Epoxy resins are key materials used in various applications, including coatings, adhesives, and composites. Tougheners, such as nanoparticles, soft polymers, elastomeric polyurethanes, and core/shell particles, have been widely applied to compensate for the brittleness of the epoxy matrix and to enhance the impact resistance. Modifying epoxy resin by reacting it with a flexible component is one of the representative methods to overcome the weakness of cured epoxy polymers upon impact. For introducing flexible parts, we synthesized three types of epoxy-modified resins by reacting acid anhydride with glycidol, followed by reaction with bisphenol [F, S, or J] glycidyl ether to produce flexible modified epoxy resins. Mechanical tests, such as flexural strength and impact resistance tests, were performed by adding various amounts of the synthesized resin to the epoxy composites. The results of these tests suggest that the modified resins were effective in improving the toughness of the epoxy matrix.

Despite their excellent properties, epoxy resins have the disadvantage of low toughness. With the aim to improve the toughness of epoxy resin, polyol and polyurethane are synthesized using bisphenol‐Z (BPZ). The synthesized material is dispersed in the epoxy resin and used as a toughening agent. Polyol (modified bisphenol‐Z [MBPZ]‐OH) is synthesized by a ring‐opening polymerization of propylene oxide and caprolactone, and polyurethane (MBPZ‐PU) is synthesized by a one‐shot method using MBPZ‐OH and hexamethylene diisocyanate (HMDI). The effect of MBPZ‐PU on the mechanical strength of the epoxy resin is analyzed by monitoring the flexural and impact characteristics. The addition of the synthesized MBPZ‐PU improves the tensile strength, flexural strength, flexibility, and impact strength of the cured epoxy composite. Field‐emission scanning electron microscopy measurements indicate that the fracture surface had a specific pattern. Thermal properties such as the curing temperature, glass transition temperature (Tg), and modulus are investigated through differential scanning calorimetry, dynamic mechanical analysis, and thermomechanical analysis. Based on the results, MBPZ‐PU is expected to be applied to a wider field for increasing the toughness of epoxy.