# Advances in Toughening Modification Methods for Epoxy Resins: A Comprehensive Review
## 1 Introduction  
Epoxy resin (EP) is one of the most widely consumed thermosetting resins among major commercial resins, distinguished by its extensive variety, flexible formulation design, and broad application range [1]. Epoxy resins are high-performance thermosetting polymers characterized by the presence of two or more epoxy groups within their molecular structure, typically based on aliphatic, cycloaliphatic, or aromatic carbon chains [2, 3, 4] (Figure 1). The epoxy groups (C\({}_{2}\)H\({}_{4}\)O-) at a molecular level provide notable reactivity and exceptional adhesive properties to the resin. The presence of the benzene ring structure adds high rigidity, heat resistance, and chemical stability, guaranteeing that cured epoxy resins maintain consistent mechanical properties even at high temperatures. Moreover, the inclusion of ether bonds (-O-) and hydroxyl groups (-OH) improves the resin's capacity tobe wetted, hence promoting stronger chemical interaction with the surfaces of substrates. Hydroxyl groups facilitate hydrogen bonding with other molecules, hence enhancing intermolecular interactions and overall material performance. Epoxy resins has exceptional properties such as strong adhesion, resistance to wear, effective electrical insulation, chemical stability, capacity to withstand high and low temperatures, dimensional stability, ease of processing, and affordability. These qualities make epoxy resins very versatile and suitable for use in a wide range of industries and applications [5; 6; 7]. Due to the presence of chemically reactive epoxy groups in its molecular structure, epoxy resin can undergo crosslinking reactions with various types of curing agents, resulting in a highly crosslinked three-dimensional network structure after curing [8]. While this structure imparts excellent performance characteristics, it also leads to significant brittleness, poor impact resistance, and a tendency to crack [9; 10]. These shortcomings have notably limited the application of epoxy resins in fields requiring high impact and fracture strength [11; 12]. The introduction of toughening agents can significantly enhance the performance of epoxy resins. Specifically, toughening agents can markedly improve the resin's toughness, reducing or preventing cracking. Additionally, they effectively increase the impact resistance of epoxy resins, making them more resilient to external shocks and vibrations. The incorporation of toughening agents also improves the processing and molding properties of epoxy resins, enhancing the flexibility of manufacturing processes. Toughened resins typically exhibit higher tensile and flexural strengths, providing more reliable and stable performance for structural and engineering applications. Many researchers have developed and studied various types of epoxy toughening agents to achieve diversified performance enhancements. Among these, the rubber toughening method, which introduces rubber particles, significantly improves the impact resistance of epoxy resins. The thermoplastic resin toughening method effectively enhances the material's plasticity and reprocessability. Nanoparticle toughening relies on nanoscale particles to strengthen the material's durability and wear resistance. Hyperbranched polymer toughening strengthens the molecular network through a hyperbranched structure, enhancing intermolecular interactions. Flexible chain segment toughening increases the flexibility of molecular chains, reducing rigid constraints between them. Table 1 provides a detailed comparison of the advantages and disadvantages of these toughening methods. Through an in-depth review of domestic and international literature, this paper comprehensively summarizes the mechanisms and latest research advancements in the toughening modification of epoxy resins using various methods such as rubber toughening, thermoplastic resin toughening, hyperbranched polymer toughening, and flexible chain segment toughening.