigate the changes
which take place during the thermal degradation of these materials when a
change on the sequence of time and temperature is introduced during the curing
this work, the thermal degradation of two epoxy systems diglycidyl ether of
bisphenol A (BADGE n=0)/1, 2 diamine cyclohexane
(DCH) cured through different sequences of time and temperature was studied
by thermogravimetric analysis in order to determine the reaction mechanism
of the degradation processes, and also to check the influence of the curing
cycle on this mechanism. Values obtained using different kinetic methods were
compared to the value obtained by Kissinger’s method (differential method
which do not require a knowledge of the n-order
reaction mechanism), and to that obtained through Flynn–Wall–Ozawa
method in a previous work.
Authors:D. Oviedo Vega, I. Garcia Alonso, and M. Velasco Villanueva
A chitin-lipoprotein complex was detected in the pre-exuvial lobster shell. The aqueous extract of this skeleton contains a lipoprotein in its highest molecular weight fraction where the phospholipids constitute the main component. Changes in the lipoprotein crystalline structure with temperature were detected, but not the formation of typical liquid crystals.
Authors:M. Villanueva, I. Fraga, J. Rodríguez-Añón, and J. Proupín-Castiñeiras
The rheological behaviour of the materials diglycidil ether of bisphenol A (n = 0), 1, 2 diamine-cyclohexane and the epoxy reactive diluent vinylcyclohexane dioxide have been studied both separately
and mixed before the beginning of the curing reaction. Different kinds of tests such as: preshear and time sweep, flow curve
and stress sweep experiments were carried out. From these experiments, interesting information about: viscosity-shear stress
and viscosity-shear rate dependences, storage modulus and the linear viscoelastic region were found out. Relationships between
concentration of diluent and the final viscosity of the mixed sample and also between percentage of diluent and storage modulus
Authors:N. Barros, J. Salgado, M. Villanueva, J. Rodriquez-Añón, J. Proupin, S. Feijóo, and M. Martín-Pastor
The environmental concern on soil exploitation, linked to global warming by the Kyoto protocol, is responsible for increasing interest in the understanding of the role of the composition and structure of the soil organic matter (SOM) on soil carbon, C, dynamics. Thermal analysis and nuclear magnetic resonance (NMR) are applied to study the thermal properties, the structure and composition of the SOM of six samples with different C contents in order to improve the interpretation of results given by thermal analysis. Results showed that the direct integral of the combustion peaks obtained by DSC and the percentage of SOM given by TG were both directly related to the quantity of total soil C. Thus, soils with higher C content showed higher energy content too. The combustion temperatures of the curves given by DSC are those reported for labile OM. NMR results indicated the presence of aliphatic C, carbohydrates, and a weak signal in the aromatic C band in all the samples that was not detected in the DSC curves. Only two samples showed carboxyl/carbonyl C which was not detected by DSC also.
Authors:A. Borhidi, de la M. Luz Arreguín Sánchez, and R. Galván Villanueva
The Herbarium of the National School of the Biological Science (ENCB) of the Mexican National Polytechnics Institute is the second collection in size and importance among the herbaria of Mexico. According to its catalogue published in 1996 more than 1,000 type specimens of vascular plants are registered in it. In the year 2003 several undetermined collections were revised by the first author belonging to the Rubiaceae family. The result of this work was the description of 7 new species: Arachnothryx latiloba, A. michoacana, A. monticola, Bouvardia lottae, Guettarda guerrerensis, Psychotria diegoae, Rogiera variiflora and Psychotria rzedowskiana as a new name for P. izabalensis subsp. oaxacana.