However, since epoxy materials are generally brittle, the mechanical durability of an unmodified, epoxy adhesive against external forces, structural deformations, and impacts is weak. Among them, epoxy-based structural adhesives are commonly adopted because of their excellent mechanical strength, chemical resistance, and environmental resistance. There are structural adhesives that are based on epoxy, urethane, and acrylic materials. Finally, a stable performance in the operating temperature range is required to prevent catastrophic performance degradation at a specific temperature. The impact resistance is required to minimize the occupant’s impact by sufficiently absorbing or dispersing the impact on the vehicle body when a contact accident occurs. The toughness is required because a vehicle in the driving state experiences various external forces. The essential basic requirements for automotive structural adhesives are as follows: an elongation performance, in addition to the adhesive strength (i.e., the toughness) an impact resistance and a stable performance in the specified operating temperature range. Simultaneously, automobile and adhesive manufacturers have carefully considered and studied the reliability of adhesives in automobiles, where human safety must be guaranteed. Due to these various advantages, the number of structural adhesives consumed in manufacturing increases every year, and the significance of adhesive material contributions to automotive performance is emerging. In addition to achieving the purpose of weight reduction, structural adhesives have the following additional advantages : uniform stress distribution on the fastening area by providing continuous bonding, improvement of fatigue resistance by minimizing the stress concentration, improvement of noise and vibration damping properties due to the relatively high energy absorption rate on the adhesive joints, securing the mechanical strength of the joint and protecting it from moisture and debris, adhesion between dissimilar materials, and prevention of the galvanic corrosion induced by intimate contact. The morphology of the fracture surface was also provided, which offered useful information for composition studies using the CSR/epoxy system. The results were discussed in relation to the changes in the properties of the bulk epoxy depending on the temperature and CSR content.

In particular, the SLJ showed excellent strength at low temperatures (32.74 MPa at 50 phr −40 ☌ (i.e., an 190% improvement compared to 17.2 MPa at 0 phr −40 ☌)), and the IWP showed excellent energy absorption at high temperatures (21.73 J at 50 phr 80 ☌ (i.e., a 976% improvement compared to 2.07 J at 0 phr 80 ☌)). As the CSR content increased, the strength of the SLJ and dynamic resistance to the cleavage of the IWP improved. To study the adhesion behavior, we performed single-lap joint (SLJ) and impact wedge-peel (IWP) tests. On the other hand, the strain at break exhibited the opposite tendency to the flexural modulus. The flexural modulus of the cured epoxies at various temperatures (−40, −10, 20, 50, and 80 ☌) showed the same tendency as the DMA results, and as the flexural strength, except at 0 phr. Modulus variations of the cured bulk epoxies were studied using a dynamic mechanical analyzer (DMA) in the dual cantilever mode.

To determine the curing conditions, we studied the curing behavior of the system by differential scanning calorimetry (DSC). A simple, single-component, epoxy system of DGEBA/dicyandiamide with a latent accelerator was adopted, where the CSR content of the system was controlled from 0 to 50 phr by the CSR mixture.

In this study, a CSR, pre-dispersed, diglycidyl epoxy of bisphenol A (DGEBA) mixture at 35 wt % (KDAD-7101, Kukdo Chemical, Seoul, Korea) was used as a toughener for an automotive structural epoxy adhesive system. Core-shell rubber (CSR) nanoparticles are known to enhance the toughness of epoxy systems. Automotive structural adhesives must show a steady toughness performance in the temperature range of −40 ☌ to 80 ☌, considering their actual usage environments.

Core shell rubber