A recent article in Scientific Reports explored the incorporation of zinc oxide (ZnO) nanoparticles into core-shell rubber (CSR) particles to improve the mechanical and thermal properties of epoxy adhesives.
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Background
Epoxy adhesives are widely used in industry due to their desirable properties, including excellent chemical and corrosion resistance, ambient curability, low shrinkage during curing, high strength, electrical insulation, and strong adhesion. Their high cross-linking density forms a robust network structure, but this also results in significant brittleness after curing.
To address this limitation, various modifiers have been investigated—such as thermoplastics, nanoparticles, toughening agents, and liquid rubbers. CSRs are a novel class of toughening agents that enhance fracture toughness while minimizing the loss of mechanical strength and thermal stability in epoxy adhesives.
However, CSRs tend to agglomerate, which limits their effectiveness. This can be mitigated by introducing appropriate surface chemical groups. In this study, ZnO nanoparticles were used in combination with CSRs to improve the toughness of epoxy adhesives.
Methods
CSR particles were synthesized using a two-step emulsion polymerization method. First, poly(butyl acrylate-divinyl benzene) elastomeric cores were prepared. Then, methyl methacrylate (MMA) and glycidyl methacrylate (GMA) were polymerized on the surface to form the shell.
The epoxy adhesives were prepared by mixing epoxy resin with xylene, followed by the addition of CSR particles and ZnO nanoparticles. The adhesives were then applied to anodized metal surfaces for testing.
To analyze the materials, the team used Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These techniques helped confirm the chemical structure, surface morphology, and internal characteristics of the CSR particles.
Mechanical properties were assessed through tensile and lap shear tests, while thermal performance was evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). DSC was also used to confirm the structure and thermal properties of the CSR particles.
In addition to experimental testing, the researchers used a design-of-experiments (DOE) approach and an artificial neural network (ANN) to model and predict the mechanical performance of the adhesive systems. A multilayer perceptron (MLP) model was trained to estimate tensile strength, modulus, toughness, and shear strength based on the composition of the adhesives.
Results and Discussion
FTIR, TEM, SEM, and DSC confirmed the successful synthesis of CSR particles. Tensile test results showed that adding 2 parts per hundred resin (phr) of ZnO nanoparticles led to a 60 % increase in tensile strength and a 37 % increase in modulus compared to epoxy without ZnO.
The specimen with 2.5 phr CSR and 2 phr ZnO had the highest tensile strength overall. For shear strength, the best performance came from the sample containing 3.75 phr CSR and 5 phr ZnO, which showed a 27 % improvement over pure epoxy.
TGA analysis revealed that the sample with 5 phr ZnO had the greatest thermal stability. SEM images of the fracture surfaces aligned with these results. The pure epoxy displayed a flat fracture surface, typical of brittle failure. In contrast, the samples with CSR and ZnO nanoparticles showed rougher surfaces, indicating more plastic deformation and improved resistance to crack growth.
Toughening mechanisms observed included crack pinning, deflection, and particle debonding. These contributed to the improved toughness and fracture resistance in the modified adhesives.
The DOE model produced four polynomials with strong correlation to the experimental data and minimal error. ANN predictions also showed a high level of agreement with the experimental results across training, validation, and test datasets.
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Conclusion
This study demonstrated the potential of combining CSR particles with ZnO nanoparticles to enhance the mechanical and thermal properties of epoxy adhesives. The CSR structure, composed of a flexible core and a rigid shell, acted as an effective toughening agent. ZnO nanoparticles further improved strength, modulus, and thermal performance.
A two-step mixing process was used to achieve better dispersion of both components. Testing on dumbbell-shaped aluminum lap-joint specimens showed that samples with 2.5 phr CSR and 2 phr ZnO delivered notable improvements—51 % higher tensile strength, 30 % higher modulus, and 218 % higher toughness than unmodified epoxy.
The thermal stability also improved with the addition of CSR and ZnO, suggesting practical potential for industrial applications. The integration of DOE and ANN approaches provided accurate models for predicting mechanical behavior, supporting further development and optimization of epoxy-based systems.
Journal Reference
Mirmohammadi, S. M., Jazani, O. M., Gharieh, A. (2025). Thermomechanical analyses and ANN modeling of novel epoxy adhesives with CSR particles and zinc oxide nanoparticles in structural bonding. Scientific Reports. DOI: 10.1038/s41598-025-96270-6, https://www.nature.com/articles/s41598-025-96270-6
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