The effect of stoichiometry on curing and properties of epoxy–clay nanocomposites |
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Authors: | M A García del Cid M G Prolongo C Salom C Arribas M Sánchez-Cabezudo R M Masegosa |
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Institution: | 1. Departamento Materiales y Producción Aeroespacial, Universidad Politécnica de Madrid, Madrid, Spain 2. Departamento Física y Química Aplicadas a la Técnica Aeronáutica, Universidad Politécnica de Madrid, Madrid, Spain
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Abstract: | Epoxy–clay nanocomposites have been prepared with an organically modified montmorillonite. The epoxy network was based on
diglycidyl ether of bisphenol A (DGEBA) cured with diaminodiphenylmethane (DDM). The stoichiometry DGEBA–DDM was varied, the
molar ratio of amine hydrogen/epoxy groups, r, ranged from 0.85 to 1.15. The influence of stoichiometry on curing and properties of the nanocomposites was studied using
differential scanning calorimetry, dynamic mechanical thermal analysis and X-ray diffraction. All nanocomposites had intercalated
clay structures. The clays accelerated the curing reaction whose rate was also increased when increasing r. The heat of reaction, −ΔH (J/g epoxy), increased as r increased, reaching a constant value for r ≥ 1. In the presence of clays −ΔH was lower than in the neat DGEBA–DDM. The glass transition temperature (T
g) of the neat epoxy thermosets reached a maximum at r = 1; however, the nanocomposites showed the T
g maximum at 0.9 < r < 1. The presence of clay lowered the T
g for r > 0.94 and raised T
g for r ≤ 0.85. The elastic modulus of neat epoxy thermosets reached a maximum in the rubber state and a minimum in the glassy state
at r = 1. The nanocomposites showed similar behavior, but the maximum and the minimum values of the elastic modulus were reached
at stoichiometry r < 1. The comparison of the properties of neat epoxy with those of the nanocomposites varying the stoichiometry indicates
that the clay itself induces stoichiometric changes in the system. |
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