Self-Healing and Flame-Retardant Modifications of Epoxy Resins by the Diels–Alder Release-Delivery Strategy for a High-Efficiency and Green Application

Abstract

The sustainability of epoxy thermosets (EPs) and EP-based composites is always limited by the evolution of internal microcracks and flammability. Herein, inspired by the drug release-delivery system for disease treatment, these symptoms of EP are solved efficiently by a multifunctional Diels–Alder (DA) release-delivery system. First, this novel system is fabricated by the cyclization reaction between the furan diene-containing phosphorus derivative and the maleimide dienophile. The resultant DA system is linked to the epoxy resin by the grafting reaction, thus ensuring its uniform dispersion. Being faced with the formation of inner microcracks, the maleimide dienophiles were released from the DA adducts under the mediated action of temperature and then were delivered to the targeted microcracks under the chemical potential difference derived from the variation of free volume. Then, the released maleimide molecules re-connect with the furan groups that existed near the microcracks to complete the repairing of the EP matrix. The skeletal structure of the EP was well preserved throughout the restoration process. Satisfactory flexural strength (105.5 MPa) and excellent crack-healing efficiency (104.5%) can be obtained for the modified EP. After three constant strain cycles, the stress retention rate can be readily maintained (more than 75%). In addition, due to the phosphorus–nitrogen synergistic flame-retardant effect from the DA system, not only the limited oxygen index of the DA-modified EP matrix can be improved to 27.5% but the lower peak release rate and lower total heat release are also observed compared with the unmodified EP.

High-Performance Recyclable and Malleable Epoxy Resin with Vanillin-Based Hyperbranched Epoxy Resin Containing Dual Dynamic Bonds

“…To estimate the dynamic properties of epoxy resin, the stress relaxation properties were evaluated at different temperature using DMA. In contrast to conventional epoxy resin, dynamically cross-linking epoxy resin can undergo imine and disulfide bond exchange reactions to restructure the cross-linking network, thereby releasing internal stresses and imparting dynamic properties to the network. ,− Specifically, in dynamic exchange reaction, new dynamic bonds are formed simultaneously with the dissociation of the old bonds so that the cross-linking density in the network remains little changed.…”

“…Many researchers have incorporated covalent adaptable networks (CANs) in synthesizing recyclable epoxy resins, − including imine exchange, , transesterification, − disulfide exchange, − boronic ester exchange, , siloxane exchange, and DA/retro-DA reaction . However, due to the introduction of CANs, the dynamic networks were susceptible to creep when used in engineering and structural fields, and creeping of these materials usually occurred around the topology freezing temperature ( T v ), which was usually much lower than the T g . − Many efforts have been made to improve the creep resistance, such as through controlling the availability of cross-linking or incorporation of permanent cross-linking. − For example, Xu et al prepared a type of dihydrazone-based dynamic covalent epoxy network containing dihydrazone with an initial creep temperature of about 105 °C .…”

Recycling of Epoxy Resins with Degradable Structures or Dynamic Cross-Linking Networks: A Review

“…(f) Self-healing behavior of the epoxy resins was shown by a bending strength test. (g) Cyclability of crack healing in an epoxy resin was shown by the stress test …”

“…Liu et al 152 have prepared a self-healing and flame-retardant epoxy system by performing a Diels−Alder addition cyclization reaction with a phosphorus derivative containing furan diene and maleimide diene (Figure 7c). The furan group introduced into the polymer chain provides an active reaction site for the maleimide derivative, ensuring good compatibility of this substance in the system, and allowing the maleimide derivative to be evenly dispersed in the system.…”

A DOPO‐based reactive flame retardant containing benzimidazole groups: Synthesis and its flame‐retardation on epoxy resin

“…However, the highly flammable nature of epoxy resins during its application process presents a significant safety concern, thereby greatly restricting its usage 12–14 . Consequently, the flame‐retardant modification of epoxy resins becomes particularly imperative 15 …”