A novel sequential dual-curable thermosetting system with a custom-tailorable structure based on a click thiol–epoxy reaction and excess epoxy is presented.

A new dual-curing, solvent-free process is described for the preparation of tailor-made materials from off-stoichiometric amine-acrylate formulations. The first stage reaction is a self-limiting click aza-Michael addition between multifunctional amine and acrylate monomers with an excess of acrylate groups. The second stage reaction is a photoinduced radical polymerization of the unreacted acrylate groups. By selecting the structure of the monomers and the stoichiometry of the formulations, mechanical and thermal characteristics of the intermediate and final materials can be tuned. The materials obtained after the first curing stage can be gelled or ungelled and loosely or tightly crosslinked at the end of the second curing stage. The methodology used allows to obtain storable and procesable intermediate polymers and final networks with optimum properties for different applications. The presence of amines in the reaction medium overcomes the intrinsic oxygen inhibition of acrylate free-radical polymerizations, resulting in a quasi complete cure. Dual curing processing, combining different and compatible polymerization mechanisms, can be used in many applications to obtain a stable material after the first stage that maintains the ability, upon application of a second stimulus, to further react and achieve the final properties desired. Nair et al. 6 developed new two-stage reactive systems based on the combination of base-catalyzed thiol-acrylate Michael addition and UV-induced free radical polymerization of excess acrylates. Peng et al. 7 prepared

No abstract

Acrylate chemistry has found widespread use in dual-curing systems over the years. Acrylates are cheap, easily handled and versatile monomers that can undergo facile chain-wise or step-wise polymerization reactions that are mostly of the "click" nature. Their dual-curing processes yield two distinct and temporally stable sets of material properties at each curing stage, thereby allowing process flexibility. The review begins with an introduction to acrylate-based click chemistries behind dual-curing systems and relevant reaction mechanisms. It then provides an overview of reaction combinations that can be encountered in these systems. It finishes with a survey of recent and breakthrough research in acrylate dual-curing materials for shape memory polymers, optical materials, photolithography, protective coatings, structured surface topologies, and holographic materials.

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.

A new family of dual-curable poly(hydroxyamine)-poly(ether) thermosets based on off-stoichiometric amine-epoxy formulations has been prepared and characterized. The first curing stage was a self-limiting click epoxy-amine polycondensation with an excess of epoxides and the second stage was an anionic homopolymerization of the unreacted epoxy groups, initiated by a latent base. The curing process was sequential with storage stable intermediate materials. The latency of these partially-cured intermediate materials was established not only by a thermally activated base generator, but also by the vitrification of the formulations. The intermediate and final materials exhibit a wide range of properties depending on the relative contribution of both curing stages. Intermediate materials can either be shape conformable solids, or liquids that are applicable as adhesives. Fully cured materials exhibit shape-memory effect.

The mechanism of thiol–epoxy reactions has been analyzed from a theoretical point of view and modelled using experimental kinetic data.

No abstract