Publication: Preparación, caracterización y análisis de materiales nanocompuestos basados en sistemas epoxi-caolín
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2010-03
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2010-05-12
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Abstract
En este trabajo se han sintetizado y caracterizado materiales compuestos de matriz polimérica termoestable tipo epoxi rellena con nanoláminas de caolín. Para ello se ha preparado un derivado de caolín organofílico, modificado con una amina alifática, la etilendiamina (EDA), que ha servido como precursor para la incorporación del caolín en la matriz epoxi. Este tratamiento se llevó a cabo con dos objetivos principales: (a) compatibilizar la superficie de las láminas de caolín con la matriz epoxi y (b) aumentar el espaciado interlaminar del caolín de partida a fin de facilitar la intercalación de la matriz polimérica y promover la deslaminación del caolín para facilitar finalmente su dispersión en el material nanocompuesto. El derivado de caolín modificado con la amina se ha denominado C-EDA. Para su obtención ha sido necesario intercalar previamente de forma consecutiva, dimetilsulfóxido (DMSO) y metanol (MeOH) entre las láminas de caolín. Los resultados de la caracterización mediante FTIR, DRX y EFEE indican que en el derivado C-EDA quedan restos de estas moléculas y confirman la intercalación, al menos parcial, de las moléculas de EDA entre las láminas de caolín. Se ha estudiado el efecto que ejerce la presencia de distintos tipos de caolín en el proceso de curado de la matriz epoxi, discutiendo la influencia de los distintos pretratamientos del caolín. El seguimiento de la reacción de curado se llevó a cabo por medio de espectroscopía infrarroja por transformada de Fourier (FT-NIR) y espectroscopía de fluorescencia de estado estacionario (EFEE), permitiendo obtener información complementaria de los sistemas bajo estudio. Los experimentos de FT-NIR revelaron que la presencia de relleno: (a) no permite una reacción completa de curado siendo este efecto mayor cuando se utiliza caolín sin modificar (este resultado se ha relacionado con las características superficiales del refuerzo) y (b) no afecta significativamente al mecanismo de reacción. Los resultados de EFEE indicaron que la reacción en la interfase avanza mediante un mecanismo distinto que la reacción en la matriz, por medio del cual la reacción se ve favorecida. Este resultado se podría explicar considerando la mayor cantidad de grupos hidroxilo en la interfase, capaces de catalizar la reacción de curado. El estudio morfológico y estructural de los materiales compuestos, llevado a cabo mediante DRX, SEM y TEM muestran que el material compuesto de matriz epoxi relleno con 2% en peso de caolín presenta una morfología múltiple, que va desde agregados micrométricos hasta láminas individuales de caolín. Las micrografías de SEM sugieren que el tratamiento aplicado al caolín para obtener el derivado C-EDA mejora la dispersión del caolín en la matriz polimérica y que disminuye el número de agregados al disminuir el porcentaje de caolín añadido a la matriz. Se ha estudiado el comportamiento termo-mecano-dinámico y dieléctrico de los materiales compuestos epoxi-caolín, evaluando la influencia de la presencia y porcentaje del relleno en los resultados obtenidos. Los resultados de los ensayos de DMTA revelaron una posible plastificación del material, que se ha atribuído a un cambio en la estequiometría de la mezcla reactiva DGEBA-amina ocasionado por la adición del derivado C-EDA. Por otro lado, no se observaron diferencias significativas entre las Tgs de los materiales compuestos hasta con un 2% en peso de caolín. Los resultados de la caracterización dieléctrica revelaron que la presencia del caolín, en los porcentajes empleados en este trabajo parece afectar muy ligeramente a la movilidad de los dipolos y a los entornos Gipolares asociados a la relajación principal α de la matriz polimérica y que no afecta de forma apreciable a la movilidad de los dipolos asociados a la relajación secundaria β de la resina epoxi. Además, se observó que las propiedades dieléctricas de estos materiales se hacen patentes a bajas frecuencias y altas temperaturas. También se han estudiado las propiedades mecánicas de estos materiales y se ha llevado a cabo un análisis fractográfico, analizándose el efecto de la presencia y porcentaje de caolín añadido. Los resultados indican que la adición del caolín modificado no afecta de forma significativa a la tenacidad a fractura, KIC de la matriz epoxídica. Sin embargo, los valores tienden a aumentar con el porcentaje de caolín añadido a la matriz, observándose la misma tendencia en el parámetro, GIC. Estos resultados sugieren que la presencia del caolín incrementa la resistencia del material a la fractura, efecto que parece aumentar con el contenido de caolín en la matriz. Por otro lado, la dureza universal, HU no varía de forma significativa aunque tiende a disminuir al aumentar el porcentaje de caolín en el material. El módulo de indentación, YHU, en principio tampoco se ve afectado por la presencia de caolín en los porcentajes estudiados. Este resultado podría deberse al balance existente entre el efecto que produce, por un lado, la adición de partículas de mayor módulo que la matriz, lo que conduciría a un aumento del módulo del material y por otro lado, la presencia de los agregados de caolín, que podrían actuar como centros concentradores de tensiones, en detrimento del módulo del material. También es probable que al llevar a cabo los ensayos de microdureza el indentador no haya encontrado en la superficie de los materiales las partículas de caolín, y por tanto su efecto reforzante no se haya reflejado en estos ensayos. El análisis fractográfico muestra que en los materiales compuestos, la presencia del caolín parece actuar dificultando la propagación de grietas. Los resultados indican que en los materiales compuestos el caolín absorbe parte de la energía de fractura en el material a través de la actuación de algunos mecanismos de aumento de la tenacidad. El mecanismo que parece dominar la fractura es el denominado “crack pinning”, cuya presencia está de acuerdo con los resultados obtenidos en los ensayos de tenacidad a fractura.-------------------------------------------------------------------------------------------------------------------------------
In the present work composite materials based on a thermosetting epoxy matrix filled with kaolin nanoplatelets were prepared and characterized. For that purpose an aminomodified kaolin has been prepared, in order to compatibilize the surface of the kaolin platelets with the epoxy matrix and to increase the interlayer space of the kaolin, promoting therefore the intercalation of the polymeric matrix and the exfoliation of the clay. The modified kaolin is based on the intercalation of ethylenediamine in between the nanoplatelets that constitute the layered silicate. The intercalation process was based on three consecutive steps of intercalation with three different molecules: i) dimethylsulfoxide (C-DMSO); ii) methanol (C-MeOH) and iii) ethylenediamine (C-EDA). The results obtained from the amino-modified kaolin (C-EDA) characterization by means of X-ray diffraction (XRD), Fourier transformed infrared spectroscopy in the middle range (FTIR) and steady state fluorescence spectroscopy. Indepth analyses of the three kaolin derivatives revealed the presence of DMSO molecules in this derivative and the intercalation of EDA molecules in between the kaolin nanolayers. The effect exerted by the presence of different types of kaolin on the epoxy curing process in the kaolin – epoxy based nanocomposites was evaluated, discussing the influence of different kaolin pre-treatments. The crosslinking reaction of the epoxy polymer matrix was monitored by using two experimental techniques: Fourier transformed infrared spectroscopy (FTIR) in the near range and steady state fluorescence spectroscopy, allowing us to obtain complementary information of the systems under study. The FTIR experiments revealed that the presence of the reinforcement does not allows a full curing of the system. In fact, this effect seems to be strongest when pristine kaolin is used. This result was interpreted in terms of the surface characteristics of the reinforcements. A kinetic analysis revealed that the presence of the reinforcement does not significantly affect the overall rate at which the curing takes place. On the other hand, the use of a fluorophore molecule allowed us to monitor the cure reaction either at the polymer bulk or at the interface of the composite materials. These results indicated that the reaction carried out at the interface proceeds by means of a different mechanism for which the reaction is favored. A possible explanation of this result may be a higher amount of hydroxyl groups capable of catalyzing the curing reaction. The morphology and structure of the composite materials were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and DRX. These results indicated that the pre-treatment done to kaolin particles to obtain the C-EDA derivative promotes a better dispersion of the filler in the epoxy matrix. On the other hand, these results revealed that the epoxy resin filled with 2% by weight of CEDA presents a multiple morphology which includes individual kaolin platelets, crystallites and micrometric aggregates distributed throughout the polymer matrix. The thermo-mechanical-dynamic and thermo-dielectric behavior of the composites was studied by means of dynamic-mechanical-thermal analysis (DMTA) and dielectric relaxation spectroscopy (DRS), evaluating the effect of the filler. The DMTA results revealed a possible plasticizer effect attributed to an off-stequiometric mixture epoxyamine due the C-EDA addition in the polymeric matrix. On the other hand, no significant differences between the glass transition temperatures (Tg) of the materials filled with C-EDA were observed (at least, up to 2% by weight of filler). The DRS results indicated that the presence of kaolin in the same amounts does not significantly affect the mobility of the dipoles associated to the β secondary relaxation of the epoxy matrix and it seems to affect very slightly the mobility of the dipoles associated to the α relaxation of the matrix. Besides, it was observed that the dielectric properties of these materials were revealed at high temperatures and low frequencies. The mechanical properties of the composite materials were also analyzed and a fractographic analysis was done, evaluating the influence of the filler. These results indicated that the addition of C-EDA does not significantly affect the fracture toughness in terms of the KIC parameter. However, an increasing trend of the fracture toughness with increasing amounts of filler was observed. The values of the GIC parameter showed a similar tendency. These results suggest that the presence of C-EDA in the thermosetting matrix increases the fracture toughness of the material. This effect seems to be more marked as the amount of kaolin increases. As regarding the indentation modulus, YHU, it was not affected by the addition of C-EDA, at least in the percentages used in this work (from 0 to 2% by weight). A possible explanation of this result may be the balance between two effects, on the one hand the presence of kaolin should increase the modulus of the material since it has a higher modulus than the epoxy matrix, but on the other, the kaolin micrometric domains could be acting as stress concentration sites in the material, decreasing the value of YHU. Also it is probably that during the microhardness experiments the indentator did not find the kaolin particles on the surface of the samples, so their reinforcement effect in the matrix was not reflected in the global results. Finally, the fractographic analysis indicated that the presence of kaolin makes more difficult crack propagation in the epoxy matrix. The results point out that kaolin domains absorb part of the fracture energy due the presence of some toughening mechanisms. Among them, the one that seems to be predominant is the crack pinning mechanism which is in good agreement with the results obtained in fracture toughness tests.
In the present work composite materials based on a thermosetting epoxy matrix filled with kaolin nanoplatelets were prepared and characterized. For that purpose an aminomodified kaolin has been prepared, in order to compatibilize the surface of the kaolin platelets with the epoxy matrix and to increase the interlayer space of the kaolin, promoting therefore the intercalation of the polymeric matrix and the exfoliation of the clay. The modified kaolin is based on the intercalation of ethylenediamine in between the nanoplatelets that constitute the layered silicate. The intercalation process was based on three consecutive steps of intercalation with three different molecules: i) dimethylsulfoxide (C-DMSO); ii) methanol (C-MeOH) and iii) ethylenediamine (C-EDA). The results obtained from the amino-modified kaolin (C-EDA) characterization by means of X-ray diffraction (XRD), Fourier transformed infrared spectroscopy in the middle range (FTIR) and steady state fluorescence spectroscopy. Indepth analyses of the three kaolin derivatives revealed the presence of DMSO molecules in this derivative and the intercalation of EDA molecules in between the kaolin nanolayers. The effect exerted by the presence of different types of kaolin on the epoxy curing process in the kaolin – epoxy based nanocomposites was evaluated, discussing the influence of different kaolin pre-treatments. The crosslinking reaction of the epoxy polymer matrix was monitored by using two experimental techniques: Fourier transformed infrared spectroscopy (FTIR) in the near range and steady state fluorescence spectroscopy, allowing us to obtain complementary information of the systems under study. The FTIR experiments revealed that the presence of the reinforcement does not allows a full curing of the system. In fact, this effect seems to be strongest when pristine kaolin is used. This result was interpreted in terms of the surface characteristics of the reinforcements. A kinetic analysis revealed that the presence of the reinforcement does not significantly affect the overall rate at which the curing takes place. On the other hand, the use of a fluorophore molecule allowed us to monitor the cure reaction either at the polymer bulk or at the interface of the composite materials. These results indicated that the reaction carried out at the interface proceeds by means of a different mechanism for which the reaction is favored. A possible explanation of this result may be a higher amount of hydroxyl groups capable of catalyzing the curing reaction. The morphology and structure of the composite materials were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and DRX. These results indicated that the pre-treatment done to kaolin particles to obtain the C-EDA derivative promotes a better dispersion of the filler in the epoxy matrix. On the other hand, these results revealed that the epoxy resin filled with 2% by weight of CEDA presents a multiple morphology which includes individual kaolin platelets, crystallites and micrometric aggregates distributed throughout the polymer matrix. The thermo-mechanical-dynamic and thermo-dielectric behavior of the composites was studied by means of dynamic-mechanical-thermal analysis (DMTA) and dielectric relaxation spectroscopy (DRS), evaluating the effect of the filler. The DMTA results revealed a possible plasticizer effect attributed to an off-stequiometric mixture epoxyamine due the C-EDA addition in the polymeric matrix. On the other hand, no significant differences between the glass transition temperatures (Tg) of the materials filled with C-EDA were observed (at least, up to 2% by weight of filler). The DRS results indicated that the presence of kaolin in the same amounts does not significantly affect the mobility of the dipoles associated to the β secondary relaxation of the epoxy matrix and it seems to affect very slightly the mobility of the dipoles associated to the α relaxation of the matrix. Besides, it was observed that the dielectric properties of these materials were revealed at high temperatures and low frequencies. The mechanical properties of the composite materials were also analyzed and a fractographic analysis was done, evaluating the influence of the filler. These results indicated that the addition of C-EDA does not significantly affect the fracture toughness in terms of the KIC parameter. However, an increasing trend of the fracture toughness with increasing amounts of filler was observed. The values of the GIC parameter showed a similar tendency. These results suggest that the presence of C-EDA in the thermosetting matrix increases the fracture toughness of the material. This effect seems to be more marked as the amount of kaolin increases. As regarding the indentation modulus, YHU, it was not affected by the addition of C-EDA, at least in the percentages used in this work (from 0 to 2% by weight). A possible explanation of this result may be the balance between two effects, on the one hand the presence of kaolin should increase the modulus of the material since it has a higher modulus than the epoxy matrix, but on the other, the kaolin micrometric domains could be acting as stress concentration sites in the material, decreasing the value of YHU. Also it is probably that during the microhardness experiments the indentator did not find the kaolin particles on the surface of the samples, so their reinforcement effect in the matrix was not reflected in the global results. Finally, the fractographic analysis indicated that the presence of kaolin makes more difficult crack propagation in the epoxy matrix. The results point out that kaolin domains absorb part of the fracture energy due the presence of some toughening mechanisms. Among them, the one that seems to be predominant is the crack pinning mechanism which is in good agreement with the results obtained in fracture toughness tests.
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Keywords
Ensayo de materiales, Materiales nanocompuestos, Sistemas epoxi-caolín