# Advances in Toughening Modification Methods for Epoxy Resins: A Comprehensive Review
## 2 Rubber Toughening
### Research Progress in Rubber Toughtening Methods
#### 2.3.2 Core-Shell Rubber Toughening Method  
The core-shell rubber toughening method has emerged as a rapidly developing approach for enhancing epoxy resins, as it addresses some limitations of liquid rubber, such  
Figure 2: Effect of various CTBN contents on the mechanical properties of composite materials: (**A**) tensile strength and tensile modulus; (**B**) elongation at break and impact strength. Reproduced from Ref. [38] with permission of MDPI. (**C**) OH group chemistry diagram of EP group and EHTPB; (**D**) tensile strength and fracture elongation. Reproduced from Ref. [40] with permission of John Wiley and Sons. as the uncontrolled particle shape, size, and distribution during reaction-induced phase separation. In core-shell rubber systems, the core is typically composed of rubber particles, while the shell can consist of one or more organic or inorganic materials. This core-shell structure allows for a synergistic effect between the two components, leveraging their individual properties [42]. The shell is often made from materials that exhibit good compatibility with the epoxy resin matrix, such as polymethyl methacrylate (PMMA). The core materials generally include substances like polybutadiene, acrylate-polyurethane, or siloxanes [43]. Under external forces, core-shell rubber particles undergo cavitation, leading to shear yielding of the matrix. The rubber particles at the crack tip are stretched to match the matrix's dimensions. The deformation and failure of the rubber particles absorb energy, effectively impeding crack propagation [44]. [PERSON] et al. [45] employed a seed emulsion polymerization method to synthesize a core-shell structured emulsion composed of poly(butyl acrylate)/poly(methyl methacrylate-methyl methacrylate glycolidyl ester) (PBMG), which was then used to toughen epoxy resin (EP). The synthesis process of PBMG is illustrated in Figure 3. Their study revealed that both the impact strength and flexural strength of the EP/PBMG composite increased with the PBMG content. When the mass fraction of PBMG reached 5 wt%, it was uniformly dispersed in the EP matrix, achieving maximum impact strength and flexural strength of 37.25 kJ/m2 and 168.4 MPa, respectively. [PERSON] et al. [46] utilized phenoxy thermoplastic rubber and core-shell rubber (CSR) particles (polybutadiene and polymethyl methacrylate) for the simultaneous modification of epoxy resin. As shown in Figure 4A, prior to curing, the epoxy resin and phenoxy thermoplastic resin are miscible, forming a homogeneous blend, while CSR particles remain insoluble throughout the curing process and ultimately aggregate around the phenoxy phase. As depicted in Figure 4B, compared to pure epoxy resin, the addition of either phenoxy or CSR-MB alone increases the critical strain energy release rate (GIC) by approximately 73%. When both toughening agents are combined, particularly with a 5/5 wt% ratio of phenoxy and CSR-MB, the critical fracture energy increases by approximately 260%, and the critical stress intensity factor (KIC) value improves by about 182%. [PERSON] et al. [47] introduced a crosslinking agent, methyl methacrylate (ALMA), into the shell and core layers of core-shell particles (CSPs) and grafted epoxy-containing glycolidyl methacrylate (GMA) onto the PMMA shell. They found that the toughening effect of the grafted GMA on CSP was superior to that achieved with the crosslinking agent alone. Compared to pure epoxy resin (EP), the KIC increased by 245%, from 0.85 MPa\(\cdot\)m\({}^{1/2}\) to 2.93 MPa\(\cdot\)m\({}^{1/2}\), and the impact strength improved by 87.5%, from 7.2 kJ/m\({}^{2}\) to 13.5 kJ/m\({}^{2}\). Figure 3: Schematic preparation of PBMG. Reproduced from Ref. [45] with permission of Royal Society of Chemistry. To simultaneously enhance the mechanical properties and fracture toughness of epoxy resin without compromising heat resistance, [PERSON] et al.