Enhanced Viscosity Control in Thermosets Derived from Epoxy and Acrylate Monomers Based on Thermoreversible Aza-Michael Chemistry

Abstract

Epoxies, epoxy acrylates, and acrylic resins account for a large portion of the thermoset market, yet their inherently highly cross-linked nature prevents them from being reprocessed or recycled. Herein, we present a simple reversible epoxy-based curing strategy that can address these issues. First, an epoxy monomer is ring-opened with ammonia to result in β-hydroxyamines through a straightforward and scalable synthesis that we have demonstrated on multiple examples. The epoxy-derived amines are then cured with bisacrylates, creating dynamic covalent β-amino ester crosslinkages through a thermoreversible aza-Michael reaction. The resulting networks show a very pronounced drop in viscosity in the temperature region of 150−180 °C, which we were able to attribute mainly to significant de-cross-linking of the amine and acrylate moieties, rather than to activation of dynamic ester bond exchanges. Nevertheless, the materials do not fully liquify and retain their structural integrity as a result of the very fast amine-acrylate rebonding kinetics. As a result, cross-linked epoxy-based materials could be obtained with simultaneously enhanced (re)processability at high temperatures and strongly inhibited deformation at lower temperatures (<120 °C). The protocol is demonstrated for both petrochemically based building blocks (such as bisphenol A), as well as for fully biobased compounds (vanillin-epoxy and a Velvetol-based bisacrylate), showing its versatility. As a result of the widespread use of epoxy resins, the ease of implementation, and its interesting temperature window for debonding/ rebonding, industrial applications can be foreseen for this thermoreversible curing strategy.

β-Amino amide based covalent adaptable networks with high dimensional stability

“…), the use of highly endothermic retro-chemistry is one of the strategies showing the greatest potential. [24][25][26][27][28][29][30] Indeed, by introducing dynamic covalent chemistries that are characterized by a highly endothermic dissociation, the exchange activation energy increases, thereby shifting the exchange away from the application conditions. Hence, an appropriate balance between (re)processability and dimensional stability should be aimed for.…”