Effect of carbonaceous filler type and content on the piezoresistive response of screen-printed sustainable adhesive epoxy composites

Thermoset materials provide exceptional mechanical and adhesive properties, making them suitable for high-performance applications. Eco-friendly epoxy resins demonstrate properties comparable to those of their petroleum-based counterparts, supporting the advancement into sustainable functional materials with sensing capabilities while maintaining strong adhesion. Bioresins containing over 50 % bio-based carbon, reinforced with nanocarbonaceous fillers with varying geometry and conductivity such as carbon nanotubes (CNT), carbon black (CB) or graphene (G) show great potential for structural health monitoring applications. These materials enable functional sensing in adhesive bonded joints through cost-effective processing techniques such as screen-printing, combining tailorable printed designs with fast production. This study investigates a bio-epoxy resin (ER) as matrix modified with CNT, CB and G, focusing on its adhesive characteristics together with piezoresistive sensing. Critical results indicate that increasing the filler content leads to a decrease of 11 orders of magnitude in resistivity- from 1 13 of resin to 100 Ω/sq. for the 7CNT/ER composite, respectively- with a percolation threshold around 3 wt% CNT. The tensile strength and elongation of the bioresin (≈40 MPa and 3 %) decrease for composites, as does the shear strength in the bonded joints, except for the 3CNT/ER (≈ 150 MPa). The adhesive composites also demonstrate sensing properties, with a gauge factor (GF) about 5 for 5CB/ER at 5-mm bending for 3CNT/ER. Piezoresistive bending enhances the sensitivity in bonded structures, with GF values exceeding 15 for 15G/ER composites. These results highlight that sustainable epoxy composites are suitable for piezoresistive adhesives with self-sensing capabilities, making them appropriate for engineering applications with reduced environmental impact. • Bioresin matrix reinforced with distinct nanocarbonaceous have been processed by screen printing. • Excellent adhesive properties with low resistivity (~ 100 Ω/sq) was reached. • Sustainable composites with piezoresistive sensitivity of GF ≈ 15 was achieved. • Tailored adhesive materials for structures with integrated self-sensing capabilities.