Piezoresistive materials, able to sense geometrical deformation through variations of the electrical resistance, attracted an increasing interest in the scientific and industrial comparts during the last forty years, which increased significantly with the advent of nanostructured carbon-based conductive materials. These light, highly conductive and easy-to-obtain fillers have broadened the spectrum of materials that had been used up to that time, opening up the possibility of greater development of multifunctional materials. In particular, the carbonaceous fillers, homogeneously dispersed within a polymer matrix, immediately represented a valid alternative to the metals used in the field of piezoresistive systems. In the context of polymer nanocomposites and piezoresistive materials, a significant challenge for the scientific community is represented the by the achievement of an effective percolation pathway, which allows the passage of an electric current at the lowest percentage of filler (percolation threshold), and provides a direct correlation of the external forces in with the electrical resistance variations. Generally, the piezoresistive materials based on the exploitation of the polymer-based composites are designed by homogeneously dispersing the carbonaceous filler in the polymeric matrix. However, it is well known that a simple approach to reduce the content of filler and realize a conductive composite can be obtained by exploiting the concept of segregation of filler in the polymeric matrix. When the filler is not randomly dispersed, but segregated to build up a three-dimensional network, the electrical conductivity can be obtained with a significantly lower content of the carbonaceous filler.
Amongst the several techniques for the realization of piezoresistive systems, in the last decade, the Additive Manufacturing (3D printing) technologies have aroused the greatest interest. The 3D printing processes lead to a considerable reduction in costs and times as compared with the traditional technologies of processing of polymers. Furthermore, as regards prototyping, they allow an almost total freedom to create even complex shapes and geometries in an automated and effective way. In particular, Selective Laser Sintering (SLS) is one of the most interesting technology, able to build up easily the segregated filler network, starting from polymeric powder adequately prepared. It is focused on the sintering of polymeric particles by a laser in the classic layer-by-layer mode. Many polymers can be used, from elastomeric to thermosetting, as well as conductive fillers.
In this PhD research project, it was investigated the possibility of obtaining piezoresistive materials printed with 3D SLS using thermoplastic polyurethane (TPU) as a polymer matrix and graphene nanoparticles (GE) and multiwalled carbon nanotubes (MWCNTs) as conductive filler. The main objective of the doctoral research was to investigate the potential of SLS to create porous conductive materials with segregated distribution of the conductive filler, by evaluating the effect of different geometries and porosities (from 20% to 80%) and different shape of the conductive filler (i.e. 1D filler and 2D filler). Again, the aim was to evaluate, based on the complete characterization of the materials, what is the effect of the technology used, finding a possible correlation with the printed geometries. Thus, in the first part of the project, porous systems were printed using TPU modified with 1wt% of GE and starting from Diamond (D), Gyroid (G) and Schwarz (S) geometries for the building up of systems with regular porosity. The resulting three-dimensional porous structures show an effective conductive network due to the segregation of the graphene nanoplatelets previously assembled on the TPU powder surface in between the sintered elastomeric particles. The results confirm that GE nanoplatelets improve the thermal stability of the TPU matrix, while also increasing its glass transition temperature. Furthermore, porous structures made from S geometry show higher elastic modulus values in comparison with D and G based structures. After cyclic compression tests, all porous structures show robust negative piezoresistive behavior, regardless of their porosity and geometry, with exceptional sensitivity to deformation. Gauge Factor (GF) values of 12.4 at 8% deformation are obtained for S structures with 40 and 60% porosity, while GF values up to 60 are obtained for deformations lower than 5%. The thermal conductivity of TPU/GE structures significantly decreases with increasing porosity, while the effect of the structure architecture is less relevant.
The second part of the project focused on the characterization of 3D printed TPU products with MWCNTs and a mixture of the two fillers, again at 1wt% but with a proportion of 70/30 wt/wt MWCNTs/GE with geometries D and G, in order to investigate a possible synergistic effect of the two conductive fillers. The results showed that the porous structures based on TPU with 1wt% MWCNTs/GE exhibit excellent electrical conductivity and mechanical strength. In particular, all the porous structures show a robust negative piezoresistive behavior, as demonstrated by the GF values that reach values of about -13 at 8% deformation. Moreover, the G20 porous structures (20% porosity) show microwave absorption coefficients ranging from 0.70 to 0.91 in the 12-18 GHz region and close 1 in the THz (300 GHz - 1 THz) frequency region. The results show that the simultaneous presence of MWCNT and GE brings a significant improvement in the specific functional properties of porous structures, which are proposed as potential piezoresistive actuators with relevant electromagnetic interference (EMI) shielding properties
