Selection of Toughening Materials for Epoxy Resins and Discussion on Toughening Technology Approaches

The technical approaches for toughening epoxy resins include chemical and physical methods. Regarding the chemically reactive toughening （CRT）methods, this study employs reactive toughening agents and compares the toughening effects of several agents such as carboxyl-terminated polyether (CTPE), carboxyl-terminated polytetrahydrofuran (CTPF), carboxyl-terminated liquid butadiene nitrile rubber (CTBN), and core-shell polymers containing polybutadiene (CSP), on epoxy resins. These toughening agents are incorporated into the epoxy resin through chemical reactions or dispersion, forming flexible segments with impact resistance. During the curing process, micro-phase separation occurs, forming an island structure that absorbs energy under stress. At equivalent dosages, all toughening agents have a significant toughening effect on epoxy resin, with a notable improvement in impact resistance. Among them, CTPF and CTBN demonstrated particularly pronounced improvements, with the impact strength of CTPF-modified resin increasing by 257%. These agents form homogeneous phases in epoxy resin with minimal impact on transparency, making them viable options for transparent toughening. CTPE and CTPF lead to a decrease in thermal resistance, while CTBN and CSP have almost no effect on thermal resistance. CTPE and CTPF exhibited decreased volume resistivity due to enhanced impurity ion migration caused by flexible polyether segments. CSP improved electrical strength by reducing the effective carrier mobility of its structure. For the physical added thoughening (PAT) methods of toughening epoxy resin, this paper adopts the physical addition approach and compares the effects of special engineering plastics (SEP) such as polyetheretherketone (PEEK), polyimide (PI), thermoplastic polyimide (TPI), and polyphenylene sulfide (PPS) on the mechanical, thermal, and electrical properties of epoxy resin. The rigid and active group-containing SEP forms a difference phase during the curing process of epoxy resin, absorbing energy under stress, preventing the propagation of microcracks, and improving the mechanical properties of epoxy resin, including tensile, compressive, and impact strength. Due to the phase separation caused by the physical toughening method, there is significant light reflection and absorption loss, making it usually difficult to be used as a transparent toughening method. SEP has better heat resistance than epoxy resin, which is beneficial for enhancing the heat resistance of epoxy resin. During the curing process of epoxy, a strong intermolecular force is generated between SEP and epoxy resin, further enhancing the heat resistance of the modified epoxy resin. Adding SEP with better insulation performance can effectively improve the insulation performance of epoxy resin.