Abstract
Automotive structural adhesives must show a steady toughness performance in the temperature range of −40 °C to 80 °C, considering their actual usage environments. Core-shell rubber (CSR) nanoparticles are known to enhance the toughness of epoxy systems. 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. 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. To determine the curing conditions, we studied the curing behavior of the system by differential scanning calorimetry (DSC). Modulus variations of the cured bulk epoxies were studied using a dynamic mechanical analyzer (DMA) in the dual cantilever mode. The flexural modulus of the cured epoxies at various temperatures (−40, −10, 20, 50, and 80 °C) showed the same tendency as the DMA results, and as the flexural strength, except at 0 phr. On the other hand, the strain at break exhibited the opposite tendency to the flexural modulus. To study the adhesion behavior, we performed single-lap joint (SLJ) and impact wedge-peel (IWP) tests. As the CSR content increased, the strength of the SLJ and dynamic resistance to the cleavage of the IWP improved. In particular, the SLJ showed excellent strength at low temperatures (32.74 MPa at 50 phr @ −40 °C (i.e., an 190% improvement compared to 17.2 MPa at 0 phr @ −40 °C)), and the IWP showed excellent energy absorption at high temperatures (21.73 J at 50 phr @ 80 °C (i.e., a 976% improvement compared to 2.07 J at 0 phr @ 80 °C)). The results were discussed in relation to the changes in the properties of the bulk epoxy depending on the temperature and CSR content. The morphology of the fracture surface was also provided, which offered useful information for composition studies using the CSR/epoxy system.
Highlights
To respond to the annually strengthening carbon dioxide emission regulations [1,2,3]and improve the mileage range of electric vehicles that will replace internal combustion engine cars [4,5], the global automotive industry continues to trend toward weight reduction [6,7]
To determine the curing temperature and time required for the Core-shell rubber (CSR)/epoxy adhesives, we evaluated the thermal behavior of each composition by differential scanning calorimetry (DSC) (DSC Q200, TA Instruments, New Castle, DE, USA)
Even when the CSR content reached 30% of the total weight composition ratio (Table 2), the peak temperature change of 3 ◦ C showed a relatively small delay compared to the toughened epoxy system using rubber-modified epoxy (RME) or urethane-modified epoxy (UME), as reported by Back et al [44]
Summary
To respond to the annually strengthening carbon dioxide emission regulations [1,2,3]and improve the mileage range of electric vehicles that will replace internal combustion engine cars [4,5], the global automotive industry continues to trend toward weight reduction [6,7]. In addition to achieving the purpose of weight reduction, structural adhesives have the following additional advantages [17]: 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. A stable performance in the operating temperature range is required to prevent catastrophic performance degradation at a specific temperature
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