Comparative Study on the Impact Wedge-Peel Performance of Epoxy-Based Structural Adhesives Modified with Different Toughening Agents

Abstract

Epoxy adhesives are widely used in various industries because of their high heat and chemical resistance, high cohesion, and minimal shrinkage. Recently, epoxy adhesives have been applied in the automotive industry as structural adhesives for lightweight vehicles. However, the brittleness of the epoxy is an obstacle for this application, since the automotive industry requires epoxy-based structural adhesives to have a high level of high-speed impact resistance. Hence, we used phenol-terminated polyurethane (PTPU) as a toughening agent for epoxy adhesives and compared the results with those that were obtained with carboxyl-terminated butadiene acrylonitrile copolymer (CTBN). The high-energy impact resistance of the epoxy adhesives was measured by the impact wedge-peel (IWP) test, and the shear strength was measured by the single lap joint test. As a result, the 20 wt % PTPU-modified epoxy adhesive showed remarkably higher total absorbed energy (25.8 J) during the IWP test and shear strength (32.3 MPa) as compared with the control epoxy adhesive (4.1 J and 20.6 MPa, respectively). In particular, the total absorbed energy of the PTPU-modified epoxy adhesive was much larger than that of the CTBN-modified epoxy adhesive (5.8 J). When more than 10 wt % PTPU was added, the modified epoxy adhesives showed stable crack growth and effectively transferred external stress to the substrate. These results were explained by changes in the glass transition temperature, crosslinking density, and morphology due to the toughening agents.

Mechanical Characterization of Core-Shell Rubber/Epoxy Polymers for Automotive Structural Adhesives as a Function of Operating Temperature

“…The shell and core of the CSR nanoparticles (200 to 450 nm diameter) were composed of poly (methyl methacrylate) (PMMA) and butadiene rubber, respectively [ 23 ]. Ground calcium carbonate (GCC (CaCO 3 ), Omyacarb 10, Omya AG, Oftringen, Switzerland) was used as the filler for the basic performance as an adhesive [ 23 , 24 , 43 , 44 ]. Table 1 shows the details of each component.…”

“…Simultaneously, automobile and adhesive manufacturers have carefully considered and studied the reliability of adhesives in automobiles, where human safety must be guaranteed. The essential basic requirements for automotive structural adhesives are as follows: an elongation performance, in addition to the adhesive strength (i.e., the toughness) [ 19 , 20 , 21 ]; an impact resistance [ 22 , 23 , 24 ]; and a stable performance in the specified operating temperature range [ 25 , 26 , 27 ]. The toughness is required because a vehicle in the driving state experiences various external forces.…”

“…To overcome this brittleness and improve their automotive applicability, it is common to improve their toughness using the following methods: formulating epoxy adhesives with urethane or rubber-based additives, adding thermoplastic or inorganic filler particles, and cross-linking with synthesized epoxy that has a urethane or rubber molecular structure. Core-shell rubber (CSR) nanoparticles are among the most widely used additives for improving the toughness of epoxy polymers since their shells and cores are made of thermoplastic and rubber-based materials, respectively [ 23 , 24 , 30 , 31 , 32 , 33 , 34 , 35 ].…”

“…Quan et al [ 36 ] also used CSR in epoxy adhesives and found that the CSR addition changed the failure mode of the single lap joint test (SLJ test) from brittle and interfacial to ductile and cohesive. Back et al [ 23 ] and Chae et al [ 24 ] also used CSR in automotive structural adhesives and characterized its impact performance using the impact wedge-peel test (IWP test, ISO 11343 [ 37 ]), since the IWP test is suitable and preferrable for automotive structural adhesives that require high impact resistance under high-speed impacts with high energies [ 22 ]. However, conventional studies on CSR nanoparticle/epoxy composite polymers have mainly considered the mechanical properties at room temperature.…”

Comparative Study of the Impact Wedge-Peel Performance of Epoxy Structural Adhesives Modified with Functionalized Silica Nanoparticles

“…In our previous study, we modified epoxy structural adhesives with soft elastomers, such as phenol-terminated polyurethane polyol (PTPU) and CTBN, and compared their impact resistance using the IWP test [ 36 ]. However, a previous study showed that PTPU interferes with the crosslinking reaction and decreases T g and crosslink density.…”