Time-temperature-transformation (TTT) diagram of a dual-curable off-stoichiometric epoxy-amine system with latent reactivity

Abstract

Dual-curing, that is, the combination of two different polymerization processes taking place simultaneously or sequentially in a curing process, is a highly advantageous technology for the processing of thermosetting systems [1,2] because of its versatility and flexibility. A significant added value of sequential dual curing is possibility of controlling the curing process sequence and the intermediate and final network structures and properties [3-5]. This has potential application in multi-stage processing of one-pot formulations, where materials can be partially pre-cured before their storage, and with tailored material properties for the final application [4-7]. Processing or assembly and the final properties can be achieved whenever desired by using heat or UV light, depending on formulation chemistry, to initiate the final curing stage. Click-type reactions are commonly used in dual-curing formulations [1], in combination with other click [8,9] or non-click reactions [3,5]. Many dual-curable systems are based on acrylate reactions [2,5], although systems based on epoxy curing chemistries such as off-stoichiometric thiol-epoxy [3] or epoxyamine [4,10] formulations, or other epoxy systems [11], are also gaining interest in the recent years. We recently developed a novel off-stoichiometric epoxy-amine system, with excess epoxy groups and latent reactivity in the intermediate state [4]. The first curing reaction was a self-limiting epoxy-amine condensation that takes place at moderate temperatures until exhaustion of reactive amine groups. In the presence of a suitable initiator such as a nucleophilic tertiary amine [10], the excess of epoxy groups can undergo anionic homopolymerization at higher temperatures. However, in this recent work we employed a latent thermal base for the epoxy homopolymerization [4], which ensured at least 7 weeks of storage stability at 30 °C for intermediate materials after the completion of the epoxy-amine reaction. Different materials, coming from a wide range of formulations with different amine-epoxy ratios and tailored intermediate and final properties, were analyzed and characterized, but the curing kinetics of Time-temperature-transformation (TTT) diagram of a dual-curable off-stoichiometric epoxy-amine system with latent reactivity Núria Areny

Sequential heat release: an innovative approach for the control of curing profiles during composite processing based on dual‐curing systems

“…Other dual‐curing thermosetting systems such as off‐stoichiometric thiol–acrylate or epoxy–amine could be used as well, providing suitable catalysts or initiators were used to activate the curing reactions in a controlled way in order to achieve a SHR and therefore enable easy thermal management during processing. We recently reported a novel off‐stoichiometric epoxy–amine system with excellent stability in the intermediate state that could be useful for that purpose …”

“…In the case of a dual‐curing process, this expression might be applied to the first part of the curing process (making use of T g0 , T gint and the ratio Δc pint / Δc p0 instead, measured before and after the first curing stage) and to the second part of the curing process (making use of T gint , T g∞ and the ratio Δc p∞ / Δc pint instead, measured before and after the second curing stage). This approach was successfully used in the analysis of the T g ( x ) relationship of an off‐stoichiometric amine–epoxy dual‐curing system . This expression was used to determine whether the curing system is in the relaxed state ( T > T g ( x )) or not during processing.…”

Time–temperature–transformation (TTT) and TTT–viscosity diagrams of a typical benzoxazine resin

“…When the viscosity of the resin continued to increase, the Equation (18) was no longer applicable. Under this circumstance, the double Arrhenius equation was modified by adding a temperature-dependent preceding factor A shown in Equation (10).…”

“…The linear fitting was carried out by lnA versus T and lnK versus 1/T based on Equations (11) and (13), respectively, to obtain Equations (19) and (20) Equation (21) was obtained by bringing Equations (18), (19), and (20) into Equation (10), and a threedimensional time-temperature-viscosity diagram was plotted in Figure S14 according to Equation (21). The relationship between temperature and time was obtained when η t was selected as 0.2 and 1.0 pa s, and the relational curves are shown in Figure 10.…”

“…15 Areny et al have elaborated a TTT diagram of an offstoichiometric amine-epoxy formulation with latent reactivity and illustrated the curing behaviors of the dualcuring formulations. 18 Restrepo-Zapata et al have introduced a TTT-viscosity diagram to fully characterize curing reactions of thermosetting and elastomeric resins. This diagram has been used to realize the curing process' on-line control and optimization.…”