Epoxy resins (EPs), accounting for about 70% of the thermosetting resin market, have been recognized as the most widely used thermosetting resins in the world. Nowadays, 90% of the world's EPs are obtained from the bisphenol A (BPA)-based epoxide prepolymer. However, certain limitations severely impede further applications of this advanced material, such as limited fossil-based resources, skyrocketing oil prices, nondegradability, and a "seesaw" between toughness and strength. In recent years, more and more research has been devoted to the preparation of novel epoxy materials to overcome the compromise between toughness and strength and solve plastic waste problems. Among them, the development of bio-based hyperbranched epoxy resins (HERs) is unique and attractive. Bio-based HERs synthesized from bio-derived monomers can be used as a matrix resin or a toughener resulting in partially or fully bio-based epoxy thermosets. The introduction of a hyperbranched structure can balance the strength and toughness of epoxy thermosets. Here, we especially focused on the recent progress in the development of bio-based HERs, including the monomer design, synthesis approaches, mechanical properties, degradation, and recycling strategies. In addition, we advance the challenges and perspectives to engineering application of bio-based HERs in the future. Overall, this review presents an up-to-date overview of bio-based HERs and guidance for emerging research on the sustainable development of EPs in versatile high-tech fields.

Bio-based hyperbranched epoxy resins: synthesis and recycling.

Large-scale applications of flame-retardant epoxy resins have caused sustainability concerns on the use of renewable resources and end-of-life wastes. New strategies are required to address chemical recycling of fire-safe epoxy resins. Here, we demonstrated recyclable flame-retardant epoxy resins (FREPs) using itaconic acid-derived hyperbranched epoxy resin (IA-EHBP) and (1,3,5-hexahydro-s-triazine-1,3,5-triyl) benzyl mercaptan (HT-BM). The unique structure enabled a closed-loop chemical recyclable network that can be degraded into monomers with up to 86% yield of recovered monomers. The hyperbranched topological structure of IA-EHBP also contributed to a significant improvement in the strength and toughness of the FREPs. By using positron annihilation lifetime spectroscopy, the effects of free-volume hole size and relative fractional free-volume on the mechanical performance and dynamic mechanical properties of FREPs were studied, and the function relations were therefore established based on the Williams–Landel–Ferry equation. The incorporation of IA-EHBP effectively promoted the formation of char residue and produced phosphorus- and sulfur-containing free radicals to prevent the generation of combustible volatiles, thus enhancing the flame retardancy. The FREPs can be recycled and reused multiple times without loss of performance. The method reported here provides a facile and closed-loop approach for high-efficiency chemical recycling and reuse of end-of-life flame-retardant materials. 

Closed-Loop Recycling of Tough and Flame-Retardant Epoxy Resins

Non-isocyanate polyurethanes (NIPUs) from renewable resources have attracted wide attention because of their remarkable benefits to sustainable development and green production. In this work, a strong, self-healing, and catalyst-free NIPU (ECMP) was prepared based on the hyperbranched bio-based cyclic carbonate (Ec-MTDA) synthesized through catalytic carbonization of 1,8-menthane diamine (MTDA) and CO2. The hyperbranched and rigid structures of ECMP enable improved mechanical properties that a high tensile strength of up to 34.9 MPa can be achieved. Benefiting from the dynamic transesterification reaction between the carbamate and hydroxyl groups, ECMP presents favorable self-healing, reprocessing properties, and shape memory. Notably, 91% of the original tensile strength can be recovered after self-healing behavior. In addition, abundant polar groups provide excellent adhesion properties for ECMP with a high shear strength of 7.09 MPa. This study provides a promising strategy for the design of bio-based NIPUs, which broadens their applications in printing, furniture, packaging, and other industries. 

High-Strength, Self-Healing, Recyclable, and Catalyst-Free Bio-Based Non-Isocyanate Polyurethane

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

The extensively used petroleum-based epoxy thermosets suffer from difficulty in recycling (reprocessing, self-healing, or welding) and biodegradation due to their permanent crosslinking network, which not only causes massive consumption of petroleum resources and shorted lifespan of epoxy thermosets, but also leads to serious environmental burden after discarding. Bio-based epoxy vitrimers with reversible dynamic covalent networks are competitive alternatives, which offer the advantages of robustness like thermosets and closed-loop recycling similar to thermoplastic, and thus agree well with the idea of carbon neutrality and sustainable development. Here, we reviewed the progress in the design and properties of bio-based epoxy vitrimers with different dynamic bonds and diverse biomass feedstock in the past two years, and also highlighted the major challenges and future prospects. 

Recyclable, repairable and malleable bio-based epoxy vitrimers：overview and future prospects

Itaconic acid-based hyperbranched polymer toughened epoxy resins with rapid stress relaxation, superb solvent resistance and closed-loop recyclability

Design, Synthesis, and Biological Evaluation of Boron-Containing β-Lactamase Inhibitors: Closed-Loop Education Experiences in an Undergraduate Medicinal Chemistry Course

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Itaconic acid-based hyperbranched polymer toughened epoxy resins with rapid stress relaxation, superb solvent resistance and closed-loop recyclability

Closed-Loop Recycling of Tough and Flame-Retardant Epoxy Resins

Design, Synthesis, and Biological Evaluation of Boron-Containing β-Lactamase Inhibitors: Closed-Loop Education Experiences in an Undergraduate Medicinal Chemistry Course