# Advances in Toughening Modification Methods for Epoxy Resins: A Comprehensive Review
## 2 Rubber Toughening
### Research Progress in Rubber Toughtening Methods
#### 2.3.1 Liquid Rubber Toughening Method  
Liquid rubber, including types such as nitrile rubber, polybutadiene, and polysulfide rubber, is commonly used in toughening epoxy resins. Among these, liquid nitrile rubber systems are the most developed [28]. To enhance the interface interaction between the rubber and the resin matrix, active functional groups that can participate in the curing reaction are typically introduced into the rubber, such as carboxyl, hydroxyl, and amino groups [37]. Liquid rubbers with active reactive groups are miscible with epoxy resins before curing. During the curing process, these rubbers undergo reaction-induced phase separation, forming a two-phase structure (sea-island morphology). Ultimately, the rubber is dispersed as micron-sized particles within the epoxy resin matrix. [PERSON] et al. [38] used carboxyl-terminated butadiene-nitrile liquid rubber (CTBN) to toughen boron nitride-epoxy resin hybrid systems and investigated the impact of varying CTBN content on the mechanical properties of the composites. As shown in Figure 2A, the tensile strength reached its maximum value of 50 MPa at a CTBN content of 5%. However, as the CTBN content continued to increase, the tensile strength gradually decreased, likely due to the aggregation of CTBN within the resin matrix, which prevented the formation of a well-dispersed structure. The elongation at break increased with higher CTBN content, reaching a maximum of 32% greater than the unmodified epoxy resin when CTBN content was 15%. At a CTBN content of 20%, the elongation at break decreased significantly, and the changes in impact strength paralleled those of the elongation at break (Figure 2B). The results indicate that the mechanical properties of the composites are optimal with CTBN content ranging from 5% to 15%. [PERSON] et al. [39] employed carboxyl-terminated nitrile rubber (CTBN) to toughen high-molecular-weight solid epoxy resins. When the CTBN content reached 10 wt%, the tensile strength increased to 105.4 MPa, the impact strength rose from 7.63 kJ/m\({}^{2}\) to 23.9 kJ/m\({}^{2}\), and the elongation at break improved from 5.4% to 8.1%. These improvements in impact strength and other properties significantly expanded the application range of the modified epoxy resin. In addition to using liquid nitrile rubber, polybutadiene is also frequently used to toughen epoxy resins. [PERSON] et al. [40] utilized non-polar epoxy-terminated hydroxyl polybutadiene (EHTPB) to toughen epoxy resins (EP), aiming to enhance both impact strength and dielectric performance. EHTPB, containing hydroxyl and epoxy groups, readily undergoes chemical reactions with EP, thereby improving interface interactions and compatibility (Figure 2C). The results demonstrated that the tensile strength reached its maximum at 5 phr EHTPB, after which it gradually decreased. This decrease is attributed to the lower mechanical strength of EHTPB compared to EP, as EHTPB is a soft elastomer. Due to good chemical bonding between EHTPB and EP, the elongation at break significantly increased, peaking at 15 phr EHTPB before starting to decline (Figure 2D). End-amine liquid nitrile rubber (ATBN), due to its active amino (-NH\({}_{2}\)) groups capable of participating in chemical reactions, is widely used for toughening and modifying epoxy resins. [PERSON] et al. [30] incorporated varying amounts of ATBN, a copolymer containing 16% acrylonitrile and end-amine butadiene-acrylonitrile, into bisphenol-A-type epoxy resin cured with multi-amine imidazoline to enhance its toughness. The study revealed that the incorporation of ATBN resulted in a reduction in tensile strength and an increase in elongation at break, with a yield phenomenon observed. As the ATBN content increased, the tensile modulus decreased slightly from approximately 1.85 GPa to 1.34 GPa. The addition of 12.5 phr ATBN resulted in a threefold increase in impact strength, reaching 2.86 and 14.26 kJ/m\({}^{2}\) for notched and unnotched samples, respectively.