Pedro Costa

Epoxy adhesives are key enablers of lightweight, high‐performance structures, providing efficient bonding of dissimilar materials for demanding engineering applications. Meanwhile, sustainable bio‐based formulations with performances comparable to petroleum‐derived systems are emerging as environmentally friendly alternatives. By incorporating carbon nanotubes (CNTs), these adhesives become multifunctional materials capable of simultaneously carrying loads and monitoring their structural state. Herein, a commercial Araldite epoxy and a bio‐based resin, together with their composites containing 1, 3, and 5 wt% CNTs, were processed using a three‐roll mill and systematically characterized. The bioresin exhibited higher tensile strength but lower strain‐at‐break than Araldite, while CNT incorporation caused only a slight reduction in the mechanical performance of both matrices. Although lap shear strength decreased after functionalization, all formulations remained suitable for semi‐structural bonding applications. Thermoresistive sensitivity was maximized near the electrical percolation threshold, reaching ST ≈ −0.29 and −0.23% °C ⁻¹ between 30°C and 100°C for Araldite and the bioresin, respectively. Load sensing was successfully demonstrated under shear loading in single‐lap joints up to 50 N, with piezoresistive sensitivities approaching 300 Ω/N for Araldite containing 3 wt% CNTs and 25 Ω/N for the bioresin with 5 wt% CNTs. These findings demonstrate that both petroleum‐based and bio‐based epoxy adhesives can be transformed into sustainable multifunctional bonding materials with integrated thermo‐ and piezoresistive sensing capabilities for next‐generation structural applications.

Bisphenol A polycarbonates are widely employed material among others, in electronics but their synthesis entails healthy concerns. Polycarbonates (PCs), thanks to their carbonate moiety, are the ideal candidates to substitute fossil feedstock by carbon dioxide (CO ₂ ) in this manner synthesizing sustainable CO ₂ derived polycarbonates (CO ₂ ‐PCs). In this work, two commercial polymers: poly(propylene)carbonate and poly(ethylene)carbonate and one synthesized in our laboratory: poly(butylene)carbonate, have been loaded with multiwalled carbon nanotubes in order to explore their use as active materials for sustainable piezoresistive applications. The mechanical properties of the polymeric matrices show high strain > 859% and young modulus ≈1.9–2.9 MPa. The electrical conductivity is dominated by the filler network, measuring electrical conductivity values in the range 0.10–0.16 of S m ⁻¹ for 3.0 wt.% MWCNTs composites. Given the similar mechanical and electrical properties, the piezoresistive effect has been measured in lab‐made matrix loaded with 0.5 and 3.0 wt.% MWCNTs. The performance characterized by gauge factors (GF) results < 4 up to 50% of strain have been measured. Herein, this study promotes material development contributing towards sustainable electronics and digitalization, as demonstrated by a life‐cycle‐oriented screening assessment.

Environmental concerns arise over traditional printed circuit boards (PCBs) due to the large amount of materials required for their production, which complicates their recyclability and degradability. In the transition toward low environmental impact materials, printed electronics (PE) allow the use of polymeric substrates, meeting eco-friendly criteria. In this context, six sustainable printable substrates have been prepared consisting of three carbon dioxide-derived polycarbonates (CO2–PCs) blended with degradable polymers. CO2–PCs are synthesized from the reaction between epoxide and CO2 offering an alternative and sustainable greener synthesis in contrast to bisphenol A polycarbonates (BPA-PCs). A selection process among the six substrates was conducted in order to determine the most suitable one for piezoresistive (PR) applications. Based on characterization results, the poly(ethylene)carbonate (PEC)/polybutylene succinate (PBS) blend, referred to as PEC 1, has emerged as the most appropriate candidate. The substrate exhibits a contact angle (CA) ≈ 61.4 ± 0.6°, a melting temperature (Tm) ≈ 89.1 °C, Young’s modulus (E) ≈ 88.8 ± 14.9 MPa, and a strain at break (εb) ≈ 946.6 ± 117.8%. On this substrate, 8- and 16-line strain gauges have been screen-printed using a water-based hydroxypropyl cellulose (HPC)/nitrogen-doped reduced graphene oxide (N-rGO) sustainable ink. The sensitivity of screen-printed gauges has been evaluated, where the 8-line strain gauge demonstrated higher linearity with electrical resistance variation under bending. As the printed substrates constitute the largest part of many sensors, this work contributed to the transition from petrochemical polymers to sustainable options in order to bridge the gap toward eco-friendly paths.

Carbon dioxide‐derived polycarbonates (CO ₂ ‐PCs) present a sustainable alternative to standard bisphenol A polycarbonates (BPA‐PCs), as they are synthesized from the reaction between epoxide and carbon dioxide (CO ₂ ). In fact, these eco‐friendly polymers are less widespread and studied than BPA‐PCs for electronic applications. Herein, in this work composites consisting of a CO ₂ ‐derived poly(cyclohexene)carbonate (PCHC) polymer matrix loaded with different weight percentages of reduced graphene oxide (rGO) and carbon black (CB) have been prepared, and their thermoresistive (TR) characteristics have been evaluated. The prepared composites show enhanced thermal stability but decreased mechanical properties with the incorporation of the filler, regardless of its type and loading, compared to the neat polymeric matrix. With respect to the electrical conductivity, PCHC composites show values comparable to those of BPA‐PCs, representing therefore a suitable replacement for electronic applications. TR characterization has been performed on the 5.0rGO and 4.0CB composites, reaching sensitivities ( S ) up to S _{40°C–100°C} = −1.3 × 10 ⁻⁴ °C ⁻¹ for 5.0rGO and S _{40°C–100°C} = 6.5 × 10 ⁻⁵ °C ⁻¹ for the 4.0CB composite. This work demonstrates the suitability of PCHC/rGO and PCHC/CB composites for TR sensing applications, opening a path towards environmentally friendlier electronic solutions.

A water-soluble hydroxypropyl cellulose (HPC) polymer matrix has been filled with different weight percentages (wt.%) of multiwalled carbon nanotubes (MWCNTs), fullerenes C₆₀, fullerenols C₆₀(OH)_24, and their combinations. We study the potential of the 0D nanoparticles for improving electrical properties of the conductive MWCNT network in a biocompatible matrix. Physicochemical effects of fillers content, both individually and in combinations (MWCNTs/C₆₀ and MWCNTs/C₆₀(OH)₂₄), for these composite systems, have been investigated. The performed SAXS analysis shows improved nanofiller dispersion for films with two fillers. The electrical percolation threshold (P_c) in MWCNTs composites occurs at ≈1.0 wt.%. A synergistic effect for binary filler composites on the electrical conductivity has been evaluated by keeping a constant amount of 0.5 wt.% MWCNTs (σ ≈ 3 × 10⁻⁹ S·m⁻¹) and increasing the amount of C₆₀ or C₆₀(OH)₂₄. A large increase in the electrical conductivity is obtained for the bifiller composites with 0.5 wt.% MWCNTs and 1.5 wt.% of C₆₀(OH)₂₄, reaching σ ≈ 0.008 S·m⁻¹. Further, the sensing properties of 4.0/1.0 MWCNT/C₆₀ nanocomposites were demonstrated by measuring both piezoresistive (PR) and thermoresistive (TR) responses. The combination of semiconductive fullerene/fullerenols combined with MWCNTs allows obtaining more homogeneous composites in comparison to single MWCNTs composites and also gives possibilities for tuning the electrical conductivity of the system. Overall, it is demonstrated that the use of bifillers with a water soluble biopolymeric matrix allows the development of eco-friendly high-performance electroactive materials for sustainable digitalization.

Epoxy resin composites produced by additive manufacturing have been developed for their use as adhesive materials with sensing functionalities as multifunctional piezoresistive and thermoresistive materials. The processing of the composites involves acetone as a solvent to tune the viscosity and achieve a homogeneous dispersion of the carbon nanotubes, resulting in a solution with suitable characteristics for application by additive manufacturing. Doctor blade and screen printing films have been prepared with a low electrical resistivity of approximately 100 Ω/sq. for composites with 10 wt% of carbon nanotubes. The largest piezoresistive sensitivity, determined by a Gauge Factor of approximately 15 under 5 mm bending, has been obtained, with suitable stability over cycling solicitation. Furthermore, the thermoresistive response exhibited a sensitivity of 0.05% °C⁻¹. Self‐sensing bonded structures have been obtained with excellent adhesive forces (approximately 20 MPa in glass fiber joints) in shear lap tests. In addition, the bonded structure demonstrated a linear response to loading–unloading up to 5 mm of bending in the cantilever mode, with cycle stability. Overall, a scalable methodology to obtain high‐performance self‐sensing adhesive materials processable by additive manufacturing is presented, which shows excellent bonding characteristics, together with mechanical and thermal sensing properties.Highlights: Epoxy resin composites have been processed by doctor blade and screen printing. Piezo‐ and thermoresistive responses of GF ≈ 15 and S ≈ −0.05% °C⁻¹ were achieved. Developed adhesive structures with integrated self‐sensing capabilities. Proof‐of‐concepts were developed demonstrating the applicability of the samples.