Development and characterization of a piezoresistive polyurethane/GNP coating for strain sensing applications

In this work a simple cost-effective process is described to obtain a highly piezoresistive coating, consisting of sprayable water-based polyurethane (PU) paint filled with graphene nanoplatelets (GNPs). We investigated the morphology of the produced nanomaterials (i.e. cross-section and top surface) using a Field Emission Scanning Electron Microscope (FE-SEM). The rheological features of the polymeric blend loaded with 3.5 wt% of GNPs were analyzed at different concentrations of water (up to 20 wt%) in order to achieve a viscosity suitable for air-spraying. Moreover, the effect of humidity on the electrical resistance variation of the cured nanocomposite films was investigated and limited through the use of a covering agent. The stability of the PU/GNP based sensors protected with the covering agent was assessed repeating the same humidity test after four months. The sensor's piezoresistive response was obtained through three-point flexural tests and dc volt-amperometric measurements. The results of the electromechanical tests showed an increasing sensitivity of the sensor with the applied deformation and a maximum gauge factor of 17 at 1% of strain, thus demonstrating the feasibility of the paint for strain sensing in structural health monitoring applications.