Next-Generation Structural Adhesives Composed of Epoxy Resins and Hydrogen-Bonded Styrenic Block Polymer-Based Thermoplastic Elastomers

Structural adhesives are currently applied in the assembly of automobiles, aircraft, and buildings. In particular, epoxy adhesives are widely used due to their excellent mechanical strength and durability. However, cured epoxy resins are typically rigid and inflexible; thus, they have low peel and impact strength. In this study, tough cured epoxy adhesives were developed by mixing a liquid epoxy prepolymer (EP) and polystyrene-<i>b</i>-polyisoprene-<i>b</i>-polystyrene (SIS). SIS is a block polymer-based thermoplastic elastomer (TPE) composed of polystyrene (S) soluble in liquid EP and polyisoprene (I) insoluble in liquid EP, where S and I have a glass transition temperature that is higher and lower than room temperature, respectively. In addition, cured adhesives tougher than the cured adhesives containing SIS were prepared by mixing liquid EP and SIS with hydrogen-bonding groups in the I block (h-SIS). Transmission electron microscopy (TEM) observations revealed mixed S/cured EP domains, with a <i>d</i>-spacing of several tens of nanometers, and cured EP domains, with diameters of one hundred to several hundred nanometers, that were macroscopically dispersed in the I or hydrogen-bonded I matrix of the cured adhesive containing SIS or h-SIS. The lap shear, peel, and impact strength of cured neat EP (EP*) were 23 MPa, 45 N/25 mm, and 0.62 kN/m, respectively. Meanwhile, the cured adhesive containing 16.5 wt % SIS exhibited the slightly lower lap shear strength of 17 MPa compared to that of cured EP*, whereas the peel and impact strength of the cured adhesive with SIS were 61 N/25 mm and 7.1 kN/m, respectively, both higher than those of EP*. Furthermore, the lap shear strength of the cured adhesive containing 15.5 wt % h-SIS was 21 MPa, which was similar to that of cured EP*. The cured adhesive with h-SIS also exhibited an excellent peel strength of 97 N/25 mm and an impact strength of 14 kN/m which was 22 times higher than that of cured EP*. Therefore, mixing liquid EP and SIS improved the cured adhesive properties and flexibility of the cured epoxy adhesives compared to the cured adhesive composed of neat EP, and further enhancement of the adhesive properties was achieved by mixing liquid EP and h-SIS with hydrogen-bonding groups instead of mixing with SIS.