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1. Core–Shell Rubber Nanoparticle-Modified CFRP/Steel Ambient-Cured Adhesive Joints: Curing Kinetics and Mechanical Behavior

   https://doi.org/10.3390/ma17030749  

   Okeola, Abass Abayomi; Hernandez-Limon, Jorge E; Tatar, Jovan (February 2024, Materials)

   Externally bonded wet-layup carbon fiber-reinforced polymer (CFRP) strengthening systems are extensively used in concrete structures but have not found widespread use in deficient steel structures. To address the challenges of the adhesive bonding of wet-layup CFRP to steel substrates, this study investigated the effect of core–shell rubber (CSR) nanoparticles on the curing kinetics, glass transition temperature (Tg) and mechanical properties of ambient-cured epoxy/CSR blends. The effects of silane coupling agent and CSR on the adhesive bond properties of CFRP/steel joints were also investigated. The results indicate that CSR nanoparticles have a mild catalytic effect on the curing kinetics of epoxy under ambient conditions. The effect of CSR on the Tg of epoxy was negligible. Epoxy adhesives modified with 5 to 20%wt. of CSR nanoparticles were characterized with improved ductility over brittle neat epoxy; however, the addition of CSR nanoparticles reduced tensile strength and modulus of the adhesives. An up to 250% increase in the single-lap shear strength of CFRP/steel joints was accomplished in CSR-modified joints over neat epoxy adhesive joints. 

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2. High-performance, multi-component epoxy resin simulation for predicting thermo-mechanical property evolution during curing

   https://doi.org/10.1038/s41428-025-01022-y  

   Patil, Sagar Umesh; Kemppainen, Josh; Maiaru, Marianna; Odegard, Gregory M (May 2025, Polymer Journal)

   Abstract High-performance epoxy systems are extensively used in structural polymer‒matrix composites for aerospace vehicles. The evolution of the thermomechanical properties of these epoxies significantly impacts the evolution of process-induced residual stresses. The corresponding process parameters need to be optimized via multiscale process modeling to minimize the residual stresses and maximize the composite strength and durability. In this study, the thermomechanical properties of a multicomponent epoxy system are predicted via molecular dynamics (MD) simulation as a function of the degree of cure to provide critical property evolution data for process modeling. In addition, the experimentally validated results of this study provide critical insight into MD modeling protocols. Among these insights, harmonic- and Morse-bond-based force fields predict similar mechanical properties. However, simulations with the Morse-bond potential fail at intermediate strain values because of cross-term energy dominance. Additionally, crosslinking simulations should be conducted at the corresponding processing temperature, because the simulation temperature impacts shrinkage evolution significantly. Multiple analysis methods are utilized to process MD heating/cooling data for glass transition temperature prediction, and the results indicate that neither method has a significant advantage. These results are important for effective and comprehensive process modeling within the ICME (Integrated Computational Materials Engineering) and Materials Genome Initiative frameworks. 

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3. Recyclable high-performance epoxy based on transesterification reaction

   https://doi.org/10.1039/c7ta06397k  

   Lu, Lu; Pan, Jian; Li, Guoqiang (January 2017, J. Mater. Chem. A)

   Self-healing thermoset epoxy based on dynamic covalent bond chemistry has been developed in the past several years, which warrants the creation of recyclable epoxy. However, the existing systems produce epoxy that has lower strength, stiffness, and glass transition temperature, making them unsuitable for load-bearing structures. In this study, we developed a new recyclable thermoset epoxy through solid form recycling. The epoxy has strength, stiffness, and glass transition temperature similar to those found in conventional thermoset epoxy. The effect of healing temperature, healing time, healing pressure, and powder size on the healing efficiency was experimentally investigated. It was found that the healing efficiency is as high as 88.1%, and the epoxy can be recycled more than one time. 

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4. A preliminary study of using film adhesives containing aligned and unaligned nanotubes and nanofibers for bonding laminates and steel plates

   Taylor, William W.; Uddin, Md N.; Islam, Mohammad R.; Dizbay-Onat, Melike; Hsiao, Kuang-Ting (May 2022, SAMPE 2022 Conferene Proceedings, Charlotte, NA, May 23-26, 2022.)

   Well-dispersed and unaligned multi-walled carbon nanotubes (MCNTs) infused liquid epoxy adhesive have been reported for significantly improving the adhesive-joint of carbon fiber reinforced polymer (CFRP) composite laminates. However, it has not been determined in the literature if the alignment of MCNTs would provide an additional improvement than the randomly aligned case. In this study, various epoxy film adhesives embedded with 1wt% through-thickness aligned MCNTs, unaligned MCNTs, aligned carbon nanofibers (CNFs), and unaligned CNFs were used for bonding CFRP laminates. These variants have been used to bond two CFRP laminates for the ASTM D5868-01 single lap test as well as a steel variant for the same bonding process. The average shear strengths of the samples bonded by the various film adhesives were compared with the samples bonded by the pure epoxy-films. Microscopic analysis has been used to examine the fracture surface after testing. It was also used to visualize how the film adhesives fail while experiencing shear. This study has investigated the effectiveness of infusing through-thickness directionally aligned vs. unaligned nanoparticles in an epoxy film adhesive for bonding CFRP laminates and steel plate. It also indicates the potential future research direction of using nanoparticles in advanced adhesive technologies. 

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5. Preliminary experimental study of using nano-enhanced epoxy adhesive to bond carbon nanofibers z-threaded CFRP laminates

   Warren, Ryan A.; Taylor, William W.; Islam, Mohammad R.; Hsiao, Kuang-Ting (October 2023, CAMX 2023 Technical Proceedings)

   Previous studies have provided evidence that reinforcement of epoxy adhesives with nanostructures such as carbon nanofibers (CNFs) produces higher strength bonded joints between carbon fiber reinforced polymer (CFRP) laminates and shifts bond-line failure modes from the adhesive into the laminate. Despite this, there has been no research dedicated to applying reinforced adhesives to the bonding of nano-reinforced CFRP such as CNF z-threaded carbon fiber reinforced polymer (ZT-CFRP) laminates, which have been proven to exhibit increased interlaminar shear strength, mode-I delamination toughness, and compressive strength over traditional CFRP. This study examined the effectiveness of using CNF reinforced epoxy adhesives for unidirectional ZT-CFRP laminate bonding through single-lap shear tests using the ASTM D5868-01 standard. Unidirectional CFRP laminate samples bonded with both epoxy adhesive and CNF reinforced epoxy adhesive were also tested for comparison. It was found that the average shear strength observed for ZT-CFRP samples bonded with CNF reinforced epoxy adhesive was approximately 44% and 26 % higher than that of CFRP samples bonded with epoxy adhesive and CNF reinforced epoxy adhesive, respectively. Microscopic image analysis was performed to examine the mode of bond failure. The roles of nanomaterials in the fracture mechanism of the adhesives and the composite laminates are also discussed. 

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