Next-Generation Structural\nAdhesives Composed of Epoxy\nResins and Hydrogen-Bonded Styrenic Block Polymer-Based Thermoplastic\nElastomers

Structural adhesives are currently applied in the assembly\nof automobiles,\naircraft, and buildings. In particular, epoxy adhesives are widely\nused due to their excellent mechanical strength and durability. However,\ncured epoxy resins are typically rigid and inflexible; thus, they\nhave low peel and impact strength. In this study, tough cured epoxy\nadhesives were developed by mixing a liquid epoxy prepolymer (EP)\nand polystyrene-<i>b</i>-polyisoprene-<i>b</i>-polystyrene (SIS). SIS is a block polymer-based thermoplastic elastomer\n(TPE) composed of polystyrene (S) soluble in liquid EP and polyisoprene\n(I) insoluble in liquid EP, where S and I have a glass transition\ntemperature that is higher and lower than room temperature, respectively.\nIn addition, cured adhesives tougher than the cured adhesives containing\nSIS were prepared by mixing liquid EP and SIS with hydrogen-bonding\ngroups in the I block (h-SIS). Transmission electron microscopy (TEM)\nobservations revealed mixed S/cured EP domains, with a <i>d</i>-spacing of several tens of nanometers, and cured EP domains, with\ndiameters of one hundred to several hundred nanometers, that were\nmacroscopically dispersed in the I or hydrogen-bonded I matrix of\nthe cured adhesive containing SIS or h-SIS. The lap shear, peel, and\nimpact strength of cured neat EP (EP*) were 23 MPa, 45 N/25 mm, and\n0.62 kN/m, respectively. Meanwhile, the cured adhesive containing\n16.5 wt % SIS exhibited the slightly lower lap shear strength of 17\nMPa compared to that of cured EP*, whereas the peel and impact strength\nof the cured adhesive with SIS were 61 N/25 mm and 7.1 kN/m, respectively,\nboth higher than those of EP*. Furthermore, the lap shear strength\nof the cured adhesive containing 15.5 wt % h-SIS was 21 MPa, which\nwas similar to that of cured EP*. The cured adhesive with h-SIS also\nexhibited an excellent peel strength of 97 N/25 mm and an impact strength\nof 14 kN/m which was 22 times higher than that of cured EP*. Therefore,\nmixing liquid EP and SIS improved the cured adhesive properties and\nflexibility of the cured epoxy adhesives compared to the cured adhesive\ncomposed of neat EP, and further enhancement of the adhesive properties\nwas achieved by mixing liquid EP and h-SIS with hydrogen-bonding groups\ninstead of mixing with SIS.