Sequential dual-curing by combination of isocyanate-epoxy and thiol click reactions

Sequential dual-curing procedures consist in the combination of two different and compatible polymerizations taking place sequentially in a well-controlled manner, without overlapping among them. The processing by sequential dual curing allows preparing stable intermediate materials, once finished the 1st curing stage, that keep the ability, after stimulation, to activate the 2nd curing stage and complete the process, leading to fully-cured materials with the desired final properties. This approach makes possible to obtain uncommon-shaped materials such as spring- or bent-shaped thermosets with optimal mechanical and, in some cases, shape-memory properties. The intermediate materials can be highly viscous liquids or deformable rubbers depending on the composition of the formulation, which can be used as coatings or adhesives or in case of rubber, they can be processed i.e. by compression-moulding. In the communication, we report the preparation and characterization of a new family of thermosets obtained by thiol-isocyanate and thiol-epoxy reactions, both activated by temperature. The sequential dual character of this curing system relies on the faster reaction kinetic of the thiol-isocyanate coupling at lower temperature, than the thiol-epoxy reaction. The extent of the first curing step is controlled by the isocyanate/thiol equivalent ratio. This proportion defines the intermediate and final material properties. Both reactions have a click character since they are specific, without the formation of undesired by-products, and they lead to homogeneous networks, which are highly adequate to be used as shape memory smart materials, because of their narrow transitions that allows quick movements and a rapid change in their properties [1, 2]. The formulations studied were formed by different proportions of diglycidylether of bisphenol A and one of the two isocyanates, hexamethylene diisocyanate and isophorone diisocyanate. Pentaerythritol tetrakis (3-mercaptopropionate), which acts as crosslinking unit of both type of network structures, was added to the formulation in stoichiometric ratio. As catalyst, 1-methylimidazole was used. The kinetics of both curing stages and the conversion achieved were studied by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). In Figure 1 we can see the DSC traces of IPDI and HDI formulations in which the appearance of two separated peaks, corresponding to both curing stages, can be observed. The lower the proportion of the catalyst the better is the separation between both processes.