An epoxy foam adhesive is an epoxy containing a foaming agent that generates a gas inside the epoxy resin or expands upon heat treatment. After heat treatment, the epoxy foam adhesive is cured and foamed, simultaneously filling the gap between two substrates and binding them [1
]. Through this process, the epoxy foam adhesive provides weight reduction, watertight property, and reinforcement effects, and can thus be applied to structural adhesives in automobiles [1
As the mechanical strength of an epoxy foam adhesive weakens through an increase in the expansion ratio [3
], it is necessary to improve the mechanical strength while preserving the expansion ratio. Particularly, since the impact resistance of an epoxy foam adhesive is important for its application to structural adhesives in automobiles, epoxy composite foam adhesives containing an additive that improves impact resistance are required [4
Rubber particles have been widely used to enhance the impact resistance of epoxy composites [5
]. However, the poor dispersion and aggregation of rubber particles in a composite decreases the impact resistance at a high content of rubber particles. Therefore, core–shell rubber (CSR) particles, in which rubber particles form the core structure and a polymer forms the shell, have been developed [9
]. Using CSR particles in a composite rather than rubber particles, the dispersion of CSR particles in the composite can be improved and impact resistance can be enhanced [9
The pore structure of epoxy composite foams is typically characterized by two-dimensional analysis, such as scanning electron microscopy (SEM) [3
]. Two-dimensional analysis can be used to observe only the sample surface, and the investigation of the internal pore structure necessitates a destructive evaluation of the epoxy composite foam adhesive. However, X-ray computed tomography (CT) can nondestructively characterize the internal pore structure of polymeric foams [1
] and can quantitatively evaluate the average pore size, standard deviation of pore size, porosity, and expansion ratio.
Further, the impact resistance of an epoxy composite containing CSR particles has been conventionally evaluated by a ballistic impact test [10
], izod impact test [11
], etc. By contrast, to evaluate the impact resistance of structural adhesives, a test specimen is adhesively bonded and impact is applied to the specimen using instruments such as an Izod impact tester [4
], impact wedge–peel tester [19
], and servohydraulic tester [21
In this study, epoxy composite foam adhesives containing epoxy resin, a foaming agent, and CSR particles were prepared, and their pore structure was characterized by X-ray CT. We used an impact wedge–peel tester and their resistance to cleavage of the epoxy composite foam adhesive under impact condition. We investigated the effect of CSR particles on the pore structure and impact resistance of the epoxy composite foam adhesive containing different amounts of foaming agent and suggested an optimal content of CSR particles to achieve a high expansion ratio and impact resistance.
CSR/epoxy composite foam adhesives were prepared with different amounts of foaming agent and CSR particles. With increasing CSR content, the curing reaction retarded, which affected the growth of the pores. The pore structure, pore size, porosity, and expansion ratio were determined by X-ray CT. The expansion ratio for 1 phr foaming agent was higher than that for 0.1 phr foaming agent. At 5 and 10 phr CSR content, the pore size and expansion ratio increased by decrease in curing rate, but, at 20 phr CSR content, the pore size and expansion ratio decreased due to the steric hindrance effect of the CSR particles. The impact resistance of the CSR/epoxy composite foam adhesive was compared in terms of EC. It was significantly enhanced by the addition of CSR particles at 0 and 0.1 phr foaming agent. However, at 1 phr foaming agent, the EC was hardly improved by the addition of CSR particle, indicating that the improvement in impact resistance is effective only at low foaming agent contents (0 and 0.1 phr). For the CSR/epoxy composite foam adhesives containing 0.1 phr foaming agent and 20 phr CSR particles, a simultaneous increase in both the expansion ratio (~148%) and impact resistance (EC = 34000 mJ/cm2) was achieved. A limitation of this study is that we only focused on the impact resistance of the CSR–epoxy composite foam adhesives at room temperature. Since CSR particles can improve impact resistance at low temperatures, it is necessary to investigate the impact resistance of CSR/epoxy composite foam adhesives at low temperatures in future research.