Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Graphene-based nanomaterials for sensors applications: development, characterization and prevention of workers’ exposure during the production process in R&D labs

In the last decades, Graphene-based materials have undergone extensive development due to the unique properties of graphene and 2D nanostructures, enabling the synergic combination of nano-scale and micro-scale effects when used as filler in polymer nanocomposites for multifunctional applications. The final properties of such nanomaterials are dramatically affected by the production process, which therefore must be defined and optimized in order to guarantee reliability and high-quality of the final product. One of the key-aspect in technology and process development is related to the evaluations and assessment of workers’ exposure to nanomaterials and nanostructures during the production process in R&D labs. This is a crucial aspect which needs to be taken into consideration even at the early stage of the development of nanotechnology-based devices, components or materials, in order to prevent risks for workers and to guarantee safety and sustainability of the whole process in line with a holistic approach to nanotechnology and the related engineering applications. Within this context, a prevention-through-design (PtD) approach has been proposed and applied during the design and production of novel graphene-based composite materials for sensor applications, aimed at the mitigation of health and safety risks for workers at the early stage of the production process development. The monitoring and evaluation of workers’ exposure were conducted with regard to the production of graphene nanoplatelets (GNPs) by thermal exfoliation of graphite intercalated compounds (GICs), the spray deposition of GNP-based polymeric coatings for strain sensors, and during the infiltration process of 3D porous structures with a GNP/ethanol solution for highly sensitive low-pressure sensors. The production process of a conductive GNP-based polymeric coating was first developed and fine-tuned in order to produce strain sensors for structural health monitoring (SHM) applications. In particular, the coating was made of a water-based polyurethane paint filled with GNPs. The piezoresistive properties of the coating, which can be easily applied by spray deposition technique, were firstly evaluated. Then, different types of sensors were produced and characterized in order to demonstrate the feasibility of the coating to be used as strain sensors for static and dynamic stimuli detection. As regard the pressure sensors, new 3D squeezable Ecoflex® open cell foams loaded with different concentrations of graphene nanoplatelets (GNPs) were fabricated and tested, in order to obtain lightweight, soft, and cost-effective piezoresistive sensors with high sensitivity in a low-pressure regime. Firstly, the morphology of the produced materials was analyzed and both mechanical and piezoresistive response of samples through quasi-static cyclic compression tests were characterized. Results indicated that sensors infiltrated with 1 mg of ethanol/GNP solution with a GNP concentration of 3 mg/mL were more sensitive and stable compared to those infiltrated with the same amount of ethanol/GNP solution but with a lower GNP concentration. The electromechanical response of the sensors showed a negative piezoresistive behavior up to ~10 kPa and an opposite trend for the 10–40 kPa range. The sensors were particularly sensitive at very low deformations, thus obtaining a maximum sensitivity of 0.28 kPa-1 for pressures lower than 10 kPa. The evaluation of workers’ exposure by inhalation to GNPs during the different productions was performed in parallel with the design and development of the described graphene-based materials and composites, in order to guarantee the sustainable design and production of the devices and propose prevention strategies for health and safety in the workplace. The exposure measurements were conducted in collaboration with the Italian Workers’ Compensation Authority (INAIL), by using a multi-metric methodology based on the harmonized tiered approach proposed by the Organisation for Economic Co-operation and Development (OECD) standard guidance and optimized for each case study. This approach includes the evaluation of multiple exposure data, collected by using a combination of different methods and instruments , including hand-held high resolution (1 Hz) devices for real-time measurements and time integrated personal samplers. Real-time instruments were used to detect the average diameter, the particle number concentration and the lung-deposited surface area of nanomaterials; personal samplers were employed for performing morphological and elemental analysis on airborne sampled materials in the workers’ personal breathing zone (PBZ) and in the workplace environment. The phases at higher risk for workers have been identified in each production process and proper prevention and protective measures were proposed to reduce such risk in the workplace.