The thermal, mechanical, and adhesive properties of nanoclaymodified adhesives were investigated. Two organically modified montmorillonites: Cloisite 93A (C93A) and Nanomer I.30E (I.30E) were used as reinforcement of an epoxy adhesive. C93A and I.30E are modified with tertiary and primary alkyl ammonium cations, respectively. The aim was to study the influence of the organoclays on the curing, and on the mechanical and adhesive properties of the nanocomposites. A specific goal was to compare

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... ir behavior with that of Cloisite30B/epoxy and Cloisitel5Aj epoxy nanocomposites that we have previously studied. Both C30B and C15A are modified with quaternary alkyl ammonium cations. Differential scanning calorimetry results showed that the clays accelerate the curing reaction, an effect that is related to the chemical structure of the ammonium cations. The three Cloisite/ nanocomposites showed intercalated clay structures, the interlayer distance was independent of the clay content. The I.30E/epoxy nanocomposites presented exfoliated structure due to the catalytic effect of the organic modifier. Clay-epoxy nanocomposites showed lower glass transition temperature (T g ) and higher values of storage modulus than neat epoxy thermoset, with no significant differences between exfoliated or intercalated nanocomposites. The shear strength of aluminum joints using clay/epoxy adhesives was lower than with the neat epoxy adhesive. The water aging was less damaging for joints with I.30E/epoxy adhesive. INTRODUCTION Epoxy resins after the crosslinking (curing) give rise to thermosetting materials. These polymers are widely used as coatings, composite matrices, and adhesives. The reason lies in their excellent properties, combining creep resistance and rigidity, mechanical and thermal resistance, environmental stability, good chemical resistance, electrical insulating performance, and high adhesive strength. Other attractive features are the absence of volatile and low shrinkage on curing. Epoxy resins are versatile and can be crosslinked in a broad range of temperatures and with different degrees of crosslinking. The curing agents used at room temperature are usually aliphatic amines, however curing with aromatic amine needs high temperatures. In general, epoxy cured at high temperature exhibits improved high temperature properties such as increased glass transition temperature (7p, higher strength, and stiffness as compared to those cured at room temperature [1] . Epoxies are used as structural adhesives, matrices for composite materials, encapsulating agents, etc. Epoxy/clay nanocomposites are materials obtained by dispersing clay platelets in epoxy thermosets. Montmorillonite is the most commonly used layered silicate in polymer matrix nanocomposites. In order to obtain a good dispersion of the clay layers in the polymer, the clays are modified by replacing the inorganic cations (located between the silicate layers) by alkyl ammonium cations [2] [3] [4] [5] . Clay/epoxy nanocomposites have been investigated in recent years [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] , because it is expected that the homogeneous dispersion of clay layers in the polymer leads to improved mechanical and thermal barrier properties compared to the pure polymer and to conventional composites with similar reinforcement content. In the nanocomposite the epoxy monomers intercalate between the clay layers, leading to an increase of the clay spacing and, in the best of cases to the exfoliation of the silicate layers that become dispersed in the polymer matrix. The epoxy curing reaction occurs inside and outside the clay galleries (space between silicate layers). It has been proposed that the exfoliation depends on the ratio between the rates of both reactions [7] . Exfoliated clay/epoxy nanocomposites are obtained using modified clays with primary and secondary alkyl ammonium cations [6, 8, 12] , whereas when modified clays with quaternary alkyl ammonium are used most research indicate the formation intercalated nanocomposites [16, 18, 21] .