Epoxy resins have been widely used in several materials including carbon fiber composites; however, they are arduous to recycle. In this study, for the first time, a Schiff base epoxy thermoset combining excellent recyclability and high performance was facilely prepared from a synthesized formyl group-containing vanillin-based monoepoxide and a diamine via in situ formation of the Schiff base structure and epoxy network. The chemical structure of the monoepoxide and its cross-linked network were characterized in detail. In addition, the thermal and mechanical properties, recyclability of the thermoset and its application in carbon fiber composite were systematically investigated. The results showed that the thermoset possessed a similar glass transition temperature of 172 °C, a tensile strength of 81 MPa and a modulus of 2112 MPa, and higher thermal stability with the degradation temperature for 5% weight loss of 323 °C and elongation at break of 15% in comparison with a bisphenol A epoxy resin. Moreover, it exhibited superior reprocessing recyclability due to the vitrimer or CAN nature of its Schiff base network. Furthermore, it could also be completely degraded under mild acidic conditions, leading to the quick and nondestructive recycling of its carbon fiber composite.

Two diepoxides containing a two-benzene-ring-conjugated Schiff base when cured with an aromatic diamine result in high-performance thermosets combining excellent controlled degradability, stability and antibacterial properties. Epoxy resins have been widely used in coatings, adhesives, composites, electronic packaging materials, etc., but they are arduous to recycle and have no antibacterial activity. In this paper, two-benzene-ring-conjugated Schiff base diepoxy monomers were synthesized and further cross-linked with 4,4-diaminodiphenylmethane. The chemical structures of the diepoxides as well as their thermosets were characterized in detail. The property investigation manifested that the thermosets possessed a far higher glass transition temperature of ∼206 °C, tensile strength of ∼122 MPa, and tensile modulus of ∼2646 MPa than a commonly used high-performance bisphenol A epoxy resin. Moreover, they possessed excellent acidity and temperature-controlled degradability, and superior stability in common solvents, thermal treatment and hygrothermal aging. Furthermore, they presented excellent coating properties (including hardness, adhesion, and solvent resistance) and antibacterial properties with a bactericidal capability of ∼91%.

This review provides an overview of the closed-loop recycling of thermosets via hydrolysis and dynamic exchange reactions.

Conventional thermosets, widely used in composites, coatings, electronic packing materials, etc., are typically made of non-sustainable fossil resources and cannot be recycled under mild conditions. Degradable thermosets are promising alternatives, while combining ready degradability and high performance is a challenge. In this paper, for the first time, a spiro diacetal structure was utilized to synthesize readily recyclable thermosetting materials with high performance. A renewable bioresource, lignin derived vanillin, was used to produce a rigid spiro diacetal trigger and further built into three thermosets. Among them, spiro diacetal epoxy resin was systematically investigated and was proved to be readily degradable under mild acidic conditions while maintaining stability under neutral or basic conditions and showing outstanding thermal stability. Meanwhile, its glass transition temperature, tensile strength, modulus, elongation at break are comparable to or even better than those of conventional bisphenol A epoxy resin. Moreover, facilely recyclable carbon fiber reinforced composites and convenient-removal coatings were achieved from this epoxy resin accompanied by excellent performances.