Publication: Curado de una Resina Epoxi con nanotubos de carbono, en molde grande y por calentamiento por efecto Joule
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2014-03
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2014-03-03
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Abstract
La mentalidad de la sociedad de ahorro de energía hace que cada vez se busquen materiales más ligeros, rígidos y resistentes. Estas propiedades las cumplen los materiales compuestos.
En este proyecto se trabaja con un material compuesto formado por una matriz de resina epoxi, reforzada con nanotubos de carbono. Normalmente, las resinas epoxi necesitan ciclos de calentamiento largos para curar y adquirir unas propiedades finales buenas. El calentamiento se suele llevar a cabo en autoclaves que proporcionan presión y temperatura.
Los nanotubos de carbono además de mejorar las propiedades mecánicas del material compuesto, le proporciona propiedades eléctricas, a partir de cierta concentración de nanotubos de carbono, lo que se conoce como percolación. Se aprovechan las propiedades eléctricas, para calentar el material por el paso de corriente eléctrica, gracias al efecto Joule, y como alternativa al calentamiento tradicional en autoclave.
Esta idea fue concebida por el equipo de nanocompuestos multifuncionales de IMDEA MATERIALES, que consiguió un curado homogéneo en muestras de pequeño tamaño. Cuando se pasa a muestras de mayor tamaño aparecen problemas debido a variaciones de la viscosidad y de la resistencia eléctrica durante el calentamiento, que da lugar a un mal curado del material.
En un molde cuadrado de 50x50 mm, el efecto que se observa cuando se pasan las corrientes eléctricas es que se calienta solo una zona de la pieza, mientras que el resto permanece a baja temperatura. Para resolver este problema y conseguir un calentamiento uniforme, se desarrolla en este proyecto un circuito electrónico con varias parejas de electrodos. El circuito electrónico está formado principalmente por termopares para medir temperaturas y relés de estado sólido para conmutar el paso de corriente. El paso de corriente a los electrodos está controlado por un regulador PI programado en LabVIEW. El circuito electrónico se puede conectar directamente a una toma de corriente alterna convencional de la red eléctrica, a 230V.
Por otro lado, se intenta encontrar la mejor forma de colocar los electrodos del circuito electrónico, en la muestra, para conseguir un calentamiento homogéneo. Se empieza con la disposición más primitiva, que es utilizar solo una pareja de electrodos, y se finaliza con el sistema, obtenido en el desarrollo de este proyecto, que es el de tres parejas de electrodos. Además se plantea un proceso de acondicionamiento de la muestra previo al calentamiento por efecto Joule que ayuda a disminuir los cambios de resistencia en la muestra.
Por último, se utiliza la calorimetría diferencial de barrido para analizar el grado de curado de las muestras que han sido calentadas con las diferentes configuraciones de los electrodos. Con esta técnica también se obtiene la temperatura de transición vítrea que relaciona con la temperatura que se ha alcanzado durante el calentamiento por efecto Joule.
The mentality of the energy-saving society increases the search of more lightweight, rigid and resistant materials. These properties are satisfied by compound materials. This project runs on an epoxy matrix composite material, which is reinforced with carbon nanotubes. Generally, epoxy resins need long heating cycles to get good final properties. Heating is usually carried out in autoclaves which provide pressure and temperature. Carbon nanotubes, besides the fact that improve mechanic properties of the composite, provide it electrical properties after certain carbon nanotubes concentration, which is known as percolation. Electrical properties are used to heat the material by the electric current, due to the Joule effect, as an alternative to the traditional autoclaving warming. This idea was conceived by the multifunctional nanocomposites team of IMDEA MATERIALES that got a homogeneous cured in small samples. But, when it is attempted in larger samples, problems arise due to variations in viscosity and electrical resistance when heated, which leads to poor curing of the material. The effect we can appreciate in a 50x50 mm square pan when electrical current passes by, is that only one zone of the piece is warmed, while the rest of it remains at a low temperature. To achieve a more homogeneous heating, an electronic circuit regulated, with several pairs of electrodes, capable of heating the entire sample, is developed. The electronic circuit is mainly composed of thermopairs to measure temperatures and solid state relays for switching the current flow. This last one passage to the electrodes is controlled by a PI controller programmed in LabVIEW. The electronic circuit can be connected directly into a conventional alternating current (Plug) of 230V. The electronic circuit is regulated by a PI controller programmed in LabVIEW. The heating system is portable and it can be directly connected to the socket. The goal is to find the best placement of the electrodes, to achieve the most homogeneous curing possible. Subsequently, the pieces curing degree is analyzed by a differential scanning calorimetry. On the other hand, the goal is to find the best placement of the electrodes into the sample’s electrical circuit, to achieve the most homogeneous heating possible. It begins with the most primitive arrangement, which is to use only a pair of electrodes, and it ends with the optimal system which has got three pairs of electrodes. Moreover, it is suggested a conditioning process of the sample, prior to Joule effect heating. It helps reducing resistance changes in the sample. Finally, the differential scanning calorimetry is used to analyze the curing degree of the samples which have been heated to different electrode configurations. With this technique, information about the glass transition temperature is also obtained, giving an indication of the temperature that is reached during Joule effect heating.
The mentality of the energy-saving society increases the search of more lightweight, rigid and resistant materials. These properties are satisfied by compound materials. This project runs on an epoxy matrix composite material, which is reinforced with carbon nanotubes. Generally, epoxy resins need long heating cycles to get good final properties. Heating is usually carried out in autoclaves which provide pressure and temperature. Carbon nanotubes, besides the fact that improve mechanic properties of the composite, provide it electrical properties after certain carbon nanotubes concentration, which is known as percolation. Electrical properties are used to heat the material by the electric current, due to the Joule effect, as an alternative to the traditional autoclaving warming. This idea was conceived by the multifunctional nanocomposites team of IMDEA MATERIALES that got a homogeneous cured in small samples. But, when it is attempted in larger samples, problems arise due to variations in viscosity and electrical resistance when heated, which leads to poor curing of the material. The effect we can appreciate in a 50x50 mm square pan when electrical current passes by, is that only one zone of the piece is warmed, while the rest of it remains at a low temperature. To achieve a more homogeneous heating, an electronic circuit regulated, with several pairs of electrodes, capable of heating the entire sample, is developed. The electronic circuit is mainly composed of thermopairs to measure temperatures and solid state relays for switching the current flow. This last one passage to the electrodes is controlled by a PI controller programmed in LabVIEW. The electronic circuit can be connected directly into a conventional alternating current (Plug) of 230V. The electronic circuit is regulated by a PI controller programmed in LabVIEW. The heating system is portable and it can be directly connected to the socket. The goal is to find the best placement of the electrodes, to achieve the most homogeneous curing possible. Subsequently, the pieces curing degree is analyzed by a differential scanning calorimetry. On the other hand, the goal is to find the best placement of the electrodes into the sample’s electrical circuit, to achieve the most homogeneous heating possible. It begins with the most primitive arrangement, which is to use only a pair of electrodes, and it ends with the optimal system which has got three pairs of electrodes. Moreover, it is suggested a conditioning process of the sample, prior to Joule effect heating. It helps reducing resistance changes in the sample. Finally, the differential scanning calorimetry is used to analyze the curing degree of the samples which have been heated to different electrode configurations. With this technique, information about the glass transition temperature is also obtained, giving an indication of the temperature that is reached during Joule effect heating.
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Keywords
Materiales compuestos, Nanotecnología, Polímeros, Resinas epoxi, Efecto Joule