Thermosetting epoxy resins are widely used in various fields owing to their high strength, corrosion resistance, and good adhesion [1–7]. However, owing to their dense three-dimensional cross-linked structure, they are difficult to recycle and reuse [8–13], and their waste causes serious environmental and resource problems [14–17].
Recycling thermosetting epoxy resins is a common problem worldwide, making it a hot topic in current research. There are two main strategies for recycling epoxy resin: one is to synthesize a special thermosetting polymer through vitrification, where the resin is introduced into the dynamic covalent bonds, such as disulfides [18], esters [19], acetal [20], Schiff bases [21], Diels–Alder addition structures [22, 23], and hemiacetal/hemiketal ester bonds [24]. Under external stimuli (e.g., heat, light, and pH), the dynamic network structure in epoxy can be cleaved or attacked by other compounds, which can change the bond arrangement and topology, resulting in the degradation of thermosetting epoxy resins. Si et al. [25] proposed a high-performance epoxy vitrification using aromatic disulfide bond cross-linking, which could be degraded by environmentally harmful thiols. Hashimoto et al. [26] established a dynamic acetal–epoxy resin system degraded by acid hydrolysis. Li et al. [27] investigated a kinetic system based on imine bonding that decomposed into small molecules in strongly acidic solvents. However, this recycling route cannot cope with the current situation of recycling waste commercial thermosetting resins.
Another route is to utilize the degradable bonds in monomers to achieve degradation recycling. Glycidyl ester epoxy resins can be degraded and recycled owing to their unique ester bond structure. Moreover, glycidyl ester epoxy resin is high performing owing to its excellent mechanical properties, heat resistance, and electrical insulation; it possesses low viscosity, high activity, low temperature resistance, and is usually used as a coating, diluent, and large-scale vacuum-infusion device [28]. Current research on the recycling of thermosetting epoxy resins includes mechanical, physical, and chemical methods, where chemical recycling is relatively effective [29, 30]. For example, Ahrens et al. [31] studied the recycling of thermosetting epoxy used for wind turbine blades and recovered bisphenol A and fiber from it. Knappich et al. [32] found that supercritical water can degrade epoxy resins, but temperatures above 400°C and pressures above 600 bar are required, resulting in high energy consumption. Liu et al. [33]studied the methanol degradation of an anhydride-cured epoxy under subcritical conditions in the presence of potassium hydroxide. Xing et al. [34] proposed a mild and effective method for the decomposition of brominated Epoxy using subcritical acetic acid. Lu et al. [35] degraded anhydride-cured epoxy in a hot solution of ethylene glycol for up to 72 h. Liu et al. [36] used an aqueous solution of environmentally friendly phosphotungstic acid as a catalyst to selectively cleave ester bonds in the cross-linked structure.
However, epoxy polymers obtained by the chemical degradation and recycling of thermosetting epoxy resins are often of low molecular weight; obtaining epoxy monomers is considerably difficult (Zhao et al. [37]). Dattilo et al. [38] found that the thermosetting resin EP can be completely degraded in the H2O2/phosphotungstic acid system at 80 ℃ over 4 h, and the macromolecular product can be used for the synthesis of supramolecular adhesive raw materials. Zhao et al. [39] degraded thermosetting epoxy in aqueous acetic acid solution at 80 ℃ and used the product to synthesize polyurethane. They added the degradates to the original epoxy resin as fillers, and the low additive amount of degradates had little effect on the thermal and mechanical properties of the resin; however, the method did not have a high degradation rate.
Owing to the difficulties in the utilization of epoxy oligomers with a low polymerization degree, in this study, we chose epoxy resin curing compositions containing ester-bonded epoxy resin monomers and amine curing agents. First, we degraded them with ethylene glycol at atmospheric pressure and then separated and recovered the epoxy degradation solution (DEP) to obtain epoxy resin monomers with the same structure. The effects of reaction temperature, reaction time, ratio of DEP to NaOH, and NaOH concentration on the purity of the product in the alkaline decomposition process of the epoxy degradation solution were explored using an orthogonal experimental design. The optimal process conditions for the preparation of sodium cyclohexene-2-carboxylate (R-THA-Na) were obtained, and the sodium salt was prepared through acidification and epoxidation treatment to obtain epoxy monomers having the same chemical structure as the original epoxy monomers. The performance of the recycled epoxy resins was comprehensively assessed, and a new method was proposed for the separation and reuse of thermosetting epoxy resins.