The effect of carbon nanotube (CNT) doping on the physical and mechanical properties of eco-friendly, natural resin-modified shellac adhesives in metal–metal bonding

This study analysed the effect of carbon nanotube (CNT) modification on the physicochemical and mechanical properties of bioadhesives based on natural resins. Twelve variants of shellac-based adhesive compounds were prepared with the addition of rosin, spruce resin, dammar and olibanum, forming a reference series and a CNT-modified series containing 0.02 g of multi-walled carbon nanotubes (MWCNTs), corresponding to approximately 0.4 wt% relative to the shellac mass. The material characterization included microstructural analysis (SEM-EDS), functional group analysis (FT-IR), and rheological testing of the compounds. The surface of the EN-AW 6082 T6 aluminium alloy was assessed based on stereometric and physicochemical measurements. Shear strength tests were carried out on lap joints bonded with adhesive. Rheological studies have shown that the incorporation of CNTs leads to an increase in the pH, density, and viscosity of the matrices, with the highest viscosity recorded for systems containing dammar. The shear strength of surface bonds for the P series was low (0.30–0.86 MPa). CNT addition significantly improved strength for untreated surfaces (up to 563%), while for sandblasted surfaces it led to a decrease in joint strength due to increased viscosity and limited surface penetration.