Thermomechanical analyses and ANN modeling of novel epoxy adhesives with CSR particles and zinc oxide nanoparticles in structural bonding

Epoxy adhesives are widely used as structural adhesives distinguished by a significant degree of cross-linking, resulting in their brittle characteristics. Some specialized applications require improved thermal stability and adhesive strength. The incorporation of zinc oxide nanoparticles into a core-shell rubber (CSR) structure composed of poly(butyl acrylate-allyl methacrylate) core and poly(methyl methacrylate-glycidyl methacrylate) shell will enhance the adhesion, toughness, and thermal stability of epoxy adhesives. We synthesized CSR particles using a two-stage emulsion polymerization method, characterizing them through Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) analyses. We synthesized epoxy adhesives with different CSR particles ratios (1.25, 2.5, and 3.75 phr) and zinc oxide nanoparticles (1, 2, and 5 phr) using mechanical stirring and ultrasonication (a two-step mixing process) to enhance dispersion. We cured the epoxy adhesive samples for 7 days for tensile tests and 2 days for lap shear tests at room temperature. We employed the tensile and lap shear tests to assess the mechanical properties of the samples. The samples underwent thermogravimetric analysis (TGA) to assess their thermal stability. We assessed the fracture surface of the optimum samples using field-emission scanning electron microscopy (FESEM). We utilized design-of-experiments (DOE) and artificial neural network (ANN) approaches to model the mechanical properties. The outcomes of FTIR, SEM, TEM and DCS analyses validated the successful synthesis of CSR particles. The tensile test findings on the dumbbell-shaped samples show a 51%, 30%, and 218% enhancement in tensile strength, modulus, and toughness for the samples containing 2.5 phr CSR particles and 2 phr zinc oxide nanoparticles, respectively. Furthermore, the lap shear tests revealed that the addition of 3.75 phr CSR particles and 5 phr zinc oxide nanoparticles increased the shear strength to 19.5 MPa. This is 127% higher than the pure epoxy. The TGA data indicated that both additions improved the thermal stability of the pure epoxy. Additionally, the predictions of shear strength, toughness, tensile modulus, and tensile strength by DOE and ANN were very close to the experimental results (R²_adj > 0.95 for DOE and MRE_ave < 3.2 for ANN).