Flame-retardant recyclable epoxy networks: Strategies, mechanisms, and future directions

Research teams from Swinburne University of Technology and University of Southern Queensland have provided a deep overview of the current state of the art of fire-retardant recyclable epoxy systems (FRREs) based on covalent adaptable networks. By integrating dynamic covalent bonds (DCBs) and flame-retardant groups into the epoxy crosslinking network can effectively improve fire safety and recyclability. However, how to balance the recyclability, flame retardancy, and network stability of FRREs remains a key challenge. This review provides valuable insights into the directional design of high-stability FRREs.

Epoxy resins (EPs) are fundament material in modern industries owing to their exceptional properties and thermal stability. These qualities make them indispensable in electronics, automotive, aerospace, and structural engineering applications. They are widely used in the fields of adhesives, coatings, electronics and electrical, construction, and carbon fibers-reinforced polymers (CFRPs) due to its excellent processability, mechanical properties, adhesive performance, corrosion resistance, and dimensional stability. However, there are two main drawbacks of EP materials in practical industrial applications: (i) high flammability due to their carbon-hydrogen-based structure; and (ii) non-recyclability/reprocessability caused by their permanently crosslinked network. These problems raise safety concerns and environmental issues. Currently, efforts to enhance fire safety often involve adding flame-retardant additives, which can compromise mechanical strength and complicate recycling processes.

Despite advancements in flame-retardant strategies, sustainability remains elusive. Conventional epoxy systems are predominantly thermosetting, rendering them difficult to recycle, and their fixed network structures tend to deteriorate over time, especially under environmental stress. The integration of reversible bonds through dynamic covalent chemistry (DCC) offers a promising avenue to imbue epoxy networks with recyclability and self-healing capabilities. Yet, achieving a delicate balance between flame retardancy, recyclability, and in-service performance has remained a significant hurdle. Depending on the difference of dynamic covalent reactions, FRREs can be categorized into carboxylic ester-based, phosphate ester-based, imine-based, disulfide-based, and Diels-Alder-mediated exchange. Nevertheless, the introduction of weak reversible linkages and labile flame-retardant structures inevitably compromise the structural integrity of the networks, resulting in reduced thermal stability, creep resistance, and long-term durability in harsh service environments. This contradiction between dynamic recyclability and network stability represents the central obstacle preventing FRREs from broader engineering applications. Following this, researchers proposed some feasible and potential solutions to construct high-stability FRREs, such as (i) controlling catalytic activity, (ii) incorporating high-energy DCBs, (iii) introducing rigid or conjugated groups, (iv) utilizing noncovalent interactions, (v) constructing hyperbranched and interpenetrating networks.

By rational structure design, future FRREs are expected to achieve the combined goals of excellent in-service performance, outstanding flame retardancy, and high-efficiency recyclability. Nevertheless, several challenges and technical barriers related to achieving an optimal balance between recyclability, flame retardancy, and long-term stability continue to hinder the large-scale application and commercialization of high-stability FRREs. To advance the practical implementation of these materials, future research should focus on optimizing molecular design strategies, conducting systematic mechanistic studies, employing theoretical modeling, and developing scalable manufacturing processes.

This review provides a comprehensive roadmap for the directional design and fabrication of FRREs, while also suggesting promising research pathways for developing high-stability FRREs.

This work has been recently published in the online edition of Materials Futures, a new international journal in the field of interdisciplinary materials science research.

Reference: Qingshan Yang, Yong Guo, Guofeng Ye, Cheng Wang, Asim Mushtaq, Min Hong, Pingan Song, Hao Wang, Siqi Huo. Fire-retardant recyclable epoxy systems based on covalent adaptable networks[J]. Materials Futures. DOI: 10.1088/2752-5724/ae17de