A Hierarchical Energy Dissipated Structure Enabled Strong, Ultra‐Tough, and Sustainable Adhesives

Abstract

Sustainable adhesives with simultaneous high strength and excellent ductility are important but still challenging. Here, an epoxy adhesive with simultaneously strong, tough, and sustainable features by designing a new hierarchical energy‐dissipated cross‐linking structure is reported: an annular structure containing quadruple internal B─N coordination boronic esters (AQBN). Under small strain, the AQBN structure is rigid to enhance the tensile strength of the adhesive. With medium strain applied, its non‐coplanar ring and internal quadruple B─N coordination can orderly open, providing mechanical energy dissipation and excellent ductility. Therefore, this multilevel structure can endow epoxy adhesives with both high strength and toughness. Besides, the rapid dynamic exchange and internal B─N coordination in boronic esters enable the adhesives to have good sustainability and environmental tolerance, respectively. As a result, the epoxy adhesives exhibit simultaneous high adhesive strength (18.5 MPa), outstanding toughness (work of debonding of 29692 N m−1), good chemical/physical recyclability, and excellent environmental reliability (humidity, water, high/ultralow temperature, and organic solvents). This work provides a new strategy for balancing the strength, toughness, and sustainability of adhesives.

Covalent Adaptable Networks Containing Nitrogen-Coordinated Boronic Ester and Imine Bonds

“…Liu and co-workers developed a series of quadruple internal B−N coordination in boronic esters cross-linked epoxy adhesive. 12 They designed a rigid but hierarchical energy-dissipated structure as the hard phase in the polymer networks, thus endowing the epoxy adhesive with a high strength of 45.5 MPa and toughness of 119.2 MJ m −3 . Zhang and co-workers demonstrated a class of closed-loop recyclable epoxy resins using itaconic acid-derived hyperbranched epoxy resin (IAEHBP) and a (1,3,5-hexahydros-triazine-1,3,5-triyl) benzyl mercaptan (HT-BM) curing agent.…”

“…Since their commercial production in the 1940s, epoxy resins have gradually become an irreplaceable thermosetting plastic product in various fields. − It is estimated that the global epoxy resin market capacity will reach 37.3 billion US dollars by 2025 . However, the fast development of epoxy resins faces tough barriers, especially the disposal of waste epoxy resins. ,− Owing to their permanent covalently cross-linked structures, the thermosetting epoxy resin products cannot be directly recycled like thermoplastics after their usage. ,− The commonly used disposal methods for these waste epoxy resin products are landfill and incineration, both of which are not environment friendly and economical. ,, To solve this issue, many researchers have constructed covalent adaptable network (CAN)-based epoxy resins using dynamic covalent bonds such as boroxines, imine bonds, boronic ester bonds, acetal linkages, and disulfide bonds. ,− ,− The resultant epoxy CANs typically exhibit promising self-healing ability, processability, and recyclability. ,,,,, Recently, many researchers have developed mechanically strong and tough epoxy CANs through the design of their chemical structures. For example, Xie and co-workers reported the design of a boronic ester bond-based epoxy CAN through the polymerization-induced nanoscale phase separation mechanism .…”

“…The obtained epoxy CAN showed an ultrahigh toughness of 108.4 MJ m –3 and tensile strength of 41.0 MPa, respectively. Liu and co-workers developed a series of quadruple internal B–N coordination in boronic esters cross-linked epoxy adhesive . They designed a rigid but hierarchical energy-dissipated structure as the hard phase in the polymer networks, thus endowing the epoxy adhesive with a high strength of 45.5 MPa and toughness of 119.2 MJ m –3 .…”

“…Epoxy-amine polymerization is commonly used to fabricate linear or cross-linked epoxy resins, ,,,− which generate β-amino diol groups during the polymerization process . The generated β-amino diol groups can be modified with boronic acids, yielding nitrogen-coordinated boronic ester bonds, which usually exhibit excellent thermal and water resistance. ,− Therefore, it is inferred that epoxy-amine-polymerized linear epoxy can be modified with boronic acid derivatives to obtain epoxy CANs with high mechanical strength and good thermal stability. Herein, we have developed mechanically strong epoxy CANs by cross-linking 4-formylphenylboronic acid (4-PBA)-modified linear epoxies produced via epoxy-amine polymerization with polyethylene polyamine (PEPA).…”

Transparent, Self-Healing, and Recyclable Biomass-Based Covalent Adaptable Networks Formed via Magnolol and Dynamic Boronate Ester

“…The introduction of exchangeable cross-linking in thermoset materials allows for favorable properties such as self-healing, remodelability, and recyclability while retaining properties such as thermal and chemical stability. Many dynamic motifs for bond exchange have been developed based on the response of exchangeable bonds to stimuli such as light, temperature, pH, and so forth, such as ester exchange reactions, boron ester exchanges, disulfide exchanges, diselenide exchanges, and imine exchanges. − …”