Incorporation of carbon-based nanoparticles into epoxy adhesives: Enhancing mechanical properties through plasma functionalization and morphology analysis

The incorporation of carbon-based nanoparticles into epoxy adhesives is a promising strategy to enhance the mechanical properties of adhesive joints, improving their reliability and competitiveness compared to traditional mechanically fastened joints. This study investigates the effect of plasma functionalization of carbon-based nanoparticles with distinct geometries (carbon nanotubes, graphene, and carbon black) on the thermal and mechanical properties of an epoxy adhesive. Plasma treatment was employed to introduce oxygen and nitrogen functional groups onto the nanoparticle surfaces, enhancing their interaction with the epoxy matrix. Among these nanoparticles, carbon black, although a traditional and widely used material in composites, often presents challenges such as poor dispersion and agglomeration, which can negatively impact adhesive properties. Plasma functionalization addresses these issues by modifying the particle surface, enhancing dispersion and interfacial bonding with the epoxy matrix. Nanocomposites were fabricated using three nanoparticle mass fractions (0.25%, 0.5%, and 1%). X-ray photoelectron spectroscopy (XPS) was used to quantify the functional group concentrations on the nanoparticle surfaces. Tensile tests and electron and scannig microscopy characterizations were conducted to assess the effects of functionalization and nanoparticle concentration on the properties of the adhesive-based nanocomposites. XPS results confirmed the incorporation of oxygen and nitrogen onto the nanoparticle surfaces at the atomic level. Among the evaluated materials, functionalized carbon nanotubes (CNT.f) exhibited the highest nitrogen content, with 0.63 at.% of N 1s and a 37% increase in O 1s concentration compared to untreated CNTs. Nanocomposites containing functionalized CNTs showed reduced nanoparticle agglomeration and enhanced interfacial interaction with the epoxy matrix. As a result, a 21% increase in ultimate tensile strength was achieved with 0.25 wt.% of CNT.f, along with a 19% improvement in elastic modulus using 0.5 wt.% of the same filler. These findings provide valuable insights into the influence of functionalization and nanoparticle morphology on the mechanical behaviour of adhesive systems for advanced composite applications.