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

Abstract

A simple and robust strategy to generate time-temperature-transformation (TTT) diagrams for dual-curable thermosets is presented. TTT diagram is built for a dual-curable amine-epoxy formulation. Both curing stages are epoxy-amine polycondensations. Whereas the first stage is carried out at relatively low temperature using a strongly nucleophilic aliphatic amine, the second is carried out at high temperature with a weakly nucleophilic aromatic amine. The good separation between curing stages enabled the individual kinetics analysis of each stage by integral isoconversional procedures. Gelation was determined by thermomechanical analysis; the glass transition temperature -conversion relationship was determined by calorimetry and was used to draw the vitrification line. Gelation and vitrification lines were checked by means of solubility tests and temperature modulated calorimetry. The results presented in the form of a TTT diagram showed the disparate reactivity of both curing stages and a substantial storage stability at the intermediate curing stage. These materials show great potential in applications where processing flexibility and intermediate stability are crucial.

Time-Temperature-Transformation, Temperature-Conversion-Transformation, and Continuous-Heating-Transformation Diagrams of Reversible Covalent Polymer Networks

“…Note that a similar effect, but in this case of two superimposed vitrification lines, was observed in the TTT diagram of an irreversible epoxy-amine thermoset, cured in offstoichiometric conditions, 12 and an epoxy-amine system cured according to a mechanism with a double set of Arrhenius parameters. 22 All gelation data tgel at Tcure of Figure 10 and Figure 11 can be calculated as gel conversions xgel at Tcure by means of the optimized cure kinetics of Table 3. The results for all reversible networks are plotted in Figure 12.…”

“…by increasing Tcure). 26,29,30,33 Up to now, TTT and CHT diagrams were only established for irreversible network systems, with some recent examples given in [20][21][22] , and not for reversible networks.…”

“…Time-temperature-transformation (TTT) and temperature-conversion-transformation (TxT) diagrams for isothermal curing and the related continuous-heating-transformation (CHT) diagrams for nonisothermal curing, as developed by Gillham and co-workers, − are excellent tools to describe the cure path of thermosetting systems in a systematic and quantitative way. − The cure kinetics linking the temperature ( T cure ) and time ( t cure ) of cure with the cure conversion ( x ), together with the relationship between the cure conversion and the glass transition temperature ( T g ) of the growing network, are necessary to develop a quantitative TTT, TxT, or CHT diagram. Important transformations or characteristics of the curing system can be indicated, such as the vitrification line, the gelation line, lines of constant viscosity before gelation, and also isoconversion or iso- T g lines (see the schematic representation in Figure ).…”

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

“…The introduction of the time variable by means of the kinetics data makes it possible, in addition, to define curing schedules according to specific process requirements, in single or multiple curing stages [45]. More exotic shapes of TTT diagram can be obtained in complex systems such as dual-curing systems [49].…”

“…Liquid materials were successfully tested as adhesives and gels were shape-conformable, with the possibility of fixing the shape permanently by triggering and carrying out the second curing stage. The two-fold intermediate stability, thanks to both the low reactivity of the aromatic amine at storage temperatures and to the vitrification of the partially-cured network, was elegantly represented by a time-temperature-transformation diagram in a subsequent publication [49].…”