Abstract
Epoxy adhesive formulations using latent imidazole metal cation complexes Complexes of 2-methylimidazole with cations from several metal sulfates were prepared and investigated as curing agents for epoxy resins. The reactivity of one-part formulations of these complexes with a bisphenol A type epoxy resin was determined by the differential scanning calorimetry and the pot life observed by viscosity measurements. Tensile lap shear tests at room temperature and at 120°C were used to evaluate the adhesive strength of the formulations directly after preparation as well as after one and three months of storage at room temperature. The DSC measurements showed much lower reactivity (7 - 32%) and higher reaction temperatures of the complex formulations in comparison to the mixtures with pure 2-methylimidazole. The viscosity of most formulations remained almost unchanged over the observed period of three months. The adhesive strength of the freshly prepared complex formulations is comparable to a formulation with pure 2-methylimidazole and decreases over time, depending on the type of metal cation and the cation-to-imidazole molar ratio. The obtained results indicate that complexes of 2-methylimidazole with cations are suitable as latent curing agents for epoxy resins.
Highlights
This work describes a one-part epoxy resin compositions with imidazole complexes as latent curing agents[1,2,3]
Epoxy resin compositions for use as adhesives or molding materials can be formulated as two-part systems, wherein the resin component and the curing agent are prepared separately and must be mixed before application, or as the one-part systems which are ready-to-use mixtures of a resin with a suitable curing agent
In this work we present the properties of epoxy adhesives hardened with imidazole complexes[1], which were obtained by the new method of synthesis and purification of these products[4,5,6,7]
Summary
This work describes a one-part epoxy resin compositions with imidazole complexes as latent curing agents[1,2,3]. Since the components of the two-part systems must be accurately measured and mixed before application, measurement errors are likely to occur which can lead to faulty adhesive joints. The quick reaction between the resin and the curing agent does not allow for the extended storage of the mixed adhesive. Premature commencement of the reaction between the epoxy resin and the curing agent may still lead to a reduction in storage stability. The most common solution to this problem are epoxy resin formulations with dicyandiamide as a curing agent which are known for long storage stability, but resin compositions with imidazoles and their complexes are used as well1, 4 – 7. Authors developed a new way to obtain the latent curing agents on the basis of imidazole and inorganic metals salts, which will be published as new patent application
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