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  • Author or Editor: Lisardo Núñez x
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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.

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Abstract  

The thermal degradation of the epoxy system diglycidyl ether of bisphenol A (BADGE n=0)/1,2-diamine cyclohexane (DCH) containing different concentrations of an epoxy reactive diluent was studied by thermogravimetric analysis in order to determine the reaction mechanism of the degradation process and to compare it with the results for the same system without diluent. The value of the activation energy, necessary for this study, was calculated using various integral and differential methods. Values obtained using the different methods were compared to the value obtained by the Flynn-Wall-Ozawa"s method (between 193-240 kJ mol-1 depending on the diluent concentration) with does not require a knowledge of the nth order reaction mechanism. All the experimental results were compared to master curves in the range of Doyle"s approximation (20-35% of conversion). Analysis of the results suggests that the reaction mechanism could be F2, F3, or A2 type.

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Journal of Thermal Analysis and Calorimetry
Authors: Lisardo Núñez, L. Fraga, M. Núñez, M. Villanueva, and B. Rial
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Journal of Thermal Analysis and Calorimetry
Authors: Lisardo Núñez Regueira, M. Núñez, M. Villanueva, and B. Rial

Abstract  

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, respectively.

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Abstract  

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 T g decreases with the increase of diluent percentage, thus corroborating DSC measurements.

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Journal of Thermal Analysis and Calorimetry
Authors: Lisardo Núñez-Regueira, J. Rodríguez-Añón, J. Proupín-Castiñeiras, Maria Villanueva-López, and O. Núñez-Fernández

Abstract  

Microcalorimetry was used to study the seasonal evolution over one year of the microbial activity in a humic-eutrophic Cambisol soil as a function of its forest cover. The study was carried out on three soils with identical origin but covered with different forest species: pine, eucalyptus, and a typical Atlantic-humid riverside forest. Some other physical, chemical and biological properties and environmental parameters, mainly humidity and environmental temperature, were considered to analyze their influence on soil microbial activity. The study was performed using a microcalorimeter Thermal Analysis Monitor 2277 in which the experiments were carried out with 1 g soil samples treated with 1.25 mg glucose g–1 soil. From the measured results it follows that pine forest soil is the least productive of the three, as it generates an average heat of 2.7 vs. 5.9 J g–1 generated by the eucalyptus forest soil and 3.1 J g–1 generated by the riverside forest soil. These results are dependent on the remaining physical, chemical and biological features analysed and because of this, pine forest soil, with a pH value 3.3 in spring, shows a small capacity to maintain a stable microbial population which is the lowest of the three (0.079108 to 0.46108 microorganisms g–1 soil) while riverside soil microbial population is in the range from 7.9108 to 17108 microorganisms g–1 soil.

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Journal of Thermal Analysis and Calorimetry
Authors: Lisardo Núñez-Regueira, J. Proupín-Castiñeiras, J. Rodríguez-Añón, Maria Villanueva-López, and O. Núñez-Fernández

Abstract  

The design of a rigorous experimental procedure is the basis for any environmental study. In this work, the basic criteria are established for determination of soil health using microcalorimetry as the main technique complemented by the study of physical (temperature, moisture, porosity, hydraulic conductivity, density and plasticity), chemical (pH and C to N ratio) and biological features (most probable number of microorganisms and organic matter content), and also environmental properties in the form of bioclimatic diagrams. The design was elaborated using as a reference a humic eutrophic-Cambisol subjected to afforestation with P. pinaster Aiton situated in Viveiro (Galicia, NW Spain). Main results of this study refer to total heat evolved during the processes (2.65 to 3.81 J g–1), time to reach the maximum of the peak from 16.17 to 19.29 h, and microbial growth rate constant from 0.0732 to 0.1043 h–1. These results change over the year as they are influenced by the action of environmental parameters over soil microbial activity. The results are in close agreement with some others previously reported using different experimental techniques.

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Journal of Thermal Analysis and Calorimetry
Authors: Lisardo Núñez-Regueira, M. Villanueva, I. Fraga, C. A. Gracia-Fernández, and S. Gómez-Barreiro

Abstract  

The thermal degradation of an epoxy system consisting of a diglycidyl ether of bisphenol A (DGEBA, n=0) and m-xylylenediamine (m-XDA) was studied by both thermogravimetric analysis (TG) and dielectric analysis (DEA). It has been checked a deviation of the typical behaviour in the Arrhenius plot expected for this kind of systems, owing to the thermal degradation. Both, structural relaxation time and conductivity values, were represented as a function of the mass loss, that allow a relationship to be obtained between characteristic relaxation time and the degree of degradation at the beginning of the degradation process.

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