Phenomenological characterization of sequential dual-curing of off-stoichiometric “thiol-epoxy” systems: Towards applicability

Abstract

An extensive characterization of a sequential dual-curing system based on off-stoichiometric "thiol-epoxy" mixtures was carried out using thiol compounds of different functionality. The intermediate and final materials obtained after each curing stages at different thiol-epoxy ratios were studied by means of thermomechanical and rheological experiments. The storage and loss modulus and the loss factor tan δ were monitored during the curing process to analyse gelation and network structure build-up. The critical ratio for gelation was determined making use of the ideal Flory-Stockmayer theory and compared with experimental results. Intermediate materials obtained in the vicinity of the theoretical critical ratio did not have the mechanical consistency expected for partially crosslinked materials, did not retain their shape and even experienced undesired flow upon heating to activate the second curing reaction. The rheological results showed that the critical ratio is higher than the predicted value and that a softening during the second curing stage affects the shape-retention at this ratio. From the thermomechanical results, a wide range of intermediate and final materials with different properties and applicability can be obtained by properly choosing the thiol-epoxy ratio: from liquid-like to highly deformable intermediate materials and from moderately crosslinked (deformable) to highly crosslinked (brittle) final materials.

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

“…Therefore, it can be assumed safely that quantitative conversion of epoxy groups takes place in any case. For DGS3‐1, T g∞ is equal to 34 °C but for the DGS3‐0.5 system it is equal to 79 °C, as reported previously, because of the contribution of the tighter network structure of the homopolymerized epoxy. In previous works we reported that this increase in T g∞ was accompanied by an increase in relaxed modulus due to the higher crosslink density caused by the homopolymerization of the excess epoxy groups in the DGS3‐0.5 formulation.…”

“…In addition, in thiol–epoxy off‐stoichiometric formulations the network structure at the end of the first processing stage can be easily tailored just by changing the thiol–epoxy ratio . In the case of DGS3‐0.5, we verified experimentally that gelation and subsequent crosslinking take place during the second curing stage . This is of crucial importance, because it does not matter if the first curing stage is not uniform at all, because the system is still liquid‐like at the end of it.…”

“…The reaction mechanisms of nucleophile‐initiated thiol–epoxy polymerization and epoxy homopolymerization have been recently discussed in detail elsewhere . Because the kinetics of the second process are much slower than the first one, it has been possible to develop recently a new family of dual‐curable thermosetting materials that have found, so far, an application in shape‐memory devices …”

“…For this expression to be valid, there should be a controlled curing sequence and therefore good separation between processes, as is commonly observed in off‐stoichiometric thiol–epoxy systems …”

“…This curing system has manifold advantages: only one initiator is required to activate both reactions, a wide range of tertiary amine initiators are available, formulations can be safely prepared due to the slow activation of the curing process and it can be used dual‐curing applications where UV irradiation is not feasible. Intermediate materials with properties ranging from viscoelastic liquid to lightly crosslinked structures can be easily designed by means of changing the monomer feed ratio and structure . It makes sense to choose a dual‐curing formulation that does not gel in the first curing stage, so that a stable liquid‐like intermediate material is obtained, and crosslinking takes place entirely in the second curing stage in a more controlled way (see Scheme ).…”