Hard epoxy thermosets obtained via two sequential epoxy-amine condensations

Abstract

A new family of dual-curable poly(β-hydroxyamine ether) thermosets based on amine-epoxy formulations has been prepared and characterized. Both curing stages consist in an epoxy-amine polycondensation, each having different reactivity. Whereas the first one is carried out at relatively low temperature using an aliphatic flexible amine, the second is carried out at high temperature with a rigid aromatic amine. The sequentiality of the curing and the stability of the intermediate materials can be established in the basis of the different nucleophilicity of the amines used. The aromatic amine, with a weakly nucleophilic character, is not able to react with epoxides during the first stage at low temperature. The intermediate and final materials exhibit a wide range of properties, thanks to the different glass transition of both epoxy-amine networks, and can be tuned by changing the relative proportion of both diamines in the formulation. The participation of the epoxy resin in both curing stages ensures the homogeneity of the prepared materials, rendering them excellent candidates for applications such as adhesives or smart materials.

Time-temperature-transformation (TTT) diagram of dual-curable epoxy thermosets obtained via two sequential epoxy-amine condensations

“…The tri-glycidyl p-aminophenol epoxy resin (TGAP hereafter) with trade name Araldite MY0510 (Huntsman Advanced Materials) and an epoxy equivalent 95 g/ee determined following standard procedures [6], was used after dried in vacuum. As hardener, 4,4-diamino diphenyl sulphone (DDS hereafter) and poly(propylene glycol) bis(2-aminopropyl ether) (Jeffamine, M w = 400 g/mol) (JEF hereafter), both supplied by Sigma-Aldrich were used as received (Scheme 1).…”

“…We studied a dual formulation coded as TGAP-JEF50DDS50 (which contains 100 epoxy, 50 JEF, and 50 DDS equivalents and stage 1 and stage 2 wt fractions of 55.4% and 44.6, respectively). This formulation was selected based on our previous results [6] and because it contains the same equivalent amount of both amines, being the most representative of dual curing strategy. For comparison purposes, the curing kinetics of both neat formulations (coded as TGAPJEF and TGAPDDS) were also studied.…”

“…Among the different strategies used, a scheme based on two sequential click reactions is the most advantageous as it affords workable intermediate materials, which, upon exposure to a second stimulus, attain their ultimate properties without producing any by-products [1]. The control of the curing sequence is of paramount importance in this strategy and can be achieved by using a latent catalyst or by different stimuli at each stage or when the reaction rates are disparate [2][3][4][5][6]. TTT cure diagram plays a pivotal role in knowing the properties of thermosets and the physical events (gelation and vitrification) that take place during curing [7][8][9][10][11].…”

“…In a previous manuscript [6] we developed a new family of dual-curable epoxy thermosets, consisting of two consecutive amine-epoxy polycondensations at different temperatures and using different diamines: one aliphatic, the other aromatic, each with a different nucleophilic character. Intermediate materials had remarkable stabilities at room temperature, thanks to the low reactivity of the aromatic amine used as stage 2 curing agent, and also to vitrification during the first curing stage or during storage.…”

The Use of Click-Type Reactions in the Preparation of Thermosets

“…Dual-curable epoxy thermosets can be obtained completely in a "click manner" by using a combination of aliphatic and aromatic diamines. By employing these two amines which are reactive at two different temperatures, an epoxide compound (triglycidyl p-aminophenol epoxy resin) was dual-cured without any catalysts [209]. Depending on the initial monomer composition, partially-cured (stage 1) materials were either liquid or gel, each of which could be used for specific applications.…”